Surface Acoustic Waves in Silicon-on-insulator Photonic Devices

Abstract: A surface acoustic wave device is demonstrated in a standard silicon photonic circuit. Modulated light generates the surface wave through absorption in a metal grating. Photo-elastic perturbations are detected in a nearby ring resonator.

Main text: Silicon-photonic devices support the optical communication and processing of data at Tbit/s rates. Certain processing functions, such as narrow-band filtering of optical waveforms, require long temporal delays. Due to the fast speed of light, long time delays are difficult to realize in photonic waveguide paths. Similar difficulties are being faced by analog electronic circuits for decades. One solution path relies on the conversion of incoming signals to the form of surface acoustic waves (SAWs), which travel at the slow speed of sound [1]. Acoustic waves are often introduced through piezo-electric transduction, which is absent in silicon. In addition, optical silicon-on-insulator (SOI) waveguides do not support guided acoustic modes, which tend to leak into the bulk substrate [2].

In this work, we demonstrate a first SAW device that is part of a photonic circuit in standard SOI. Light that is modulated by a radio-frequency sine wave of frequency f illuminates an input grating target of aluminum stripes, with spatial period L. The absorption of the modulated optical beam leads to periodic heating and cooling of the metal pattern, which in turn induces an alternating thermal strain pattern in the underlying silicon layer [3]. If the modulation frequency is chosen so that f = v/L, where v is the speed of SAWs in the SOI layer stack, a surface wave may propagate away from the metallic grating region. The surface wave induces photo-elastic perturbation to the transmission spectrum of a nearby ring resonator. Therefore, probe light that is tuned to the spectral slope of the ring resonator transmission undergoes modulation by the surface wave.

The stimulation and detection of SAWs was experimentally demonstrated using several metal gratings with different periods L. The frequencies of efficient stimulation consistently match an acoustic velocity of 3,700 m/s, which is close to the speed of shear sound waves in the silica layer of the SOI stack. The devices already serve as narrow-band microwave-photonic filters in SOI. They serve as proof of principle for SAW-photonic processing.

References

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• Nardi et al., Nano Letters 11, 4126-4133 (2011).