We report on an opto-mechanical resonator with vibration excited by compressive radiation pressure via stimulated Brillouin scattering [SBS]. We experimentally excite a mechanical whispering-gallery mode (WGM) from an optical WGM and detect vibration via the red Doppler shifted (Stokes) light it scatters. We numerically solve the stress-strain equation to calculate the circumferentially circulating mechanical WGM and reveal mechanical WGMs with a variety of transverse shapes. Frequency in our device is limited by the shortest optical wavelength it can transmit, irrespective of device size.

Although bolometric- and ponderomotive-induced deflection of device boundaries are widely used for laser cooling, the electrostrictive Brillouin scattering of light from sound was considered an acousto-optical amplification-only process(1-7). It was suggested that cooling could be possible in multi-resonance Brillouin systems(5-8) when phonons experience lower damping than light(8). However, this regime was not accessible in electrostrictive Brillouin systems(1-3,5,6) as backscattering enforces high acoustical frequencies associated with high mechanical damping(1). Recently, forward Brillouin scattering(3) in microcavities(7) has allowed access to low-frequency acoustical modes where mechanical dissipation is lower than optical dissipation, in accordance with the requirements for cooling(8). Here we experimentally demonstrate cooling via such a forward Brillouin process in a microresonator. We show two regimes of operation for the electrostrictive Brillouin process: acoustical amplification as is traditional and an electrostrictive Brillouin cooling regime. Cooling is mediated by resonant light in one pumped optical mode, and spontaneously scattered resonant light in one anti-Stokes optical mode, that beat and electrostrictively attenuate the Brownian motion of the mechanical mode

stimulated Brillouin interaction between sound and light, known to be the strongest optical nonlinearity common to all amorphous and crystalline dielectrics, has been widely studied in fibres and bulk materials but rarely in optical microresonators. The possibility of experimentally extending this principle to excite mechanical resonances in photonic microsystems, for sensing and frequency reference applications, has remained largely unexplored. The challenge lies in the fact that microresonators inherently have large free spectral range, whereas the phase-matching considerations for the Brillouin process require optical modes of nearby frequencies but with different wave vectors. Here we rely on high-order transverse optical modes to relax this limitation and report the experimental excitation of mechanical resonances ranging from 49 to 1,400 mHz by using forward Brillouin scattering. These natural mechanical resonances are excited in ~100 µm silica microspheres, and are of a surface-acoustic whispering-gallery type.

Whispering gallery mode resonators (WGMRs) take advantage of strong light confinement and long photon lifetime for applications in sensing, optomechanics, microlasers and quantum optics. However, their rotational symmetry and low radiation loss impede energy exchange between WGMs and the surrounding. As a result, free-space coupling of light into and from WGMRs is very challenging. In previous schemes, resonators are intentionally deformed to break circular symmetry to enable free-space coupling of carefully aligned focused light, which comes with bulky size and alignment issues that hinder the realization of compact WGMR applications. Here, we report a new class of nanocouplers based on cavity enhanced Rayleigh scattering from nano-scatterer(s) on resonator surface, and demonstrate whispering gallery microlaser by free-space optical pumping of an Ytterbium doped silica microtoroid via the scatterers. This new scheme will not only expand the range of applications enabled by WGMRs, but also provide a possible route to integrate them into solar powered green photonics.

Currently, optical or mechanical resonances are commonly used in microfluidic research. However, optomechanical oscillations by light pressure were not shown with liquids. This is because replacing the surrounding air with water inherently increases the acoustical impedance and hence, the associated acoustical radiation losses. Here, we bridge between microfluidics and optomechanics by fabricating a hollow-bubble resonator with liquid inside and optically exciting vibrations with 100 MHz rates using only mW optical-input power. This constitutes the first time that any microfluidic system is optomechanically actuated. We further prove the feasibility of microfluidic optomechanics on liquids by demonstrating vibrations on organic fluids with viscous dissipation higher than blood viscosity while measuring density changes in the liquid via the vibration frequency shift. Our device will enable using cavity optomechanics for studying non-solid phases of matter, while light is easily coupled from the outer dry side of the capillary and fluid is provided using a standard syringe pump. Keywords: nonlinear optics; optical materials and devices; optomechanics INTRODUCTION A major application of optical resonators is in the field of sensing where microresonators were used to sense biomarkers in serum 1 and to detect viruses and nanoparticles 2-4 in an aqueous environment. With similar motivation and in a parallel effort, mechanical resonators were used to weigh biomolecules and cells. 5,6 Just like we use more than one sense (e.g., eyes and ears) to detect hazards, optomechanics might suggest a bridge between the seemingly parallel optical and mechanical detection fields. The recent availability of liquid containing bubble shaped resonators, 7,8 in combination with optomechanical vibrations at .GHz rate, 9-12 might pave the way for ultrasound investigations on analytes in liquids. Nevertheless, cavity optomechanics on non-solid phases of material was never before demonstrated.One of the major 'show stoppers' on the way to microfluidic optomechanics originates from the fact that water has acoustical impedance that is more than 4000 times larger than air. Hence, naively immersing optomechanical devices in water will accordingly increase the acoustical radiation losses. Liquid submerged optomechanical oscillators are therefore challenging, as sound will tend to escape from the cavity by radiating out rather than being confined to the resonator. Here, we confine the high-impedance water inside 6,13 a silica-bubble resonator so that its acoustical quality factor is minimally affected. In this manner, when both mechanical and optical quality factors, Q m and Q O , of microdevices are sufficiently leveraged, their optical and mechanical modes can be parametrically coupled, allowing the optical excitation of vibrations. 14 In spherical shapes 9 such as our bubble,