We demonstrate switchable polarized thermal emission from VO2 nanofin stacks fabricated by co-deposition, etching, and oxidation. We find that reverse switching of the thermal emission is enabled by a reflecting underlayer and induced by either short oxidation time or additional deposition of a reflecting underlayer. Observed thermal emission is well explained by a biaxial Bruggeman effective medium model, which predicts the strong polarization change for aligned fin layers in the micron thickness range. The dominant polarization of the emission is modulated by the presence of a reflector, oxidation of the fins, fin fill-factor, and structural anisotropy. Normal incidence polarized emittance change of up to 0.6 is theoretically possible, and we were able to demonstrate a change of 0.34, similar to that predicted by the model.
Here we present our recent developments in temperature dependent ellipsometry, FTIR and emittance measurements of flat and structured vanadium dioxide (VO2) surfaces allowing significant control of switchable radiative cooling beyond that attainable via traditional VO2 surfaces. VO2 undergoes a metal-insulator transition at a critical temperature of ~ 68°C; previous work has investigated tuning of this critical temperature over a wide range of temperatures. Here we exploit the shift in optical properties to produce surfaces with various emittance temperature profiles that modulate the thermal radiative transfer to/from a surface.
Designing surfaces with different temperature emittance profiles requires accurate optical/thermal characterisation of materials. VO2 is produced by sputtering of vanadium followed by post deposition annealing in a 0.1Torr to 0.3Torr Air atmosphere at 450°C to 550°C, in-situ optical monitoring allows for accurate termination of the annealing process once the desired optical response is achieved.
Angled columnar structures produced by oblique angle deposition possess useful optical polarization effects. It is well known that this is due to structural anisotropy but the relative contributions of factors affecting this anisotropy are not fully understood in all cases. Serial bideposited films where the azimuth is changed during deposition can have greater birefringence if the azimuths are directly opposed. In contrast, in this article the properties of perpendicular azimuth films are studied: silicon films at tilt angles 50-80° were deposited and analyzed. Electron microscopy confirmed that the silicon nanostructures were formed off-axis, meaning they did not develop along the deposition axes but followed the averaged azimuth. Optical measurements confirm that the maximum birefringence occurs closer to glancing angles, and optical modelling demonstrates that in contrast to fixed azimuth films the birefringence in these perpendicular azimuth films is primarily modulated by depolarization factors.
The optical and electrical responses of open, nanoscale, metal networks are of interest in a variety of technologies including transparent conducting electrodes, charge storage, and surfaces with controlled spectral selectivity. The properties of such nanoporous structures depend on the shape and extent of individual voids and the associated hyper-dimensional connectivity and density of the metal filaments. Unfortunately, a quantitative understanding of this dependence is at present only poorly developed. Here we address this problem using numerical simulations applied to a systematically designed series of prototypical sponges. The sponges are produced by a Monte Carlo simulation of the dealloying of Ag-Al alloys containing from 60% to 85% Al. The result is a series of Ag sponges of realistic morphology. The optical properties of the sponges are then calculated by the discrete dipole approximation and the results used to construct an 'effective medium' model for each sponge. We show how the resulting effective medium can be correlated with the associated morphological characteristics of each target and how the optical properties are primarily controlled by the density of the sponge and its state of percolation.
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