Nils T. Otterstrom

On-chip inter-modal Brillouin scattering

Brillouin nonlinearities—which result from coupling between photons and acoustic phonons—are exceedingly weak in conventional nanophotonic silicon waveguides. Only recently have Brillouin interactions been transformed into the strongest and most tailorable nonlinear interactions in silicon using a new class of optomechanical waveguides that control both light and sound. In this paper, we use a multi-mode optomechanical waveguide to create stimulated Brillouin scattering between light-fields guided in distinct spatial modes of an integrated waveguide for the first time. This interaction, termed stimulated inter-modal Brillouin scattering, decouples Stokes and anti-Stokes processes to enable single-sideband amplification and dynamics that permit near-unity power conversion. Using integrated mode multiplexers to address separate optical modes, we show that circulators and narrowband filters are not necessary to separate pump and signal waves. We also demonstrate net optical amplification and Brillouin energy transfer as the basis for flexible on-chip light sources, amplifiers, nonreciprocal devices and signal-processing technologies.

A silicon Brillouin laser

Making silicon shine bright Silicon is the workhorse of the semiconductor electronics industry, but its lack of optical functionality is a barrier to developing a truly integrated silicon-based optoelectronics platform. Although there are several ways of exploiting nonlinear light-matter interactions to coax silicon into optical functionality, the effects tend to be weak. Otterstrom et al. used a suspended silicon waveguide racetrack structure to stimulate the stronger nonlinear effect of Brillouin scattering and achieve lasing from silicon. The ability to engineer the nonlinearity and tune the optical response through the design of the suspended cavity provides a powerful and flexible route for developing silicon-based optoelectronic circuits and devices. Science, this issue p. 1113 Brillouin scattering is exploited to develop a silicon laser. Brillouin laser oscillators offer powerful and flexible dynamics as the basis for mode-locked lasers, microwave oscillators, and optical gyroscopes in a variety of optical systems. However, Brillouin interactions are markedly weak in conventional silicon photonic waveguides, stifling progress toward silicon-based Brillouin lasers. The recent advent of hybrid photonic-phononic waveguides has revealed Brillouin interactions to be one of the strongest and most tailorable nonlinearities in silicon. In this study, we have harnessed these engineered nonlinearities to demonstrate Brillouin lasing in silicon. Moreover, we show that this silicon-based Brillouin laser enters a regime of dynamics in which optical self-oscillation produces phonon linewidth narrowing. Our results provide a platform to develop a range of applications for monolithic integration within silicon photonic circuits.

Non-reciprocal interband Brillouin modulation

Non-reciprocal light propagation is essential to control optical crosstalk and back-scatter in photonic systems. However, realizing high-fidelity non-reciprocity in low-loss integrated photonic circuits remains challenging. Here, we experimentally demonstrate a form of non-local acousto-optic light scattering to produce non-reciprocal single-sideband modulation and mode conversion in an integrated silicon photonic platform. In this system, a travelling-wave acoustic phonon driven by optical forces in a silicon waveguide spatiotemporally modulates light in a separate waveguide through linear interband Brillouin scattering. This process extends narrowband optomechanics-based schemes for non-reciprocity to travelling-wave physics, enabling large operation bandwidths of more than 125 GHz and up to 38 dB of non-reciprocal contrast between forward- and backward-propagating optical waves. The modulator operation wavelength is tunable over a 35-nm range by varying the optical drive wavelength. Such travelling-wave acousto-optic interactions provide a promising path toward the realization of broadband, low-loss isolators and circulators within integrated photonics.Non-reciprocal single-sideband modulation and mode conversion are realized in a low-loss integrated silicon waveguide, enabling >125 GHz operation bandwidths and up to 38 dB of non-reciprocal contrast between forward- and backward-propagating waves.

Brillouin lasers and amplifiers in silicon photonics

Using a new class of optomechanical waveguides that produce large Brillouin nonlinearities, we realize Brillouin lasers, Brillouin amplifiers, and Brillouin-based signal processing technologies in silicon photonics. Counterintuitively, the same nanophotonic silicon waveguides that greatly enhance both Kerr and Raman nonlinearities exhibit vanishingly small Brillouin nonlinearities. Only with the advent of new optomechanical waveguides—that guide both light and sound—have Brillouin interactions been transformed into the strongest and most tailorable nonlinearities in silicon. We summarize progress in the rapidly growing field of integrated Brillouin photonics, and explain how a variety of simulated lightscattering processes can be engineered to (1) create Brillouin-based optical amplifiers, (2) tailor optical susceptibility, and (3) create new signal processing technologies in silicon photonics. Finally, we harness Brillouin-based opticalamplification to create the first silicon-based Brillouin lasers and we discuss their performance characteristics.