PICOSECOND PROCESS OF NONLINEAR ABSORPTION IN HIGHLY EXCITED CdS SINGLE CRYSTAL

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PICOSECOND PROCESS OF NONLINEAR

ABSORPTION IN HIGHLY EXCITED CdS SINGLE

CRYSTAL

H. Jingyu, D. Rensong, X. Xurong

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C7, supplément au n°10, Tome 46, octobre 1985 page C7-185

PICOSECOND PROCESS OF NONLINEAR ABSORPTION IN HIGHLY EXCITED CdS SINGLE CRYSTAL

H. Jingyu, D. Rensong and X. Xurong

Changchun Institute of Physios, The Academy of Sciences of China, Changchun, China

Résumé - Il est trouvé que l'absorption nonlinéaire dans le monocristal de CdS dépend du temps par la mesure de transmission avec la technique de cor-rélation d'impulsion égale de PS. Nous résolvons la variation de l'absorp-tion en foncl'absorp-tion du temps en deux composantes : un processus "rapide" et ce-lui "lent" qui sont attribués à l'interaction d'exciton-exciton et à celle d'exciton-électron respectivement.

Abstract - The nonlinear absorption in CdS single crystal is found to be time dependent by measurement of transmission with PS equal-pulse

correlation technique. The variation of nonlinear absorption with time can be divided into two components! a 'fast' process and a 'slow' one, which are ascribed to exciton-exciton interaction and exciton-electron interaction, respectively.

I - INTRODUTION

Many workers have studied the nonlinear absorption phenomenon in CdS at low temper-ature(4.2K) at exciton energy or less. Several explanations have been given out:

(a) Two-photon absorption due to formation of excitonic molecule /l/.

(b) The inverse process of exciton scattering-induced absorption (e,h;hy-ex;ex)/ 2/.

(c) The Mott transition from an exciton gas to an electron-hole plasma/3,4/. (d) Exciton-exciton interaction/5/.

Experimentally, we studied the nonlinear transmission characteristics of a thin sample of CdS. Using equal-pulse correlation technique/6/, we measured the excited--state relaxation in the picosecond scale, which revealed time dependent light transmission. We find a'fast' and a 'slow' processes and ascribe them to exciton— exciton interaction and exciton-electron interaction respectively.

II - EXPERIMENT AND DISCUSSION

+ ° Mode-locked Ar laser is used as pump source. Laser wave lenghth^;=5145 A (2.41eV).

Pulse width is 160 PS . Repeating rate is 82 MHz. The pulses is splitted into two beams of orthogonal polarizations and recombined collinearly with one beam delayed with respect to the other by a time interval (160PS,270PS). Since the two pulses are orthogonally polarized there is no coherent effect. Laser spot is focused to <f> 0.1 mm. the thickness of CdS single crystal plate is about 100 urn. The experiment arrangement is shown in Fig. 1

J O U R N A L D E PHYSIQUE

Fig. 1 Eqxxirrental arrangement.

l . W e - l o c k e d Af+ laser; 2,Optical delay l i n e ; 3.Sample holder; 4. 404

m r

mter; 5 . S p e c t m t e r ; 7. X-Y recorder; 8.

2

p l a t e . 6 Photomultiplier

b

A t rocin temperature band gap of CdS Fg=2.423eV,exciton binding energy Ex=28meV/10/, s o t h a t under given condition t h e absorption is due t o t h e formation of exciton. When a s i n g l e beam is used in e x c i t a t i o n , t h e dependence of transmitted l i g h t

I on the incident l i g h t E is shown in Fig. 2. I n the case of E 4 1 5 I W / e I increa- ses with E and absorption coeEficent d = const. I n t h e case of E

>

15IW/CM: I w i l l decrease with E. Obviously,nonlinear absorption happens in CdS when e x c i t a t - ion i n t e n s i t y is =eater than 15IU&M?

Here, the nonlinearity of absorption process

is

not due t o two-photon absorption. For two photon t r a n s i t i o n , t h e s e l e c t i o n r u l e A 1=0,2 is not s a t i s f i e d . In our case,excited dense Bxcitons play an important r o l e .

0

Fig. 3 The r e l a t i o n between T and the r e l a t i v e transmission of l a s e r

7

.

I(260PS)-I( ) a: E=12; b: E=21; c: E=34; d: E=50 (~0.76IW/&

.

? =

I(260PS)

Measure the transmission of the t o t a l power of two beams I

a t

d i f f e r e n t delay time T

.

I n the case of E

>

15KN/Qrl; t h e r e e x i s t absorption peaks in about 150PS,

when T

2

167PS I does not change with

r

; I n the case of E < 1 5 K N / e I=const. (Fig. 3 ) . The appearance of deficiency of transmitted l i g h t is due t o the existence of a ' f a s t ' absorption process.

Next,= set t h e delay time 'C a t 167PS ,where ' f a s t ' process has already f i n i s h e d . Then.1 does not change with

r

.

I f each beam of incident l i g h t v a r i e s £ram a value 151W/Qr12(that is in the c a s e oElineartransmission),We f i n d the n o n l i n e a r i t y of transmission happens s o long as the t o t a l incident i n t e n s i t y is greater than

23KW/CM2 ( F i g , 4 ) . Let's suppose Z can be set long enough t h a t t h e e f f e c t of t h e

f i r s t pulse on sample completly disappears. Then,no nonlinear absorption should appear,because each

beam

i t s e l f gives only linear transmission. Now,we can make t h e conclusion: After 167PS. the e x c i t e d s t a t e fo& by f i r s t pulse i n t e r a c t s with the second pulse s t i l l . The nonlinear absorption a r i s e s from a slow r e l a x a t i o n process.

I

CdS

C7-188 JOURNAL

DE

PHYSIQUE

Experiment shows a broad emission band of CdS centered a t 5300

1

with long-wave length t a i l a t low energy side. This peak w a s observed

i

n

cathodolminesence and photoluminescence by various authors/7,8/ and w a s ascribed t o stimulated emission of exciton-exciton s c a t t e r i n g (x-x) process. While according t o Koch/9/ and Klingshirn/lO/ exciton-electron s c a t t e r i n g (x-e) process is

mre

important than (x-x) process a t

rocm

t-rature and has l o w r thresold of exciton density than

x-x

process.

Since excitons are neutral p a r t i c l e s , t h e interaction between them Fs short ranged. It is strong within an exciton Bohr radius and w i l l decay CrJickly a f t e r several

Bohr r a d i i / l l / . It must happen under highly excited condition. In our case,if 80% photons a r e absgrbed by CdS p l a t e , under the excitation l e v e l E = ~ ~ K W / C M : we have 1 photon/( IOOA*

Fig. 5

---

Absorption peak ( e r i m e n t ) E = ~ ~ K W / Q V I '

-

.

- Convolution of autocorrelation of t h e laser pulse. (calculated)

A T (7) with b=0.8 (PS)-!(calculated)

There are s u f f i c i e n t l y large n m b r of exciton t o produce t h e interaction between the excitons transiently. Hanamura/lZ/ pointed out t h e o r e t i c a l l y that when the s p e c i f i c photon density

N / v ~ o ' ~ % M ~

t h e nonlinear absorption due t o t h e formation of excitonic m l e c u l e s

is

greatly enhanced due t o giant o s c i l l a t i o n . There a r e no excitonic m l e c u l e s a t roan-temperature, but t h e excitons may i n t e r a c t each other and r e s u l t i n apperance of additional absorption i.e. t h e ' f a s t ' process i n nonlinear absorption. Since x-x interaction decays with time,

dn

32-

d t ( l+bt)'

f o r b h l e c u l a r reaction, t h e absorption peak must be wider than t h a t of the autoconvolution of t h e pulses. Let x-x interaction decay according t o then, the change of transmission power with 'Cis ( l + b t )z

1

n T ( c ) 4

7

C ( r - t )

-

(I+bt)zdt

0

Time dependenttransmissionor nonlinear absorption i n CdS gives u s information about relaxation of excited system. Under Kighly excited condition, there a r e various kinds of interactions between quasi-particles with d e f i n i t e relaxation t i m e . W e find, there a r e ' f a s t ' and 'slow' compnents i n non l i n e a r Bbsorption process. Here we ascribe t h e ' f a s t ' and 'slow' absorption process to

x-x

and x-e interaction respectively.

REk'ERDKX

/1/ R u c h m , I., Phy. S t a t . Sol. b87 N O . ~ (1978) Klll

/2/ Kushida,T. and Moriya,T.,Phys.Stat.Sol. b72 No.1 (1975) 385

/3/ Kllngshirn,C. e t c . , Phil. Trans. R. Sot-lond A313 (1984) 269

/4/,80hnert, K. ect., Appl. Phys.Lett.

3

(1984) 1088 /5/ Nguyen, H.

X.

e t c . , Phys.Stat.So1.

b124

(1984) 191 /6/ Taylor, A.J. ect.,Appl. Phys. Lett.

9

(1983) 989 /7/ Nicoll, F. H., Appl. Phys. Lett.

10

(1967) 69 /8/ Warn, J.M., Phys. Rev.

g

(1971) 4459

/9/ Koch, S. W., Phys. S t a t . Sol.

p&3

(1978) 431

/lo/ Klingshirn, C. and Hang, H., Phys. Report (Review Section of "Phys. Lett.")

70 No5 (1981) 365

/lr

Brinlaan, W. F. e c t . , Phys. Rev. 3(1973) 1570