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  1. 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 600C 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 500C, 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 450C, 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.

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  2. 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. 
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  5. Silicon carbide (SiC) is rapidly emerging as a leading platform for the implementation of nonlinear and quantum photonics. Here, we find that commercial SiC, which hosts a variety of spin qubits, possesses low optical absorption that can enable SiC integrated photonics with quality factors exceeding107. We fabricate multimode microring resonators with quality factors as high as 1.1 million, and observe low-threshold (8.5±<#comment/>0.5mW) optical parametric oscillation using the fundamental mode as well as optical microcombs spanning 200 nm using a higher-order mode. Our demonstration is an essential milestone in the development of photonic devices that harness the unique optical properties of SiC, paving the way toward the monolithic integration of nonlinear photonics with spin-based quantum technologies.

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  6. The exceptional stability required from high finesse optical cavities and high precision interferometers is fundamentally limited by Brownian motion noise in the interference coatings of the cavity mirrors. In amorphous oxide coatings these thermally driven fluctuations are dominant in the high index layer compared to those in the low index SiO2layer in the stack. We present a systematic study of the evolution of the structural and optical properties of ion beam sputtered TiO2-doped Ta2O5films with annealing temperature. We show that low mechanical loss in TiO2-doped Ta2O5with a Ti cation ratio = 0.27 is associated with a material that consists of a homogeneous titanium-tantalum-oxygen mixture containing a low density of nanometer sized Ar-filled voids. When the Ti cation ratio is 0.53, phase separation occurs leading to increased mechanical loss. These results suggest that amorphous mixed oxides with low mechanical loss could be identified by considering the thermodynamics of ternary phase formation.

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  7. We present the optical and structural characterization of films ofTa2O5,Sc2O3, andSc2O3dopedTa2O5with a cation ratio around 0.1 grown by reactive sputtering. The addition ofSc2O3as a dopant induces the formation of tantalum suboxide due to the “oxygen getter” property of scandium. The presence of tantalum suboxide greatly affects the optical properties of the coating, resulting in higher absorption loss atλ<#comment/>=1064nm. The refractive index and optical band gap of the mixed film do not correspond to those of a mixture ofTa2O5andSc2O3, given the profound structural modifications induced by the dopant.

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  9. Amorphous tantala (Ta2O5) thin films were deposited by reactive ion beam sputtering with simultaneous low energy assistAr+orAr+/O2+bombardment. Under the conditions of the experiment, the as-deposited thin films are amorphous and stoichiometric. The refractive index and optical band gap of thin films remain unchanged by ion bombardment. Around 20% improvement in room temperature mechanical loss and 60% decrease in absorption loss are found in samples bombarded with 100-eVAr+. A detrimental influence from low energyO2+bombardment on absorption loss and mechanical loss is observed. Low energyAr+bombardment removes excess oxygen point defects, whileO2+bombardment introduces defects into the tantala films.

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