Cryogenic cavity turns light into mechanical displacement in a bulk acoustic wave device

Known for applications ranging from distinguishing cell phone signals to pressure sensors to improved clocks, bulk acoustic wave (BAW) resonators made from quartz provide stable acoustic frequency references that are automatically coupled to electronics through the piezoelectricity of the quartz crystal. Recent work by Bon et al. leveraged the low-noise advantages of cryogenic environments and cutting-edge laser technology to develop a new type of BAW device that can transfer optical signals into mechanical ones.

The authors demonstrated the new optomechanical device that uses radiation pressure from an optical source to efficiently actuate a mechanical displacement in quartz plano-convex bulk cavities chilled to nearly absolute zero. The resonator traps light inside the cavity with a thin metallic film, serving both as optical mirrors and a piezoelectric electrode. It loses only one billionth of the stored mechanical energy per oscillation cycle, a key characteristic for a device that deals with low radiation pressures and energy levels.

When in resonance, incident laser beams become increasingly intense inside the cavity, exerting a radiation pressure and exciting the cavity’s vibration at its eigenfrequency. This generates a stable frequency tone at its piezoelectric electrode. The authors found that this might work even better at an overtone of the fundamental mechanical resonance frequency, thus further boosting the device’s performance.

Non-piezoelectric materials, such as sapphire, calcium fluoride, and diamond, that can exhibit even lower mechanical loss than quartz, could then be used in opto-mechanical clocks. The results suggest that better oscillators might be on the horizon, potentially leading to better clocks, GPS devices and astronomical measurements. The group will also further investigate the photoelastic and thermal effects that continue to make optomechanical actuation difficult.

Source: “Cryogenic optomechanic cavity in low mechanical loss material,” by Jérémy Bon, Leonhard Neuhaus, Samuel Deléglise, Tristan Briant, Philippe Abbé, Pierre-François Cohadon, and Serge Galliou, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5042058 .