Preparation And Properties Of One-Dimensional Photonic Crystals Consisting Of Au Nanoparticles/Silica Nanocomposites And Silica Films

Bragg reflectors have been widely used in various fields such as resonant cavity devices,light-emitting diodes and surface emitting lasers.Among them,the vertical cavity surface emitting laser contain a Bragg reflector composed of two different materials with various refractive indexes.Their reflectivity is directly associated with the refractive index contrast of the constituent materials and the precise thickness control of each layer.As for precise control of layer thickness,method as Epitaxial Growth has been usually selected to fabricate the constituent materials.However,due to the epitaxial growth constraints for matching the material atomic lattice,the refractive index contrast of the two combined materials is invariably small.Consequently,in order to obtain very high reflectivity(?99.5%),a rather large number(up to 20-40)of pairs of the Bragg reflectors is necessary.Hence,fabrication of a Bragg reflector with a small number of pairs but a high reflectivity based on dielectric materials is still a great challenge.In 2003,A.Christ et al.prepared a gold nanowire array structure on an ITO substrate by spin coating.The transmittance of the periodic structure was spectacularly enhanced through coupling the waveguide mode into the plasmon resonance mode.Hence,we propose a method that ion implantation is used to introduce gold nanoparticles into one-dimensional photonic crystal constituent materials,and the strong scattering effect of gold nanoparticles as well as the radiative dipolar coupling of inter-laminar metallic nanoparticles are used to greatly increase the reflectivity of the structure.(1)The periodic structure of Au NPs/SiO₂ nanocomposites and SiO₂ multilayer films with 4 pairs was fabricated on the 0.5-mm-thick SiO₂ slabs by alternately combining Au ion implantation with magnetron sputtering of SiO₂.The implantation fluence and energy of gold ions were 30 keV and 6×10¹⁶ cm^-2,respectively,the SiO₂ film with thickness of about 215.0nm was then deposited through the multi-target magnetron sputtering system.The optical spectra were recorded by Fiber Optic Spectrometer.One can find that a wide absorption peak occurs at about 528.0nm while,a reflection peak at about 550.0nm appears in the reflection spectrum.The reflection spectrum after each gold ion implantation showed several Bragg reflection peaks corresponding to different orders.These peaks are actually the result of the interaction between light and periodic structures.In addition,the peak reflectivity at central wavelength increases as the number of periods increases,and the reflectivity grows linearly with the number of nanocomposite layers.So,we can guess that the reflectivity around 100% could be achieved if the layer number of nanocomposites is increased to six.We have also simulated the reflection spectrum of the periodic structure with the same refractive index contrast(0.26)and the period as our case without presence of the Au NPs by using the Transfer Matrix Methods.It can be seen from the simulated results that without Au NPs,the reflectivity is smaller than that obtained in our case.The result clearly reveals that the formed Au NPs in the nanocomposite layer should play a significant role in enhancement of reflectivity.(2)In order to explore and verify the key role of the introduction of gold nanoparticles on the periodic structure,we separately prepared two sets of Au/SiO₂-SiO₂ Bragg reflectors with central wavelengths at?=450nm and?=650nm.The reflection spectrum at vertical angle and other angles are measured separately.The measurement content exhibit that two reflection peaks has appeared around Bragg central wavelength.The result means that as long as the center wavelength is properly designed,the two peaks can coincide in one position,which could greatly improves the optical performances of the periodic structures.Subsequently,we improved the simulated reflectance spectrum by taking into account the effects of gold nanoparticles and redefining the effective refractive index of the Au/SiO₂ nanocomposite layers.The simulated reflection spectrum calculated by improved average refractive index theory is coincident well to the experimental results on the location and peak reflectivity at central wavelength.And for the Bragg reflection peaks of other orders,the simulation results and experimental results coincide well in their peak positions.