Peter F. Braunlich

Contributions of multiphoton absorption to laser−induced intrinsic damage in NaCl

Intrinsic damage of NaCl upon exposure to intense laser pulses is discussed in terms of electron kinetic processes. Avalanche ionization is confirmed to be the dominant mechanism at the ruby frequency. However, compared to pure avalanche breakdown, both five−photon absorption and, to a lesser extent, the presence of F centers reduce the damage threshold by providing a large concentration of free carriers at the onset of avalanche ionization. Three−photon absorption contributes extensively to damage at the doubled ruby frequency. The dependence of the damage threshold on the laser pulse length is calculated and it is shown that its measurement can provide information on the role of multiphoton absorption in the damage process. At a given wavelength multiphoton contributions become increasingly more important as the laser pulse length decreases.

Statistics in laser‐induced dielectric breakdown

The statistical aspects of laser‐induced breakdown of transparent dielectrics are reexamined and it is found that the experimentally observed independence of the damage probability on spot size is inconsistent with previous theory. In an attempt to find spot‐size‐independent contributions to damage statistics, modification of the electronic properties of the material caused by intense photon fluxes below the one‐shot damage threshold is considered. This modification takes the form of absorbing defects (color centers). The case of high‐quality NaCl (containing 5×1016 Cl− vacancies) is discussed in detail. At the ruby frequency, it may require up to three identical 60‐nsec shots at the same sample site to cause damage at field strengths below the one‐shot threshold. The results exhibit the proper volume dependence and they indicate that laser‐induced material modifications prior to breakdown may indeed contribute to damage statistics.

Starting times of laser−induced intrinsic damage in NaCl

The starting time for laser−induced intrinsic damage in NaCl is calculated as a function of the rms optical field strength at the peak of the laser pulse. Three different damage mechanisms are considered: (i) avalanche ionization for λ⩾1 μ, (ii) five−photon assisted avalanche ionization at λ=0.694 μ, and (iii) three−photon assisted avalanche ionization at λ=0.347 μ. It is shown that the probability to measure the true damage threshold E*th is zero. (E*th is the optical peak field strength of a laser pulse that heats a small sample volume so that the melting point is reached exactly at the end of the pulse.) For this reason a ’’practical’’ damage threshold E*T is defined which is the peak field strength of a pulse that causes damage by melting at the time the laser flux reaches its peak.

Trap-level spectroscopy by thermally stimulated release of trapped carriers

Abstract Investigations of thermoluminescence (TL), thermally stimulated conductivity (TSC), thermally stimulated capacitance (TSCAP), and edge region TSCAP have been carried out over the last thirty years to study thermal release of trapped carriers. The goal of this work was and remains the measurement of spectroscopic trap level data: energy levels, transition probabilities and trap concentrations. Little, if any, information on the physical nature of traps can be obtained by these methods. Discrimination between electron and hole traps is possible with TSCAP. The analysis of experimental thermal release curves or “glow curves” is traditionally based on phenomenological theories which take the form of a set of coupled nonlinear rate equations. The complete description of thermal emission of trapped carriers and their subsequent recombination or retrapping for an arbitrary distribution of trap levels and recombination centers of arbitrary concentration has proved intractable. In order to remove the mathematical complexity it became necessary to resort to various approximations. As a result, the model descriptions available until recently were only applicable to the simplest trapping and recombination conditions which can rarely be related to a real material. Exact solutions of the commonly employed so-called “single trap level model” have become available only in the last five years. A number of other conceivable models involving several different electron and hole traps pose no principle mathematical problems any longer. After examination of the vast variety of different thermal emission curves obtained from the physically meaningful range of trapping parameters, it became evident that it is extremely difficult to correlate theory and experiment with any degree of confidence. Even the simultaneous investigation of TL and TSC does not yield sufficient information to determine uniquely the kinetical mechanism of the thermally stimulated recombination process. It is at best possible to test the validity of a given model when most of the spectroscopic trap level parameters are known from independent measurement techniques. Several of those techniques have now been developed. The most important of them is deep-level transient spectroscopy (DLTS). By proper choice of the experimental conditions it is possible to measure thermal emission rates, activation energies, trapping rates, concentration profiles of traps in the depletion region of Schottky barriers or in p - n junctions as well as to discriminate between hole traps and electron traps. The analysis of the experimental data makes use of only the most general principles of thermal release of carriers from traps.

Laser-Stimulated Exoelectron Emission

Laser-stimulated exoelectron emission is discussed as a new technique to measure the electron affinity, the electron concentration in the conduction band, and the relaxation time of free electrons in exicted states. A simple theory is presented and applied to NaCl exposed to CO2-laser photons of 10.6 µm wavelength during a thermally stimulated relaxation experiment.

The probability to avoid optical breakdown at 6943 Å in NaCl

Computer‐simulated ’’one‐shot‐on‐one‐site’’ damage events in NaCl at the ruby wavelength are illustrated as deterministic survival curves. Multiphoton‐assisted avalanche breakdown is considered for varying laser peak flux densities. For a fixed laser output power level, the effects are calculated for various initial free and trapped carrier concentrations in the presence and absence of multiphoton absorption. The results are in reasonable agreement with recent measurements obtained with LiF and Ar and N2 at intermediate pressures. Experimental observations in NaCl are not presently available.