Viewpoint “ Snowflake Crystal ” Traps Light and Sound

Research on cavity optomechanics, which concerns the interaction between light and mechanical vibrations in confined geometries, has blossomed during the past few years [1]. Potential applications include coherent microwave–optical conversion, sensitive mechanical measurements, quantum information processing, mechanical storage of light pulses, and coupling between different quantum systems, as well as new tests of the foundations of quantum mechanics. Of the many different optomechanics platforms, one of the most promising comprises “optomechanical crystals.” In Physical Review Letters, Amir Safavi-Naeini et al.[2] and his colleagues in Oskar Painter’s group at the California Institute of Technology, Pasadena, exhibit a novel two-dimensional (2D) structure of this type. The engineering of wave propagation by means of periodic patterning of materials has been applied for some time to optical and acoustic waves, leading to photonic and phononic crystals, respectively. Optomechanical crystals that combine the two have emerged in the last few years, as shown earlier by the Painter group [3]. They demonstrated that optical and vibrational modes of high quality can be created in this manner at the same micrometer-sized spot, producing an optomechanical coupling that exceeded that of previous devices by orders of magnitude. This has already been employed successfully in the demonstration of optomechanical laser cooling to phonon numbers below unity [4], close to the ground state, as well as other tasks. These first devices were fabricated by one-dimensional (1D) periodic patterning of freestanding nanobeams. Extending optomechanical crystal structures to two dimensions would allow a greater variety of designs to be built, and the first steps have already been taken. For example, “phonon shields” reduce energy loss in mechanical resonators, as reflected in their “quality factor,” which is roughly the number of oscillations before the energy leaks away. Researchers achieved this by surrounding the resonators with structures that have an acoustic band gap FIG. 1: A planar slab of silicon with a periodic arrangement of snowflake-shaped holes (here viewed from above) forms a two-dimensional optomechanical crystal. (Left) By disrupting the alignment between successive horizontal lines and (not visible) locally altering the structure along that linear defect, Safavi-Naeini et al. produced a single localized defect mode (indicated here by a shaded ellipse) for optical radiation at the same place as a localized vibrational mode of the structure. (Right) In the future, a periodic arrangement of such defects could produce an “optomechanical array,” with photons and phonons tunneling between each pair of neighboring sites (exemplified by the arrow) and interacting with each other. (APS/Florian Marquardt)