Four-wave mixing spectroscopy of Cr-doped laser crystals

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FOUR-WAVE MIXING SPECTROSCOPY OF Cr-DOPFD LASER CRYSTALS Emerald, alexandrite, garnets and rubs * Richard C. POWELL, Andrzej SUCHOCKI, Gu~D. GIl LII \Nl) and (iregor\ 3. ()l J \RLFS l)eparttrts’ni of Ph isis s Uk/ultooiu Ssa~

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Four—wase nosing techniques sseie used to establish and probe population gratings ol ( r sins in difk’i’ni is pes ‘f isei host stals between (I and 100 K ~Ihe ariation of the signal decas rate ss ith grating spacing was used to cbs date the properties iii long—range spatial energs migration in these samples. These properties wei e found to he quite dO 0~reiitfoi the sat ions samples iris estigated. especialls svith respect to the role plas ed bs phonon processes I he satiation sit the signal intensits ss ith 551 ste heans crwsing angle was used to determine the pump hand to metastable state radsat’.snless cli’c as rate [hese were tound t’~he i’, ‘he ps time range and to approxirnatels follow ait exponential energ\ gap Ian ss oh eneras diff’i ens e hetss een the zero tslionusn I ines of the if and E les els We recentis reported the results of four-wave mtxtng FWM ) measurements on alexandrite cr)stals 1] BeAI ~ . ( r ) and demonstrated hoss these results could he used to characterize two important ph~siealpro cesses: spatial energ\ migration among the ( r ions. and the pump band to metastable rate radiationless relaxa lion rate. Similar measurements hase now been made on other Cr-doped laser cr) stals including rubs A! 0,: Cr ). emerald (Be 5Al~(Si05 I Cr ). and the garnets GSGG 1~).(Gd1Sc ~Ga 0 here ( r a )comparison and GGG We present sit (Gd5Ga 0 results Cr obtained on these materials, sonic of the

Ftgure I show the pail of the Sugano [striabe diagiam ss ttlt the energs les els lot ( i tims reles ant to this stud\ The maior difference in the samples is the St s’ngth of the octahedral component of the c rs stal field at the site of the ~r tons [his alters the splitting ( U. ) of the I pump hand and the E metasrable state. The dashed s eitical lines show the posittons for rtibs, emerald, and the miri sit and insersion Sites iii alexanclrtte The gar net hosts has c’ es en smaller salites for If lit order to insinestigate the characteristics cii spatial en erg\ migration these samples. laser-induced popula lion gratings of ( r . tons were produced with an argon laser-pumped ring cisc lasei tuned to resonance with ab

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angle of the write beams was measured as a function of temperature. The results weie interpreted using the the ois of Kc’nkre 12 to describe FWM stgnals in lit’ pres ence of spatial energs migration. I spical results are shossis in fig. 2 where theoreto al anal) sts of the data was used 1st deter mine the exciton clif’fuston coefficient 1) 1 lie strong I s’lcf sites such as tubs and inset ston sites in a li’xand i itc slioss un long range energ) inigi ation I tic’ ale~anditie mirror sites shoss a phonon limited energs mtgratinn ansi

Sample a. Emerald b. Alexandrite—M c. Ruby d. Alexandrite — I Fig. I Partial Sugano Tanahe diagram for ( host crsstals *

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the emerald shows a phonon-assistcd enei’g\ migisition. Sinitlai measurements on 0500 and (rUG shoss phonon assisted and phonon-lumited energs migration respec tis els. \ model is being deseloped to explatn the different emtergv transfer character tstics of these materials t nder similar esperintental conditions. meastti enseitts were made of the FWM signal effictencs as a ftinction of write beam crossing angle. J’s pical results foi nibs sirs’ shown in fig. ‘1 \ conspctter fit of these curses san be oh

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3000 5000 1! ~E 1cm Fig. 4. Dephasing time for the FWM signal pumping the 4T~band in rub), emerald, and alexandrite (both Sites) plotted as a fune tion of splitting betsveen the 4T~and -E levels.

Fig. 2. Temperature dependences of the exciton diffusion coefficients.

tamed by writing the expressions for the interacting laser beams in the sample. treating the coupling parameters as adjustable parameters [I]. Then, assuming the laser beams interact xvith a two-level atomic system, the coupling parameters can be used to determine the laser-induced modulations ofthe absorption and dispersion parts ofthe complex index of refraction. The dephasing time of the atomic system is then given by = (2w/c)(4,i/icx ) ( w — O)~~) where 0)12 is the center frequency of the atomic transition, The solid line in flg ~ represents the best fit to the data generated in this was, and )ie!ds a value of T2=4.5 ps. The broken and dotted hines are for values of 13.3 and 1,17 ps, respectively, and show the sensitivit) ofthe fitting procedure to the choice of parameters, Figure 4 shows example values obtained T2 at room 4Lfor~Esplitting, temperature, plotted as a function of theinto components, The dephasing rate can be separated T = (2T 1) I + y, where T1 represents dephasing due to

radiationless decay processes and ~‘ represents dephasing due to phonon scattering processes. In this case the radiationless decay from the 4T, pump band to the ~Emetastable state will dominate the dephasing [31.Using this method to determine T 1 for this transition gives results for ruby in relatively good agreement with those obtained from pulse-probe measurements [4]. The results are not consistent with pulse-probe results for alexandrite [41. The measurements were repeated on alexandrite mirror sites at several temperatures between 300 and 35 K. Both 4n and ala were found to vary with temperature as the fluorescence lifetime varies, but their ratio remains constant thus leading to a temperature-independent value for T~.This is consistent with the predictions of decay by a muitiphonon relaxation process across an 800cm energyband gap. did Also,not pumping different wavelengths within the alter theatvalues determined for the dephasing times except through the detuning factor in the 5T~T,band notThis dominate equation for givendoes abase. shows the that dephasing relaxation with the process.

References

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Fig. 3. FWM signal efficiency at room temperature for ruby as a function of crossing angle of the write beams (see text for an cxplanation ofthe theoretical lines).

[2] A V.M. Kenkre Schmidand Ph)5 Rev. B 31 (1985) 2430. [1] Suchocki, Uand D.D.(Jullutand R.C. Powell, Phvs. Rex. 35 (1987) 5830.NA. [3] ID. Abella, Kurnit and SR. 1-lartmann, Phys. Rev. 14 (1966) 391.

14] S.K. Gayen, W.B. Wang, V. Petricevic, R. Dorsmnville and R R. ~lfano, AppI. Phys. Lett. 47 (1985) 455: S.K. Gayen, W.B. Wang, V. Petricevuc and R.R. Alfano, AppI. Phys. Lett 49 (1986) 437.