Luke A. Emmert

Modeling the effect of native and laser-induced states on the dielectric breakdown of wide band gap optical materials by multiple subpicosecond laser pulses

A model for the multiple-pulse laser-induced breakdown behavior of dielectrics is presented. It is based on a critical conduction band (CB) electron density leading to dielectric breakdown. The evolution of the CB electron density during the pulse train is calculated using rate equations involving transitions between band and mid-gap states (native and laser-induced). Using realistic estimations for the trap density and ionization cross-section, the model is able to reproduce the experimentally observed drop in the multiple-pulse damage threshold relative to the single-pulse value, as long as the CB electron density is controlled primarily by avalanche ionization seeded by multiphoton ionization of the traps and the valence band. The model shows that at long pulse duration, the breakdown threshold becomes more sensitive to presence of traps close (within one photon energy) to the CB. The effect of native and laser-induced defects can be distinguished by their saturation behavior. Finally, measurements of ...

TixSi1−xO2 optical coatings with tunable index and their response to intense subpicosecond laser pulse irradiation

Ion-beam sputtered TixSi1−xO2 binary-oxide films of high optical quality with tunable bandgap and refractive index were produced using zone targets. The suitability of the films for high-power subpicosecond laser applications is explored by laser breakdown measurements. The observed scaling laws of the single-pulse breakdown threshold—a power law with respect to pulse duration and a linear law with respect to bandgap energy—are similar to results obtained with high-quality simple oxides. The single- and multiple-pulse breakdown behaviors of these binary films indicate only slightly larger defect densities than found in simple oxides.

Investigations on SiO2/HfO2 mixtures for nanosecond and femtosecond pulses

After several investigations in laser induced damage behavior of oxide mixtures of different compositions, also HfO2 could be steplessly mixed with SiO2. A study of SiO2/HfO2 IBS single layers and high reflectors is presented. Damage testing has been performed at 800nm and 355nm on an extensive set of single layers employing different mixture ratios of silica and hafnia. The analysis of the response of optical single layer coatings to femtosecond and nanosecond pulse exposure provides input for further coating designs, in particular for the optimization in respect to the damage threshold properties. A deeper understanding of the damage mechanisms is gained by comparing the ns and fs pulse results as a function of the mixing ratio.

Point defects in Sc 2 O 3 thin films by ion beam sputtering

We show that the concentration of oxygen interstitials trapped in Sc2O3 films by ion beam sputtering from metal targets can be controlled by modifying deposition conditions. We have identified point defects in the form of oxygen interstitials that are present in Sc2O3 films, in significantly high concentrations, i.e., ∼10(18)  cm(-3). These results show a correlation between the increase of oxygen interstitials and the increase in stress and optical absorption in the films. Sc2O3 films with the lowest stress and optical absorption loss at 1 μm wavelength were obtained when using a low oxygen partial pressure and low beam voltage.

Comparative study of femtosecond and nanosecond laser-induced breakdown spectroscopy of depleted uranium

We present spectra of depleted uranium metal from laser plasmas generated by nanosecond Nd:YAG (1064 nm) and femtosecond Ti:sapphire (800 nm) laser pulses. The latter pulses produce short-lived and relatively cool plasmas in comparison to the longer pulses, and the spectra of neutral uranium atoms appear immediately after excitation. Evidence for nonequilibrium excitation with femtosecond pulses is found in the dependence of spectral line intensities on the pulse chirp.

Spatio-TEmporally REsolved Optical Laser Induced Damage (STEREO LID) technique for material characterization

A technique for measuring the ablation and laser-induced damage threshold (LIDT) by identifying the temporal onset of damage and location of initiation within the beam profile is demonstrated. This new method, dubbed Spatio-TEmporally REsolved Optical Laser Induced Damage (STEREO LID), is compared to traditional damage tests and its advantages are exemplified.

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.

Studies of femtosecond laser induced damage of HfO2 thin film in atmospheric and vacuum environments

The single pulse femtosecond laser induced damage threshold (LIDT) of hafnia and silica films is not affected by the ambient gas pressure. In vacuum, the multiple pulse LIDT drops to ~10% (~10%) of its atmospheric value for hafnia (silica). The water vapor content of the ambient gas was found to control the change in the LIDT. The LIDT of bulk fused silica surfaces did now show any dependence on the ambient gas pressure. Hydrocarbons (toluene) did not change the multiple pulse LIDT for Hafnia films

Advances in ion beam sputtered Sc2O3 for optical interference coatings

Scandium oxide is an attractive candidate for the engineering of interference coatings, although not widely explored. This paper describes the ion beam sputtering of Sc2O3. It is shown that the structural properties of the material are affected by the deposition conditions. Laser damage in different regimes of pulsewidths is investigated. These results show that the 1-on-1 laser damage fluence, in both the thermal and deterministic regimes, varies with deposition conditions but this is not the case for S-on-1, indicating that laser-induced defects are important.

The role of native and photoinduced defects in the multi-pulse subpicosecond damage behavior of oxide films

A model for wide bandgap materials was developed to study the breakdown behavior under multiple subpicosecond laser pulse illumination. While this model has been applied to the study oxide materials, it is general enough to be used with any wide bandgap material. The model distinguishes two types of midgap trapping states - shallow and deep traps (defects), which can be native or laser induced. Excitation of these midgap states enhances the seed for the avalanche ionization process that causes breakdown, lowering the damage fluence for pulses later in the train. A set of rate equations for the conduction band electron density and population dynamics of the trap states was solved numerically to predict the damage threshold as a function of pulse number F(M). The effect of trap level parameters such as density, absorption cross-section, and the initial population on the shape of F(M) is discussed. Comparison is made to experimental data for oxide thin films.