Design of Hyperspectral Tunable Filters by Polymer-Stabilized Oblique Helicoidal Cholesteric Liquid Crystal

Abstract

Hyperspectral filters play a significant role in sorting specific wavelength information from incident light. However, previous filters (e.g., pigment-based color filter, acousto-optic filter, Lyot filter) provide limited access to high color purity, tunability, and transmittance (1, 2). Here, we report a hyperspectral tunable filter via polymer-stabilized oblique helicoidal cholesteric (ChOH) liquid crystals (LCs) (Fig. 1a). Due to the internal pseudo-layers arising from cone angles of LC molecules, ChOH LCs can precisely and freely control Bragg reflection across a wide spectral range in response to electric field (E) and temperature (3). By leveraging these properties, we demonstrate ChOH LC to control wavelength of transmitted light in whole visible range, while blocking unwanted spectral region. Firstly, we broaden reflection bandwidth of ChOH LC by forming a density gradient of polymer networks which induces gradients on the pitch of pseudo-layers (Fig. 1a). Furthermore, we add a chain extender, capable of elongating polymer chains, to introduce elasticity in polymer networks. This allows the pitch gradients in polymer-stabilized ChOH LC to be freely manipulated by E, leading to a tunable Bragg reflection in a wide spectral range. Lastly, we prepare bilayer ChOH LC films, each reflecting different spectral ranges, resulting in the transmission of a specific wavelength of light where both ChOH LC films do not reflect (Fig. 1b). The transmitted light exhibits a narrow bandwidth (~20 nm) and can be tuned in the entire visible range with respect to E (Fig. 1c). Fig.