David L. McAuslan

Strong-coupling cavity QED using rare-earth-metal-ion dopants in monolithic resonators: What you can do with a weak oscillator

We investigate the possibility of achieving the strong coupling regime of cavity quantum electrodynamics using rare-earth-metal-ions as impurities in monolithic optical resonators. We conclude that due to the weak oscillator strengths of the rare-earth-metals, it may be possible but difficult to reach the regime where the single photon Rabi frequency is large compared to both the cavity and atom decay rates. However, reaching the regime where the saturation photon and atom numbers are less than one should be much more achievable. We show that in this “bad cavity” regime, transfer of quantum states and an optical phase shift conditional on the state of the atom is still possible and suggest a method for coherent detection of single dopants.

Back-scatter based whispering gallery mode sensing

Whispering gallery mode biosensors allow selective unlabelled detection of single proteins and, combined with quantum limited sensitivity, the possibility for noninvasive real-time observation of motor molecule motion. However, to date technical noise sources, most particularly low frequency laser noise, have constrained such applications. Here we introduce a new technique for whispering gallery mode sensing based on direct detection of back-scattered light. This experimentally straightforward technique is immune to frequency noise in principle, and further, acts to suppress thermorefractive noise. We demonstrate 27 dB of frequency noise suppression, eliminating frequency noise as a source of sensitivity degradation and allowing an absolute frequency shift sensitivity of 76 kHz. Our results open a new pathway towards single molecule biophysics experiments and ultrasensitive biosensors.

Optomechanical magnetometry with a macroscopic resonator

We demonstrate a centimeter-scale optomechanical magnetometer based on a crystalline whispering gallery mode resonator. The large size of the resonator allows high magnetic field sensitivity to be achieved in the hertz to kilohertz frequency range. A peak sensitivity of 131 pT per root Hz is reported, in a magnetically unshielded non-cryogenic environment and using optical power levels beneath 100 microWatt. Femtotesla range sensitivity may be possible in future devices with further optimization of laser noise and the physical structure of the resonator, allowing applications in high-performance magnetometry.

Laser cooling and control of excitations in superfluid helium

It takes extreme sensitivity to measure the elementary excitations in liquid helium-4. An optomechanical cavity with a thin film of superfluid inside can be used to both observe and control phonons in real time.

Minimum requirements for feedback enhanced force sensing

The paper theoretically shows that for linear oscillators neither stationary nor non-stationary linear feedback provide any sensitivity enhancement over that possible with estimation alone. Importantly this result holds true in both the quantum and classical regime and in the presence of non-Gaussian noise terms. It is experimentally demonstrated that detection of a small stationary incoherent force can be enhanced by estimation in the same way as feedback cooling. The system presented is a cavity opto-electromechanical system (COEMS) consisting of an evanescently coupled silica microtoroid integrating high Q mechanical and optical modes. The incoherent signal, which is uncorrelated to the thermal noise, was applied via an electrode which facilitated strong electrical actuation through strong electrical gradient forces. The effect of the incoherent force on the mechanics manifests as an increase in the temperature of the mechanical oscillator. Energy averaging is used to resolve the temperature discrepancies and detect the signal. Enabling force sensitivity through post-processing without the need for feedback techniques greatly simplifies the role of micromechanical oscillators in sensing applications and fundamental research.

High bandwidth on-chip capacitive tuning of microtoroid resonators

We report on the design, fabrication and characterization of silica microtoroid based cavity opto-electromechanical systems (COEMS). Electrodes patterned onto the microtoroid resonators allow for rapid capacitive tuning of the optical whispering gallery mode resonances while maintaining their ultrahigh quality factor, enabling applications such as efficient radio to optical frequency conversion, optical routing and switching applications.

Microphotonic forces from superfluid flow

In cavity optomechanics, radiation pressure and photothermal forces are widely utilized to cool and control micromechanical motion, with applications ranging from precision sensing and quantum information to fundamental science. Here, we realize an alternative approach to optical forcing based on superfluid flow and evaporation in response to optical heating. We demonstrate optical forcing of the motion of a cryogenic microtoroidal resonator at a level of 1.46 nN, roughly one order of magnitude larger than the radiation pressure force. We use this force to feedback cool the motion of a microtoroid mechanical mode to 137 mK. The photoconvective forces demonstrated here provide a new tool for high bandwidth control of mechanical motion in cryogenic conditions, and have the potential to allow efficient transfer of electromagnetic energy to motional kinetic energy.

Theoretical framework for thin film superfluid optomechanics: towards the quantum regime

Excitations in superfluid helium represent attractive mechanical degrees of freedom for cavity optomechanics schemes. Here we numerically and analytically investigate the properties of optomechanical resonators formed by thin films of superfluid He covering micrometer-scale whispering gallery mode cavities. We predict that through proper optimization of the interaction between film and optical field, large optomechanical coupling rates g0 > 2π × 100 kHz and single photon cooperativities C0 > 10 are achievable. Our analytical model reveals the unconventional behaviour of these thin films, such as thicker and heavier films exhibiting smaller effective mass and larger zero point motion. The optomechanical system outlined here provides access to unusual regimes such as g0 > ΩM and opens the prospect of laser cooling a liquid into its quantum ground state. PACS numbers: 67.25.dt, 67.25.dp, 42.60.Da, 42.82.Et

Optical detection of ultrasound using AFC-based quantum memory technique in cryogenic rare earth ion doped crystals

We present results of a novel and highly sensitive technique for the optical detection of ultrasound using the selective storage of frequency shifted photons in an inherently highly efficient and low noise atomic frequency comb (AFC) based quantum memory. The ultrasound ‘tagged’ optical sidebands are absorbed within a pair of symmetric AFCs, generated via optical pumping in a Pr3+:Y2SiO5 sample (tooth separation Δ = 150 kHz, comb finesse fc ~ 2 and optical depth αL ~ 2), separated by twice the ultrasound modulation frequency (1.5 MHz) and centered on either side of a broad spectral pit (1.7 MHz width) allowing transmission of the carrier. The stored sidebands are recovered with 10-20% efficiency as a photon echo (as defined by the comb parameters), and we demonstrate a record 49 dB discrimination between the sidebands and the carrier pulse, high discrimination being important for imaging tissues at depth. We further demonstrate detector limited discrimination (~29 dB) using a highly scattered beam, confirming that the technique is immune to speckle decorrelation. We show that it also remains valid in the case of optically thin samples, and thus represents a significant improvement over other ultrasound detection methods based on rare-earth-ion-doped crystals. These results strongly suggest the suitability of our technique for high-resolution non-contact real-time imaging of biological tissues.

Superfluid helium optomechanics

By evanescently coupling a helium film to a high-Q optical resonator we make the first observation of superfluid helium Brownian motion. This new system has applications in quantum optomechanics, and in understanding superfluid helium properties.