Zhanliang Sun

Generic incubation law for laser damage and ablation thresholds

In multi-pulse laser damage and ablation experiments, the laser-induced damage threshold (LIDT) usually changes with the number of pulses in the train, a phenomenon known as incubation. We introduce a general incubation model based on two physical mechanisms—pulse induced change of (i) absorption and (ii) critical energy that must be deposited to cause ablation. The model is applicable to a broad class of materials and we apply it to fit data for dielectrics and metals. It also explains observed changes of the LIDT as a function of the laser repetition rate. We discuss under which conditions the crater-size method to determine LIDTs can be applied in multi-pulse experiments.

Characterization of laser-induced air plasmas by third harmonic generation.

Third harmonic generation by a femtosecond probe intersecting a laser-induced air plasma is measured and a general model developed to describe such signal. The electron density distribution and nonlinear coefficient of the plasma are determined.

Laser damage in dielectric films: What we know and what we don't

Damage mechanisms in thin films are reviewed from femtosecond pulse to CW laser illumination. Special emphasis is given to the role of native and laser induced defects, recent successes and the need for better diagnostic tools.

Optical coatings excited by femtosecond lasers near the damage threshold: challenges and opportunities

The push for ever more powerful and shorter laser pulse sources demands optical coatings that can handle large bandwidths while operating near the threshold of permanent damage. We review some of the current challenges in the understanding of film behavior under load and opportunities that arise from tailored coating stacks for nonlinear optical harmonic generation.

Determination of multi-pulse damage thresholds from crater size measurements

A well-known method for the determination of the threshold fluence for laser-induced damage (LIDT) and ablation with single laser pulses is the measurement of the ablated crater area as a function of incident fluence and extrapolation to zero crater area. In this paper, we introduce a general incubation model for a large class of materials, for example dielectric materials and metals, and we compare it to experimental data. We conclude that multi-pulse LIDT measurements can also be based on crater size measurements similar to 1-on-1 damage tests if the incubation does not involve coupling between spatial and temporal processes.

Femtosecond to nanosecond laser damage in dielectric materials

We present a consistent model supported by experimental data of damage of dielectric films by femtosecond to nanosecond pulses. Special emphasis is given to the role of defects and transient processes in the material. New imaging and diagnostic techniques are discussed to characterize the film performance.

Confocal photothermal microscopy of thin films based on etalon and thermal lensing effects

A photothermal apparatus with submicron resolution (~0.5 μm) using a focused 532 nm pump beam is demonstrated. A coaxial probe (633 nm HeNe) detects changes in reflectivity. In addition to detecting modulation of the probes total reflected power caused by perturbation of the Fabry-Perot resonance, a pinhole in confocal geometry detects changes to the probe beam caused by thermal lensing. Based on signal-to-noise measurements made with a film of known absorption, a sensitivity of ~10 ppm is predicted with 200 mW of pump power. The instrument is applied to mapping of local absorption spikes in a Sc2O3 film and characterization of laser-induced ripple pattern in a HfO2 film.