Analysis of enhanced stimulated Brillouin scattering in silicon slot waveguides

Impacts of spatial symmetries on stimulated Brillouin scatterings in nanoscale silicon waveguides: a theoretical and numerical study considering material anisotropies

Fully considering the mechanical and photoelastic anisotropies of monocrystalline silicon, the impacts of spatial symmetries on the stimulated Brillouin scatterings (SBSs) in nanoscale suspended silicon waveguides are studied theoretically and numerically based on group theory. First, starting from an assumption that the principal material coordinate system can be arbitrarily orientated in a waveguide with fixed geometry, the silicon waveguides are systematically classified into a number of point groups according to their spatial symmetry features. Thereafter, the symmetry characteristics of physical fields and SBS opto-mechanical coupling characteristics in the silicon waveguides belonging to different point groups are further examined, and the major new findings can be summarized as follows: The SBS opto-mechanical couplings in several kinds of silicon waveguides with certain nontrivial symmetry features exhibit relatively predictable behaviors in that the opto-mechanical coupling coefficients can be deterministically vanishing or nonvanishing under very few constraints, which can thus serve as general symmetry selection rules for SBSs in suspended silicon waveguides. The results obtained in the present study could be a useful theoretical reference for the design of novel SBS-active silicon photonic devices.

Mode multiplexer for guided optical and acoustic waves

Integrated acousto-optic (AO) devices utilize the strong overlap of acoustic and optical fields in a waveguide to facilitate efficient photon-phonon (Brillouin) interactions. For example, acoustic waves offer a lossless modulation mechanism for light. "Brillouin active" photonic platforms are currently being developed that may see optical, acoustic, and AO waveguide circuits on the same chip, where guided light and sound come together in active interaction regions. A key missing component for such a platform is a device that can multiplex modes across these two physical domains. We propose and describe a new class of optical and acoustic components, the "acoustic-optical mode multiplexer" (AOMM), a device that takes respective optical and acoustic waveguides as input ports and couples their excited guided modes into a single, joint output waveguide. We show an example suspended silicon-silicon dioxide design that combines two optical modes and a spatially separate acoustic mode into a single, co-guided output port with low insertion loss down to 0.3 dB for both optical and acoustic modes, and reflection below -20dB and -11dB, respectively. The AOMM may enable new, efficient integrated AO devices, such as isolators and circulators, where the acoustic wave generation and opto-acoustic interaction are separated.

Design of a hybrid on-chip waveguide with giant backward stimulated Brillouin scattering

On-chip waveguides on insulator with high stimulated Brillouin gain have wide potential application prospects in the field of nanophotonic structures. We propose a new on-chip hybrid silicon-chalcogenide slot waveguide structure consisting of a chalcogenide As₂S₃ rectangle core with an air slot and a wrapping layer of silicon. In the new hybrid waveguide, the high radiation pressure and electrostriction force, determined by pump and Stokes optical waves, and the acoustic displacement, determined by acoustic wave, can be achieved by adjusting the dimensions of rectangle core, the thickness of wrapping layers and the width of air slot. Therefore, a strong optomechanical coupling between high radiation pressure and transverse acoustic displacement will be generated. In such a way, a nonlinear gain for backward stimulated Brillouin scattering can be theoretically achieved with a high gain coefficient of 2.88×10⁴ W^-1m^-1. The enhanced gain coefficient in the proposed waveguide is around 2.4 times as that in an on-chip silicon-chalcogenide hybrid slot waveguide on insulator without the wrapping layer. The Stokes amplification reaches 85.7 dB with the waveguide length of 2.5 cm. Therefore, this method provides a new idea to design nanophotonic waveguides for giant backward stimulated Brillouin scattering.

Plasmonic waveguide design for the enhanced forward stimulated brillouin scattering in diamond

We propose a scheme of metal/dielectric/metal waveguide for the enhanced forward stimulated Brillouin scattering (FSBS) in diamond that is mediated by gap surface plasmons. Numerical results based on finite-element method show that the maximum Brillouin gain in the small gap (~100 nm) can exceed 10⁶ W⁻¹ m⁻¹, which is three orders of magnitude higher than that in diamond-only waveguides. It is found that the radiation pressure that exists at the boundaries of metal and diamond plays a dominant role in contributing to the enhanced forward stimulated Brillouin gain, although electrostrictive forces interfere destructively. Detailed study shows that high FSBS gain can still be obtained regardless of the photoelastic property of the dielectric material in the proposed plasmonic waveguide. The strong photon-phonon coupling in this gap-surface-plasmon waveguide may make our design useful in the development of phonon laser, RF wave generation and optomechanical information processing in quantum system.

Strong forces in optomechanically actuated resonant mass sensor

As resonance mass sensors shrink in order to improve the sensitivity, traditional methods of transduction and readout struggle to keep up with increasing resonance frequencies and decreasing feature sizes. In this work we demonstrate an all-photonically transduced resonant mass sensor that manages to deal with these problems. The strong optomechanical force in slot waveguides is used to drive the mechanical resonanator giving a good signal to noise ratio at low optical powers. Using 120 µW of modulated optical power we measure a frequency noise equivalent to being able to resolve 500 kDa.

Photon-phonon Interaction in a Microfiber Induced by Optical and Electrostrictive Forces

Stimulated Brillouin scattering (SBS) via electrostrictive force is a fundamental interaction between light and sound which limits the power in conventional optical fibers. The emergence of optical microfibers with subwavelength diameter, ultralight mass and an intense light field, provides a new platform for photon-phonon coupling, resulting in the radiation pressure mediated contribution of SBS. This study examines the optomechanical system in cylindrical coordinates, reveals the theoretically radiation pressure induced analogous, and demonstrates contrary effect compared with electrostrictive force in solid or hollow silica microfibers. The finding shows that the photon-phonon coupling, which is related to SBS, can be suppressed in a solid microfiber, and even be completely cancelled in a hollow microfiber.

Stimulated Brillouin scattering in silicon/chalcogenide slot waveguides

We study slot waveguide geometries, comprising a combination of soft glasses and high-index guiding structures, for enhancing stimulated Brillouin scattering (SBS). We show that strong optical and acoustic mode confinement in these waveguides can lead to a substantial increase in SBS gain, comparable to or greater than recently proposed suspended silicon nanowire structures. We compute the optimal parameters of the structure and examine the physics of optical and acoustic confinement within slot waveguides. Finally, we compute the effects of linear and nonlinear loss mechanisms on optimum pump/Stokes powers and waveguide lengths.