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In this work we report an experimental investigation of subsurface damage (SSD) in conventionally polished fused silica (FS), sapphire substrates, and YAG crystals which are widely used in laser applications and directly influence performances of critical ultrahigh intensity and high average power laser system optics. Two surface treatment procedures were tested: 1 – plasma treatment, 2 – chemical treatment. Plasma and chemical treatments were applied to fused silica substrates. The laser induced damage threshold (LIDT) FS substrates were studied as the function of etching depth.

Metal oxides, Ta 2 O 5 , HfO 2 and SiO 2 nanoceramics are used in photonic structures as they are transparent at wavelength between 0.3-5 µm. Extremely high optical quality thin films by ion beam sputtering are demonstrated for applications in multilayer interference coating structures.

Laser damage resistance of a transparent optical component significantly depends on its substrate surface quality. In this work low energy oxygen plasma etching of fused silica (FS) substrates was investigated in terms of plasma ion energy and etching depth. Laser induced damage threshold (LIDT) and surface roughness of experimental samples were measured and compared. The LIDT of plasma treated uncoated FS substrate increased from 5.0 ± 0.3 J/cm2 up to 75 ± 4 J/cm2 for 355 nm laser ns pulses without any surface roughness deterioration. Anti-reflective (AR) optical interference coating for 355 nm wavelength was deposited on plasma etched and non-etched substrates. Measured LIDT of plasma etched and AR coated component was 3.4 times higher comparing to coated non-etched substrate case and reached 14 ± 0.7 J/cm2. These results demonstrate successful application of low energy oxygen plasma etching for improving laser induced damage performance of uncoated and AR coated fused silica substrate without deterioration of surface roughness. This application could be also successfully extended for production of other transparent high power UV laser optics like polarizers, beam splitters, etc.

Scandium oxide films were deposited on fused silica substrates by reactive pulsed DC magnetron sputtering. The use of feed-back optical emission monitoring enabled high-rate reactive deposition of films with tunable stoichiometry and properties. The under-stoichiometric, stoichiometric and over-stoichiometric scandium oxide films were prepared. The compressive stress in films was between 235 and 530 MPa. We showed that phase structure, density, surface roughness and optical properties of the scandium oxide are affected by the film stoichiometry and deposition conditions. Transparent scandium oxide films were slightly hydrophobic (94 ± 3°), homogeneous with a crystallite size of 20 ± 5 nm. The lowest extinction coefficient 0.7 × 10 −3 , the highest refractive index 2.08 (both quantities at the wavelength of 355 nm) and the highest density 4.1 ± 0.1 g cm −3 exhibited film prepared with the stoichiometric composition. Stoichiometric scandium oxide can be used in various optical applications as high refractive index and wide bandgap material. Transitions to under- or over-stoichiometry lead to a decrease of film density, refractive index and increase of the extinction coefficient.

Diffraction gratings that do not reach >99% diffraction efficiency (DE) are a major source of losses in chirped pulse amplification systems in current ultrashort pulse lasers. Here, we suggest an optimization route for diffraction gratings based on a commercially available, 1064 nm laser line high reflectivity (HR) multilayer-dielectric mirror. We modeled optical response of 1700 lines/mm gratings in Littrow configuration imposed on this typical quarter wavelength-based stack structure. We demonstrate that theoretically DE of 99.995% at a single wavelength and >99% in 1003–1034 nm wavelength range can be achieved using the original dielectric mirror layer setup. The high DE spectral range could be broadened up to 992–1060 nm while still maintaining >99% DE, when the dielectric stack with a thinner topmost layer is considered. In both cases the diffraction grating grooves must span the two topmost layers, including a higher refractive index material, i.e. Nb2O5. Alternatively, >99% DE in 1009–1033 nm wavelength range can be also obtained using a thicker topmost layer. In this case, the grating groove spanning just the SiO2 layer is sufficient. However, twice as deep grooves and an accurate control of the filing factor is required. We show through numerical simulations that this structure is more sensitive to angular positioning error. The modelled electric field distributions showed reduced laser damage probability in the latter case. All investigated structures can sustain the bandwidth of sub-100 fs pulse length laser pulses. The proposed optimization route can serve as a guideline to design any diffraction grating based on commercially available dielectric mirrors.

This paper investigates niobia/silica mixed oxide thin films deposited by reactive pulse-DC/RF magnetron co-sputtering of Nb and Si metal targets at room temperature. The reactive gas flow during the sputtering processes was either controlled by direct mass flow rate (i.e. constant reactive gas flow) or by an active feedback process control system. 61% and 137% higher deposition rates of Nb2O5 and SiO2 layers, respectively, were obtained using the latter technique as compared to constant reactive gas flow. Films exhibited bulk or near-bulk density. All mixture films produced in this study had an amorphous structure. A volume law of mixtures was used to determine the coating composition. It is shown that the fraction of SiO2 or/and Nb2O5 has a linear dependency on sputter target power density. On this basis, rugate filter coating designs can be easily deposited, where refractive index gradually varies between that of pure Nb2O5 and pure SiO2. Substantially less inhomogeneity of coating mixtures was found in films produced using a reactive sputtering process with feedback-control.

Target voltage ( V T ) feedback-based closed-loop gas flow control is one of the most cost-effective and well-known methods to regulate reactive oxide thin film sputtering processes. However, it can be difficult to perform due to target voltage non-linearity, changes in signal direction and drifts in the case of Zr and Hf sputtering in an Ar/O 2 ambient. We show in this paper that the use of the ratio of V T over the O* (oxygen plasma emission, or alternatively oxygen partial pressure) as a feedback signal in a closed-loop control, enables stable operation anywhere in the ‘transition’ region (the region between ‘metal’ and ‘fully poisoned’ sputtering target states) as well as extending into the ‘forbidden’ parts of it. The result of using V T over the O* ratio as the main feedback signal is a stable process exhibiting constant sputter target state and partial pressure of oxygen as well as high deposition rate (e.g. 6.4 A/s for ZrO 2 and 9.2 A/s for HfO 2 at D T–S = 20 cm by pulse-DC sputtering). Such a process is able to deliver ZrO 2 and HfO 2 coatings that exhibit a low extinction coefficient in the near UV (0.0002 for HfO 2 and 0.001 for ZrO 2 at 350 nm) and a high refractive index (2.05 for HfO 2 and 2.16 for ZrO 2 at 1000 nm), without any ion/plasma assistance, at low substrate temperature (20–50 °C) and no nitrogen addition.

ZrO2 exhibits low optical absorption in the near-UV range and is one of the highest laser-induced damage threshold (LIDT) materials; it is, therefore, very attractive for laser optics applications. This paper reports explorations of reactive sputtering technology for deposition of ZrO2 films with low extinction coefficient k values in the UV spectrum region at low substrate temperature. A high deposition rate (64 % of the pure metal rate) process is obtained by employing active feedback reactive gas control which creates a stable and repeatable deposition processes in the transition region. Substrate heating at 200 °C was found to have no significant effect on the optical ZrO2 film properties. The addition of nitrogen to a closed-loop controlled process was found to have mostly negative effects in terms of deposition rate and optical properties. Open-loop O2 gas-regulated ZrO2 film deposition is slow and requires elevated (200 °C) substrate temperature or post-deposition annealing to reduce absorption losses. Refractive indices of the films were distributed in the range n = 2.05–2.20 at 1,000 nm and extinction coefficients were in the range k = 0.6 × 10−4 and 4.8 × 10−3 at 350 nm. X-ray diffraction analysis showed crystalline ZrO2 films consisted of monoclinic + tetragonal phases when produced in Ar/O2 atmosphere and monoclinic + rhombohedral or a single rhombohedral phase when produced in Ar/O2 + N2. Optical and physical properties of the ZrO2 layers produced in this study are suitable for high-power laser applications in the near-UV range.

Atomic layer deposition was used to grow HfO2 thin films at different process temperatures. Optical constants and laser induced damage thresholds were determined and compared with thin films deposited by ion beam sputtering technology.

The nonlinear LBO (LiB3O5) crystal is widely used in many Nd : YAG, Yb : KGW and Nd : YLF lasers in order to generate higher optical harmonics. However, the most limiting factor in such applications is the optical resistance of their coated surfaces described by the so-called Laser Induced Damage Threshold (LIDT) parameter. In this work we investigate the “fatigue” (multi-pulse) behaviour of LIDTs in LBO crystals coated with different types of (single AR@355 nm and triple AR@355+532+1064 nm wavelength) anti-reflective coatings while optimising the refractive index designs and selecting appropriate sub-layer materials. All the coatings were produced of different oxide materials (ZrO2, Al2O3, SiO2) and ZrO2-SiO2 mixtures by using the ion beam sputtering (IBS) deposition technique. The optical damage resistance of both fixed and transient refractive index coatings is of special interest. Besides the spectral properties, the resistance to laser irradiation is characterised at the wavelength of 355 nm with laser pulses of nanosecond duration. The conclusions are drawn about the AR coated LBO with the most successful designs by the means of optical resistance.

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.

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.

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 HfO 2 and SiO 2 coatings. Monolayer thin films containing different fractions of HfO 2 are deposited with ion beam sputtering technology (IBS.) All samples are post annealed at different temperatur es and optimal regimes are defined. Re fractive 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 for ce microscopy (AFM). Microstr uctural changes are identified from changes in surface topography. Furt her, optical resistance was characterized by 5.7 ns duratio n 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 HfO

Optical and physical properties of magnetron sputtered Nb2O5-SiO2 mixtures are reported. Deposition rate and properties of mixtures produced were improved significantly by using fast feedback reactive sputtering process control.

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 de position techniques are exam ined 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 SiO 2 monolayer by Ion Beam Sputte ring (IBS) technique. Su rface 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 characteri zed 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. Keywords: laser damage threshold, total integrated scattering, polishing, cleaning, etching, coating, ion-beam sputtering.

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.

Commercially polished fused silica substrates were etched and their properties were investigated. Resistance to laser radiation was significantly enhanced and no degradation of surface roughness, flatness or loss in transmission spectra was observed.