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Spectroscopic properties of thulium doped (Lu0.25Gd0.75)2SiO5 (LGSO) single crystals
Journal of Luminescence 220 (2020) 116962
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Spectroscopic properties of thulium doped (Lu0.25Gd0.75)2SiO5 (LGSO) single crystals Adam Strzęp a, b, *, Michał Głowacki c, Maksymilian Szatko a, d, Karolina Potrząsaj a, d, Radosław Lisiecki a, Witold Ryba-Romanowski a a
Institute of Low Temperature and Structure Research PAS, Okolna 2, 50-422, Wroclaw, Poland Chimie-ParisTech (ENSCP), PSL Research University, 11 rue Pierre et Marie Curie, 75005, Paris, France Institute of Physics PAS, Al. Lotnik� ow 32/46, 02-668, Warsaw, Poland d Wroclaw University of Science and Technology, Wybrzerze Wyspianskiego 27, 50-370, Wroclaw, Poland b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Thulium Oxyorthosilicate LSO GSO Single crystal Spectroscopy Luminescence
In this work, spectroscopic properties of (Lu0.25Gd0.75)2(1-x)Tm2xSiO5 (x ¼ 0.04 and 0.005) single crystals were investigated. The material was found to be biaxial, exhibiting strong optical anisotropy, as clearly demonstrated in polarized absorption and emission spectra. Data from room temperature polarized absorption spectra were used in calculations within the phenomenological Judd-Ofelt model. The intensity parameters Ωt were estab lished at Ω2 ¼ 3.24 (�0.58), Ω4 ¼ 2.96 (�0.24) and Ω6 ¼ 0.86 (�0.14) [10 20 cm2]. The values of fluorescence lifetimes of the 1D2 and 1G4 levels were measured and subsequently compared with those calculated using the Judd-Ofelt theory. The strong absorption bands located at wavelengths around 355, 460 and 790 nm, related to the 3H6 → 1D2, 1G4, 3H4 transitions, were found suitable for optical pumping of the investigated material. The characteristics of emission spectra recorded in the blue (ca. 455 nm) and IR (ca. 1750 nm) regions indicate that further investigations of stimulated emission within those wavelength ranges are well justified and can be considered promising.
1. Introduction Oxyorthosilicates with the general formula RE2SiO5 form a group of known but still intensively studied materials. Lu2SiO5 or Gd2SiO5 crys tals doped with cerium or praseodymium ions, usually denoted as LSO (Ce, Pr) or GSO (Ce, Pr) respectively, belong to the most common scintillation materials [1]. The Rare Earths oxyorthosilicates doped with the lanthanide ions such as Nd3þ, Sm3þ, Eu3þ, Tb3þ, Dy3þ, Er3þ, Yb3þ can find potential application as phosphors [2] or laser materials [3]. Rare Earths oxyorthosilicates RE2SiO5 (RE ¼ Sc3þ, Y3þ, Gd3þ, Lu3þ) crystalize in the monoclinic system. Most of the RE2SiO5 crystallize in the C2/c space group. Exception is the Gd2SiO5 which crystallizes in the P21/c space group. The LSO and GSO systems exhibit a continuous solubility in solid state. However, due to the different space groups of pure components, phase type of the (LuxGd1-x)2SiO5 crystal change, when x is close to 0.17 [4]. The great disadvantage of a solution crys tallized in the P21/c space group, is its tendency to crack along a cleavage plane, what leads to the problems during mechanical pro cessing of this material. On the other hand, the LSO melts at 2060 � C.
This temperature is close to a thermal breakdown of iridium crucibles and a heat insulation of stabilized zirconia ceramics. Introduction of the solid state solution of GSO and LSO provides the two advantages. Firstly, the melt temperature of the solution can be reduced by ca. 200 K in comparison to the melt temperature of a pure LSO crystal [4]. Secondly, a huge amount of an expensive lutetium oxide can be substituted by a relatively cheaper gadolinium oxide. Sidletsky et al. [5] discuss the other practical aspects of substitution of Lu3þ ions by Gd3þ ones. They suggested that the substitution of Lu3þ by Gd3þ will efficiently reduce the difference between ionic radii of the dopant and the constituent ions. This should move the segregation coefficient closer to the unity resulting in a more homogenous distribution of the dopants along the pulled crystals. Trivalent thulium ions, due to their energy level structure, can emit light at various wavelengths in the IR, VIS and UV regions. The energy separations between the luminescent levels are sufficiently high to suppress the non-radiative relaxation virtually regardless of the matrix into which Tm3þ ions were doped. Tm3þ ions in crystals are known to lase over a relatively wide wavelength range (from ~0.348 to ~2.46
* Corresponding author. Institute of Low Temperature and Structure Research PAS, Okolna 2, 50-422, Wroclaw, Poland. E-mail address: [email protected] (A. Strzęp). https://doi.org/10.1016/j.jlumin.2019.116962 Received 16 May 2019; Received in revised form 9 December 2019; Accepted 10 December 2019 Available online 16 December 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
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Journal of Luminescence 220 (2020) 116962
μm) [6]. It is also known that the tunable stimulated emission in the
Table 1 The crystallographic characteristic of the two Lu3þ sites in Lu2SiO5 taken from Ref. [9] *Available r(Ln) was calculated by subtraction of 1.40 (value of ionic Radius of O2 ) from value of the mean Ln–O distance.
vacuum and far UV regions can be obtained by the interconfigurational 4f115d1–4f12 allowed transition of Tm3þ. This emission was achieved in the several Tm3þ doped anisotropic fluoride crystals [7]. This work is aimed at the assessment of the spectroscopic properties of the thulium doped (Lu0.25Gd0.75)2SiO5 single crystals as the potential laser material in IR or VIS regions. The absorption and emission spectra as well as the luminescence kinetics were measured and analyzed under various pumping conditions and temperatures. The influence of optical anisotropy on the spectroscopic properties was investigated in details. 2. Experimental
Single crystals of (Lu0.25Gd0.75)2SiO5 doped with 0.5 at.% and 4 at.% of Tm3þ were grown by the Czochralski method. The purity grade of the starting materials were: Gd2O3 (5 N), Lu2O3 (4 N), SiO2 (4N5), Tm2O3 (5 N). The detailed procedure of a crystal growth is described in the paper written by Glowacki et al. [4]. Orthorhombic, monoclinic and triclinic crystals are optically biaxial. In the monoclinic system one of the optical indicatrix is collinear with the crystallographic b axis. The obtained crystal boule was cut into a slices with the planes perpendicular to the crystallographic b axis. The slices were further polished for the optical measurements. A crosspolarized polariscope was used to determine the directions in which the crystal slice does not pass light (the directions of the two other indicatrices have been revealed – hereafter denoted as X1 and X2). The samples in the form of 5 � 5 � 4 mm3 cubes were cut along the deter mined directions out of the crystal slice and all walls were polished for the measurements. Due to lack of sufficient equipment we were unable to determine the exact values of nα, nβ, and nγ, so we can’t assign if the investigated crystal is optically positive or negative, nor we can estimate the value of the inclination angle between the optical axes. The absorption spectra were recorded employing an Agilent Cary5000 UV-VIS-NIR spectrophotometer. The instrument spectral bandwidth was set to 0.25 nm in UV-VIS (300–800 nm) and 1 nm in IR (800–2500 nm) regions. The emission and lifetime measurements were performed using a Dongwoo Optron DM750 monochromator coupled with a Hamamatsu R928 photomultiplier or an Electro-Optical Systems INC PbS photodiode. The signal from the detectors were gathered and averaged by a Tektronix MDO-3052B Oscilloscope. An Opotec Opolette tunable pulsed laser system was used as the excitation source. For the low temperature measurements a continuous – flow liquid helium cryostat (OXFORD INSTRUMENTS model CF 1204) equipped with a temperature controller was used. The polarized spectra were measured using Harrick PGT-S1V Glan-Taylor polarizers.
Parameter
Ln1
Ln2
Coordination number (CN) Local symmetry Ln–O mean [Å] Ln–O min [Å] Ln–O max [Å] Ln–O stand. dev. (σ) [Å] LnX–LnX min distance [Å] Ln1–Ln2 min distance [Å] Available r(LnX)* [Å]
7 C1 2.325 2.160 2.616 0.142 3.587 3.345 0.925
6 C1 2.228 2.166 2.262 0.032 3.425 0.828
the Ln-O min distance is the Ln-O minimum distance, the Ln-O max distance is the Ln-O maximum distance, the Ln-O stand dev is the Ln-O distance standard deviation. The available lanthanide radius was calculated by subtraction of 1.40 from the Ln-O mean distance. The value of 1.40 is a value of the ionic radius of O2 ligand, that was given and used by Shannon and Prewitt [8]. Taking into consideration the standard deviations of Ln–O distance in the both sites, one can assume, that the Ln–O distances in the “Ln2” site are quite uniform, while the Ln–O distances in the “Ln1” site are more heterogeneous. The non-uniform distribution of oxygen ions in the “Ln1” site should result in a better adaptation of the oxygen ligands for ions with the bigger ionic radius (like Gd3þ), while the “Ln2” sites should be preferentially occupied by the smaller Lu3þ ions (in case of the investigated material). The mismatch between the ionic radii of Gd3þ, Tm3þ and Lu3þ will slightly affect the symmetry of the crystal envi ronment around neighbor and next neighbor “Ln1” and “Ln2” sites. Since the ligands geometry modifies local crystal field - energy of the 2Sþ1 LJ(α) Stark levels of lanthanide ions incorporated into the matrix will also be modified for the each “Ln” site. The abovementioned energy differences lead to the broadening of the absorption and emission bands, what in extremely case, can change the shape of bands from “crystalline” to “glassy” type (this behavior can be clearly noticed, when comparing the absorption spectra of the dysprosium doped (LuxGd1-x)2SiO5 crystals shown in the work of Dominiak-Dzik et al. [10]). The arguments described above lead to a conclusion that in the investigated material, two groups of crystalline environment are present instead of two strictly defined sites. Again we remind, that those are only the theoretical considerations, and are not based on the experimental data. 4. Absorption characteristics This section summarizes the results of measurements and analyzes their relevance. The presented data confirm, that the Tm3þ ions locate in at least two distinct crystallographic sites. The room temperature and the low temperature (10 K) polarized absorption spectra recorded for the samples doped with 4 at.% of Tm3þ are shown on Figs. 1 and 2 respectively. They reveal slight but observable optical anisotropy of an investigated material. Changes in the absorption bands shape and in tensities are clearly seen especially for the 3H6 → 3H4, 3F2,3 transitions. For the polarizations E||X1 and E||X2 the absorption is more intense for 3 H6 → 3F2,3 transition, while for the E||B polarization the 3H6 → 3H4 transition is more prominent. Also a shape of band related to the 3H6 → 3 F2,3 transition differs for the E||X1 and E||X2 polarizations. For the former polarization this transition is dominated by the strong line at 12642 cm 1 (791.0 nm). Second transition is composed of numerous lines, among which 5 have similar intensity, and are partially over lapped. Those lines occur at 12642, 12702, 12890, 12961, and 13067 [cm 1] (791.0; 787.3; 775.8; 771.5; 762.5 [nm] respectively). The very similar observation was made in the thulium doped Y2SiO5 crystals by Li et al. [11] since the investigated material and Y2SiO5 share similar crystal structure.
3. Crystallographic and optical characterization To fully understand the spectroscopic features of the investigated material, the short introduction into its structural properties is needed. This section touches only the theoretical considerations. It was intro duced for better understanding of the multisite location of Tm3þ ions in the investigated matrix. The crystal structure of Lu2SiO5 (investigated material is isostructural to this compound) possesses the two different crystallographic sites of Lu3þ. They differ in the size and the coordina tion number. Gadolinium ions possess a little bit higher ionic radius than Tm3þ and Lu3þ (0.94, 0.88 to 0.86 [Å] respectively for Coordination Number ¼ VIII [8]). Since almost ¾ of the ions in the investigated crystal are the “big” Gd3þ ions we assume that the sites we denote as “Ln1” will be preferentially occupied by gadolinium, while the smaller “Ln2” sites will accept Lu3þ and Tm3þ. Occurrence of the Tm3þ in the “Ln1” sites can’t be excluded however. The crystallographic features of those sites are characterized in a Table 1. The data gathered in Table 1 were ob tained on basis of the following assumptions: The coordination number and the local symmetry were taken directly from the appropriate CIF file. The Ln-O mean distance is the arithmetic mean of all Ln-O distances, 2
A. Strzęp et al.
Journal of Luminescence 220 (2020) 116962
Fig. 1. Polarized room temperature absorption spectra of the investigated crystal.
Fig. 2. Polarized low temperature (10 K) absorption spectra of the investigated crystal.
At room temperature the strongest absorption occurs for the E||X1 polarization for IR, VIS and UV bands, reaching absorption coefficient of 14.29 cm 1 at 8271 cm 1 (1209 nm). The strong absorption band related to the 3H6 → 1D2 transition, with maximum at 28189 cm 1 (354.75 nm) for all polarizations could be used for optical pumping of
this material by the third harmonic of the easily commercially available Nd:YAG lasers. The appropriate absorption coefficients are equal to 5.57, 8.37 and 4.69 cm 1 for the E||B, E||X1 and E||X2 polarizations, respectively. Going down with temperature to the 10 K, the absorption lines 3
Journal of Luminescence 220 (2020) 116962
A. Strzęp et al.
(shown on Fig. 2.) become narrower. It is shown that the intensity of some absorption lines vary. The suppressed lines belong to transitions from the higher Stark components of the 3H6 ground state. Fortunately the lines at ca. ~355 nm belong to transitions from the lowest Stark component of ground state. With decrease of the temperature their in tensities increase reaching an absorption coefficient of 19.28 cm 1 at 28153 cm 1 (355.2 nm) at 10 K (for E||X1 polarization). This line is quite broad, so the small discrepancy between exact wavelength of 3rd harmonic of Nd:YAG (354.67 nm) does not matter significantly. In fact, the absorption coefficient at the exact Nd:YAG 3rd harmonic wavelength equals to 18.37 cm 1. The other lines suitable for the optical pumping of the material employing lasers could be found at ca. 800 nm. At the low temperature (10 K) the two narrow and intense lines are present for the E||B and E||X1 polarizations in this region. For the E||B polarization the absorption coefficients equal to 6.97 and 13.18 [cm 1] for lines at 12645 cm 1 (790.8 nm) and 12703 cm 1 (787.2 nm), respectively. For the E||X1 polarization those coefficients equal to 11.92 and 11.79 [cm 1], respectively. Based on the low temperature absorption spectra, energy of the Stark levels of several Tm3þ multiplets were determined. Their complete list is shown in Table 2. For each multiplet a number of the observed Stark components exceed theoretical number of the predicted Stark compo nents for the C1 symmetry. This means that the thulium ions must be located in both crystal sites present in this crystal.
transitions, due to the fact that they overlap. This operation was made in order to reduce experimental uncertainties. The mean oscillator strength was calculated according to following formula: Pmean ¼ (PE||B þ PE||X1 þ PE||X2)/3. Obtained values of the mean oscillator strengths were then used as an input data for the calculations in the framework of the phenomenological Judd – Ofelt method. The obtained Ωt (t ¼ 2, 4, 6) parameters are gathered in Table 3. The Judd-Ofelt model considers only transitions of the Electric Dipole type (hereafter denoted as ED). However for the transitions ful filling following rule ΔJ ¼ 0, �1 (except 0↔0) a Magnetic Dipole type transition contributes, too. Thus, it should be mentioned, that for the absorption spectra of Tm3þ both 3H6 → 3H5 and 3H6 → 1I6 transitions are mixed ED and MD in nature. The latter transition was not observed during this study, however. The oscillator strength of the absorption transition could be calculated based on the experimental data and aforementioned equation, however it could be also calculated knowing the value of the so called “line strength” of selected transition. The equation to do it is as follows: F¼
The transition intensities of Tm3þ ions in the (Lu0.25Gd0.75)2SiO5 crystal were analyzed in the framework of the Judd-Ofelt phenomeno logical model [12,13]. The polarized absorption spectra recorded at room temperature were used to calculate the oscillator strengths PE||B, PE||X1 and PE||X2. The intensities of absorption bands were evaluated by means of the numerical integration employing following equationEq 1: Z 4:303x10 9 υ2 FE ¼ αðυÞdυ (1) c υ1
SMD ¼
Barycenter
ΔE
Nexp
Ntheor
3
5705, 5754, 5709, 5814, 5844, 5858, 5970, 6086, 6112, 6161, 6188, 6219, 6309 8271, 8285, 8467, 8503, 8591, 8673, 8703, 8741, 8865, 8920 12636, 12650, 12687, 12706, 12714, 12815, 12877, 12966, 12993, 13051, 13073, 13219 14603, 14652, 14695, 14751, 14779, 14825, 14876, 15181, 15285, 15406, 15463, 15598 21017, 21052, 21088, 21137, 21213, 21436, 21598, 21667, 21739, 21795, 21819, 21981, 21987 27862, 27909, 27995, 28153, 28200, 28296, 28409
5979
604
13
2�9
8602
649
10
2�11
12866
583
12
2�9
15010
995
12
2�(5 þ 7)
21502
970
13
2�9
28118
547
7
2�5
F4
3
H5
3
H4
3
F2,3
1
G4
1
D2
(3)
1. Case 1: J’ ¼ J �r � JðJ þ 1Þ þ SðS þ 1Þ LðL þ 1Þ 2JðJ þ 1ÞðJ þ 2Þ ðψ JkLþ2Skψ ’ J ’ Þ¼ℏ 1þ 2JðJ þ 1Þ
Table 2 Values of the energies, barycenters, energy splitting and number of experi mentally observed Stark levels of selected multiplets in investigated crystal. Energy [cm 1]
h2 ðψ JkL þ 2Skψ ’ J ’ Þ 16π2 m2 c2
The numerical value of h2/16π2m2c2 is equal to 3.73 � 10 22. The value of (ΨJkLþ2SǁΨ’J’) could be calculated with use of the following equations.
where FE means the oscillator strength at appropriate polarization, c means Tm3þ ions concentration in moles/dm3 and α(υ) means absorp tion coefficient at appropriate wavenumber υ. The integration takes place for a whole band related to the investigated transition between υ1 and υ2. This relation was applied to the six bands located in the 5000–29000 cm 1 spectral range and the procedure was repeated for each polarization state. For one band related to 3H6 -> 3F2, 3F3 transi tions the oscillator strengths were calculated holistically for both
H6 →
(2)
where m is an electron mass (9.11 � 10 28 g), c is a velocity of light in vacuum (2.997 � 1010 cm/s), and h is a Planck Constant (6.626 � 10 27 erg/s). Numerical value of 8πmc/3h equals to 1.08 � 1011. All those values are given in the nowadays abandoned CGS system, which was quite often used when Judd and Ofelt separately proposed their models. To preserve compatibility with the old data still CGS system is used. The barycenter of transition wavenumber is denoted as υ. J is the total angular momentum quantum number of the initial multiplet, χ is a Lorentz correction factor that is n for MD transitions or (n2þ2)2/9 for ED transition (n is refractive index). S is called a “line strength” and it equals Ω2U2þΩ4U4þΩ6U6 for the ED transitions. For the MD transition it could be calculated directly from the equationEq 3:
5. Judd – Ofelt analysis
4
8π2 mc υ χS 3h 2J þ 1
2. Case 2: J’ ¼ J-1 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi JÞðJ þ S LÞðJ þ L SÞ ℏ ðS þ L þ J þ 1ÞðS þ L þ 1 ’ ’ ðψ JkLþ2Skψ J Þ¼ 4J
3. Case 3: J’ ¼ Jþ1 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi LÞðL þ J þ 1 SÞðS þ L JÞ ℏ ðS þ L þ J þ 2ÞðS þ J þ 1 ’ ’ ðψ JkLþ2Skψ J Þ¼ 4ðJ þ 1Þ with knowledge of the line strength of MD we could get back to equation (2) and calculate MD part of the oscillator strength for the appropriate absorption transition. Subtracting a MD part from experimental value should leave only ED part, suitable for the further Judd-Ofelt analysis. All aforementioned equations were taken from the work of Carnall, Fields and Wybourne [14]. Considering calculated values of the oscillator strength (gathered in 4
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Journal of Luminescence 220 (2020) 116962
Table 3 Calculated oscillator strengths for the absorption bands and the Judd-Ofelt analysis for (Lu0.25Gd0.75)1.92Tm0.08SiO5 single crystal. 3
Barycenter [cm 1]
Fosc �10 EkB
EkX1
EkX2
mean
ED
MD
J-O
3
6000 8440 12820 14710 21440 27980
2.47 2.02 2.53 2.05 1.03 3.24
3.77 2.71 4.07 4.67 1.72 5.02
3.25 2.44 3.64 4.88 1.30 4.07
3.16 2.39 3.41 3.87 1.35 4.11
3.16 2.11 3.41 3.87 1.35 4.11
– 0.28 – – – –
3.60 1.97 3.00 4.08 1.22 4.19
H6 ->
F4 3 H5 3 H4 3 F2.3 1 G4 1 D2
6
Error [%]
Ω2 ¼ 3.24 (�0.58); Ω4 ¼ 2.96 (�0.24); Ω6 ¼ 0.86 (�0.14) [�10
20
þ13.6 6.7 11.2 þ5.8 9.4 þ4.9
cm2]; AVG err ¼ 13.0%
a Table 3) we can perceive that the biggest errors occur for the transi tions 3H6 → 3F4, 3H4, 1G4. For all those transitions ΔJ ¼ 2. The transi tions fulfilling this rule are called the hypersensitive ones. Their intensity strongly depends on the crystal field around the lanthanide ion and usually those transitions are badly described by the Judd-Ofelt theory [15]. However in the case of investigated material obtained er rors are within acceptable values. The omega parameters gathered in a Table 3 were used to determine the emission characteristic of the system under study. The radiative transition rates, luminescence branching ratios, and radiative lifetime of excited states were estimated based on the Judd - Ofelt model. The obtained results are collected in a Table 4. Similarly to the absorption calculations, emission was also corrected for the MD type transitions. There is no comparison between the two concentrations, because at the low doping levels, when dopant does not force structural change, the absorption bands are proportional to the concentration. And since the oscillator strength does not depend on the concentration we does not think it is necessary to repeat the calculation of JO parameters for the second sample – thus results should be the same within experimental error. The reason why second sample was prepared (0.5 at%. Tm3þ) was to check the lifetime values of metastable levels and how they are affected by the concentration. Especially the triplets F4 and H4. Optical excitation of 3H4 could lead to the “2 for 1” pumping scheme via the efficient cross-relaxation. Thus, it is important to check, if for a high dopant concentration, the lifetime of 3H4 decreases significantly while lifetime of the 3F4 remains the same. This is the case for an efficient 2 for 1 scheme pumping.
6. Luminescence characteristics Thulium ions exhibit luminescence from up to 5 levels, namely 3P0, D2, 1G4, 3H4 and 3F4. Since in this matrix the 3P0 absorption line overlaps with the absorption lines of Gd3þ ions, this level wasn’t investigated. The other four luminescent multiplets are responsible for the emission in UV/blue, blue/red or IR parts of the spectrum respec tively. Among those transitions an emission in the IR region is with high industrial/commercial significance, due to its use in an industrial, telecommunication or medical lasers. The emission usually spans over a several hundred nanometers centering around 1900 nm.
1
6.1. Luminescence from the 3F4 level To estimate the laser potential of the investigated material the polarized emission spectra in 300 K (red) and 10 K (black) were measured. They are presented in Figs. 3 and 4. The room temperature spectra do not show significant anisotropy. Maximum of the emission emerges at ca. 1820–1850 nm. Decrease of temperature to the 10 K leads to more discrete spectra however. For each polarization the strong band component at 1756 nm (5727 cm 1) could be observed. Spectral posi tion of the second most intense peak depends on the polarization. The most prominent line is in fact a 0-0 transition as can be seen from the comparison of the absorption and emission spectra. The positions of second most intense peaks are 1966 nm (5086 cm 1), 1942 nm (5149 cm 1) and 1897 nm (5271 cm 1) respectively. To evaluate the possi bility of a laser action generation in this matrix the effective stimulated emission cross sections have been calculated on the basis of the
Table 4 Experimental and calculated luminescence characteristics of (Lu0.25Gd0.75)1.92Tm0.08SiO5 single crystal. Trans
kU2k
Barycenter [cm 1]
kU4k
kU6k
SED
[nm]
[�10
SMD 21
cm2]
AED
AMD
A
[s 1]
В [%]
1
D2 → G4 6720 F3,2 13345 3 H4 15370 3 H5 19620 3 F4 22100 3 H6 28060 ΣA ¼ 46425 s 1. τrad ¼ 22 μs 1 G4 → 3 F3.2 6625 3 H4 8650 3 H5 12900 3 F4 15380 3 H6 21340 ΣA ¼ 5459 s 1. τrad ¼ 183 μs 3 H4 → 3 H5 4250 3 F4 6730 3 H6 12690 ΣA ¼ 2098 s 1. τrad ¼ 477 μs 3 F4 → 3 H6 5960 ΣA ¼ 421 s 1. τrad ¼ 2373 μs 1 3
1488 749 651 510 452 356
0.1874 0.2280 0.1257 0 0.5686 0
0.1799 0.3771 0.0124 0.0012 0.0961 0.3131
0.0022 0 0.2300 0.0182 0.0215 0.0958
11.42 18.55 6.42 0.19 21.45 10.09
– 2.05 – – – –
302 3848 2034 127 20210 19460
– 445 – – – –
302 4293 2034 127 20210 19460
0.7 9.2 4.4 0.3 43.5 41.9
1509 1156 775 650 469
0.0212 0.0200 0.0773 0.1645 0.0464
0.1592 0.0182 0.0078 0.0052 0.0747
0.3685 0.0693 0.5633 0.4114 0.0100
8.57 1.78 7.58 9.02 3.80
1.37 5.01 0 5.01 –
121 56 789 1591 1791
20 165 0 926 –
141 221 789 2517 1791
2.6 4.0 14.5 46.1 32.8
2353 1486 788
0.0131 0.1275 0.2672
0.4762 0.1311 0.1650
0.0095 0.2113 0.5704
14.60 9.83 18.45
2.13 4.01 –
54 145 1828
8 62 –
63 207 1828
3.0 9.9 87.1
1678
0.5395
0.7261
0.2421
41.05
–
421
–
421
100
5
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Journal of Luminescence 220 (2020) 116962
Fig. 3. Overlap of the low temperature (10 K) absorption and emission spectra related to the 3H6 – 3F4 transition – bands relevant for IR lasing channel of Tm3þ ion.
Fig. 4. Overlap of the room temperature absorption and emission spectra related to the 3H6 – 3F4 transition – bands relevant for IR lasing channel of Tm3þ ion.
reciprocal method. The results are gathered in Fig. 5. As could be seen the polarization plays important role in both possible efficiency and lasing wavelength of this material. However further laser experiments are needed to estimate the real gain and lasing wavelengths. A chance of obtaining the laser action in this material is rather high, since laser ac tion in the similar RE2SiO5 crystals have already been obtained. Bin Liu et al. have reported on the slope efficiency up to 26% in the Tm doped Sc2SiO5 crystals [16]. Yao et al. have reported on the slope efficiency up
to 21% in the Tm doped Lu2SiO5 crystals [17]. The broad emission spectrum at the room temperature could potentially find application in the short pulse lasers, while when operated at the cryogenic tempera tures this laser could be operated in the traditional single-wavelength scheme. The absorption spectra measured at the temperature of liquid nitrogen shows, that the shape of spectrum strongly resembles those observed at 10 K rather than those at 300 K. This means that an efficient laser operation could be potentially obtained with use of a much cheaper 6
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Journal of Luminescence 220 (2020) 116962
Fig. 5. Calculated effective stimulated emission cross-sections (300 K) for the 3F4–3H6 transition for various partition coefficient values – influence of polarization.
6.2. Luminescence from the 1G4 level
liquid nitrogen or a stack of Peltier Modules, instead of the expensive helium equipment. Moreover on the base of the low temperature absorption and emis sion spectra only one 0-0 transition could be observed what suggests that the 0-0 transitions for both sites are present however they do overlap spectrally.
The second aforementioned luminescent level, 1G4, emits in the cyan and red parts of the visible spectrum. The room temperature absorption/ emission features of this level are shown in Fig. 6. The luminescence from this level was observed under an excitation at 463 nm laser with pulses delivered by OPO. The emission spectra were not corrected for
Fig. 6. Spectral features of absorption/emission related to 1G4 level recorded at 300 K 7
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spectral response of a measurement system. The absorption and emis sion spectra are characterized by the significant anisotropy. At the room temperature bands are composed of broad overlapping lines. The one of the most prominent emission line in each polarization corresponds to the similar line in the absorption spectrum. Since those transitions possess neighbors on both right and left sides it is hard to say if they are a 00 transition. Especially if we consider that the aforementioned maximum lies at 477 nm in the case of E||X1 and E||X2 and at 474 nm in the case of E||B polarization. The small absorption coefficient contrary to emission intensity at ca. 490 nm may suggest future laser experiment at this wavelength, under pumping by the more and more popular laser diodes operating at a 473 nm or 460 nm. To increase chance of the laser generation success a cryogenic temperature (~100 K) could be helpful. For those experiments especially the E||X2 polarization could be useful.
level operation scheme, thus the pumping power delivered must be high. On the other hand laser oscillations on the 1D2→3F4 transition may be difficult to achieve due to the spectral overlap between this emission and the 3H6→1G4 absorption bands. The decrease of temperature most probably will increase the spectral overlap. Additionally much longer lifetime of the 3F4 level contrary to the 1D2 level will not allow to obtain a CW operation. The lifetime in the order of several milliseconds will restrict the maximum pulse repetition rate to several Hz. The spectral overlaps are presented in graphs enclosed in the “supplementary ma terials” Fig. s1. 7. Excited state relaxation kinetics In this section we will present the results on our study on the metastable levels lifetimes. Influence of the temperature and concen tration of dopant ions on the metastable level lifetime will be shown and discussed. Especially crucial and important is characterization of the 3F4 level since it is a metastable level for the laser channel 3F4 → 3H6 that gives IR radiation ca. 1.8–1.9 μm. The luminescence kinetics of the excited levels were investigated as a function of the temperature within a 10–300 K range. Investigations of luminescence kinetics could bring information concerning the path of relaxation that the electron follows from the excited level down to the luminescent one. All those information is present in the rise and decay parts of the luminescence kinetics curve after a short pulse excitation. Those investigations also provide information on the inter-ionic in teractions among lanthanides like the energy transfer (especially crossrelaxation). The cross-relaxation processes may be responsible for diminishing of luminescence intensity for a particular excited level. On the other hand they are able to provide an efficient way of excitation of other metastable levels. For example, the cross relaxation process pro vides extremely efficient transfer of energy accumulated on the 3H4 pump level to the metastable 3F4 level in thulium lasers operating near 1700–1900 nm (the two-for-one pumping scheme). In the investigated material the four luminescent levels were observed. They are the 1D2,
6.3. Luminescence from the 1D2 level The last mentioned luminescent level possesses the highest energy. Its absorption spectrum clearly suggest possible pumping by a common 3rd harmonic of Nd:YAG lasers at a 355 nm. With use of this excitation we can achieve an efficient emission in UV (~365 nm) and royal-blue (~455 nm) regions. The emission spectra measured at room tempera ture are shown in Fig. 7. Those spectra were recorded separately and were not corrected for a spectral response. All of the spectra presented in Fig. 7 were normalized to 1. Comparison of the spectral bandwidths (in cm 1) of lines for bands related to the 1D2→3H4 and 1D2→3F4 transitions leads to conclusion that the latter ones are much narrower. Additionally anisotropy plays much more role for these transitions than for the 1D2→3H6 one. Likely in the 1 D2→3F4 transition only the lowest Stark components of the 3F4 level are involved. As it was frequently experimentally proven, in this material also, the lowest Stark components are weakly coupled with the matrix phonons. Considering the laser feasibility of transitions from this level theoretically both transitions could be used for generation. However, for both cases the transition under resonant pumping needs the quasi three-
Fig. 7. Spectral features of absorption/emission related to 1D2 level recorded at 300 K 8
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G4 3H4 and 3F4 levels. The radiative properties of those levels were predicted on basis of the Judd-Ofelt model, and are gathered in afore mentioned tables. After short laser pulses with wavelength tuned to absorption of the 3 H6→1D2 transition (355 nm, UV light), the intense blue emission ca. 460 nm could be observed. This light is generated by a the 1D2→3F4 transition. The predicted radiative lifetime amounts to 22 μs. The observed lifetimes equal to 14 μs for the 0.5% Tm sample and 2 μs for the 4%Tm sample. Based on the energy levels structure of the Tm3þ ions it is easy to find a potential path for the concentration quenching of the 1D2 level. The transitions 1D2→3H4 and 3H6→3F3 are in resonance. Thus Tm3þ ion excited to the 1D2 level, could share part of the energy with neighboring Tm3þ ion in ground state. After cross relaxation one ion is in the 3H4 and second in the 3F3 state – efficiently depleting the 1D2 level. This mechanism could be responsible for efficient decrease of the radi ative properties of this level. To check this cross-relaxation contribution a sample with lower concentration of Tm was grown. For the (Lu0.25Gd0.75)1.99Tm0.01SiO5 sample under the same pumping and observation conditions the luminescence lifetime value was τ1 ¼ 14 μs. This clearly confirms that in the investigated crystal Tm3þ concentration is too high for the efficient emission from this level. The luminescence kinetics of the 1G4 level was also investigated. The observed curves were single exponential for sample containing the lower Tm3þ concentration. Comparison of the theoretical radiative lifetimes and experimental 226 μs for the 0.5 at.% Tm sample and to 9 μs for the 4 at.% Tm sample leads to the conclusion that this level is strongly quenched by the cross relaxation in the heavier doped sample. The possible cross-relaxation channels could be 1G4(1)þ3H6(2)→ 3 H5(1)þ3H4(2) or 3H4(1)þ3H5(2) or 3F3(1)þ3F4(2). Diversity of a crossrelaxation channels is most probably responsible for the efficient depopulation of the excited state. Experimental lifetimes of the 3H4 level equal 16 μs for the 4 at.% Tm and 145 μs for the 0.5 at.% Tm. Shortening of the lifetime is an evidence of the strong cross-relaxation - the only possible relaxation path is 3H4 þ 3 H6 – 2 � 3F4. This shows the efficient way to populate 3F4 upper laser level. The one pumping photon can be converted into the two emission photons. This process could highly increase a possible laser emission efficiency. The lifetimes of the 3F4 level does not show such dependency on concentration. For the sample doped with 0.5 at.% Tm the lifetime value is 2.21 ms while for the 4 at.% Tm sample it is 1.64 ms. This clearly shows that the higher concentrations are better suited for laser active material since the relaxation from the pumping level to the metastable level is faster, while high luminescence efficiency is still maintained. All decay curves that were utilized to estimate aforementioned experimental lifetimes are included in the supplementary materials.
Pr- and Ce-doped mixed lutetium-gadolinium oxyorthosilicates have been previously investigated. It was found that the thulium ions occupy both of the available lanthanide sites in this matrix. A strong energy transfer between the sites is bound to occur, since the observation of the two different emission spectra under various excitations was not possible. The lifetime measurements showed that 4 at.% doping is relatively high in the case of this matrix and lower concentration values should be considered in order to increase the luminescence quantum efficiency for the 1D2, 1G4 and 3H4 levels. On the other hand, heavier doping is favorable for the intensity of the emission from the 3F4 level, when pumping of the 3H4 level ca. 800 nm is considered. Further studies regarding the generation of laser action are required to decisively confirm or rule out the usability of this material for laser applications. Declaration of competing interest All authors declare that there was no conflict of interest while doing research described in this manuscript. CRediT authorship contribution statement Adam Strzęp: Methodology, Validation, Formal analysis, Investiga tion, Resources, Writing - original draft, Writing - review & editing, Visualization. Michał Głowacki: Investigation, Resources. Maksymi lian Szatko: Investigation, Formal analysis, Resources, Visualization. Karolina Potrząsaj: Investigation, Formal analysis, Resources, Visual ization. Radosław Lisiecki: Formal analysis, Visualization. Witold Ryba-Romanowski: Conceptualization, Supervision, Project adminis tration, Funding acquisition. Acknowledgement The work received financial support from National Science Centre, Poland under grant no DEC 2016/21/B/ST5/00890. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jlumin.2019.116962. References [1] B. Liu, M. Gu, Z. Qi, C. Shi, M. Yin, G. Ren, Journal of. Luminescence 127 (2007) 645. [2] A. Strzep, W. Ryba-Romanowski, M. Berkowski, J. Lumin. 153 (2014) 242–244. [3] M. Jacquemet, et al., J. Appl. Phys. B 80 (2005) 171. [4] M. Glowacki, et al., J. Solid State Chem. 186 (2012) 268–277. [5] O. Sidletskiy, et al., J. Cryst. Growth 312 (2010) 601. [6] A.A. Kaminskii, Laser Photonics Rev. 1 (2) (2007). [7] K.H. Yang, J.A. DeLuca, Appl. Phys. Lett. 29 (1976) 499. [8] R.D. Shannon, Acta Crystallogr. 32 (1976) 751–767. [9] T. Gustafsson, et al., Acta Crystallogr. C 57 (2001) 668. [10] G. Dominiak-Dzik, et al., Cryst. Growth Des. 10 (2010) 3521. [11] C. Li, et al., J. Lumin. 62 (1994) 157–171. [12] B.R. Judd, Phys. Rev. 127 (1962) 750. [13] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [14] W.T. Carnall, P.R. Fields, B.G. Wybourne, J. Chem. Phys. 42 (1965) 3797. [15] R.D. Peacock, Chem. Phys. Lett. 16 (1972) 590–592. [16] B. Liu, et al., Chin. Phys. B 26 (2017), 084203. [17] B.Q. Yao, et al., Laser Phys. Lett. 5 (2008) 714–718.
8. Conclusions The motivation behind growing this crystal was the pursuit of novel, highly efficient thulium-based laser materials. It was found that thulium-doped (Lu0.25Gd0.75)2SiO5 crystals are characterized by high optical anisotropy. The chemical stability, moderate thermal conduc tivity and broad region of transparency make LGSO a good candidate material for a laser matrix considered for doping with various lantha nide ions. In the recent studies of our group, the properties of Dy-, Sm-,
9
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Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Spectroscopic properties of thulium doped (Lu0.25Gd0.75)2SiO5 (LGSO) single crystals Adam Strzęp a, b, *, Michał Głowacki c, Maksymilian Szatko a, d, Karolina Potrząsaj a, d, Radosław Lisiecki a, Witold Ryba-Romanowski a a
Institute of Low Temperature and Structure Research PAS, Okolna 2, 50-422, Wroclaw, Poland Chimie-ParisTech (ENSCP), PSL Research University, 11 rue Pierre et Marie Curie, 75005, Paris, France Institute of Physics PAS, Al. Lotnik� ow 32/46, 02-668, Warsaw, Poland d Wroclaw University of Science and Technology, Wybrzerze Wyspianskiego 27, 50-370, Wroclaw, Poland b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Thulium Oxyorthosilicate LSO GSO Single crystal Spectroscopy Luminescence
In this work, spectroscopic properties of (Lu0.25Gd0.75)2(1-x)Tm2xSiO5 (x ¼ 0.04 and 0.005) single crystals were investigated. The material was found to be biaxial, exhibiting strong optical anisotropy, as clearly demonstrated in polarized absorption and emission spectra. Data from room temperature polarized absorption spectra were used in calculations within the phenomenological Judd-Ofelt model. The intensity parameters Ωt were estab lished at Ω2 ¼ 3.24 (�0.58), Ω4 ¼ 2.96 (�0.24) and Ω6 ¼ 0.86 (�0.14) [10 20 cm2]. The values of fluorescence lifetimes of the 1D2 and 1G4 levels were measured and subsequently compared with those calculated using the Judd-Ofelt theory. The strong absorption bands located at wavelengths around 355, 460 and 790 nm, related to the 3H6 → 1D2, 1G4, 3H4 transitions, were found suitable for optical pumping of the investigated material. The characteristics of emission spectra recorded in the blue (ca. 455 nm) and IR (ca. 1750 nm) regions indicate that further investigations of stimulated emission within those wavelength ranges are well justified and can be considered promising.
1. Introduction Oxyorthosilicates with the general formula RE2SiO5 form a group of known but still intensively studied materials. Lu2SiO5 or Gd2SiO5 crys tals doped with cerium or praseodymium ions, usually denoted as LSO (Ce, Pr) or GSO (Ce, Pr) respectively, belong to the most common scintillation materials [1]. The Rare Earths oxyorthosilicates doped with the lanthanide ions such as Nd3þ, Sm3þ, Eu3þ, Tb3þ, Dy3þ, Er3þ, Yb3þ can find potential application as phosphors [2] or laser materials [3]. Rare Earths oxyorthosilicates RE2SiO5 (RE ¼ Sc3þ, Y3þ, Gd3þ, Lu3þ) crystalize in the monoclinic system. Most of the RE2SiO5 crystallize in the C2/c space group. Exception is the Gd2SiO5 which crystallizes in the P21/c space group. The LSO and GSO systems exhibit a continuous solubility in solid state. However, due to the different space groups of pure components, phase type of the (LuxGd1-x)2SiO5 crystal change, when x is close to 0.17 [4]. The great disadvantage of a solution crys tallized in the P21/c space group, is its tendency to crack along a cleavage plane, what leads to the problems during mechanical pro cessing of this material. On the other hand, the LSO melts at 2060 � C.
This temperature is close to a thermal breakdown of iridium crucibles and a heat insulation of stabilized zirconia ceramics. Introduction of the solid state solution of GSO and LSO provides the two advantages. Firstly, the melt temperature of the solution can be reduced by ca. 200 K in comparison to the melt temperature of a pure LSO crystal [4]. Secondly, a huge amount of an expensive lutetium oxide can be substituted by a relatively cheaper gadolinium oxide. Sidletsky et al. [5] discuss the other practical aspects of substitution of Lu3þ ions by Gd3þ ones. They suggested that the substitution of Lu3þ by Gd3þ will efficiently reduce the difference between ionic radii of the dopant and the constituent ions. This should move the segregation coefficient closer to the unity resulting in a more homogenous distribution of the dopants along the pulled crystals. Trivalent thulium ions, due to their energy level structure, can emit light at various wavelengths in the IR, VIS and UV regions. The energy separations between the luminescent levels are sufficiently high to suppress the non-radiative relaxation virtually regardless of the matrix into which Tm3þ ions were doped. Tm3þ ions in crystals are known to lase over a relatively wide wavelength range (from ~0.348 to ~2.46
* Corresponding author. Institute of Low Temperature and Structure Research PAS, Okolna 2, 50-422, Wroclaw, Poland. E-mail address: [email protected] (A. Strzęp). https://doi.org/10.1016/j.jlumin.2019.116962 Received 16 May 2019; Received in revised form 9 December 2019; Accepted 10 December 2019 Available online 16 December 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
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μm) [6]. It is also known that the tunable stimulated emission in the
Table 1 The crystallographic characteristic of the two Lu3þ sites in Lu2SiO5 taken from Ref. [9] *Available r(Ln) was calculated by subtraction of 1.40 (value of ionic Radius of O2 ) from value of the mean Ln–O distance.
vacuum and far UV regions can be obtained by the interconfigurational 4f115d1–4f12 allowed transition of Tm3þ. This emission was achieved in the several Tm3þ doped anisotropic fluoride crystals [7]. This work is aimed at the assessment of the spectroscopic properties of the thulium doped (Lu0.25Gd0.75)2SiO5 single crystals as the potential laser material in IR or VIS regions. The absorption and emission spectra as well as the luminescence kinetics were measured and analyzed under various pumping conditions and temperatures. The influence of optical anisotropy on the spectroscopic properties was investigated in details. 2. Experimental
Single crystals of (Lu0.25Gd0.75)2SiO5 doped with 0.5 at.% and 4 at.% of Tm3þ were grown by the Czochralski method. The purity grade of the starting materials were: Gd2O3 (5 N), Lu2O3 (4 N), SiO2 (4N5), Tm2O3 (5 N). The detailed procedure of a crystal growth is described in the paper written by Glowacki et al. [4]. Orthorhombic, monoclinic and triclinic crystals are optically biaxial. In the monoclinic system one of the optical indicatrix is collinear with the crystallographic b axis. The obtained crystal boule was cut into a slices with the planes perpendicular to the crystallographic b axis. The slices were further polished for the optical measurements. A crosspolarized polariscope was used to determine the directions in which the crystal slice does not pass light (the directions of the two other indicatrices have been revealed – hereafter denoted as X1 and X2). The samples in the form of 5 � 5 � 4 mm3 cubes were cut along the deter mined directions out of the crystal slice and all walls were polished for the measurements. Due to lack of sufficient equipment we were unable to determine the exact values of nα, nβ, and nγ, so we can’t assign if the investigated crystal is optically positive or negative, nor we can estimate the value of the inclination angle between the optical axes. The absorption spectra were recorded employing an Agilent Cary5000 UV-VIS-NIR spectrophotometer. The instrument spectral bandwidth was set to 0.25 nm in UV-VIS (300–800 nm) and 1 nm in IR (800–2500 nm) regions. The emission and lifetime measurements were performed using a Dongwoo Optron DM750 monochromator coupled with a Hamamatsu R928 photomultiplier or an Electro-Optical Systems INC PbS photodiode. The signal from the detectors were gathered and averaged by a Tektronix MDO-3052B Oscilloscope. An Opotec Opolette tunable pulsed laser system was used as the excitation source. For the low temperature measurements a continuous – flow liquid helium cryostat (OXFORD INSTRUMENTS model CF 1204) equipped with a temperature controller was used. The polarized spectra were measured using Harrick PGT-S1V Glan-Taylor polarizers.
Parameter
Ln1
Ln2
Coordination number (CN) Local symmetry Ln–O mean [Å] Ln–O min [Å] Ln–O max [Å] Ln–O stand. dev. (σ) [Å] LnX–LnX min distance [Å] Ln1–Ln2 min distance [Å] Available r(LnX)* [Å]
7 C1 2.325 2.160 2.616 0.142 3.587 3.345 0.925
6 C1 2.228 2.166 2.262 0.032 3.425 0.828
the Ln-O min distance is the Ln-O minimum distance, the Ln-O max distance is the Ln-O maximum distance, the Ln-O stand dev is the Ln-O distance standard deviation. The available lanthanide radius was calculated by subtraction of 1.40 from the Ln-O mean distance. The value of 1.40 is a value of the ionic radius of O2 ligand, that was given and used by Shannon and Prewitt [8]. Taking into consideration the standard deviations of Ln–O distance in the both sites, one can assume, that the Ln–O distances in the “Ln2” site are quite uniform, while the Ln–O distances in the “Ln1” site are more heterogeneous. The non-uniform distribution of oxygen ions in the “Ln1” site should result in a better adaptation of the oxygen ligands for ions with the bigger ionic radius (like Gd3þ), while the “Ln2” sites should be preferentially occupied by the smaller Lu3þ ions (in case of the investigated material). The mismatch between the ionic radii of Gd3þ, Tm3þ and Lu3þ will slightly affect the symmetry of the crystal envi ronment around neighbor and next neighbor “Ln1” and “Ln2” sites. Since the ligands geometry modifies local crystal field - energy of the 2Sþ1 LJ(α) Stark levels of lanthanide ions incorporated into the matrix will also be modified for the each “Ln” site. The abovementioned energy differences lead to the broadening of the absorption and emission bands, what in extremely case, can change the shape of bands from “crystalline” to “glassy” type (this behavior can be clearly noticed, when comparing the absorption spectra of the dysprosium doped (LuxGd1-x)2SiO5 crystals shown in the work of Dominiak-Dzik et al. [10]). The arguments described above lead to a conclusion that in the investigated material, two groups of crystalline environment are present instead of two strictly defined sites. Again we remind, that those are only the theoretical considerations, and are not based on the experimental data. 4. Absorption characteristics This section summarizes the results of measurements and analyzes their relevance. The presented data confirm, that the Tm3þ ions locate in at least two distinct crystallographic sites. The room temperature and the low temperature (10 K) polarized absorption spectra recorded for the samples doped with 4 at.% of Tm3þ are shown on Figs. 1 and 2 respectively. They reveal slight but observable optical anisotropy of an investigated material. Changes in the absorption bands shape and in tensities are clearly seen especially for the 3H6 → 3H4, 3F2,3 transitions. For the polarizations E||X1 and E||X2 the absorption is more intense for 3 H6 → 3F2,3 transition, while for the E||B polarization the 3H6 → 3H4 transition is more prominent. Also a shape of band related to the 3H6 → 3 F2,3 transition differs for the E||X1 and E||X2 polarizations. For the former polarization this transition is dominated by the strong line at 12642 cm 1 (791.0 nm). Second transition is composed of numerous lines, among which 5 have similar intensity, and are partially over lapped. Those lines occur at 12642, 12702, 12890, 12961, and 13067 [cm 1] (791.0; 787.3; 775.8; 771.5; 762.5 [nm] respectively). The very similar observation was made in the thulium doped Y2SiO5 crystals by Li et al. [11] since the investigated material and Y2SiO5 share similar crystal structure.
3. Crystallographic and optical characterization To fully understand the spectroscopic features of the investigated material, the short introduction into its structural properties is needed. This section touches only the theoretical considerations. It was intro duced for better understanding of the multisite location of Tm3þ ions in the investigated matrix. The crystal structure of Lu2SiO5 (investigated material is isostructural to this compound) possesses the two different crystallographic sites of Lu3þ. They differ in the size and the coordina tion number. Gadolinium ions possess a little bit higher ionic radius than Tm3þ and Lu3þ (0.94, 0.88 to 0.86 [Å] respectively for Coordination Number ¼ VIII [8]). Since almost ¾ of the ions in the investigated crystal are the “big” Gd3þ ions we assume that the sites we denote as “Ln1” will be preferentially occupied by gadolinium, while the smaller “Ln2” sites will accept Lu3þ and Tm3þ. Occurrence of the Tm3þ in the “Ln1” sites can’t be excluded however. The crystallographic features of those sites are characterized in a Table 1. The data gathered in Table 1 were ob tained on basis of the following assumptions: The coordination number and the local symmetry were taken directly from the appropriate CIF file. The Ln-O mean distance is the arithmetic mean of all Ln-O distances, 2
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Journal of Luminescence 220 (2020) 116962
Fig. 1. Polarized room temperature absorption spectra of the investigated crystal.
Fig. 2. Polarized low temperature (10 K) absorption spectra of the investigated crystal.
At room temperature the strongest absorption occurs for the E||X1 polarization for IR, VIS and UV bands, reaching absorption coefficient of 14.29 cm 1 at 8271 cm 1 (1209 nm). The strong absorption band related to the 3H6 → 1D2 transition, with maximum at 28189 cm 1 (354.75 nm) for all polarizations could be used for optical pumping of
this material by the third harmonic of the easily commercially available Nd:YAG lasers. The appropriate absorption coefficients are equal to 5.57, 8.37 and 4.69 cm 1 for the E||B, E||X1 and E||X2 polarizations, respectively. Going down with temperature to the 10 K, the absorption lines 3
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(shown on Fig. 2.) become narrower. It is shown that the intensity of some absorption lines vary. The suppressed lines belong to transitions from the higher Stark components of the 3H6 ground state. Fortunately the lines at ca. ~355 nm belong to transitions from the lowest Stark component of ground state. With decrease of the temperature their in tensities increase reaching an absorption coefficient of 19.28 cm 1 at 28153 cm 1 (355.2 nm) at 10 K (for E||X1 polarization). This line is quite broad, so the small discrepancy between exact wavelength of 3rd harmonic of Nd:YAG (354.67 nm) does not matter significantly. In fact, the absorption coefficient at the exact Nd:YAG 3rd harmonic wavelength equals to 18.37 cm 1. The other lines suitable for the optical pumping of the material employing lasers could be found at ca. 800 nm. At the low temperature (10 K) the two narrow and intense lines are present for the E||B and E||X1 polarizations in this region. For the E||B polarization the absorption coefficients equal to 6.97 and 13.18 [cm 1] for lines at 12645 cm 1 (790.8 nm) and 12703 cm 1 (787.2 nm), respectively. For the E||X1 polarization those coefficients equal to 11.92 and 11.79 [cm 1], respectively. Based on the low temperature absorption spectra, energy of the Stark levels of several Tm3þ multiplets were determined. Their complete list is shown in Table 2. For each multiplet a number of the observed Stark components exceed theoretical number of the predicted Stark compo nents for the C1 symmetry. This means that the thulium ions must be located in both crystal sites present in this crystal.
transitions, due to the fact that they overlap. This operation was made in order to reduce experimental uncertainties. The mean oscillator strength was calculated according to following formula: Pmean ¼ (PE||B þ PE||X1 þ PE||X2)/3. Obtained values of the mean oscillator strengths were then used as an input data for the calculations in the framework of the phenomenological Judd – Ofelt method. The obtained Ωt (t ¼ 2, 4, 6) parameters are gathered in Table 3. The Judd-Ofelt model considers only transitions of the Electric Dipole type (hereafter denoted as ED). However for the transitions ful filling following rule ΔJ ¼ 0, �1 (except 0↔0) a Magnetic Dipole type transition contributes, too. Thus, it should be mentioned, that for the absorption spectra of Tm3þ both 3H6 → 3H5 and 3H6 → 1I6 transitions are mixed ED and MD in nature. The latter transition was not observed during this study, however. The oscillator strength of the absorption transition could be calculated based on the experimental data and aforementioned equation, however it could be also calculated knowing the value of the so called “line strength” of selected transition. The equation to do it is as follows: F¼
The transition intensities of Tm3þ ions in the (Lu0.25Gd0.75)2SiO5 crystal were analyzed in the framework of the Judd-Ofelt phenomeno logical model [12,13]. The polarized absorption spectra recorded at room temperature were used to calculate the oscillator strengths PE||B, PE||X1 and PE||X2. The intensities of absorption bands were evaluated by means of the numerical integration employing following equationEq 1: Z 4:303x10 9 υ2 FE ¼ αðυÞdυ (1) c υ1
SMD ¼
Barycenter
ΔE
Nexp
Ntheor
3
5705, 5754, 5709, 5814, 5844, 5858, 5970, 6086, 6112, 6161, 6188, 6219, 6309 8271, 8285, 8467, 8503, 8591, 8673, 8703, 8741, 8865, 8920 12636, 12650, 12687, 12706, 12714, 12815, 12877, 12966, 12993, 13051, 13073, 13219 14603, 14652, 14695, 14751, 14779, 14825, 14876, 15181, 15285, 15406, 15463, 15598 21017, 21052, 21088, 21137, 21213, 21436, 21598, 21667, 21739, 21795, 21819, 21981, 21987 27862, 27909, 27995, 28153, 28200, 28296, 28409
5979
604
13
2�9
8602
649
10
2�11
12866
583
12
2�9
15010
995
12
2�(5 þ 7)
21502
970
13
2�9
28118
547
7
2�5
F4
3
H5
3
H4
3
F2,3
1
G4
1
D2
(3)
1. Case 1: J’ ¼ J �r � JðJ þ 1Þ þ SðS þ 1Þ LðL þ 1Þ 2JðJ þ 1ÞðJ þ 2Þ ðψ JkLþ2Skψ ’ J ’ Þ¼ℏ 1þ 2JðJ þ 1Þ
Table 2 Values of the energies, barycenters, energy splitting and number of experi mentally observed Stark levels of selected multiplets in investigated crystal. Energy [cm 1]
h2 ðψ JkL þ 2Skψ ’ J ’ Þ 16π2 m2 c2
The numerical value of h2/16π2m2c2 is equal to 3.73 � 10 22. The value of (ΨJkLþ2SǁΨ’J’) could be calculated with use of the following equations.
where FE means the oscillator strength at appropriate polarization, c means Tm3þ ions concentration in moles/dm3 and α(υ) means absorp tion coefficient at appropriate wavenumber υ. The integration takes place for a whole band related to the investigated transition between υ1 and υ2. This relation was applied to the six bands located in the 5000–29000 cm 1 spectral range and the procedure was repeated for each polarization state. For one band related to 3H6 -> 3F2, 3F3 transi tions the oscillator strengths were calculated holistically for both
H6 →
(2)
where m is an electron mass (9.11 � 10 28 g), c is a velocity of light in vacuum (2.997 � 1010 cm/s), and h is a Planck Constant (6.626 � 10 27 erg/s). Numerical value of 8πmc/3h equals to 1.08 � 1011. All those values are given in the nowadays abandoned CGS system, which was quite often used when Judd and Ofelt separately proposed their models. To preserve compatibility with the old data still CGS system is used. The barycenter of transition wavenumber is denoted as υ. J is the total angular momentum quantum number of the initial multiplet, χ is a Lorentz correction factor that is n for MD transitions or (n2þ2)2/9 for ED transition (n is refractive index). S is called a “line strength” and it equals Ω2U2þΩ4U4þΩ6U6 for the ED transitions. For the MD transition it could be calculated directly from the equationEq 3:
5. Judd – Ofelt analysis
4
8π2 mc υ χS 3h 2J þ 1
2. Case 2: J’ ¼ J-1 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi JÞðJ þ S LÞðJ þ L SÞ ℏ ðS þ L þ J þ 1ÞðS þ L þ 1 ’ ’ ðψ JkLþ2Skψ J Þ¼ 4J
3. Case 3: J’ ¼ Jþ1 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi LÞðL þ J þ 1 SÞðS þ L JÞ ℏ ðS þ L þ J þ 2ÞðS þ J þ 1 ’ ’ ðψ JkLþ2Skψ J Þ¼ 4ðJ þ 1Þ with knowledge of the line strength of MD we could get back to equation (2) and calculate MD part of the oscillator strength for the appropriate absorption transition. Subtracting a MD part from experimental value should leave only ED part, suitable for the further Judd-Ofelt analysis. All aforementioned equations were taken from the work of Carnall, Fields and Wybourne [14]. Considering calculated values of the oscillator strength (gathered in 4
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Journal of Luminescence 220 (2020) 116962
Table 3 Calculated oscillator strengths for the absorption bands and the Judd-Ofelt analysis for (Lu0.25Gd0.75)1.92Tm0.08SiO5 single crystal. 3
Barycenter [cm 1]
Fosc �10 EkB
EkX1
EkX2
mean
ED
MD
J-O
3
6000 8440 12820 14710 21440 27980
2.47 2.02 2.53 2.05 1.03 3.24
3.77 2.71 4.07 4.67 1.72 5.02
3.25 2.44 3.64 4.88 1.30 4.07
3.16 2.39 3.41 3.87 1.35 4.11
3.16 2.11 3.41 3.87 1.35 4.11
– 0.28 – – – –
3.60 1.97 3.00 4.08 1.22 4.19
H6 ->
F4 3 H5 3 H4 3 F2.3 1 G4 1 D2
6
Error [%]
Ω2 ¼ 3.24 (�0.58); Ω4 ¼ 2.96 (�0.24); Ω6 ¼ 0.86 (�0.14) [�10
20
þ13.6 6.7 11.2 þ5.8 9.4 þ4.9
cm2]; AVG err ¼ 13.0%
a Table 3) we can perceive that the biggest errors occur for the transi tions 3H6 → 3F4, 3H4, 1G4. For all those transitions ΔJ ¼ 2. The transi tions fulfilling this rule are called the hypersensitive ones. Their intensity strongly depends on the crystal field around the lanthanide ion and usually those transitions are badly described by the Judd-Ofelt theory [15]. However in the case of investigated material obtained er rors are within acceptable values. The omega parameters gathered in a Table 3 were used to determine the emission characteristic of the system under study. The radiative transition rates, luminescence branching ratios, and radiative lifetime of excited states were estimated based on the Judd - Ofelt model. The obtained results are collected in a Table 4. Similarly to the absorption calculations, emission was also corrected for the MD type transitions. There is no comparison between the two concentrations, because at the low doping levels, when dopant does not force structural change, the absorption bands are proportional to the concentration. And since the oscillator strength does not depend on the concentration we does not think it is necessary to repeat the calculation of JO parameters for the second sample – thus results should be the same within experimental error. The reason why second sample was prepared (0.5 at%. Tm3þ) was to check the lifetime values of metastable levels and how they are affected by the concentration. Especially the triplets F4 and H4. Optical excitation of 3H4 could lead to the “2 for 1” pumping scheme via the efficient cross-relaxation. Thus, it is important to check, if for a high dopant concentration, the lifetime of 3H4 decreases significantly while lifetime of the 3F4 remains the same. This is the case for an efficient 2 for 1 scheme pumping.
6. Luminescence characteristics Thulium ions exhibit luminescence from up to 5 levels, namely 3P0, D2, 1G4, 3H4 and 3F4. Since in this matrix the 3P0 absorption line overlaps with the absorption lines of Gd3þ ions, this level wasn’t investigated. The other four luminescent multiplets are responsible for the emission in UV/blue, blue/red or IR parts of the spectrum respec tively. Among those transitions an emission in the IR region is with high industrial/commercial significance, due to its use in an industrial, telecommunication or medical lasers. The emission usually spans over a several hundred nanometers centering around 1900 nm.
1
6.1. Luminescence from the 3F4 level To estimate the laser potential of the investigated material the polarized emission spectra in 300 K (red) and 10 K (black) were measured. They are presented in Figs. 3 and 4. The room temperature spectra do not show significant anisotropy. Maximum of the emission emerges at ca. 1820–1850 nm. Decrease of temperature to the 10 K leads to more discrete spectra however. For each polarization the strong band component at 1756 nm (5727 cm 1) could be observed. Spectral posi tion of the second most intense peak depends on the polarization. The most prominent line is in fact a 0-0 transition as can be seen from the comparison of the absorption and emission spectra. The positions of second most intense peaks are 1966 nm (5086 cm 1), 1942 nm (5149 cm 1) and 1897 nm (5271 cm 1) respectively. To evaluate the possi bility of a laser action generation in this matrix the effective stimulated emission cross sections have been calculated on the basis of the
Table 4 Experimental and calculated luminescence characteristics of (Lu0.25Gd0.75)1.92Tm0.08SiO5 single crystal. Trans
kU2k
Barycenter [cm 1]
kU4k
kU6k
SED
[nm]
[�10
SMD 21
cm2]
AED
AMD
A
[s 1]
В [%]
1
D2 → G4 6720 F3,2 13345 3 H4 15370 3 H5 19620 3 F4 22100 3 H6 28060 ΣA ¼ 46425 s 1. τrad ¼ 22 μs 1 G4 → 3 F3.2 6625 3 H4 8650 3 H5 12900 3 F4 15380 3 H6 21340 ΣA ¼ 5459 s 1. τrad ¼ 183 μs 3 H4 → 3 H5 4250 3 F4 6730 3 H6 12690 ΣA ¼ 2098 s 1. τrad ¼ 477 μs 3 F4 → 3 H6 5960 ΣA ¼ 421 s 1. τrad ¼ 2373 μs 1 3
1488 749 651 510 452 356
0.1874 0.2280 0.1257 0 0.5686 0
0.1799 0.3771 0.0124 0.0012 0.0961 0.3131
0.0022 0 0.2300 0.0182 0.0215 0.0958
11.42 18.55 6.42 0.19 21.45 10.09
– 2.05 – – – –
302 3848 2034 127 20210 19460
– 445 – – – –
302 4293 2034 127 20210 19460
0.7 9.2 4.4 0.3 43.5 41.9
1509 1156 775 650 469
0.0212 0.0200 0.0773 0.1645 0.0464
0.1592 0.0182 0.0078 0.0052 0.0747
0.3685 0.0693 0.5633 0.4114 0.0100
8.57 1.78 7.58 9.02 3.80
1.37 5.01 0 5.01 –
121 56 789 1591 1791
20 165 0 926 –
141 221 789 2517 1791
2.6 4.0 14.5 46.1 32.8
2353 1486 788
0.0131 0.1275 0.2672
0.4762 0.1311 0.1650
0.0095 0.2113 0.5704
14.60 9.83 18.45
2.13 4.01 –
54 145 1828
8 62 –
63 207 1828
3.0 9.9 87.1
1678
0.5395
0.7261
0.2421
41.05
–
421
–
421
100
5
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Journal of Luminescence 220 (2020) 116962
Fig. 3. Overlap of the low temperature (10 K) absorption and emission spectra related to the 3H6 – 3F4 transition – bands relevant for IR lasing channel of Tm3þ ion.
Fig. 4. Overlap of the room temperature absorption and emission spectra related to the 3H6 – 3F4 transition – bands relevant for IR lasing channel of Tm3þ ion.
reciprocal method. The results are gathered in Fig. 5. As could be seen the polarization plays important role in both possible efficiency and lasing wavelength of this material. However further laser experiments are needed to estimate the real gain and lasing wavelengths. A chance of obtaining the laser action in this material is rather high, since laser ac tion in the similar RE2SiO5 crystals have already been obtained. Bin Liu et al. have reported on the slope efficiency up to 26% in the Tm doped Sc2SiO5 crystals [16]. Yao et al. have reported on the slope efficiency up
to 21% in the Tm doped Lu2SiO5 crystals [17]. The broad emission spectrum at the room temperature could potentially find application in the short pulse lasers, while when operated at the cryogenic tempera tures this laser could be operated in the traditional single-wavelength scheme. The absorption spectra measured at the temperature of liquid nitrogen shows, that the shape of spectrum strongly resembles those observed at 10 K rather than those at 300 K. This means that an efficient laser operation could be potentially obtained with use of a much cheaper 6
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Journal of Luminescence 220 (2020) 116962
Fig. 5. Calculated effective stimulated emission cross-sections (300 K) for the 3F4–3H6 transition for various partition coefficient values – influence of polarization.
6.2. Luminescence from the 1G4 level
liquid nitrogen or a stack of Peltier Modules, instead of the expensive helium equipment. Moreover on the base of the low temperature absorption and emis sion spectra only one 0-0 transition could be observed what suggests that the 0-0 transitions for both sites are present however they do overlap spectrally.
The second aforementioned luminescent level, 1G4, emits in the cyan and red parts of the visible spectrum. The room temperature absorption/ emission features of this level are shown in Fig. 6. The luminescence from this level was observed under an excitation at 463 nm laser with pulses delivered by OPO. The emission spectra were not corrected for
Fig. 6. Spectral features of absorption/emission related to 1G4 level recorded at 300 K 7
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Journal of Luminescence 220 (2020) 116962
spectral response of a measurement system. The absorption and emis sion spectra are characterized by the significant anisotropy. At the room temperature bands are composed of broad overlapping lines. The one of the most prominent emission line in each polarization corresponds to the similar line in the absorption spectrum. Since those transitions possess neighbors on both right and left sides it is hard to say if they are a 00 transition. Especially if we consider that the aforementioned maximum lies at 477 nm in the case of E||X1 and E||X2 and at 474 nm in the case of E||B polarization. The small absorption coefficient contrary to emission intensity at ca. 490 nm may suggest future laser experiment at this wavelength, under pumping by the more and more popular laser diodes operating at a 473 nm or 460 nm. To increase chance of the laser generation success a cryogenic temperature (~100 K) could be helpful. For those experiments especially the E||X2 polarization could be useful.
level operation scheme, thus the pumping power delivered must be high. On the other hand laser oscillations on the 1D2→3F4 transition may be difficult to achieve due to the spectral overlap between this emission and the 3H6→1G4 absorption bands. The decrease of temperature most probably will increase the spectral overlap. Additionally much longer lifetime of the 3F4 level contrary to the 1D2 level will not allow to obtain a CW operation. The lifetime in the order of several milliseconds will restrict the maximum pulse repetition rate to several Hz. The spectral overlaps are presented in graphs enclosed in the “supplementary ma terials” Fig. s1. 7. Excited state relaxation kinetics In this section we will present the results on our study on the metastable levels lifetimes. Influence of the temperature and concen tration of dopant ions on the metastable level lifetime will be shown and discussed. Especially crucial and important is characterization of the 3F4 level since it is a metastable level for the laser channel 3F4 → 3H6 that gives IR radiation ca. 1.8–1.9 μm. The luminescence kinetics of the excited levels were investigated as a function of the temperature within a 10–300 K range. Investigations of luminescence kinetics could bring information concerning the path of relaxation that the electron follows from the excited level down to the luminescent one. All those information is present in the rise and decay parts of the luminescence kinetics curve after a short pulse excitation. Those investigations also provide information on the inter-ionic in teractions among lanthanides like the energy transfer (especially crossrelaxation). The cross-relaxation processes may be responsible for diminishing of luminescence intensity for a particular excited level. On the other hand they are able to provide an efficient way of excitation of other metastable levels. For example, the cross relaxation process pro vides extremely efficient transfer of energy accumulated on the 3H4 pump level to the metastable 3F4 level in thulium lasers operating near 1700–1900 nm (the two-for-one pumping scheme). In the investigated material the four luminescent levels were observed. They are the 1D2,
6.3. Luminescence from the 1D2 level The last mentioned luminescent level possesses the highest energy. Its absorption spectrum clearly suggest possible pumping by a common 3rd harmonic of Nd:YAG lasers at a 355 nm. With use of this excitation we can achieve an efficient emission in UV (~365 nm) and royal-blue (~455 nm) regions. The emission spectra measured at room tempera ture are shown in Fig. 7. Those spectra were recorded separately and were not corrected for a spectral response. All of the spectra presented in Fig. 7 were normalized to 1. Comparison of the spectral bandwidths (in cm 1) of lines for bands related to the 1D2→3H4 and 1D2→3F4 transitions leads to conclusion that the latter ones are much narrower. Additionally anisotropy plays much more role for these transitions than for the 1D2→3H6 one. Likely in the 1 D2→3F4 transition only the lowest Stark components of the 3F4 level are involved. As it was frequently experimentally proven, in this material also, the lowest Stark components are weakly coupled with the matrix phonons. Considering the laser feasibility of transitions from this level theoretically both transitions could be used for generation. However, for both cases the transition under resonant pumping needs the quasi three-
Fig. 7. Spectral features of absorption/emission related to 1D2 level recorded at 300 K 8
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G4 3H4 and 3F4 levels. The radiative properties of those levels were predicted on basis of the Judd-Ofelt model, and are gathered in afore mentioned tables. After short laser pulses with wavelength tuned to absorption of the 3 H6→1D2 transition (355 nm, UV light), the intense blue emission ca. 460 nm could be observed. This light is generated by a the 1D2→3F4 transition. The predicted radiative lifetime amounts to 22 μs. The observed lifetimes equal to 14 μs for the 0.5% Tm sample and 2 μs for the 4%Tm sample. Based on the energy levels structure of the Tm3þ ions it is easy to find a potential path for the concentration quenching of the 1D2 level. The transitions 1D2→3H4 and 3H6→3F3 are in resonance. Thus Tm3þ ion excited to the 1D2 level, could share part of the energy with neighboring Tm3þ ion in ground state. After cross relaxation one ion is in the 3H4 and second in the 3F3 state – efficiently depleting the 1D2 level. This mechanism could be responsible for efficient decrease of the radi ative properties of this level. To check this cross-relaxation contribution a sample with lower concentration of Tm was grown. For the (Lu0.25Gd0.75)1.99Tm0.01SiO5 sample under the same pumping and observation conditions the luminescence lifetime value was τ1 ¼ 14 μs. This clearly confirms that in the investigated crystal Tm3þ concentration is too high for the efficient emission from this level. The luminescence kinetics of the 1G4 level was also investigated. The observed curves were single exponential for sample containing the lower Tm3þ concentration. Comparison of the theoretical radiative lifetimes and experimental 226 μs for the 0.5 at.% Tm sample and to 9 μs for the 4 at.% Tm sample leads to the conclusion that this level is strongly quenched by the cross relaxation in the heavier doped sample. The possible cross-relaxation channels could be 1G4(1)þ3H6(2)→ 3 H5(1)þ3H4(2) or 3H4(1)þ3H5(2) or 3F3(1)þ3F4(2). Diversity of a crossrelaxation channels is most probably responsible for the efficient depopulation of the excited state. Experimental lifetimes of the 3H4 level equal 16 μs for the 4 at.% Tm and 145 μs for the 0.5 at.% Tm. Shortening of the lifetime is an evidence of the strong cross-relaxation - the only possible relaxation path is 3H4 þ 3 H6 – 2 � 3F4. This shows the efficient way to populate 3F4 upper laser level. The one pumping photon can be converted into the two emission photons. This process could highly increase a possible laser emission efficiency. The lifetimes of the 3F4 level does not show such dependency on concentration. For the sample doped with 0.5 at.% Tm the lifetime value is 2.21 ms while for the 4 at.% Tm sample it is 1.64 ms. This clearly shows that the higher concentrations are better suited for laser active material since the relaxation from the pumping level to the metastable level is faster, while high luminescence efficiency is still maintained. All decay curves that were utilized to estimate aforementioned experimental lifetimes are included in the supplementary materials.
Pr- and Ce-doped mixed lutetium-gadolinium oxyorthosilicates have been previously investigated. It was found that the thulium ions occupy both of the available lanthanide sites in this matrix. A strong energy transfer between the sites is bound to occur, since the observation of the two different emission spectra under various excitations was not possible. The lifetime measurements showed that 4 at.% doping is relatively high in the case of this matrix and lower concentration values should be considered in order to increase the luminescence quantum efficiency for the 1D2, 1G4 and 3H4 levels. On the other hand, heavier doping is favorable for the intensity of the emission from the 3F4 level, when pumping of the 3H4 level ca. 800 nm is considered. Further studies regarding the generation of laser action are required to decisively confirm or rule out the usability of this material for laser applications. Declaration of competing interest All authors declare that there was no conflict of interest while doing research described in this manuscript. CRediT authorship contribution statement Adam Strzęp: Methodology, Validation, Formal analysis, Investiga tion, Resources, Writing - original draft, Writing - review & editing, Visualization. Michał Głowacki: Investigation, Resources. Maksymi lian Szatko: Investigation, Formal analysis, Resources, Visualization. Karolina Potrząsaj: Investigation, Formal analysis, Resources, Visual ization. Radosław Lisiecki: Formal analysis, Visualization. Witold Ryba-Romanowski: Conceptualization, Supervision, Project adminis tration, Funding acquisition. Acknowledgement The work received financial support from National Science Centre, Poland under grant no DEC 2016/21/B/ST5/00890. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jlumin.2019.116962. References [1] B. Liu, M. Gu, Z. Qi, C. Shi, M. Yin, G. Ren, Journal of. Luminescence 127 (2007) 645. [2] A. Strzep, W. Ryba-Romanowski, M. Berkowski, J. Lumin. 153 (2014) 242–244. [3] M. Jacquemet, et al., J. Appl. Phys. B 80 (2005) 171. [4] M. Glowacki, et al., J. Solid State Chem. 186 (2012) 268–277. [5] O. Sidletskiy, et al., J. Cryst. Growth 312 (2010) 601. [6] A.A. Kaminskii, Laser Photonics Rev. 1 (2) (2007). [7] K.H. Yang, J.A. DeLuca, Appl. Phys. Lett. 29 (1976) 499. [8] R.D. Shannon, Acta Crystallogr. 32 (1976) 751–767. [9] T. Gustafsson, et al., Acta Crystallogr. C 57 (2001) 668. [10] G. Dominiak-Dzik, et al., Cryst. Growth Des. 10 (2010) 3521. [11] C. Li, et al., J. Lumin. 62 (1994) 157–171. [12] B.R. Judd, Phys. Rev. 127 (1962) 750. [13] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [14] W.T. Carnall, P.R. Fields, B.G. Wybourne, J. Chem. Phys. 42 (1965) 3797. [15] R.D. Peacock, Chem. Phys. Lett. 16 (1972) 590–592. [16] B. Liu, et al., Chin. Phys. B 26 (2017), 084203. [17] B.Q. Yao, et al., Laser Phys. Lett. 5 (2008) 714–718.
8. Conclusions The motivation behind growing this crystal was the pursuit of novel, highly efficient thulium-based laser materials. It was found that thulium-doped (Lu0.25Gd0.75)2SiO5 crystals are characterized by high optical anisotropy. The chemical stability, moderate thermal conduc tivity and broad region of transparency make LGSO a good candidate material for a laser matrix considered for doping with various lantha nide ions. In the recent studies of our group, the properties of Dy-, Sm-,
9