A. E. Siegman

Surface ripples on silicon and gallium arsenide under picosecond laser illumination

We report experimental results on spontaneous periodic surface structures or ripples which occur on the surface of crystalline or ion‐implanted semiconductors, using high‐power picosecond laser pulses. We suggest that these surface ripples develop as the result of an exponentially growing interaction between the incident laser wave front and a scattered surface optical wave.

Observations of higher-order laser-induced surface ripples on 〈111〉 germanium

We report the observation and analysis of higher-order optical diffraction patterns arising from linear combinations of the primary laser-induced gratings or ripples on germanium surfaces. These higher-order surface structures presumably arise from nonlinear interactions between superimposed primary gratings with different grating wave vectors. For gratings produced by normally incident laser beams on Ge 〈111〉 surfaces, the diffraction patterns exhibit a strongly hexagonal symmetry.

Lasers without photons — or should it be lasers with too many photons?

Expanding the fields of a laser cavity in a set of orthonormal modes is a standard technique in laser theory. Expansion in a normal mode set is also the basis of the concept of “photons”. A substantial number of practical lasers do not, however, support any kind of normal or orthogonal cavity modes, and thus, their fields cannot be represented (at least not easily) in terms of normal modes, or photons. This leads to a number of unusual results, including situations in which the lowest-order mode of a cavity can contain substantially more energy than the total energy in the cavity, as well as enhanced quantum spontaneous emission far stronger than the “single extra photon” level characteristic of an ordinary laser oscillator. We review the theoretical origins of these unusual effects and present experimental confirmation of greatly enhanced Schawlow-Townes fluctuations in an unstable-resonator laser with a Petermann-noise enhancement factor of several hundred times.

LARGE-SIGNAL RESULTS FOR DEGENERATE FOUR-WAVE MIXING AND PHASE CONJUGATE RESONATORS

A numerical calculation procedure and various large-signal numerical solutions are presented for degenerate four-wave mixing in optical Kerr media, and for phase conjugate resonators using degenerate four-wave mixing. The solutions presented take full account of nonlinear refractive index changes, pump depletion, signal saturation, distributed losses, and possible external mirrors with laser gain. We find that including the nonlinear index change generally causes little change in the reflectivity or power output of degenerate four-wave mixing devices, at least with symmetric pumping. The optimum power output from a phase conjugate resonator with and without a laser gain medium is calculated. The results provide some theoretical guidance for designing phase conjugate resonators.

Axial modes in a grating-dispersed laser cavity

Two recent papers by Stankov and the author have predicted that unusual axial mode properties should occur in a laser cavity containing an intracavity dispersive grating pair. The analysis in these reports is unfortunately incorrect, and the corrected properties of the dispersive cavity are found to be less interesting than at first predicted.

Evidence for a dense electron‐hole plasma close to the melting phase transition in silicon

We have studied both theoretically and experimentally the dynamics of laser‐induced electron‐hole plasmas created at the surface of silicon using the strong synergy observed during simultaneous illumination by two picosecond pulses of different wavelengths. In a range of intensities up to those capable of producing a phase transition, we find good agreement between the results of our experiments and the predictions of a conventional model for pulsed laser heating.

Periodic Ripple Structures on Semiconductors Under Picosecond Pulse Illumination

We can routinely generate spontaneous periodic surface structures or ripples on both semiconductors and metals, using either single or multiple TEM00 Nd:YAG pulses with durations of ≤ 100 ps at 1.06 μm or 80 ps at 532 nm [1]. We believe these ripples evolve through stimulated growth of surface corrugations on molten surfaces, and offer theoretical evidence to this end. Results of two-pulse experiments which test another recently suggested model are also presented.

Laser-Induced Surface Ripples: What is Understood and What is Not

The most predominant ripple properties can be explained by existing perturbative theories. We present experimental results on novel types of ripples, which suggest that theory has to go beyond first-order perturbation and that the materials response is not well understood.

Ultrafast Spectroscopy of Very Dense and Hot Electron-Hole Plasmas in Crystalline and Amorphized Semiconductors

The physics of highly excited semiconductors has recently been the subject of many studies, especially in connection with the problem of pulsed laser annealing. Careful experiments [1–3] have now been performed with the temporal resolution required to demonstrate that one single picosecond or femtosecond pulse can produce melting at the surface of silicon. Since the carrier relaxation time is very short ( 1020 cm−3) and hot (Te > 2000°K) plasma close to but below the melting phase transition in Si and GaAs.