Simultaneous detection of complex refractive index and temperature using a compact side-coupled photonic crystal nanobeam cavity

In this paper, we theoretically propose a compact photonic crystal nanobeam cavity (PCNC) side coupled with a waveguide for sensing complex refractive index (CRI) and temperature ( T ). Two Fano resonances are achieved by coupling two discrete states (zeroth-order mode and first-order mode) in a PCNC and the continuous state in the waveguide with a partial transmitting element. The transmission expression is derived based on the temporal coupled-mode theory. By measuring the wavelength shifts and shape changes of two Fano resonances in the transmission spectrum, not only the real part of the refractive index (RRI) and the imaginary part of the refractive index (IRI) can be sensed simultaneously, but also the effect of T on CRI is considered. The sensitivities of RRI for zeroth-order mode and first-order mode are 152.7 nm/refractive index unit (RIU) and 183.7 nm/RIU, respectively. The sensitivities of IRI are − 461 / R I U and − 198 / R I U for zeroth-order mode and first-order mode, respectively. The T sensitivities for zeroth-order mode and first-order mode are 58.6 pm/K and 55.6 pm/K, respectively. The fringe visibility values of both modes are more than 0.5. The relationship between structural parameters and coupling effect is studied and the effect of T on detection accuracy is discussed. To the best of our knowledge, this is the first geometry to simultaneously sense CRI and T . Moreover, the total footprint of the device is only 11 × 1.45 × 0.22 ( l e n g t h × w i d t h × h e i g h t ) µ m 3 , which will contribute to the large-scale integrated sensor array on chip.