R. Tommasi

High repetition rate nonlinear generation of synchronized frequency tunable femtosecond pulses

Abstract Pulses delivered by a high power Ti:sapphire femtosecond oscillator are frequency broadened in an optical fiber and subsequently filtered to generate two synchronized frequency tunable femtosecond pulses at a repetition rate of 76 MHz. The created pulses are transform limited with a duration in the range 70–120 fs, and independently tunable over 220 nm. The possibilities offered by this source for high sensitivity two-color femtosecond spectroscopy are demonstrated in GaAs.

Cold-phonon effect on electron heating in GaAs

Abstract Heating of cold electrons is investigated in bulk GaAs at room temperature using a femtosecond high sensitivity absorption saturation technique. The electron thermalization time is found to increase with the photoexcited carrier density in the range 5 × 1015−3 × 1017cm−3. This effect is attributed to LO phonon underpopulation and is shown to be controlled by the LO phonon lifetime in agreement with a numerical simulation of the nonequilibrium carrier dynamics.

Optical Phonon Coherence and Population Decays in III–V Semiconductors

Investigations of optical phonon coherence and population decays in bulk III–V semiconductors are discussed. The results are compared to our lifetime determinations using a non phonon-mode selective technique based on precise measurement of electron-lattice thermalization dynamics. The measured LO phonon lifetimes were found to be consistent with the dephasing times measured in intrinsic samples. In the presence of carriers (for the density range 1016–1017cm-3), due to LO phonon-plasmon hybridization, the dephasing time is strongly reduced and becomes much smaller than the lifetime which is almost not altered. The roles of pure dephasing and population decay mechanisms in optical phonon relaxation are discussed for various carrier environment.

Ultrafast hole relaxation in III-V semiconductors

Ultrafast relaxation of photoexcited nonequilibrium holes is selectively investigated in bulk GaAs and InP using a high- sensitivity two-color absorption saturation technique. Measurements of the hole characteristic thermalization time as a function of the lattice temperature and of the carrier density and initial average energy show that nonequilibrium hole relaxation is dominated by hole-optical phonon interactions in the range 100 - 300 K. Comparison of the experimental results with a numerical model of carrier dynamics permits the determination of the optical deformation potential in both of these compounds.

ULTRAFAST HOLE-PHONON INTERACTIONS IN GAAS

Ultrafast heating of cold holes is investigated in bulk GaAs using a high-sensitivity two-color absorption saturation technique. Measurements performed as a function of the lattice temperature and of the carrier excess energy show that absorption of optical phonons is the main hole heating mechanism for the investigated temperatures in the range 100–300 K. Using a numerical model for carrier dynamics, the optical deformation potential is estimated to be d0∼40 eV.

Ultrafast Hole Heating in Intrinsic GaAs

Ultrafast interactions of free carriers between themselves and with their environment is a central problem of semiconductor physics, both from a fundamental point of view and for their technological implications. Ultrafast carrier dynamics has thus been extensively studied in semiconductors using time resolved femtosecond techniques, yielding important information on carrier scattering processes1–3. In direct gap semiconductors, most of the investigations have focused on electron relaxation dynamics and although electron scattering processes are relatively well characterized, little is known about hole relaxation dynamics. Because of their larger occupation number close to the band edge and/or of their generally slower thermalization dynamics, electrons usually dominate the probed transient semiconductor properties and experimental conditions have to be specifically chosen in order to access nonequilibrium hole dynamics.