High pressure optical studies of doped YAG

JOURNAL OF

LUMINESCENCE ELSEVIER

Journal of Luminescence 60&ôl (1994) 188—191

High pressure optical studies of doped YAG P.R. Wamsleya, K.L. Brayb.* aDepartment of Phisics, Unirersitv 0/ Wisconsin. 1415 Johnson Driue,ivladison, WI 53706, (JS.4 hDepart?nent of Chemical Engineering and Materials Science Program, finirersitv ~?tWisconsin. /415 Jo/cn,wn Drirc, Madison. WI 53706, USA

Abstract

YAG and Cr3 + Tm3 : YAG was studied. Shifts in emission The ranging effect of from pressure on the emission spectra Tm3 : in peaks —0.14 to 0.54 A/kbar were of observed Tm33 :YAG. The ability of pressure to resolve competing absorption of overlapping Cr33 (4T 3F (1G 3~ 1) and Tm 4) levels has allowed us to identify previously unreported Tm (‘G 3H 4 5) emission. *

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2E 2 and In this paper we continue our study by considering the effect of pressure on Tm3 + : YAG and Cr3 + Tm3 + : YAG. The energies and intensities of the visible luminescence transitions of Tm33 (iG 3F 3H 3H 3 (4T 6) and Cr 2 A2, 2E 4 4A 4, 4 2) in YAG have 3~ been measured as a funcYAG we find shifts in tion of pressure. Tm luminescence peakIn positions ranging from —0.14 to 0.54A/kbar. Studies of Cr3~:Tm3~:YAG reveal the presence of previously unreported 3H ‘G4 5 emission. alevels decrease in ~.the extent of mixing of the of Cr2

1. Introduction

Pressure is a useful tool for studying energy transfer in co-doped solid state laser materials because of its ability to alter interionic separations, energies of electronic transitions, and spectral overlap. We are studying the effect of pressure on en3 + Tm3 + YAG. In a previous ergy transfer in Cr paper we considered of pressure on Cr~:YAG and found the thateffect pressure destabilizes the 4T 3~relative to the 2E level [1]. 2 level of Cr results in a systematic decrease This destabilization in the thermal population of the 4T 2 level and a consequent 4T continuous decrease and eventual 4A elimination of 2 2 emission intensity. A de33 R-lines 2 4 in the separation of the Cr crease E A 2) and a significant increase in luminescence lifetime were also observed. These effects were attributed to a pressure-induced decrease of + the trigonal distortion at the Cr site in YAG and —~

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The visible—near-IR emission of Tm~ YAG was measured after 466 nm excitation into the G4 level. The ambient pressure spectrum is shown in Fig. 1 and consists of emission bands centered near 3H 3H generally 6550 and 8000 A. 3F These bands are attributed to ‘G4 4 and 4 6 emission,

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P.R. Wamslev, K.L. Bray

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Journal of Luminescence 60&61 (1994)188—191

energies of the Stark components of the33F ‘1’—Tm3 4 level+ cross-relaxation (3H of the 3F Tm 3H are critical to the process effectiveness 4 —+ 4, 6 ~

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this observation suggests that pressurecross-relaxmay have 3 ~—Tm3~ a strongThis influence on thewill Tm be pursued in future ation. possibility work. The similar shift behavior observed near 8000 A may likewise be due to an increase in the overall Stark splitting of the 3H 6 level with pressure. However, we present evidence below for an alternative explanation which contends that the significantly

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shifting transitions at 8015 A and 8140 A (three 3H peaks) 3Hat ambient pressure are not due 3Hto 4 -~ 6 emission, but rather to ‘G4 5 emission.

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3’1’:Tm3’1’:YAG(O.7%,2%) 3. Cr The emission spectrum of Cr3” Tm3’’ :YAG at various pressures is shown in Fig. 2. The effect of pressure on Cr3’1’ emission (Fig. 2(a)) in Cr3 + : Tm3 + : YAG is similar to the effect observed in Cr3 + : YAG. In addition to the emission features observed in Cr3 + : YAG, we observe a small absorption feature (arrow, lower trace of Fig. 2(a)) in the anti-Stokes sideband of the 2E —~ 4A 2 4T feature2E levels of Cr31’ to the 3F transitton. This is the result of energy transfer the3 + [2]. 2 and 3 levelfrom of Tm We observed a continuous loss of Cr3 + intensity, in addition to the loss associated with destabilizing the 4T 2 level [1], due to ahigher pressure-induced shiftdeof 4T~pump level energies. The the creased pump efficiency results in a decrease in Cr3 + emission and a reduced energy transfer contribution to Tm31’ emission in Cr3’4’ Tm3’’ :YAG at high pressure. The effect of pressure on Tm3’1’ emission is shown in Fig. 2(b). The shifts of the emission peaks agree with those observed in Tm3 + : YAG. The most significant effect of pressure is to induce the growth of new features in the Tm3 + emission spectrum (arrows, Fig. 2(b)). Based on a compari3 + : YAG, the intense peak at 8075 A son100 to kbar Tm corresponds to the peak marked with at .

3~: YAG Fig. Room-temperature emission spectrum of Tm (2 /o). 1.Pressure shifts are indicated.

respectively [2]. The been effect measured of pressurefor on the emission spectrum has pressures up to 170 kbar. The primary effect of pressure is to induce shifts in the energies of the emission peaks. To a good approximation, all shifts observed were linear with pressure. One noteworthy observation about the shifts is that in the two regions of the spectrum shown in Fig. 1, shorter-wavelength transitions exhibit little or no shift with pressure while the longerwavelength transitions exhibit pronounced red shifts. In the case of ‘G 3F —+ overall 4 emission, this result indicates an increase in 4the Stark splitting of the 3F 4 level with increasing pressure. Since the

an asterisk in the 3 kbar spectrum. The peak shifts

190

P.R. Warns/cc, K.I,. Bray

Journal of Luminescence 60&6/

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spectrum at 100 kbar upon 458, 477, and 496nm excitation is3’shown in Fig.The 2(b).Tm3’ The excitation emission. emissionis wavelength of the Tmdependence of the indicated features striking and is consistent with an interpretation which contends that the features arise from directly excited Tm33. The 458 and 477 nm lines are cx-

496 nm 100 kbar

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pected The 496to nmdirectly excitation excite is not thebecause ‘G it is outside 33. of the 1G 4 level support of Tm is 4 absorption band [2]. Additional 3 portion of the emission spectrum found at highin pressure the Cr (upper trace of Fig. 2(a)). New features emerge near 6500A. The positions and intensities of these features match those of the iG~ 3F 33 :YAG at the same 4 transition Tm evidence for direct pressure and provide infurther 1G 4 excitation. As the pressure is raised above 100 kbar, the emerging peaks in both spectral regions continue to gain intensity relative to the rest of the spectrum and spectrum approaches 3 the YAG observed at high the spectrum of Tm We have established that the emerging Tm3 emission peaks in Cr3 : Tm3 YAG are pressure. due to directly excited Tm3 and that the peaks appear because pressure has reduced the amount of Cr3 excitation relative to the direct Tm3 ~ excitation. We continue by discussing the assignment of these peaks. Since the peaks are present +

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and gains intensity with pressure. Argon laser excitation, in addition to exciting the 4T~ level of Cr33, is capable of exciting the ‘G 4 level 3 ~ Consequently, the Tm3 emission in of Tm Cr3’’ :Tm3’1 : YAG contains contributions from +

both energy transfer and direct excitation. We showed above that the contribution due to energy transfer decreases with increasing because 3’1 pressure as the 4T~level less energy absorbed Cr shifts blue. isWe thereforebyattribute the emerging emission peaks to transitions from the iG 4 level of 3’1. directly excited Support for Tm this conclusion can be obtained from the excitation wavelength dependence

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upon 458 and 477 nm excitation, but not upon 496 nm excitation, they must arise from Tm3’1 states accessible to 458 and 477 nm, but not 496 nm. excitations. Because of the energy transfer process, 3F 3F 33 3 and all lower energy of Tm are7, accessible to all three lines. levels Con-sequently, the only Tm3’’ level accessible to 458 and 477nm. butthe notenergy 496nm, excitations is ‘G4.YAG A consideration of levels [3] of Tm3 reveals that the only transition from iG 3H 4 capable of appearing near 8000 A is ‘G4—s 5. We therefore conclude that 1G the emerging peaks shown in Fig. 2(b) are due 3H to the4 emission 5 emission. further that peaks at We 8015A and conclude 8l40A (three peaks) in Tm3 : YAG at ambient pressure 3H are note also due ~ assignment 5 emission. ofFinally, we that totheiG4new these transitions explains why, at ambient pressure, the spectra of Tm3’1 :YAG (Fig. 1(b)) and +

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P.R. Warnsley, K.L. Brat

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Journal of’ Luminescence 60&61 (1994) 188—191

Cr34’ : Tm3’’ : YAG (Fig. 2(b)) near 8000 A are dissimilar. Acknowledgement We gratefully acknowledge financial support from the American Chemical Society Petroleum Research Fund under grant # 25077—G6, 3.

191

References [1] P.R. Wamsley and K.L. Bray, J. Lumin., 59(1994)11. [2] G. Armagan, B. DiBartolo and AM. Buoncristiani, J. Lumin. 44 (1989) 129. [3] J. Gruber, M. Hills, R. MacFarlane, C Morrison, G Turner, G. Quarles, G. Kintz and L. Esterowitz, Phys. Rev, B 40 (1989) 9464.