E. Krous

Large area high efficiency broad bandwidth 800 nm dielectric gratings for high energy laser pulse compression.

We have demonstrated broad bandwidth large area (229 mm × 114 mm) multilayer dielectric diffraction gratings for the efficient compression of high energy 800 nm laser pulses at high average power.

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.

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

Influence of process conditions on the optical properties of HfO2/SiO2 coatings for high-power laser coatings

We investigate the variations that occur with changes in the number of layers and with the use of the assist beam main and assist beam energy on the morphology of HfO2/SiO2 quarter wave stacks deposited by dual ion beam sputtering. We show how the addition of sequential HfO2/SiO2 bilayers, up to eight, affects the surface roughness and micro-crystallinity of the top HfO2 layer. We also show that use of the assist source significantly smooths the surface while simultaneously reducing microcrystallinity. The HfO2/SiO2 structures are very robust and can withstand fluences in excess of 3 J/cm2 generated by 1ps pulses from a chirped amplified Ti:Sapphire laser.

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.

What role do defects play in the laser damage behavior of metal oxides

We have investigated the role of native point defects in the laser damage behavior of amorphous thin films of Sc2O3 deposited by ion beam sputtering. Through systematic characterization and detailed modeling we show that native defects influence the 1-on-1 laser induced damage threshold fluence of the Sc2O3. This effect, as shown by the model and confirmed by experiments, is pulse duration dependent.

Assessing the impact of atomic oxygen in the damage threshold and stress of Hafnia films grown by ion beam sputter deposition

Hafnium oxide (HfO2) is undoubtedly one of the most desirable high-index optical coatings for high power laser applications. One of the key goals in the fabrication of oxide films with high Laser Induced Damage Threshold (LIDT) is to minimize the number of film imperfections, in particular stoichiometric defects. For HfO2 films deposited by ion beam (reactive) sputtering (IBS) of a hafnium metal target, stoichiometry is controlled by the injection of molecular oxygen, either close to the substrate or mixed with the sputtering gas or some other combination. Good stoichiometry is important to reduce the density of unoxidized particles buried in the coatings, which affect the LIDT. This work evaluates the potential advantages of using pre-activation of oxygen in the IBS of HfO2, with special emphasis on its impact on LIDT and film stress. For the experiments, oxygen was activated by an independent plasma source and then introduced into a commercial IBS chamber. The optical properties of the films were characterized using spectrophotometry and ellipsometry. Their structural quality and composition were determined from x-ray diffraction and x-ray photoelectron emission spectroscopy. The stress was determined from interferometer measurements. For optimized conditions, 2.5 J/cm2 LIDT was measured on HfO2 films at λ=800 nm with 1 ps and 25 mJ pulses from a chirped amplification Ti:Sapphire laser. In the range of oxygen variations under consideration the effects on LIDT are shown to be minimal.

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.

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.