1. Sample : GaAs (8nm) / Al 0.3 Ga 0.7 As (10nm) ×20 multiple quantum wells Light source : Mode-locked femtosecond Ti-sapphire laser Detection : Balancing Photo-diodes Accuracy : Less than the 10 -2 degree Biexcitonic Faraday Rotation in GaAs / AlGaAs Quantum wells Y Hashimoto*, T Kuroda, F Minami Department 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 Motivation Experimental Set Up 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. Photo-induced Faraday rotation measurement Probe beam (linear polarized) Pump beam (circularly polarized) Sample in 5K Polarization beam splitter Balancing unit OUT Result & Discussion Temporal and spectral resolved Faraday rotation Fitting to the experimental data Fitting results Why the biexciton state induce the Faraday rotation ? Assuming the situation under  + polarized excitation. Then, the  + (left picture) and  - (right picture) polarized component of probe beam cause a transition forming the antibonding (2x) and bonding biexciton (B x ), respectively. This difference generates the remarkable circular dichroism and induce the Faraday rotation. Abstract 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. 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. Temporal profile in the hh exciton resonance 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. Spectral profile at t = 6ps 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. 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). conclusions 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. : A fit considering the single exciton and biexciton Excellent agreement The Biexcitonic Faraday rotation : A fit considering only the single exciton Not match On the other hand, The parameter used in the fit considering the exciton and bieciton ( ) The fitting parameter exciton : Extracted from the transmission spectrum f 0,  0,  00 biexciton :  : the value of exciton  0  0, f : free parameter mirror lens slit grating mirror probe beam Amplitude Resonance wavelength Line Width n+n+ n-n- By use of spin selective photo-excitation, a remarkable nonlinear birefringence due to resonant creation of bonding and antibonding biexcitons is induced. 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. H-255