Chiral-magic angle of nanoimprint meta-device

Abstract

The magic angle of Twistronics has attracted a lot of attention because of its peculiar electrical characteristics. Moiré patterns formed by the superlattice of a twisted bilayer change overall physical properties. Circular dichroism can also be manipulated through the generated moiré pattern. Here, we report a polymer-based twisted bilayer meta-device fabricated by multilayer nanoimprint technology and study the magic angle of chirality. The superlattice of the bilayer meta-device creates moiré patterns and brings unique chiral optical responses. The bilayer nanoimprint technology is developed for metasurfaces with relative twist angles. Via the twist angle control, polymer materials with a low refractive index can manipulate the electric field of the light and reveal the chiral magic angle. Moreover, the shape of the meta-atoms plays a key role in chiral magic angle tuning. The chirality engineering by the reported nanoimprint technology and chiral meta-devices may contribute to applications in chiral imaging, biomedical sensing, lasing, and tunable optical devices.

Keywords: Moirépatterns; chirality; meta-device; metasurface; nanoimprint; twisted bilayer meta-devices.

Figure 1:

Schematic diagram of the twisted bilayer meta-devices. (a) Side view of the twisted bilayer meta-devices. “α” is the twist angle of the two layers of the meta-device. The illuminated light is a board band white light source. (b) Top view of the twisted bilayer meta-devices. “L” is 300 nm, which is the length of the square nano-hole of the meta-device. “A” is 535 nm, which is the period of the nano-holes, and the hexagonal blue dash line is the lattice arrangement. (c) The zoomed-in side view of the twisted bilayer meta-devices. “h” is 280 nm, which is the height of the nano-hole. “a” and “b” are 280 and 415 nm, which are the thicknesses of the top and bottom support layers, respectively. (d) Illustration of in-plane wave vectors of the twisted bilayer meta-devices with different twist angles. The blue triangles (g ₁) and green circles (g ₂) denote the reciprocal lattices of the two layers, respectively. The red arrows and red stars (m _α ) are the first-order moiré wave vectors, which are dependent on the twist angles shown on the z-axis. The dotted circles are the unit circle.

Figure 2:

Fabrication technology and scanning electron micrographs for twisted bilayer meta-device. (a) Nano-holes in SU-8 polymer as the bottom layer were nanoimprinted with simultaneous UV exposure. (b) The top layer with nano-holes was reversal nanoimprinted onto the bottom layer with a twist angle. (c) Single polymer layer with nano-holes. (d) 64.4° twisted bilayer meta-device. (e) Tilted view of the bilayer meta-device in SU-8. (f) Cross-section of the bilayer meta-device with an intermediate layer.

Figure 3:

Optical micrographs and the simulated electric field distribution of moiré patterns. The twist angles are (a) 20.3°, (b) 56.8°, (c) 60.9°, and (d) 64.4°. The inserts show the simulated electric near-field distribution of the twisted bilayer meta-devices. The yellow dashed circles highlight the lattices of the measured moiré patterns and simulated electric near-field distribution.

Figure 4:

Light manipulation and subwavelength features by the twisted bilayer meta-devices. (a) and (b) The distribution of the Poynting vector at the surface of the twisted bilayer meta-device when the polarization states of the incident beam are LCP (a) and RCP (b), respectively. The wavelength of the normal incident light is 535 nm, and the twist angle of the meta-device is 65°. (c) and (d) The magnitude of the Poynting vector, P _LCP, and P _RCP, under the incidence of LCP beam (c) and RCP beam (d), when the twist angle is 60°. (e) The spatial variations of the magnitude of the Poynting vector versus the spin states are calculated as γ = (P _LCP − P _RCP)/(P _LCP + P _RCP). (f) A detailed view of the spatial variations across the black dashed line is shown in (e). The absolute value of the spatial variations |γ| is shown to facilitate the quantitative characterization of the feature size.

Figure 5:

Circular dichroism (CD) of the twisted bilayer meta-devices. (a) Simulation of the CD signal map as a function of wavelength and twist angle. (b)–(e) The simulation and experimental CD signal for the twist angles of 20.3° (b), 56.8° (c), 60.9° (d), and 64.4° (e), respectively. The 3D distribution is the simulation results, and the black plan on edge is the simulation results of 20.3° (b), 56.8° (c), 60.9° (d), and 64.4° (e), respectively. The 2D colored curve is the experimental CD signal results of 20.3° (b), 56.8° (c), 60.9° (d), and 64.4° (e), respectively.

Figure 6:

Chiral magic-angle of the twisted bilayer meta-devices. The orange lines are the merged points with zero CD signal, which is the chiral magic angles.

Figure 7:

CD signal versus the shape of the unit cell. (a) Round holes with D = 300 nm. (b) Squared holes with W1 = L1 = 300 nm. (c) Rectangles with W1 = 300 nm and L2 = 370 nm. (d) Rectangles with one side length W3 = 300 nm and L3 = 430 nm. (e) The standard deviation of 30°, 60° and 90° symmetry axis versus the length of the rectangle nano-holes.