Stress reduction in ion beam sputtered mixed oxide films

Development of Wide-Angle Short-Wave Pass Thin Film Based on the Ultra-Thin Silicate Glass

With the rapid development of laser medicine, there are higher requirements placed on the performance of optical components in various medical systems. This paper is aimed at exploring the critical optical devices of medical equipment for treating periodontitis and gingivitis. The cathode sputtering method was used to produce the wide-angle short-wave pass filter, and a hundreds grid fastness test was conducted to detect the occurrence of film peeling. Considering the results of SEM, transmission spectrum, and stress test of the sample, an analysis was conducted as to the cause of poor bonding force for the film. By increasing the amount of argon gas and adjusting the baking temperature, the problem of film peeling was resolved. Besides, a short-wave pass filter film with good bonding and low roughness was obtained to meet the requirements of laser medical equipment.

Laser-induced layers peeling of sputtering coatings at 1064 nm wavelength

Large-scale layers peeling after the laser irradiation of dual ion beam sputtering coatings is discovered and a model is established to explain it. The laser damage morphologies relate to the laser fluence, showing thermomechanical coupling failure at low energy and coating layers separation at high energy. High-pressure gradients appear in the interaction between laser and coatings, resulting in large-scale layer separation. A two-step laser damage model including defect-induced damage process and ionized air wave damage process is proposed to explain the two phenomena at different energy. At relatively high energies (higher than 20 J/cm²), ionization of the air can be initiated, leading to a peeling off effect. The peeling effect is related to the thermomechanical properties of the coating materials.

Comparison of damage and ablation dynamics of multilayer dielectric films initiated by few-cycle pulses versus longer femtosecond pulses

The importance of high intensity few- to single-cycle laser pulses for applications such as intense isolated attosecond pulse generation is constantly growing, and with the breakdown of the monochromatic approximation in field ionization models, the few-cycle pulse (FCP) interaction with solids near the damage threshold has ushered a new paradigm of nonperturbative light-matter interaction. In this Letter, we systematically study and contrast how femtosecond laser-induced damage and ablation behaviors of SiO₂/HfO₂-based reflective multilayer dielectric thin film systems vary between FCP and 110 fs pulses. With time-resolved surface microscopy and ex situ analysis, we show that there are distinct differences in the interaction depending on the pulse duration, specifically in the "blister" morphology formation at lower fluences (damage) as well as in the dynamics of debris formation at higher fluences (ablation).

Varying stress of SiOxCy thin films deposited by plasma polymerization

SiO_xC_y thin films were deposited by plasma polymerization. The stress of the deposited SiO_xC_y thin films can be modified by adjusting the beam current, the anode voltage, and the flow rate of hexamethyldisiloxane (HMDSO) gas and oxygen. Reducing the beam current or increasing the flow rate of HMDSO gas increased the linear/cage structure ratio and turned the stress of the SiO_xC_y thin films from compressive to tensile. The linear/cage structure ratio can be adjusted by changing the composite parameter, W[FM]_c/[FM]_m, to control the stress of the deposited plasma polymer films. Multilayers of TiO₂/SiO₂/TiO₂ were coated on a SiO_xC_y plasma polymer film herein, reducing their stress by 70% from 0.06 to 0.018 GPa. The refractive index is 1.55, and the absorption coefficient is less than 10^-4 at 550 nm of the SiO_xC_y films. Superior optical performances of SiO_xC_y thin films make their use in optical thin films.

Reflective coating optimization for interferometric detectors of gravitational waves

Brownian fluctuations in the highly reflective test-mass coatings are the dominant noise source, in a frequency band from a few tens to a few hundreds Hz, for Earth-bound detectors of Gravitational Waves. Minimizing such noise is mandatory to increase the visibility distance of these instruments, and eventually reach their quantum-limited sensitivity. Several strategies exist to achieve this goal. Layer thickness and material properties optimization have been proposed and effectively implemented, and are reviewed in this paper, together with other, so far less well developed, options. The former is the simplest option, yielding a sensible noise reduction with limited technological challenges; the latter is more technologically demanding, but is needed for future (cryogenic) detectors.

Thickness-dependent crystallization on thermal anneal for titania/silica nm-layer composites deposited by ion beam sputter method

Crystallization following thermal annealing of thin film stacks consisting of alternating nm-thick titania/silica layers was investigated. Several prototypes were designed, featuring a different number of titania/silica layer pairs, and different thicknesses (in the range from 4 to 40 nm, for the titania layers), but the same nominal refractive index (2.09) and optical thickness (a quarter of wavelength at 1064 nm). The prototypes were deposited by ion beam sputtering on silicon substrates. All prototypes were found to be amorphous as-deposited. Thermal annealing in air at progressive temperatures was subsequently performed. It was found that the titania layers eventually crystallized forming the anatase phase, while the silica layers remained always amorphous. However, progressively thinner layers exhibited progressively higher threshold temperatures for crystallization onset. Accordingly it can be expected that composites with thinner layers will be able to sustain higher annealing temperatures without crystallizing, and likely yielding better optical and mechanical properties for advanced coatings application. These results open the way to the use of materials like titania and hafnia, that crystallize easily under thermal anneal, but ARE otherwise promising candidate materials for HR coatings necessary for cryogenic 3rd generation laser interferometric gravitational wave detectors.

Optical properties and laser damage threshold of HfO(2)-SiO(2) mixed composite thin films

HfO(2)-SiO(2) mixed composite thin films have been deposited on fused silica substrate by co-evaporation of HfO(2) and SiO(2) through the reactive electron-beam evaporation technique. The composition-dependent refractive index and the absorption coefficient have been analyzed using different effective medium approximation (EMA) models in order to evaluate the suitability of these models for such mixed composite thin films. The discrepancies between experimentally determined and EMA-computed values are explained through microstructural and morphological evolutions observed in these mixed composite films. Finally, the dependence of the laser damage threshold as a function of silica content has been investigated, and the improved laser-induced damage threshold for films having more than 80% silica content has been explained through the defect-assisted multiphoton ionization process.

Laser-induced damage thresholds of bulk and coating optical materials at 1030 nm, 500 fs

We report on extensive femtosecond laser damage threshold measurements of optical materials in both bulk and thin-film form. This study, which is based on published and new data, involved simple oxide and fluoride films, composite films made from a mixture of two dielectric materials, metallic films, and the surfaces of various bulk materials: oxides, fluorides, semiconductors, and ionic crystals. The samples were tested in comparable conditions at 1030 nm, 375 to 600 fs, under single-pulse irradiation. A large number of different samples prepared by different deposition techniques have been tested, involving classical materials used in the fabrication of optical thin film components (Ag, AlF3, Al2O3, HfO2, MgF2, Nb2O5, Pt, Sc2O3, SiO2, Ta2O5, Y2O3, and ZrO2) and their combination with codeposition processes. Their behaviors are compared with the surfaces of bulk materials (Al2O3, BaF2, CaF2, Ge, KBr, LiF, MgF2, NaCl, Quartz, Si, ZnS, ZnSe, and different silica glasses). Tabulated values of results are presented and discussed.

Laser damage resistance of ion-beam sputtered Sc2O3/SiO2 mixture optical coatings

We report on the correlation between the laser damage resistance, the optical and the physical properties of Sc(2)O(3)/SiO(2) mixture coatings. Several sets of samples with ten different mixture ratios have been prepared by ion-beam sputtering. The atomic compositions of the mixture thin films are quantified employing x-ray photoelectron spectroscopy depth profiles. Laser-induced damage thresholds are determined with single subpicosecond pulses (500 fs) at 1030 nm. Furthermore, Son1 multishot measurements are realized in the ultraviolet wavelength range (355 nm) at pulse durations of 5 ns. In addition, the influence of two different substrate polishing qualities on the radiation resistance of the composite thin films is discussed.

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.

Mixed oxide coatings for optics

Material mixtures offer new possibilities for synthesizing coating materials with tailored optical and mechanical properties. We present experimental results on mixtures of HfO2, ZrO2, and Al2O3, pursuing applications in UV coating technology, while the mixtures are prepared by magnetron sputtering, ion beam sputtering, plasma ion-assisted deposition (PIAD), and electron beam evaporation without assistance. The properties investigated include the refractive index, optical gap, thermal shift, and mechanical stress. The first high reflectors for UV applications have been deposited by PIAD.

Charles Keith Carniglia (1944-2006): in memoriam

Chuck Carniglia was an industrial scientist, an educator, and a friend to all he met. He had an important impact on the optical thin-film community. This article highlights his career and accomplishments.

TiO2--Ta2O5 composite thin films deposited by radio frequency ion-beam sputtering

TiO2--Ta2O5 composite films were prepared by a radio frequency ion-beam sputtering deposition process, and the refractive indices and extinction coefficients of the composite films were found to be between those of the TiO2 and Ta2O5 films. The structure of the as-deposited films was amorphous, and the surface roughness was approximately 0.1 nm. The residual stress of the composite films was less than that of pure TiO2 film. The structure of the composite films after annealing was amorphous, with low surface roughness and slightly increased residual stress. The film containing 6.3% TiO2 displayed better properties than either the pure TiO2 or the pure Ta2O5 film.

X-ray photoelectron spectroscopy study of thin TiO2 films cosputtered with Al

In this study, titanium dioxide (TiO(2)) films were fabricated by cosputtering of a titanium (Ti) target and an aluminum (Al) slice in a smaller area by an ion-beam sputtering deposition method. The sputtered films were postannealed at 450 degrees C. The x-ray photoelectron spectroscopy spectra were categorized by their oxygen bonding variations, which include high-binding-energy oxygen, (HBO), bridging oxygen, low-binding-energy oxygen, and shifts of the binding energies (BEs) of oxygen (O) and Ti signals. The enhancement of HBO and higher BE shifts of the O 1s spectra as a function of cosputtered Al in the film imply the formation of an Al-O-Ti linkage. Corresponding changes in the Ti 2p spectra further confirm the modification of properties of the cosputtered film that results from the variation of the chemical bonding environment. An observed correlation between the chemical structure and optical absorption of the Al cosputtered films can be used to modify the optical properties of the film.

Rugate filter made with composite thin films by ion-beam sputtering

Composite films of Ta-Si oxide with refractive indices that varied from 1.48 to 2.15 were realized by using rf ion-beam sputtering. All the composite films were amorphous and had a surface roughness of less than 0.3 nm. The inhomogeneity of the composite was discussed, and a rugate filter was designed and fabricated by automatic computer control.

Titania, silicon dioxide, and tantalum pentoxide waveguides and optical resonant filters prepared with radio-frequency magnetron sputtering and annealing

Mixing dielectric materials in solid-thin-film deposition allows the engineering of thin films' optical constants to meet specific thin-film-device requirements, which can be significantly useful for optoelectronics devices and photonics technologies in general. In principle, by use of radio-frequency (rf) magnetron sputtering, it would be possible to mix any two, or more, materials at different molar ratios as long as the mixed materials are not chemically reactive in the mixture. This freedom in material mixing by use of magnetron sputtering has an advantage by providing a wide range of the material optical constants, which eventually enables the photonic-device designer to have the flexibility to achieve optimal device performance. We deposited three combinations from three different oxides by using rf magnetron sputtering and later investigated them for their optical constants. Each two-oxide mixture was done at different molar ratio levels. Moreover, postdeposition annealing was investigated and was shown to reduce the optical losses and to stabilize the film composition against environmental effects such as aging and humidity exposure. These investigations were supported by the fabricated planar waveguides and optical resonant filters.

Direct observation of waveguided scattered light in multilayer dielectric thin films

When a focused laser beam falls on a multilayer thin-film coating, light is scattered from the volume over which the beam intersects the coating. Some of the light may be scattered into directions that correspond to guided modes of the thin-film structure. We report the observation of scattered light at locations removed from the region of incidence of the light on the coating, resulting from the secondary scatter of the guided scattered light. The shape of the pattern observed by secondary scatter clearly resembles the pattern expected for Rayleigh scatter from a point source. This observation has important implications for the design of scatter-measuring instruments as well as for the theoretical treatment of scatter from multilayer coatings.

Optical scatter characteristics of high-reflectance dielectric coatings and fused-silica substrates

Optical scatter characteristics for high-reflectance dielectric coatings and polished fused-silica substrates are measured. The coating materials are tantala/silica, titania/silica, and zirconia/silica. The coatings are deposited on substrates that are conventionally polished, superpolished, and irradiated with a CO(2) laser before deposition. The measurements are made at the design wavelengths of the coatings, i.e., 633 and 1320 nm, and consist of bidirectional-reflectance distribution-function angle scans and spatial mappings. The substrate scatter is lower for superpolished surfaces, and the coating scatter is reduced by the use of superpolished substrates. Scatter levels are strongly influenced by coating design and materials.