Actuation of micro-optomechanical systems via cavity-enhanced optical dipole forces

Abstract

We demonstrate an optomechanical system employing a movable, micron-scale waveguide optically-coupled to a high-Q optical microresonator. We show that milliwatt-level optical powers create micron-scale displacements of the input waveguide. The displacement is caused by a cavity-enhanced optical dipole force (CEODF) on the waveguide, arising from the stored optical field of the resonator. The CEODF is used to demonstrate tunable cavity-waveguide coupling at sub-mW input powers, a form of all-optical tunable filter. The scaling properties of the CEODF are shown to be independent of the intrinsic Q of the optical resonator and to scale inversely with the cavity mode volume.Although light is usually thought of as imponderable, carrying energy but relatively little momentum, light can exert a large force per photon if confined to small structures. Such forces have recently been proposed 1,2 as a means to construct novel optomechanical components such as tunable filters, couplers, and lasers. Other theoretical studies of the nonlinear dynamics of these systems have shown them to be useful for performing optical wavelength conversion and efficient optical-to-mechanical energy conversion 3,4 . In the field of quantum physics, there has also been recent interest in using radiation pressure forces within micro-optomechanical resonators to help cool macroscopic mechanical oscillators to their quantum-mechanical ground state 5,6,7,8 . Here, we demonstrate an optomechanical system employing a movable, micron-scale waveguide optically-coupled to a high-Q optical microresonator. We show that milliwatt-level optical powers create micron-scale displacements of the input waveguide. The displacement is caused by a cavity-enhanced optical dipole force (CEODF) on the waveguide, arising from the stored optical field of the resonator. The CEODF is used to demonstrate tunable cavity-waveguide coupling at sub-mW input powers, a form of all-optical tunable filter. Finally, the scaling properties of the CEODF are shown to be independent of the intrinsic Q of the optical resonator, and to scale inversely with the cavity mode volume, indicating that such forces may become even more effective as devices approach the nanoscale.The ponderomotive effects of light within optical resonators have long been considered in the field of high-precision measurement 9 . The canonical system, shown in Fig. 1a, consists of a Fabry-Perot (FP) resonant cavity formed between a rigid mirror and a movable mirror attached to a spring or hung as a pendulum 10 . A nearly-resonant optical field builds up in amplitude as it bounces back-and-forth between the mirrors and pushes on the movable mirror with each reflection, which detunes the FP cavity. The nonlinear dynamics associated with the displacement of the mirror and the build-up of internal cavity energy result in an "optical spring" effect 11 . Under conditions in which the optical field cannot adiabatically follow the mirror movement, the radiation pressure force can drive or dampen oscillations of the positi...

Observation of thermal feedback on the optical coupling noise of a microsphere attached to a low-spring-constant cantilever

“…Eichenfield et al [23] used the optical gradient force between a tapered optical fiber and a microresonator to pull a movable tapered fiber towards the resonator. An input power of less than 370 μW was used to displace the tapered fiber by about 120 nm.…”

“…The dipole or gradient force from one microresonator can be used to amplify or dampen the mechanical motion of another microscopic device, such as a nanostring [21] or a microdisk [22], as well as provide a way to precisely control the gap between them [3,4,6,7]. For example, Eichenfield et al [23] have shown how the cavity-enhanced optical dipole force from a microdisk can be used to control the relative position of a movable, tapered optical fiber. Povinelli et al [3] discussed the possibility of observing both positive and negative optical forces between two evanescently coupled microspheres, sometimes termed a "photonic molecule."…”

Controlling photonic structures using optical forces

“…More recently, the downscaling of optical systems to the micro and nano-scale resulted in very compliant systems with nanogram-scale masses, rendering them susceptible to optical forces 3,4,5,6 . In fact, optical forces have been exploited to demonstrate chaotic quivering of microcavities 7 , optical cooling of mechanical modes 8,9,10 , actuation of a tapered-fiber waveguide 11 , and excitation of the mechanical modes of silicon nano-beams 12 .…”

Transformation optics beyond the manipulation of light trajectories

“…Hence, these methods allow for amplification of the optical force per milliwatt of input power, although-locally-the force per milliwatt of optical power remains the same. We mention the so-called resonator-based enhancement and the slow-light enhancement techniques as two examples [88,89].…”