Structured Metallic Films for Enhanced Light Transmission and Absorption
Sammanfattning: Photonic devices such as light emitters, detectors, solar cells etc. are playing increasingly important roles in modern society. Yet their structures and designs are still under constant improvement, driven mostly by advances in material science and progress in integration techniques. A common challenge in these devices is to precisely manage transmission and absorption of light when it enters or escapes active regions. In certain devices it is even required to have both high optical transmission as well as high electrical conductivity. The current thesis investigates how structured metallic films can be devised to meet these challenges. Our work demonstrates that nanostructured noble metals can be tailored as either transparent or highly absorptive at wavelengths of interest, with or without electrical conductivity.In the first part of this work, metallic photonic crystals in the form of a thin gold film with an array of holes were fabricated with various geometrical parameters (hole size, pitch and metal thickness) on different substrate materials to investigate the impact of each parameter on the transmittance spectra at mid- to long-wave infrared wavelengths. Pitch size is shown to be the dominating factor for the high-transmittance band positions. Fill factor and metal thickness collectively define the selectivity of the pass bands. The selective transmission of infrared light can be used to improve the performance of infrared detectors. In the second part of this work, a thin-film multilayer structure based on two coupled metal-insulator-metal optical resonators was investigated for achieving a transparent conductor at visible wavelength range. The fabricated silver-based sample has a figure of merit (transmissivity-over-resistance) comparable to that of the traditionally used indium tin oxide. Such structures can potentially be used in light-emitting diodes and displays. In the third part of this work, a thin gold nanoparticle layer is obtained from a thermal annealing process. The interplay between this nanoparticle layer and a substrate metal reflector gives rise to broadband extinction of light at the near-infrared wavelength range. Specular and diffuse reflectances were singled out. Samples with high absorption or high diffuse reflection are identified. The structures can potentially be incorporated in solar cells as diffuse back reflectors or as spectrally selective absorbers for solar thermal collectors. In the fourth part of this work, the possibility of using a metal-insulator-metal structure (based on titanium, alumina, and aluminum) for achieving artificial coloration is explored. Through a diffusion-assisted deposition procedure, the dielectric spacer has a laterally varying thickness. Thereby the sample exhibits a continuum of visible colors. The reflectance spectra of the fabricated sample in the visible range were measured, and agreement to theoretical calculation is found to be very good. The artificial colors can be patterned at various geometries. Their potential application, besides functioning as spectrally selective absorbers in optoelectronic devices, can be used for security applications of consumer and artistic products.
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