Wavelength- and Helicity-Reconfigurable Circularly Polarized Optical Diodes Enabled by Molecular Photoswitches.

Abstract

Achieving nonreciprocal transmission of circularly polarized light (CPL) is critical for chiral photonics and spin-selective optical communication, yet existing optical diodes typically operate at fixed wavelengths. Here, we report an all-optical, photoswitchable diode-type device for tunable nonreciprocal (asymmetric) transmission of CPL across a broad spectrum of light using molecular motor (photoswitch)-doped cholesteric liquid crystals (CLCs) sandwiched between polyvinyl alcohol (PVA) and polydopamine (PDA)-PVA composite layers. Under ultraviolet irradiation, the photoswitches continuously modulate the helical twisting power of the CLC, enabling reversible tuning of the photonic bandgap (PBG) and, at higher irradiation levels, helix inversion with opposite handedness. Thus, we demonstrate dynamic switching of both the operating wavelength and the isolated CPL helicity within a single device architecture. The asymmetric PDA@PVA photothermal interface enables forward photothermal activation of the cholesteric-isotropic transition while maintaining spin-selective Bragg reflection under backward incidence. This mechanism enables wavelength-tunable and chirality-adaptive nonreciprocal transmission for both right- and left-handed CPL. The device achieves isolation ratios exceeding 15.10 dB at all tested wavelengths, with a maximum value of 15.80 dB at 980 nm, while maintaining insertion losses below 2.89 dB, reaching as low as 1.05 dB. Furthermore, the operating thresholds and thermal tolerance can be tailored through liquid-crystal composition.