Hyperspectral Tunable Filters via Polymer-Stabilized Oblique Helicoidal Cholesteric Liquid Crystals

Spectral filters play an important role in sorting specific wavelength information from incident light. However, conventional filters, such as pigment-based color filters, acousto-optic filters, and Lyot filters, have limitations in terms of poor color purity, low tunability, and low transmittance [1,2]. The experimental discovery of oblique helicoidal cholesteric (Ch_OH) liquid crystals (LCs), with their periodic pseudo-layers, realizes highly tunable Bragg reflectors across a wide spectral range in response to electric field (E) and temperature [3,4].

Here, we report a hyperspectral tunable filter using polymer-stabilized Ch_OH LCs. Specifically, we broaden reflection bandwidth of Ch_OH by forming a density gradient of polymer networks which induces gradients on the pitch of periodic oblique helicoidal structures. Additionally, we introduce elasticity into the polymer networks to enable the polymer-stabilized Ch_OH to be freely tuned by E, leading to tunable reflection wavelengths in a wide spectral range. By preparing bilayer Ch_OH films that reflect different spectral ranges, a certain wavelength of light is transmitted where both Ch_OH films do not reflect. The transmitted light shows high level tuning in wavelength with narrow bandwidth (< 20 nm) covering the whole visible range with respect to E. The proposed method has potential in various applications including image sensors and smart windows. This work was supported by the Korean National Research Foundation (RS-2023-00212739).