Julius Mirauskas

Femtosecond laser damage resistance of oxide and mixture oxide optical coatings

We report on the laser damage resistance of ion beam-sputtered oxide materials (Al2O3, Nb2O5, HfO2, SiO2, Ta2O5, ZrO2) and mixtures of Al2O3-SiO2, Nb2O5-SiO2, HfO2-SiO2, Ta2O5-SiO2, and ZrO2-SiO2, irradiated by single 500 fs pulses at 1030 nm. Laser-induced damage threshold (LIDT), refractive index, and bandgaps of the single-layer coatings are measured. For pure oxide materials a linear evolution of the LIDT with bandgap is observed. The results are in accordance with our simulations based on photo-ionization and avalanche-ionization. In the case of mixtures, however, deviations from the previous behaviors are evidenced. The evolution of the LIDT as a function of the refractive index is analyzed, and an empirical description of the relation between refractive index and LIDT is proposed.

Characterization of zirconia– and niobia–silica mixture coatings produced by ion-beam sputtering

ZrO2-SiO2 and Nb2O5-SiO2 mixture coatings as well as those of pure zirconia (ZrO2), niobia (Nb2O5), and silica (SiO2) deposited by ion-beam sputtering were investigated. Refractive-index dispersions, bandgaps, and volumetric fractions of materials in mixed coatings were analyzed from spectrophotometric data. Optical scattering, surface roughness, nanostructure, and optical resistance were also studied. Zirconia-silica mixtures experience the transition from crystalline to amorphous phase by increasing the content of SiO2. This also results in reduced surface roughness. All niobia and silica coatings and their mixtures were amorphous. The obtained laser-induced damage thresholds in the subpicosecond range also correlates with respect to the silica content in both zirconia- and niobia-silica mixtures.

Laser-induced damage of hafnia coatings as a function of pulse duration in the femtosecond to nanosecond range

Laser-damage thresholds and morphologies of hafnia single layers exposed under femtosecond, picosecond, and nanosecond single pulses (1030/1064 nm) are reported. The samples were made with different deposition parameters in order to study how the damage behavior of the samples evolves with the pulse duration and how it is linked to the deposition process. In the femtosecond to picosecond regime, the scaling law of the laser-induced damage threshold as a function of pulse duration is in good agreement with the models of photo and avalanche ionization based on the rate equation for free electron generation. However, differences in the damage morphologies between samples are shown. No correlation between the nanosecond and femtosecond/picosecond laser-damage resistance of hafnia coatings could be established. We also report evidence of the transition in damage mechanisms for hafnia, from an ablation process linked to intrinsic properties of the material to a defect-induced process, that exists between a few picoseconds and a few tens of picoseconds.

Transient interference implications on the subpicosecond laser damage of multidielectrics

Laser-induced damage in optical thin films with subpicosecond pulses is investigated. A model dedicated to optical interference coatings and based on the rate equation for free electron generation is introduced. It takes into account the transient interference effects induced by changes in the dielectric function during the laser pulse and its feedback effect on the electron density distribution in the multilayer stack. Simulations are compared to experiments on HfO2 and Ta2O5 films with pulses ranging from 45 fs to 1 ps. It is shown that this approach can improve the interpretation of femtosecond and picosecond laser induced damage in thin films.

Adaptive laser-induced damage detection

Laser-Induced Damage Threshold (LIDT) measurements are typically performed in order to characterize the optical resistance of laser components. However the sensitivity of online damage detection techniques is often limiting factor for the accuracy and reproducibility of so called S-on-1 and 1-on-1 measurements. In fact the sensitivity of damage detection has the biggest impact to precision of these experiments. In this paper we describe the idea of making improvements on scattered light registration based damage detection. It was learned, that scattered light intensity is linearly proportional to incident energy of laser pulses while material is not damaged. However in case of induced damage the linear proportionality becomes nonlinear: this feature is used in order to detect optically induced surface and bulk changes in material. According to the base of those theoretical considerations the adaptive scattering detector was proposed and made up. It was put into practice by projecting it on discrete element schematics. The improved sensitivity of laserinduced damage detection was reached. This technique helps to avoid degradation of damaged site and pollution of surrounding area due to laser ablation during the S-on-1 tests since it allows blocking the repetitive irradiation immediately when damage appears. Numerous tests were made, that shows, that adaptive scattering detector can precisely detect damages in their initiation state, independently from material ability of light scattering. Calibration of this detector can be automated, therefore the influence of human factor is minimized. This fact opens up the possibility to run whole damage threshold measurement procedure automatically.

Automated test station for characterization of optical resistance with ultrashort pulses at multi kilohertz repetition rates

In this paper new laser-induced damage threshold testing system operating in broad range of pulse repetition rates (from 0.02 Hz up to 200 kHz) is introduced. The system is capable to test either bare or coated optical components, used for high average and peak power femtosecond laser applications. Pulses of tunable duration (300 - 5000 fs) from diode pumped Yb:KGW solid state laser are employed at fundamental wavelength (1030 nm) and its II-IV harmonics (515 nm, 343 nm and 258 nm). Thanks to advanced adaptive damage detection technique so called S-on-1 tests are performed with single shot resolution. The capabilities of the system were characterized and demonstrated on niobia and zirconia - single layer dielectric coatings at different repetition rates.

Study of the laser matter interaction in the femtosecond regime: application to the analysis of the laser damage of optical thin films

We report an experimental investigation in the laser-induced damage threshold (LIDT) of optical coatings materials. The samples are single layers of Al2O3, Nb2O5, HfO2, SiO2, Ta2O5, ZrO2 deposited through different deposition techniques (evaporation or sputtering with/without ion assistance) and mixtures of Al2O3/SiO2, Nb2O5/SiO2, HfO2/SiO2, Ta2O5/SiO2 and ZrO2/SiO2 on silica substrates. The LIDT is measured at 1030nm, 500fs in single shot mode. The results are expressed and compared in term of LIDT as a function of bandgap and LIDT as a function of refractive index.

Complex study of zirconia-silica and niobia-silica composite coatings produced by ion beam sputtering

In this study, we report on our recent progress in research of single layer mixed zirconia-silica and niobia-silica composite coatings prepared by Ion Beam Sputtering technique. All coatings of the same optical thickness were characterized in terms of reflection/transmission spectrometry, X-ray diffraction, atomic force and optical microscopy, optical back-scattering and optical resistance (laser-induced damage threshold - LIDT) in subpicosecond mode. The optical resistance, TIS and LIDT results reveal clear dependence on high refractive index material content in composite coating and its crystalline structure. The results are interpreted and discussed by the means of different models available in literature.

Investigation of laser damage in single layer coatings with pulse durations from 45fs to 24ps

Single shot LIDT of single layer coatings of different deposited materials (SiO2, HfO2, Ta2O5 and Nb2O5) have been studied. We report dependence of the damage threshold with different operational and material parameters (pulse duration, nature of the deposited material, deposition process or thickness of the layer). For interpretation a model dedicated to optical coatings and based on the conduction band electron rate equation is used. The simulations are compared to experiments. The theoretical approach is in good accordance to the experimental data.

Laser conditioning of high reflectivity mirrors used in OPOs by 266 and 355 nm nanosecond pulses

Experimental investigation of the laser conditioning efficiency by nanosecond pulses at 266 and 355 nm in high reflectivity mirrors used in optical parametric oscillators (OPOs) is present in this report. The high reflection coatings were deposited on the fused silica substrates. The materials used for e-beam coating deposition were ZrO2 and SiO2. Laser conditioning was investigated as function of number of pulses, wavelength and conditioning protocol. Ramped-fluence pre-exposure was used as a method to explore optimal improvement to the damage performance at 266 and 355 nm. No conditioning effect was observed using nanosecond pulses at 266 nm, but the mirror conditioning with 355 nm pulses increased the laser-induced damage threshold (LIDT) by 2.5-3 times. The experimental results support the concept that the laser conditioning effect observed in high quality optical thin films is associated with intrinsic electronic defects in the films.