Direction-Dependent Plasmonic Chiroptical Response With High Asymmetry and Dynamic Tunability.

A direction-dependent chiroptical response breaks forward-backward symmetry, introducing an additional degree of freedom beyond conventional circular dichroism (CD) modulation in wavelength, intensity, and handedness. However, most direction-asymmetric chiroptical materials rely on isotropic films incorporating chiral molecules, showing a relatively weak signal. Here, a plasmonic-based chiroptical film with a direction-dependent CD signal is achieved via linear assembly of gold nanospheres (AuNSs) on an elastomeric polymer substrate. This film demonstrates reversal of CD handedness upon changing the light propagation direction from the forward to the backward configuration, accompanied by a significantly enhanced CD intensity from -0.8 to 10.8 deg. This behavior originates from wrinkle-induced geometrical chirality in the AuNSs assemblies under mechanical stretching, as well as the interplay between the linear dichroism of the AuNSs chains and the strain-induced linear birefringence of the polymer film. Furthermore, the direction-dependent response is programmable by varying the mechanical stretching ratio, applied angle, and x-/y-orientation, enabling dynamical tuning of signal on/off, magnitude of ellipticity, and reversible handedness. This plasmonic chiroptical film not only provides a strong direction-dependent CD response, exceeding the performance of most reported systems by 2-3 orders of magnitude, but also possesses multiple degrees of freedom for dynamical chiroptical modulation, presenting a promising platform for circularly-polarized light-based information encryption.