Duy N. Nguyen

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.

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.

Third harmonic microscopy of intrinsic and induced material anisotropy in dielectric thin films

Abstract. Third harmonic (TH) microscopy with circularly polarized illumination directly reveals material anisotropy owing to suppression of background optical signals from isotropic media. Because optical thin films and their substrates are expected to be highly isotropic, TH microscopy presents a path to study induced and intrinsic anisotropy in films, providing both insight into laser-induced material modification that precedes damage and feedback about the deposition process. Because nanoscale defects and material strain influence the damage behavior of films, we examined TH sensitivity to similar sources of contrast. We demonstrate imaging of individual 10 nm colloidal gold nanoparticles and 100 mN nanoindentations in fused silica both with signal-to-noise ratio (SNR)≥100∶1. We present TH images (SNR≥210∶1) of sites exposed to femtosecond laser pulses below damage in 100 nm HfO2 films that are barely visible (SNR≤2.3∶1) with Nomarski and polarization imaging, traditional microscopic techniques known to display contrast for material anisotropy. At our detection limit (320 mW, 50 fs, 790 nm, ≈106 photomultiplier tube gain), we examined root mean square in the TH image of nascent films that correlated to the film’s macrostrain. TH microscopy presents a relatively simple all-optical method to monitor nanoscale anisotropy in thin films during exposure to high-intensity radiation and during deposition.

The Effect of Annealing on the Subpicosecond Breakdown Behavior of Hafnia Films

Subpicosecond laser induced breakdown of dielectric films has gained a great deal of attention in laser nano- and micromachining and in the development of optical coatings for the next generation of high-power ultrafast laser system. The understanding of the fundamental processes affecting the breakdown behavior and how they depend on the material properties and the film deposition is highly desirable for improving the coating performance. In the present work we compare the single and multiple pulse damage behavior of as-grown and annealed HfO2 films. Annealing can reduce the film absorption near the band edge but its impact on the single and multiple femtosecond pulse damage behavior remained open. Damage measurements with pairs of pulses of variable subpicosecond delay in bulk fused silica revealed a partial recovery toward single pulse behavior on a few hundred fs time scale. We investigate if such behavior also occurs in hafnia films and identify the time scale for a full recovery. Our experimental results are compared with existing theoretical models[1], which allows us to suggest microscopic changes that occur during the annealing process.

Femtosecond pulse S on 1 LIDT in dielectric materials: comparison of experiment and theory

The multiple-pulse laser-induced breakdown behavior of dielectrics is modeled. The model 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 for the occupation and ionization of band and midgap 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 conduction band. The effect of native and laser-induced defects can be distinguished by their saturation behavior. The model explains the principal behavior of the LIDT of a pair of pulses as a function of the temporal separation. Using the model, the observed transients can be related to rate constants of electrons leaving the CB and midgap states.

Fundamental processes controlling the single and multiple femtosecond pulse damage behavior of dielectric oxide films

In this contribution we will summarize the fundamental mechanisms that lead to subpicosecond laser damage in dielectric films, discuss the resulting scaling laws of single pulse (1-on-1) damage with respect to pulse duration and bandgap, of the multiple pulse (S-on-1) damage threshold as a function of pulse number, and compare these findings to recent experimental results.

Experimental and theoretical studies of subpicosecond laser damage in Ti x Si 1-x O 2 composite films

The scaling law of subpicosecond laser induced damage (LID) with respect to pulse duration and band gap for TixSi1-xO2 composite films is studied. The band gap in these materials can be changed gradually by varying the composition pa-rameter x. Damage is very deterministic and scaling laws with respect to pulse duration and band gap energy derived previously for pure materials are found to apply to composite films. The scaling can be explained theoretically by using a modified Keldysh theory. The composite materials also show a dependence of the damage threshold as a function of pulse number F(N) (incubation) that is similar to observations in pure dielectric oxides. The measured F(N) is explained with a theoretical model that assumes the formation of an intermediate sample state that increases the absorption of sub-sequent pulses in the train.

Scandium Oxide Thin Films Deposited by Dual Ion Beam Sputtering for High-Power Laser Applications

Scandium oxide films were deposited using reactive dual ion beam puttering. At 1 micron, the refractive index of the films is 1.95 and the absorption loss is 18.5 ppm. X-ray photoelectron spectroscopy showed oxygen defects.