Topologically Enhanced Circular Dichroism from Metasurfaces

Circular dichroism (CD) is one of the most distinctive properties of chiral materials. In recent decades, chiral metasurfaces have been regarded as a promising framework to realize chiral optical devices with ultrathin thickness and compact footprints. Various efforts have been devoted to increase the CD response from such metasurfaces. Here we present a method to enhance the CD effect from metasurfaces utilizing a topological concept. Two multilayered photonic crystals with different topological invariants are adopted and put in contact leading to the appearance of a topological edge state and a distinct electromagnetic field concentration at the interface. By embedding L-shaped chiral metasurfaces into the interface, the CD response is observed to be obviously boosted. We find that due to the robustness feature of the topological edge state, the spectral position of the CD peak is fixed at the edge-state wavelength and is immune to structural variation of the metasurface array. Considering that the multilayered films are easy to fabricate, such a topological concept could act as a simple and feasible strategy for enhancing chiral optical response from nanoscales, and could find wide applications in realizing ultracompact polarization-selective optical components.

Figure 1

Topological edge state in a multilayer film. (a) Schematic of a topological heterostructure formed by interfacing two PCs (labeled as PC1 and PC2). Layer $A$ is ${\mathrm{Ti}\mathrm{O}}_2$ with a thickness of 60 nm and $B$ is ${\mathrm{Si}\mathrm{O}}_2$ with a thickness of 170 nm. Both PCs show inversion symmetry with two inversion centers corresponding to the centers of layers $A$ and $B$. The two PCs are essentially the same except that their unit cells are defined in different ways: the unit cell of PC1 arranges layer $B$ at the center, while the unit cell of PC2 centers at layer $A$. The unit cells of the two PCs are indicated by orange and red dashed rectangles, and the interface is indicated by a dashed line. (b) SEM image of cross section of the multilayer film. Scale bar is 500 nm. (c) Band structures of PC1 (left panel) and PC2 (right panel). Zak phase of each band is indicated by numbers (0 or $\pi$). Magenta and gray rectangles present a band gap with positive and negative sign of topological invariants, respectively. An edge mode would exist in the lowest gap (frequency of approximately 0.2875) where the topological invariants have opposite signs in the two PCs. (d) Transmission of the topological photonic crystal. Red and black lines give simulated and experimental results. The yellow triangle represents a topological transmission peak and the green circle represents a nontopological transmission peak. (e) Calculated electric field distributions corresponding to the nontopological peak at 575 nm and the topological peak at 800 nm. (f) Field enhancement factor ($|E/E_0|$) at the interfacial layer $A$ of the PCs as a function of wavelength, where $E$ is local field and $E_0$ is incident field. A distinct field enhancement peak appears at around 800 nm overlapping the topological state spectral position.

Figure 2

L-shaped metasurface and its chiral optical response. (a) Schematic diagram of one unit cell of the metasurface on a ${\mathrm{Si}\mathrm{O}}_2$ substrate. Geometrical parameters include vertical arm length $L_y$, horizontal arm length $L_x$, arm width $w$, and thickness $h$. (b) Numerically simulated spectra for the metasurface with $L_y$ of 150 nm and $L_x$ of 100 nm. Inset shows the designed schematic of the unit cell. Blue and red curves represent the transmission spectra for LCP and RCP incidence, respectively. CD spectrum is given by the black curve, which shows a weak resonance peak of less than 1% at around 680 nm. (c) Experimentally measured transmission and CD spectra. A SEM image of the fabricated metasurface is given as an inset, for which the scale bar is 100 nm.

Figure 3

Topologically enhanced metasurface and its chiral optical response. (a) Schematic of the topologically enhanced metasurface. The L-shaped nanostructures are embedded at the position of the topological edge state. (b) Numerically simulated spectra for the metasurface with $L_y$ of 150 nm and $L_x$ of 100 nm. Blue and red curves represent the transmission spectra for LCP and RCP incidence, respectively. CD spectrum is given by the black curve, which shows an obvious resonance peak around 800 nm consistent with the topological resonance peak in Fig. 1. (c) Experimentally measured transmission and CD spectra of the fabricated sample. Scale bar in the SEM image is 100 nm.

Figure 4

Evolution of chiral optical response as a function of geometric variation of metasurfaces. (a) Top-view SEM images of the fabricated metasurfaces. $L_x$ is fixed as 100 nm, while $L_y$ is changed from 100 to 350 nm in steps of 50 nm in different metasurface arrays. Scale bars are 100 nm. (b) Simulated comparison between CD magnitudes of the metasurfaces before (red curves) and after (blue curves) topological enhancement. Vertical scale is shown in the top-left corner. (c) Experimentally measured CD spectra of the metasurfaces before and after the topological enhancement.