Research On Plasmonic Gratings Fabrication By Pulsed Laser On The Surface Of Silicon

When the wave vector of the incident light along the metal surface is equal to that of the surface plasmon（SPP）wave vector,the resonance will be excited,which will cause the absorption of light wave at a specific wavelength or angle and the local electromagnetic field will be greatly enhanced.This phenomenon is called surface plasmon resonance（SPR）.In the past decades,this characteristic of SPR has been applied in many fields such as SPR sensor,surface enhanced Raman scattering（SERS）,surface enhanced fluorescence spectroscopy（SPFS）,enhanced nonlinearity and so on.In recent years,with the rapid development of nano-machining technology,the miniaturized grating coupled SPR（GCSPR）sensor with simple structure,low cost and easy integration has attracted wide attention.Although there are currently many methods to fabricate periodic micro-nano structures,such as electron beam lithography（EBL）,focused ion beam etching（FIB）,nanoimprint lithography,self-assembly and so on,they have the limitations of high cost,long time and lack of flexibility,etc.How to fabricate high-quality plasmonic gratings at a large scale and low cost is still a challenge.Compared with other traditional machining methods,pulsed laser provides a mask free,non-vacuum and low-cost fabrication for preparing periodic structures on sample surfaces.In this paper,two methods to fabricate high quality plasma gratings with low cost and controllable morphology based on pulsed laser are studied,and apply the prepared nano gratings to SPR refractive index sensors.The main contents of this thesis are as follows:（1）With the popularization of femtosecond lasers,laser-induced periodic surface structures（LIPSS）have become a hot topic in the field of micro-nano fabrication.However,the micro-nano structures are generated because the material is removed by evaporation in general LIPSS,so it will bring a lot of surface debris and defects so that greatly affects the quality of the stripes.Therefore wet-etching-assisted fabrication is adopted in this paper:in the non-ablative,femtosecond laser scanning the silicon surface at a certain speed can induce alternating crystal-amorphous stripes without removing almost any material.Then,based on the significant difference of etching rates between the two crystalline silicon in KOH solution,uniform and regular subwavelength periodic nano-gratings were etched.This can greatly reduce the generation of debris and defects and improve the quality of the nano-gratings,and its morphology can be further regulated by etching.In the experiment,uniform nano-gratings with a size of 3×6 mm² were successfully fabricated under the condition of sample scanning speed of 0.9 mm/s and laser power of 0.115 W.In addition,the effects of laser polarization pulse energy scanning speed and etching time on the grating morphology are investigated.After that,the optical properties of the metal grating were measured and analyzed by a set of SPR measuring device.The results show that the metal grating prepared by this method can effectively excite SPR,and its sensitivity,quality factor and refractive index resolution are707.4 nm/RIU,13.37 and 5.3×10^-4 RIU,respectively.The preparation method is simple and rapid,and the grating morphology can be flexibly modulated by etching.This kind of periodic nanostructure with controllable morphology has broad application prospect in optical communication and optical sensor.In addition,plasmonic nanostructures are often used to enhance nonlinearity,so the enhanced frequency doubling effect of gold films is also measured in this chapter.（2）The fabrication of gratings by nanosecond laser interference lithography was studied and improved.Firstly,the effects of different energy and pulse quantity on the depth of the nano gratings were analyzed,and the two-dimensional lattice structure was fabricated by two orthogonal exposures.Due to ablation,the period and depth range of the grating prepared by direct writing are limited,and the morphology is not very good.Therefore,on the basis of the above methods,this paper studied the processing method combined with chemical treatment,and prepared highly uniform nano gratings with a period greater than 383.5 nm and a depth adjustable from 0 to 200 nm.Atomic force microscopy（AFM）showed that the quality of the gratings was significantly improved,and the range of morphology control was further increased.Then,a self-built SPR device was used to measure the optical properties of the metal gratings.The sensitivity of the metal grating prepared by this method is 527.7 nm/RIU,the quality factor is 20.6,and the refractive index resolution is 3.1×10^-5 RIU.The properties of periodic nano gratings prepared in this experiment are close to those of commercial metal gratings such as DVD or CD.The two methods proposed in this paper can produce high quality metal gratings with controllable morphology quickly and at low cost,which provides a powerful preparation technology for optimizing the miniaturization of GCSPR sensors.In this paper,a low-cost and efficient method for fabricating long-range ordered and high-uniform periodic surface structures is proposed,which can be widely used in optical devices and sensors.