Large Chiroptical Effects in Planar Chiral Metamaterials

Chiroptical effects characterized by different optical responses for left- (LCP) and right-handed circularly polarized light (RCP) are powerful and valuable tools in optics with wide applications in polarization-resolved imaging and sensing. Previously observed strong chiroptical effects are limited to metamaterials with complex three-dimensional chiral structures at the subwavelength scale. Although asymmetrical transmission of LCP and RCP have been investigated in planar chiral metasurfaces, the observed weak chiroptical effects result from anisotropic Ohmic dissipation of the metal constituents. Here, we demonstrate by theory and proof-of-concept experiments that a large difference in transmittances of LCP and RCP can be attained in a single-layer planar chiral metamaterial with a subwavelength thickness. Without violating the reciprocity and mirror symmetry, the strong chiroptical effect, independent of dielectric loss, arises from a mechanism of multimode interference. The described effect may lead to a gateway towards chiral manipulations of light and chiral optical devices.

Figure 1

Schematic illustration and theoretical demonstration of the strong chiroptical effect in the planar chiral metamaterial. The proposed PCMM (a) consists of single-layer $\mathsf{L}$-shaped gold nanoantennas in a square lattice. Where RCP at normal incidence is converted into LCP when transmitted, LCP is reflected into LCP. The period $a$, thickness $H$, length $L$, widths of the two arms $W_x$ and $W_y$ of the chiral $\mathsf{L}$-shaped gold nanoantenna (b) are 730, 580, 360, 246, and 160 nm, respectively. Calculated transmittance (c) and circular-polarization-resolved transmittance (d) spectra for RCP and LCP at normal incidence to the proposed PCMM, respectively. $T_{\mathrm{LR}}$ ($T_{\mathrm{LL}}$) and $T_{\mathrm{RR}}$ ($T_{\mathrm{RL}}$) denote the LC- and RC-polarized transmittance of an incident RCP (LCP), and (e),(f) are the reflectance and circular-polarization-resolved reflectance spectra. $R_{\mathrm{LR}}$ ($R_{\mathrm{LL}}$) and $R_{\mathrm{RR}}$ ($R_{\mathrm{RL}}$) denote the LC- and RC-polarized reflectance of an incident RCP (LCP).

Figure 2

Eigenmodes of $\mathsf{L}$-shaped gold periodic waveguide array. Real part (a) and imaginary part (b) of the effective mode index $n_{\mathrm{eff}}$ of the four eigenmodes supported by the $\mathsf{L}$-shaped gold periodic waveguide array with the lowest cutoff frequencies denoted as the $Y$-arm, $X$-arm, $X$-end, and $Y$-end modes according to their transverse-electric-field distribution (c)–(f) at the wavelength of 1500 nm. The small arrows denote the directions of the transverse-electric-field vectors, and the different color hues denote the relative amplitudes of the electric fields.

Figure 3

Chiroptical effects in the PCMM with different thicknesses studied by the scattering-matrix method. (a) Scattering-matrix model of the proposed PCMM. $R^{\pm }$ ($L^{\pm }$) denotes the electric fields of the $\pm z$-directionally-propagating RCP and LCP in the semi-infinite ${\mathrm{SiO}}_2$ layer. The electric fields in the PCMM denoted by subscripts $Xa$, $Ya$, $Xe$, and $Ye$ are that of the four eigenmodes supported by the $\mathsf{L}$-shaped gold periodic waveguide array shown in Fig. 2 ($X$-arm, $Y$-arm, $X$-end, and $Y$-end modes, respectively). Transmittance (b) and reflectance (d) for RCP and LCP with the wavelength of 1500 nm at normal incidence on the PCMM with different thicknesses $H$. Ellipticity of high-transmittance (c) and high-reflectance (e) circular polarization with different thicknesses. The lines and symbols (square, triangle, and circle) are the results calculated by using the scattering-matrix method and COMSOL simulations, respectively.

Figure 4

Chiroptical effects in the PCMM studied by the multiple-beam interference model. Transmission and reflection matrix model (a) is used to describe the transmitted and reflected light. The circular-polarization-resolved transmission coefficient $t$ (b) and reflection coefficient $r$ (c) for RCP and LCP with the wavelength of 1500 nm at normal incidence on the PCMM with 360 nm thickness, which varies with the incremental maximum of the round-trip from 0 to 20.

Figure 5

Experimental demonstration of chiroptical effects in the PCMM. SEM images of the fabricated $\mathsf{L}$-shaped gold nanoantennas with the sample tilted by 52° (a) and 0° (b), respectively. Measured overall transmittance spectra (c) and circular-polarization-resolved transmittance spectra (d) for RCP and LCP at normal incidence (inc.) on the fabricated PCMM. The dashed lines in (c) are the calculated transmittances of the PCMM consisting of $\mathsf{L}$-shaped gold nanoantennas with trapezoid cross sections.