Studies Of Impedance Matching Anti-reflection Coatings And Anomalous Dispersion Materials

An impedance matching anti-reflection?IMAR?layer is expected to achieve an ideal antireflection performance for broad spectrum and incident angles.To realize IMAR,anomalous dispersion is necessary and the refractive index is a function of thickness and wavelength.Most existing materials in nature do not have anomalous and strong dispersion.Therefore,it is of great significance to study the approaches to tailor the material dispersion for IMAR.Making use of the effective medium theory?EMT?,the composite materials,consisting of metal nanoparticles and dielectric oxides,and more complicated metamaterials could be the good candidates.Based on the IMAR theory,this thesis systematically studied the refractive index profiles of IMAR coatings,and the effects of thicknesses and substrates.Two kinds of nanocomposite materials,SiO₂-Ag and AZO-Ag composite thin films,were prepared for the realization of the anomalous dispersion.Eventually,the multilayered anti-reflection?AR?coatings we designed and fabricated for the initial studies on the anomalous dispersion materials and the experimental realization of IMAR coatings.The main conclusions were summarized as follows:1.The IMAR theory was investigated in detail and applied to the reflection suppression of glass/air and silicon/air interfaces.Varying the thickness of IMAR coatings from 100 nm to 800 nm,the refractive index profiles of IMAR coatings were varied,correspondingly.When the thickness was increased,the profiles became similar to that of traditional gradient index AR coatings;when the thickness was small,the profile was strongly dispersive and discontinuous at the interfaces.Using the EMT,such as the linear and the Bruggeman models,the dispersive properties of SiO₂-Ag and AZO-Ag nanocomposite thin films were investigated.2.Combining the EMT and IMAR,a five layered AR coating was proposed and simulated.Tailoring the dispersive refractive index by varying the filling ratio of each composite,the influences of the central wavelength and incident angles on AR properties were studied.According to the IMAR design,the refractive index at the central wavelength was determined in each layer.The reflectance spectra of the multilayered AR coatings were calculated and optimized using the TFCalc software.3.Using a sol-gel process,the SiO₂-Ag nanocomposite thin films were prepared by adding AgNO₃ into alkaline solid sphere silica sol and acidic silica sol,followed by annealing at 200 degrees Celsius,which showed an anomalous dispersion.The influences of Ag/Si mole ratio in precursor sol?0,1/20,1/10,1/5,1/2 and 1/1?on the dispersion properties were further studied.Spectroscopic ellipsometry measurements showed that the refractive index of the nanocomposite thin films decreased and anomalous dispersion range became broadened with increasing the content of Ag.When the Ag/Si mole ratio was 1:1,an anomalous dispersion range of 400 nm?1.14?-700 nm?1.21?could be obtained.The EDX results further confirmed the existence of Ag nanoparticles.Correspondingly,the UV-vis spectra showed an absorption peak of Ag nanoparticle at 410 nm.4.The AZO-Ag nanocomposite thin films were deposited on the silicon and glass substrates using a modified co-sputtering method.Fixing AZO sputtering power?200 W,RF?and varying silver sputtering power?0 W,1 W,2 W,5 W and 10 W,DC?,five series of nanocomposite thin films with different silver doping ratio were prepared.Spectroscopic ellipsometry measurements showed that the anomalous dispersion range became larger when increasing the Ag doping ratio.When silver sputtering power was 10 W?DC?,the anomalous dispersion range of 400 nm?1.90?-700 nm?2.20?could be obtained.The retrieved extinction coefficient implied that the absorption peak appeared at 415 nm.The influences of different anneal temperatures?200 ?,300 ? and 400 ??on transmittance,refractive index and conductivity were systematically studied.5.Based on the aforementioned nanocomposite thin films with the anomalous dispersion,the two and three layered IMAR coatings were designed.The three layered AR coatings,consisting of anomalous and normal dispersion materials,were deposited on the silicon substrates and the average reflectance dropped from 37.6% to 7.1% at the wavelength from 400 to 900 nm.The two layered IMAR coatings were also fabricated on the silicon substrates,in which the bottom layer was prepared by co-sputtering of AZO and Ag,and the top layer was deposited by sol-gel dip-coating of hollow silica nanospheres.The average reflectance further decreased to 2.9% at the wavelength from 400 to 900 nm.