Polarization choices in exciton-biexciton system of GaAs quantum wells S. Adachi,* T. Miyashita, S. Takeyama, and Y. Takagi Department of Material Science, Faculty of Science, Himeji Institute of Technology, 1479-1 Kamigori, Harima Science Garden City, Hyogo 678-12, Japan A. Tackeuchi Fujitsu Laboratories Ltd., Morinosato-Wakamiya, Atsugi, Kanagawa 243-01, Japan M. Nakayama Department of Applied Physics, Faculty of Engineering, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558, Japan Received 1 August 1996; revised manuscript received 20 September 1996 We study the induced absorption due to biexciton formation in absorption bleaching pump-probe signals and the quantum beats between exciton and biexciton in the four-wave-mixing signals. The origin is confirmed by the polarization dependence of the signals in both experiments. The definite discrimination between exciton- biexciton quantum beats and the others by the polarization choices is demonstrated clearly. We also show the well-width dependence of the biexciton binding energy that is determined by the period of the quantum beats in the range of well width 45–150 Å of GaAs/Al x Ga 1-x As ( x =0.3–1.0 multiple quantum wells. S0163-18299700104-5 I. INTRODUCTION Excitons essentially dominate the linear and nonlinear op- tical properties of GaAs quantum wells QW’s near the band edge, but stabilize easily into biexcitons in the resonant excitation. 1,2 In the range probed by ultrafast spectroscopy using femtosecond pulse lasers, the intense excitation condi- tion is easily achieved. As a result, it is much more likely to observe the modified exciton dynamics by the biexcitonic contributions. 3–5 In fact, recent studies of the exciton dynam- ics have shown that nonlinear interaction in QW’s could not be described by a simple assembly of two-level systems be- cause the exciton-exciton interaction 6 and exciton-biexciton interaction 7 should be taken into account. Biexcitons have been studied in various semiconductor materials and in heterostructures. In bulk crystals, for ex- ample, Si and CuCl, the creation of biexcitons is clearly demonstrated. Theoretical work 8 predicts a significant en- hancement of the biexciton binding energy in quasi-two- dimensional systems. For GaAs QW’s, several experimental works related to biexcitons 1–4,6,7,9–19 have been published and established the existence of biexcitons. Most of these experimental studies indicate that the biexciton binding en- ergy is 1–2 meV, which is about one order of magnitude larger than that in bulk GaAs and confirms the theoretical prediction in a sense, but its well-width dependence needs continued investigation. The binding energy of quantum-well biexcitons in the measurement by Miller et al. 9 agrees with the calculation by Kleinman 8 for the well thickness below 150 Å. But the observed binding energies for thicker QW’s do not agree with the theoretical result and recent experimental studies also indicate a larger binding energy. An explanation for the discrepancy between theory and experiment has not been given yet. Recently, Birkedal et al. 20 performed a systematic experimental study of the well-width dependence of biexci- ton binding energy for a range of well thickness 80–160 Å. In most of the previous works the biexciton binding energy is deduced from the energy separation of the doublet struc- ture in the photoluminescence PL spectrum or the period of the quantum beat on the basis of a superlinear power depen- dence of the lower-energy peak in the PL spectrum. On the other hand, Osborne et al. 21 warn strongly that the lower-energy peak of the doublet structure in the low- temperature PL tends to be easily assigned to the biexciton origin. They observed strikingly similar features to the biex- citonic PL in the cw PL spectra of GaAs/Al 0.33 Ga 0.67 As mul- tiple quantum wells MQW’s, such as a tailed component in the lower energy of the doublet structure, a superlinear growth of the peak against laser intensity, a binding energy of 1–2 meV, and a well-width dependence. The origin of the lower-energy PL component is attributed not to biexcitons but to positively charged excitons that are excitons bound to holes in the QW originating from the background concentra- tion of acceptors in Al x Ga 1 -x As barriers. They also empha- size that the superlinear power dependence in the PL spectra is not necessarily indicative of biexciton formation, as ar- gued in previous works, 2,3,9 because the positive-charged ex- citon shows the same power dependence as that of biexciton. Therefore, we require a definite discrimination between biex- citons and, for example, charged excitons or excitons bound to some localized states. In the present work we observe the quantum beats be- tween exciton and biexciton states and the induced absorp- tion due to biexciton formation, respectively, in the four- wave-mixing FWM and pump-probe nonlinear transmission-reflection measurements in GaAs MQW’s. The pump-probe technique is used for the confirmation of the biexciton formation and the polarization selection rule. The definite discrimination between biexcitons and other excitons by the FWM measurement is demonstrated on the basis of the polarization selection rule. Finally, we show the well- PHYSICAL REVIEW B 15 JANUARY 1997-I VOLUME 55, NUMBER 3 55 0163-1829/97/553/16547/$10.00 1654 © 1997 The American Physical Society