A Solution for Detection of Ethanol and Methanol With Overlapping Refractive Indexes Based on Photonic Crystal Ring Resonator Optical Sensors

In this article, a selective and sensitive photonic crystal (PC) sensor is designed to detect two types of alcohol, namely, ethanol and methanol, and to measure their concentration percentages. The use of different concentrations of each compound leads to a change in their densities that have definite correlations with the refractive indexes based on the Gladstone–Dale theory. According to the experimental results and the Gladstone–Dale theory, the value of the refractive index in different percentages of alcohols overlaps, which makes it difficult to detect them with optical techniques. We present a new solution to solve this problem, which is very interesting. The full width half maximum (FWHM) for spectrum of the ethanol and methanol mixtures with water are obtained as 1.2 and 1.4 nm, respectively. In this way, it is possible to distinguish between two types of alcohols. After this process and by diluting the solution of each of the alcohols below 30%, the overlap of the refractive indexes is prevented and the obtained resonance wavelength can be attributed to a determined percentage of alcohol. The performance parameters of the proposed sensor for both alcohols are quite promising so that the transmission, the quality factor, the sensitivity, and the figure of merit (FOM) are above 97%, 1092, 756 nm/RIU, and 593.9 RIU $^{-{1}}$ , respectively. Even, when two typical samples have the same refractive indexes, the sensor can distinguish the alcohol type and the sample's concentration. This capability along with the small size ( $123~\mu \text{m}^{{2}}{)}$ and high sensitivity is the special feature of the proposed PC sensor.