High pressure luminescence and time resolved spectra of La2Be2O5:Pr3+

Optical Materials 34 (2011) 164–168

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High pressure luminescence and time resolved spectra of La2Be2O5:Pr3+ S. Mahlik a, M. Malinowski b, M. Grinberg a,⇑ a b

´ sk, Wita Stwosza 57, 80-952 Gdan ´ sk, Poland Institute of Experimental Physics, University of Gdan Institute of Micro- and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 13 April 2011 Received in revised form 17 July 2011 Accepted 10 August 2011 Available online 9 September 2011 Keywords: La2Be2O5:Pr3+ High pressure spectroscopy Impurity trapped exciton luminescence

a b s t r a c t We present luminescence, luminescence excitation and luminescence time resolved spectra of La2Be2O5:Pr3+ system. We used high pressure spectroscopy approaches, with high pressure applied in diamond anvil cell (DAC) and sapphire anvil cell (SAC), for detailed analysis of luminescence related to the 4f5d ? 4f2 and 4f2 ? 4f2 transitions. We present effect of up-converted luminescence related to 4f5d ? 4f2 transition excited with 488 nm. We also discussed possibility of existence of praseodymium trapped exciton (PTE) states in La2Be2O5:Pr3+ system. Lack of the PTE is attributed to high quantity of bulk modulus of this material. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Lanthanum berylate La2Be2O5 (BLO) crystals have been investigated due to potential application as laser material. BLO:Nd3+ has been shown to have good lasing properties, [1], in the IR visible region and BLO:Ho3+ was also investigated as potential laser emitter at the 3 lm wavelength, [2]. Energy structure and luminescence properties of Pr3+ in crystals are determined by two electrons that are located at the ground electronic configuration 4f2 or the excited electronic configuration 4f5d. The emission of Pr3+ ions depends on crystal host and extends from UV to IR, [3,4]. The broad band parity allowed UV emission is related to 4f5d ? 4f2 transition and appears when the lowest state of the excited electronic configuration 4f5d has energy lower than the highest state of the ground electronic configuration 4f2, the singlet 1S0. In other cases only sharp lines related to parity forbidden f–f transitions are observed, [5]. In the case when energy of the 1S0 state is lower than the energy of 4f5d, effect of quantum cutting (QC) or photon cascade (PC) effect i.e. emission of two photons by single Pr3+ ion, could be observed. The violet photon is emitted due to 1S0 ? 1I6 transition in the first step and green or red photon is emitted due to the 3P0 ? 3H4 or 1D2 ? 3H4 transition, respectively in the second step, [5]. The PC effect has been discussed in the case of excitation in the 4f5d band by UV radiation as well as under excitation by X-ray, [6]. The weak sharp-line absorption in the visible spectral region attributed to Pr3+ impurity is related to internal f–f transitions. Much stronger absorption bands in the UV region are related to

⇑ Corresponding author. E-mail address: fi[email protected] (M. Grinberg). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.08.010

4f2 ? 4f5d transition. In some oxides the charge transfer (CT) transition that shifts electron from ligands to the Pr3+ ion results in broad absorption band with energy smaller than energy of 4f2 ? 4f5d transition. The alternative effect the Pr3+ trapped exciton states (PTE) called also Pr-metal charge transfer states (CTS), [7], or intervalence charge transfer (IVCT) state, [8,9] contributes to additional absorption bands below or above the CT band. Specifically it has been shown that PTE states are responsible for quenching of blue luminescence related to spin allowed 3P0 ? 3H4 transition in Pr3+, [9]. A model of PTE has been proposed by Reut and Ryskin, [10], as transfer of electron from Pr3+ to a reducible lattice cation Mn+ and formation of Pr4+ + M(n1)+ system. The alternate model of PTE has been proposed by Gryk et al. [11] for LiTaO3:Pr3+ and LiNbO3:Pr3+ where one deals with Pr4+ and an electron bounded by long range Coulomb potential at quasi shallow donor states. The PTE and CT have not been observed in BLO:Pr3+. One of the purpose of this contribution is to check if the high pressure can create the PTE effect. Polarized optical absorption and luminescence spectra, emission lifetimes and time-resolved excited state absorption (ESA) in BLO:Pr3+ have been recently reported in Ref. [12]. It has been shown that UV induced transient absorption of color centers and ESA prevent lasing in the visible part of the spectrum. Nevertheless BLO:Pr3+ was still investigated as possible infra-red to blue and orange to blue upconverter, [13] and as scintillator due fast UV emission related to d–f parity allowed transition [14]. In this paper we focus on energetic structure and on up conversion processes of Pr3+ in BLO crystals. Specifically we present the effect of up-converted luminescence related to 4f5d ? 4f2 transition excited with 488 nm emission. We also discuss possibility of existence of PTE states in BLO:Pr3+ system. We have used high pressure spectroscopy approaches, with high pressure applied in diamond anvil cell

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2.1. Preparation The BLO:Pr3+ sample was provided by prof. G.A. Skripko from Byelorussian Polytechnical Institute, Minsk. The BLO crystal has monoclinic unit cell with space group C2h. The atomic arrangement in equilibrium phase consists of beryllium-oxygen tetrahedtra and lanthanum ions irregularly coordinated up to ten oxygen, [15]. The Pr3+ ions occupy the La3+ sites of the C1 symmetry. 2.2. Spectroscopy Luminescence excitation spectra was measured using a system consisting of a Xe lamp (450 W), two monochromators SPM2 (one in the excitation and one in the detection line) and two photomultipliers working in photon counting regime (the first for reference signal detection and the second one for the luminescence). High resolution luminescence spectra were excited with the 488 nm radiation generated by an Ar laser. Luminescence spectra were measured using a PGS2 spectrometer working as a monochromator and a R943-2 photomultiplier working in photon counting regime. A closed cycle helium cryostat type DE-202 (Cryogenics Inc.) was used for all low-temperature measurements. The experimental setup for luminescence kinetics [16] consists of PL 2143 A/SS laser and the optical parametric generator PG 401/ SH that generate 30 ps laser pulses, with frequency of 10 Hz at spectral range from 210 nm to 2400 nm. The detection part of the system consists of the spectrograph 2501S (Bruker Optics) and the Hamamatsu Streak Camera model C4334-01. The time resolved luminescence spectra were collected by integration of the streak camera pictures over the time intervals, whereas the luminescence decays were collected by integration of the streak camera pictures over the wavelength intervals. Pressure was applied in DAC and SAC of Merrill Bassett type [17]. Poly(dimethylsiloxane) oil was used as the pressure-transmitting medium, and a ruby crystal was used as the pressure detector. 3. Results and discussion Ambient pressure and ambient temperature luminescence and luminescence excitation spectra of the BLO:Pr3+ are presented in Fig. 1a. Emission spectra excited with 265 nm pulse excitation (solid curve) consists of the sharp lines related f–f transitions and broad bands related to d–f luminescence. The most intensive are the lines related to the spin-allowed transitions from the excited 3 P0 state to the ground state 3H4 and higher triplets, and two broad bands peaked at 28,000 cm1 and 34,000 cm1, related to 4f5d ? 4f2(3F3, 3F4) and 4f5d ? 4f2(3H4) transitions, respectively. In Fig. 1a the up-converted emission related to 4f5d ? 4f2(3F3, 3 F4) and 4f5d ? 4f2(3H4) transitions obtained under excitation 488 nm (20,491 cm1) is presented by dash–dotted curve. This emission is excited in two steps. The first one can be attributed to the 3H4 ? 3P1, 3P2 transitions and the non-radiative relaxation to the 3P0 state. The second step is the 3P0 ? 4f5d transition. One cannot exclude also the non-radiative relaxation to 1D2 the and the second step is then 1D2 ? 4f5d transition. The excitation spectrum of the emission monitored at 16,390 cm1 (610 nm) related to 3P0 ? 3H6 transition is presented by dashed curve, whereas excitation spectrum of emission monitored at 28,000 cm1 (357 nm) related 4f5d ? 4f2(3F3, 3F4) transition is presented by dotted curve. In both cases the spectrum

intensity (arb. units)

2. Experiments

a λexc= 265nm

λobs= 610nm λobs= 357nm

λexc= 488nm

f-f d-f

20000

30000

40000

50000

-1

wavenumber (cm )

b intensity (arb. units)

(DAC) and sapphire anvil cell (SAC), for detailed analysis of 4f5d ? 4f2 luminescence.

3

P0

3

3

3

3

3

F 4 F 3 F 2 H6

3

H5

H4

294K 1

200K

D2

3

3

H4 P1

3

H5

100K 10K 14000

16000

18000

20000

-1

energy (cm ) Fig. 1. (a) Ambient temperature emission and excitation spectra of the La2Be2O5:Pr3+. Emission spectra excited at 265 nm (solid curve) and at 488 nm (dash–dotted curve), time acquisition 100 ls. Excitation spectra of the emission monitored at 16,390 cm1 related to f–f transition (dash curve), and at 28,000 cm1 related to d–f transition (dotted curve). (b) high resolution luminescence spectra of the La2Be2O5:Pr3+ for different temperatures obtained under excitation at 488 nm (CW laser Ar+).

consists of single broad band peaked at 37,800 cm1 that energy corresponds to 4f2(3H4) ? 4f5d transition. In the case of excitation of f–f luminescence (3P0 ? 3H4 transition) the sharp lines structure in the spectral region 19,900–22,750 cm1 is related to transition from 3H4 to 3P0,1,2 states. In the f–f luminescence excitation spectrum additional bump in the spectral region 27,000–35,000 cm1 appears and can be related to excited state absorption (ESA). Energetically this process corresponds to 4f2(1G4) ? 4f5d transition. More detailed luminescence spectra related to f–f transitions were measured under CW excitation with 488 nm of Ar+ laser. The results obtained at different temperatures are presented in Fig. 1b. One notices that at low temperature the spectra consist mainly with the emission related to transitions from the excited 3 P0 state. Only two lines related to the transitions from the lower excited state 1D2 to the ground 3H4 multiplet, peaked at 16,454 cm1 and 16,482 cm1 are observed. Typical temperature evolution of the spectra is observed. With increase temperature the emission lines become broader, additionally above 200 K anty-Stokes emission lines related to 3P1 ? 3H5 transition appears. In Fig. 2a and b time evolution of luminescence obtained at ambient pressure and at 132 kbar are presented, respectively. The emission is a superposition of 3P0 ? 3F2, 3H6 and 3H5, and 1 D2 ? 3H4 transitions. Actually emission weakly depends on pressure. One notices that the streak related to emission due to transitions from 3P0 state decay much faster than the streaks related to the 1D2 ? 3H4 emission. Since the 1D2 ? 3H4 transition is spin forbidden the respective luminescence decays with longer lifetime than emission from 3P0 state. As the result the luminescence signal collected in the time interval 0–2 ls after excitation corresponds to transitions form the 3P0 state, whereas luminescence collected

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a

3

b

3

P0

F2 1

3

P0

3

F2 1

13000

15000

17000

tim

e

(μ

s)

intensity

intensity

0

100

3

D2 H4

3

D2 H4

0

100 13000

19000

15000

17000

tim

e

(μ

s)

19000

wavenumber (cm-1)

wavenumber (cm-1)

Fig. 2. Time resolved luminescence spectra of La2Be2O5:Pr3+ under ambient pressure (a) and pressure of 132 kbar (b). The sample was excited with wavelength 488 nm. (YAG:Nd pulsed laser and OPG system). All spectra were recorded at room temperature.

22kbar

sapphire anvil cell

14kbar ambient

28000

32000

36000 -1

wavenumber (cm ) 2 3

Fig. 4. 4f5d ? 4f ( F3, pressures.

3

F4) and 4f5d ? 4f2(3H4) luminescence for different

R

IðtÞ  t  dt s¼ R

ð1Þ

IðtÞdt

The results are presented in Table 1. Pressure causes decrease of the luminescence of the emission related to 1D2 ? 3H4 and 3 P0 ? 3F2 transitions, whereas luminescence lifetime of the 4f5d ? 4f2(3F3, 3F4) transition is almost independent of pressure. One notices that pressure 164 kbar reduces the 1D2 lifetime by 11% and the 3P0 lifetime by 33%.To analyze the dependence of luminescence lifetimes on pressure one should consider the kinetics of the populations of the 3P0 and 1D2 states.Population of the 3 P0 state decays in two different pathways. The first one is related to the radiative transitions to the lower triplet states and the second one is the non-radiative transition to the 1D2 state. The nonradiative 3P0 ? 1D2 transition is related to multiphonon process.

163 kbar 132 kbar 100 kbar 60 kbar 30 kbar

intensity (arb. units)

b

intensity (arb. units)

a

λ exc= 250nm

intensity (arb. units)

after the time longer than 2 ls after excitation corresponds to the emission from the 1D2 state. To follow the changes in the emission spectrum related to increase of pressure we present the luminescence obtained for different pressures in Fig. 3a and b. Fig 3a presents the time resolved luminescence obtained for time acquisition 0–2 ls, that corresponds to the emission from 3P0 (3P0 ? 3F2 transition). Fig. 3b presents the luminescence obtained for time acquisition 4–200 ls and represents the emission from 1D2 state (1D2 ? 3H4 transition). All spectra presented in Fig. 3a and b were recorded at room temperature. One observes a small pressure shifts of the emission towards to lower energy, equal to 0.6 cm1/kbar for 1D2 ? 3H4 transition and equal to 0.9 cm1/ kbar for 3P0 ? 3F2 transition. The shifts of these lines are related to increase of the 4f2 electrons Coulomb and exchange interactions with increasing pressure. Luminescence spectra related to the 4f5d ? 4f2(3F3, 3F4) and 4f5d ? 4f2(3H4) transitions under excitation with 250 nm, at different pressure obtained in SAC are presented in Fig. 4. SAC was used because diamonds are not transparent for UV (250 nm) radiation. Emission related to the 4f5d ? 4f2(3F3, 3F4) and 4f5d ? 4f2(3H4) transitions peak at 28,840 cm1 and 33,690 cm1, respectively. One notices the strong shift of the spectrum toward lower energy equal to 9.9 cm1/kbar for the 4f5d ? 4f2(3F3, 3F4) transition (the dominant band). This effect is related to the increase of the crystal field splitting of the 4f5d electronic manifold with pressure [18]. Decays of the luminescence related to the 1D2 ? 3H4 transition, 3 P0 ? 3F2 transitions and 4f5d ? 4f2(3F3, 3F4) transition in Pr3+ are presented in Fig. 5a–c, respectively. In all cases the decays are not single exponential. One can calculate the effective decay time s using following formula:

163 kbar 132 kbar 100 kbar 60 kbar 30 kbar ambient

ambient 14000

16000

18000 -1

energy (cm )

20000

14000

16000

18000

20000

-1

energy (cm )

Fig. 3. Time resolved luminescence spectra of La2Be2O5:Pr3+ under high pressure: (a) time acquisition 0–2 ls, (b) time acquisition 4–200 ls. The sample was excited with wavelength 488 nm (YAG:Nd pulsed laser and OPG system). All spectra were recorded at room temperature.

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b

1000 100

ambient 30kbar 60kbar 100kbar 132kbar 164kbar

10 1 0,1 0,01

intensity (arb. units)

intensity (arb. units)

a

ambient 30kbar 60kbar 100kbar 132kbar 164kbar

100

10

1 50

100

150

200

250

0

1

time (µs)

intensity (arb. units)

c

2

time (µs)

1000 100

164kbar

10

132kbar

1

100kbar 0,1

60kbar

0,01

30kbar ambient

1E-3 10

20

30

40

50

60

time (ns) Fig. 5. Decay curves under different pressures for luminescence: (a) monitored at 16,260–16,393 cm1 (610–615 nm) that corresponds to 1D2–3H4 Pr3+ emission, (b) monitored at 15,037–15,267 cm1 (655–665 nm) that corresponds to 3P0–3F2 transition of the Pr3+ emission, and (c) monitored at 27,027–29,411 cm1 (340–370 nm) that corresponds to d–f Pr3+ emission. The sample was excited at 488 nm. All spectra were recorded at room temperature.

4. Summary and conclusions

Table 1 Luminescence lifetimes. Pressure

Ambient 30 kbar 60 kbar 100 kbar 132 kbar 164 kbar

Transition 4f65d1 ? 4f7 (ns)

1

D2 ? 3H4 (ls)

3

P0 ? 3F2 (ls)

10.27 10.86 10.96 11.63 10.76 10.92

57.3 55.2 53.48 52.4 51.7 50.94

0.78 0.71 0.66 0.64 0.57 0.52

Simultaneously this is the main pathway of population of the 1D2 state. The population of the 1D2 state decreases due to the radiative spin-forbidden transitions to the lower triplet states and is additionally diminished by cross-relaxation non-radiative process.One of possible reason of decrease of the luminescence lifetimes of the 1D2 ? 3H4 and 3P0 ? 3F2 transitions is an increase of radiative transition probability due to pressure induced increase of the strength of the odd parity crystal field that makes the f–f radiative transitions probable. This effect should be similar for transitions from the 3P0 and 1D2 states. Increasing pressure can also weakly increase the cross-relaxation process.Pressure causes increase of probability of non-radiative 3P0 ? 1D2 transition due to increase of the energy of phonon modes with increasing pressure. Effect of pressure induced increase energy of phonons was analysed in several ions and materials in paper [19]. The non-radiative depopulation pathway due to multiphonon process is not active in the case of 1D2 state, therefore the 3P0 lifetime decrease much stronger then the 1D2 lifetime. Thus our results confirmed that the influence of pressure on radiative transition probability and cross-relaxation is weaker then influence on muliphonon non-radiative 3P0 ? 1D2 process.

Luminescence and luminescence excitation spectra of Pr3+ ions in BLO crystal were investigated in conditions of direct and up-conversion pumping. High-pressure photoluminescence spectra of Pr3+ activated BLO crystals were measured at pressures from ambient to 20 kbar and exhibited evident red shift of emission bands related to 4f5d ? 4f2(3F3, 3F4) and 4f5d ? 4f2(3H4) transitions. Decay profiles of the 4f5d ? 4f2 luminescence were measured at pressure up to 164 kbar using two-photon excitation. We have found that 4f5d ? 4f2 luminescence decay did not depend of pressure. Luminescence from excited states of 4f2 electronic configuration was observed at different pressure. The emission lines shift weakly to the lower energies with increasing pressure and strong diminishing of the luminescence lifetimes were observed. The lifetime of the luminescence related to transitions from the 3P0 state decreased with increase pressure from 0.78 ls at ambient pressure to 0.52 ls at 164 kbar. Emission from the 1D2 state is less dependent on pressure. Two-photon excited d–f emission was observed under CW and under 30 ps pulse excitation. We considered two two-photon excitation pathways. In both the first is the excitation of the 3PJ (J = 0, 1, 2) and the second the excited state absorption (ESA) from the 3P0 and 1D2 stats. One notices that ESA from both the 3P0 and 1D2 stats is active under CW excitation and ESA only from 3P0 state is active for pulse excitation. No PTE phenomena, that could influence of the Pr+ emission lineshape and decay, was evidenced for considered pressure range. It has been discussed that existence of PTE needs large lattice relaxation (the large shift of the ligands ions) that strength is proportional to compressibility (inverse of bulk modulus) of the material [20]. The same dependence on bulk modulus concerns to the energy of lattice relaxation, which is responsible for diminishing of the energy of PTE below the energy of excited electronic configuration 4f5d, [20,21]. The hosts where PTE has been observed after

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doping with Pr3+ were characterized by quantity of bulk modulus smaller than 1500 kbar: LiNbO3 – 1058 kbar, LiTaO3 – 1057 kbar [22], and YVO4 – 1380 kbar [23]. The materials where PTE was not observed or were very high, [24] are characterized by large quantities of bulk modulus (see for instance Y3Al5O12 (YAG) – 1870 kbar [25], Gd3Ga5O12 (GGG) – 1710 kbar [26]). Quantity of bulk modulus of BLO is not available. On the other hand the hardness of BLO is of the same order as hardness of garnets and about twice as YVO4 [27], thus one can consider that their bulk modulus it is close to the bulk modulus of garnets. Lack of PTE confirm the small compressibility (large bulk modulus) of BLO. Thus our results confirm the prediction that PTE states appear only in relatively soft materials. Acknowledgements This work was supported by Polish National Center for Research and Development Grant No. NR 15 02906 and Polish Ministry of Science and Higher Education by grants active in years 2008–2011. References [1] L. Goldberg, M.K. Chum, Appl. Phys. Lett. 55 (1989) 18. [2] S.A. Payne, L.K. Smith, W.F. Krupke, J. Appl. Phys. 77 (1995) 4274. [3] A.A. Kaminskii, in: A. Pinto, T.Y. Fan (Eds.), Advanced Solid State Lasers, vol. 15, OSA, Washington DC, 1993, p. 266. [4] A.A. Kaminskii, Phys. Status Solidi A 148 (1995) 9.

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