Steerable chiral optical responses unraveled in planar metasurfaces via bound states in the continuum

Chiral metasurfaces, with appealing properties for studying light-matter interactions at the nanoscale, have emerged as a promising platform for the realization of chiral optical responses, thereby showing advantages in chirality-related applications. The conventional approaches primarily concentrate on circular dichroism and the high $Q$ factor of the chiral resonances, while little attention has been paid to the aspects of flexibility and controllability in the modulation of optical chirality, further inhibiting the implementation of tunable and multifunctional chiral metadevices. Here, we employ a planar chiral silicon metasurface governed by bound states in the continuum (BICs) to unravel steerable chiral optical responses. In particular, the BIC-based intrinsic and extrinsic planar chiralities can be precisely steered by breaking the in-plane symmetry and the illumination symmetry, respectively. Moreover, a hybrid $\mathrm{Si}\text{−}{\mathrm{VO}}_2$ metasurface, manifested by the chiral coupled-mode theory, showcases the feasibility of actively tuning the dissipative loss while maintaining chiral quasi-BICs, then yielding desired loss-steered optical chirality. Our results provide alternative insights into tunable optical chirality and pave the way for advancements in chiroptical applications.

Figure 1

(a) Schematic illustration of a planar silicon metasurface supported chiral BIC, which consists of double-semicircle unit cells deposited on a quartz substrate. The incident circularly polarized light acts vertically, along the negative-$Z$ direction, on the metasurface. (b) Zoomed view and the related geometric parameters for the unit cell.

Figure 2

The simulated band structure and $Q$ factor of the related eigenmodes of the planar silicon metasurface near the $\mathrm{\Gamma }$ point, as well as eigenpolarization maps with different values of $L$. (a) and (b) The band structure and $Q$ factor of the eigenmodes in the vicinity of the $\mathrm{\Gamma }$ point with $L=0\mathrm{nm}$. (c) Top view of the unit cell suffering from in-plane geometry symmetry breaking that results from the asymmetric parameter $\mathrm{\Delta }S$. (d)–(f) Eigenpolarization maps, where the polarization states are represented by blue and red ellipses corresponding to left-handed and right-handed states in $\mathit{k}$ space with $L=0\mathrm{nm}, L=10\mathrm{nm}$, and $L=50\mathrm{nm}$, respectively.

Figure 3

(a) The side view of the unit cell in the planar silicon metasurface under normal incidence. (b) The transmittance Jones matrix spectra of $T_{RR}, T_{RL}, T_{LR}$, and $T_{LL}$ and the CD spectrum of the chiral q-BIC metasurface with symmetry breaking ($L=50\mathrm{nm}$). (c) The Cartesian multipole expansion of the scattering cross-section spectra of the chiral q-BIC mode in (b). (d) The electric field distribution (color) and magnetic fields $H_x$ and $H_y$ (vector) of the chiral q-BIC mode governed by the electric quadrupole. (e) The evolution of CD spectra by varying $L$. (f) Dependence of the $Q$ factor of the chiral q-BIC modes on the in-plane asymmetric parameter $\mathrm{\Delta }S/S$. The orange solid line exhibits an inverse quadratic fitting.

Figure 4

(a) The side view of the unit cell in the planar silicon metasurface under oblique incidence. (b) The transmittance Jones matrix spectra and the CD spectrum of the chiral q-BIC metasurface. (c) The Cartesian multipole expansion of the scattering cross-section spectra of the chiral q-BIC mode in (b). (d) The electric field distribution (color) and magnetic fields $H_x$ and $H_y$ (vector) of the chiral q-BIC mode predominated by the EQ. (e) The evolution of the CD spectra as a function of $\theta$. (f) The $Q$ factor of the chiral q-BIC modes for the out-of-plane asymmetry parameter $sin\theta$, with the orange solid line being an inverse quadratic fitting. With a fixed incident angle $\theta =5^{\circ }$, (g) the CD spectra versus the wavelength and the conical angle $\phi$ and (h) the related maximum and minimum of the CD spectra with respect to the conical angle $\phi$.

Figure 5

(a) Sketch of the hybrid $\mathrm{Si}\text{−}{\mathrm{VO}}_2$ metasurface. (b) Refractive index of ${\mathrm{VO}}_2$ with different temperatures from 500 to $2000\mathrm{nm}$. (c) Transmittance of LCP and RCP by altering the temperatures.

Figure 6

(a)–(c) The Jones matrix spectra of $T_{LL}, T_{RR}, T_{RL}$, and $T_{LR}$ and the CD spectrum of the actively steerable chiral q-BIC metasurface suffering from symmetry breaking ($L=50\mathrm{nm}$) for different temperatures. (d) The CD spectrum and chiral enhancement (inset) of the actively steerable chiral q-BIC metasurface for different temperatures. (e) and (f) The CD spectra versus $L$ and the height $h$ of the ${\mathrm{VO}}_2$ thin film, respectively, for different temperatures. The inset in (f) is the $Q$ factor as a function of $h$ at different temperatures.