Thermal noise and mechanical loss of SiO 2 /Ta 2 O 5 optical coatings at cryogenic temperatures

Mechanical loss of dielectric mirror coatings sets fundamental limits for both gravitational wave detectors and cavity-stabilized optical local oscillators for atomic clocks. Two approaches are used to determine the mechanical loss: ringdown measurements of the coating quality factor and direct measurement of the coating thermal noise. Here we report a systematic study of the mirror thermal noise at 4, 16, 124, and 300 K by operating reference cavities at these temperatures. The directly measured thermal noise is used to extract the mechanical loss for$SiO2/Ta2O5$coatings, which are compared with previously reported values.

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Publication Date:
NSF-PAR ID:
10210894
Journal Name:
Optics Letters
Volume:
46
Issue:
3
Page Range or eLocation-ID:
Article No. 592
ISSN:
0146-9592; OPLEDP
Publisher:
Optical Society of America
1. Optical coatings formed from amorphous oxide thin films have many applications in precision measurements. The Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and Advanced Virgo use coatings of$SiO2$(silica) and$TiO2:Ta2O5$(titania-doped tantala) and post-deposition annealing to 500°C to achieve low thermal noise and low optical absorption. Optical scattering by these coatings is a key limit to the sensitivity of the detectors. This paper describes optical scattering measurements for single-layer, ion-beam-sputtered thin films on fused silica substrates: two samples of$Ta2O5$and two of$TiO2:Ta2O5$. Using an imaging scatterometer at a fixed scattering angle of 12.8°, in-situ changes in the optical scatter of each sample were assessed during post-deposition annealing to 500°C in vacuum. The scatter of three of the four coated optics was observed to decrease during the annealing process, by 25–30% for tantala and up to 74% for titania-doped tantala, while the scatter frommore »
2. We present the optical and structural characterization of films of$Ta2O5$,$Sc2O3$, and$Sc2O3$doped$Ta2O5$with a cation ratio around 0.1 grown by reactive sputtering. The addition of$Sc2O3$as 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 at$λ<#comment/>=1064nm$. The refractive index and optical band gap of the mixed film do not correspond to those of a mixture of$Ta2O5$and$Sc2O3$, given the profound structural modifications induced by the dopant.
3. Amorphous tantala ($Ta2O5$) thin films were deposited by reactive ion beam sputtering with simultaneous low energy assist$Ar+$or$Ar+/O2+$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-eV$Ar+$. A detrimental influence from low energy$O2+$bombardment on absorption loss and mechanical loss is observed. Low energy$Ar+$bombardment removes excess oxygen point defects, while$O2+$bombardment introduces defects into the tantala films.
4. Temperature scaling of collisional broadening parameters for krypton (absorber)$4p6S01→<#comment/>5p[3/2]2$electronic transition centered at 107.3 nm in the presence of major combustion species (perturber) is investigated. The absorption spectrum in the vicinity of the transition is obtained from the fluorescence due to the two-photon excitation scan of krypton. Krypton was added in small amounts to major combustion species such as$CH4$,$CO2$,$N2$, and air, which then heated to elevated temperatures when flowed through a set of heated coils. In a separate experimental campaign, laminar premixed flat flame product mixtures of methane combustion were employed to extend the investigations to higher temperature ranges relevant to combustion. Collisional full width half maximum (FWHM) ($wC$) and shift ($δ<#comment/>C$) were computed from the absorption spectrum by synthetically fitting Voigt profiles to the excitation scans, and their corresponding temperature scaling was determined by fitting power-law temperature dependencies to the$wC$and$δ<#comment/>C$data for each perturber species. The temperature exponents of$wC$and$δ<#comment/>C$for all considered combustion species (perturbers) were$−<#comment/>0.73$and$−<#comment/>0.6$, respectively. Whereas the temperature exponents of$wC$are closer tomore »
5. Beam-steering devices such as optical phased arrays (OPAs) are key components in the applications of solid-state Lidar and wireless communication. The traditional single-layer OPA results in a significant energy loss due to substrate leakage caused by the downward coupling from the grating coupler structure. In this work, we have investigated a structure based on a multi-layer$Si3N4/SiO2$platform that can form a 3D OPA to emit light from the edge of the device with high efficiency; a 2D converged out-coupling beam will be end-fired to the air. High efficiency and wide horizontal beam steering are demonstrated numerically, and the influence of vertical crosstalk, delay length, and number of waveguide layers are discussed, as well as the fabrication feasibility.