Scientists at the US National Institute of Standards and Technology (NIST) have demonstrated controlled mode competition in a phonon laser, or a laser based on sound waves. The process occurs routinely in optical lasers, but has never before been seen in analogous optomechanical oscillator systems, which have come to be known as phonon lasers.
The scientists at NIST’s Physical Measurement Laboratory (PML) and the Joint Quantum Institute (JQI) used a Fabry-Perot optical cavity, operated in vacuum, in which one mirror is a thin, specially patterned, silicon nitride membrane.
Using the NIST nanofabrication facility (CNST), the researchers patterned the membrane so as to form a sub-wavelength diffraction grating offering a reflectivity of 99.4 per cent. In addition to the optical role the membrane plays in the cavity, it is a mechanical oscillator supporting hundreds of drumhead modes with frequencies upwards of 130kHz.
When light is injected into the cavity, the circulating power becomes very large, and is correlated with the membrane motion. When detuned to the high-frequency side of a cavity resonance, the radiation pressure associated with the circulating power provides mechanical gain. If the gain is sufficient to overcome the intrinsic damping of a particular mechanical mode of the membrane, that mode starts to oscillate. The membrane motion is probed by an auxiliary interferometer beam, not involving the cavity that keeps track of its displacement over time.
The researchers found that, despite the fact that the mechanical gain couples to hundreds of membrane modes, steady-state operation is always characterised by a single mechanical mode. In other words, their phonon laser behaves in a manner analogous to that of an ideal, homogeneously broadened conventional laser.
‘Over the last 50 years, optical lasers have become the staple for optical metrology requiring coherent light at a well-defined frequency,’ said Garnett Bryant, leader of PML’s Quantum Processes and Metrology Group. ‘Nanoscale optical lasers have become ubiquitous for commercial applications with the same needs. The results presented here show the way toward the same goal of single-mode operation for applications and metrology where sources of mechanical vibrations are needed.’