Leveraging low index contrast to reduce the polarization anisotropy in one-dimensional photonic crystals

One-dimensional photonic crystals (1DPCs) are widely used optical platforms for guiding, filtering, and enhancing light at the nanoscale. Traditionally, designs have favored high refractive index contrast to maximize the size of the photonic band gap. Here, we demonstrate that low index contrast systems offer a powerful and underexplored route to achieving improved optical functionalities in truncated 1DPCs. In particular, we show that these systems enable closely aligned photonic band structures for transverse electric/magnetic (TE/TM) polarizations, allowing for broadband superposition of TE/TM Bloch surface waves (BSWs). As a demonstration of this approach, we design 1DPCs capable of generating planar superchiral fields, which require simultaneous excitation of TE/TM BSWs, for enhanced circular dichroism spectroscopy. To design such structures, we introduce an automated framework based on multi-objective genetic optimization. Comparing optimized designs in both high and low index contrast regimes, we find the low index contrast systems yield significantly larger optical chirality across the operational bandwidth due to the greater overlap between TE/TM BSW dispersion relations and the pronounced reduction in optical anisotropy. Furthermore, numerical simulations reveal that low index contrast structures offer improved robustness to fabrication tolerances and support a wider range of chiral analyte concentrations. In addition to their optical performance advantages, low index contrast systems are naturally compatible with polymeric materials, which offer benefits such as low cost, environmental sustainability, and mechanical flexibility. While this work focuses on surface wave behavior, the underlying design principle has broad implications for polarization-independent photonic technologies, including optical sensing, computing, and spectral filtering.