Gintarė Batavičiūtė

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...

Characterization of photopolymers used in laser 3D micro/nanolithography by means of laser-induced damage threshold (LIDT)

An ISO certified laser-induced damage threshold testing method was applied to characterize photopolymers widely used in 3D laser micro/nano-lithography. For the first time, commercial as well as custom made materials, including epoxy based photoresist (SU-8), hybrid organic-inorganic polymers (OrmoComp and SZ2080), thermopolymer (PDMS) and pure acrylate (PMMA), are investigated and directly compared. The presence of photoinitiator molecules within host matrix clearly indicating the relation between damage threshold and absorption of light is revealed. To simulate single- and multiphoton absorption processes optical resistance measurements were carried out at both fundamental (1064 and 1030 nm) and second harmonic (532 and 515 nm) wavelengths with laser pulse duration’s representing nanosecond and femtosecond regimes. Damage morphology differences from post mortal microscopic analysis were used to enrich the discussion about the possible breakdown mechanisms. The obtained characteristic values of damage threshold reveal potential of photopolymers and their possible applications in high power lasers.

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.

Direct comparison of defect ensembles extracted from damage probability and raster scan measurements

The presented study addresses the characterization of nanometer sized defects acting as damage precursors in nanosecond laser pulse duration regime. Two approaches are used to extract distributions of localized damage precursors, namely, damage probability and damage density measurements. Testing is performed on uncoated and SiO2 monolayer film deposited fused silica substrate exposed with pulsed UV irradiation (355 nm, 4.8 ns). Then, a direct comparison of damage precursor ensembles obtained from both methods is carried out. Our analysis indicates apparent differences between both methods that are discussed in detail. Contamination by ablation products is identified as one of the key factors that influence damage density measurements.

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.

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.

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.

Characterization and application of HfO2 - SiO2 mixtures produced by ion-beam sputtering technology

In the past years the usage of mixed oxides coatings lead to an important improvement of laser damage threshold and quality of optical elements. In this study influence of post treatment procedure - ex-situ annealing - is examined in terms of quality, optical constants and laser induced damage threshold (LIDT) of mixed HfO2 and SiO2 coatings. Monolayer thin films containing different fractions of HfO2 are deposited with ion beam sputtering technology (IBS.) All samples are post annealed at different temperatures and optimal regimes are defined. Refractive index and absorption coefficient dispersion is evaluated from transmission spectra measurements. Surface roughness of all samples is characterized before and after deposition and annealing, using atomic force microscopy (AFM). Microstructural changes are identified from changes in surface topography. Further, optical resistance was characterized by 5.7 ns duration pulses for 355 nm wavelength laser radiation, performing 1-on-1 sample exposure tests with high resolution micro-focusing approach for monolayer samples and S-on-1 tests for multilayer reflectors. Morphology of damaged sites was analyzed through optical microscopy. Finally, conclusions about annealing effect for mixed HfO2 and SiO2 monolayer and multilayer coatings are made.

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.

Enhancement of laser-induced damage threshold in chirped mirrors by electric field reallocation

In this paper, the relation between the laser-induced damage threshold (LIDT) and the electric field intensity (EFI) distribution inside a CM is investigated experimentally. We show that it is possible to increase the LIDT values by slightly modifying the electric field of a standing wave distribution without loss of spectral and dispersion performance. Suggested CM design improvement could increase reliability and LIDT performance of both CM elements and high-power systems they are used in.