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

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

The scaling of the single-pulse laser threshold fluence for dielectric breakdown with respect to pulse duration and material band gap energy was investigated in the subpicosecond pulse regime using oxide films (${\mathrm{TiO}}_{2}$, ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$, ${\mathrm{HfO}}_{2}$, ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{SiO}}_{2}$). A phenomenological model attributes the pulse duration dependence to the interplay of multiphoton ionization, impact ionization, and subpicosecond electron decay out of the conduction band. The observed linear scaling of the breakdown fluence with band gap energy can be explained within the framework of this model by invoking the band gap dependence of the multiphoton absorption coefficient from Keldysh photoionization theory. The power exponent $\ensuremath{\kappa}$ of the observed dependence of the breakdown threshold fluence ${F}_{\mathit{th}}$ on pulse duration ${\ensuremath{\tau}}_{p}$, ${F}_{\mathit{th}}\ensuremath{\propto}{\ensuremath{\tau}}_{p}^{\ensuremath{\kappa}}$, is independent of the material and is attributed to photoionization seeded avalanche ionization.

Scaling laws of femtosecond laser pulse induced breakdown in oxide films

Advanced polymeric dielectrics for high energy density applications

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.

T. Holstein

Papers

Theory of electron-avalanche breakdown in solids

Theory of electron-avalanche breakdown in solids