Ultrafast phenomena in gallium arsenide/aluminum gallium arsenide multiple quantum well waveguide structures using a near infrared femtosecond laser system.

Abstract

A near infrared hybridly mode-locked dye laser system consisting of a femtosecond oscillator and a high repetition rate dye amplifier was designed and built. This system was then applied to the study of room temperature below-bandgap femtosecond switching and coherent pulse propagation in GaAs/GaAlAs multiple quantum well waveguides. The noise properties of the oscillator output were studied using radio frequency spectrum analysis techniques. Two distinct modes of operation were identified: The first is characterized by the shortest pulse duration and its real-time autocorrelation signal appears more strongly modulated. The second mode of operation, which exhibits a slightly longer pulse duration and a smoother real-time autocorrelation signal, is obtained for a relative cavity length detuning of ΔL = -0.7 μm. Unexpectedly, the second mode features larger pulse duration fluctuations than the first mode and self-pulsing, while the pulse repetition timing and pulse energy fluctuations were found to be similar in both cases, making the first mode preferable for use in time-resolved experiments. Femtosecond all-optical switching under off-resonance room temperature excitation was demonstrated in a passive GaAs/AlGaAs multiple quantum well directional coupler for the first time. The required phase mismatch originates from an ultrafast refractive index change caused by the optical Stark effect. The main obstacle regarding practical device applications is its low transmission (less than 10%). The use of electrically pumped semiconductor waveguides that provide gain promises to remove this disadvantage. Below-resonance, coherent pulse breakup in a room temperature semiconductor waveguide was observed for the first time. Numerical simulations of the coupled semiconductor Maxwell-Bloch equations show that the light-matter interaction can induce enough chirp through self-phase modulation during propagation in order to violate the initial adiabatic following regime and cause pulse breakup. This coherent effect is distinctly different from self-induced transparency, because it does not involve Rabi-oscillations at the start of propagation, from temporal solitons, because it does not require group velocity dispersion, and from self-steepening. However, it should be ubiquitous under off-resonance pulse propagation with a pulse duration less than the polarization dephasing time.