Egidijus Pupka

Wavelength dependence of femtosecond laser-induced damage threshold of optical materials

An experimental and numerical study of the laser-induced damage of the surface of optical material in the femtosecond regime is presented. The objective of this work is to investigate the different processes involved as a function of the ratio of photon to bandgap energies and compare the results to models based on nonlinear ionization processes. Experimentally, the laser-induced damage threshold of optical materials has been studied in a range of wavelengths from 1030 nm (1.2 eV) to 310 nm (4 eV) with pulse durations of 100 fs with the use of an optical parametric amplifier system. Semi-conductors and dielectrics materials, in bulk or thin film forms, in a range of bandgap from 1 to 10 eV have been tested in order to investigate the scaling of the femtosecond laser damage threshold with the bandgap and photon energy. A model based on the Keldysh photo-ionization theory and the description of impact ionization by a multiple-rate-equation system is used to explain the dependence of laser-breakdown with the...

Sculptured anti-reflection coatings for high power lasers

Achieving higher optical power in UV laser systems is a challenging task due to the limited performance of their built-in optical elements. As a rule of thumb, interference coatings of such elements are found to be the weakest links by the means of laser-induced damage threshold (LIDT). The optical resistance is directly attributed to the fundamental absorption properties of deposited layers. Unfortunately, there are only a limited set of available materials with discrete refractive indices that are also compatible with UV applications. In this study, an attempt is made to employ sculptured layers in order to produce durable anti-reflective (AR) coatings by using the so-called glancing angle deposition (GLAD) method. Spectral, structural, mechanical and stress properties of GLAD coatings were investigated in detail. AR coatings produced by GLAD were found to be three times more laser damage resistant at 355 nm wavelength as compared to those prepared by ion beam sputtering (IBS).

Effect of conventional fused silica preparation and deposition techniques on surface roughness, scattering, and laser damage resistance

Despite the growing improvement in optical polishing and deposition technologies optical resistance of the laser components used for high-power UV applications remains insufficient in many cases. In this study influence of different fused silica substrate preparation, post treatment processing and deposition techniques are examined in terms of surface roughness, optical scattering and laser damage performance. The conventional techniques of polishing, etching, and finally surface cleaning of substrates have been investigated. Further, a part of samples were also coated with SiO2 monolayer by Ion Beam Sputtering (IBS) technique. Surface quality was characterized prior to and after the treatment and deposition processes by the means of total integrated scattering (TIS) and atomic force microscopy (AFM). The experimental results of surface roughness measurements exhibited a good correlation between AFM and TIS methods. Further optical resistance was characterized with 10 ns duration pulses for 355 nm wavelength laser radiation performing 1-on-1 sample exposure test with high resolution micro-focusing approach. A dominating damage precursor ensembles produced during manufacturing processes were identified and directly compared. Finally, the conclusions about the quality influencing factors of investigated processes were drawn.

Investigation of subsurface damage impact on resistance of laser radiation of fused silica substrates

In this work we report an experimental investigation of subsurface damage (SSD) in conventionally polished fused silica (FS) substrates which are widely used in laser applications and directly influence performances of optical elements. Two procedures were developed: 1 - acid etching and 2 - superpolishing. Additionally, surface roughness and total integrated scattering (TIS) measurements were performed to find correlation between the main surface properties and laser induced damage threshold (LIDT) as circumstantial evidence of elimination of SSD. Different durations of acid etching have been used to study LIDT of FS substrates. These experiments revealed that the optimal etching time is ~1 min. for a given acid concentration. Laser induced damage threshold of etched and SiO2 layer coated FS samples increased ~3 times, while of the ones that were not coated - 4 times. It has been revealed that for nonetched surface a single nano- to micro-scale absorbing defect ensemble most likely associated with polishing particles within Beilby layer was dominant, while damage morphology in ~1 min etched FS sample had no point defects observed. More than 5 times lower roughness value (RMS) was obtained by superpolishing procedure using colloidal silica abrasive particles. LIDT of such superpolished fussed silica substrates was also strongly increased and compared with conventional CeO2 abrasive polishing.

Effect of longitudinal laser mode beating in damage probability measurements

In this study influence of temporal effects are investigated within a context of laser-induced damage threshold (LIDT) measurements. 1-on-1 LIDT testing has been performed with laser operating in single- and multilongitudinal mode regimes. Four fused silica samples were chosen for investigation. Qualitative differences in the damage morphology and damage probability curve have been observed. Analysis of these phenomena was performed by employing Monte Carlo simulations representing the statistical interaction between laser irradiation and randomly distributed damage precursors. The results and findings of this study are reported and discussed in detail.

Comprehensive studies of IR to UV light intensification by nodular defects in HfO2/SiO2 multilayer mirrors

Nodular defects tend to limit laser-induced damage threshold (LIDT) of multilayer dielectric coatings frequently used for laser applications. Cross-sections of localized damage morphologies correlate well with light intensifi- cation patterns caused by defect geometries. In vast majority of studies electric field enhancement in nodular defects was investigated for infrared spectral region. In this work theoretical analysis has been extended for IR - UV range. Light intensification in HfO2/SiO2 multilayer mirror coating was studied numerically. The analysis of obtained results indicates that phenomena is very sensitive to almost every investigated parameter. It was also found that field enhancement effect can be reached within distinct material layers (either of low or high refractive index). The discussion and insights complementing existing knowledge on nodular defects were made.

Parametric analysis of damage probability: a tool to identify weak layers within multilayer coatings

The role of defects, inherent to fused silica substrate due to polishing and deposition processes, is interpreted in terms of laser-induced damage probability. Changes of damage threshold behavior are observed in bare substrate, monolayer, and multilayer coatings after irradiation with UV (355 nm) nanosecond laser pulses at different angles of incidence (0° and 45°) and polarizations (s and p). Statistical damage probability models are constructed for experimental data approximation. Effects of light intensification by standing waves within multilayer coatings and localization of the defects (surface, interface, and bulk) are considered as key factors within this work. Polishing defects are shown to be the limiting factor in the case of uncoated fused silica sample, as well as SiO₂ and HfO₂ monolayer coated substrates. The obtained results also suggest that damage threshold of almost identical sublayers constituting highly reflective multilayer HfO₂/SiO₂ coating with central 355 nm wavelength is a function of sublayer depth.

High LIDT mirrors for 355nm wavelength based on combined ion beam sputtering and glancing angle deposition technique

Laser induced damage of optical coatings has been one of the most important targets during many decades of intensive research. Different techniques were used and explored with the aim to increase the resistance of multilayer systems to laser pulses. In this work, LIDT results of different “base” structures made by ion beam sputtering of Al2O3, SiO2 and their mixtures are presented, and further enhancement possibilities are discussed by applying additional layer structure using higher bandgap material – fluorides and glancing angle deposited SiO2.

Next-generation all-silica coatings for UV applications

Band-gap and refractive index are known as fundamental properties determining intrinsic optical resistance of multilayer dielectric coatings. By considering this fact we propose novel approach to manufacturing of interference thin films, based on artificial nano-structures of modulated porosity embedded in high band-gap matrix. Next generation all-silica mirrors were prepared by GLancing Angle Deposition (GLAD) using electron beam evaporation. High reflectivity (HR) was achieved by tailoring the porosity of highly resistant silica material during the thin film deposition process. Furthermore, the proposed approach was also demonstrated to work well in case of anti-reflection (AR) coatings. Conventional HR HfO2 and SiO2 as well as AR Al2O3 and SiO2 multilayers produced by Ion Beam Sputtering (IBS) were used as reference coatings. Damage performance of experimental coatings was also analyzed. All-silica based GLAD approach resulted in significant improvement of intrinsic laser damage resistance properties if compared to conventional coatings. Besides laser damage testing, other characteristics of experimental coatings are analyzed and discussed – reflectance, surface roughness and optical scattering. We believe that reported concept can be expanded to virtually any design of thin film coatings thus opening a new way of next generation highly resistant thin films well suited for high power and UV laser applications.

Toward separation of bulk and interface defects: damage probability analysis of thin film coatings

Nanosecond laser - induced damage threshold (LIDT) of dielectric coatings is limited by absorption of nanometer sized defects inherent to optics manufacturing process. Herewith theoretical and experimental efforts were made in order to characterize internal damage thresholds of defects introduced during substrate polishing and coating deposition processes. For this purpose LIDT testing was performed under UV (355 nm, 4.8 ns) irradiation on three different types of samples by varying irradiation conditions such angle of incident (0°, 45°, 56°) and polarization (s, p). Experimentally obtained damage probability curves were analyzed numerically by employing model considering relative electric field distributions and randomly distributed defect ensembles attributed to distinct manufacturing processes. An attempt is made to identify the layers with the weakest optical resistance.