Theoretical Model of a RI THz Sensor Realized by Coherent Perfect Absorption With Optical Phase Modulation

In this work, a theoretical model for the refractive index (RI) detection that employs phase modulation for absorption compensation based on the mechanism of coherent perfect absorption (CPA) is presented. Different from most sensors using frequency modulation, the proposed sensor measures RI with phase modulation in a narrow frequency range. Due to the fragility of CPA, the narrow absorption peak is highly sensitive to changes in RI values of the dielectric layers inserted in the structure. Benefiting from this, the proposed sensing method is a feasible scheme of high-precision and high-sensitivity detection. By filling the structure with the substance to be detected, the measurement and analysis of substances with RI of 3.510-3.565 and 1.45-1.52 can be accurately achieved, respectively. For the measurement of RI from 3.51 to 3.565, the maximum sensitivity (S) is up to 3749.79 °/RIU, and the limit of detection (LoD) is $2.4\times10$ ⁻⁴ RIU on average. If this sensor is used to measure RI of 1.45-1.52, the maximum S is 2775.1 °/RIU, and the average LoD is $2.8\times10$ ⁻⁴ RIU. The structure consists of ferrites and other conventional materials. The magnetic field applied to the ferrite layers is proved to provide a sensor calibration function. The effects of the thicknesses of the dielectric layers and the incident angle on the sensing performance are also discussed. The suggested theoretical model has a better performance compared with the conventional sensors. This theoretical study introduces a novel idea for RI sensing and expands on the use of CPA in sensors.