In-vacuum measurements of optical scatter versus annealing temperature for amorphous Ta 2 O 5 and TiO 2 :Ta 2 O 5 thin films

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 from more »

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Publication Date:
NSF-PAR ID:
10217577
Journal Name:
Journal of the Optical Society of America A
Volume:
38
Issue:
4
Page Range or eLocation-ID:
Article No. 534
ISSN:
1084-7529; JOAOD6
Publisher:
Optical Society of America
1. 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.
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. 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.
4. Electro-optic quantum coherent interfaces map the amplitude and phase of a quantum signal directly to the phase or intensity of a probe beam. At terahertz frequencies, a fundamental challenge is not only to sense such weak signals (due to a weak coupling with a probe in the near-infrared) but also to resolve them in the time domain. Cavity confinement of both light fields can increase the interaction and achieve strong coupling. Using this approach, current realizations are limited to low microwave frequencies. Alternatively, in bulk crystals, electro-optic sampling was shown to reach quantum-level sensitivity of terahertz waves. Yet, the coupling strength was extremely weak. Here, we propose an on-chip architecture that concomitantly provides subcycle temporal resolution and an extreme sensitivity to sense terahertz intracavity fields below 20 V/m. We use guided femtosecond pulses in the near-infrared and a confinement of the terahertz wave to a volume of$VTHz∼<#comment/>10−<#comment/>9(λ<#comment/>THz/2)3$in combination with ultraperformant organic molecules ($r33=170pm/V$) and accomplish a record-high single-photon electro-optic coupling rate of, 10,000 times higher than in recent reports of sensing vacuum field fluctuations in bulk media. Via homodyne detection implemented directly on chip, the interaction results into an intensity modulation of the femtosecond pulses. The single-photon cooperativity is$C0=1.6×<#comment/>10−<#comment/>8$, and the multiphoton cooperativity is$C=0.002$at room temperature. We show$><#comment/>70dB$dynamic range in intensity at 500 ms integration under irradiation with a weak coherent terahertz field. Similar devices could be employed in future measurements of quantum states in the terahertz at the standard quantum limit, or for entanglement of subsystems on subcycle temporal scales, such as terahertz and near-infrared quantum bits.
5. Metasurfaces with dynamic optical performance have the potential to enable a broad range of applications. We computationally investigate the potential of dielectric Huygens metasurfaces, supporting both electric and magnetic dipole resonances, as a candidate platform for dynamic tuning. The asymmetric response of the two dipole resonances to changes in geometric and material parameters, and the potential for separate control of amplitude and phase, is analyzed. A review of dynamic materials, and their promise and limitations for use in dynamic Huygens metasurfaces, is discussed. Vanadium dioxide ($VO2$) is recognized as a singularly interesting material, due to its variable refractive index and optical absorption in response to several stimuli. Transmitted phase modulation of$±<#comment/>π<#comment/>$is computationally demonstrated as a function of decaying resonance utilizing only the first 5% of the insulator-metal transition, corresponding to a temperature change of$<<#comment/>10∘<#comment/>C$. As another case study utilizing asymmetric resonance tuning in response to changing incidence angle, phase modulation ($2π<#comment/>$range for reflected light and$><#comment/>1.5π<#comment/>$for transmitted light) and amplitude modulation (from$R=1$to$T=1$) are demonstrated using a simplemore »