Abstract
It is interesting to pose the question: How best to design an optomechanical device, with no electronics, optical cavity, or laser gain, that will self-oscillate when pumped in a single pass with only a few mW of single-frequency laser power? One might begin with a mechanically resonant and highly compliant system offering very high optomechanical gain. Such a system, when pumped by single-frequency light, might self-oscillate at its resonant frequency. It is well-known, however, that this will occur only if the group velocity dispersion of the light is high enough so that phonons causing pump-to-Stokes conversion are sufficiently dissimilar to those causing pump-to-anti-Stokes conversion. Recently it was reported that two light-guiding membranes 20 μm wide, ∼500 nm thick and spaced by ∼500 nm, suspended inside a glass fiber capillary, oscillated spontaneously at its mechanical resonant frequency (∼6 MHz) when pumped with only a few mW of single-frequency light. This was surprising, since perfect Raman gain suppression would be expected. In detailed measurements, using an interferometric side-probing technique capable of resolving nanoweb movements as small as 10 pm, we map out the vibrations along the fiber and show that stimulated intermodal scattering to a higher-order optical mode frustrates gain suppression, permitting the structure to self-oscillate. A detailed theoretical analysis confirms this picture. This novel mechanism makes possible the design of single-pass optomechanical oscillators that require only a few mW of optical power, no electronics nor any optical resonator. The design could also be implemented in silicon or any other suitable material.
Highlights
The intriguing dynamics of light-sound interactions have been attracting increasing attention due to remarkable advances in experimental techniques, and a wide range of potential applications are emerging.[1]
Suppression of giant Raman-like gain in optomechanical systems is expected if the group velocity dispersion is negligible over the device length for the frequency bandwidth considered
This is the case in dual-nanoweb fiber, where the Raman-like resonant frequency is ∼6 MHz and the device length ∼10 cm
Summary
The intriguing dynamics of light-sound interactions have been attracting increasing attention due to remarkable advances in experimental techniques, and a wide range of potential applications are emerging.[1]. Diagram of the associated phonon is very flat, i.e., its wavevector can be freely chosen while keeping its frequency fixed For this reason this phenomenon is referred to as stimulated Raman-like scattering (SRLS).[13] It has been observed in a photonic crystal fiber with a solid core ∼1 μm in diameter and a vibrational frequency of a few GHz,[13] and in a dual-nanoweb fiber structure with very strong optomechanical nonlinearity and a resonant frequency of ∼6 MHz.[14] Recently we reported mechanical self-oscillation of such a dual-nanoweb system when pumped by a few mW of narrow-line single-frequency laser light.[15] This came as a surprise, partly because in previous experiments a dual-frequency pump had always been needed to obtain oscillation, and for a more subtle reason: Raman gain suppression. Experimental observations, made by scanning the side-probing beam (Fig. 2) along the fiber, provide convincing evidence for this SIMS/SRLS mechanism of self-oscillation
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