Marcelo Wu

Dissipative and Dispersive Optomechanics in a Nanocavity Torque Sensor

Sensors in optical cavities can be used for measuring acceleration, fields, and particles. New research reveals a record sensitivity for detecting small amounts of torque within optical cavities, useful for detecting magnetic fields.

Nonlinear optomechanical paddle nanocavities

Nonlinear cavity optomechanics is a promising platform for realizing experiments which reveal the quantum properties of nanomechanical structures. Here we introduce an optomechanical system combining strong nonlinear optomechanical coupling, low mass and large optical mode spacing. This nanoscale “paddle nanocavity” supports mechanical resonances with hundreds of fg mass which couple nonlinearly to optical modes with a single photon quadratic optomechanical coupling rate that is four orders of magnitude higher than competing systems. This coupling relies on strong interactions between the nanocavity optical field and mechanical excitations in a structure whose optical mode spectrum is highly non-degenerate. Nonlinear optomechanical readout of thermally driven motion in these devices should be observable for temperatures above 50 mK, and measurement of phonon shot noise is achievable. This work shows that strong nonlinear effects can be realized without relying on coupling between nearly degenerate optical modes, thus avoiding parasitic linear coupling present in two mode systems.

Nanocavity optomechanical torque magnetometry and radiofrequency susceptometry

Nanophotonic optomechanical devices allow the observation of nanoscale vibrations with a sensitivity that has dramatically advanced the metrology of nanomechanical structures and has the potential to impact studies of nanoscale physical systems in a similar manner. Here we demonstrate this potential with a nanophotonic optomechanical torque magnetometer and radiofrequency (RF) magnetic susceptometer. Exquisite readout sensitivity provided by a nanocavity integrated within a torsional nanomechanical resonator enables observations of the unique net magnetization and RF-driven responses of single mesoscopic magnetic structures in ambient conditions. The magnetic moment resolution is sufficient for the observation of Barkhausen steps in the magnetic hysteresis of a lithographically patterned permalloy island. In addition, significantly enhanced RF susceptibility is found over narrow field ranges and attributed to thermally assisted driven hopping of a magnetic vortex core between neighbouring pinning sites. The on-chip magnetosusceptometer scheme offers a promising path to powerful integrated cavity optomechanical devices for the quantitative characterization of magnetic micro- and nanosystems in science and technology.

Optomechanical torsional sensing in photonic crystal split-beam nanocavities

Photonic crystal split-beam nanocavities are proposed and fabricated for optomechanical detection of torsional motion. Large optomechanical transduction around 20 GHz/nm allows for predicted torsional sensitivities down to 10-20 N·m√Hz.

Photonic crystal paddle nanocavities for optomechanical torsion sensing

Photonic crystal nanocavities with suspended central elements suitable for optomechanical detection of torsional forces are designed and fabricated. This “floating” low mass nanocavity may be mechanically coupled to nanomagnetic structures.

Nanoscale optomechanical sensors: Split-beam photonic crystal nanocavities

Optomechanical nanocavities allow nanomechanical resonances to be measured optically with high sensitivity. We have created a new type of photonic crystal nanocavity optomechanical sensor optimized for detecting sources of torque and other forces which can deflect nanoscale cantilevers. This nanocavity consists of two precisely engineered photonic Bragg mirrors patterned in silicon cantilevers and separated by a 50-100 nm wide gap. Simulations of the optical and mechanical modes predict that mechanical displacements of the sub-picogram cantilevers will shift the optical nanocavity resonance frequency at a rate exceeding 20 GHz / nm, and that the nanocavity optical mode may have a quality factor Qo > 106 in optimized devices.

Design and experimental demonstration of optomechanical paddle nanocavities

We present the design, fabrication, and initial characterization of a paddle nanocavity consisting of a suspended sub-picogram nanomechanical resonator optomechanically coupled to a photonic crystal nanocavity. The optical and mechanical properties of the paddle nanocavity can be systematically designed and optimized, and the key characteristics including mechanical frequency can be easily tailored. Measurements under ambient conditions of a silicon paddle nanocavity demonstrate an optical mode with a quality factor Qo∼6000 near 1550 nm and optomechanical coupling to several mechanical resonances with frequencies ωm/2π∼ 12−64 MHz, effective masses meff∼350−650 fg, and mechanical quality factors Qm∼ 44−327. Paddle nanocavities are promising for optomechanical sensing and nonlinear optomechanics experiments.

Photonic crystal split-beam nanocavities for torsional optomechanics

A novel type of photonic crystal nanocavity nanocavity tailored to sensitively measure torques is theoretically investigated. Suspended low-mass elements (< pg) in the nanomechanical resonator are sensitive to environmental stimuli, such as a magnetic field from external sources or from embedded nanomagnetic systems. The torsional mechanical motion of these elements directly influences the optical field concentrated inside the optical nanocavity, resulting in a strong cavity optomechanical coupling rate up to 90 GHz/nm. The actuation of the mechanical resonator is readout with high sensitivity using evanescent coupling between the photonic crystal nanocavity and an optical fiber taper. A sub-100nm physical air gap in the middle of the nanobeam cavity allows torsional mechanical degrees of freedom as well as strong optical field confinement in a small mode volume. Numerical simulations show that high-Q ~ 106 optical cavities with a gap are possible. Potential applications incorporating these devices include sensitive magnetometry and probing the quantum properties of nanomagnetic systems.