Multi-mode optomechanical systems have formed the basis of recent proposals and experiments, enabling optical frequency translation and hybridization of near-resonant mechanical modes. An important question is how to control the internal mechanical states of such systems using laser light. Such control enables engineering of effective nonlinearities for phonons, allowing phonon-phonon frequency translation, mechanical entanglement, and precision metrology. On-chip engineered nanostructures are particularly suitable for exploring multi-mode systems.
Here, we consider a silicon nanobeam optomechanical crystal with two mechanical modes coupled to a common optical mode. Simulations of the phonon-phonon scattering parameters of the system suggest that large conversion efficiency can be obtained at cryogenic temperatures. We show that remarkably, phonon-phonon conversion efficiency near unity is achievable, even when the loss rate of the intermediate optical mode dominates all other rates in the system by several orders of magnitude. This counter-intuitive phenomenon is the result of a long-lived mechanical dark state of the system that arises in the optical pumping scheme being used. We experimentally demonstrate two GHz frequency mechanical modes, separated by nearly 300 MHz, coupled to a first-order common optical TE mode with vacuum coupling rates of nearly 500 kHz. By optically driving the optomechanical crystal with two tones separated by the mechanical difference frequency we present evidence for optically induced phonon-phonon interactions at room temperature. We will present results of measurements in a cryogenic environment, operating at 4 Kelvin demonstrating improved large phonon-phonon conversion efficiency.
The optical and mechanical properties of silicon and silica glass make the silicon-on-insulator material system a platform natural for photonics and challenging for phononics. High index-contrast enables index-guiding in silicon waveguides on glass, but silicon's relative stiffness and high sound velocity hampers analogous efforts to ``index-guide'' acoustic waves. Waveguide geometry plays fundamentally different roles in the dispersion of mechanical and optical waves, enabling radiation-free waveguiding in high aspect-ratio cantilevers defined in silicon. We fabricate silicon fins, here 80 nm wide in 340 nm SOI, that exhibit low-loss mechanical resonances at 600-700 MHz.
We present designs, numerical studies, and the first measurements of release-free optomechanical “fin cavities” in 340 nm SOI. The dispersion of flexural fin mechanical modes is readily engineered by variation of the fin's width. TE and TM optical cavities at telecom frequencies are made with an adjoined nanobeam. Nanobeam geometry independently influences the optics decoupling optical and mechanical design problems. Optical and mechanical modes can be colocalized with a simple cavity where a parabolically curved fin is placed near a photonic crystal waveguide. We simulate and measure optical and mechanical spectra of these devices. Optomechanical interaction rates ranging from low kHz to 500 kHz for the fin cavities are demonstrated. Furthermore, by analyzing the interaction rates we identify the different optical modes of these structures. The demonstrated SOI fin cavities create new opportunities for quantum optomechanical sensing in a truly CMOS-compatible setting.