W.L. Nighan

The properties of free carriers in amorphous silicon

The properties of free carriers photogenerated in the extended states of hydrogenated amorphous silicon have been investigated using the techniques of femtosecond time-resolved spectroscopy. The optical susceptibility of free carriers can be described by a Drude model with a relaxation time shorter than 1 fs. This relaxation time seems to remain constant for various experimental situations. It implies a very small mobility (∼ 6 cm 2 /V s) in the extended states. The hot carriers thermalize quickly to the mobility edge by emission of phonons. The thermalization rate is found to be ≥ 1 eV/ps. In addition, the decay time of optic phonons into acoustic phonons is ≤ 100 fs. The photogenerated carriers recombine non-radiatively in a time that can be as short as 1 ps at very large injected density ( N ≤ 10 21 cm −3 ). Several regimes are distinguished, depending on the value of N . In general, the characteristic times for all these processes are much shorter in a-Si:H than in a typical direct-gap crystalline semiconductor such as GaAs. The difference can be traced to the lack of momentum conservation in amorphous semiconductors.

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.

Trapping time in processed polycrystalline silicon measured by picosecond time-resolved reflectivity

We have used the methods of picosecond time‐resolved reflectivity to measure the carrier lifetime in fine grain polycrystalline silicon films grown by low pressure chemical vapor deposition at 625 °C. After monatomic hydrogen diffusion or implantation with phosphorus ions followed by high temperature annealing (1150 °C), the trapping time τ increased from 40 to 150 ps, consistent with passivation of the grain boundaries or an increase in grain size, respectively. If implantation was not followed by annealing, τ decreased to less than 10 ps, while if it was followed by low temperature annealing (900 °C), which approximately restored the original grain size, τ recovered to 50 ps, very close to the trapping time measured in the as‐grown samples. In all cases, we found indications that trapping of carriers was much faster than their subsequent recombination.

Femtosecond spectroscopic determination of the properties of free carriers in a-Si:H

Abstract High intensity femtosecond pulses have been used to inject free carriers in the extended states of a-Si:H where they are maintained for a time of the order of one picosecond. Their optical properties are measured by pump and probe femtosecond spectroscopy. We find that the momentum scattering time that enters in the Drude model and in the expression for the mobility is less than one femtosecond for electrons. The upper bound for the mobility of electrons above the mobility edge is 6 cm2/V sec.

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 thermalization processes in a-Si:H

Abstract We report measurements in a-Si:H of the free carrier thermalization rate and the lattice heating rate following energy transfer from non-radiative carrier recombination. Both processes appear to be more rapid than in crystalline materials.

The mechanism of subnanosecond carrier recombination in a-Si:H

We report on the non-radiative picosecond recombination of photogenerated (free or trapped) carriers in a-Si:H and alloys. The mechanism is bimolecular multiphonon, with a limited number of recombining states. The recombination efficiency of tail states is discussed.

Characterization of ultrashort laser pulses by the method of self‐diffraction

The pulsewidth and coherence time of a picosecond Nd:YAG laser has been measured by the method of self‐diffraction, either in the two pulses or in the three pulses configuration. Experimental results have been obtained with thin dye cells or thin semiconductor films. A theoretical model has been developed that explains our results.

Ultrafast relaxation dynamics of photoexcited carriers in GaAs

We report time-resolved pump-probe measurements of both absorption and refractive index for GaAs thin films with 80 fs time resolution. The probe photon energies are near the band-edge and near the initial excited states, and photoexcited carrier densities are varied from 1017 to 1019 cm3. Our results reveal the instantaneous band gap renormalization and the importance of carrier-carrier scattering and intervalley scattering. Mechanisms which cause an ultrafast change in refractive index are also discussed.

High efficiency, high energy optical amplifier for femtosecond pulses

We report on the design and performance of an improved copper vapor laser pumped amplifier for femtosecond optical pulses. Consideration of the large duration mismatch between pump and seed pulses, the short energy storage time of the gain dye, spatial mode matching, and polarization effects led to an amplifier design of 1.5% efficiency, which is greater than previous designs. Pulses of > 10 tJ energy and -65 fsec duration are generated that are focussable to less than twice the diffraction limit. The multiple pass amplifier incorporates optimized mode matching, an antireflection coated flowing dye cell, and a polarized, spatially smoothed pump beam.