Research On Bound States In The Continuum In Photonic Crystal Cavity And Its Sensing

Bound state in the continuum(BIC)is considered as a new mechanism to achieve ultra-high Q due to its extremely narrow linewidth and infinite quality factor(Q),which has attracted a lot of attention from domestic and international researchers.At present,BIC has been proved to exist in structures such as metasurfaces,optical waveguides,gratings and photonic crystals,and has realized many high-performance applications such as nonlinear enhancement,ultra-sensitive sensing,low-threshold lasers and so on.Among them,photonic crystals have become one of the ideal platforms for optical sensors,optical switches,and lasers due to their unique photonic control properties(photonic band gap and photonic localization).In particular,one-dimensional(1D)photonic crystals have attracted the research interest of many scholars in ultra-sensitive optical sensing because of their compact structure,easy integration with bus waveguides,and CMOS compatible characteristics.Compared with conventional photonic crystal sensors,BIC-based sensors can be directly excited with external light sources without special coupling mechanisms.Therefore,the study of BIC in 1D photonic crystals provides a new method for high-sensitivity optical sensing.This paper verifies the existence of BIC based perfect state 1D photonic crystals using three-dimensional finite-difference time-domain method(3D-FDTD).On this basis,we propose defective state 1D photonic crystal(i.e.,1D photonic crystal cavity)by introducing defects to the above structure,which verify the existence of quasi-BIC and study their sensing properties.The specific research work is as follows:Firstly,we proposed and numerically proved the existence of BIC in the structure based on perfect state 1D photonic crystal.The effects of 1D photonic crystal structure parameters(hole length,hole width,period and number of holes)on resonant mode are studied based on 3D-FDTD simulation platform,and the corresponding 1D photonic crystal is designed.Then,the variation law of quality factor Q and plane wave incident angle is studied.The results show that with the incident angle(0)close to 0°,the Q factor can be close to infinity,which proves the existence of BIC.On this basis,the shift of resonant wavelength caused by the change of refractive index and the sensor performance are studied based on the mode shift mechanism,and the sensitivity of different BIC to θ the is analyzed.The sensitivity and figure of merit(FOM)of 274nm/RIU(θ=15°)and 368 are realized respectively.Compared with 1D photonic crystal sensor based on forbidden bands mode,the 1D photonic crystal based on BIC not only has ultra-high Q,and it has the characteristics of direct excitation by external light source and free space coupling,which provides a new method to realize ultra-high sensitivity refractive index sensor.Secondly,we prove the existence of quasi-BIC based on defective state 1D photonic crystals.By introducing defects(hole length,hole width and period)into the above perfect 1D photonic crystal,the effects of Q factors on the asymmetric parameters(α)are studied.The high Q quasi BIC(～4.1 ×107)is realized when introducing hole length defects(α=0.001 μm).On this basis,its sensing performance is further analyzed,and the refractive index sensing with sensitivity and figure of merit(FOM)of 82.5 nm/RIU and 3.2×106 are realized respectively.And it is equivalent to the traditional 1D photonic crystal cavity sensors.The 1D photonic crystal cavity based on quasi-BIC is considered to have great potential in the application of ultra-high sensitivity refractive index sensing,and provides a unique opportunity for applications requiring ultra-high Q factor.