AlGaAs quantum wells under high power excitation

AlGaAs quantum wells under high power excitation

Volume 7 1,number OPTICS 6 DYNAMIC RESPONSE OF EXCITON ABSORPTION UNDER HIGH POWER EXCITATION Toshio KATSUYAMA and Kensuke 15 June 1989 COMMUNIC...

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Volume 7 1,number

OPTICS

6

DYNAMIC RESPONSE OF EXCITON ABSORPTION UNDER HIGH POWER EXCITATION Toshio KATSUYAMA

and Kensuke

15 June 1989

COMMUNICATIONS

IN GaAs/AlGaAs

QUANTUM

WELLS

OGAWA

Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan Received

25 May 1988; revised manuscript

received

27 January

1989

Bleaching of exciton absorption in GaAs/AlGaAs quantum wells under high-power excitation below the band edge is studied. It is found that the exciton bleaching associated with both slow recovery caused by the creation of the long-lived carriers and fast recovery related to the fast relaxation process is drastically enhanced by decreasing temperature. The bleaching accompanied by slow recovery can be explained by the thermodynamic quasi-equilibrium model of excitons and electron-hole plasma. The temperature dependence of the bleaching with fast recovery, on the other hand, suggests that the other quasi-particles such as longitudinal-optical phonons may supply the energies to bleaching process.

1. Introduction

Recently there has been a great deal of interest in the nonlinear optical properties of excitons in semiconductor quantum wells. These properties can be applied to high-speed light modulation in systems for both optical communications and optical processing [ 11. For instance, the dynamic behavior of exciton absorption in GaAs/AlGaAs quantum wells under high-power excitation has been extensively studied. This had led to the discovery of strong bleaching of exciton absorption [ 2 1. It is particularly interesting that bleaching and peak shift of exciton absorption under excitation below the absorption edge exhibit an ultrafast recovery on the order of less than a picosecond [ 3,4]. Mysyrowicz et al. and von Lehmen et al. have explained these phenomena in terms of the ac Stark effect, which is derived from the concept of “dressed atoms” [ 3-5 1. Furthermore, it has been recently reported that the oscillatory structures in the probe transmission spectra around the exciton can be observed even if the probe pulse precedes the excitation pump pulse [ 61. This phenomenon is referred as “coherent transients in semiconductors”

2. Experimental setup Experiments used picosecond optical measurement techniques based on a Nd-YAG laser system, as shown in fig. 1. Tunable laser light from a synchronously-pumped mode-locked dye laser (Spectra-physics 3600 and 375B) was amplified by a dye amplifier (Quanta-Ray PDA-1). The dye, DCM, produced high-power laser light (peak power 100 MW, pulse duration 10 ps, repetition 10 Hz, and tunable wavelength 620-670 nm). The laser light was separated into two beams. One was focused into a Raman cell (length 60 mm) containing ethanol. High-power light was generated through an induced Raman process. The wavelength of the Raman light was about 800 nm. The pulse duration was almost

GaAs-AKiaAsQW

[61. This paper deals with the temperature dependence of such exciton bleaching and recovery. 0 030-4018/89/$03.50 Q Elsevier Science Publishers ( North-Holland Physics Publishing Division )

OMA

Fig. 1. Experimental setup for measuring time-resolved tion coefficient under high-power light excitation.

B.V.

absorp-

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the same as that of the input laser light. The line width of the Raman light was 2.3 nm. The measured Stokes shift was 2963.0 cm-‘. The conversion efficiency was about 10% at 10 MW input power. This Raman light was used for high-power excitation. The other beam was introduced into a cell containing a dye solution. An induced emission was obtained, which can be used as a probe light pulse. Its wavelength region was relatively wide, 30 nm, because no cavity structure was used. Time resolved absorption spectra under high-power light excitation were obtained by varying the light path difference between the above two light beams. Linear polarization light was used for both probe and excitation pulses. The direction of the polarization of the probe light was arranged to be just the same as that of the excitation light. Measured samples consisted of 50 periods of alternating layers of GaAs (thickness 50 A) and A1,,,Gao.,As (thickness 50 A). These layers were grown on a GaAs substrate by molecular-beam epitaxy. The substrate was then removed by selectively etching part of the sample.

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500

600 700 Wavelength

9 0

800

Fig. 2. Absorption spectrum at room temperature for GaAs/ Al,,Gaa,,As (50 A/50 A) quantum wells. Absorption spectra for both GaAs and AlO.SGa,,SAs bulk samples are also shown for comparison.

o,c ,

GoAs/AlOsGaO& QW,

Resonant

Ngn-resonant 1‘

3. Results and discussion To verify the existence of excitons in our GaAs/ AlGaAs quantum wells, we measured the absorption spectrum at room temperature with a Hitachi double-beam monochromator. The measured absorption spectra for the GaAs/AlGaAs quantum well, and for both GaAs and AlO.SGaO.SAs bulk samples, are shown in fig. 2. The figure shows steep absorption due to excitons consisting of heavy-holes and electrons at a wavelength of 79 1 nm, and absorption due to light-hole excitons at 770 nm. Furthermore, absorptions due to heavy-hole excitons were found at 751 nm at liquid nitrogen temperature and at 746 nm at liquid helium temperature. Absorptions due to light-hole excitons were at 733 and 728 nm at these temperatures. We then measured the bleaching of the exciton absorption under high-power light excitation below exciton absorption. We used the picosecond absorption measurement techniques described in section 2. Fig. 3 shows the relation between the reduction of peak absorption (in optical density) of heavy-hole 352

20

30

Excitatron

LO

50

light detuning Am.’ A0

ops 0 :loopS

60

70

(meV)

after pumping

Fig. 3. The change in peak absorption (in optical density) of heavy-hole exciton versus excitation-light detuning. Circles, squares, and triangles represent room temperature, liquid nitrogen temperature, and liquid helium temperature, respectively. The pulse duration of excitation light is 10 ps.

exciton and the excitation-light detuning at 0 ps and 100 ps after excitation. The peak power of the excitation pulse was 2 MW/cm’, and the time duration was about 10 ps. On the other hand, the suppression of the probe pulse power is very important in reducing the disturbance caused by the probe light it-

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self. Thus the peak power of the probe pulse was maintained to be sufficiently low. The bleaching of the exciton absorption could not be observed in the absorption spectrum measured by the broad-band probe pulse with no excitation pulse. It is estimated that the probe pulse created the carriers with density of only 107- 1O9 cm’. The time duration of the probe pulse was about 12 ps. By comparing the peak absorption changes for 0 ps and 100 ps after excitation, we can see that exciton bleaching recovery is relatively slow in the resonant detuning region, e.g. in detuning below 35 meV for room temperature. By contrast, recovery is as fast as 100 ps or less in the non-resonant region, which is restricted to within the 35 to 55 meV detuning at room temperature. Fig. 3 also indicates that as temperature decreases, the amount of bleaching drastically increases. Furthermore, it is particularly worth noting that, as temperature decreases, the non-resonant detuning region becomes far from the exciton absorption, although the exciton absorption becomes much steeper. The ratio of bleaching at resonant 20 meV detuning to stationary exciton absorption is replotted as a function of temperature in fig. 4. This behavior can be explained using the thermodynamic quasi-equilibrium model of excitations and electron-hole plasma, excitons (electron + hole). The basic idea of this model was introduced by Chemla et al. [ 7 ] to explain the light-power dependence of exciton bleaching. In our case, the model is somewhat modified as follows. Exciton bleaching is assumed to be

l/T

( K-‘)

Fig. 4. Ratio of bleaching at 20 meV detuning to stationary exciton peak absorption as a function of temperature. Dots show experimental results. The solid line shows calculated results.

proportional to the ratio of exciton the total amount of created carriers, of excitons and electrons. With this can estimate the ratio of bleaching tionary exciton absorption by using tion in two dimensions [ 71: NN a=$kTexp NX

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number against i.e., the number assumption, we amount to stathe Saha equa-

(1)

with N, excitons per unit area, N, electrons per unit area, Nh holes per unit area, rn: electron effective mass, fi Planck’s constant/2a, B exciton binding energy, k Boltzmann’s constant, and T absolute temperature. The numerical values used in the calculation are as follows; B=9 meV [ 11, rn: co.067 m, (m, free space electron mass) [ 7 1. Furthermore, total carriers per unit area N,, = N, + N, can be estimated around 10”-10’2 cm-’ at 2 MW/cm2 excitation. The results thus calculated are shown as the solid line in fig. 4. As can be seen, this estimate quantitatively coincides with the experimental results. Therefore, it is strongly suggested that the existence of excitons plays an important role in the resonant bleaching of exciton absorption. Candidate mechanism may include the band-filling and phase-space filling effects of excitons, which is caused by electrons and holes composing excitons [ 11. On the other hand, we believe that the bleaching occurring in the non-resonant detuning region may be associated with the fast process, which was discussed for the first time in the framework of the ac Stark effect by Mysyrowicz et al. [ 31 and von Lehmen et al. [ 41 and later by Fluegel et al. [ 61, because of its fast recovery of the exciton bleaching. However, in our experiment, the Stark shift of the exciton absorption could not be clearly observed because of the insufficient measurement accuracy. As shown in fig. 3, the non-resonant detuning region becomes far from the exciton absorption with decreasing temperature, in spite of the fact that the exciton absorption becomes much steeper. This behavior suggests from the viewpoint of the energy conservation law that the other quasi-particles such as longitudinaloptical phonons may supply the energies to bleaching process. 353

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4. Summary Bleaching of exciton absorption in GaAs/ Alo.sGao.sAs (50 A/50 A) quantum wells under highpower excitation below the band edge was studied. A picosecond time-resolved absorption measurement technique was used. It was found that the exciton bleaching associated with both slow recovery caused by the creation of the long-lived carriers and fast recovery related to the fast relaxation process is drastically enhanced by decreasing temperature. The bleaching accompanied by slow recovery could be explained by the thermodynamic quasi-equilibrium model of excitons and electron-hole plasma. The existence of excitons plays an important role in the bleaching with slow recovery. On the other hand, the temperature dependence of the bleaching with fast recovery suggested that the other quasi-particles such as longitudinal-optical phonons may supply the energies to bleaching process. Acknowledgements The authors wish to thank Dr. Hiroyoshi Matsumura of the Hitachi Central Research Laboratory for

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his encouragement throughout this work. Thanks are also due to Dr. Tadashi Fukuzawa, Takao Kuroda, and Dr. Hiroaki Inoue for their fruitful discussions. This work is supported in part by the Ministry of International Trade and Industry of Japan.

References [l] D.S.ChemlaandD.A.B.Miller, J.Opt.Soc.Am.B2 (1985) 1155. [2] D.A.B. Miller, D.S. Chemla, D.J. Eilenberger, P.W. Smith, A.C. Gossard and W.T. Tsang, Appl. Phys. Lett. 41 ( 1982) 679. [ 31 A. Mysyrowicz, D. Hulin, A. Antonetti and A. Migus, Phys. Rev. Lett. 56 (1986) 2748. [4] A. von Lehmen, D.S. Chemla, J.E. Zucker and J.P. Heritage, Optics Lctt. 11 (1986) 609. [ 51 S. Schmitt-Rink and D.S. Chemla, Phys. Rev. Lctt. 57 ( 1986) 2752. [ 61 B. Pluegel, N. Peyghambarian, G. Olbright, M. Lindberg, S.W. Kock, M. Joffre, D. Hulin, A. Migus and A. Antonetti, Phys. Rev. Lett. 59 (1987) 2588. [7] D.S. Chemla, D.A.B. Miller, P.W. Smith, A.C. Gossard and W. Wiegmann, IEEE J. Quantum Electron. QE-20 (1984) 265.