Breakup of a below-resonance femtosecond pulse is observed in a room-temperature GaAs/AlGaAs multiple-quantum-well waveguide using cross-correlation techniques. The breakup is due to neither self-induced transparency nor temporal solitons. Instead, calculations based on the coupled semiconductor Maxwell-Bloch equations show that coherent self-phase-modulation during propagation drives the system out of the initial adiabatic following regime into excitation density oscillations and eventually pulse-shape modulations.PACS numbers: 42.50.Hz, 42.50.Md Coherent pulse propagation, including self-induced transparency and pulse breakup, has been studied extensively in passive atomic media [1][2][3]. However, such coherent processes have only recently been considered for semiconductors where the phase relaxation times are very rapid [4,5]. Here we report the first experimental and theoretical investigations showing coherent pulse breakup in a semiconductor waveguide for photon energies below the lowest (Is) exciton resonance. In this spectral region the optical Stark effect [6] should produce an ultrafast change in the nonlinear refractive index with a response time limited only by the polarization dephasing time. This suggests that optical solitons may be formed in conjunction with the group-velocity dispersion of the semiconductor waveguide system. However, for the femtosecond experiments reported here the input pulses are shorter than the polarization dephasing so that the coherent nature of the light-semiconductor interaction becomes of paramount importance. Coherent propagation is then shown to drive the system out of the initial adiabatic following regime which leads to pulse breakup and prevents soliton formation.The experiments have been performed with an amplified (1 kHz repetition rate) hybridly mode-locked dye laser. The center wavelength is X = 870 nm and the pulse duration is t p -100 fs full width at half maximum (FWHM) before entering the optics used for coupling into the waveguide. The time-bandwidth product Avt p =0.2 is indicative of transform-limited pulses with asymmetric temporal shape that has been verified using standard cross-correlation techniques [7]. The waveguide is inserted into one arm of the cross-correlator. The 1.2-/im-thick 100-A GaAs/AlGaAs multiple-quantum-well guiding region is sandwiched between AlGaAs layers for vertical confinement with ridges in the top layer for horizontal confinement supporting a single transverse guided mode. The light is polarized in the growth direction which simplifies the problem by allowing only the lighthole excitonic transition. Figures I (a) and 1 (b) show the experimentally measured cross-correlations of the transmitted pulses for both low and high intensity, respectively. These results were obtained using a 0.37-mm-long waveguide, and peak intensities of 8.5 GW/cm 2 [Fig. Kb)] and 0.8 GW/cm 2 [Fig.

All-optical modulation is observed at room temperature in a GaAs/AlGaAs multiple quantum well nonlinear directional coupler using femtosecond pulses. The origin of the ultrafast (<500 fs) recovery of the device is attributed to the optical Stark effect.

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