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Optical spectroscopic properties, 0.946 and 1.074 μm laser performances of Nd3+-doped Y2O3 transparent ceramics
Journal of Alloys and Compounds 711 (2017) 446e454
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Optical spectroscopic properties, 0.946 and 1.074 mm laser performances of Nd3þ-doped Y2O3 transparent ceramics Suchinda Sattayaporn a, Gerard Aka a, *, Pascal Loiseau a, Akio Ikesue b, Yan Lin Aung b a b
PSL Research University, Chimie ParisTech, CNRS, 75005, Paris, France World-Lab Co., Ltd., CSJ 308 1-2-19 Mutsuno, Atsuta-ku, Nagoya, 456-0023, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2017 Received in revised form 22 March 2017 Accepted 29 March 2017 Available online 31 March 2017
0.4% Nd3þ-doped Y2O3 transparent ceramics have been investigated from their optical absorption and emission properties to the laser performances at two different wavelengths, 0.946 mm and 1.074 mm. Absorption spectra in the range 300e1000 nm was measured to perform Judd-Ofelt analysis: the calculated spectroscopic parameters U2, U4 and U6 are 7.59 1020, 4.04 1020 and 1.40 1020 cm2 respectively. The measured fluorescence and radiative lifetime of 4F3/2 manifold are determined to be 283 ms and 328 ms respectively. The branching ratios of 4F3/2 / 4I9/2 and 4F3/2 / 4I11/2 transitions are 57% and 38% with corresponding stimulated emission cross sections of 1.02 1020 and 1.16 1020 cm2 respectively. Laser experiments, under 0.808 mm laser diode pumping source, were carried out using a plano-concave resonating cavity. At 1.074 mm, the maximum output power is 3.5 W for an absorbed power of 13.0 W. The optical-to-optical and slope efficiencies reached 26.9 and 31.1%, respectively. For laser operation at 0.946 mm, the maximum output power reached 1.0 W for 13.0 W of absorbed power. The optical-to-optical conversion efficiency and slope efficiency were 7.9 and 12.4%, respectively. © 2017 Elsevier B.V. All rights reserved.
Keywords: Near-infrared Ceramic transparent lasers Neodymium Judd-Ofelt analysis Spectroscopy
1. Introduction High-power laser sources operating in the blue spectral range of 440e490 nm are of great interest for several applications such as display technology, submarine communications or medical treatment. First, the blue emission can be directly realized by doping appropriate lanthanide ions likes Pr3þ, Tb3þ or Ce3þ into a crystalline host. Among these lanthanide ions, Pr3þ exhibits for example the outstanding spectroscopic properties of high absorption and emission cross sections in blue region [1]. Nevertheless, the direct laser emitting is not very efficient. At present, the frequency doubling or second harmonic generation is frequently applied to realize laser emission which is hardly accessible with aforementioned direct method. There are many reports on high-efficient green laser tunable around 0.532 mm by doubling infrared emission around 1.064 mm from Nd3þ ions [2e5]. As Nd3þ ions can provide near-infrared emission at shorter wavelength around 0.946 mm (4F3/2 / 4I9/2), the same principle could be also used to reach high-power blue laser emission [6e8]. Therefore, it is interesting to begin with Nd3þ solid-state materials for high
* Corresponding author. E-mail address: [email protected] (G. Aka). http://dx.doi.org/10.1016/j.jallcom.2017.03.343 0925-8388/© 2017 Elsevier B.V. All rights reserved.
power 0.946 mm laser. Using commercial LD pump at 0.808 mm, Nd3þ ion can be easily excited from ground state 4I9/2 to 4F5/2, 2H29/2 upper state with high output power pump. Nd3þ ions generate infrared emission along 4F3/2 / 4I9/2 and 4F3/2 / 4I11/2 transition, leading to near-infrared emissions at 0.946 mm and 1.074 mm respectively. Since many decades, Nd3þ-doped various hosts have been fabricated and studied for their optical, spectroscopic properties and laser performances for 0.946 mm especially, Nd doped garnet (Y3Al5O12, YAG), perovskite (YAlO3, YAP) and vanadate (YVO4, YVO) hosts [9e11]. According to the referred works, Nd: YAG, have delivered 2.2 Watts of the maximum output at 0.946 mm with an absorbed power of 11 Watts. However, its calculated ratio U4/U6 (c), relating to the probability ratio of 4F3/2 / 4I9/2 on 4F3/ 4 2 / I11/2 emission is much relatively weak (0.6) [12] when comparing with the other oxide as Y2O3 (2.2) [13]. Especially, the good thermomechanical properties of Nd: Y2O3, associated with its optical spectroscopic data comparable to Nd: YAG [14e16], should allow laser generation with better performances on the 4F3/2 / 4I9/ 2 channel. Nevertheless, the Nd: Y2O3 laser performances on the 4 F3/2 / 4I9/2 transition have not been yet studied. It is important to point out that the growth of Y2O3 single crystal is rather difficult to achieve with large size and high optical quality using conventional technique like Czochralski because of its high
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Fig. 1. The crystal structure of Y2O3 showing the YO6 unit (left) and the two inequivalent Y ions in C2 and C3i symmetries (Pink and blue balls, respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
melting point (2430 C). Iridium crucible, with a melting point of 2450 C, is not appropriate for the Y2O3 crystal growth. Additionally, phase transition of polymorphic phase occurs during the growth process from cubic phase to hexagonal phase at high temperature around 2280 C [17]. This phase transition leads to twinning and crystal defects formation. The most suitable solution is to prepare alternatively cubic Y2O3 transparent ceramic since lower temperature fabrication methods can provide Y2O3 transparent ceramics with an optical quality, comparable to Y2O3 single crystal. In this work, we studied Nd doped Y2O3 transparent ceramics and reported the optical, spectroscopic properties as well as laser performance at two different wavelengths 0.946 mm and 1.074 mm under the excitation at 0.808 mm by LD pumping. 2. Experimental section Yttrium oxide (Y2O3) belongs to cubic system with Ia-3 space group and has bixbyite structure. Y ions are coordinated with 6 oxygen atoms and locate in two inequivalent cation sites with different symmetries: C2 (24 sites) and C3i (8 sites). The C2 site can be described as a cube structure with two oxygen vacancies on a body diagonal. Whereas, the C3i sites have two oxygen vacancies on a face diagonal of a cube as shown in Fig. 1 (right). 0.4, 0.8 and 1 at% Nd doped Y2O3 ceramic samples (see Fig. 2) in our work were prepared and supplied by World-Lab Co., Ltd. from Japan. The optical homogeneity of sample was thoroughly investigated with the help of micro-polarizer plate and transmission polarized optical microscope. In order to determine the
447
concentration of Nd ions, the surface of sample was polished and coated with graphite. The chemical compositions of Nd3þ-doped Y2O3 were analyzed by using an electron probe microanalysis (EPMA) technic with CAMECA SX100 instrument. The thermal conductivity measurements of 0.4% Nd: Y2O3 and pure Y2O3 ceramic samples were performed at room temperature with a Ctherm TCi Thermal Conductivity Analyzer apparatus (TCA). Moreover, the refractive index of Y2O3 transparent ceramic was measured in the visible-infrared range by the minimum deviation technique, using a prism of pure Y2O3 transparent ceramic to determine the associated sellmeier coefficients. 0.4 at% Nd: Y2O3 with a thickness of 3.2 mm was used to collect the optical absorption and emission spectra. The non-polarized absorption spectra were recorded in the 300e1000 nm range at 10 K and room temperature by using a Cary 6000i UVeViseNIR spectrophotometer. A cryostat was used for sample absorption spectra acquisition at 10 K. Room temperature absorption spectra were also used to realize Judd-Ofelt computation with the help of Relic 1.0 software developed by Markus P. Hehlen from Los Alamos National Laboratory [18]. Furthermore, fluorescence emission spectra in the range of 850e1200 nm were recorded at 77 K by dipping a sample in liquid nitrogen and at room temperature with 808 nm as excited wavelength by using two sets of equipment. The first one is composed with an optical parametric oscillator (OPO) as pulsed exciting source, a monochromator Acton SP2300 from Princeton Instrument and an InGaAs photodiode camera. The second set uses a cw Ti: Sapphire laser Coherent 890 pumped with an OPSL at 540 nm, an ARC Spectra Pro-7510 monochromator and a cooled InGaAs photodiode. In addition, the emission decay of 4F3/2 excited state at room temperature was also recorded to determine the fluorescence lifetime of the Nd: Y2O3 transparent ceramic samples. Fig. 3 presents the plano-concave resonating cavity set up of laser experiments at 0.946 mm and 1.076 mm. The fiber-coupled Laser diode (LD) from LIMO was employed as cw pumping source at 808 nm, delivering a maximum output power of 35 W. The laser diode fiber core size is 100 mm with a 0.22 of numerical aperture. The LD pump laser was collimated and focused into the Nd: Y2O3 ceramic with two lenses f1: f2 of 75: 150 focusing lengths in mm. The Nd: Y2O3 sample with an aperture of 5 5 mm2 was wrapped in indium foil and placed in a water-cooled petlier copper block set at 5 C. Both faces of transparent ceramic were polished and antireflection (AR) coated at 0.808 mm and 0.946 mm or 1.074 mm M1 is a plane mirror with high-reflection (HR) coating for 0.946 and 1.074 mm and high-transmission (HT) at 0.808 mm. The M2 output coupler is a concave mirror with a radius of curvature R ¼ 50 mm. Various M2 output coupler transmissions were used for laser experiments: 3 or 5%TOC at 0.946 mm, 2, 6 or 10% TOC at 1.074 mm. Laser
Fig. 2. 0.4%, 0.8% and 1 at% Nd doped Y2O3 ceramic samples.
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Fig. 3. Configuration of the experimental laser setup.
Table 1 Initial and measured compositions of 0.4 %Nd: Y2O3 samples. Composition
Nd
Y
Initial Measured
0.008 0.007
1.992 1.993
emission spectra were recorded using a spectrometer HR4000 from Ocean Optics.
3. Results and discussion 3.1. Electron microprobe analysis and thermal conductivity measurements The chemical composition of 0.4% Nd: Y2O3 sample was determined by using the technique of electron microprobe analysis (EMPA). The initial doping level of Nd3þ ions in raw material is 0.4 at.%, corresponding to a composition of Y1.992Nd0.008O3. Table 1 presents measured composition in ceramic sample which is Y1.993Nd0.007O3. As the thermal properties have an influent on laser performance, it is necessary to investigate the thermal conductivity of 0.4% Nd: Y2O3 and pure Y2O3 ceramics at room temperature. At room temperature, the thermal conductivity of 0.4% Nd: Y2O3 and pure Y2O3 are measured to be 8.3 and 9.5 Wm1K1 respectively. The doped one has thermal conductivity slightly lower than that reported in the literature [14] and that of the pure one. As compared to various hosts, Y2O3 exhibits a thermal conductivity as high as YAG ceramic (10 Wm1K1). Consequently, this advantageous property makes Nd: Y2O3 an attractive host for laser operation.
Table 2 Measured and calculated refractive index data from the fitted Sellmeier equation in the range of 0.405 mme1.020 mm. Wavelength (mm)
0.405 0.408 0.468 0.480 0.509 0.546 0.577 0.579 0.589 0.644 0.980 1.000 1.020
Refractive index Measured
Calculated
1.9549 1.9541 1.9379 1.9348 1.9283 1.9216 1.9171 1.9165 1.9153 1.9092 1.8914 1.8904 1.8897
1.9556 1.9545 1.9368 1.9339 1.9280 1.9216 1.9172 1.9170 1.9157 1.9097 1.8911 1.8904 1.8898
Table 3 The fitted values with standard deviation of Sellmeier's coefficients of Y2O3 ceramic in the range of 0.405e1.020 mm. Sellmeier's coefficients
Fitted value
A B C
3.537 ± 0.004 0.0473 ± 0.0008 0.011 ± 0.004
3.2. Refractive index dispersion The prepared prism of Y2O3 shown in Fig. 7 (onset) was used to measure the refractive index by minimum deviation method. The apex angle of the prism is 50 . The room temperature refractive index of Y2O3 prism were measured by using gas-discharge lamps and OPOs pumping source in the range from 0.4 to 1.02 mm. The eleven values of measured refractive index are all presented in Table 2. The fit of measured data was realized by using the non-linear curve presented in Fig. 7 (red line), leading to the determination of Sellmeier coefficients (A, B and C) from Equation (1) given below:
n2 ¼ A þ
B
l2
C l2
(1)
where, n is refractive index, A, B and C represent Sellmeier coefficients and l is wavelength in micrometer. A, B and C coefficients, obtained from the fitting curve, are given in Table 3. 3.3. Optical measurement of Nd: Y2O3 transparent ceramics The optical homogeneity of the 0.4% Nd: Y2O3 transparent ceramic with dimensions of 5 5 3 mm3 was measured by micro-polarizer plate. In addition, transmission polarized optical microscope was also used to check optical quality inside the transparent ceramic material. These results are summarized in Fig. 4 (upper and lower). Obviously, there were no optical stresses in the whole position of the samples. As shown in the microscopic image, there were no double refractions due to grain boundary phases or secondary phases or voids inside the sample. This suggested that only cubic Y2O3 phase was formed. Shadow graph of the prepared 0.4% Nd: Y2O3 (5 5 3 mm3) ceramics was measured by Schlieren imaging system. The result is shown in Fig. 5 with the shadow graph of commercial 1% Nd: YAG single crystal as a reference. Both samples showed homogeneous distribution of refractive index throughout both samples, suggesting that the sample 0.4% Nd: Y2O3 is optically homogeneous. In addition, transmission spectrum of the prepared 0.4% Nd: Y2O3 (5 5 3 mm3) ceramics is shown in Fig. 6. By using the value of refractive index at laser oscillation wavelength (~1 mm) ranges, Fresnel loss can be calculated. The theoretical transmission after subtracting the Fresnel loss was around 81.9% by using the set of refractive index we measured. At 0.946 mm, the transmittance was
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Fig. 4. Observation under micro-polarizer (upper) and transmission polarized optical microscope (lower) for the prepared 0.4% Nd: Y2O3 (5 5 3 mm3) ceramic sample.
Fig. 5. Shadowgraph of the prepared 0.4% Nd: Y2O3 ceramics (right) and commercial 1% Nd: YAG single crystal (left) measured by Schlieren imaging system.
Fig. 6. In-line transmittance curve of the prepared 0.4% Nd: Y2O3 ceramics (length of 3.2 mm with optical polished surfaces) recorded by spectrophotometer.
slightly higher than 80%, indicating an excellent optical quality of Y2O3 ceramic. That is the reason why Nd: Y2O3 transparent ceramics prepared by World Lab company can achieve laser emission at 0.946 mm and reported for the first time. In the present paper there was no report on success of laser demonstration on the 4 F3/2 / 4I9/2 transition when using Nd3þ-doped other ceramics or Y2O3 transparent ceramics but prepared by other methods. A possible hypothesis is that the transmittance of other ceramics around 0.946 mm is significantly lower than that of our samples, probably due to the presence of many scattering centers. To our best knowledge, our ceramics exhibited the highest transmittance for Nd: Y2O3 transparent ceramics. For example, the one fabricated recently by using La2O3 and ZrO2 additives still had the transmittance spectra lower than 80% in near-infrared spectral region [19]. The transmittance of our sample near laser oscillation wavelength ranges was very close to the theoretical value and it can be
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Fig. 7. Y2O3 prism and measured refractive index in the range from 0.4 to 1.0 mm.
judged that the optical loss of the prepared sample is very low. The theoretically calculated transmission at 500 nm is approximately 80%, nevertheless the actual measured transmittance was around 78%. This suggested that the optical quality of the sample is influenced by Rayleigh's scattering. 3.4. Spectroscopic characterizations The non-polarized absorption spectra of 0.4% Nd: Y2O3 were recorded at 10 K and room temperature in the range of 300e1000 nm. All the obtained spectra were recorded with a resolution of 0.1 nm - 0.2 nm. In order to identify the number of Nd crystallographic sites in Y2O3 structure, absorption spectra in the particular 434e450 nm range were recorded at various temperatures from 25 K to 300 K. The corresponding transition is 4I9/2 / 2P1/2 channel. Here, one absorption line is expected per Nd site. Fig. 8 shows five absorption spectra in the range of 434e439 nm, recorded at 25, 44, 77, 150 and 300 K. At room temperature, two broad absorption peaks were found (green spectra). When the temperature decreased, these peaks became sharper and the other peaks, which have been overlapped, were better resolved at 25 K. Four peaks were found
Fig. 8. Absorption spectra of 0.4% Nd: Y2O3 from 434 to 439 nm at various temperatures.
here. Furthermore, the peak intensities are temperature dependent. In fact, the evolution of peak intensity with temperature can be attributed to thermal population. Therefore, there are two crystallographic sites for Nd3þ ions, relating to the two different lowest energy levels marked Y1 and Y2 on Fig. 1. This obtained result is in agreement with previous work [15]. The two Nd sites have different symmetries, C2 (Y2) and C3i (Y1). The C3i symmetry site is weakly active for lanthanide ions. We also focused on near-infrared absorption in the range of 700e950 nm as shown in Fig. 9 to study laser performances of Nd: Y2O3 at 0.946 mm and 1.074 mm. In this spectral region, the strongest band could be attributed to 4I9/2 / 4F5/2 and 4I9/2 / 4H9/2 transitions. The two strongest absorption bands are peaked at 806 and 821 nm with high absorption cross sections of 2.2 1020 and 3.8 1020 cm2 respectively. Moreover, the full width at half maximum (FWHM) of these two peaks are 1.8 and 2.4 cm1, respectively. The peak located at 806 nm is the most attractive because of the availability of commercial LD pumping sources. By comparing the fluorescence emission in the spectral range 850e1000 nm at 77 K and 300 K (Fig. 10 a), the stark energy of 4I9/2 state could be investigated. Further, the room temperature fluorescence spectra, recorded with excitation at wavelength 808 nm in the range of 850e1200 nm is shown in Fig. 10 b, demonstrating the stimulated emission cross sections of 4F3/2 / 4I9/2 and 4F3/2 / 4I11/2 transitions. The splitting energy of 4I9/2 manifold is determined to be 641 cm1, which is in good agreement with ref [14], reporting the stark value of 643 cm1. The strongest emission peak is located at 1.074 mm corresponding to 4F3/2 / 4I11/2 transition. The stimulated emission cross section was calculated to be 3.5 1020 cm2. Additionally, the concentration dependency of fluorescent lifetime was also studied in this work. The room temperature fluorescence decay of 4F3/2 metastable state for 0.4, 0.8 and 1.0 at. % Nd: Y2O3 ceramic were collected at wavelength 1074 nm with a digital oscilloscope. The logarithm of fluorescence intensity plotted as a function of time is shown in Fig. 11 (left). The linear fitting leaded to the determination of fluorescence lifetimes of 4F3/2 manifold which were 283, 249 and 217 ms for 0.4, 0.8 and 1.0 at.% Nd: Y2O3 ceramics respectively. As shown in Fig. 10 (right), the measured lifetime decreased dramatically with Nd3þ concentration probably due to cross-relaxation process. This evolution is in good accordance with the literature reporting a decreasing from 270 ms (1% Nd) to 153 ms (1.5% Nd) of the 4F3/2 state
Fig. 9. Room temperature absorption spectral of 0.4% Nd: Y2O3 in the range from 700 nm to 975 nm.
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Fig. 10. Emission spectra of 0.4% Nd: Y2O3 in the range of 850e1000 nm at 77 K and 300 K (a) and 850e1200 nm at 300 K (b).
lifetime of Nd: Y2O3 ceramics prepared by other methods [13,17]. Based on the lifetime results, laser experiments will be performed by employing samples with two Nd3þ concentrations: 0.4 and 0.8 Nd at. %. 3.5. Judd-Ofelt analysis The optical absorption spectra and refractive index, experimentally obtained and demonstrated in the previous section were used to perform Judd-Ofelt analysis. All absorption data including the mean wavelength, refractive index, integrated area and oscillator strength were derived and summarized in Table 4. The JuddOfelt approach provides the calculation of electric dipole line strengths of each transition, the spectroscopic intensity parameters Ut, with t ¼ 2, 4 and 6, the branching ratios, the radiative transition probabilities as well as the radiative lifetime. Firstly, based on ten absorption bands deduced from absorption spectra of 0.4% Nd: Y2O3, the experimental electric dipole line strength from initial state J to final state J0 , so-called Sexp (J/J0 ), were calculated for each band with Equation (2):
0 Sexp J/J ¼
27hcð2J þ 1Þn 2 8p3 Nƛe2 n2 þ 2
Z
aðlÞdl
(2)
451
Fig. 11. Room temperature fluorescence decay in a logarithmic scale for 0.4, 0.8 and 1 at% Nd: Y2O3 (left) and the decreasing lifetime as a function of at% Nd doped Y2O3 (right).
where, c is the speed of light in vacuum, h is Planck's constant, e is charge of one electron, J and J0 are the angular momentum of initial and final states respectively. N corresponds to Nd3þ concentration (ions per cm3), ƛ is mean wavelength of absorption band in nanometer, n refractive index, a is absorption coefficient in cm1. On the other hand, the calculated electric dipole line strengths (Scalc) can be linearly expressed by computing three parameters Ut by using Equation (3) given below:
2 X 0 Scalc J/J ¼ Ut < JJ U t J0 J 0 >
(3)
t¼2;4;6
where Ut is tensor operator of rank t ¼ 2, 4 and 6, the term jj2 represents the reduced matrix elements of the tensor Ut which relates to transitions between initial and final manifolds. The RELIC 1.0 software was used to realize the Judd-Ofelt computation of experimental data presented in Table 3. According to Judd-Ofelt theory, the values of three Ut values and Scalc can be determined by assuming an equivalence of experimental and calculated line strengths, Sexp ¼ Scalc. The three calculated parameters Ut were fitted to be U2 ¼ 7.59 1020 cm2, U4 ¼ 4.04 1020 cm2 and U6 ¼ 1.40 1020 cm2. In fact, the U2 parameter depends significantly on site symmetry. The U2 value of
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Table 4 Absorption bands, experimental and calculated line strengths of 0.4% Nd doped Y2O3 ceramic. Transition from 4I9/2 ground state
ƛ (nm)
m
ʃA(l)dl
Sexp (1020cm2)
Salc (1020cm2)
4
357 437 480 534 592 689 752 812 892 1581
1.981 1.945 1.933 1.923 1.914 1.905 1.901 1.898 1.895 1.885
4.149 0.292 0.369 2.882 13.171 0.471 3.324 4.854 1.836 0.410
0.3272 0.2624 0.3094 2.1986 8.9838 0.3005 1.6090 2.4677 0.8413 0.1078
0.1522 0.1535 0.3132 2.0264 9.3398 0.2161 1.1023 2.2376 1.0168 0.0640
D1/2 þ 4D3/2 þ4D5/2 þ2I11/2 P1/2 þ 2D5/2 2 G11/2þ2K15/2þ2D3/2þ2G9/2 4 G9/2 þ 2K13/2 þ4G7/2 2 G7/2 þ4G5/2 4 F9/2þ2H11/2 4 F7/2 þ 4S3/2 4 F5/2 þ 2H9/2 4 F3/2 4 I15/2 2
0.4% Nd: Y2O3 is quite large due to the low symmetry C2 of active Y site. Whereas, the parameters U4 and U6 are characteristic factors usually applied to predict the intensity of stimulated emission for different transitions as 4F3/2 / 4I9/2 and 4F3/2 / 4I11/2 by considering the ratio of c ¼ U4/U6. These two factors vary with different hosts. Indeed, the U4 parameter relates significantly to the probability of 4F3/2 / 4I9/2 transition, emitting at wavelength of 0.946 mm. The U6 parameter correspond to the other emission at wavelength 1.074 mm (4F3/2 / 4I11/2). The calculated c value of 0.4% Nd: Y2O3 was 2.9, which is relatively high as compared to values already reported in the literature as presented in Table 5. The high value of c related to the strong emission from excited state 4F3/2 to ground state 4I9/2 and makes 0.4% Nd: Y2O3 a promising material for laser generation at 0.946 mm. Furthermore, the accuracy of the Judd-Ofelt fitting can be expressed in the term of root mean square deviation (Drms) with the usual Equation (4).
Drms ¼
"P
ðSmeas Scalc Þ2 pq
#1=2 (4)
where p is the number of absorption bands included in the JuddOfelt fit and q is the number of spectroscopic intensity parameters U. In this work, the p and q are 10 and 3 respectively. Finally, the rms deviation was 0.1 1020 cm2. 3.6. Radiative properties From the three Ut parameters, the emission line strength (Sems) from 4F3/2 manifold to lower manifolds such as 4I9/2, 4I11/2, 4I13/2 and 4 I15/2, can be also calculated by using Equation (3) previously mentioned in Judd-Ofelt analysis section. The emission line strengths Sems were used to find the value of radiative transition rate between two states (J to J0 ), AJ/J0 with Equation (5) given below:
2 64p4 e2 m m2 þ 2 0 0 A J/J ¼ Sems J/J 3 27hð2J þ 1Þƛ
Table 5 The spectroscopic intensity parameters Ut
(t¼2, 4, 6)
(5)
Table 6 Calculated emission cross section, radiative lifetime, fluorescence branching ratio of 0.4% Nd: Y2O3 ceramic. Transition
ƛ (nm)
Sems (1020cm2)
A(J/J0 ) (s1)
b(J/J0 )
tr (ms)
4
889 1062 1341 1840
1.02 1.16 0.30 0.04
1741 1151 147 8
57% 38% 0.5% 0.2%
328
F3/2 F3/2 F3/2 4 F3/2 4 4
/ / / /
4
I9/2 I11/2 I13/2 4 I15/2 4 4
where J ¼ 3/2 and J’ ¼ 9/2, 11/2, 13/2 and 15/2. The radiative lifetime (tr) of the excited metastable state 4F3/2 was then calculated by using Equation (6). On the other hand, the fluorescence branching ratios (b) are expressed as the ratio of a radiative transition to the sum of all radiative transitions as shown in Equation (7). Table 6 presents all emission spectroscopic parameters determined by Judd-Ofelt analysis.
tr ¼ P
1 0 AðJ/J Þ
(6)
0 A J/J 0 AðJ/J Þ
bðJ/J 0 Þ ¼ P
(7)
The calculated branching ratios of 4F3/2 / 4I9/2 and 4F3/2 / 4I11/2 transitions are determined to be 57% and 38% respectively. The radiative lifetime is approximatively 328 ms, which is in good agreement of the experimental value (283 ms). Therefore, the radiative quantum efficiency (ƞ), defined as the ratio of the experimental value to the calculated lifetime reached up to 86%. 4. Laser performance The polished 0.4% Nd and 0.8% Nd: Y2O3 samples with dimension of 5 5 8 mm3 or 5 5 4 mm3 respectively were used to operate laser experiments under the excitation at 0.808 mm by LD pump for two different emissions at 0.946 mm and 1.074 mm. The plano-concave resonating cavity and experimental conditions of laser operation were previously described in Fig. 1. The Nd: Y2O3 sample was placed between M1 and M2 mirrors with the cavity
of various Nd3þ doped hosts.
Material
Type
U2 1020cm2
U4 1020cm2
U6 1020cm2
c
Reference
Nd: Nd: Nd: Nd: Nd: Nd: Nd:
S.C S.C S.C S.C ceramic ceramic ceramic
4.05 0.50 2.19 1.30 4.36 8.84 7.59
2.82 3.35 8.16 2.29 3.61 9.82 4.04
2.67 5.16 8.57 2.42 2.92 4.44 1.40
1.0 0.6 0.9 0.9 1.2 2.2 2.9
[20] [12] [21] [22] [16] [13] This work
GdTaO4 Y3Al5O12 CaYAlO4 ASL Y2O3 by X. Hou Y2O3 by G. Singh Y2O3
*S.C ¼ single crystal.
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length of 4 mm. 4.1. Laser emission at 1.074 mm
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were 11.2 and 14.2%, respectively. Concentration quenching effect is probably responsible of this lower output power.
4.2. Laser emission at 0.946 mm
0.4% and 0.8% Nd: Y2O3 samples with lengths of 8.3 and 4.3 mm respectively had the same level of absorption efficiency around 70e74%. The experimental results with three different transmission rates of the output coupler (2, 6 and 10%) were shown in Fig. 12. In the case of 0.4% Nd: Y2O3, the laser thresholds pump were 0.8, 1.7 and 2.0 W for 2, 6 and 10% TOC respectively. The maximum output power and laser threshold increased significantly with % TOC. Using output coupler of 10% transmission, the maximum output power was 3.5W with 13.0 W of absorbed power. Further, the opticaloptical conversion efficiency and slope efficiency were determined to be 27.0 and 31.1% respectively. On the other hand, Fig. 12 (b) presents the laser performance of 0.8% Nd: Y2O3 sample at the same wavelength. The laser thresholds were similar to those of 0.4% Nd: Y2O3. The increasing trend of laser performance was also exhibited with % TOC of M2. Despite of the same absorbance efficiency, the laser output powers of 0.8% Nd: Y2O3 were much lower than those of 0.4% Nd under the same conditions of experiment. When using 10% TOC of output coupler mirror, the maximum output power was 1.3 W with the absorbed power of 11.6 W. The opticaloptical quantum efficiency and slope efficiency of 0.8% Nd: Y2O3
According to the best results of laser experiment at 1.074 mm, 0.4%Nd: Y2O3 ceramics were chosen for laser operation at 0.946 mm. In addition, the samples with lengths of 4.5 and 8.3 mm were prepared to have corresponding absorption efficiencies of 53 and 74%. To our best knowledge, laser operation at 0.946 mm had never been reported yet for Nd: Y2O3 transparent ceramics. The same type of plano-concave cavity was set up with M1 and M2 adapted to emission at 0.946 mm. The transmission output coupler of M2 mirror were 3 and 5%. The excitation was also performed in CW mode at 0.808 mm with the same LD pump. The input-output curves of 4 mm- and 8 mm-long ceramics at 0.946 mm are shown in Fig. 13 (a) and (b) respectively. For 4 mm-long sample and 3% output coupling, the laser threshold was found to be 3.5 W. The maximum output was limited at 0.41 W with 7.7 W of absorbed power when using M2 with coupling rate of 3%. The optical-optical conversion efficiency and slope efficiency were 5.3% and 9.6%, respectively. For the 8 mm-long sample, with the same output coupling rate of 3%, the optical-optical and slope efficiencies were 6.9 and 9.3% respectively: the maximum output power was 0.9 W with absorbed
Fig. 12. Output power of 0.4% Nd (left) and 0.8% Nd (right): Y2O3 at 1.074 mm versus absorbed power at 0.808 mm.
Fig. 13. Output power of 0.4%Nd: Y2O3 at 0.946 mm versus absorbed power at 0.808 mm.
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power of 13.0 W. Obviously, 53% of absorption efficiency was too low to optimize laser performance. Therefore, 8 mm-long sample was further employed with 5% output coupling. The maximum output power of 1 W was then obtained. The optical-optical conversion and slope efficiencies were 7.8% and 12.5%, respectively. Even though the calculated c value derived from Judd-Ofelt computation is high (2.7) and very encouraging to achieve the strong emission at 0.946 mm, laser experiments show that laser performance of 0.4% Nd: Y2O3 at 0.946 mm is significantly less efficient than that at 1.074 mm. The transmittance spectrum of 0.4% Nd: Y2O3 transparent ceramic at 1.074 mm (%T ¼ 81.4) is slightly higher than that observed at 0.946 mm (%T ¼ 80.9). Consequently, the optical loss at 0.946 nm has to be considered as sligthly higher and could affect 4F3/2 / 4I9/2 laser efficiency. These results could show the crucial role of transparent ceramic transmittance on the laser demonstration of the 4F3/2 / 4I9/2 channel.
5. Conclusions We studied and reported the spectroscopic properties and laser performance of Nd3þ-doped Y2O3 transparent ceramic host, especially for laser operation at 0.946 mm along the 4F3/2 / 4I9/2 channel, which is reported for the first time in this work. This success relies on a good optical quality of Nd: Y2O3 transparent ceramic. Owing to Judd-Ofelt computation by RELIC program, the spectroscopic parameters Ut (t ¼ 2, 4, 6) were determined to be 7.59 1020, 4.04 1020 and 1.40 1020 cm2 for t ¼ 2, 4 and 6, respectively. The absorption cross section at 0.808 mm is 2.2 1020 cm2 with FWHM of 1.8 nm. The fluorescence decay lifetime of 3F4 manifold is 283 ms at room temperature for 0.4% Nd doping level. The laser operation at 1.074 mm exhibits the maximum output power of 3.5 W with an optical-optical conversion efficiency of 27.0% and slope efficiency of 31.1%. For laser experiments at 0.946 mm, the maximum output power is around 1 W with an absorbed power of 13 W. An optical-optical conversion efficiency and slope efficiency are respectively 7.8% and 12.5%. From these results, Nd: Y2O3 transparent ceramic can be an excellent material for laser generating at 1.074 mm and the results of laser operation at 0.946 mm is encouraging to develop a new LD pumped solid state laser. It should be noted that the optimization of 0.946 mm laser performance of Nd: Y2O3 transparent ceramic strongly depends on the improvement of the optical quality of the transparent ceramic material. However, this first laser performance demonstrated on the 4F3/2 / 4I9/2 channel is very encouraging.
Acknowledgements Authors would like to thanks Royal Thai Government for supporting the fellowship of PhD student involved in this work.
References [1] D. Marzahl, F. Moglia, P.W. Metz, Out of the blue: semiconductor laser pumped visible rare-earth doped lasers, Laser Photonics Rev. 21 (2016) 1e21. [2] W. YAO, J. GAO, et al., Continuous-wave yellowegreen laser at 0.56 mm based on frequency doubling of a diode-end-pumped ceramic Nd: YAG laser, Appl. Opt. 54 (2015) 1e5. [3] P.F. Zhu, C.M. Zhang, P. Song, et al., All-solid-state dual end pumped Nd: YAG/ LBO yellow green laser with 10.8 W output power at 561 nm, Opt. Spectrosc. 118 (2015) 623e626. [4] J. Gao, L. Zhang, H. Sun, X. Dai, X. Wu, High-power continuous-wave yellowgreen laser at 558 nm under in-band pumping, Opt. Commun. 319 (2014) 110e112. [5] B. Li, L. Zhao, Y.B. Zhang, Q. Zheng, Y. Zhao, Y. Yao, All-solid-state continuouswave frequency doubling Nd: LuVO4/LBO laser with 2.17 W output power at 543 nm, Opt. Spectrosc. 22 (2012) 1759e1762. [6] Z. Ling, Y. Yi, Z. Yang, Z. Quan, All solid state dual end pumped YVO4: Nd/LBO blue laser with 21.8 W output power at 457 nm, Opt. Spectrosc. 116 (2014) 470e472. [7] Y.F. Lü, X.D. Yin, J. Xia, R.G. wang, D. Wang, Efficient continuous-wave intracavity frequency-doubled Nd: YAG - LBO blue laser at 473 nm under diode pumping directly into the emitting level, Laser Phys. Lett. 7 (2010) 24e28. [8] C. Czeranowsky, E. Heumann, G. Huber, All-solid-state continuous-wave frequency-doubled Nd: YAG-BiBO laser with 2.8 W output power at 473nm, Opt. Lett. 28 (6) (2003) 432e434. [9] H.Y. Zhu, C.W. Xu, J. Zhang, D.Y. Tang, D.W. Luo, Y.M. Duan, Highly efficient continuous-wave Nd:YAG ceramic lasers at 946 nm, Laser Phys. Lett. 10 (1e4) (2013) 75802. [10] Y. Lü, L. Zhao, P. Zhai, J. Xia, X. Fu, S. Li, Simultaneous three-wavelength continuous wave laser at 946 nm, 1319 nm and 1064 nm in Nd: YAG, Opt. Commun. 286 (2013) 257e260. [11] D. Krennrich, R. Knappe, B. Henrich, R. Wallenstein, J.A. L'Huillier, A comprehensive study of Nd: YAG, Nd: YAlO3, Nd:YVO4 and Nd:YGdVO4 lasers operating at wavelengths of 0.9 and 1.3mm. Part 1: cw-operation, Appl. Phys. B 92 (2008) 175e183. [12] H. Guo, M. Zhang, J. Han, Y. Nie, Crystal growth, JuddeOfelt analysis and radiative properties of Nd: YAG single crystal grown by HDS, J. Luminescence 140 (2013) 135e137. [13] G. Singh, V.S. Tiwari, P.K. Gupta, Spectroscopic analysis on the basis JuddeOfelt theory of Nd3þ in (Y0.985Nd0.015)2O3: a transparent laser-host ceramic, Mater. Res. Bull. 60 (2014) 838e842. [14] B.M. Walsh, N.P. Barnes, B.M. Walsh, J.M. Mcmahon, R.W. Equall, R.L. Hutcheson, Spectroscopic characterization of Nd: Y2O3: application toward a differential absorption lidar system for remote sensing of ozone, Opt. Soc. Am. B 19 (2002) 2893e2903. [15] A. Lupei, V. Lupei, A. Ikesue, C. Gheorghe, Spectroscopic and energy transfer investigation of Nd/Yb in Y2O3 transparent ceramics, Opt. Soc. Am. B 27 (5) (2010) 1002e1010. [16] X. Hou, S. Zhou, T. Jia, H. Lin, H. Teng, Effect of Nd concentration on structural and optical properties of Nd: Y2O3 transparent ceramic, J. Luminescence 131 (2011) 1953e1958. [17] G.A. Kumar, Jianren Lu, Alexander A. Kaminskii, Ken-Ichi Ueda, Hideki Yagi, Takakimi Yanagitani, Spectroscopic and stimulated emission characteristics of Nd3þ in transparent Y2O3 ceramics, IEEE J. Quantum Electron. 42 (7) (2006). [18] M.P. Hehlen, M.G. Brik, K.W. Kr€ amer, 50th anniversary of the Judd- ofelt Theory: an Experimentalist's view of the formalism and its application, J. Luminescence 136 (2013) 221e239. [19] L. Zhang, Z. Huang, W. Pan, High transparency Nd: Y2O3 ceramics prepared with La2O3 and ZrO2 additives, J. Am. Ceram. Soc. 98 (2015) 824e828. [20] F. Peng, H. Yang, Q. Zhang, Spectroscopic properties and laser performance at 1.066 mm of a new laser crystal Nd: GdTaO4, Appl. Phys. B 118 (2015) 549e554. [21] D.Z. Li, X.D. Xu, S.S. Cheng, D.H. Zhou, Polarized spectral properties of Nd3þ ions in CaYAlO4 crystal, Appl. Phys. B 101 (2010) 199e205. [22] L. Zheng, P. Loiseau, G. Aka, Diode-pumped laser operation at 1053 and 900 nm in Sr1xLax-yNdyMgxAl12xO19 (Nd: ASL) single crystal, Laser Phys. 23 (1e6) (2013) 095802.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Optical spectroscopic properties, 0.946 and 1.074 mm laser performances of Nd3þ-doped Y2O3 transparent ceramics Suchinda Sattayaporn a, Gerard Aka a, *, Pascal Loiseau a, Akio Ikesue b, Yan Lin Aung b a b
PSL Research University, Chimie ParisTech, CNRS, 75005, Paris, France World-Lab Co., Ltd., CSJ 308 1-2-19 Mutsuno, Atsuta-ku, Nagoya, 456-0023, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2017 Received in revised form 22 March 2017 Accepted 29 March 2017 Available online 31 March 2017
0.4% Nd3þ-doped Y2O3 transparent ceramics have been investigated from their optical absorption and emission properties to the laser performances at two different wavelengths, 0.946 mm and 1.074 mm. Absorption spectra in the range 300e1000 nm was measured to perform Judd-Ofelt analysis: the calculated spectroscopic parameters U2, U4 and U6 are 7.59 1020, 4.04 1020 and 1.40 1020 cm2 respectively. The measured fluorescence and radiative lifetime of 4F3/2 manifold are determined to be 283 ms and 328 ms respectively. The branching ratios of 4F3/2 / 4I9/2 and 4F3/2 / 4I11/2 transitions are 57% and 38% with corresponding stimulated emission cross sections of 1.02 1020 and 1.16 1020 cm2 respectively. Laser experiments, under 0.808 mm laser diode pumping source, were carried out using a plano-concave resonating cavity. At 1.074 mm, the maximum output power is 3.5 W for an absorbed power of 13.0 W. The optical-to-optical and slope efficiencies reached 26.9 and 31.1%, respectively. For laser operation at 0.946 mm, the maximum output power reached 1.0 W for 13.0 W of absorbed power. The optical-to-optical conversion efficiency and slope efficiency were 7.9 and 12.4%, respectively. © 2017 Elsevier B.V. All rights reserved.
Keywords: Near-infrared Ceramic transparent lasers Neodymium Judd-Ofelt analysis Spectroscopy
1. Introduction High-power laser sources operating in the blue spectral range of 440e490 nm are of great interest for several applications such as display technology, submarine communications or medical treatment. First, the blue emission can be directly realized by doping appropriate lanthanide ions likes Pr3þ, Tb3þ or Ce3þ into a crystalline host. Among these lanthanide ions, Pr3þ exhibits for example the outstanding spectroscopic properties of high absorption and emission cross sections in blue region [1]. Nevertheless, the direct laser emitting is not very efficient. At present, the frequency doubling or second harmonic generation is frequently applied to realize laser emission which is hardly accessible with aforementioned direct method. There are many reports on high-efficient green laser tunable around 0.532 mm by doubling infrared emission around 1.064 mm from Nd3þ ions [2e5]. As Nd3þ ions can provide near-infrared emission at shorter wavelength around 0.946 mm (4F3/2 / 4I9/2), the same principle could be also used to reach high-power blue laser emission [6e8]. Therefore, it is interesting to begin with Nd3þ solid-state materials for high
* Corresponding author. E-mail address: [email protected] (G. Aka). http://dx.doi.org/10.1016/j.jallcom.2017.03.343 0925-8388/© 2017 Elsevier B.V. All rights reserved.
power 0.946 mm laser. Using commercial LD pump at 0.808 mm, Nd3þ ion can be easily excited from ground state 4I9/2 to 4F5/2, 2H29/2 upper state with high output power pump. Nd3þ ions generate infrared emission along 4F3/2 / 4I9/2 and 4F3/2 / 4I11/2 transition, leading to near-infrared emissions at 0.946 mm and 1.074 mm respectively. Since many decades, Nd3þ-doped various hosts have been fabricated and studied for their optical, spectroscopic properties and laser performances for 0.946 mm especially, Nd doped garnet (Y3Al5O12, YAG), perovskite (YAlO3, YAP) and vanadate (YVO4, YVO) hosts [9e11]. According to the referred works, Nd: YAG, have delivered 2.2 Watts of the maximum output at 0.946 mm with an absorbed power of 11 Watts. However, its calculated ratio U4/U6 (c), relating to the probability ratio of 4F3/2 / 4I9/2 on 4F3/ 4 2 / I11/2 emission is much relatively weak (0.6) [12] when comparing with the other oxide as Y2O3 (2.2) [13]. Especially, the good thermomechanical properties of Nd: Y2O3, associated with its optical spectroscopic data comparable to Nd: YAG [14e16], should allow laser generation with better performances on the 4F3/2 / 4I9/ 2 channel. Nevertheless, the Nd: Y2O3 laser performances on the 4 F3/2 / 4I9/2 transition have not been yet studied. It is important to point out that the growth of Y2O3 single crystal is rather difficult to achieve with large size and high optical quality using conventional technique like Czochralski because of its high
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Fig. 1. The crystal structure of Y2O3 showing the YO6 unit (left) and the two inequivalent Y ions in C2 and C3i symmetries (Pink and blue balls, respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
melting point (2430 C). Iridium crucible, with a melting point of 2450 C, is not appropriate for the Y2O3 crystal growth. Additionally, phase transition of polymorphic phase occurs during the growth process from cubic phase to hexagonal phase at high temperature around 2280 C [17]. This phase transition leads to twinning and crystal defects formation. The most suitable solution is to prepare alternatively cubic Y2O3 transparent ceramic since lower temperature fabrication methods can provide Y2O3 transparent ceramics with an optical quality, comparable to Y2O3 single crystal. In this work, we studied Nd doped Y2O3 transparent ceramics and reported the optical, spectroscopic properties as well as laser performance at two different wavelengths 0.946 mm and 1.074 mm under the excitation at 0.808 mm by LD pumping. 2. Experimental section Yttrium oxide (Y2O3) belongs to cubic system with Ia-3 space group and has bixbyite structure. Y ions are coordinated with 6 oxygen atoms and locate in two inequivalent cation sites with different symmetries: C2 (24 sites) and C3i (8 sites). The C2 site can be described as a cube structure with two oxygen vacancies on a body diagonal. Whereas, the C3i sites have two oxygen vacancies on a face diagonal of a cube as shown in Fig. 1 (right). 0.4, 0.8 and 1 at% Nd doped Y2O3 ceramic samples (see Fig. 2) in our work were prepared and supplied by World-Lab Co., Ltd. from Japan. The optical homogeneity of sample was thoroughly investigated with the help of micro-polarizer plate and transmission polarized optical microscope. In order to determine the
447
concentration of Nd ions, the surface of sample was polished and coated with graphite. The chemical compositions of Nd3þ-doped Y2O3 were analyzed by using an electron probe microanalysis (EPMA) technic with CAMECA SX100 instrument. The thermal conductivity measurements of 0.4% Nd: Y2O3 and pure Y2O3 ceramic samples were performed at room temperature with a Ctherm TCi Thermal Conductivity Analyzer apparatus (TCA). Moreover, the refractive index of Y2O3 transparent ceramic was measured in the visible-infrared range by the minimum deviation technique, using a prism of pure Y2O3 transparent ceramic to determine the associated sellmeier coefficients. 0.4 at% Nd: Y2O3 with a thickness of 3.2 mm was used to collect the optical absorption and emission spectra. The non-polarized absorption spectra were recorded in the 300e1000 nm range at 10 K and room temperature by using a Cary 6000i UVeViseNIR spectrophotometer. A cryostat was used for sample absorption spectra acquisition at 10 K. Room temperature absorption spectra were also used to realize Judd-Ofelt computation with the help of Relic 1.0 software developed by Markus P. Hehlen from Los Alamos National Laboratory [18]. Furthermore, fluorescence emission spectra in the range of 850e1200 nm were recorded at 77 K by dipping a sample in liquid nitrogen and at room temperature with 808 nm as excited wavelength by using two sets of equipment. The first one is composed with an optical parametric oscillator (OPO) as pulsed exciting source, a monochromator Acton SP2300 from Princeton Instrument and an InGaAs photodiode camera. The second set uses a cw Ti: Sapphire laser Coherent 890 pumped with an OPSL at 540 nm, an ARC Spectra Pro-7510 monochromator and a cooled InGaAs photodiode. In addition, the emission decay of 4F3/2 excited state at room temperature was also recorded to determine the fluorescence lifetime of the Nd: Y2O3 transparent ceramic samples. Fig. 3 presents the plano-concave resonating cavity set up of laser experiments at 0.946 mm and 1.076 mm. The fiber-coupled Laser diode (LD) from LIMO was employed as cw pumping source at 808 nm, delivering a maximum output power of 35 W. The laser diode fiber core size is 100 mm with a 0.22 of numerical aperture. The LD pump laser was collimated and focused into the Nd: Y2O3 ceramic with two lenses f1: f2 of 75: 150 focusing lengths in mm. The Nd: Y2O3 sample with an aperture of 5 5 mm2 was wrapped in indium foil and placed in a water-cooled petlier copper block set at 5 C. Both faces of transparent ceramic were polished and antireflection (AR) coated at 0.808 mm and 0.946 mm or 1.074 mm M1 is a plane mirror with high-reflection (HR) coating for 0.946 and 1.074 mm and high-transmission (HT) at 0.808 mm. The M2 output coupler is a concave mirror with a radius of curvature R ¼ 50 mm. Various M2 output coupler transmissions were used for laser experiments: 3 or 5%TOC at 0.946 mm, 2, 6 or 10% TOC at 1.074 mm. Laser
Fig. 2. 0.4%, 0.8% and 1 at% Nd doped Y2O3 ceramic samples.
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Fig. 3. Configuration of the experimental laser setup.
Table 1 Initial and measured compositions of 0.4 %Nd: Y2O3 samples. Composition
Nd
Y
Initial Measured
0.008 0.007
1.992 1.993
emission spectra were recorded using a spectrometer HR4000 from Ocean Optics.
3. Results and discussion 3.1. Electron microprobe analysis and thermal conductivity measurements The chemical composition of 0.4% Nd: Y2O3 sample was determined by using the technique of electron microprobe analysis (EMPA). The initial doping level of Nd3þ ions in raw material is 0.4 at.%, corresponding to a composition of Y1.992Nd0.008O3. Table 1 presents measured composition in ceramic sample which is Y1.993Nd0.007O3. As the thermal properties have an influent on laser performance, it is necessary to investigate the thermal conductivity of 0.4% Nd: Y2O3 and pure Y2O3 ceramics at room temperature. At room temperature, the thermal conductivity of 0.4% Nd: Y2O3 and pure Y2O3 are measured to be 8.3 and 9.5 Wm1K1 respectively. The doped one has thermal conductivity slightly lower than that reported in the literature [14] and that of the pure one. As compared to various hosts, Y2O3 exhibits a thermal conductivity as high as YAG ceramic (10 Wm1K1). Consequently, this advantageous property makes Nd: Y2O3 an attractive host for laser operation.
Table 2 Measured and calculated refractive index data from the fitted Sellmeier equation in the range of 0.405 mme1.020 mm. Wavelength (mm)
0.405 0.408 0.468 0.480 0.509 0.546 0.577 0.579 0.589 0.644 0.980 1.000 1.020
Refractive index Measured
Calculated
1.9549 1.9541 1.9379 1.9348 1.9283 1.9216 1.9171 1.9165 1.9153 1.9092 1.8914 1.8904 1.8897
1.9556 1.9545 1.9368 1.9339 1.9280 1.9216 1.9172 1.9170 1.9157 1.9097 1.8911 1.8904 1.8898
Table 3 The fitted values with standard deviation of Sellmeier's coefficients of Y2O3 ceramic in the range of 0.405e1.020 mm. Sellmeier's coefficients
Fitted value
A B C
3.537 ± 0.004 0.0473 ± 0.0008 0.011 ± 0.004
3.2. Refractive index dispersion The prepared prism of Y2O3 shown in Fig. 7 (onset) was used to measure the refractive index by minimum deviation method. The apex angle of the prism is 50 . The room temperature refractive index of Y2O3 prism were measured by using gas-discharge lamps and OPOs pumping source in the range from 0.4 to 1.02 mm. The eleven values of measured refractive index are all presented in Table 2. The fit of measured data was realized by using the non-linear curve presented in Fig. 7 (red line), leading to the determination of Sellmeier coefficients (A, B and C) from Equation (1) given below:
n2 ¼ A þ
B
l2
C l2
(1)
where, n is refractive index, A, B and C represent Sellmeier coefficients and l is wavelength in micrometer. A, B and C coefficients, obtained from the fitting curve, are given in Table 3. 3.3. Optical measurement of Nd: Y2O3 transparent ceramics The optical homogeneity of the 0.4% Nd: Y2O3 transparent ceramic with dimensions of 5 5 3 mm3 was measured by micro-polarizer plate. In addition, transmission polarized optical microscope was also used to check optical quality inside the transparent ceramic material. These results are summarized in Fig. 4 (upper and lower). Obviously, there were no optical stresses in the whole position of the samples. As shown in the microscopic image, there were no double refractions due to grain boundary phases or secondary phases or voids inside the sample. This suggested that only cubic Y2O3 phase was formed. Shadow graph of the prepared 0.4% Nd: Y2O3 (5 5 3 mm3) ceramics was measured by Schlieren imaging system. The result is shown in Fig. 5 with the shadow graph of commercial 1% Nd: YAG single crystal as a reference. Both samples showed homogeneous distribution of refractive index throughout both samples, suggesting that the sample 0.4% Nd: Y2O3 is optically homogeneous. In addition, transmission spectrum of the prepared 0.4% Nd: Y2O3 (5 5 3 mm3) ceramics is shown in Fig. 6. By using the value of refractive index at laser oscillation wavelength (~1 mm) ranges, Fresnel loss can be calculated. The theoretical transmission after subtracting the Fresnel loss was around 81.9% by using the set of refractive index we measured. At 0.946 mm, the transmittance was
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Fig. 4. Observation under micro-polarizer (upper) and transmission polarized optical microscope (lower) for the prepared 0.4% Nd: Y2O3 (5 5 3 mm3) ceramic sample.
Fig. 5. Shadowgraph of the prepared 0.4% Nd: Y2O3 ceramics (right) and commercial 1% Nd: YAG single crystal (left) measured by Schlieren imaging system.
Fig. 6. In-line transmittance curve of the prepared 0.4% Nd: Y2O3 ceramics (length of 3.2 mm with optical polished surfaces) recorded by spectrophotometer.
slightly higher than 80%, indicating an excellent optical quality of Y2O3 ceramic. That is the reason why Nd: Y2O3 transparent ceramics prepared by World Lab company can achieve laser emission at 0.946 mm and reported for the first time. In the present paper there was no report on success of laser demonstration on the 4 F3/2 / 4I9/2 transition when using Nd3þ-doped other ceramics or Y2O3 transparent ceramics but prepared by other methods. A possible hypothesis is that the transmittance of other ceramics around 0.946 mm is significantly lower than that of our samples, probably due to the presence of many scattering centers. To our best knowledge, our ceramics exhibited the highest transmittance for Nd: Y2O3 transparent ceramics. For example, the one fabricated recently by using La2O3 and ZrO2 additives still had the transmittance spectra lower than 80% in near-infrared spectral region [19]. The transmittance of our sample near laser oscillation wavelength ranges was very close to the theoretical value and it can be
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Fig. 7. Y2O3 prism and measured refractive index in the range from 0.4 to 1.0 mm.
judged that the optical loss of the prepared sample is very low. The theoretically calculated transmission at 500 nm is approximately 80%, nevertheless the actual measured transmittance was around 78%. This suggested that the optical quality of the sample is influenced by Rayleigh's scattering. 3.4. Spectroscopic characterizations The non-polarized absorption spectra of 0.4% Nd: Y2O3 were recorded at 10 K and room temperature in the range of 300e1000 nm. All the obtained spectra were recorded with a resolution of 0.1 nm - 0.2 nm. In order to identify the number of Nd crystallographic sites in Y2O3 structure, absorption spectra in the particular 434e450 nm range were recorded at various temperatures from 25 K to 300 K. The corresponding transition is 4I9/2 / 2P1/2 channel. Here, one absorption line is expected per Nd site. Fig. 8 shows five absorption spectra in the range of 434e439 nm, recorded at 25, 44, 77, 150 and 300 K. At room temperature, two broad absorption peaks were found (green spectra). When the temperature decreased, these peaks became sharper and the other peaks, which have been overlapped, were better resolved at 25 K. Four peaks were found
Fig. 8. Absorption spectra of 0.4% Nd: Y2O3 from 434 to 439 nm at various temperatures.
here. Furthermore, the peak intensities are temperature dependent. In fact, the evolution of peak intensity with temperature can be attributed to thermal population. Therefore, there are two crystallographic sites for Nd3þ ions, relating to the two different lowest energy levels marked Y1 and Y2 on Fig. 1. This obtained result is in agreement with previous work [15]. The two Nd sites have different symmetries, C2 (Y2) and C3i (Y1). The C3i symmetry site is weakly active for lanthanide ions. We also focused on near-infrared absorption in the range of 700e950 nm as shown in Fig. 9 to study laser performances of Nd: Y2O3 at 0.946 mm and 1.074 mm. In this spectral region, the strongest band could be attributed to 4I9/2 / 4F5/2 and 4I9/2 / 4H9/2 transitions. The two strongest absorption bands are peaked at 806 and 821 nm with high absorption cross sections of 2.2 1020 and 3.8 1020 cm2 respectively. Moreover, the full width at half maximum (FWHM) of these two peaks are 1.8 and 2.4 cm1, respectively. The peak located at 806 nm is the most attractive because of the availability of commercial LD pumping sources. By comparing the fluorescence emission in the spectral range 850e1000 nm at 77 K and 300 K (Fig. 10 a), the stark energy of 4I9/2 state could be investigated. Further, the room temperature fluorescence spectra, recorded with excitation at wavelength 808 nm in the range of 850e1200 nm is shown in Fig. 10 b, demonstrating the stimulated emission cross sections of 4F3/2 / 4I9/2 and 4F3/2 / 4I11/2 transitions. The splitting energy of 4I9/2 manifold is determined to be 641 cm1, which is in good agreement with ref [14], reporting the stark value of 643 cm1. The strongest emission peak is located at 1.074 mm corresponding to 4F3/2 / 4I11/2 transition. The stimulated emission cross section was calculated to be 3.5 1020 cm2. Additionally, the concentration dependency of fluorescent lifetime was also studied in this work. The room temperature fluorescence decay of 4F3/2 metastable state for 0.4, 0.8 and 1.0 at. % Nd: Y2O3 ceramic were collected at wavelength 1074 nm with a digital oscilloscope. The logarithm of fluorescence intensity plotted as a function of time is shown in Fig. 11 (left). The linear fitting leaded to the determination of fluorescence lifetimes of 4F3/2 manifold which were 283, 249 and 217 ms for 0.4, 0.8 and 1.0 at.% Nd: Y2O3 ceramics respectively. As shown in Fig. 10 (right), the measured lifetime decreased dramatically with Nd3þ concentration probably due to cross-relaxation process. This evolution is in good accordance with the literature reporting a decreasing from 270 ms (1% Nd) to 153 ms (1.5% Nd) of the 4F3/2 state
Fig. 9. Room temperature absorption spectral of 0.4% Nd: Y2O3 in the range from 700 nm to 975 nm.
S. Sattayaporn et al. / Journal of Alloys and Compounds 711 (2017) 446e454
Fig. 10. Emission spectra of 0.4% Nd: Y2O3 in the range of 850e1000 nm at 77 K and 300 K (a) and 850e1200 nm at 300 K (b).
lifetime of Nd: Y2O3 ceramics prepared by other methods [13,17]. Based on the lifetime results, laser experiments will be performed by employing samples with two Nd3þ concentrations: 0.4 and 0.8 Nd at. %. 3.5. Judd-Ofelt analysis The optical absorption spectra and refractive index, experimentally obtained and demonstrated in the previous section were used to perform Judd-Ofelt analysis. All absorption data including the mean wavelength, refractive index, integrated area and oscillator strength were derived and summarized in Table 4. The JuddOfelt approach provides the calculation of electric dipole line strengths of each transition, the spectroscopic intensity parameters Ut, with t ¼ 2, 4 and 6, the branching ratios, the radiative transition probabilities as well as the radiative lifetime. Firstly, based on ten absorption bands deduced from absorption spectra of 0.4% Nd: Y2O3, the experimental electric dipole line strength from initial state J to final state J0 , so-called Sexp (J/J0 ), were calculated for each band with Equation (2):
0 Sexp J/J ¼
27hcð2J þ 1Þn 2 8p3 Nƛe2 n2 þ 2
Z
aðlÞdl
(2)
451
Fig. 11. Room temperature fluorescence decay in a logarithmic scale for 0.4, 0.8 and 1 at% Nd: Y2O3 (left) and the decreasing lifetime as a function of at% Nd doped Y2O3 (right).
where, c is the speed of light in vacuum, h is Planck's constant, e is charge of one electron, J and J0 are the angular momentum of initial and final states respectively. N corresponds to Nd3þ concentration (ions per cm3), ƛ is mean wavelength of absorption band in nanometer, n refractive index, a is absorption coefficient in cm1. On the other hand, the calculated electric dipole line strengths (Scalc) can be linearly expressed by computing three parameters Ut by using Equation (3) given below:
2 X 0 Scalc J/J ¼ Ut < JJ U t J0 J 0 >
(3)
t¼2;4;6
where Ut is tensor operator of rank t ¼ 2, 4 and 6, the term j
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Table 4 Absorption bands, experimental and calculated line strengths of 0.4% Nd doped Y2O3 ceramic. Transition from 4I9/2 ground state
ƛ (nm)
m
ʃA(l)dl
Sexp (1020cm2)
Salc (1020cm2)
4
357 437 480 534 592 689 752 812 892 1581
1.981 1.945 1.933 1.923 1.914 1.905 1.901 1.898 1.895 1.885
4.149 0.292 0.369 2.882 13.171 0.471 3.324 4.854 1.836 0.410
0.3272 0.2624 0.3094 2.1986 8.9838 0.3005 1.6090 2.4677 0.8413 0.1078
0.1522 0.1535 0.3132 2.0264 9.3398 0.2161 1.1023 2.2376 1.0168 0.0640
D1/2 þ 4D3/2 þ4D5/2 þ2I11/2 P1/2 þ 2D5/2 2 G11/2þ2K15/2þ2D3/2þ2G9/2 4 G9/2 þ 2K13/2 þ4G7/2 2 G7/2 þ4G5/2 4 F9/2þ2H11/2 4 F7/2 þ 4S3/2 4 F5/2 þ 2H9/2 4 F3/2 4 I15/2 2
0.4% Nd: Y2O3 is quite large due to the low symmetry C2 of active Y site. Whereas, the parameters U4 and U6 are characteristic factors usually applied to predict the intensity of stimulated emission for different transitions as 4F3/2 / 4I9/2 and 4F3/2 / 4I11/2 by considering the ratio of c ¼ U4/U6. These two factors vary with different hosts. Indeed, the U4 parameter relates significantly to the probability of 4F3/2 / 4I9/2 transition, emitting at wavelength of 0.946 mm. The U6 parameter correspond to the other emission at wavelength 1.074 mm (4F3/2 / 4I11/2). The calculated c value of 0.4% Nd: Y2O3 was 2.9, which is relatively high as compared to values already reported in the literature as presented in Table 5. The high value of c related to the strong emission from excited state 4F3/2 to ground state 4I9/2 and makes 0.4% Nd: Y2O3 a promising material for laser generation at 0.946 mm. Furthermore, the accuracy of the Judd-Ofelt fitting can be expressed in the term of root mean square deviation (Drms) with the usual Equation (4).
Drms ¼
"P
ðSmeas Scalc Þ2 pq
#1=2 (4)
where p is the number of absorption bands included in the JuddOfelt fit and q is the number of spectroscopic intensity parameters U. In this work, the p and q are 10 and 3 respectively. Finally, the rms deviation was 0.1 1020 cm2. 3.6. Radiative properties From the three Ut parameters, the emission line strength (Sems) from 4F3/2 manifold to lower manifolds such as 4I9/2, 4I11/2, 4I13/2 and 4 I15/2, can be also calculated by using Equation (3) previously mentioned in Judd-Ofelt analysis section. The emission line strengths Sems were used to find the value of radiative transition rate between two states (J to J0 ), AJ/J0 with Equation (5) given below:
2 64p4 e2 m m2 þ 2 0 0 A J/J ¼ Sems J/J 3 27hð2J þ 1Þƛ
Table 5 The spectroscopic intensity parameters Ut
(t¼2, 4, 6)
(5)
Table 6 Calculated emission cross section, radiative lifetime, fluorescence branching ratio of 0.4% Nd: Y2O3 ceramic. Transition
ƛ (nm)
Sems (1020cm2)
A(J/J0 ) (s1)
b(J/J0 )
tr (ms)
4
889 1062 1341 1840
1.02 1.16 0.30 0.04
1741 1151 147 8
57% 38% 0.5% 0.2%
328
F3/2 F3/2 F3/2 4 F3/2 4 4
/ / / /
4
I9/2 I11/2 I13/2 4 I15/2 4 4
where J ¼ 3/2 and J’ ¼ 9/2, 11/2, 13/2 and 15/2. The radiative lifetime (tr) of the excited metastable state 4F3/2 was then calculated by using Equation (6). On the other hand, the fluorescence branching ratios (b) are expressed as the ratio of a radiative transition to the sum of all radiative transitions as shown in Equation (7). Table 6 presents all emission spectroscopic parameters determined by Judd-Ofelt analysis.
tr ¼ P
1 0 AðJ/J Þ
(6)
0 A J/J 0 AðJ/J Þ
bðJ/J 0 Þ ¼ P
(7)
The calculated branching ratios of 4F3/2 / 4I9/2 and 4F3/2 / 4I11/2 transitions are determined to be 57% and 38% respectively. The radiative lifetime is approximatively 328 ms, which is in good agreement of the experimental value (283 ms). Therefore, the radiative quantum efficiency (ƞ), defined as the ratio of the experimental value to the calculated lifetime reached up to 86%. 4. Laser performance The polished 0.4% Nd and 0.8% Nd: Y2O3 samples with dimension of 5 5 8 mm3 or 5 5 4 mm3 respectively were used to operate laser experiments under the excitation at 0.808 mm by LD pump for two different emissions at 0.946 mm and 1.074 mm. The plano-concave resonating cavity and experimental conditions of laser operation were previously described in Fig. 1. The Nd: Y2O3 sample was placed between M1 and M2 mirrors with the cavity
of various Nd3þ doped hosts.
Material
Type
U2 1020cm2
U4 1020cm2
U6 1020cm2
c
Reference
Nd: Nd: Nd: Nd: Nd: Nd: Nd:
S.C S.C S.C S.C ceramic ceramic ceramic
4.05 0.50 2.19 1.30 4.36 8.84 7.59
2.82 3.35 8.16 2.29 3.61 9.82 4.04
2.67 5.16 8.57 2.42 2.92 4.44 1.40
1.0 0.6 0.9 0.9 1.2 2.2 2.9
[20] [12] [21] [22] [16] [13] This work
GdTaO4 Y3Al5O12 CaYAlO4 ASL Y2O3 by X. Hou Y2O3 by G. Singh Y2O3
*S.C ¼ single crystal.
S. Sattayaporn et al. / Journal of Alloys and Compounds 711 (2017) 446e454
length of 4 mm. 4.1. Laser emission at 1.074 mm
453
were 11.2 and 14.2%, respectively. Concentration quenching effect is probably responsible of this lower output power.
4.2. Laser emission at 0.946 mm
0.4% and 0.8% Nd: Y2O3 samples with lengths of 8.3 and 4.3 mm respectively had the same level of absorption efficiency around 70e74%. The experimental results with three different transmission rates of the output coupler (2, 6 and 10%) were shown in Fig. 12. In the case of 0.4% Nd: Y2O3, the laser thresholds pump were 0.8, 1.7 and 2.0 W for 2, 6 and 10% TOC respectively. The maximum output power and laser threshold increased significantly with % TOC. Using output coupler of 10% transmission, the maximum output power was 3.5W with 13.0 W of absorbed power. Further, the opticaloptical conversion efficiency and slope efficiency were determined to be 27.0 and 31.1% respectively. On the other hand, Fig. 12 (b) presents the laser performance of 0.8% Nd: Y2O3 sample at the same wavelength. The laser thresholds were similar to those of 0.4% Nd: Y2O3. The increasing trend of laser performance was also exhibited with % TOC of M2. Despite of the same absorbance efficiency, the laser output powers of 0.8% Nd: Y2O3 were much lower than those of 0.4% Nd under the same conditions of experiment. When using 10% TOC of output coupler mirror, the maximum output power was 1.3 W with the absorbed power of 11.6 W. The opticaloptical quantum efficiency and slope efficiency of 0.8% Nd: Y2O3
According to the best results of laser experiment at 1.074 mm, 0.4%Nd: Y2O3 ceramics were chosen for laser operation at 0.946 mm. In addition, the samples with lengths of 4.5 and 8.3 mm were prepared to have corresponding absorption efficiencies of 53 and 74%. To our best knowledge, laser operation at 0.946 mm had never been reported yet for Nd: Y2O3 transparent ceramics. The same type of plano-concave cavity was set up with M1 and M2 adapted to emission at 0.946 mm. The transmission output coupler of M2 mirror were 3 and 5%. The excitation was also performed in CW mode at 0.808 mm with the same LD pump. The input-output curves of 4 mm- and 8 mm-long ceramics at 0.946 mm are shown in Fig. 13 (a) and (b) respectively. For 4 mm-long sample and 3% output coupling, the laser threshold was found to be 3.5 W. The maximum output was limited at 0.41 W with 7.7 W of absorbed power when using M2 with coupling rate of 3%. The optical-optical conversion efficiency and slope efficiency were 5.3% and 9.6%, respectively. For the 8 mm-long sample, with the same output coupling rate of 3%, the optical-optical and slope efficiencies were 6.9 and 9.3% respectively: the maximum output power was 0.9 W with absorbed
Fig. 12. Output power of 0.4% Nd (left) and 0.8% Nd (right): Y2O3 at 1.074 mm versus absorbed power at 0.808 mm.
Fig. 13. Output power of 0.4%Nd: Y2O3 at 0.946 mm versus absorbed power at 0.808 mm.
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power of 13.0 W. Obviously, 53% of absorption efficiency was too low to optimize laser performance. Therefore, 8 mm-long sample was further employed with 5% output coupling. The maximum output power of 1 W was then obtained. The optical-optical conversion and slope efficiencies were 7.8% and 12.5%, respectively. Even though the calculated c value derived from Judd-Ofelt computation is high (2.7) and very encouraging to achieve the strong emission at 0.946 mm, laser experiments show that laser performance of 0.4% Nd: Y2O3 at 0.946 mm is significantly less efficient than that at 1.074 mm. The transmittance spectrum of 0.4% Nd: Y2O3 transparent ceramic at 1.074 mm (%T ¼ 81.4) is slightly higher than that observed at 0.946 mm (%T ¼ 80.9). Consequently, the optical loss at 0.946 nm has to be considered as sligthly higher and could affect 4F3/2 / 4I9/2 laser efficiency. These results could show the crucial role of transparent ceramic transmittance on the laser demonstration of the 4F3/2 / 4I9/2 channel.
5. Conclusions We studied and reported the spectroscopic properties and laser performance of Nd3þ-doped Y2O3 transparent ceramic host, especially for laser operation at 0.946 mm along the 4F3/2 / 4I9/2 channel, which is reported for the first time in this work. This success relies on a good optical quality of Nd: Y2O3 transparent ceramic. Owing to Judd-Ofelt computation by RELIC program, the spectroscopic parameters Ut (t ¼ 2, 4, 6) were determined to be 7.59 1020, 4.04 1020 and 1.40 1020 cm2 for t ¼ 2, 4 and 6, respectively. The absorption cross section at 0.808 mm is 2.2 1020 cm2 with FWHM of 1.8 nm. The fluorescence decay lifetime of 3F4 manifold is 283 ms at room temperature for 0.4% Nd doping level. The laser operation at 1.074 mm exhibits the maximum output power of 3.5 W with an optical-optical conversion efficiency of 27.0% and slope efficiency of 31.1%. For laser experiments at 0.946 mm, the maximum output power is around 1 W with an absorbed power of 13 W. An optical-optical conversion efficiency and slope efficiency are respectively 7.8% and 12.5%. From these results, Nd: Y2O3 transparent ceramic can be an excellent material for laser generating at 1.074 mm and the results of laser operation at 0.946 mm is encouraging to develop a new LD pumped solid state laser. It should be noted that the optimization of 0.946 mm laser performance of Nd: Y2O3 transparent ceramic strongly depends on the improvement of the optical quality of the transparent ceramic material. However, this first laser performance demonstrated on the 4F3/2 / 4I9/2 channel is very encouraging.
Acknowledgements Authors would like to thanks Royal Thai Government for supporting the fellowship of PhD student involved in this work.
References [1] D. Marzahl, F. Moglia, P.W. Metz, Out of the blue: semiconductor laser pumped visible rare-earth doped lasers, Laser Photonics Rev. 21 (2016) 1e21. [2] W. YAO, J. GAO, et al., Continuous-wave yellowegreen laser at 0.56 mm based on frequency doubling of a diode-end-pumped ceramic Nd: YAG laser, Appl. Opt. 54 (2015) 1e5. [3] P.F. Zhu, C.M. Zhang, P. Song, et al., All-solid-state dual end pumped Nd: YAG/ LBO yellow green laser with 10.8 W output power at 561 nm, Opt. Spectrosc. 118 (2015) 623e626. [4] J. Gao, L. Zhang, H. Sun, X. Dai, X. Wu, High-power continuous-wave yellowgreen laser at 558 nm under in-band pumping, Opt. Commun. 319 (2014) 110e112. [5] B. Li, L. Zhao, Y.B. Zhang, Q. Zheng, Y. Zhao, Y. Yao, All-solid-state continuouswave frequency doubling Nd: LuVO4/LBO laser with 2.17 W output power at 543 nm, Opt. Spectrosc. 22 (2012) 1759e1762. [6] Z. Ling, Y. Yi, Z. Yang, Z. Quan, All solid state dual end pumped YVO4: Nd/LBO blue laser with 21.8 W output power at 457 nm, Opt. Spectrosc. 116 (2014) 470e472. [7] Y.F. Lü, X.D. Yin, J. Xia, R.G. wang, D. Wang, Efficient continuous-wave intracavity frequency-doubled Nd: YAG - LBO blue laser at 473 nm under diode pumping directly into the emitting level, Laser Phys. Lett. 7 (2010) 24e28. [8] C. Czeranowsky, E. Heumann, G. Huber, All-solid-state continuous-wave frequency-doubled Nd: YAG-BiBO laser with 2.8 W output power at 473nm, Opt. Lett. 28 (6) (2003) 432e434. [9] H.Y. Zhu, C.W. Xu, J. Zhang, D.Y. Tang, D.W. Luo, Y.M. Duan, Highly efficient continuous-wave Nd:YAG ceramic lasers at 946 nm, Laser Phys. Lett. 10 (1e4) (2013) 75802. [10] Y. Lü, L. Zhao, P. Zhai, J. Xia, X. Fu, S. Li, Simultaneous three-wavelength continuous wave laser at 946 nm, 1319 nm and 1064 nm in Nd: YAG, Opt. Commun. 286 (2013) 257e260. [11] D. Krennrich, R. Knappe, B. Henrich, R. Wallenstein, J.A. L'Huillier, A comprehensive study of Nd: YAG, Nd: YAlO3, Nd:YVO4 and Nd:YGdVO4 lasers operating at wavelengths of 0.9 and 1.3mm. Part 1: cw-operation, Appl. Phys. B 92 (2008) 175e183. [12] H. Guo, M. Zhang, J. Han, Y. Nie, Crystal growth, JuddeOfelt analysis and radiative properties of Nd: YAG single crystal grown by HDS, J. Luminescence 140 (2013) 135e137. [13] G. Singh, V.S. Tiwari, P.K. Gupta, Spectroscopic analysis on the basis JuddeOfelt theory of Nd3þ in (Y0.985Nd0.015)2O3: a transparent laser-host ceramic, Mater. Res. Bull. 60 (2014) 838e842. [14] B.M. Walsh, N.P. Barnes, B.M. Walsh, J.M. Mcmahon, R.W. Equall, R.L. Hutcheson, Spectroscopic characterization of Nd: Y2O3: application toward a differential absorption lidar system for remote sensing of ozone, Opt. Soc. Am. B 19 (2002) 2893e2903. [15] A. Lupei, V. Lupei, A. Ikesue, C. Gheorghe, Spectroscopic and energy transfer investigation of Nd/Yb in Y2O3 transparent ceramics, Opt. Soc. Am. B 27 (5) (2010) 1002e1010. [16] X. Hou, S. Zhou, T. Jia, H. Lin, H. Teng, Effect of Nd concentration on structural and optical properties of Nd: Y2O3 transparent ceramic, J. Luminescence 131 (2011) 1953e1958. [17] G.A. Kumar, Jianren Lu, Alexander A. Kaminskii, Ken-Ichi Ueda, Hideki Yagi, Takakimi Yanagitani, Spectroscopic and stimulated emission characteristics of Nd3þ in transparent Y2O3 ceramics, IEEE J. Quantum Electron. 42 (7) (2006). [18] M.P. Hehlen, M.G. Brik, K.W. Kr€ amer, 50th anniversary of the Judd- ofelt Theory: an Experimentalist's view of the formalism and its application, J. Luminescence 136 (2013) 221e239. [19] L. Zhang, Z. Huang, W. Pan, High transparency Nd: Y2O3 ceramics prepared with La2O3 and ZrO2 additives, J. Am. Ceram. Soc. 98 (2015) 824e828. [20] F. Peng, H. Yang, Q. Zhang, Spectroscopic properties and laser performance at 1.066 mm of a new laser crystal Nd: GdTaO4, Appl. Phys. B 118 (2015) 549e554. [21] D.Z. Li, X.D. Xu, S.S. Cheng, D.H. Zhou, Polarized spectral properties of Nd3þ ions in CaYAlO4 crystal, Appl. Phys. B 101 (2010) 199e205. [22] L. Zheng, P. Loiseau, G. Aka, Diode-pumped laser operation at 1053 and 900 nm in Sr1xLax-yNdyMgxAl12xO19 (Nd: ASL) single crystal, Laser Phys. 23 (1e6) (2013) 095802.