1. Thomas J. Watson Sr Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA

Correspondence to: Oskar Painter¹ Correspondence and requests for materials should be addressed to O.P. (Email: opainter@caltech.edu).

Periodicity in materials yields interesting and useful phenomena. Applied to the propagation of light, periodicity gives rise to photonic crystals¹, which can be precisely engineered for such applications as guiding and dispersing optical beams^{2, 3}, tightly confining and trapping light resonantly⁴, and enhancing nonlinear optical interactions⁵. Photonic crystals can also be formed into planar lightwave circuits for the integration of optical and electrical microsystems⁶. In a photonic crystal, the periodicity of the host medium is used to manipulate the properties of light, whereas a phononic crystal uses periodicity to manipulate mechanical vibrations^{7, 8, 9, 10, 11, 12, 13}. As has been demonstrated in studies of Raman-like scattering in epitaxially grown vertical cavity structures¹⁴ and photonic crystal fibres¹⁵, the simultaneous confinement of mechanical and optical modes in periodic structures can lead to greatly enhanced light–matter interactions. A logical next step is thus to create planar circuits that act as both photonic and phononic crystals¹⁶: optomechanical crystals. Here we describe the design, fabrication and characterization of a planar, silicon-chip-based optomechanical crystal capable of co-localizing and strongly coupling 200-terahertz photons and 2-gigahertz phonons. These planar optomechanical crystals bring the powerful techniques of optics and photonic crystals to bear on phononic crystals, providing exquisitely sensitive (near quantum-limited), optical measurements of mechanical vibrations, while simultaneously providing strong nonlinear interactions for optics in a large and technologically relevant range of frequencies.