Ultrahigh- <i>Q</i> and ultrasensitive quasi-bound states in the continuum via asymmetric environment engineering of a photonic crystal slab

High-performance quasi-bound states in the continuum (QBICs) are pivotal for amplifying light-matter interactions, enabling low-threshold lasers, ultrasensitive sensors, and enhanced nonlinear effects. However, achieving QBICs typically requires breaking in-plane or vertical structural symmetry within the photonic system itself. Here, we present an approach to achieve ultrahigh <i>Q</i>-factors and highly sensitive QBICs by introducing controlled perturbations in the surrounding environment of a photonic crystal slab. Using mode expansion theory and a standing-wave guidance condition, we numerically demonstrate that diverse QBICs types under TE and TM polarizations can be accessed by engineering the asymmetric environments to control dominant radiation channels. Eigenmode analysis reveals that transverse modal properties of QBICs states critically govern their spectral shifts and quality factors. Furthermore, dual accidental QBICs spectral features can also be observed through simultaneous tuning of both the photonic crystal slab and its background parameters. Crucially, the transverse and longitudinal modal profiles of accidental QBICs states lead to a substantial enhancement in their refractive-index sensitivities to the surrounding environment. Our research results offer significant potential for ultrasensitive sensing and efficient frequency conversion applications.