P. H. Kim

Nanoscale torsional optomechanics

Optomechanical transduction is demonstrated for nanoscale torsional resonators evanescently coupled to optical microdisk whispering gallery mode resonators. The on-chip, integrated devices are measured using a fully fiber-based system, including a tapered and dimpled optical fiber probe. With a thermomechanically calibrated optomechanical noise floor down to 7 fm/Hz, these devices open the door for a wide range of physical measurements involving extremely small torques, as little as 4×10−20N·m.

Nonlinear optomechanics in the stationary regime

We have observed nonlinear transduction of the thermomechanical motion of a nanomechanical resonator when detected as laser transmission through a sideband unresolved optomechanical cavity. Nonlinear detection mechanisms are of considerable interest as special cases allow for quantum nondemolition measurements of the mechanical resonators energy. We investigate the origin of the nonlinearity in the optomechanical detection apparatus and derive a theoretical framework for the nonlinear signal transduction, and the optical spring effect, from both nonlinearities in the optical transfer function and second order optomechanical coupling. By measuring the dependence of the linear and nonlinear signal transduction -- as well as the mechanical frequency shift -- on laser detuning from optical resonance, we provide estimates of the contributions from the linear and quadratic optomechanical couplings.

Multidimensional optomechanical cantilevers for high-frequency force sensing

High-frequency atomic force microscopy has enabled extraordinary new science through large bandwidth, high-speed measurements of atomic and molecular structures. However, traditional optical detection schemes restrict the dimensions, and therefore the frequency, of the cantilever?ultimately setting a limit to the time resolution of experiments. Here we demonstrate optomechanical detection of low-mass, high-frequency nanomechanical cantilevers (up to 20 MHz) and anticipate their use for single-molecule force measurements. These cantilevers achieve 2 fm displacement noise floors, and force sensitivity down to 132 aN . Furthermore, the ability to resolve both in-plane and out-of-plane motion of our cantilevers makes them excellent candidates for ultrasensitive multidimensional force spectroscopy, and optomechanical interactions, such as tuning of the cantilever frequency in situ, provide opportunities in high-speed, high-resolution experiments.

On-chip cavity optomechanical coupling

AbstractBackgroundOn-chip cavity optomechanics, in which strong co-localization of light and mechanical motion is engineered, relies on efficient coupling of light both into and out of the on-chip optical resonator. Here we detail our particular style of tapered and dimpled optical fibers, pioneered by the Painter group at Caltech, which are a versatile and reliable solution to efficient on-chip coupling. A brief overview of tapered, single mode fibers is presented, in which the single mode cutoff diameter is highlighted.MethodsThe apparatus used to create a dimpled tapered fiber is described, followed by a comprehensive account of the procedure by which a dimpled tapered fiber is produced and mounted in our system. The custom-built optical access vacuum chambers in which our on-chip optomechanical measurements are performed are then discussed. Finally, the process by which our optomechanical devices are fabricated and the method by which we explore their optical and mechanical properties is explained.ResultsUsing this method of on-chip optomechanical coupling, angular and displacement noise floors of 4 nrad/ and 2 fm/ have been demonstrated, corresponding to torque and force sensitivities of and 132 aN/ , respectively.ConclusionThe methods and results of our on-chip optomechanical coupling system are summarized. It is our expectation that this manuscript will enable the novice to develop advanced optomechanical experiments.PACS codes07.60.-j; 07.10.Cm; 42.50.Wk

Approaching the standard quantum limit of mechanical torque sensing.

Reducing the moment of inertia improves the sensitivity of a mechanically based torque sensor, the parallel of reducing the mass of a force sensor, yet the correspondingly small displacements can be difficult to measure. To resolve this, we incorporate cavity optomechanics, which involves co-localizing an optical and mechanical resonance. With the resulting enhanced readout, cavity-optomechanical torque sensors are now limited only by thermal noise. Further progress requires thermalizing such sensors to low temperatures, where sensitivity limitations are instead imposed by quantum noise. Here, by cooling a cavity-optomechanical torque sensor to 25 mK, we demonstrate a torque sensitivity of 2.9 yNm/. At just over a factor of ten above its quantum-limited sensitivity, such cryogenic optomechanical torque sensors will enable both static and dynamic measurements of integrated samples at the level of a few hundred spins.

Optomechanics and thermometry of cryogenic silica microresonators

We present measurements of silica optomechanical resonators, known as bottle resonators, passively cooled in a cryogenic environment. These devices possess a suite of properties that make them advantageous for preparation and measurement in the mechanical ground state, including high mechanical frequency, high optical and mechanical quality factors, and optomechanical sideband resolution. Performing thermometry of the mechanical motion, we find that the optical and mechanical modes demonstrate quantitatively similar laser-induced heating, limiting the lowest average phonon occupation observed to just ~1500. Thermalization to the 9 mK thermal bath would facilitate quantum measurements on these promising nanogram-scale mechanical resonators.

Optical microscope and tapered fiber coupling apparatus for a dilution refrigerator.

We have developed a system for tapered fiber measurements of optomechanical resonators inside a dilution refrigerator, which is compatible with both on- and off-chip devices. Our apparatus features full three-dimensional control of the taper-resonator coupling conditions enabling critical coupling, with an overall fiber transmission efficiency of up to 70%. Notably, our design incorporates an optical microscope system consisting of a coherent bundle of 37,000 optical fibers for real-time imaging of the experiment at a resolution of ∼1 μm. We present cryogenic optical and optomechanical measurements of resonators coupled to tapered fibers at temperatures as low as 9 mK.

Ultralow-Dissipation Superfluid Micromechanical Resonator

Micro and nanomechanical resonators with ultra-low dissipation have great potential as useful quantum resources. The superfluid micromechanical resonators presented here possess several advantageous characteristics: straightforward thermalization, dissipationless flow, and in situ tunability. We identify and quantitatively model the various dissipation mechanisms in two resonators, one fabricated from borosilicate glass and one from single crystal quartz. As the resonators are cryogenically cooled into the superfluid state, the damping from thermal effects and from the normal fluid component are strongly suppressed. At our lowest temperatures, damping is limited solely by internal dissipation in the substrate materials, and reach quality factors up to 913,000 at 13 mK. By lifting this limitation through substrate material choice and resonator design, modelling suggests that the resonators should reach quality factors as high as 10

Magnetic actuation and feedback cooling of a cavity optomechanical torque sensor

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Broadband optomechanical transduction of nanomagnetic spin modes

at 100 mK, putting this architecture in an ideal position to harness mechanical quantum effects.