arXiv subjects

Garrett D. Cole

Publications and source records attributed to Garrett D. Cole.

At least 19 recordsLinked to original sources

arXiv2026

Cryogenic Mechanical Loss of GaAs/AlGaAs Crystalline Coatings

Optical coatings with minimal thermal noise are essential for high-precision, laser-based metrology experiments. Substrate-transferred crystalline GaAs/Al$_{0.92}$Ga$_{0.08}$As coatings have the lowest thermal noise for high-reflectivity mirror coatings with $λ=900$ nm -- 12 $μ$m. Time-frequency stabilization experiments have demonstrated that GaAs/AlGaAs mirror coatings have a lower noise floor than amorphous mirror coatings. These mirrors are now being developed at sizes $\ge 300$ mm for gravitational-wave detectors. In order to understand the coating's fundamental loss mechanisms, which cause thermal noise, we have performed the first measurement of the mechanical loss at cryogenic temperatures using a cryogenic, multimodal, gentle nodal suspension system, which operates from 12 K to room temperature. This initial measurement used a silicon substrate whose asymmetry produced excess friction with the nodal support. Accounting for this friction and Akhiezer loss in the substrate and coating, the mechanical loss agrees well with the thermal noise observed in experiments using cryogenic fixed-cavities with GaAs/AlGaAs mirrors.

physics.ins-det↗

arXiv2025

Non-uniform Birefringence in Highly-reflective Substrate-Transferred GaAs/Al$_{0.92}$Ga$_{0.08}$As Coatings at 1064 nm

Using a custom-built scanning system, we generated maps of birefringence on reflection at $λ=1064$~nm from single-crystal GaAs/Al$_{0.92}$Ga$_{0.08}$As Bragg reflectors (henceforth ``AlGaAs coatings''). Ten coatings were bonded to fused silica substrates and one remained on the epitaxial growth wafer. The average phase difference on reflection between beams polarized along the fast and slow axes of the coating was found to be $ψ= 1.09 \pm 0.18$~mrad, consistent with values observed in high-finesse optical reference cavities using similar AlGaAs coatings. Scans of substrate-transferred coatings with diameters between 18 and 194 millimeters showed birefringence non-uniformity at a median level of $0.1$~mrad. A similar epitaxial multilayer that was not substrate transferred, but remained on the growth wafer, had by far the least birefringence non-uniformity of all mirrors tested at $0.02$~mrad. On the other hand, the average birefringence of the epi-on-wafer coating and substrate-transferred coatings was indistinguishable. Excluding non-uniformity found at the location of crystal and bonding defects, we conclude that the observed non-uniformity was imparted during the substrate transfer process, likely during bonding. Quantifying the impact on the scatter loss in a LIGO-like interferometer, we find that birefringence non-uniformity at the levels seen here is unlikely to have a significant impact on performance. Nonetheless, future efforts will focus on improved process control to minimize and ultimately eliminate the observed non-uniformity.

physics.optics↗

arXiv2025

Ion-Beam-Sputtered Mid-Infrared Coatings for Hybrid Supermirrors

We report on the development of low-loss ion-beam-sputtered (IBS) mid-infrared coatings for hybrid supermirrors. Two highly reflective designs were realized: HR1, a 4-period a-Si/SiO$_2$ DBR with an Al$_2$O$_3$ bonding layer, and HR2, a 6-period a-Si/Ta$_2$O$_5$ DBR with an a-Si terminating layer for bonding. Combined with a GaAs/AlGaAs crystalline mirror, HR2-based hybrids yielded a total loss of 9.3 ppm and excess loss of 6.8 ppm at 4.45 $μ$m, with cavity finesse up to 396000. For the first time, we used IBS-deposited a-Si directly as a bonding layer, verified its sub-angstrom roughness, and demonstrated excellent performance. This establishes a clear path toward scalable coatings for longer mid-infrared wavelengths, building on prior results in Nat. Commun. 14, 7846 (2023).

physics.optics↗

arXiv2025

Thermal Noise Measurement Below the Standard Quantum Limit

We present a method characterizing thermal noise in an optical cavity independent from quantum noise despite the thermal noise falling below the quantum noise limit. Using this method, we measured the thermal noise contribution from a GaAs AlGaAs micro-mirror suspended on a GaAs cantilever microresonator when brought to a cryogenic temperature (approximately 25 K) and incorporated into a Fabry Perot cavity. An optical spring is formed in this optical cavity. Previously, this setup exploited an optical spring to produce a displacement sensitivity falling 2.8 dB below the free-mass standard quantum limit (SQL), as reported by Cullen et. al. Here we use a similar setup to measure thermal noise which fell a maximum of 5 dB below the SQL. This measurement, in turn, allowed for an investigation of quantum noise suppression resulting from the optical spring effect, falling a maximum of 10 dB below the SQL.

quant-ph↗

arXiv2025

Conditional Motional Squeezing of an Optomechanical Oscillator Approaching the Quantum Regime

Squeezed mechanical states are a highly coveted resource for quantum-enhanced sensing and serve as a compelling platform for probing the interplay between gravity and quantum mechanics. It has been predicted that a mechanical oscillator can be prepared into a quantum squeezed state if the applied measurement rate is fast relative to its mechanical resonance frequency. However, the experimental feasibility of this protocol has remained uncertain because of the difficulty in achieving low-frequency oscillators with sufficiently strong read-out. Here, we demonstrate that a careful selection of parameters in an optomechanical system, combined with optimal filtering techniques, enables the preparation of a 50 ng GaAs cantilever in a conditional classical squeezed state, achieving a minimum uncertainty of just 1.07 plus/minus 0.04 times the zero-point fluctuation level. This minimum variance is 3 orders of magnitude smaller than what has been achieved in previous experiments using the same protocol. Although we do not fully achieve the quantum squeezed regime, our demonstration provides definitive evidence that a measurement-based protocol is a practical and effective approach for the real-time preparation of macroscopic oscillators in quantum squeezed states.

quant-ph↗

arXiv2024

Optical Spring Tracking for Enhancing Quantum-Limited Interferometers

Modern interferometers such as LIGO have achieved sensitivities limited by quantum noise, comprised of radiation pressure and shot noise. To mitigate this noise, a static system is employed that minimizes the quantum noise within the measurement band. However, since gravitational wave inspiral signals are a single frequency changing over time, only noise at the chirp frequency needs to be minimized. Here we demonstrate dynamically tracking a target signal using an optical spring, resulting in an increased signal to noise ratio (SNR). We report on a SNR increase by up to a factor of 40 when compared to a static configuration.

quant-ph↗

arXiv2024

Wafer-Scale Fabrication of InGaP-on-Insulator for Nonlinear and Quantum Photonic Applications

The development of manufacturable and scalable integrated nonlinear photonic materials is driving key technologies in diverse areas such as high-speed communications, signal processing, sensing, and quantum information. Here, we demonstrate a novel nonlinear platform -- InGaP-on-insulator -- optimized for visible-to-telecommunication wavelength $χ^{\left(2\right)}$ nonlinear optical processes. In this work, we detail our 100-mm wafer-scale InGaP-on-insulator fabrication process realized via wafer bonding, optical lithography, and dry-etching techniques. The resulting wafers yield 1000s of components in each fabrication cycle, with initial designs that include chip-to-fiber couplers, 12.5-cm-long nested spiral waveguides, and arrays of microring resonators with free-spectral ranges spanning 400-900 GHz. We demonstrate intrinsic resonator quality factors as high as 324,000 (440,000) for single-resonance (split-resonance) modes near 1550 nm corresponding to 1.56 dB cm$^{-1}$ (1.22 dB cm$^{-1}$) propagation loss. We analyze the loss versus waveguide width and resonator radius to establish the operating regime for optimal 775-to-1550 nm phase matching. By combining the high $χ^{\left(2\right)}$ and $χ^{\left(3\right)}$ optical nonlinearity of InGaP with wafer-scale fabrication and low propagation loss, these results open promising possibilities for entangled-photon, multi-photon, and squeezed light generation.

physics.optics↗

arXiv2023

Simultaneous Measurement of Mid-Infrared Refractive Indices in Thin-Film Heterostructures: Methodology and Results for GaAs/AlGaAs

We present our results for simultaneous measurement of the refractive indices of gallium arsenide (GaAs) and aluminum gallium arsenide (Al$_\mathrm{x}$Ga$_\mathrm{1-x}$As) from $2.0$ to $7.1\,\mathrm{μm}$ ($5000$ to $1400\,\mathrm{cm^{-1}}$). We obtain these values from a monocrystalline superlattice Bragg mirror of excellent purity (background doping $\leq 1 \times 10^{-14}\,\mathrm{cm^{-3}}$), grown via molecular beam epitaxy. To recover the refractive indices over such a broad wavelength range, we fit a dispersion model for each material. In a novel combination of well-established methods, we measure both a photometrically accurate transmittance spectrum of the Bragg mirror via Fourier-transform infrared spectrometry and the individual physical layer thicknesses of the structure via scanning electron microscopy. To infer the uncertainty of the refractive index values, we estimate relevant measurement uncertainties and propagate them via a Monte-Carlo method. This highly-adaptable approach conclusively yields propagated relative uncertainties on the order of $10^{-4}$ over the measured spectral range for both GaAs and Al$_{0.929}$Ga$_{0.071}$As. The fitted model can also approximate the refractive index for MBE-grown Al$_\mathrm{x}$Ga$_\mathrm{1-x}$As for $0\leq x \leq 1$. Both these updated values and the measurement approach will be essential in the design, fabrication, and characterization of next-generation active and passive optical devices in a spectral region that is of high interest in many fields, e.g., laser design and cavity-enhanced spectroscopy in the mid-infrared spectral region.

physics.optics↗

arXiv2023

Precise Measurement of Refractive Indices in Thin Film Heterostructures

We present a robust, precise, and accurate method to simultaneously measure the refractive indices of two transparent materials within an interference coating. This is achieved by measuring both a photometrically accurate transmittance spectrum and the as-grown individual layer thicknesses of a thin-film multilayer structure. These measurements are used for a TMM-based curve-fitting routine which extracts the refractive indices and their measurement uncertainties via a Monte-Carlo-type error propagation. We demonstrate the performance of this approach by experimentally measuring the refractive indices of both, GaAs and Al$_{0.929}$Ga$_{0.071}$As, as present in an epitaxial distributed Bragg reflector. A variety of devices can be used to obtain the transmittance spectrum (e.g., FTIR, grating-based spectrophotometer) and layer thicknesses (e.g., SEM, TEM, AFM), the discussed approach is readily adaptable to virtually any wavelength region and many transparent material combinations of interest. The subsequent model-fitting approach yields refractive index values with $10^{-4}$-level uncertainty for both materials.

physics.optics↗

arXiv2022

Study on electro-optic noise in crystalline coatings toward future gravitational wave detectors

Thermal noise in high-reflectivity mirror coatings is a limiting factor in ground-based gravitational wave detectors. Reducing this coating thermal noise improves the sensitivity of detectors and enriches the scientific outcome of observing runs. Crystalline gallium arsenide and aluminum-alloyed gallium arsenide (referred to as AlGaAs) coatings are promising coating candidates for future upgrades of gravitational wave detectors because of their low coating thermal noise. However, AlGaAs-based crystalline coatings may be susceptible to an electro-optic noise induced by fluctuations in an electric field. We investigated the electro-optic effect in an AlGaAs coating by using a Fabry-Perot cavity, and concluded that the noise level is well below the sensitivity of current and planned gravitational-wave detectors.

physics.ins-det↗

arXiv2022

MAQRO -- BPS 2023 Research Campaign Whitepaper

The objective of the proposed MAQRO mission is to harness space for achieving long free-fall times, extreme vacuum, nano-gravity, and cryogenic temperatures to test the foundations of physics in macroscopic quantum experiments. This will result in the development of novel quantum sensors and a means to probe the foundations of quantum physics at the interface with gravity. Earlier studies showed that the proposal is feasible but that several critical challenges remain, and key technologies need to be developed. These new technologies will open up the potential for achieving additional science objectives. The proposed research campaign aims to advance the state of the art and to perform the first macroscopic quantum experiments in space. Experiments on the ground, in micro-gravity, and in space will drive the proposed research campaign during the current decade to enable the implementation of MAQRO within the subsequent decade.

quant-ph↗

arXiv2018

Quantum back action cancellation in the audio band

We report on the cancellation of quantum back action noise in an optomechanical cavity. We perform two measurements of the displacement of the microresonator, one in reflection of the cavity, and one in transmission of the cavity. We show that measuring the amplitude quadrature of the light in transmission of the optomechanical cavity allows us to cancel the back action noise between 1 kHz and 50 kHz, and obtain a more sensitive measurement of the microresonator's position. To confirm that the back action is eliminated, we measure the noise in the transmission signal as a function of circulating power. By splitting the transmitted light onto two photodetectors and cross correlating the two signals, we remove the contributon from shot noise and measure a quantum noise free thermal noise spectrum. Eliminating the effects of back action in this frequency regime is an important demonstration of a technique that could be used to mitigate the effects of back action in interferometric gravitational wave detectors such as Advanced LIGO.

quant-ph↗

arXiv2018

Broadband reduction of quantum radiation pressure noise via squeezed light injection

We present the reduction and manipulation of quantum radiation pressure noise (QRPN) in an optomechanical cavity with the injection of squeezed light. The optomechanical system consists of a high-reflectivity single-crystal microresonator which serves as one mirror of a Fabry-Perot cavity. The experiment is performed at room temperature and is QRPN dominated between 10 kHz and 50 kHz, frequencies relevant to gravitational wave observatories. We observed a reduction of 1.2 dB in the measurement noise floor with the injection of amplitude squeezed light generated from a below-threshold degenerate optical parametric oscillator. This experiment is a crucial step in realizing the reduction of QRPN for future interferometric gravitational wave detectors and improving their sensitivity.

quant-ph↗

arXiv2018

Room temperature optomechanical squeezing

The radiation-pressure driven interaction of a coherent light field with a mechanical oscillator induces correlations between the amplitude and phase quadratures of the light. These correlations result in squeezed light -- light with quantum noise lower than shot noise in some quadratures, and higher in others. Due to this lower quantum uncertainty, squeezed light can be used to improve the sensitivity of precision measurements. In particular, squeezed light sources based on nonlinear optical crystals are being used to improve the sensitivity of gravitational wave (GW) detectors. For optomechanical squeezers, thermally driven fluctuations of the mechanical oscillator's position makes it difficult to observe the quantum correlations at room temperature, and at low frequencies. Here we present a measurement of optomechanically (OM) squeezed light, performed at room-temperature, in a broad band near audio-frequency regions relevant to GW detectors. We observe sub-poissonian quantum noise in a frequency band of 30 kHz to 70 kHz with a maximum reduction of 0.7 $\pm$ 0.1 dB below shot noise at 45 kHz. We present two independent methods of measuring this squeezing, one of which does not rely on calibration of shot noise.

quant-ph↗

arXiv2018

Stress control of tensile-strained In$_{1-x}$Ga$_{x}$P nanomechanical string resonators

We investigate the mechanical properties of freely suspended nanostrings fabricated from tensile-stressed, crystalline In$_{1-x}$Ga$_{x}$P. The intrinsic strain is a consequence of the epitaxial growth given by the lattice mismatch between the thin film and the substrate which is confirmed by x-ray diffraction measurements. The flexural eigenfrequencies of the nanomechanical string resonators reveal an orientation dependent stress with a maximum value of 650 MPa. The angular dependence is explained by a combination of anisotropic Young's modulus and a change of elastic properties caused by defects. As a function of the crystal orientation a stress variation of up to 50 % is observed. This enables fine tuning of the tensile stress for any given Ga content $x$, which implies interesting prospects for the study of high Q nanomechanical systems.

cond-mat.mes-hall↗

arXiv2018

Mechanical Ringdown Studies of Large-Area Substrate-Transferred GaAs/AlGaAs Crystalline Coatings

We investigated elastic loss in GaAs/AlGaAs multilayers to help determine the suitability of these coatings for future gravitational wave detectors. We measured large ($\approx 70$-mm diameter) substrate-transferred crystalline coating samples with an improved substrate polish and bonding method. The elastic loss, when decomposed into bulk and shear contributions, was shown to arise entirely from the bulk loss, $ϕ_{\mathrm{Bulk}} = (5.33 \pm 0.03)\times 10^{-4}$, with $ϕ_{\mathrm{Shear}} = (0.0 \pm 5.2) \times 10^{-7}$. These results predict the coating loss of an 8-mm diameter coating in a 35-mm long cavity with a 250-$μ$m spot size (radius) to be $ϕ_{\mathrm{coating}} = (4.78 \pm 0.05) \times 10^{-5}$, in agreement with the published result from direct thermal noise measurement of $ϕ_{\mathrm{coating}} = (4 \pm 4) \times 10^{-5}$. Bonding defects were shown to have little impact on the overall elastic loss.

cond-mat.mtrl-sci↗

arXiv2018

Thermal noise in complex systems

We present a method to calculate the power spectral density of Brownian noise in complex optomechanical systems using Levin's approach of virtual pressure and present first mechanical loss measurements for high-purity GaAs over a wide temperature range from 7 K to 250 K. The loss reveals three Debye loss peaks. Each peak corresponds to an Arrhenius-like relaxation process with activation energies of 17.9 meV, 65.4 meV and 123 meV respectively. Additional light induced damping was observed for photon energies below and above the fundamental gap of GaAs in contrast to observations by Okamoto et al.

physics.app-ph↗

arXiv2018

Observation of a room-temperature oscillator's motion dominated by quantum fluctuations over a broad audio-frequency band

We report on the broadband measurement of quantum radiation pressure noise (QRPN) in an optomechanical cavity at room temperature over a broad range of frequencies relevant to gravitational-wave detectors. We show that QRPN drives the motion of a high-reflectivity single-crystal microresonator, which serves as one mirror of a Fabry-Perot cavity. In our measurements QRPN dominates over all other noise between 10 kHz and 50 kHz and scales as expected with the circulating power inside the cavity. The thermal noise of the microresonator, the largest noise source next to the QRPN, is measured and shown to agree with a structural damping model from 200 Hz to 30 kHz. By observing the effects of QRPN in the audio-band, we now have a testbed for studying techniques to mitigate back-action, such as variational readout and squeezed light injection, that could be used to improve the sensitivity of gravitational-wave detectors.

quant-ph↗