HAL Id: jpa-00225071
https://hal.archives-ouvertes.fr/jpa-00225071
Submitted on 1 Jan 1985
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of
sci-entific research documents, whether they are
pub-lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
NON-EQUILIBRIUM PHONONS IN OPTICALLY
PUMPED YAG-Cr3+
R. Goossens, E. Koldenhof, J. Dijkhuis, H. de Wijn
To cite this version:
NON-EQUILIBRI UM PHONONS IN OPTICALLY PUMPED
YAG-c
r3'R.J.G. Goossens, E.E. Koldenhof, J.I. Dijkhuis and H.W. de Wijn F y s i s c h h b o r a t o r i w n , R i j k s u n i v e r s i t e i t U t r e c h t ; P.O. Box 80.000 3508 TA U t r e c h t , T h e NetherZands
Abstract
-
Tne o p t i c a l generation and the dynamics of high-frequency phonons i n diluted YAG:&* a t 1.5 K a r e discussed. The experiments show severe bottlenecking of the 19.5-cm-' phonons and surprisingly strong Raman processes induced by non- equilibrium very-high-frequency phonons.I n t h i s paper w report on the dynamics of nonequilibrium phonons interacting with excited C r Y centers i n yttrium aluminum F n e t (YAG, Y 3 y l 2 ) . m e pbnons a r e generated, i n the usual way, i n the f i n a l s t e p X ( E) + E('E) of the nonradiative decay following o p t i c a l excitation with an argon laser. Ihe energy of the resonant phonons is 19.5 cm-l. A primary w t i v a t i o n of t h i s study i s t o see what m d i f i c a t i o n s i n the phonon decay are introduced a s compared t o the well-studied case of ruby 11-51. In YAG:&>, Raman t r a n s i t i o n s connecting ~ ( ~ 3 ) and fi('E), which effectively provide an additional feeding into z ( ~ E ) , a r e shown to be e f f i c i e n t already a t low excitation.
The experiments a r e concerned with measuring, under stationary o p t i c a l pumping, the r a t i o of the luminescent i n t e n s i t i e s R2/R1, which is a d i r e c t measure f o r the population r a t i o N2
1%
-A2
/1,2/, and, upon r e w v a l of the o p t i c a l pumping, the relaxation time Teff of the 2A( E) p o p l a t i o n , which is a measure for the phonon decay /3,4/. The diameter of the l a s e r beam i n the c r y s t a l was 50 vm. The experiments are carried out a s a function of the excited-state concentration N*. A technique of wdulated pumping was employed, which has the advantage of allowing %/R1 and Teff to be measured s h l t a n e o u s l y , i.e., a t i d e n t i c a l conditions. The t o t a l o p t i c a l feeding into 2A is then derived, under appropriate circumstances, by calculating the r a t i o Na/%Teff. For d e t a i l s of t h i s technique a s w11 a s the experimental set-up reference i s made to Ref.5. All experiments were carried out a t 1.5 K t o e l e n a t e relaxation by tk?nnal phonons, and a t such high pumping powers t h a t the 19.5-cn-' phonons generated In the decay following o p t i c a l excitation may be considered to be bottlenecked, i.e., t h e i r occupation i s determined by the dynamical equilibrium with the E ( ~ E ) - ~ ~ ( ~ E ) system. The sample i s do d with &* a t a concentration of approximately 100 ppm, and has
Y
dimensions 1.2 x 5 x 5 mm.
In IYg. 1, the dependence is shown of NZA/% on the excited s t a t e population N* under stationary o p t i c a l pumping. From the known absorption cross section /6/, we estimate
8
-
6 x 1017 a t the highest pumping i n an excited pencil of 50 pm diameter. A quadratic dependence is found, which can be a t t r i b u t e d , a s we w i l l see below, to bottlenecking of the d i r e c t decay z ( ~ E )*
E ( ~ E ) and Raman processes from E('E) + s('E) induced by high- frequency phonons generated i n the nonradiative decay following o p t i c a l pumping i n 4 ~ 2JOURNAL DE PHYSIQUE
Fig. 1
-
The r a t i o of populations of ~ A ( ~ E ) and ~ ( ' 3 ) v s t h e excited-state population8
f o r 100 p p Y A G : & ~ a t 1.5 K. Calibration o f8
is uncertain within h a l f an order of magnitude. [cf. Refs /2,4/]. It i s noted t h a t t h e r a t i o N2&/NE, h i c h equals t h e occupations of t h e resonant 19.5 cm-' phonons / I / , reaches a value of 4 x low3, f a r above t h e thennal value. In i n t e r p r e t i n g these r e s u l t s we f i r s t note that a11 d a t a points a r e i n t h e regime8
>>
pAv. From the v e l o c i t y v = 5 x lo5 an s-' of t h e transverse m d e s /7/ and a width Av assumed t o be comparable with ruby (Av -- 0.02 cm-l) /8/ we have, i n t h e Debye approximation, pAv -4 4 10l6 cm-*. The absolute magnitude of Teff may be used to derive an estimate f o r the l i f e t i m e T of t h e 19.5-cml phonons i n t h e excited zone v i a t h e r e l a t i o n151
I n Fig.2, r ~ e present t h e r e s u l t s derived from t h e temporal decay of Z A ( ~ E ) versus t h e
JOURNAL
DE
PHYSIQUEappropriate to the regime
$
>>
pAv. A t X( w 3 x 1017 cm-3 we then find a -60 ns. This value i s f a r too short t o be due t o anharmonic decay, which i s known t o be a t l e a s t several microseconds a t these frequencies i n c r y s t a l s of t h i s type. %reover, p l r e anharmonic decay m u l d provide a s t r i c t l y l i n e a r increase of Toe, with$.
The mst l i k e l y explanation f o r a therefore is, a s i n ruby /5/, decay of thephone';
by i n t e r a c t i o n with weakly exchange-coupled Cr* ions combined with r e s i d u a l boundary-limited s p a t i a l diffusion a t t h e lowerEf.
Aprecise and d e f i n i t i v e calculation of these e f f e c t s can however not be performed a s long a s the matrix element connecting ZA('E) and E('E) i s not available.
As has been pointed o u t e a r l i e r 141, i n the regime
8
>>
pAv the quantity Na/% Teffl which can be determined i n a modulated pumping experiment, equals the t o t a l feeding i n t o 2A( E) per excited CrS per second,$%!
In Fig. 3, the feeding r a t e$%:
derived from t h e data points of Fig. 2 and the corresponding NZ/ not shown here, i s plotted vs the excited s t a t e%
population
$.
'Itu, contributions to gZAt a r e t o be discerned, viz., the d i r e c t o p t i c a l feeding i n t o 2A and Raman t r a n s i t i o n s from E('E) t o ZA('E) induced by o p t i c a l l y generated high-frequency phonons. The former amunts to l / a R = 175 s-' a t maxi-, and corresponds to the $ E t a t N*, while the l a t t e r contribution, of course, s c a l e s with$.
The is over an order of magnitude larger than the d i r e c t o p t i c a l feeding, and apparently Raman feeding is predominant a t these$.
Remarkably, the Raman feeding i s of a strength similar to the case of ruby 141, suggesting strong enhancement by non-equilibrium o p t i c a l phonons. This implies t h a t Cr* a l s o is a s e n s i t i v e detector f o r near-zone-boundary acoustic pbnons and possibly o p t i c a l phonons i n Y3A15012/lo/.
Optical detection of high- frequency phonons i n j e c t e d i n Cr3:yAG by thin-film heaters has already been c a r r i e d o u t /11,12/, and revealed broad phonon pulses a r r i v i n g with delays several times longer than according to the sound velocity. These r e s u l t s e r e interpreted assuming resonant detection only and continuous thermalization of the phonon p l l s e i n the course of its t r a v e l t o the detection wlume.me
present experiments suggest t h e a l t e r n a t i v e explanation t h a t a t heater temperatures of 20 K i n YAG already a good f r a c t i o n of the phonons i s generated i n the dispersive part of the Brillouin zone.Acknowledgement
The Netherlands Foundations FOM and ZW3 f i n a n c i a l l y supported t h i s m r k .
REFERENCES
/ I / J.I. Dijkhuis, A. van der Pol, and H.W. de Wijn, Phys. Rev. Lett. x ( 1 9 7 6 ) 1554. /2/ J.I. Dijkhuis and H.W. de Wijn, Phys. Rev. B E(1979) 1844.
/3/ R.S. Meltzer, J.E. Rives, and W.C. Egbert, Phys. Rev. B 5(1982) 3026. / 4 / R.J.G. Goossens, J.I. Dijkhuis, and H.W. d e Wijn, Phys. Rev. B.
/5/ R.J.G. Goossens, J.I. Dijkhuis, and H.W. de Wijn, Phys. Rev. B.
161 D.L. Wood, J. Ferguson, K. Knox, and J.F. Dillon J r . , J. Chem. Phys.
2
(1963) 890. /7/ E.G. Spencer, R.T. Denton, T.B. Bateman, W.B. Snow, and L.G. van U i t e r t , J. Appl. Phys.34 (1963) 3059.
-
181 B. Halperin, D. Nicollin, and J.A. Koningstein, Chem. Phys.
42
(1979) 277.191
J.G.M. van Miltenbutg, J.I. Dijkhuis, and H.W. de Wijn, submitted t o Phys. Rev. B./lo/
R.J.G. Goossens, J.I. Dijkhuis, and H.W. de Wijn, J. Turnin.22
(1985)./11/ A.P. Abramv, I.N. Abramva, I. Ya. Gerlovin, and I.K. Ra-va, Fiz. Tverd. Tela
