Optomechanics in an ultrahigh-Q two-dimensional photonic crystal cavity

Abstract

We demonstrate an ultrahigh-Q slotted two-dimensional photonic crystal cavity capable of obtaining strong interaction between the internal light field and the mechanical motion of the slotted structure. The measured optical quality factor is Q = 1.2ϫ 10 6 for a cavity with an effective modal volume of V eff = 0.04͑ ͒ 3 . Optical transduction of the thermal motion of the fundamental in-plane mechanical resonance of the structure ͑ m = 151 MHz͒ is performed, from which a zero-point motion optomechanical coupling rate of g ‫ء‬ / 2 = 320 kHz is inferred. Dynamical back-action of the optical field on the mechanical motion, resulting in cooling and amplication of the mechanical motion, is also demonstrated. © 2010 American Institute of Physics. ͓doi:10.1063/1.3507288͔The strength of the interaction between light and matter, which is fundamental to many applications in nonlinear and quantum optics, depends on the ability to create a large optical energy density, either through increased photon number or photon localization. This may be achieved by creating optical cavities with large quality factors Q and simultaneously small modal volumes V eff . The mode volume V eff in particular can be decreased through the introduction of slots, increasing the electric field intensity in low-index regions of the device. As such, slotted photonic crystal cavities 1 and waveguides 2 have been previously proposed and applied to create highly sensitive detectors of motion 3,4 and molecules. 5 They have also more recently been studied in the context of Purcell enhancement of spontaneous emission from embedded quantum dots. 6 In the canonical optomechanical system, consisting of a Fabry-Perot resonator with an oscillating end-mirror, 7 the radiation pressure force per cavity photon is given by បgwhere o is the cavity resonance frequency, x is the position of the end mirror, and L OM is approximately equal to the physical length of the cavity. In the quantum realm, one is interested in the zero-point motion coupling rate, which is given by g = g OM ͱ ប / 2m eff m , where m eff is the effective motional mass and m is the mechanical resonance frequency. Large optomechanical coupling, approaching g OM = o / , has recently been realized in several different guided wave optical cavity geometries utilizing nanoscale slots. 3,4,8 In this work we design, fabricate, and measure the optomechanical properties of a slotted twodimensional ͑2D͒ photonic crystal cavity formed in a Silicon membrane. Due to the strong optical confinement provided by a sub-100 nm slot and a 2D photonic band gap, this cavity structure is demonstrated to have an optical quality factor Q Ͼ 10 6 and a coupling rate of g / 2 = 320 kHz.A common approach to forming photonic crystal optical circuits is to etch a pattern of holes into a thin dielectric film such as the top Silicon device layer in a Silicon-On-Insulator ͑SOI͒ microchip. An effective means of forming resonant cavities in such quasi-2D slab photonic crystal structures is to weakly modulate the properties of a line-de...

Wavelength-sized GaAs optomechanical resonators with gigahertz frequency

“…7,10 For disk 1, this second method yields an experimental optomechanical coupling of g 0 e = 0.8 MHz ͑g om e = 485 GHz/ nm͒, a factor 1.4 times smaller than the simulated value. Similar discrepancies were observed in photonic crystals structures [7][8][9] and probably stem from an imperfect control in the geometry of the nanofabricated resonator. The associated irregularity is difficult to capture in numerical simulations.…”

“…Large optomechanical coupling, approaching g om = o / with the wavelength of photons, was recently obtained in photonic-crystal based structures. [7][8][9] In this work, we scale the size of GaAs optomechanical disk resonators 10 down to and show that we can this way naturally couple an ͑over͒ GHz mechanical oscillator to an optical cavity of high quality factor Q and subwavelength mode volume. Thanks to the strong confinement of the optical and mechanical modes in a nanoscale disk, we approach g om = o / and obtain a vacuum coupling rate g 0 = 0.8 MHz.…”

Optomechanic interactions in phoxonic cavities

“…Safavi-Naeini et al gave theoretical values of 1.8 MHz for a 'snowflake' crystal slab structure 15 and 2 MHz in a slotted two-dimensional photonic crystal cavity. 39 The highest value was reported by Chan et al for a 1D nanobeam structure 14 with a theoretical value of 5.4 MHz and experimental value of 1.1 MHz.…”

Non-classical correlations between single photons and phonons from a mechanical oscillator

“…One of the outstanding challenges is to achieve single-particle coupling rates that are sufficiently large to alleviate effects of optical and mechanical decoherence in the system, that is, single-photon strong co-operativity. Some of the largest optomechanical couplings have been reported in nanomechanical photonic crystal cavities [25], but are still two orders of magnitude short of that regime. Although low coupling rates can be overcome in principle by a strong and detuned coherent drive field [15], such measures typically result in unwanted heating of the mechanical device (see Methods).…”