Interference-enhanced chirality-reversible dichroism metalens imaging using nested dual helical surfaces

Abstract

Conventional chiral metalenses based on helicoidal structures suffer from low energy efficiency and fixed chirality due to the extremely low conversion efficiency of cross-circular polarization in helicity-matched structures. Here, we report on high-efficiency and chirality-reversible metalens imaging using nested dual helical surfaces. The high-efficiency chiral metalenses were implemented by splitting one conventional helical surface into two nested ones with independently controllable parameters. When the relative orientations of the two nested helical surfaces were twisted at certain angles, the conversion efficiency of cross-circular polarization (i.e., the effective polarization component in imaging) could be significantly enhanced by one order of magnitude (from 4.5% to 45%) due to constructive interference of surface plasmonic polaritons between the two nested helical surfaces with a single pitch. Furthermore, the chirality of the metalens could be reversed by manipulating the twist angle even though the helicity of the surface is unchanged. Experimental verifications were performed using two-photon laser direct writing, and chiral imaging in the infrared wavelength range of 3–5 µm was successfully realized via lock-in thermography. This demonstration of the high-efficiency and chirality-reversible metalens provides what we believe is a new method to enhance chiral imaging efficiencies and the design possibilities for practical applications.