H. M. van Driel

High-power, 62-fs infrared optical parametric oscillator synchronously pumped by a 76-MHz Ti:sapphire laser

We demonstrate a producing KTiOPO4-based optical parametric oscillator with external-pulse compression, 62-fs pulses at 76 MHz and 175-mW average power (37-kW peak power). The oscillator is pumped synchronously with 800-mW, 110-fs pulses from a Kerr lens mode-locked Ti:Al2O3 laser operating at λ = 765 nm. The oscillator cavity length is actively stabilized, and the total amplitude noise is <1% (rms). With a single mirror set, the signal-beam wavelength is tunable between 1.20 and 1.34 μm. The idler beam tunes from 2.1 to 1.78 μm with an expected (uncompressed) pulse width of <400 fs and ~100-mW average power. Tuning throughout the bandwidth from 0.9 to 4.5 μm should be possible with additional mirror sets.

Laser‐induced periodic surface damage and radiation remnants

We report a detailed experimental study of the periodic surface damage induced by laser irradiation at 1.06 μm on nominally smooth Ge samples; we include novel observations of the damage obtained by studying its optical diffraction pattern, which reveals a previously unappreciated richness in the damage structure. The usual ’’surface scattered wave’’ explanation of the damage is criticized; we argue that the damage results from the electromagnetic fields generated by surface inhomogeneities but that, due to the presence of the interface between vacuum and bulk Ge, these fields are not radiative; these ’’radiation remnants’’ are discussed.

Quantum interference control of electrical currents in GaAs

In an earlier publication, preliminary observations of the generation of electrical currents were reported in GaAs and low-temperature-grown GaAs (LT-GaAs) at 295 K using quantum interference control of single- and two-photon band-band absorption of 1.55- and 0.775-/spl mu/m ultrashort optical pulses. Time-integrated currents were measured via charge collection in a metal-semiconductor-metal (MSM) electrode structure. Here we present detailed characteristics of this novel effect in terms of a simple circuit model for the MSM device and show how the injected current depends on MSM parameters as well as optical coherence, power, and polarization. For picosecond pulse excitation with peak irradiance of only 30 MW/cm/sup -2/ (1.55 /spl mu/m) and 9 kW/cm/sup -2/ (0.775 /spl mu/m), peak current densities of /spl sim/10 A/cm/sup -2/ at peak carrier densities of 10/sup 15/ cm/sup -3/ are inferred from the steady-state signals. This compares with 50 A/cm/sup -2/ predicted theoretically; the discrepancy mainly reflects inefficient charge collection at the MSM electrodes.

Kerr lens mode locking of a diode-pumped Nd:YAG laser.

We demonstrate Kerr lens mode locking of a diode-pumped Nd:YAG laser by using the gain medium as the Kerr medium and no intracavity slit. Kerr self-focusing within the Nd:YAG rod is believed to improve matching between the cavity mode and the aperture created by thermal lens aberration and thereby discriminate against lower (cw) intensities. The laser produces 8.5-ps, 100-MHz pulses with 1 W of average output power and a 35% slope efficiency. Kerr lens mode locking is initiated by slight mechanical perturbation.

Attenuation of optical transmission within the band gap of thin two-dimensional macroporous silicon photonic crystals

The transmissivity within the photonic band gap of two-dimensional photonic crystals of macroporous silicon is reported as a function of crystal thickness. Measurements were carried out for crystals of nominally 1, 2, 3, and 4 crystal layers using a commercial parametric source, with a wavelength tunable from 3 to 5 μm. For wavelengths well within the 3–5 μm photonic band gap, attenuation of approximately 10 dB/crystal layer is obtained, in agreement with calculations based on plane wave expansion methods. For these materials, one should be able to achieve photonic crystal functionality in many applications with very small crystal volumes.

A model system for two-dimensional and three-dimensional photonic crystals: macroporous silicon

A review of the optical properties of two-dimensional and three-dimensional photonic crystals based on macroporous silicon is given. As macroporous silicon provides structures with aspect ratios exceeding 100, it can be considered to be an ideal two-dimensional photonic crystal. Most of the features of the photonic dispersion relation have been experimentally determined and were compared to theoretical calculations. This includes transmission and reflection of finite and bulk photonic crystals and their variation with the pore radius to determine the gap map. All measurements have been carried out for both polarizations separately since they decouple in two-dimensional photonic crystals. Moreover, by inhibiting the growth of selected pores, point and line defects were realized and the corresponding high-Q microcavity resonances as well as waveguiding properties were studied via transmission. The tunability of the bandgap was demonstrated by changing the refractive index inside the pores caused by an infiltrated liquid crystal undergoing a temperature-induced phase transition. Finally different realizations of three-dimensional photonic crystals using macroporous silicon are discussed. In all cases an excellent agreement between experimental results and theory is observed.

Time-dependent second-harmonic generation from the Si–SiO 2 interface induced by charge transfer

We have observed that the second-harmonic signal generated from oxidized Si(001) varies on a time scale of several seconds in experiments involving a fundamental beam of lambda = 770 nm, 110-fs pulses at 76 MHz. We suggest that the temporal behavior arises from absorption of weak (<100-fW average power) third-harmonic light generated in air or in the sample, inducing charge transfer across the Si-SiO(2) interface and trapping in the oxide layer. Detrapping has been determined to take several minutes.

Enhanced second-harmonic generation from planar photonic crystals

Strongly enhanced second-harmonic generation is observed from a two-dimensional square lattice GaAs/AlGaAs photonic crystal waveguide when the fundamental beam, the second-harmonic beam, or both beams resonantly couple to a leaky eigenmode. P-polarized second-harmonic spectra are obtained for s-polarized, 150-fs pump pulses that are tuned from 5000 to 5600 cm(-1) and directed along the gamma-chi direction of the crystal for various angles of incidence. Compared with off-resonant conditions, enhancements of >1200x in the second-harmonic conversion are observed for resonant coupling of both the fundamental and the second-harmonic fields to leaky eigenmnodes. The angular and spectral positions of the peaks are in good agreement with simulations.

Rectification and shift currents in GaAs

We observe second-order rectification and shift currents in GaAs at 295 K using 150 fs pulses at 1.55 and 0.775 μm, respectively. For the same low pump intensity, 100 MW cm−2, the shift current density is 570 times larger than the rectification current density. At high intensity, the shift current is strongly affected by carrier screening and dephasing and is in phase quadrature with the rectification current. A maximum shift current density of 60 kA cm−2 is inferred for a pump intensity of 500 MW cm−2.

Optical effective mass of high density carriers in silicon

The optical effective masses of electrons and holes in silicon are calculated for carrier densities up to 2×1021 cm−3 and temperatures up to 3000 K. This evaluation, which is based on the band structure as determined by the local empirical pseudopotential method, indicates that the carrier masses for high density plasmas can be up to two times larger than the corresponding low density values. For electrons, this occurs primarily because of increasing warpage of the (low density) ellipsoidal constant energy surfaces with increasing energy, while for holes the effect is primarily due to nonparabolicity of the valence bands. Carrier‐carrier interactions and carrier‐lattice screening do not influence the effective masses significantly.