Photoinduced modulation of refractive index in Langmuir-Blodgett films of Azo-based H-shaped liquid crystal molecules

TL;DR

This study demonstrates how UV light can induce reversible changes in the refractive index of Langmuir-Blodgett films of azo-based H-shaped liquid crystal molecules, enabling potential applications in optical switching and waveguiding.

Contribution

It introduces a novel H-shaped liquid crystal molecule with azo groups for photoinduced refractive index modulation in LB films, studied using SPR.

Findings

UV irradiation causes trans-cis isomerization of azo groups.

Refractive index changes are detectable via shifts in SPR resonance angle.

The LB films exhibit reversible optical switching behavior.

Abstract

The development of optically active area consisting of organic molecules are essential for the devices like optical switches and waveguides, as it can be easily maneuvered by the application of suitable electromagnetic (EM) waves. In this article, we report the development of a photoactive surface by the deposition of a single layer of Langmuir-Blodgett (LB) film of a novel H-shaped liquid crystal (HLC) molecule. The synthesized HLC molecules possess azo-groups and nitro-groups. The azo-group can be isomerized (trans-cis transformation) by irradiating them with ultraviolet (UV) light. The nitro-group can provide sufficient amphiphilicity to the HLC molecules to form a stable Langmuir monolayer at air-water interface. The Langmuir monolayer of the HLC molecules exhibited gas and liquid-like phases. A single layer of LB film of HLC molecules was deposited on a gold chip of a home-built surface plasmon resonance (SPR) instrument. The azo-groups of the molecules in LB film was excited by UV irradiation leading to a change in morphology due to trans-cis transformation. Such a change in morphology can lead to a miniscule change in refractive index (RI) of the LB film. SPR is a label free and highly sensitive optical phenomenon for the measurement of such changes in RI. In our studies, we found systematic changes in the resonance angle of the LB film of HLC molecules as a function of intensity of the UV irradiation. We measured switch-on and switch-off intensity which may suggest that the LB film of HLC molecules can find applications in optical switches or waveguides.