Thermo-elastic gigahertz-frequency oscillator through surface acoustic wave-silicon photonics.

Opto-electronic oscillators are sources of microwave-frequency tones that may reach very low noise levels. Much effort is being dedicated to the realization of oscillators based on photonic integrated devices. In this work, we propose and demonstrate a thermo-elastic opto-electronic oscillator at 2.213 GHz frequency based on a standard silicon-photonic integrated circuit. A microwave-frequency electrical signal modulates an optical pump wave carrier. The modulated waveform launches surface acoustic waves in a silicon-on-insulator substrate, through absorption in a metallic grating and thermo-elastic actuation. The waveform is reconverted to the optical domain through photoelastic modulation of an optical probe wave carrier in a standard racetrack resonator waveguide. Both the thermo-elastic actuation and the photoelastic modulation are radio-frequency selective. The output probe wave is detected, and the receiver voltage is amplified and fed back to modulate the optical pump input. Sufficient gain drives the loop into oscillations. The oscillator does not involve piezoelectricity and can be realized on any substrate. Long acoustic delays may be implemented in compact devices. The frequency of operation is scalable to tens of GHz. The principle may be useful in integrated microwave-photonic signal processing and in the elastic analysis of surfaces and thin layers.

Role of surface passivation in integrated sub-bandgap silicon photodetection.

We study experimentally the effect of oxide removal on the sub-bandgap photodetection in silicon waveguides at the telecom wavelength regime. Depassivating the device allows for the enhancement of the quantum efficiency by about 2-3 times. Furthermore, the propagation loss within the device is significantly reduced by the oxide removal. Measuring the device 60 days after the depassivation shows slight differences. We provide a possible explanation for these observations. Clearly, passivation and depassivation play an essential role in the design and the implementation of such sub-bandgap photodetector devices for applications such as on-chip light monitoring.