A Multi-material Low Phase Noise Optomechanical Oscillator
Technical Report Identifier: EECS-2014-235
Abstract: Recent advancements in cavity optomechanics have allowed researchers to exploit coupling between the optical field and mechanical motion of an optical cavity to affect cooling or amplification of mechanical motion. Cooling the mechanical motion of microscale objects has been of high scientific interest, since it facilitates observation and exploration of certain quantum phenomena, e.g., the standard quantum limit of detection. On the other hand, amplification of the mechanical motion allows realization of microscale devices for practical applications, such as light driven low phase noise signal generation by Radiation Pressure driven OptoMechanical Oscillators (RP-OMO). The ability to achieve self-sustained oscillation with no need for feedback electronics makes an RP-OMO compelling for on-chip applications where directed light energy, e.g., from a laser, is available to fuel the oscillation, such as Chip Scale Atomic Clocks (CSAC). Indeed, an RP-OMO can substantially reduce power consumption of a CSAC by replacing its power-hungry conventional quartz-based microwave synthesizer but this requires that the RP-OMO output be sufficiently stable, as gauged over short time spans by its phase noise. This motivates a focus on achieving a high mechanical Q (Qm) RP-OMO to have low phase noise while maintaining a reasonably high optical Q (Qo) for low power operation-a challenge in previous OMOs that had to trade-off Qm and Qm mainly because they use a single material to set both its Qm and Qo. The work in this report presents a multi-material RP-OMO that circumvents this limitation by combining a silicon nitride optical material with a lower mechanical loss polysilicon material to attain simultaneous high Qm and Qo. The multi-material RP-OMO structure boosts the Qm of a silicon nitride RP-OMO by more than 2× toward realization of the simultaneous high Qm greater than 22,000 and Qo greater than 190,000 needed to maximize RP-OMO performance. With its high Qm, the multi-material RP-OMO exhibits a best-to-date phase noise of -114 dBc/Hz at 1 kHz offset from its 52-MHz carrier—a 12 dB improvement from the previous best by an RP-OMO constructed of silicon nitride alone. The doped polysilicon structure and electrodes additionally allow tuning of the RP-OMO’s oscillation frequency via DC voltage, enabling future deployment of the multi-material RP-OMO as a locked oscillator in a target low-power CSAC application.