Enhanced sensitivity of refractive-index-sensing optical frequency comb using frequency multiplication capability of photocarrier terahertz frequency comb for rapid measurement

We propose a refractive index (RI) sensing method that leverages the frequency multiplication capability of a photocarrier terahertz (PC-THz) comb to enhance sensitivity and speed. By incorporating a PC-THz comb as a frequency multiplier for a RI-sensing optical frequency comb (OFC), we achieve significant amplification of the RI-dependent mode spacing (<i>f_rep</i> ) shift. Our results show a 3100-fold increase in sensitivity, confirmed through the accurate measurement of ethanol-water solutions with varying concentrations. Additionally, we demonstrate the superiority of beat frequency measurement between PC-THz comb of <i>f_rep</i> and a reference THz wave over the direct <i>f_rep</i> measurements, particularly in high-speed scenarios, such as real-time gas and biosensing. The ability to perform high-resolution RI measurements on the millisecond timescale, without being constrained by RF frequency counter limitations, highlights the potential of this approach. This technique paves the way for enhanced sensitivity and speed in dynamic sensing applications using RI-sensing OFC.