Electromechanical Brillouin scattering in integrated optomechanical waveguides

Qiyu Liu,Mo Li,Huan Li

doi:10.1364/optica.6.000778

Abstract

In the well-known stimulated Brillouin scattering (SBS) process, spontaneous acoustic phonons in materials are stimulated by laser light and scatter the latter into a Stokes sideband. SBS becomes more pronounced in optical fibers and has been harnessed to amplify optical signals and even achieve lasing. Exploitation of SBS has recently surged on integrated photonics platforms as simultaneous confinement of photons and phonons in waveguides leads to drastically enhanced interaction. Instead of being optically stimulated, coherent phonons can also be electromechanically excited with very high efficiency as has been exploited in radiofrequency acoustic filters. Here, we demonstrate electromechanically excited Brillouin scattering in integrated optomechanical waveguides made of piezoelectric material aluminum nitride (AlN). Acoustic phonons of 16 GHz in frequency are excited with nanofabricated electromechanical transducers to scatter counter-propagating photons in the waveguide into a single anti-Stokes sideband. We show that phase-matching conditions of Brillouin scattering can be tuned by varying both the optical wavelength and the acoustic frequency to realize tunable single-sideband modulation. Combining Brillouin scattering photonics with nanoelectromechanical systems, our approach provides an efficient interface between microwave and optical photons that will be important for microwave photonics and potentially quantum transduction.

Highlights

The scattering of light by the sound wave, namely Brillouin scattering, is of technological importance because this three-wave mixing process provides transduction between the fastmoving photons and the slow-moving phonons [1,2,3,4,5,6,7]
We demonstrate electromechanical Brillouin scattering in optomechanical waveguides in which photons and acoustic phonons are co- or counter-propagating
We build the entire device on the aluminum nitride (AlN) on silicon platform, where both the optical waveguides and the piezoelectric transducers are fabricated on the 330 nm thick AlN layer [36,37,38,39,40]

Summary

Introduction

The scattering of light by the sound wave, namely Brillouin scattering, is of technological importance because this three-wave mixing process provides transduction between the fastmoving photons and the slow-moving phonons [1,2,3,4,5,6,7]. Realizing on-chip Brillouin scattering using electromechanically excited acoustic waves, as opposed to optical stimulation in SBS, will provide the complementary and important transduction from the microwave to the optical domain. The ultrahigh frequency phonons provide the momentum required by the phase-matching condition to backscatter counter-propagating photons into an anti-Stokes sideband (ASB). This acousto-optic scattering process distinguishes from those in prior work in guided-wave acousto-optics [31,32,33,34,35], in which acoustic wave of much lower frequency is used to deflect light by only a small angle in a 2D waveguide system. The ultrahigh frequency enabled the unprecedented backward Brillouin scattering configuration in the OM waveguide, which, when fully optimized, can naturally achieve tunable single-sideband modulation within a compact device footprint

Electromechanical Brillouin scattering in an optomechanical waveguide

Experimental demonstration of electromechanical Brillouin scattering

RF photonic link using electromechanical Brillouin scattering

Conclusion

Device fabrication