Amir H. Safavi-Naeini

Squeezed light from a silicon micromechanical resonator

Monitoring a mechanical object’s motion, even with the gentle touch of light, fundamentally alters its dynamics. The experimental manifestation of this basic principle of quantum mechanics, its link to the quantum nature of light and the extension of quantum measurement to the macroscopic realm have all received extensive attention over the past half-century. The use of squeezed light, with quantum fluctuations below that of the vacuum field, was proposed nearly three decades ago as a means of reducing the optical read-out noise in precision force measurements. Conversely, it has also been proposed that a continuous measurement of a mirror’s position with light may itself give rise to squeezed light. Such squeezed-light generation has recently been demonstrated in a system of ultracold gas-phase atoms whose centre-of-mass motion is analogous to the motion of a mirror. Here we describe the continuous position measurement of a solid-state, optomechanical system fabricated from a silicon microchip and comprising a micromechanical resonator coupled to a nanophotonic cavity. Laser light sent into the cavity is used to measure the fluctuations in the position of the mechanical resonator at a measurement rate comparable to its resonance frequency and greater than its thermal decoherence rate. Despite the mechanical resonator’s highly excited thermal state (104 phonons), we observe, through homodyne detection, squeezing of the reflected light’s fluctuation spectrum at a level 4.5 ± 0.2 per cent below that of vacuum noise over a bandwidth of a few megahertz around the mechanical resonance frequency of 28 megahertz. With further device improvements, on-chip squeezing at significant levels should be possible, making such integrated microscale devices well suited for precision metrology applications.

Observation of Quantum Motion of a Nanomechanical Resonator

We present optical sideband spectroscopy measurements of a mesoscopic mechanical oscillator cooled near its quantum ground state. The mechanical oscillator, corresponding to a 3.99GHz acoustic mode of a patterned silicon nanobeam, is coupled via radiation pressure to a pair of co-localized 200THz optical modes. The mechanical mode is cooled close to its quantum ground state from a bath temperature of 18K using radiation pressure back-action stemming from the optical pumping of one of the optical cavity resonances. An optical probe beam, resonant with the second optical cavity resonance, is used to transduce the mechanical motion and determine the phonon occupancy of the mechanical mode. Measurement of the asymmetry between up-converted and down-converted photons of the probe beam yields directly the displacement noise power associated with the quantum zero-point motion of the mechanical oscillator, and provides an absolute calibration of the average phonon occupancy of the mechanical mode.

Coherent optical wavelength conversion via cavity optomechanics

Both classical and quantum systems utilize the interaction of light and matter across a wide range of energies. These systems are often not naturally compatible with one another and require a means of converting photons of dissimilar wavelengths to combine and exploit their different strengths. Here we theoretically propose and experimentally demonstrate coherent wavelength conversion of optical photons using photon-phonon translation in a cavity-optomechanical system. For an engineered silicon optomechanical crystal nanocavity supporting a 4-GHz localized phonon mode, optical signals in a 1.5 MHz bandwidth are coherently converted over a 11.2 THz frequency span between one cavity mode at wavelength 1,460 nm and a second cavity mode at 1,545 nm with a 93% internal (2% external) peak efficiency. The thermal- and quantum-limiting noise involved in the conversion process is also analysed, and in terms of an equivalent photon number signal level are found to correspond to an internal noise level of only 6 and 4 × 10(-3) quanta, respectively.

Proposal for an optomechanical traveling wave phonon–photon translator

In this paper, we describe a general optomechanical system for converting photons to phonons in an efficient and reversible manner. We analyze classically and quantum mechanically the conversion process and proceed to a more concrete description of a phonon–photon translator (PPT) formed from coupled photonic and phononic crystal planar circuits. The application of the PPT to RF-microwave photonics and circuit QED, including proposals utilizing this system for optical wavelength conversion, long-lived quantum memory and state transfer from optical to superconducting qubits, is considered.

Optimized optomechanical crystal cavity with acoustic radiation shield

We present the design of an optomechanical crystal nanobeam cavity that combines finite-element simulation with numerical optimization, and considers the optomechanical coupling arising from both moving dielectric boundaries and the photo-elastic effect. Applying this methodology results in a nanobeam with an experimentally realized intrinsic optical Q-factor of 1.2×10^6, a mechanical frequency of 5.1 GHz, a mechanical Q-factor of 6.8×10^5 (at T = 10 K), and a zero-point-motion optomechanical coupling rate of g = 1.1 MHz.

Slowing and stopping light using an optomechanical crystal array

One of the major advances needed to realize all-optical information processing of light is the ability to delay or coherently store and retrieve optical information in a rapidly tunable manner. In the classical domain, this optical buffering is expected to be a key ingredient of managing the flow of information over complex optical networks. Such a system also has profound implications for quantum information processing, serving as a long-term memory that can store the full quantum information contained in an optical pulse. Here, we suggest a novel approach to light storage involving an optical waveguide coupled to an optomechanical crystal array, where light in the waveguide can be dynamically and coherently transferred into long-lived mechanical vibrations of the array. Under realistic conditions, this system is capable of achieving large bandwidths and storage/delay times in a compact, on-chip platform.

Optomechanics in an ultrahigh-Q two-dimensional photonic crystal cavity

We demonstrate an ultrahigh-Q slotted two-dimensional photonic crystal cavity capable of obtaining strong interaction between the internal light field and the mechanical motion of the slotted structure. The measured optical quality factor is Q = 1.2×10^6 for a cavity with an effective modal volume of V_(eff) = 0.04(λ)^3. Optical transduction of the thermal motion of the fundamental in-plane mechanical resonance of the structure (ν_m = 151 MHz) is performed, from which a zero-point motion optomechanical coupling rate of g∗/2π = 320 kHz is inferred. Dynamical back-action of the optical field on the mechanical motion, resulting in cooling and amplication of the mechanical motion, is also demonstrated.

Enhanced Quantum Nonlinearities in a Two-Mode Optomechanical System

In cavity optomechanics, nanomechanical motion couples to a localized optical mode. The regime of single-photon strong coupling is reached when the optical shift induced by a single phonon becomes comparable to the cavity linewidth. We consider a setup in this regime comprising two optical modes and one mechanical mode. For mechanical frequencies nearly resonant to the optical level splitting, we find the photon-phonon and the photon-photon interactions to be significantly enhanced. In addition to dispersive phonon detection in a novel regime, this offers the prospect of optomechanical photon measurement. We study these quantum nondemolition detection processes using both analytical and numerical approaches.

Two-Dimensional Phononic-Photonic Band Gap Optomechanical Crystal Cavity

We present the fabrication and characterization of an artificial crystal structure formed from a thin film of silicon that has a full phononic band gap for microwave X-band phonons and a two-dimensional pseudo-band gap for near-infrared photons. An engineered defect in the crystal structure is used to localize optical and mechanical resonances in the band gap of the planar crystal. Two-tone optical spectroscopy is used to characterize the cavity system, showing a large coupling (g0/2π≈220  kHz) between the fundamental optical cavity resonance at ωo/2π=195  THz and colocalized mechanical resonances at frequency ωm/2π≈9.3  GHz.

Modeling dispersive coupling and losses of localized optical and mechanical modes in optomechanical crystals.

Periodically structured materials can sustain both optical and mechanical excitations which are tailored by the geometry. Here we analyze the properties of dispersively coupled planar photonic and phononic crystals: optomechanical crystals. In particular, the properties of co-resonant optical and mechanical cavities in quasi-1D (patterned nanobeam) and quasi-2D (patterned membrane) geometries are studied. It is shown that the mechanical Q and optomechanical coupling in these structures can vary by many orders of magnitude with modest changes in geometry. An intuitive picture is developed based upon a perturbation theory for shifting material boundaries that allows the optomechanical properties to be designed and optimized. Several designs are presented with mechanical frequency approximately 1-10 GHz, optical Q-factor Qo > 107, motional masses meff approximately 100 femtograms, optomechanical coupling length LOM < 5 microm, and clampinig losses that are exponentially suppressed with increasing number of phononic crystal periods (radiation-limited mechanical Q-factor Qm > 107 for total device size less than 30 microm).