Liquid crystal integrated photonic circuit for on-chip tunable optical filters

Abstract

There has been tremendous advancements in silicon photonics over decades, thanks to compatibility of fabrication processes involved, with the well-developed and readily available CMOS (Complementary Metal Oxide Semiconductor) technology already established in the microelectronics industry. Consequently, it is possible to realize photonic integrated circuits (PICs) on a large scale with low costs. Silicon photonics finds applications in optical communications, quantum photonics, bio-medical sensing, security, etc. However, current photonics circuits and devices are designed speci fically for particular functions, due to their limited intricacy, and thus unsuitable for applications that require enormous complexity in terms of designs and functionalities. Programmable or recon figurable integrated photonics, on the other hand, are emerging paradigms that can transform the application-specific present photonic devices into more flexible systems that can allow a series of applications on a common hardware.

Liquid crystals could be a promising candidate towards the development of programmable integrated photonics because of their large anisotropy in their optical properties and electro-optic property which can be manipulated by an external electric eld. These properties can be exploited to develop photonic circuits which can be precisely controlled by external electrical signals. In this thesis, we studied the integration of liquid crystals on silicon-based photonic devices.

In the first part, we studied the effects of the anisotropic properties of liquid crystals on optical modes in guided wave systems on silicon-on-insulator and silicon nitride platforms. Changes in the effective refractive index ) of optical modes propagating through Si and SiN rectangular waveguides with liquid crystals in the upper cladding region are computationally studied using commercial software Lumerical Mode solutions. We found that the effective refractive index of modes is sensitive to liquid crystal properties and waveguide geometry. The second part of the work aimed to develop liquid crystals based on-chip integrated tunable optical fi lters. We fabricated the passive optical fi lter based on ring resonators and integrated the liquid crystals on top of it as upper cladding material. We characterized the device and showed the active tuning of resonant wavelength in the presence of varying static electric fi elds. Further study will be devoted to develop suitable designs that can fully exploit the tunability of the device.