Bound-state-in-continuum guided modes in a multilayer electro-optically active photonic integrated circuit platform

TL;DR

This paper demonstrates low-loss bound states in the continuum (BIC) guided modes in multilayer electro-optic photonic circuits, enabling efficient modulators and potential advances in integrated photonics and quantum applications.

Contribution

It introduces a novel multilayer platform supporting low-loss BIC modes and demonstrates a high-performance electro-optic modulator based on these states.

Findings

Low-loss TE-polarized BIC modes with <1.4 dB/cm loss.

A Mach-Zehnder modulator with 2.3 dB insertion loss and 25 dB extinction.

Theoretical model enabling BIC-based photonic functions.

Abstract

Bound states in the continuum (BICs) are localized states existing within a continuous spectrum of delocalized waves. Emerging multilayer photonic integrated circuit (PIC) platforms allow implementation of low index 1D guided modes within a high-index 2D slab mode continuum; however, conventional wisdom suggests that this always leads to large radiation losses. Here we demonstrate low-loss BIC guided modes for multiple mode polarizations and spatial orders in single- and multi-ridge low-index waveguides within a two-layer heterogeneously integrated electro-optically active photonic platform. The transverse electric (TE) polarized quasi-BIC guided mode with low, <1.4 dB/cm loss enables a Mach-Zehnder electro-optic amplitude modulator comprising a single straight Si3N4 ridge waveguide integrated with a continuous LiNbO3 slab layer. The abrupt optical transitions at the edges of the slab function as compact and efficient directional couplers eliminating the need for additional components. The modulator exhibits a low insertion loss of 2.3 dB and a high extinction ratio of 25 dB. The developed general theoretical model may enable innovative BIC-based approaches for important PIC functions, such as agile spectral filtering and switching, and may suggest new photonic architectures for quantum and neural network applications based on controlled interactions between multiple guided and delocalized modes.