Abstract
Radio-frequency communication systems have long used bulk- and surface-acoustic-wave devices supporting ultrasonic mechanical waves to manipulate and sense signals. These devices have greatly improved our ability to process microwaves by interfacing them to orders-of-magnitude slower and lower loss mechanical fields. In parallel, long-distance communications have been dominated by low-loss infrared optical photons. As electrical signal processing and transmission approaches physical limits imposed by energy dissipation, optical links are now being actively considered for mobile and cloud technologies. Thus there is a strong driver for wavelength-scale mechanical wave or "phononic" circuitry fabricated by scalable semiconductor processes. With the advent of these circuits, new micro- and nanostructures that combine electrical, optical and mechanical elements have emerged. In these devices, such as optomechanical waveguides and resonators, optical photons and gigahertz phonons are ideally matched to one another as both have wavelengths on the order of micrometers. The development of phononic circuits has thus emerged as a vibrant field of research pursued for optical signal processing and sensing applications as well as emerging quantum technologies. In this review, we discuss the key physics and figures of merit underpinning this field. We also summarize the state of the art in nanoscale electro- and optomechanical systems with a focus on scalable platforms such as silicon. Finally, we give perspectives on what these new systems may bring and what challenges they face in the coming years. In particular, we believe hybrid electro- and optomechanical devices incorporating highly coherent and compact mechanical elements on a chip have significant untapped potential for electro-optic modulation, quantum microwave-to-optical photon conversion, sensing and microwave signal processing.
Highlights
Microwave-frequency acoustic or mechanical wave devices have found numerous applications in radio-signal processing and sensing
The g0/(2π) ≈ 1 MHz measured in silicon optomechanical crystals [15] is via equation 9 in correspondence with the GB/Qm ≈ 10 W−1m−1 measured in silicon nanowires at slightly higher frequencies [14, 29]
In three-wave Sum-frequency driving (SFD), two microwave photons with a frequency below the phonon frequency ωm excite mechanical motion at the sum-frequency ω + ω = Ω ≈ ωm [3]. Such interactions can be realized in capacitive electromechanics, where the capacitance of an electrical circuit depends on mechanical motion
Summary
Microwave-frequency acoustic or mechanical wave devices have found numerous applications in radio-signal processing and sensing. The fortuitous matching of length scales was used to demonstrate the first 2D- and 3Dconfined systems in which both photons and phonons are confined to the same area or volume [Fig. 1] These measurements have been enabled by advances in low-loss photonic circuits that couple light to material deformations through boundary and photoelastic perturbations. We primarily consider recent advances in gigahertz-frequency phononic devices These devices have been demonstrated mainly in the context of photonic circuits, and share many commonalities with integrated photonic structures in terms of their design and physics. Despite recent demonstrations of confined phonon devices operating at gigahertz frequencies and coupled to optical fields, phononic circuits are still in their infancy, and applications beyond those of interest in integrated photonics remain largely unexplored. While not strictly limited to the material silicon, its goal is to develop a platform whose fabrication is in principle scalable to many densely integrated mechanical devices
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