Research On Methods Of Light Modulations Based On Metallic Nanostructures

Metallic structures have plenty of special electromagnetic properties andpromising potential applications in optical sensing, biomedical detection, photovoltaicdevices and surface-enhanced Raman scattering. They are also the important parts ofnano-photonic devices for the next generation, which are the key components toovercome optical diffraction limit and construct intergrated optical systems with highperformance and miniaturization. These functions are related to the surface plasmonswhich are excited at the metal surface. Utilizing the properites of high localization andenhancement of SPPs, we can realize controlling and modulation of light atnano-scales. So some convential optical devices may be replaced by new smallerfunctional nano-devices and even achieve some special capabilities which can not bedone by convential devices. In revent years, there are significant progresses in theresearch of plasmonic optical devices based on metallic nanostructures as thedevelopment of nano-fabrication and measurment technologies. However, theresearch on light modulation with metallic nanostructures is still limited in the lab.Plenty of plasmonic related phenomena require deep understandings and clearexplanations, more new concepts and effects are worthy to be exploited. Beseds, thefabrication processes of many proposed nanostructure based devices are verycomplicated and there is no efficient low cost baich fabrication method, which greatlylimits the applications of nanostructure devices in the industrial areas.The project "Research on theories and applications of light field modulationbased on SPPs in metallic nanostructures" mainly focuses on the optical properties ofmetal/dielectric multialyer nanostructures and take systematic research on twopromising areas: plasmonic interference lithography and light absorption from thepoint of view of near-field and far-field of the structures. The results play a positiverole in promoting the development of nano-optical devices and relatednano-technologies in our country.The main research contents and innovative works include:1. We discussed the limitations of effective media theory (EMT) on analyse ofmetal/dielectric structure finited layers from the prespective of SPP propagationproperty, reflection sprctrum and distribution characteristics of interference patterns.These results based EMT are compared with rigirous electromagnetic analysis methodand provide useful guidelines for the optimization andpractical application ofplasmonic interference devices. The propagation properties of SPP in metal/dielectricmultilayer film structures are analysed from the ponit of view of waveguidecoupling and the expression of beam spreading angle and conditions of uniforminterference patterns are obtained. A subwavelength plasmonic interferencelithography method is proposed and demonstrtated by numerical simulations based on above analyses, which can significantly increase the contrast and field depth of theinterference patterns and decrease the fabrication difficulty of grating mask. Thefeature size of the uniform periodic patterns is1/8of incident wavelength, the contrastis as high as0.8and the depth is80nm by introducing the SPP coupling from thebottom of photoreisit.2. A broadband plasmonic absorber in visible range based onmetal/dielectric/metal structure is proposed and experimentally demonstrated. Twokinds of structures are designed and fabricated. Structure-I with single layer ofgratings has a theoretic absorption of nearly100%(measured averaged value of80%)from a broadband range of400nm t0600nm for TM polarized light and thestructure-II with double layer gratings can further extend the absorption spectrum tothe whole visible range. The mechanism of broadband absorption is analysed by boththeoretic calculations and numerical simulations and the special vertical dispertion ofthe MIM structure is found. The angle response and robustness of the structure arealso discussed. Compared to previously reported structures, our designed structure hasmore flat absorption spectrum and high absorption ratio. Besides, its structure issimple and stable, suitiable for the large area fabrication with roll to roll nanoimprinttechnology.3. A light funneling effect in metal/dielectric mulitlayer films is proposed anddemonstrated by simulation and experiemtal method. Incident light at the resonantwavelength can be completely funneled into dielectric layers through a narrow groovethat only covers12.5%of the surface area within one period and absorbed by aresonant cavity composed of metal/dielectric multilayer films. A narrower resonantdip is observed than that produced by bulk metals with the same thickness andgrooves. The mechanism and influencing factors of the reflection spectrum, includinggroove widths, layer numbers and profile of groove’s side wall are comprehensivelyanalyzed. Coupling between the adjacent grooves with different depths are alsodiscussed. Besides, we expand the application scope of light funneling effect in singlelayer metal film by studying the reflction spectrum of strucuture with two grooveswith different depths. Our study can be applied in the applications of biologicalsensing and infrared detectors.