STRONG LIGHT-SEMICONDUCTOR INTERACTIONS IN CdS AND GaSe, AND OPTICAL BISTABILITY

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STRONG LIGHT-SEMICONDUCTOR

INTERACTIONS IN CdS AND GaSe, AND OPTICAL BISTABILITY

V. Dneprovskii, A. Furtichev, V. Klimov, Shen Li, E. Nazvanova, D.

Okorokov, U. Vandishev

To cite this version:

V. Dneprovskii, A. Furtichev, V. Klimov, Shen Li, E. Nazvanova, et al.. STRONG LIGHT- SEMICONDUCTOR INTERACTIONS IN CdS AND GaSe, AND OPTICAL BISTABILITY. Journal de Physique Colloques, 1988, 49 (C2), pp.C2-241-C2-246. �10.1051/jphyscol:1988257�. �jpa-00227674�

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STRONG LIGHT-SEMICONDUCTOR INTERACTIONS IN CdS AND GaSe, AND OPTICAL BISTABILITY

V. DNEPROVSKII, A. FURTICHEV, V. KLIMOV, Shen LI, E. NAZVANOVA, D. OKOROKOV and U. VANDISHEV

Physical Department, Moscow S t a t e U n i v e r s i t y , Moscow 119899, USSR

Abstract

-

Changes in absorption and refractive index caused by absorption of a laser beam in CdS and GaSe semiconductors were investigated and used to excite absorptive resonatorless bistability in CdS and dispersive bistability in a GaSe Pabry-Perot cavity.

1

-

INTRODUCTION

Optical properties of sen;iconductors may be significantly changed in the case of laser light absorption. Different physical processes leading to strong

nonlinearities compete and coexist. Ronlinear effects become very large due to the resonant enhancement near the sharp band edge. Strong nonlinearities arise in the case of thermal shift of the semiconductor\8 band edge,collision

broadening and "saturation" of excitonic transition,renormalization of the energy gap at high density of the carrier8,Burstein-Moas bleaching effect etco The

interplay of optical nonlinearity and feedback (intrinsic or external) leads to optical bistability (OB).

The report is related to optical nonlinearities in CdS and GaSe (80K) inauced by exciton-axciton interaction leading to the collision broadening and

"saturation" of the exciton absorption line and to the properties of OB a% low pumping intensities /1,2/.The replacement of the thermal absorptive QB in CdS (80K) by OB of excitonic origin was observed when the duration of the input pumping pulse was diminished.

2

-

ABSORPTIVE BISTABILITY IN CdS AT LOW PUBGPING INTENSITIES

The platelet crystals of CdS ( 1 - 5 7 ) cooled to the liquid nitrogen temperature were resonantly excited by Argon laser

( A

⁼⁴⁸⁸^nm,x

1)

c,c-the optical axis of the crystal).It was possible to change the initial transmission (to chooae the working point of the crystal) by changing the polarisation of the input laser beam. The pulses of tunable duration ( 7 p s

-

³⁰ms) were created with the help of electrooptical modulator. The diameter of the focused beam on the surface of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988257

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C2-247 JOURNAL DE PHYSIQUE

the sample was 10

-

³ ⁰

.

^~

Thermal bistability and transient phenomena (nonlinearity arises from the thermal shift of the gap) could be clearly seen when the duration of the input pulse was tuned from 0.5 m5 to 10 ms.The form of the hysteresis loop was

essentially changed due to the transient effect (Fig.?) when the input pulse duration was les~t than 1 ms.

Fig.1

-

The oscii.lograms of the input (S) and output (ST) pulses and the hysteresi-s loop in the case of transient effect.

Thermal OB (the siteady-state behaviour of the device) arises for longer input pulses (Fig.2)

1; ST,arb.un.

\. -

2 4 t,ms 0.2

1

0,4 S,k'#/cm 2 Fig.2

-

The osci.Llograms of the input and output pulses and the hysteresis

loop in the case of thermal OB.

Transverse effects could be seen in the case of thermal 033. P special electronic image transforme:r allowed to measure the change of the probe beam intensity profile in space and in time simultaneously. The diameter of the probe beam exceeded the diameter of the pumping beam. The switching waves were observed.

The switched area was greater than the initial spot size of the pumping beam (Fig.,?)

The sharp decrease of transmission in the vicinity of the absorption edge of CdS ( 8 0 K ) and abrlorptive resonstorless 03 of nontherxial origin was observed at the intensity of short Argon laser pumping pulses less than 10 m/crn2 (the pulse

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Fig.3

-

The change of the transmission of the probe bean in space and in time.

The initial diameter of the pumping beam xo=207.

Optical properties of semiconductors way be significantly changed in the case of resonant excitation of excitons. The density-dependent collision broadening of the free-exciton resonance /3,4/ probably leads to the nonlinear rise of absorption in the vicinity of the absorption edge of semiconductor. The resonatorless OB based on increasing nonlinear absorption may exist /5-7/.Tho intrinsic feedback arises from the dynamic relation of the generation and recombination rates of the particles.The resonatorless absorptive OB was previously observed in the cooled CdS samples at rather high excitation intensities ( w i LiW/cm 2 ) /5-7/.

The indused absorption was attributed to the formation of electron-hole plasma and renormalization of the energy gap at high density of the carriers.

The oscillograms of the input and transmitted pulses and the kjsteresis loop are presented in Fig.4 for =?/US. The fttst switching of t n e bistable system

o

I

o

t

,ps

5 10 s,kw/cmz

Pig.$

-

The ~scihiu~raris of the input and output pulses and the hysteresis loop for the short input pulses (CaS,BoK, T<l~ps).

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C2-244 JOURNAL DE PHYSIQUE

(the switching on and off times are not kcnger than the time-resolution of the applied photoreceivers

-

1-2 s) is duo to the increasing absorption in the

Y

vicinity of the absorption edge of CdS C80X). The increasing absorption may arise due to the followl.ng processes: l.the collision broade~ing of the free-exciton resonance; 2.band gap shrinkage; 3.excitation-induced heating of the sample.

The analysis of the luminescent spectra allow to suppose /2/ that at such manner of excitation the fast change of the absorption is due to the processes of the exciton-exc:iton interaction. The P-band of luminescence arises at the values of S used f'or the excitation of bistable mode of transmission. The spectral position of P-band ( the red shift from the free exciton energy is about the binding energy of the exciton),the superlinear dependence of its intensity upon the pumping intensity S,the difference of the shape of P-band from the shape of the phonon-replica of A-exciton (A-LO band appears at low intensities and has a high energy shoulder),the appearance of stimulated emission in the vicinity of the red shoulder of P-band allow to explal$e it by the process of exciton-exciton interaction: ex+ex = hy +(e+h) (e+h- is the electron-hole pare). The estimated density of excitons for switching of bistable system was about or less than 1016 cmo3. Thus the nonlinear increasing of the absorption can 't be explaintpb-by the renormalization of the energy gap. To switch the system one needs to "shiftw the absorption edge (the **shiftff must be about 10- 13 meV).Such shift may be achieved in CdS at the density of the carriers n >

loq7

cme3 /8/.The observed absorptive OB can't be explained by the laser Lnduced heating of CdS. The duration of the pumping pulse ((lops) was rather short to create thermal birstability. The measured time of the sample's cooling was 50

-

100 s. The red shift of the A-LO band (the heating of the sample) was not

P

observed at the intensities of excitation used.

It is possible to see the competition and coexistence of two types of the indused absorption ("fas.tV,which arises due to the exciton-exciton interaction,and "slown one due to the hefating of the sample) increasing the input pulse duration.For longer pumping pu:lses the fast fall of the transmission (Fig.5a) is probably due to the exciton-exciton interaction in the sample heated to higher temperature

than that presented in Fig.5b (at lower S the crystal is heated to higher temperature for longer time passes till the moment of fiexcitonicn switching).

Fig.5

-

The oscil~lb~rams of the input and output pulses and hysteresis loops for =joys;

smax

= 4 kw/cm2 (a),15 k~/cm2 (b).

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of excitons). At higher S (Fig.5b) the switching off time is faster than the

switching on time. It may be explained by the absence of feedback for wexcitonic**

effects during the switching on process and by fast *excitonicw switching off (the crystal is heated and the absorption spectrum is tuned to the region where

the feedback for the wexcitonicw switching off exists).

3

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STRONG NONLINEARITIES AND OB IS GaSe.

The layered crystals GaSe,GaSSe are convenient for creation of OB devices /9/.

These semiconductors have strong nonlinearities which exist in the case of resonant excitation of excitons.

The excite-and-probe transmission measurements at the absorption edge of GaSe were performed. Rhodamine dye-laser with tunable frequency ( r-730 ns) was utilired for excitation of high density excitons.The synchronously pumped cell with appropriate dye was used for creating a probe beam. The samples of 2

-

^{1 0 )}

thickness were cleft from g-GaSe monocrystal.The changes in the transmission spectra of GaSe at different intensities of the pumping pulse in the case of resonant excitation of excitons may be seen (Pi&.6a).The bleaching of the exciton absorption,the broadening of the exciton line and the increase of absorption at the red shoulder become noticeable at the intensities above 50 kW/cm 2

.

Fig.6

-

a.The transmission spectra of GaSe (80K3 at different pum ing intensity (resonant excitation qf excitons) : 1.-S=0,2.-S=100 kW/cm

S

,3. -S=780 kw/cm2 ,4.-S-250 kW/cm ; b.The luminescent spectra of GaSe (80K) at different pumping intensifies (interband excitation,

1

=583 nm, 3-400 kw/cm2. P

The luminescent spectra of GaSe (80K) show (Fig.6b) that simultaneously with A ~ - line (the luminescence of free direct excitons, 4,;590 nm) proband arises at high excitation.The intensity of A P -line Iex- S'

-

^{O * l}^at

^s.10 - ^loo

^kWicm2^{m d}

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C2-246 JOURNAL DE PHYSIQUE

-

^{at S}

^>

100 kw/cm2 ,such dependence indicates at the change of the I,,

linear recombina.tion by the process of bimolecular recomDination. The red shift of pr-band relatively to Ar-line is about the binding energy of the direct exciton. The intensity of pr-band 1~41:~ at all values of pumping intensities used. Thus ~ ~ - l : ~ (the intensity of the exciton luminescent line Ie~NeX'where N e x is the density of the direct excitons). E r -band may be attributed to the process of excitvor.-exclton interection: err+ ear=n)/

+

^(e

r +

h),where exr is the direct exciton ~~tr-point of tile Brillouin zone,hy -tne energy of the emitted photon,and (er+ h)

-

tne electron-hole pair.

The observed bleaching and broadening of the exciton absorption line in GaSe (80K) with increasing resonant excitation seems to arise mainly due to the process of exciton-exciton interaction.

Summarizing,strong.nonlinearities and different types of optical bistability may exist in CdS ancl GaSe due to the effective exciton-exciton interaction.

It is possible 1;o prepare zero demensional structures (semiconductor microcrystals in the transparent glass matrix /lo/ with designed band structure) which

have strong norllinearities and probably may be used in OB devices.A significant excitation-induced blue shift of the absorption (more than 30 meV) was obsemed in CdSe microcrystals /11/.The crystallite size (radius adl20A) was about the exciton Bohr raclius of CdSe (60A) in a host material.The shift of the absorption edge must arise even in the presence of the second electron-hole pair excited in the microcrystal by the laser beam.

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