Self-scattering effects in femtosecond laser nanoablation.

We experimentally investigate the self-reflectivity of intense strongly focused femtosecond laser pulses used for single-shot femtosecond laser ablation of silicon-on-insulator (SOI). We model the self-reflectivity using 2D finite-difference time-domain simulations of a single femtosecond laser pulse interacting with a submicrometer-sized time- and space-dependent plasma induced by the incident pulse itself and find excellent agreement with our experimental results. The simulation shows that the laser-induced plasma scatters the incident pulse into the guided modes of the device layer of the SOI.

Room-temperature laser emission of ZnO nanowires explained by many-body theory.

Are excitons involved in lasing in ZnO nanowires or not? Our recently developed and experimentally tested quantum many-body theory sheds new light on this question. We measured the laser thresholds and Fabry-Pérot laser modes for three radically different excitation schemes. The thresholds, photon energies, and mode spacings can all be explained by our theory, without invoking enhanced light-matter interaction, as is needed in an earlier excitonic model.

Ultrafast all-optical shutter based on two-photon absorption.

An ultrafast all-optical shutter is presented, based on a simple two-color, two-photon absorption technique. For time-resolved luminescence measurements, this shutter is an interesting alternative to the optical Kerr gate. The rejection efficiency is 99%; the switching-off and switching-on speeds are limited by the pulse length only; the rejection time is determined by the crystal slab thickness; and the bandwidth spans the entire visible spectrum.

Coherent interactions of terahertz strain solitons and electronic two-level systems in photoexcited ruby.

We observe coherent interactions between an ultrashort, longitudinal acoustic soliton train and the 29-cm(-1) electronic transition in photoexcited ruby. Propagation of the strain pulses over millimeter distance through an excited zone reveals striking behavior of the induced electronic population, which has been explained by impulsive excitation of the two-level systems, combined with the nonlinear properties of the solitons in the resonant medium. This opens up new possibilities for coherent manipulation of ultrashort acoustic pulses by local electronic centers.

High amplitude, ultrashort, longitudinal strain solitons in sapphire.

We demonstrate the development of high-amplitude picosecond strain pulses in a sapphire single crystal into an ultrafast compressional soliton train. For this purpose, large-intensity light pulses were used to excite a metal film, yielding a 2 orders of magnitude higher strain than that achieved in earlier studies. Propagation of the packets is monitored over a distance of several millimeters by means of Brillouin light scattering.

Ultrafast vibrational dynamics and stability of deuterated amorphous silicon.

Infrared four-wave mixing experiments performed upon deuterated amorphous silicon layers (a-Si:D) reveal profound differences in the dynamics of Si-D stretch vibrations compared to those of analogous Si-H vibrational modes in hydrogenated amorphous silicon (a-Si:H). Remarkably, transient-grating measurements of the population decay rate of the Si-D vibrations show single-exponential decay directly into collective modes of the a-Si host, bypassing the local bending modes of the defect into which the Si-H vibrations decay. Photon-echo measurements of the vibrational dephasing suggest at low temperature contributions from TO nonequilibrium phonons and at elevated temperatures elastic phonon scattering of TA phonons.