Ramutis Drazdys

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.

Flat Focusing Mirror

The control of spatial propagation properties of narrow light beams such as divergence, focusing or imaging are main objectives in optics and photonics. In this letter, we propose and demonstrate experimentally a flat focusing mirror, based on an especially designed dielectric structure without any optical axis. More generally, it also enables imaging any light pattern in reflection. The flat focusing mirror with a transversal invariance can largely increase the applicability of structured photonic materials for light beam propagation control in small-dimension photonic circuits.

Next generation highly resistant mirrors featuring all-silica layers

A principal possibility to overcome fundamental (intrinsic) limit of pure optical materials laser light resistance is investigated by designing artificial materials with desired optical properties. We explore the suitability of high band-gap ultra-low refractive index material (n less than 1.38 at 550 nm) in the context of highly reflective coatings with enhanced optical resistance. The new generation all-silica (porous/nonporous) SiO2 thin film mirror with 99% reflectivity was prepared by glancing angle deposition (GLAD). Its damage performance was directly compared with state of the art hafnia/silica coating produced by Ion-Beam-Sputtering. Laser-Induced Damage Thresholds (LIDT) of both coatings were measured in nanosecond regime at 355 nm wavelength. Novel approach indicates the potential for coating to withstand laser fluence of at least 65 J/cm2 without reaching intrinsic threshold value. Reported concept can be expanded to virtually any design thus opening a new way of next generation thin film production well suited for high power laser applications.

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.

Argon plasma etching of fused silica substrates for manufacturing high laser damage resistance optical interference coatings

The laser damage resistance of an optical element in high power laser systems depends significantly on the surface quality of the optical substrate. In this experiment, commercially polished fused silica substrates were etched in argon plasma generated by a RF source and their surface roughness, flatness and optical properties were investigated. This method can be applied in a vacuum chamber prior to deposition of the multilayer coatings without breaking the vacuum. It was shown that by etching the resistance to 355 nm, laser radiation could be improved more than 8 times. However, it strongly related with primary substrate quality. The etching depth from 100 nm suggests the optimum performance of surface quality in terms of surface low roughness, high flatness, and high laser damage threshold. These results are of significant importance for the manufacture of high quality laser optics on fused silica substrates. As an example of an application of our technology, anti-reflective and polarizing optical interference coatings were deposited on etched substrates and the increase of their resistance to laser radiation was measured.

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.

Investigation in oxide mixture coatings with adapted gradient index profiles

The stability of thin film coatings for applications especially in the UV spectral range is oftentimes a limiting factor in the further development of radiation sources and beam delivery systems. Particularly, functional coatings on laser and conversions crystals as well as resonator mirrors show an insufficient lifetime due to laser-induced degradation. Previous investigations in the power handling capability of UV coatings mostly concentrate on the properties of pure oxide materials and particle mitigation. Recent innovations in ion beam sputtering technology enabled efficient deposition of mixture coatings of different oxide materials. In combination with an advanced thickness monitoring equipment, the described IBS deposition systems are capable of employing designs with sub-layers of a few nm thickness. In the present investigation, the stability of classical designs using pure oxide materials is compared with gradient index design concepts based on mixture materials. Reflecting and transmitting thin film coatings employing classical and gradient index approaches manufactured under comparable conditions are characterized in respect to their power handling capability. The results are analyzed before the background of theoretical expectations regarding contributions from field enhancement and absorptance effects.

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.