Researchers: Karen Grutter
Funding Agency: DARPA MTO ORCHID Program
Light forces within micromechanical systems have attracted considerable interest due to their ability to amplify or cool mechanical motion in resonators. This optomechanical interaction is greatly enhanced when high finesse optical cavities are combined with high-Q mechanical resonators. Potential applications of optomechanics include displacement and acceleration sensing, optical mixing, and RF oscillators. In particular, we are focusing on developing an optomechanical oscillator that can be used to replace bulky and power-hungry microwave oscillators in chip-scale atomic clocks. Two of the most important performance metrics for achieving such an oscillator are low threshold power and low phase noise.
Figure 1: PSG optomechanical spoke-supported ring resonator coupled to integrated waveguide and grating couplers
The optical threshold power is strongly dependent on the optical quality factor of the cavity. Thus, we are seeking to maximize optical Q in our device by exploring the use of low-loss materials such as phosphosilicate glass and silicon nitride. To date, we have achieved a maximum optical Q of 6.5 million.
A high mechanical quality factor is required to achieve low phase noise. We are pursuing high mechanical Q in our optomechanical devices by choosing a spoke-supported ring geometry. To further increase mechanical Q, we are also coupling our optical rings to higher-Q poly-Si and diamond mechanical resonators to “boost” the mechanical Q of the system. The best phase noise we have achieved to date is -102 dBc/Hz at a 1kHz offset from the 74MHz carrier.
Figure 2:Phase noise measurements of some of our highest-performance optomechancial oscillators