AVS2013 Session MN+NS-MoA: Optomechanics, Photonics, and Quantum Nanosystems

In the past few years nanophotonic transduction, in which a nanomechanical resonator is coupled to a high finesse optical cavity and its displacement is monitored through the cavity near-field has been demonstrated as a highly flexible, ultra-sensitive, wide bandwidth scheme for nanomechanical resonator displacement readout. A common design of the so-called nano-optomechanical devices (NOMS) involves either releasing a part of a nanophotonic waveguide in a race-track optical cavity, so the evanescent field of the guided mode is coupling to the remaining substrate [1], or laterally coupling a doubly clamped mechanical beam to an optical cavity [2]. In both cases, only the fundamental flexural mode is usually investigated [3].

The quest for large bandwidth mass sensors draws attention to higher flexural modes of nanomechanical resonators. Compared to the fundamental flexural mode, higher modes of a mechanical beam offer much higher operating bandwidth while incurring only a modest (or no) increase in its effective mass, depending on the boundary conditions. Higher modes of nanomechanical devices are also of interest for realizing cavity-optomechanics in the resolved-sideband limit [4], and nonlinear modal coupling among different mechanical modes has recently attracted renewed attention [5]. Hence, there is a demand to fully understand the behaviour of higher flexural modes in NOMS devices.

In this work, we fabricate NOMS devices by releasing parts of a nanophotonic waveguide in a race-track optical cavity. Sensitive photonic transduction allows thermomechanical noise of odd flexural modes (up to 50 MHz limited by our measurement electronics) to be observed. However, the transduction responsivity diminishes for even modes, due to the geometrical symmetry of the device design. We show that breaking this symmetry can increase the transduction responsivity for even flexural modes. The nanophotonically transduced displacement responsivity for different modes is also compared to that measured using a free-space interferometry setup. We further discuss the internal stress of the devices and its influence on the mode frequency, shape and transduction responsivity.

[1] W. H. P. Pernice et al.,Opt. Express17, 12424 (2009).

[2] O. Basarir et al.,Opt. Express20, 4272 (2012).

[3] M. Li et al.,Appl. Phys. Lett.97, 183110 (2010).

[4] G. Anetsberger et al.,Nature Physics5, 909 (2009).

My group at UC Santa Barbara has been developing a chip-based, fully integrated microwave-to-optical frequency up-converter based on a piezoelectrically-actuated optomechanical crystal. The device is designed to use the 1550 nm telecommunications wavelength band, trapping photons of that wavelength in an optomechanics crystal, then modulating the frequency of these photons through interactions with GHz-frequency phonons, the latter generated using the piezoelectric response of the crystal. We hope to use this device to create optical frequency entangled photons, produced using entangled microwave-frequency phonon states, generated by a superconducting qubit. This will enable the transfer of quantum information from a millikelvin cryostat to a fiber optic transmission line, with the potential of coupling hybrid quantum systems. I will report on our progress in developing this novel device.

We report on the first experimental demonstration of an ultrasensitive laser optical interferometric technology based on thin-film silicon carbide (SiC) micromechanical and nanomechanical resonant systems, which offer motion transduction with displacement sensitivities down to the sub-10fm/rtHz level, at room temperature.

Position and motion detection with advanced optical techniques have been widely used for studying the static and dynamic motions of various systems, ranging from the classical scanning probe microscopes to the emerging resonant nano/microelectromechanical systems (NEMS/MEMS). In particular, in recent years significant efforts and advances [1,2] have been made in developing laser optical interferometric systems based on various NEMS/MEMS resonators, with constituting materials in Si, SiN, GaAs, AlN, and more recently two-dimensional (2D) crystals such as graphene and MoS₂. These advances have kept enabling very sensitive detection of motions in various NEMS/MEMS resonators, with ever improving displacement sensitivities, from ~nm/rtHz to ~pm/rtHz levels. In many of these systems, there are limitations intrinsic to the device structures and constituting materials. For instance, light absorption and parasitic heating effects can compromise these interferometric systems from achieving better sensitivities.

In this work, we explore new optical interferometric techniques by exploiting some unique properties of SiC thin films, particularly the high transparency and ultralow photon absorption (60 times lower than Si) in the wide visible range, as well as the excellent thermal conductivity. The SiC thin films are prepared by low-pressure chemical vapor deposition (LPCVD) on various substrates, which enables us to develop a novel SiC-on-SiO₂ material platform. The suspended MEMS/NEMS devices fabricated in this thin-film platform all share important features such as smooth surfaces and a uniform interferometric gap. These structural features, combined with SiC’s outstanding physical properties, have permitted us to demonstrate unprecedented displacement sensitivities at ~5-10fm/rtHz levels, better than other MEMS/NEMS-based optical interferometric techniques reported to date. We demonstrate such ultrasensitive techniques for motion detections in high frequency SiC microdisk resonators and nanocantilever resonators. In both systems, we have measured the undriven, intrinsic thermomechanical resonances up to high-order modes.