High Quality Lithium Niobate Devices

Method and device for fabricating waveguides for frequency conversion

Institute Reference: 2-23013

Background

Optical frequency conversion based upon a quadratic optical nonlinearity is a powerful technology to transfer high-coherence laser radiation into new frequencies. For thin-film lithium-niobate-on-insulators (LNOI’s), the efficiency of frequency conversion generally relies on periodic electric poling (periodic ferroelectric domain inversion) of the LN crystal. Conventionally, a set of comb-like electrodes is deposited on the surface of an LN thin film, after which the material is periodically poled, the electrodes are removed, and the optical waveguide is defined and patterned on the poled region. Unfortunately, due to the selective etching of poled LN, the waveguide becomes zigzag-shaped, which seriously degrades its optical quality, leading to significant propagation loss. As the selective etching is intrinsic to the LN material, this problem is challenging to resolve. One way to eliminate waveguide quality dependence on the poling process is to first define and pattern the waveguide on the thin-film LN and then deposit and eventually remove the poling electrodes on the surface. In this case, LN’s high dielectric constant would cause difficulty for the poling electric field to penetrate the LN waveguide uniformly, leading to degradation of poling quality. Thus, this method requires considerable layer thickness of the waveguide wing, seriously limiting the engineering flexibility and functionality of the device. This particularly becomes problematic in the visible or UV spectral regions where a thin or even completely removed waveguide wing is needed for dispersion engineering.

Technology Overview

Researchers have invented a novel approach for poling thin-film lithium niobate nonlinear devices while retaining high optical quality. This setup involves utilization and removal of a high-dielectric cladding, which dramatically improves the penetration of the poling electric field inside the waveguide core, leading to very uniform high-quality poling of the waveguide directly. The high-dielectric-constant medium could be either one or a mixture of liquids; this approach is completely different from the liquid (such as silicone oil) currently used in the poling process, where the medium is solely used to preventing the dielectric breakdown of air. This approach can be applied to any waveguide structure, geometry, and/or any thickness of waveguide wing.

Benefits

LNOI’s are inherently a very promising platform for nonlinear photonic integrated circuits (PIC) as they not only boost frequency conversion efficiency via strong mode confinement but also enable multifunctionality via photonic integration with diverse active and passive functional elements. This invented approach is able to achieve simultaneously high poling quality and high optical quality (low propagation loss), which is not possible in currently existing approaches. Researchers expect the invented approach to have profound impact on the commercialization of thin-film lithium niobate frequency conversion devices.