Optically Modulated Split Ring Resonator Sensor for Optical Density Analysis of Liquid Analytes in Microwave Regime

Conventional optical sensors are insensitive to the variations in the dielectric properties of materials, while typical microwave sensors are insensitive to the variations in the material’s optical properties. This work presents a photoresponsive microwave split ring resonator (SRR) sensor designed for the simultaneous analysis and characterization of both the dielectric and optical properties of the liquid analytes. Through the integration of a light-sensitive element (photoresistor) into the SRR, the proposed method enables the microwave sensor to characterize the optical density of the liquid analyte while distinguishing the dielectric properties of the sample. In the designed system, optical illumination with blue light (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda = 460$ </tex-math></inline-formula> nm) and red light (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda = 630$ </tex-math></inline-formula> nm) affects the conductivity of the photoresistor integrated with the microwave SRR, depending on the optical density of the liquid analyte, thus affecting the sensor’s <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{21}$ </tex-math></inline-formula> response. The developed system demonstrates a sensitivity of −0.57 dB per log10(Colorant Volume) for red-light illumination passing through blue-colored samples and −0.17 dB per log10(Colorant Volume) for the blue-light illumination in red-colored liquids. The presented system promises potential application in industries, including chemical, biological, pharmaceutical, and food processing.