Cholesteric liquid crystals enable polarization switching for photonic application

Chiral photonic crystals contain materials with periodic and helical variations in their index of refraction. These properties result in striking transmission and reflection characteristics and allow the crystals to rotate and switch the plane of polarization of incident light or modulate laser pulse duration.

But current chiral photonic crystals face large optical losses, limited tunability, narrow operation bandwidth, and insufficient optical thickness for practical implementation.

Cholesteric liquid crystals (CLCs), which self-assemble in a helical arrangement, could overcome some of these obstacles. However, their fluidic nature allows fabrication of well-aligned helices by molecular self-assembly to a thickness of, at most, tens of microns. Polarization rotation and switching and very short laser pulse modulations require thicknesses of hundreds of microns or more.

To this end, Chen et al. studied the dynamical evolution of cholesteric helical self-assembly and defect formation in the bulk of thick CLCs under various applied electric field conditions.

Previously demonstrated field-assisted self-assembly techniques can fabricate thick enough crystals, but the heating and cooling required leads to undesirable side effects.

“In contrast, we have developed a room temperature dual-frequency field assembly technique that enables robust fabrication of stable well-aligned CLCs to unprecedented thicknesses demanded by many photonic applications,” said author Iam-Choon Khoo.

The method successively applies low and high frequency electric fields. The former renders the crystal mixture into a state with completely randomly oriented helices. Then the latter reorients all the helices into a uniform standing alignment.

“Our technique employs readily available liquid crystal constituents and electrical voltage generators and can be easily incorporated in laboratory or industrial settings,” said Khoo.

Source: “Massive, soft, and tunable chiral photonic crystals for optical polarization manipulation and pulse modulation,” by Chun-Wei Chen, Ting-Mao Feng, Chih-Wei Wu, Tsung-Hsien Lin, and Iam-Choon Khoo, Applied Physics Reviews (2023). The article can be accessed at https://doi.org/10.1063/5.0139168 .