Nanoscale modal confinement is known to radically enhance the effect of intrinsic Kerr and Raman nonlinearities within nanophotonic silicon waveguides. By contrast, stimulated Brillouin-scattering nonlinearities, which involve coherent coupling between guided photon and phonon modes, are stifled in conventional nanophotonics, preventing the realization of a host of Brillouin-based signal-processing technologies in silicon. Here we demonstrate stimulated Brillouin scattering in silicon waveguides, for the first time, through a new class of hybrid photonic–phononic waveguides. Tailorable travelling-wave forward-stimulated Brillouin scattering is realized—with over 1,000 times larger nonlinearity than reported in previous systems—yielding strong Brillouin coupling to phonons from 1 to 18 GHz. Experiments show that radiation pressures, produced by subwavelength modal confinement, yield enhancement of Brillouin nonlinearity beyond those of material nonlinearity alone. In addition, such enhanced and wideband coherent phonon emission paves the way towards the hybridization of silicon photonics, microelectromechanical systems and CMOS signal-processing technologies on chip.

Strong Brillouin coupling has only recently been realized in silicon using a new class of optomechanical waveguides that yield both optical and phononic confinement. Despite these major advances, appreciable Brillouin amplification has yet to be observed in silicon. Using a new membrane-suspended silicon waveguide we report large Brillouin amplification for the first time, reaching levels greater than 5 dB for modest pump powers, and demonstrate a record low (5 mW) threshold for net amplification. This work represents a crucial advance necessary to realize highperformance Brillouin lasers and amplifiers in silicon.Both Kerr and Raman nonlinearities are radically enhanced by tight optical-mode confinement in nanoscale silicon waveguides [1][2][3][4]. Counter-intuitively, Brillouin nonlinearities are exceedingly weak in these same nonlinear waveguides [5]. Only recently have strong Brillouin interactions been realized in a new class of optomechanical structures that control the interaction between guided photons and phonons [5][6][7]. With careful design, such Brillouin nonlinearities overtake all other nonlinear processes in silicon [6,7]; these same Brillouin interactions are remarkably tailorable, permitting a range of hybrid photonic-phononic signal processing operations that have no analog in all-optical signal processing [8][9][10][11][12]. Using this physics, the rapidly growing field of silicon-based Brillouin-photonics has produced new frequency agile RFphotonic notch filters [8,10,13,14] and multi-pole bandpass filters [12] as the basis for radio-frequency photonic (RF-photonic) signal processing. Beyond these specific examples, the potential impact of such Brillouin interactions is immense; frequency combs [13,15,16], ultra-low phasenoise lasers [17][18][19], sensors [9,12,20], optical isolation [21][22][23][24], and an array of signal processing technologies [8,[12][13][14][25][26][27]] may be possible in silicon with further progress.However, strong Brillouin amplification-essential to many new Brillouin-based technologies-has yet to be realized in silicon photonics. Despite the creation of strong Brillouin nonlinearities in a range of new structures [6,7], nonlinear losses and free carrier effects have stifled attempts to demonstrate net optical amplification. Only recently, Van Laer et al. reported 0.5 dB (12%) amplification [28] using suspended silicon nanowire structures. Even with superb dimensional control, amplification diminishes with longer interaction lengths [28], highlighting the problem of dimensionally induced inhomogenous broadening [29]. Careful theoretical analyses by Wolff et al., suggest that large net amplification is fundamentally challenging to achieve in silicon nanowires at near-IR wavelengths due to nonlinear absorption [30].In this paper, we report large Brillouin amplification in silicon through an alternative device paradigm; using a new all-silicon membrane structure (Fig. 1) that permits independent design of photonic and phononic modes, we demonstrate net amplification a...

Rapid progress in integrated photonics has fostered numerous chip-scale sensing, computing and signal processing technologies. However, many crucial filtering and signal delay operations are difficult to perform with all-optical devices. Unlike photons propagating at luminal speeds, GHz-acoustic phonons moving at slower velocities allow information to be stored, filtered and delayed over comparatively smaller length-scales with remarkable fidelity. Hence, controllable and efficient coupling between coherent photons and phonons enables new signal processing technologies that greatly enhance the performance and potential impact of integrated photonics. Here we demonstrate a mechanism for coherent information processing based on travelling-wave photon–phonon transduction, which achieves a phonon emit-and-receive process between distinct nanophotonic waveguides. Using this device, physics—which supports GHz frequencies—we create wavelength-insensitive radiofrequency photonic filters with frequency selectivity, narrow-linewidth and high power-handling in silicon. More generally, this emit-receive concept is the impetus for enabling new signal processing schemes.

We develop a general framework of evaluating the Stimulated Brillouin Scattering (SBS) gain coefficient in optical waveguides via the overlap integral between optical and elastic eigen-modes. This full-vectorial formulation of SBS coupling rigorously accounts for the effects of both radiation pressure and electrostriction within micro- and nano-scale waveguides. We show that both contributions play a critical role in SBS coupling as modal confinement approaches the sub-wavelength scale. Through analysis of each contribution to the optical force, we show that spatial symmetry of the optical force dictates the selection rules of the excitable elastic modes. By applying this method to a rectangular silicon waveguide, we demonstrate how the optical force distribution and elastic modal profiles jointly determine the magnitude and scaling of SBS gains in both forward and backward SBS processes. We further apply this method to the study of intra- and inter-modal SBS processes, and demonstrate that the coupling between distinct optical modes are necessary to excite elastic modes with all possible symmetries. For example, we show that strong inter-polarization coupling can be achieved between the fundamental TE- and TM-like modes of a suspended silicon waveguide.