Dsign And Fabrcation Of Broadband Absorber Using Metal-dielectric Stacks

Electromagnetic absorbers have wide applications in many fields. Highefficiency, broadband absorbers can be used in the fields of solar energyharvesting, photonic detection and so on. Broadband absorbers based onone-dimensional (1D) planar stacks have emerged in the last few years due totheir simple geometries and low-cost benefits.In this thesis, the1D planar metallic-dielectric absorbers are studiedtheoretically and experimentally. Theoretically, we simulated the opticalproperties and optimized the structures of the multilayer metal-dielectricabsorber with the program written by MATLAB according to the transfer matrixtheory. Experimentally, we fabricated the broadband absorber based on a stackof periodic alternating Ag nanoparticles and SiO2slabs on top of a reflective Agsubstrate prepared by electron beam evaporation. The absorber is easy to befabricated and its optical stability is relatively good.1．Compared to other simulation software, the MATLAB program based onthe transfer matrix theory runs faster to calculate the optical properties of thematerials and its simulation results are consistent with those using othersoftware. An exhaustive simulation is performed for the optimal material, metal thickness, dielectric thickness, and the number of periods to have a best spectralperformance. This cycle is repeated until a desired spectral selectivity isobtained. Finally, we found the material (W and MgF2) have the best spectralperformance. The absorption efficiency of this structure is98.1%in thewavelength range of300-2100nm. In addition, it also has a good performancein the thermophotovoltaic applications.2．High efficiency, broadband plasmonic absorbers are constructed based ona stack of alternating metallic nanoparticle layers (MNLs) and SiO2slabs on topof a reflective Ag substrate. Experimental results show that the stacks with thickMNLs absorb light better than those with thin MNLs when the number ofMNLs/SiO2cell (N) is small (e.g.,1or2), but the situation gets reversed when Nis greater than3. When the nominal thickness of MNL is as thin as5nm, theacquired Ag nanoparticles are small so that light penetration through all of thestacked MNLs in the proposed design is possible. Thus, an increase in N leads toa growing number of light trapping elements. Our simulation reveals that the Agnanoparticles at different layers are hybridized to excite rich localized plasmonicresonances, resulting in multiple absorption peaks at optical frequencies andthus a broader absorption band. The broadband absorbers with an integratedabsorption efficiency of96%over the300-1100nm wavelength range wereachieved by stacking18MNL/SiO2cells. The proposed absorbers can be usedfor applications in solar energy harvesting and thermal emission tailoring, due totheir easy fabrication procedure and excellent optical properties.