Release-free lithium niobate electromechanical crystals

Doctoral thesis, 2026

Preserving mechanical motion requires careful design to reduce damping by the environment. One approach is to suspend a mechanical resonator using narrow supports, which lets it oscillate for a long time with little interaction with the environment. But this strong isolation from the environment also makes it difficult for heat to escape. This is an issue in devices which heat up during operation, as is the case for transducers that convert between light and microwave signals. At room temperature in air, radiation and convection help remove heat, but these cooling mechanisms are much less effective in the cold vacuum environment used for quantum experiments.

Here, to help heat escape, we instead attach the mechanical resonators directly to the chip surface rather than suspending them, in a release-free approach. However, to preserve the mechanical motion, we now need to prevent vibrations from escaping into the underlying chip. We achieve this through total internal reflection within mechanical waveguides. To confine the mechanical waves, we pattern the waveguide with a repeating structure that forms acoustic mirrors, trapping vibrations in a small region. To excite the mechanical motion in these devices, we create the waveguides out of the piezoelectric material lithium niobate and place electrodes on top to apply an oscillating electric field. We call these clamped acoustic devices release-free lithium niobate electromechanical crystals.

These devices form the electromechanical interface of a release-free piezo-optomechanical transducer: a device that converts microwave quantum signals into light, using mechanical vibrations as an intermediate step. In this work, we describe the design, fabrication and characterization of these electromechanical devices and their integration with superconducting microwave circuits.