“Snowflake Crystal” Traps Light and Sound

Abstract

We present the fabrication and characterization of an artificial crystal structure formed from a thin film of silicon that has a full phononic band gap for microwave X-band phonons and a two-dimensional pseudoband gap for near-infrared photons. An engineered defect in the crystal structure is used to localize optical and mechanical resonances in the band gap of the planar crystal. Two-tone optical spectroscopy is used to characterize the cavity system, showing a large coupling (g 0 =2π ≈ 220 kHz) between the fundamental optical cavity resonance at ω o =2π ¼ 195 THz and colocalized mechanical resonances at frequency ω m =2π ≈ 9.3 GHz. DOI: 10.1103/PhysRevLett.112.153603 PACS numbers: 42.50.Wk, 42.65.-k, 62.25.-g The control of optical [1,2] and mechanical waves [3,4] by periodic patterning of materials has been a focus of research for more than two decades. Periodically patterned dielectric media, or photonic crystals, have led to a series of scientific and technical advances in the way light can be manipulated and have become a leading paradigm for onchip photonic circuits [5,6]. Periodic mechanical structures, or phononic crystals, have also been developed to manipulate acoustic waves in elastic media, with myriad applications from radio-frequency filters [7] to the control of heat flow in nanofabricated systems [8]. It has also been realized that the same periodic patterning can simultaneously be used to modify the propagation of light and acoustic waves of similar wavelength [9,10]. Such phoxonic or optomechanical crystals can be engineered to yield strong optoacoustic interactions due to the colocalization of optical and acoustic fields [11][12][13][14].Utilizing silicon-on-insulator (SOI) wafers, similar to that employed to form planar photonic crystal devices [6], patterned silicon nanobeam structures have recently been created in which strong driven interactions are manifest between localized photons in the λ ¼ 1500 nm telecom band and GHz-frequency acoustic modes [13,15,16]. These quasi-one-dimensional (1D) optomechanical crystal (OMC) devices have led to new optomechanical effects, such as the demonstration of slow light and electromagnetically induced amplification [15], radiation pressure cooling of mechanical motion to its quantum ground state [16], and coherent optical wavelength conversion [17]. Although two-dimensional (2D) photonic crystals have been used to study localized phonons and photons [18][19][20], in order to create circuit-level functionality for both optical and acoustic waves, a planar 2 D crystal structure with simultaneous photonic and phononic band gaps [21-23] is strongly desired. In this Letter, we demonstrate a 2D OMC structure formed from a planar "snowflake" crystal [23] that has both an in-plane pseudoband gap for telecom photons and a full three-dimensional band gap for microwave X-band phonons. A photonic and phononic resonant cavity is formed in the snowflake lattice by tailoring the properties and inducing a defect in a bandgap-guided waveguide for optical and acoust...