Microstructured Optical Fiber Sensor for Detection of Temperature-Dependent Light-Induced Ionic Liquid Molecular Polarizability

The deformation effect of an external electric field on the framework of ionic liquid molecules has a significant impact on their molecular polarizability, which is also dependent on environmental temperature. In this paper, by exploiting the resonance coupling between the fiber fundamental core mode and the liquid rod modes in side-hole microstructured optical fibers, a compact fiber-optic sensor is proposed to characterize the temperature-dependent light-induced molecular polarizabilities of imidazole ionic liquids. Experimental results show a maximum resonance dip wavelength sensitivity up to −7.63 nm/°C for the temperature range of 25 °C to 70 °C, and according to the refractive indices of ionic liquids at different temperatures, temperature-dependent molecular polarizability can be acquired by using Lorentz-Lorentz equation. High consistence between the experimental and theoretical results has been verified by the simulation work based on density functional theory. Our proposed fiber-optic sensor possesses several desirable merits such as compact size, ease of fabrication, low cost, and considerable spectral repeatability. And moreover, the sensing scenario based on resonance mechanism provides an efficient approach to characterize the polar properties of ionic liquids, which would be of importance for various ionic-liquid-related applications, such as molecular design of ionic liquids, Raman spectroscopic analysis, and activation energy estimation of photocatalytic organic reactions.

I. Introduction

Due to their non-volatility under room temperature, ionic liquids (ILs) have become the subject of extensive studies in the field of chemistry and material sciences as one type of important green chemical organic solvents [1], [2], [3]. The two major components of ILs are inorganic or organic anions with smaller molecular mass and organic cations with larger molecular mass, which result in a liquid molten phase under room temperature. Therefore ionic liquids are often referred to room-temperature ionic liquids (RTILs) [4]. In contrast to ordinary molecular liquids, ionic liquids exhibit intriguing dipole interactions due to the significant difference in geometric size between anions and cations, resulting in high molecular polarizability, which provides an ideal material for research on intermolecular forces, dispersion forces [5] and Raman-spectra-related phenomenon [6]. In addition, their distinct physical as well as chemical properties, including no-vapor pressure [7], high conductivity [8], and optical nonlinearity [9], make ILs indispensable functional materials in various scientific and engineering applications such as gas separation [10], energy storage [11], [12] sensing arrays [13], and life science [14], [15], etc.