Chiro-optics has lately emerged as an important research area in photonics. The ability to distinguish between left- and right-handed circularly polarized (LCP/RCP) lights due to the lack of mirror symmetry in the system holds immense promise for transformative applications. These include ultra-sensitive biomolecular sensing [1], [2], robust quantum information processing using spin-polarized photons [3], [4], and the creation of novel optical components like compact circular polarizers and polarization converters [5], [6]. This potential has driven intense research into engineered metamaterials and metasurfaces designed to exhibit strong chiro-optical responses, such as CD and optical activity, exceeding those found in natural materials.

Chirality in photonic systems can arise from different mechanisms. In general, chiral responses are classified as intrinsic and extrinsic chirality. In intrinsic chirality the structure used itself usually lacks mirror symmetry. Meanwhile in extrinsic chirality, or also known as pseudo-chirality, the structure can be symmetrical and normally shows no chirality, however then the symmetry can be broken externally by using oblique incident angle of excitation. Further, it has recently been shown that chiral optical responses, particularly in systems supporting bound states in the continuum (BIC) modes, is associated with modal and topological properties of electromagnetic eigenstates wherein polarization vortices and topological charges play a crucial role in determining the handedness of radiation [7], [8], [9]. Latest works in chiro-photonics leverage BIC modes since these modes can cause extremely strong confinement of electromagnetic fields within the photonics structure which intensifies the light-matter interaction and thus provides an efficient way for controlling the optical response of the system. In particular, the q-BIC mode can be utilized to control exciton-photon interaction and chiral high-order harmonic generation simply by tuning the photonic spin state of the excitation [10], [11], [12].

The state-of-the-art in chiral metasurfaces predominantly employs geometrically asymmetric building blocks to simultaneously achieve the q-BIC and the intrinsic chirality [13]. Accordingly, high CD has been obtained by using gammadions [14], twisted meta-atoms [15], and stacked nano-helices [16] structures. However, this approach often necessitates complex, sub-wavelength 3D structures that may pose significant fabrication challenges, especially in the optical regime, limiting scalability and practical deployment. On the other hand, various works highlight that extrinsic chirality can also provide strong and measurable CD signals which can be realized with simpler and more fabrication-friendly platform thus enabling dynamic tuning through the illumination angle or polarization state [17], [18], [19], [20], [21], [22], [23], [24], [25]. Therefore, the research on extrinsic chirality remains of significant scientific and practical interest. Interestingly, while the q-BIC mode can actually be observed in simple structures like an all-dielectric one-dimensional grating [26], [27], its potential for chiro-optical applications remains underexplored.

Most works on chiral metasurface usually use plasmonic or high-index dielectric materials. Plasmonic materials like gold and silver generate strong chiro-optical signals by concentrating light into deep-subwavelength volumes, leveraging intense local field enhancements [28]. Similarly, high-index dielectrics like silicon support strong Mie resonances that enhance light-matter interactions [29]. However, the potential of other complementary metal–oxide semiconductor (CMOS) materials such as titanium dioxide (TiO2) or aluminum oxide (Al2O3) to serve as the material for building the chiro-optical metasurfaces is rarely addressed.

In this work, we will demonstrate computationally that a simple one-dimensional (1D) periodic grating of titanium dioxide (TiO2) on an aluminum oxide (Al2O3) thin film layer deposited on a quartz substrate can show a strong chiro-optical response. Further analysis shows that polarization conversion process takes place in the system and non-zero CD can be obtained only if the excitation angle is tuned, confirming that the chirality of the system is extrinsic [13]. Accordingly, a nearly maximum CD as high as 0.97 can be achieved by the grating at the q-BIC resonance wavelength. Furthermore, the strong chiro-optical response can be maintained for a range of illumination angle, thus allowing less stringent system configuration. The extrinsic CD can also be engineered to have positive or negative values simply by inverting the azimuthal angle of incidence. Through analysis of the near-field, we identify that the strong chirality of the system is accompanied by the formation of superchiral field at the grating surface caused by the q-BIC, in which the local optical chirality density is enhanced substantially as compared to circularly polarized plane waves. Further, it will also be shown that it is the presence of the Al2O3 layer that tailors the polarization conversion process in the system, leading to the nearly maximum CD with the simple structure. These findings signifies the simple grating systems as a promising platform for advanced chiral light-matter interactions.