Coupling-enabled chirality in terahertz metasurfaces

Abstract

Chirality prevails in nature and is of great value for molecular biology, medicine, and bioscience. Due to the enhancement of chiroptical responses, chiral metasurfaces has attracted enormous attentions. In this paper, some novel polarization-sensitive transmission effects in terahertz chiral metasurfaces are exhibited. In the chiral metasurfaces whose unit cell consists of two basic resonators - a wire and a split ring resonator (SRR), we observe the asymmetrical transmission for circularly polarized state from the circular cross-polarization conversion spectra and the circular conversion dichroism (CCD). More importantly, we verify that the chiroptical activities can be affected by the coupling between the two resonators by simply moving their relative position in the terahertz metasurfaces. From the experimental and simulated results, we observe the distinguished variation in the circular cross-polarization conversion spectra and CCD, and combining with the theoretical analysis using coupled mode theory, we reveal that the chirality of the metasurfaces is strongly correlated to the coupling between the two modes determined by the wire and SRR. Finally, we demonstrate the coupling-enabled chirality by investigating the dependence of CCD on the coupling discrepancy with different relative positions of the two resonators. These findings offer the insights into the relationship between chirality and mode coupling and provide a theoretical method to design chiral metasurfaces and enhance the circular conversion dichroism, which have potential applications in the fields such as optical sensing, polarization imaging, and biological/chemical detection.

Keywords: chirality; coupled-mode theory; coupling; metasurface.

Figure 1:

Schematic diagrams and the transmittance of the chiral metasurfaces. (a) Schematic diagram of the chiral metasurfaces. (b) Micrographs of type-I (left) and its mirror structure type-II (right) and their unit cells. (c) Simulated transmittance of bare wire and bare SRR in the unit cell under circularly polarized wave incidence.

Figure 2:

Schematic diagram of the experimental measurement.

Figure 3:

The simulated (solid lines) and experimental (dotted lines) spectra of the metasurfaces for (a) type-I and (b) type-II with varying dy. The black and red lines represent the transmittances of T ₊₋ and T ₋₊ respectively. For clarification, each spectrum is vertically shifted with 0.25 in sequence.

Figure 4:

CCD of the chiral metasurfaces. (a) Simulated and (b) experimental CCD spectra of the metasurfaces type-I (solid lines) and type-II (dashed lines) with varying dy.

Figure 5:

Theoretical analysis results of the coupling enabled chirality. (a) Fitted (solid line) and simulated (open dotted) spectra of the metasurfaces type-I with varying dy. (b) Spectral splittings and the coupling coefficients of T ₊₋ and T ₋₊. (c) CCD and the coupling discrepancy Δg.

Figure 6:

Normalized electric field distributions at different dy under RCP (upper panel) and LCP (lower panel) incident waves.