Effect Of Nano-Porous Structure On Infrared Broadband Light Response Of Niobium Nitride Films

For systems with Quantum Phase Transition(QPT),QPT-based detection and measurement technology has broad prospects because it is very sensitive to small changes in external physical parameters when offset near the quantum critical point.In the field of infrared photoelectric detection,as a typical example of QPT system,nano-porous thin film materials with Superconductor-Insulator Transition(SIT)have shown the potential to open a new path in this field.However,the study of the infrared broadband light response characteristics is still in its infancy,and there are many key problems and unknown rules to be solved and explored.One of the effects of nano-porous structure on the infrared broadband light response of thin films is that of nano-porous structure.For this reason,this paper takes nano-porous niobium nitride(NbN)thin film as the main research object,and takes the aperture and period as the main parameters of the porous structure,systematically analyses the influence of the thickness of NbN thin film on the infrared broadband optical response.On this basis,based on the existing research experience of our group,we use the equivalent media theory to equivalent porous thin films with different structures to uniform thin films with different material parameters,and then compare and analyze the applicability of three different equivalent media models.Finally,thanks to the convenience of the equivalent model,the original three-dimensional structure design of the detector loaded with nano-porous NbN film can be simplified to one-dimensional multi-layer structure design,which facilitates the optimization of the light absorption rate of the device.The specific work is as follows:1.Analysis of IR broadband light response characteristics of nano-porous NbN thin films with different pore structures.Firstly,by optimizing Drude model parameters,the fitting/fitting accuracy of the measured optical parameters of NbN thin films in the literature in the range of 270-1750nm was further improved,and the mid-infrared band was extrapolated to 1750-5000nm.Furthermore,the light response of nano-porous NbN thin films is modeled by Finite Difference Time Domain(FDTD)with the range of parameters of the actual porous structure.The propagation and reflection of incoming light and the distribution of electric field in the thin film are simulated with different aperture,period and thickness of the thin film in the near and mid-infrared wavelength range of 780-5000nm.The influence of nano-porous structure on the infrared broadband optical response of NbN thin films was systematically analyzed.The results show that for thin films with different thickness,the light absorbance decreases with the increase of pore size and period in the target wavelength range.The law of transmittance change is opposite to that of absorption,but the law of reflectance is not obvious.The electric field intensity is higher at the edge of the hole and lower at the superconducting island area between the hole and the hole,but with the increase of the aperture and the period,the electric field intensity increases at both places.2.Equivalent media model of nano-porous NbN film with different pore structure.Firstly,the results of the simulation of the nano-porous NbN film obtained in the previous section are compared with those of the same thickness uniform NbN film.It is also found that the porous structure has no disruptive effect on the optical properties of the film,which is consistent with the previous research experience of our group and proves that it is reasonable to continue to simplify the porous film using the equivalent media theory within a larger range of pore structure changes.Then,the applicability of Maxwell-Garnett equivalent model,Bruggeman equivalent model,Strong Fluctuation Theory(SFT)equivalent model is compared and analyzed.The results show that,on the one hand,Bruggeman equivalent model has the highest equivalent accuracy among the three equivalent models,and shows good adaptability for porous thin films with different periods and apertures.On the other hand,the equivalent precision of Bruggeman model decreases when the period size increases,and decreases to a greater extent when the period reaches 400nm.3.Optimal design of high light absorption structure for nano-porous NbN film devices loaded with different pore structures was carried out.Firstly,the back-facing optical device structure loaded with the optical cavity is designed.The incident light is incident from one side of the MgO substrate onto the porous film.The optical cavity composed of SiO2 and Si3N4 double layers and Au reflector is set on the other side of the film.Furthermore,using Bruggeman equivalent model,the porous thin film is equivalent to a uniform thin film with certain parameters,and using Particle Swarm Optimization(PSO)with variable inertia coefficients,the four thicknesses of MgO substrate,NbN film,SiO2 layer and Si3N4 layer are set as optimization variables.The device is designed with wide band and high light absorption in the near infrared and mid infrared bands under different pore structure conditions.The optimized results show that the near infrared band of 1100-1750nm can achieve no less than 88%light absorption.The mid-infrared band at 3000-5000nm achieves a light absorption of at least 87.66%.