T. Gong

HOT-CARRIER COULOMB EFFECTS IN GAAS INVESTIGATED BY FEMTOSECOND SPECTROSCOPY AROUND THE BAND EDGE

We report the first femtosecond measurements of carrier‐induced changes in the absorption and refractive index of GaAs thin films. Absorption measurements in the vicinity of the band edge indicate instantaneous band‐gap renormalization as well as instantaneous plasma screening of electron‐hole interactions. Band filling and plasma screening appear to be the dominant contributions to the changes of refractive index.

Self-diffraction: A new method for characterization of ultrashort laser pulses

Abstract We show that the duration and coherence time of ultrashort laser pulses can be measured by a transient grating technique. In the method, a self-diffracted signal is recorded as a function of delay between two identical pulses that induce the grating in an absorbing medium. An integrated-intensity grating analysis that considers fourth-order partial-coherence effects describes the diffraction efficiency of the grating. This method, which is easily implemented, inexpensive, and can be used at any wavelength, is demonstrated with low-repetition-rate picosecond lasers.

Femtosecond refractive-index spectral hole burning in intrinsic and doped GaAs

The temporal and spectral dependence of carrier‐induced changes of the refractive index in intrinsic, n‐type and p‐type GaAs samples is measured using femtosecond pump and continuum‐probe techniques. We observe, for the first time, a refractive‐index spectral hole burning arising from a nonthermal carrier distribution generated around the initial excited states. Such spectral hole burning is not present within our time resolution in the n‐type sample when the injected carrier density is low. The extremely fast initial scattering time in the presence of cold electrons is attributed to relaxation through the emission of phonon‐plasmon coupled modes.

Generation and control of solitons and solitonlike pulses in a femtosecond ring dye laser.

We control the generation of solitons and solitonlike pulses in a passively mode-locked dye laser by adjustment of group-velocity dispersion, self-phase modulation, and spectral filtering. Without spectral filtering, periodic pulse shaping reminiscent of higher-order solitons is observed. The pulses differ from the classic solitons because of additional shaping mechanisms. With spectral filtering, pulses are generated that can be described analytically as asymmetric N = 2 solitons. The results indicate an effect analogous to the soliton self-frequency shift observed in optical fibers. The remarkable stability achieved allows for accurate characterization and control.

Femtosecond optical response of Y-Ba-Cu-O films and their applications in optoelectronics

We report systematic studies of the femtosecond transient reflectivity in Y-Ba-Cu-O (YBCO) thin films at temperatures ranging from 12 to 300 K. By using a 2-eV-pump/white-light-probe technique, we have very accurately measured the ultrafast optical response of YBCO as a function of the optical frequency, film oxygen content, and excitation intensity. Our results shed new light on the thermomodulation model, routinely used to interpret time-domain reflectivity data in high-temperature superconductors. We also review the current state of the art in the ultrafast optoelectronic response of YBCO.

Femtosecond reflectivity of 60 K Y‐Ba‐Cu‐O thin films

We report systematic studies of the femtosecond transient reflectivity at 2 eV in partially oxygen‐depleted Y‐Ba‐Cu‐O thin films exhibiting the resistive superconducting transition at 60 K. Our measurements, performed at room temperature, reveal that in 60 K films, the Fermi level in the Cu‐O2 plane lies approximately 2 eV above the filled copper d9/d10 band. By mapping the transient optical response across a 1 cm2 film, we show that the position of the Fermi level is very sensitive to the apparent local variations of the film oxygen content, which gives rise to dramatically different transient optical responses. We also observe a strong transient reflectivity dependence on intensity, which can be attributed to the shift of the Fermi level caused by nonequilibrium hole heating.

Subpicosecond hot hole dynamics in highly excited GaAs

The buildup of band‐edge gain is observed on a subpicosecond time scale in highly excited intrinsic GaAs. Using a kinetic model to fit the experimentally determined ‘‘gain time’’ at different wavelengths, the transient behavior of the optically injected hole temperature is determined. The data are not consistent with a simple decay of the hot hole temperature due to phonon emission processes. We show that a simple estimate of the energy transfer from electrons to holes via Coulomb scattering qualitatively accounts for the observed behavior of the hole temperature.

Carrier-carrier interactions in GaAs investigated by femtosecond spectroscopy

We present systematic femtosecond pump-probe studies of the initial scattering and cooling processes of hot electrons in intrinsic, n-type, and p-type GaAs for carrier densities from 8 X 1015 to 1019 cm-3 excited at 2 eV. The role of electron- electron scattering in intra-(Gamma) -valley equilibrium, its dependence on the injected carrier density, and its influence on the amplitude and recovery rate of the initial absorption bleaching are established. For a low density (approximately 3 X 1016 cm-3), the electron-electron scattering is observed to be most efficient when all the electrons are in Bloch states with the same (large) kinetic energy. The importance and time scales of electron-hole and electron-plasmon interactions are also revealed.

Hot carrier dynamics near the Fermi edge of n-doped GaAs

Femtosecond bleaching measurements are performed in n-doped GaAs at both 300 K and 14 K using low densities of carriers injected at 2 eV. The dwell time of electrons excited far above the Fermi sea is reduced because of hot-electron/cold-electron interactions. The response at 14 K is much stronger and slower than that at 300 K. This is attributed to: (i) the proximity to the Fermi level of the conduction band states probed via split-off valence band coupling; (ii) slow Landau damping of non-equilibrium plasmons, primarily by the minority holes which then heat the majority electrons.

Femtosecond dynamics of hot carriers in GaAs

The pump-probe technique is used to perform a series of measurements on intrinsic GaAs samples at room temperature with a temporal resolution of 75 - 100 fs. Changes of both absorption coefficient and refractive index are measured over a wide spectral region (550 - 950 nm) for various carrier densities (<1016 to 1019 cm-3) injected at 2 eV. These measurements provide insight on the fundamental properties of nonequilibrium carriers, including electron-electron scattering, electron-hole scattering, electron-phonon scattering, hole-phonon scattering, band-gap renormalization, plasma screening of Coulomb interactions, and free-carrier absorption.