In TERRIFIC, Lithium Niobate on insulator was combined with a silicon nitride layer to form strip-loaded waveguides that can take advantage of the strong nonlinear optical coefficients of lithium niobate and of the high refractive contrast and ease of processing of silicon nitride. This approach has the advantage of being substrate based (as opposed to membraned or free standing structures) increasing the potential for integration with other devices and systems. The Fellow developed, fabricated and characterised a Fabry-Perot micro-cavity in such material for the first time. The Fabry-perot micro-cavity offers a high ease of fabrication, even mode spacing and excellent control over the free spectral range, which is advantageous for applications such as parametric down conversion amongst others. Distributed Bragg Reflectors (DBR) providing mirrors on both sides of the cavity in SiN are in the form of sidewall-width modulated structures with rectangular corrugations were used in our work. The design trade-offs implicit in the realisation of such microcavities of this nature were studied using the commercial software PhotonDesign FIMMPROP.
To fabricate the designed devices, the Fellow was trained to use an electron-beam lithography machine, inductively coupled plasma etching system and dielectric coater. On successful fabrication of devices, they were characterised using end-fire set up. The devices demonstrated an experimental quality factor of 2300, which was the highest experimentally reported result for such type of waveguide based Fabry-Perot cavity.
The results were published in peer-reviewed article titeld “Lithium Niobate Fabry-Perot microcavity based on strip loaded waveguides”, Photonics and Nanostructures - Fundamentals and Applications, Volume 43 (100886), 2021.
Micro-transfer printing was then chosen as alternative and more optimal way of combining LN and SiN. Micro-transfer printing is a technique for the heterogeneous integration of functional components fabricated on one substrate with those fabricated on another substrate. Thus, greater functionality is obtained than is possible with either substrate alone. The transfer printing technique is highly scalable, being based on the parallel transfer of the thin components using a stamping process. Crucially each material system can be optimized independently of the other. For instance, the silicon nitride may be processed at high temperatures or CMOS processes that would be impossible if lithium niobate were present. In this case, a thermal oxidised silicon wafer with thin Silicon Nitride (SiN) top layer from LIONIX was combined with a transfer printed layer of LN to form strip-loaded waveguides. The F-P micro-cavity with DBRs in SiN with transfer-printed LN was designed, fabricated and characterised in TERRIFIC. An experimentally measured intrinsic Q-factors of 50,000 was obtained. We believed this was the first realization of such type of a micro-cavity.
The results of this work were presented at the conference Optica Advanced Photonics Congress 2022 by the oral talk titled "Realization of a micro-cavity via the integration of Silicon Nitride and Lithium Niobate using micro transfer printing". A journal paper is in preparation.
