This special issue is devoted to (ultrafast) laser-induced material modifications. it is limited in scope by considering for the most part materials in the solid state. Researchers from the international scene have been invited to review a topic from their current research interests in this area. But review or not, most of the papers herein contain new and interesting results. Fortuitously, the 12 papers accepted readily fit into three sections.

Optical bandgap excitation can cause permanent or transient displacement of isolated ionic constituents in halide crystal lattices. The point defects that are formed can be utilized in some applications, but more generally they are detrimental to transmissive optics and dielectric films made from metal halide materials. The transient optical response to impulse excitation is the main subject of the work presented here. New measurements of defect formation with 2 ps resolution in KBr and RbBr are described. A survey of transient optical absorption from picoseconds to milliseconds is presented for alkali halides and partially for alkaline earth halides. This paper is intended to provide a basis for predicting the defect optical response of these materials to generation of electron-hole pairs, whether by multiphoton effects, direct ultra-violet absorption, or more conventional ionizing radiation.

Experimental observations of the consequences of generation of excited states of self-trapped excitons in insulators such as alkali halides and alkaline earth fluorides are surveyed. Three types of excited states are distinguished: electron-excited and hole-excited states of self-trapped exciton and host-excited states bound by a self-trapped exciton. The most extensive studies have been carried out for electron-excited states, which are deexcited dominantly by nonradiative transitions. A few examples are given of the photoinduced transformation of an excited self-trapped exciton in these states to a Frenkel pair in alkali halides and alkaline earth fluorides. It is emphasized that under laser irradiation with subgap photon energies, the self-trapped excitons and the Frenkel pairs generated by multiphoton excitation act as a source of one-photon absorption.

In this paper we provide a comprehensive review of our recent work on the nonlinear interaction between high intensity pulsed laser beams and transparent solids. New experimental techniques used to measure multiphoton absorption and energy deposition in wide-gap alkali halides in the prebreakdown regime have led to hard evidence refuting the avalanche model of laser-induced damage at visible laser wavelengths. These measurements, performed in specially purified materials, have allowed the discovery of the roles of laser-induced excitations in energy absorption, leading to the conclusion that virtually all lattice heating occurs via a nonlinear absorption of laser photons by multi-photon-excited free electrons. These results yield an experimentally confirmed theoretical definition of intrinsic, single pulse laser damage thresholds at 532 nm wavelength in three- and four-photon bandgap alkali halides. Extending this work to multipulse effects in the subthreshold intensity regime, we have formulated a new model of bulk damage based on thermomechanical stress induced by accumulation of multiphoton-generated lattice defects.

Recent progress in picosecond photoconductivity is reviewed. The relevance of the techniques is discussed in terms of laser-induced material modifications in wide-band gap alkali-halide crystals.

Some applications of picosecond, time-resolved Raman scattering in the field of nonequilibrium semiconductor physics are reviewed. A brief, nonexhaustive survey of progress in this field over the past 10 years serves to introduce the general experimental techniques and to indicate the range of physical phenomena that have been studied. More detailed coverage of work on nonequilibrium coupled plasmon-LO phonon modes in InP and GaAs and of optical phonon dynamics in Ge and GeSi alloys is given. Emphasis is placed on comparing the use of picosecond Raman scattering to study nonequilibrium phenomena in group IV and group III-V semiconductors.

When solids are exposed to intense pulsed laser radiation, highly excited electronic states are created that are of both practical and theoretical interest. Time-resolved photoelectron spectroscopic methods developed in recent years provide an effective approach to this problem and have yielded considerable information, particularly on semiconductors. The experiment reported on here uses a short, strongly absorbed laser pulse to excite electrons to intermediate states. Then a probe pulse that may be coincident or delayed relative to the exciting pulse and may have the same or higher photon energy ejects electrons whose energy distribution is measured. This distribution can be related directly to the distribution in the intermediate states. We describe applications of the general technique to observe normally unoccupied states between the Fermi level and vacuum level, to measure dynamics of surface space charge layers and surface recombination, to measure hot-electron temperatures, and to determine the rates of electron energy relaxation processes. Results on Si(111) 7 x 7 and cleaved ZnTe(110) and CdTe(110) surfaces are described.

The carrier dynamics of highly excited (N > 1020 cm-3) crystal-line and amorphous silicon have been studied by picosecond and femto-second pump and probe measurements. The Auger coefficient, trapping time, and other parameters relevant for transport have been obtained. At still higher excitation, melting occurs. The optical constants of liquid Si have been measured, and evidence for superheating of the liquid phase above the boiling temperature is presented.

Time-resolved photoluminescence is universally regarded as a very useful probe in the investigation of the dynamic behavior of high density electron-hole systems in direct-gap semiconductors, in which carrier lifetimes are of the order of nanoseconds or less. Two applications of this technique include (1) the observation of excitons in which a conduction electron and a valence hole are bound to one another via the long-range Coulomb interaction and (2) the formation of excitonic molecules (or biexcitons), which is caused by the attractive covalent bonding between two single excitons. This paper reviews transient photoluminescence techniques used recently in the study of such highly excited systems. Interaction between excitons at high concentration is discussed, and some experimental results for ZnSe and GaAs are presented.

We present a review of newly observed liquid crystal nonlinear responses induced by visible and infrared lasers and discuss some recently observed optical wave-mixing processes. Specifically, the dynamics of inter- and intramolecular energy transfer, response times, infrared refractive indices and their temperature dependences, and the basic principles of newly observed processes are described. These processes, in conjunction with cw or pulsed lasers spanning the visible through the infrared regime, will find applications in a great variety of nonlinear-optics-based devices.

An ultrashort light pulse illuminating a metal surface interacts primarily with the electrons. During a transient phase the electrons can be much hotter than the lattice. A two-temperature model predicts that the optical damage threshold of metals will be independent of the optical pulse duration for short pulses. Experiments performed with picosecond 10 um pulses confirm this prediction. The model also predicts, and we experimentally observe, electron emission during the transient phase even for illumination intensities below the damage threshold.

Pulsed laser radiation in the visible spectral range is used to investigate and to modify the surface of wide bandgap materials, in particular fluoride crystals. The processes studied are mainly based on multiphoton interaction and comprise optical second harmonic generation, surface ionization, desorption of positive ions, and the generation of a microplasma. All effects are facilitated by surface defect states, and the materials can be grouped into two classes of different response, according to their defect density.

We review the influence of self-focusing on the measurement of bulk laser-induced-damage (LID) thresholds in normally transparent optical mate-rials. This role is experimentally determined by measuring the spot size and polarization dependence of LID and by observing beam distortion in the far field. Utilizing these techniques, we find that by using a tight focusing geometry in which the breakdown power is below P2, the effects of self-focusing can be practically eliminated in an LID experiment. P2 is the so-called second critical power for self-focusing, and P2 = 3.77P1, where P1 = cX2/327r2n2, where c is the speed of light in vacuum, X is the laser wavelength and n2 is the nonlinear index of refraction. This is in accordance with numerical calculations by J. H. Marburger [in Progress in Quantum Electronics, J. H. Sanders and S. Sten-holm, eds., Vol. 4, Part 1, pp. 35-110, Pergamon, Oxford (1975)]. With this knowledge we determine that damage is only partially explained by avalanche ionization and that the initiation of damage is strongly influenced by extrinsic processes. We therefore conclude that we are measuring extrinsic LID.

I am writing this ditorialll to inform you about a number of significant changes in Optical Engineering editorial positions that either have already taken place or that will take place in the not-too-distant future. To begin with, Dixie Cheek has taken over as Managing Editor from Eric Pepper, who has moved on to another Managing Editor position: that of SPIE's Optical Engineering Press. Eric has done a superb job for these past several years and, because of his professionalism and cooperative nature, it has been a real pleasure to work with him. I wish him well in his new duties, and I offer my condolences to Dixie-who has inherited me.

Peter Milonni, staff member at Los Alamos National Laboratories and professor of physics at the University of Arkansas, and Joseph Eberly, professor of physics and optics at the University of Rochester, have written Lasers, an enlightening text dealing with laser physics. Lasers comprises 18 chapters, presenting both the classical and quantum models where appropriate (although not necessarily side by side).