Sapphire substrates for lithium niobate films promise circuits with broadband applications

Lithium niobate (LN) is a material with strong piezoelectric coupling and other qualities required for broadband and efficient acousto-optic modulators and deflectors. High-quality single-crystal films of LN have seen increasing use in electro-optic and nonlinear optical devices. While much research has been done on devices made from suspended LN films, suspension creates fabrication difficulties that inhibit further development of circuits or other complex systems.

An alternative approach has considered LN-on-insulator devices, but these have a fundamental drawback, since energy leaks out of the LN and into the silica when the mechanical wavelength approaches the film thickness. Instead, Sarabalis et al. fabricated acousto-optic devices using LN on a sapphire substrate. The platform has properties that make it promising for broadband piezoelectric and acousto-optic device applications.

The authors considered the use of sapphire, a material with higher sound velocities, as the substrate for an LN film.

“By moving to sapphire substrates, the LN film guides shear waves which are very piezoelectric,” said author Christopher Sarabalis. “They’re easy to transduce over a broad band and can modulate optical waveguides patterned in LN.”

The investigators compared measurements of the piezoelectric coupling coefficient and the optomechanical coupling coefficient to simulations. These two coefficients can characterize the quality of the bonded film. To make small, broadband transducers, a large piezoelectric coupling is required, and for efficient acousto-optic devices, a large optomechanical coupling.

“The coefficients reported here for a range of crystal orientations in X-cut LN confirm the quality of these bonded films and demonstrate the potential of the material platform,” said Sarabalis. This enhances the prospects for integrating acousto-optics alongside high-speed modulators, combs and frequency doublers into complex circuits and systems.

Source: “Acousto-optic modulation in lithium niobate on sapphire,” by Christopher J. Sarabalis, Timothy P. McKenna, Rishi N. Patel, Raphaël Van Laer, and Amir H. Safavi-Naeini, APL Photonics (2020) The article can be accessed at https://doi.org/10.1063/5.0012288 .