Chiroptical properties of plasmonic nanohole arrays.

Recent advances in chiral nanophotonics in man-made metasurfaces have highlighted the importance of chirality in enhancing light-matter interactions, manipulating light’s polarization states, detecting and analyzing chemical substances, and many more. A typical probe of chirality is circular dichroism CD in optical properties like absorption, reflection, and transmission. Through this work, two chiral plasmonic metasurfaces were analyzed to enhance the CD -in particular, absorption CD and hence increase the chiral sensitivity of the system. The metasurfaces are arranged in a square lattice within a metallic layer placed on a glass substrate. The first metasurface consists of elliptical nanohole arrays realized in different metals such as silver, gold, and aluminum. This plasmonic metasurface exhibits an intrinsic chiral effect due to reduced symmetry via the elliptical shape of the nanoholes and their rotation. The findings reveal that strong CD occurs in the absorption and transmission spectra at wavelengths where extraordinary optical transmission EOT is observed. This indicates a significant resonant coupling between light and surface plasmon polaritons SPPs at the metal/glass and metal/air interfaces. The elliptical shape of the nanoholes leads to splitting of absorption peaks at each SPP resonance, while tilting of the nanohole axis produces an imbalance between the fields originating from linear polarizations along the long and short axes; as a result, they sum up with a phase delay of -90 (LCP) or +90(RCP), ending up in a CD for tilted holes and explaining the strongly dispersive features of CD spectra. The study also explores how to optimize CD by varying parameters such as the diameters and tilt of the elliptical nanoholes, the thickness of the metallic layer, and the lattice constant. This optimization is crucial for designing metasurfaces with robust and tunable CD properties. The results indicate that silver (Ag) and gold (Au) metasurfaces are effective for CD resonances above 600~nm. At the same time, aluminum (Al) metasurface is suitable for strong CD resonances in the shorter wavelength range of the visible spectrum and near-UV. The UV region is particularly noteworthy as it encompasses the absorption spectrum of various chiral biomolecules. Promising applications for these elliptical plasmonic metasurfaces can be in sensing the chiral molecules and detecting and analyzing biological substances that exhibit chirality. The second metasurface is a gold metasurface with nanoholes deformed from circular to oval shape. The metasurface with circular holes supports a bound state in the continuum BIC at the lower energy side of the SPP. Upon deforming the circular holes to oval holes, this metasurface displays an extrinsic chiral response primarily dominated by a quasi-BIC with nearly maximal values of the absorption CD at small angles of incidence, which are found to be almost independent of the specific deformation of the holes in the metasurface. This suggests that the design can be flexible while still achieving strong chiral effects. The research also highlights the demonstration of a strong CD in emission, indicating that the enhanced chiral response is not limited to absorption but extends to emitted light. This broadens the potential applications of the findings in chiral light sources. The extrinsically chiral response arises from a symmetry-broken BIC, associated with significantly enhancing the local electric field. This insight provides a deeper understanding of the mechanisms behind chiral responses in plasmonic metasurfaces. Moreover, we theoretically investigated the chiral plasmonic BIC in the strong coupling regime, wherein we found evidence of plasmon-polariton BIC modes coupled to the resonance of the active medium.