Sample : GaAs (8nm) / Al 0.3 Ga 0.7 As (10nm) ×20 multiple quantum wells

1. grating mirror OUT Pump beam (circularly polarized) Balancing unit Polarization beam splitter Sample in 5K slit lens mirror Probe beam (linear polarized) probe beam n- n+ : A fit considering only the single exciton Not match Amplitude Photo-induced Faraday rotation measurement Resonance wavelength Line Width Temporal profile in the hh exciton resonance The parameter used in the fit considering the exciton and bieciton ( ) Spectral profile at t = 6ps Temporal profile of the PIFR is shown. In this figure the probe energy is tuned to be just below the hh exciton line. Occurrence of the symmetric profile with respect to the pump polarization supports that the signal originates from the photo-induced magnetic momenta. The spectrally profile of the PIFR at 6ps is displayed. The FR signal emerges only at the exciton resonance region, and becomes negligible when the probe energy is far from the exciton. This fact indicates that the dominant origin for the FR is the excitonic nonlinearity. Biexcitonic Faraday Rotation in GaAs / AlGaAs Quantum wells Y Hashimoto*, T Kuroda, F MinamiDepartment of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan*Present address: Graduate school of Science and Technology, Chiba University, Chiba 263-8522, Japan e-mail: hashimoto@physics.s.chiba-u.ac.jp H-255 Abstract Why the biexciton state induce the Faraday rotation ? Fitting to the experimental data The polarization rotation introduced by the resonant pulse excitation is investigated in nonmagnetic GaAs quantum wells. The Faraday rotation signal is resolved both in spectral and temporal domain, showing a clear excitonic and biexcitonic resonance profile. The Faraday rotation spectra are well reproduced by a calculation based on the multiple transition model, in which the two-exciton complex of singlet and triplet geometry are taken into account. The resonance energies of the two-exciton states, extracted through the present fitting, are consistent with results of spin-dependent nonlinear absorption. By use of spin selective photo-excitation, a remarkable nonlinear birefringence due to resonant creation of bonding and antibonding biexcitons is induced. Assuming the situation under s+ polarized excitation. Then, the s+ (left picture) and s- (right picture) polarized component of probe beam cause a transition forming the antibonding (2x) and bonding biexciton (Bx), respectively. This difference generates the remarkable circular dichroism and induce the Faraday rotation. The fitting parameter exciton : Extracted from the transmission spectrum f0, g0, w00 biexciton : g : the value of exciton g0 w0, f : free parameter Motivation Although photo-induced Faraday rotation (PIFR) has been observed in a number of semiconductor materials, such as diluted magnetic systems and nonmagnetic quantum structures , an origin of the PIFR has not been fully clarified. Therefore, we performed a detail study of PIFR induced by the excitonic nonlinearrity. Result & Discussion Temporal and spectral resolved Faraday rotation Fitting results Experimental Set Up On the other hand, : A fit considering the single exciton and biexciton Excellent agreement The Biexcitonic Faraday rotation Sample : GaAs (8nm) / Al0.3Ga0.7As (10nm) ×20 multiple quantum wells Light source : Mode-locked femtosecond Ti-sapphire laser Detection : Balancing Photo-diodes Accuracy : Less than the 10-2degree For the probe pulse, we inserted a wavelength variable spectral filter of Fourier-transform-limited type (left picture), so that the band width was narrowed down to 1/30 of the laser output. The probe energy was varied above the hh exciton resonance region. To confirm the formation of the biexciton states, transient absorption measurements were performed. An example of the transient absorption spectrum at t = 6 ps with counter-circularly polarized pumping and probing is shown. A fit considering the single exciton and biexciton gives the value of 2.8±0.6meV for the binding energy of the bonding biexciton, which agrees well with the parameter given by the analysis of FR spectra (2.8±0.2meV). The absorption spectrum of the present sample at 5 K is indicated, together with the spectrum of the pump pulse and that of the spectrally narrowed probe one for comparison. The pump spectrum covers the hh exciton absorption. conclusions We observed the biexcitonic Faraday rotation in semiconductor quantum wells. The spectral profile shows the a good agreement with the calculation, where the formation of the bonding, and antibonding biexciton are taken into account.