Mechanical-optical-mechanical phononic frequency comb generation at GHz range

Abstract

Light carries momentum which can give rise to radiation pressure forces. The idea of electromagnetic radiation exerting a force onto physical objects was already postulated in the 17th century by Kepler, who observed that the tail of comets always point away from the Sun during their transit. This phenomenon is now used for a variety of applications such as the cooling of atomic motion with lasers, the cooling vibrational motion of mechanical resonators, and the induction of mechanical oscillations in a system. This lattermost application is in accordance with the work of V.B. Braginsky which predicted that circulating radiation in a Fabry-Pérot cavity could induce a pressure that is able to couple optical modes to the mechanical modes of the structure. Mechanical oscillation generation has significant potential for applications in information processing and data communication. This is because phonons have the possibility to be used as carriers of information signals in the MHz and GHz domain which allows them to connect the operating regimes of electronics and optics. In 2007, P. Del’Haye et al. employed a silica micro-toroid resonator and demonstrated that by coupling the optical modes of the structure to the mechanics of the platform, an equally spaced frequency spectra appeared. This was the first demonstration of the so called Kerr frequency combs, a type of optical comb generated via the Kerr nonlinearity. Frequency combs, more specifically, optical frequency combs have revolutionized the field of frequency metrology and spectroscopy and are enabling components for a variety of applications due to their ability to allow for precise measurement of optical frequencies. Applications include comb-calibration of tunable lasers, direct comb spectroscopy, arbitrary waveform generation, and advanced telecommunications. Although the formation of mechanical combs has also been observed, they have not received as much attention as their optical counterparts. The present work presents an optomechanical system based on two phononic crystals with a guided mode around 6.8 GHz placed alongside an air-slot which serves as the optical cavity. A vibrational mode in the MHz has also been reported in the structure. The coupling of an optical mode to the mechanical modes of the structure allows for the amplification of the oscillations in the platform via the radiation pressure force. This Mechanical-Optical-Mechanical (M-O-M) coupling between light and mechanics allows the observation of a phononic periodic spectral feature centered at 6.78 GHz with a width of around 2 GHz when the radiation pressure is increased. This study reports a self-sustained phononic comb formation in the GHz range.