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Due to its excellent optical properties, such as low absorption and scattering, amorphous ${\text{Ta}}_2{\mathrm{O}}_5$is commonly used as an optical coating material, often in combination with ${\mathrm{SiO}}_2$layers to produce a highly reflective stack. However, the high mechanical loss of ${\text{Ta}}_2{\mathrm{O}}_5$limits the thermal-noise performance of such coatings when used in precision measurement applications. Doping with ${\mathrm{TiO}}_2$has previously been shown to slightly reduce the mechanical loss, but it is still very high compared to many other materials, particularly at low temperatures. In this paper, we present a detailed study of different heat treatment temperatures and of Ti concentrations of up to nominally 75%. We show a significant mechanical-loss reduction for the mixture with the highest Ti cation content, which crystallized after heat treatment at 500°C. The resulting loss is much lower than that of pure ${\mathrm{TiO}}_2$or that of ${\text{Ta}}_2{\mathrm{O}}_5$after crystallization, making further studies highly interesting, in particular investigations of scattering which may pose a major drawback for optical applications. 

The mirror suspensions in gravitational wave detectors demand low mechanical loss jointing to ensure good enough detector performance and to enable the detection of gravitational waves. Hydroxide catalysis bonds have been used in the fused silica suspensions of the GEO600, Advanced LIGO, and Advanced Virgo detectors. Future detectors may use cryogenic cooling of the mirror suspensions and this leads to a potential change of mirror material and suspension design. Other bonding techniques that could replace or be used alongside hydroxide catalysis bonding are of interest. A design that incorporates repair scenarios is highly desirable. Indeed, the mirror suspensions in KAGRA, which is made from sapphire and operated at cryogenic temperatures, have used a combination of hydroxide catalysis bonding and gallium bonding. This Letter presents the first measurements of the mechanical loss of a gallium bond measured between 10 K and 295 K. It is shown that the loss, which decreases with temperature down to the level of $(1.8\pm 0.3)\times {10}^{-4}$at 10 K, is comparable to that of a hydroxide catalysis bond. Published by the American Physical Society2024 

Abstract Blistering is a phenomenon sometimes observed in sputtered-deposited thin films but seldom investigated in detail. Here, we consider the case of titania-doped germania (TGO)/silica multilayers deposited by ion beam sputtering. TGO is a candidate as high refractive index material in the Bragg mirrors for the next iteration of gravitational waves detectors. It needs to be annealed at 600^∘C for 100 h in order to reach the desired relaxation state. However under some growth conditions, in 52-layer TGO/silica stacks, blistering occurs upon annealing at a temperature near 500^∘C, which corresponds to the temperature where Ar desorbs from TGO. In order to better understand the blistering phenomenon, we measure the Ar transport in single layers of TGO and silica. In the case of $<$1µm-thick TGO layers, the Ar desorption is mainly limited by detrapping. The transport model also correctly predicts the evolution of the total amount of Ar in a 8.5µm stack of TGO and silica layers annealed at 450^∘C, but in that case, the process is mainly limited by diffusion. Since Ar diffusion is an order of magnitude slower in TGO compared to silica, we observe a correspondingly strong accumulation of Ar in TGO. The Ar transport model is used to explain some regimes of the blisters growth, and we find indications that Ar accumulation is a driver for their growth in general, but the blisters nucleation remains a complex phenomenon influenced by several other factors including stress, substrate roughness, and impurities. 

Abstract Second-order nonlinear optical processes convert light from one wavelength to another and generate quantum entanglement. Creating chip-scale devices to efficiently control these interactions greatly increases the reach of photonics. Existing silicon-based photonic circuits utilize the third-order optical nonlinearity, but an analogous integrated platform for second-order nonlinear optics remains an outstanding challenge. Here we demonstrate efficient frequency doubling and parametric oscillation with a threshold of tens of micro-watts in an integrated thin-film lithium niobate photonic circuit. We achieve degenerate and non-degenerate operation of the parametric oscillator at room temperature and tune its emission over one terahertz by varying the pump frequency by hundreds of megahertz. Finally, we observe cascaded second-order processes that result in parametric oscillation. These resonant second-order nonlinear circuits will form a crucial part of the emerging nonlinear and quantum photonics platforms. 

We discuss wavelength conversion of a selected signal spatial mode, which preserves its quantum state and does not disturb other signal spatial modes. We present the results for a lithium niobate waveguide and a few-mode-fiber. 

Glasses are nonequilibrium solids with properties highly dependent on their method of preparation. In vapor-deposited molecular glasses, structural organization could be readily tuned with deposition rate and substrate temperature. Here, we show that the atomic arrangement of strong network-forming GeO 2 glass is modified at medium range (<2 nm) through vapor deposition at elevated temperatures. Raman spectral signatures distinctively show that the population of six-membered GeO 4 rings increases at elevated substrate temperatures. Deposition near the glass transition temperature is more efficient than postgrowth annealing in modifying atomic structure at medium range. The enhanced medium-range organization correlates with reduction of the room temperature internal friction. Identifying the microscopic origin of room temperature internal friction in amorphous oxides is paramount to design the next-generation interference coatings for mirrors of the end test masses of gravitational wave interferometers, in which the room temperature internal friction is a main source of noise limiting their sensitivity.