A Study On Raman Spectroscopy Sensing And Key Waveguide Devices Based On Silicon Nitride

Trace substance detectiong technology has been applied in the fields of food safety,environmental inspection,border security inspection,military defense,etc.It has become a rapidly growing research topic in the fields of chemistry,physics,life sciences and nanotechnology.Especially,Raman spectroscopy technology is widely used,due to its fingerprint ability,which can provide a qualitative and quantitative measurement for the analytes.Nowadays,with the development of the Internet of Things based on 5G technology,the spectrum detector with properties of a small size,flexible use,a compact volume,a lower power sonsumption and an integration capability with wireless communication is potential to be used in many applications,expecially in trace air pollutant monitoring.As for this requirement,it is meaningful and necessary to investigate an on-chip Raman spectrum detection system.Silicon-based materials are widely used to make optical waveguides with excellent performance,but the short wavelength limit of which it can transmit is about 1100nm.Due to its strong absorption in the 532nm,633nm and 785nm wavelenth regions of the common Raman spectrum detection band system,so it is not suitable for waveguide devices of these wavelengths.The transmission short-wavelength limit of silicon nitride（Si₃N₄）is about 300nm,which can be used to make short-wavelength optical waveguides,but there are still many scientific,technical and technological aspects about realizing Raman sensing and on-chip spectral detection by silicon nitride waveguide.Among them,the design and realization of key devices are the key points that need to be broke through.As an explore work of our group on silicon nitride waveguide-based Raman sensing,there are three parts.Firstly,the mechanism of waveguide coupled SERS is discussed.Secondly,some key waveguide devices including waveguide coupled gratings,microring resonators,and waveguide Mach Zendel interferometer（MZI）,etc.are studied in detailed.Thirdly,a handmake experimental system for waveguide devices are complished,and the corresponding measurements and expeiments are carried on.The major work is as follows.（1）A study on the mechanism of waveguide coupled SERS（surface-enhanced Raman scattering）.Based on the theory of optical waveguide coupling,a theoretical model of waveguide-enhanced Raman scattering and waveguide coupled SERS was established,and Raman properties of single molecule and multi-molecule have been analyzed.Furthermore,COMSOL Multiphysics 5.5 is used to analyze the electronic field distribution of strip waveguide and slot waveguide under non-SERS and SERS conditions.（2）A study on key waveguide devices.Firstly,an optical waveguide transmission theory is used to establish the optical transmission and coupling characteristic models of coupled gratings,curved waveguides,microring resonators,MZIs,etc.The effects of structural parameters on optical transmission and coupling characteristics are analyzed.The overall scheme of spectrum acquisition based on silicon nitride waveguide is further studied,and its feasibility is also analyzed theoretically and numerically.（3）A measurement and experimental system for waveguide devices.An overall design of the experimental system for waveguide devices was carried on.The selection of key components was completed,including the light source,the polarizer/polarizer,coupled fibers,microscope,precision adjustment equipment,etc.The performance of this measurement platform was invesitigated.（4）Preparation and measurement for the waveguide devices.According to the theoretical analysis and optimal design of the aforementioned key devices,the mask layout of the waveguide device was designed,and the silicon nitride processing developed by the traditional silicon-based etching process was used.Multiple sets of waveguide devices（coupled gratings,MRRs,MZI,Raman sensing parts）were fabricated.The surface morphology was characterized by SEM（scanning electron microscope）and EDS（energy dispersive spectroscopy）.The effects of the duty cycle of the grating coupler,the test method,the convergence angle of the converter,and the number of grating periods on its coupling efficiency were studied.The optical transmission characteristics and coupling characteristics of the microring resonators were experimentally studied,and the MRR-Through spectral signal reduction method was proposed.A light transmission characteristic model based on the FP effect was proposed with the Add-Drop type as the MRR structure,and it was clear that the spurious peaks originate from the multiple reflections generated by the end face of the grating coupler,and it was proposed to add an anti-reflection coating on the surface of the grating coupler to reduce spurious peaks The analysis results showed that when the optical thickness of Si O₂was set to 150 nm and the wavelength range was 0.448～0.911μm,the reflectivity R was reduced to 0.05,and the error value of the resonance wavelength was 11.08%.At this time,the spectral signal at the MRR-Drop end was unaffected by the waveguide F-P cavity formed by grating coupler end face.（5）An exploration of waveguide Raman sensing and dynamic MZI spectrum acquisition.Combined with the optimized grating coupler and MZI,the MZI output spectrum can be temperature-modulated through a power supply voltage.The relationship between the power supply voltage（V）and the temperature（T）on the waveguide chip was complished.By detecting the spectral period of the MZI output,the waveguide refractive index under different voltages（temperatures）was calculated and the thermal optical coefficient（TOC）was established.According to the sampling theorem,we set the refractive index interval stepΔn_step of 0.0065,the maximum refractive index changeΔn_maxof 0.3554,and the reduction spectrum resolution can be obtained as 30 nm.Meanwhile,the laser wavelength in Raman measurement is 785 nm.The graphene was transferred and R6G（rhodamine 6G）molecules with different concentrations were dropped on the Si₃N₄ waveguide,respectively.The corresponding characteristic peaks of graphene and R6G were seen clearly.