Laser-induced breakdown and damage to transparent materials has remained an active area of research for four decades. In this paper we review the basic mechanisms that lead to laser-induced breakdown and damage and present a summary of some open questions in the field. We present a method for measuring the threshold intensity required to produce breakdown and damage in the bulk, as opposed to on the surface, of the material. Using this technique, we measure the material band-gap and laser-wavelength dependence of the threshold intensity for bulk damage using femtosecond laser pulses. Based on these thresholds, we determine the relative role of different nonlinear ionization mechanisms for different laser and material parameters.

https://iopscience.iop.org/article/10.1088/0957-0233/12/11/305/pdf

Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses

Laser-induced damage threshold measurements were performed on homogeneous and multilayer dielectrics and gold-coated optics at 1053 and 526 nm for pulse durations τ ranging from 140 fs to 1 ns. Gold coatings were found, both experimentally and theoretically, to be limited to 0.6 J/cm2 in the subpicosecond range for 1053-nm pulses. In dielectrics, we find qualitative differences in the morphology of damage and a departure from the diffusion-dominated τ1/2 scaling that indicate that damage results from plasma formation and ablation for τ ≤ 10 ps and from conventional heating and melting for τ > 50 ps. A theoretical model based on electron production by multiphoton ionization, joule heating, and collisional (avalanche) ionization is in quantitative agreement with both the pulse-width and the wavelength scaling of experimental results.

Optical ablation by high-power short-pulse lasers

Since approximately 1950 an increasing portion of experimental solid state physics research has been concerned with studying defects in crystals. This trend might appear to be a rather belated recognition that most of the materials we come into contact with have a random structure. In fact the theoretical understanding and controlled preparation of compounds with defects or random structure has been very slow in developing. The present paper examines and reviews our knowledge of the lattice vibrations associated with defects. The coverage is extremely broad, as shown by the table of contents. It includes localized and resonant modes of isolated defects as well as the modes in highly disordered mixed crystals and glasses. It is primarily a review of experimental work but theoretical results are included where the latter explain or predict significant features. In order to be self-contained several sections of the paper deal entirely with theoretical matters. There is a chapter on explicit solutions of the linear chain vibration problem and a short chapter on Green's function methods. The review emphasizes the infrared absorption and Raman scattering of defects. This is simply because other techniques have not yielded nearly so much information. Neutron scattering and electron tunneling are referred to only where they have shed light on certain systems. Extensive tables of defects and mode frequencies are included for each type of solid. The major solids which are reviewed include semiconductors, ionic compounds, organic compounds, and amorphous insulators.

Optical studies of the vibrational properties of disordered solids

There is a strong deviation from the usual τ1/2 scaling of laser damage fluence for pulses below 10 ps in dielectric materials. This behavior is a result of the transition from a thermally dominated damage mechanism to one dominated by plasma formation on a time scale too short for significant energy transfer to the lattice. This new mechanism of damage (material removal) is accompanied by a qualitative change in the morphology of the interaction site and essentially no collateral damage. High precision machining of all dielectrics (oxides, fluorides, explosives, teeth, glasses, ceramics, SiC, etc.) with no thermal shock or distortion of the remaining material by this mechanism is described.

Ultrashort-pulse laser machining of dielectric materials

An investigation is conducted of problems which are related to a use of measured optical constants in the simulation of the optical constants of real atmospheric aerosols. The techniques of measuring optical constants are discussed, taking into account transmission measurements through homogeneous and inhomogeneous materials, the immersion of a material in a liquid of a known refractive index, the consideration of the minimum deviation angle of prism measurement, the interference of multiply reflected light, reflectivity measurements, and aspects of mathematical analysis. Graphs show the real and the imaginary part of the refractive index as a function of wavelength for aluminum oxide, NaCl, and ammonium sulfate. Tables are provided for the dispersion parameters and the optical constants.

The optical constants of several atmospheric aerosol species: Ammonium sulfate, aluminum oxide, and sodium chloride

Electron-avalanche breakdown in solids is explained by a theory that is predictive and agrees with experimental results for the magnitude of the breakdown field and its temperature dependence, pulse-duration dependence, material-to-material variation, and wavelength dependence for λ > 1 μm. The good agreement between experiment and theory with no parameters adjusted is obtained by using improved magnitudes and energy dependences of the electron-phonon relaxation frequencies. The contributions of both optical and acoustical phonons to electron loss and diffusion must be included. The breakdown field E B is calculated by solving an eigenvalue equation obtained from the diffusion transport equation. Simple models and interpretations of the diffusion equation afford physical insight into breakdown and render the breakdown conditions predictable. Preliminary results indicate that the diffusion approximation fails for wavelengths considerably shorter than 1 μm, but that E B decreases with decreasing X as a result of multiphoton absorption before the diffusion approximation fails.

Theory of electron-avalanche breakdown in solids

Electron-avalanche breakdown in solids is explained by a theory that agrees with experimental results for the magnitude of the breakdown field and its temperature dependence, pulse-duration dependence, material-to-material variation, and wavelength dependence for $\ensuremath{\lambda}\ensuremath{\ge}1$ \ensuremath{\mu} m. The good agreement between experiment and theory with no parameters adjusted is obtained by using improved magnitudes and energy dependences of the electron-phonon relaxation frequencies. The contributions of both optical and acoustical phonons to electron loss and energy-space diffusion must be included. The breakdown field ${E}_{B}$ is calculated by solving an eigenvalue equation obtained from the diffusion-transport equation. Simple models and interpretations of the diffusion equation afford physical insight into breakdown and render the breakdown conditions predictable. Preliminary results indicate that the diffusion approximation fails for wavelengths considerably shorter than 1 \ensuremath{\mu} m, where multiphoton absorption must also be considered.

Theory of Multiphonon Absorption in Insulating Crystals

Exponential frequency dependence of multiphonon-summation infrared absorption☆

The nearly exponential frequency dependence of the infrared absorption coefficient $\ensuremath{\beta}$ observed by Rupprecht, Deutsch, and others has been explained previously by evaluating the individual $n$-phonon contributions to $\ensuremath{\beta}$, summing the results, and noting that the sum was nearly exponential over a fairly wide range of frequencies. A new derivation of the multiphonon absorption coefficient yields $\ensuremath{\beta}\ensuremath{\sim}{e}^{\ensuremath{-}\ensuremath{\omega}\ensuremath{\tau}}$ directly, rather than as a sum on $n$, and provides a prescription for estimating the range of $\ensuremath{\omega}$ over which the nearly exponential behavior extends.

M. Sparks

Papers

Theory of electron-avalanche breakdown in solids

Theory of electron-avalanche breakdown in solids

Theory of Multiphonon Absorption in Insulating Crystals

Exponential frequency dependence of multiphonon-summation infrared absorption☆

Explicit exponential frequency dependence of multiphonon infrared absorption