Thermal intermodulation backaction in a high-cooperativity optomechanical system

The pursuit of room temperature quantum optomechanics with tethered nanomechanical resonators faces stringent challenges owing to extraneous mechanical degrees of freedom. An important example is thermal intermodulation noise (TIN), a form of excess optical noise produced by mixing of thermal noise peaks. While TIN can be decoupled from the phase of the optical field, it remains indirectly coupled via radiation pressure, implying a hidden source of backaction that might overwhelm shot noise. Here we report observation of TIN backaction in a high-cooperativity, room temperature cavity optomechanical system consisting of an acoustic-frequency Si$_3$N$_4$ trampoline coupled to a Fabry-P\'{e}rot cavity. The backaction we observe exceeds thermal noise by 20 dB and radiation pressure shot noise by 40 dB, despite the thermal motion being 10 times smaller than the cavity linewidth. Our results suggest that mitigating TIN may be critical to reaching the quantum regime from room temperature in a variety of contemporary optomechanical systems.Comment: 8 pages, 5 figure

Clamp-tapering increases the quality factor of stressed nanobeams

Stressed nanomechanical resonators are known to have exceptionally high quality factors ($Q$) due to the dilution of intrinsic dissipation by stress. Typically, the amount of dissipation dilution and thus the resonator $Q$ is limited by the high mode curvature region near the clamps. Here we study the effect of clamp geometry on the $Q$ of nanobeams made of high-stress $\mathrm{Si_3N_4}$. We find that tapering the beam near the clamp - and locally increasing the stress - leads to increased $Q$ of MHz-frequency low order modes due to enhanced dissipation dilution. Contrary to recent studies of tethered-membrane resonators, we find that widening the clamps leads to decreased $Q$ despite increased stress in the beam bulk. The tapered-clamping approach has practical advantages compared to the recently developed "soft-clamping" technique. Tapered-clamping enhances the $Q$ of the fundamental mode and can be implemented without increasing the device size

Towards cavity-free ground state cooling of an acoustic-frequency silicon nitride membrane

We demonstrate feedback cooling of a millimeter-scale, 40 kHz SiN membrane from room temperature to 5 mK (3000 phonons) using a Michelson interferometer, and discuss the challenges to ground state cooling without an optical cavity. This advance appears within reach of current membrane technology, positioning it as a compelling alternative to levitated systems for quantum sensing and fundamental weak force measurements.Comment: To be published in the Applied Optics special issue: James C. Wyant College of Optical Science

Searching for vector dark matter with an optomechanical accelerometer

We consider using optomechanical accelerometers as resonant detectors for ultralight dark matter. As a concrete example, we describe a detector based on a silicon nitride membrane fixed to a beryllium mirror, forming an optical cavity. The use of different materials gives access to forces proportional to baryon (B) and lepton (L) charge, which are believed to be coupling channels for vector dark matter particles ("dark photons"). The cavity meanwhile provides access to quantum-limited displacement measurements. For a centimeter-scale membrane pre-cooled to 10 mK, we argue that sensitivity to vector B-L dark matter can exceed that of the E\"{o}t-Wash experiment in integration times of minutes, over a fractional bandwidth of $\sim 0.1\%$ near 10 kHz (corresponding to a particle mass of $10^{-10}$eV/c$^2$). Our analysis can be translated to alternative systems such as levitated particles, and suggests the possibility of a new generation of table-top experiments

Thermal intermodulation noise in cavity-based measurements

Thermal frequency fluctuations in optical cavities limit the sensitivity of precision experiments ranging from gravitational wave observatories to optical atomic clocks. Conventional modeling of these noises assumes a linear response of the optical field to the fluctuations of cavity frequency. Fundamentally, however, this response is nonlinear. Here we show that nonlinearly transduced thermal fluctuations of cavity frequency can dominate the broadband noise in photodetection, even when the magnitude of fluctuations is much smaller than the cavity linewidth. We term this noise "thermal intermodulation noise" and show that for a resonant laser probe it manifests as intensity fluctuations. We report and characterize thermal intermodulation noise in an optomechanical cavity, where the frequency fluctuations are caused by mechanical Brownian motion, and find excellent agreement with our developed theoretical model. We demonstrate that the effect is particularly relevant to quantum optomechanics: using a phononic crystal $Si_3N_4$ membrane with a low mass, soft-clamped mechanical mode we are able to operate in the regime where measurement quantum backaction contributes as much force noise as the thermal environment does. However, in the presence of intermodulation noise, quantum signatures of measurement are not revealed in direct photodetectors. The reported noise mechanism, while studied for an optomechanical system, can exist in any optical cavity

Suppression of extraneous thermal noise in cavity optomechanics

Extraneous thermal motion can limit displacement sensitivity and radiation pressure effects, such as optical cooling, in a cavity-optomechanical system. Here we present an active noise suppression scheme and its experimental implementation. The main challenge is to selectively sense and suppress extraneous thermal noise without affecting motion of the oscillator. Our solution is to monitor two modes of the optical cavity, each with different sensitivity to the oscillator's motion but similar sensitivity to the extraneous thermal motion. This information is used to imprint "anti-noise" onto the frequency of the incident laser field. In our system, based on a nano-mechanical membrane coupled to a Fabry-P\'{e}rot cavity, simulation and experiment demonstrate that extraneous thermal noise can be selectively suppressed and that the associated limit on optical cooling can be reduced.Comment: 27 pages, 14 figure

Upper Mantle Structure of Central and West Antarctica from Array Analysis of Rayleigh Wave Phase Velocities

The seismic velocity structure of Antarctica is important, both as a constraint on the tectonic history of the continent and for understanding solid Earth interactions with the ice sheet. We use Rayleigh wave array analysis methods applied to teleseismic data from recent temporary broadband seismograph deployments to image the upper mantle structure of central and West Antarctica. Phase velocity maps are determined using a two–plane wave tomography method and are inverted for shear velocity using a Monte Carlo approach to estimate three-dimensional velocity structure. Results illuminate the structural dichotomy between the East Antarctic Craton and West Antarctica, with West Antarctica showing thinner crust and slower upper mantle velocity. West Antarctica is characterized by a 70–100 km thick lithosphere, underlain by a low-velocity zone to depths of at least 200 km. The slowest anomalies are beneath Ross Island and the Marie Byrd Land dome and are interpreted as upper mantle thermal anomalies possibly due to mantle plumes. The central Transantarctic Mountains are marked by an uppermost mantle slow-velocity anomaly, suggesting that the topography is thermally supported. The presence of thin, higher-velocity lithosphere to depths of about 70 km beneath the West Antarctic Rift System limits estimates of the regionally averaged heat flow to less than 90 mW/m2. The Ellsworth-Whitmore block is underlain by mantle with velocities that are intermediate between those of the West Antarctic Rift System and the East Antarctic Craton. We interpret this province as Precambrian continental lithosphere that has been altered by Phanerozoic tectonic and magmatic activity

Airborne gravity and precise positioning for geologic applications

Airborne gravimetry has become an important geophysical tool primarily because of advancements in methodology and instrumentation made in the past decade. Airborne gravity is especially useful when measured in conjunction with other geophysical data, such as magnetics, radar, and laser altimetry. The aerogeophysical survey over the West Antarctic ice sheet described in this paper is one such interdisciplinary study. This paper outlines in detail the instrumentation, survey and data processing methodology employed to perform airborne gravimetry from the multiinstrumented Twin Otter aircraft. Precise positioning from carrier-phase Global Positioning System (GPS) observations are combined with measurements of acceleration made by the gravity meter in the aircraft to obtain the free-air gravity anomaly measurement at aircraft altitude. GPS data are processed using the Kinematic and Rapid Static (KARS) software program, and aircraft vertical acceleration and corrections for gravity data reduction are calculated from the GPS position solution. Accuracies for the free-air anomaly are determined from crossover analysis after significant editing (2.98 mGal rms) and from a repeat track (1.39 mGal rms). The aerogeophysical survey covered a 300,000 km2 region in West Antarctica over the course of five field seasons. The gravity data from the West Antarctic survey reveal the major geologic structures of the West Antarctic rift system, including the Whitmore Mountains, the Byrd Subglacial Basin, the Sinuous Ridge, the Ross Embayment, and Siple Dome. These measurements, in conjunction with magnetics and ice-penetrating radar, provide the information required to reveal the tectonic fabric and history of this important region