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.
The scope for maximizing the albedo of a painted surface to produce low cost new and retro-fitted super-cool roofing is explored systematically. The aim is easy to apply, low cost paint formulations yielding albedos in the range 0.90 to 0.95. This requires raising the near-infrared (NIR) spectral reflectance into this range, while not reducing the more easily obtained high visible reflectance values. Our modified version of the four-flux method has enabled results on more complex composites. Key parameters to be optimized include; fill factors, particle size and material, using more than one mean size, thickness, substrate and binder materials. The model used is a variation of the classical four-flux method that solves the energy transfer problem through four balance differential equations. We use a different approach to the characteristic parameters to define the absorptance and scattering of the complete composite. This generalization allows extension to inclusion of size dispersion of the pigment particle and various binder resins, including those most commonly in use based on acrylics. Thus, the pigment scattering model has to take account of the matrix having loss in the NIR. A paint ranking index aimed specifically at separating paints with albedo above 0.80 is introduced representing the fraction of time at a sub-ambient temperature.
There has been continued recent interest in radiative sky cooling of coated flat surfaces that are able to passively attain sub-ambient temperatures. As the lowest incoming infrared radiation from a clear sky occurs at the zenith, a surface which sees mainly this region of the sky will receive much lower levels of sky radiation than one which views the whole sky, since the near-horizon contains significantly more incoming radiation. Two approaches to extra cooling are thus angular selectivity, which limits oblique outgoing as well as incoming radiation, and macroscopic reflectors which block oblique incoming sky radiation, while directing most outgoing emitted radiation towards the near zenith. This work focuses on the second of these techniques. We maximise cooling potential via coated 3D printed structures which can passively maintain a thermal reservoir below ambient temperature throughout the night and day. Novel design methods are used to fabricate and test structures which maximise outgoing thermal radiation from a surface, while minimising its illumination by incoming radiation from the sky and sun. Preliminary results gave 10°C below ambient both day and night during a Sydney spring. 3D printing allows the production of complex designed mirror cones with relatively low thermal conductivity. Post processing of the 3D printed structures allows the desired surface textures and optical properties to be created.
Cermet coatings based on nanoparticles of Au or Ag in a stable dielectric matrix provide a combination of spectral-selectivity and microstructural stability at elevated temperatures. The nanoparticles provide an absorption peak due to their localized surface plasmon resonance and the dielectric matrix provides red-shifting and intrinsic absorption from defects. The matrix and two separated cermet layers combined add mechanical support, greater thermal stability and extra absorptance. The coatings may be prepared by magnetron sputtering. They have solar absorptance ranging between 91% and 97% with low thermal emittance making them suitable for application in solar thermal conversion installations.
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.
Silver is considered as one of the most desirable materials for plasmonic devices due to it having low loss, low epsilon2, across the visible spectrum. In addition, silver nanotriangles can self-assemble into complex structures that can include tip-totip or base-to-base arrangements. While the optical properties of tip-to-tip dimers of nanotriangles have been quite intensively studied, the geometric inverse, the base-to-base configuration, has received much less attention. Here we report the results of a computational study of the optical response of this latter configuration. Calculations were performed using the discrete dipole approximation. The effect of gap size and substrate are considered. The results indicate that the base-to-base configuration can sustain a strong coupled dipole and various multimode resonances. The pairing of the parallel triangle edges produces a strongly capacitive configuration and very intense electric fields over an extended volume of space. Therefore, the base-to-base configuration could be suitable for a range of plasmonic applications that require a strong and uniform concentration of electric field. Examples include refractometeric sensing or metal-enhanced fluorescence.
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.
Titanium nitride is a golden-colored semiconductor with metallic optical properties. It is already widely used in room temperature spectrally-selective coatings. In contrast, aluminum nitride is a relatively wide-band gap, non-metallic material. Both nitrides have exceptional thermal stability, to over 1000 °C, but are susceptible to oxidation. We will show here that composite coatings consisting of these materials and their complex oxides have considerable potential for spectrally-selective applications, including at elevated temperatures. In particular, we examine the metastable materials produced by magnetron sputtering. The effective dielectric functions of these materials can be tuned over a wide range by manipulation of their microstructure. This provides a strategy to assemble materials with tunable dielectric functions using a 'bottom-up' approach. The results are compared to those achievable by conventional, 'top-down', planar optical stacks comprised of alternating layers of TiNx and AlN.
The optical resonances that occur in nanostructured metal layers are modulated in thin film stacks if the nanostructured layer is separated from a reflecting conducting layer by various thicknesses of thin dielectric. We have measured and modeled the optical response of interacting silver layers, with alumina spacer thickness ranging from a few nm to 50 nm, for s- and p-polarized incident light, and a range of incident angles. Standard thin film models, including standard effective medium models for the nanostructured layer, will break down for spacer thickness below a critical threshold. For example, with polarisation in the film plane and some nano-islands, it may occur at around 10 nm depending on spacer refractive index. Of particular interest here are novel effects observed with the onset of percolation in the nanolayer. Hot spot effects can be modified by nearby mirrors. Other modes to consider include (a) a two-particle mode involving a particle and its mirror image (b) A Fano resonance from hybridisation of localized and de-localised plasmon modes (c) a Babinet’s core-(partial) shell particle with metal core-dielectric shell in metal (d) spacing dependent phase modulation (e) the impact of field gradients induced by the mirror at the nano-layer.
The grating-mirror geometry is a particularly rich plasmonic system due to the coupling of localized and global modes, and it is applicable to negative index materials, plasmonic imaging, and spectral filters. Recently absorption in sub-percolative films was found to be greatly enhanced by the addition of a mirror - a situation that is also reasonably modeled by a grating-mirror geometry. A great deal of attention has been focused on the coupling of barely-sub-wavelength periodic grating modes to surface plasmon polaritons that exhibit sharp spectral features. In contrast, island films have only quasi-periodicity at a few tens of nanometers, and produce broader spectral features, suggesting the influence of localized surface plasmons. In this work we examine how absorption is affected by variations in geometry of grating-mirror systems, to identify basic physics for future investigations of particle-mirror systems.
Near-field imaging through plasmonic 'superlensing' layers can offer advantages of improved working distance (i.e.
introducing the equivalent of a focal length) and control over image intensity compared to simple near-field imaging. In
a photolithographic environment at ultra-violet (UV) wavelengths the imaging performance of single- and multi-layer
silver plasmonic superlenses has been studied both experimentally and via computer simulations. Super-resolution
imaging has been demonstrated experimentally, with the sub-100 nm resolution currently being limited by issues of
roughness in the silver layers and the ability to deposit high-quality silver-dielectric multilayers. The simulation studies
have shown that super-resolved imaging should be possible using surprisingly thick silver layers (>100 nm), with the
cost of much reduced image intensity, which is something that is yet to be shown experimentally. The use of multilayer
plasmonic superlenses also introduces richness to the imaging behaviour, with very high transmission possible for certain
spatial frequency components in the image. This has been widely touted as a means for improving image resolution, but
the complexity of the spatial-frequency transfer functions for these systems does not make this a universal fact for all
classes of objects. Examples of imaging situations are given where multi-layer superlenses are actually detrimental to
the image quality, such as the case of closely-separated dark-line objects on an otherwise bright background.
Optical and electron microscopies reveal complexity in the multilayered chiral coatings that produce green metallic-like reflections from manuka (scarab) beetles. In particular the reflectors are shown to have the form of small concave pits and troughs that are filled with contouring chiral material. Each chiral micro-reflector presents a range of pitch and tilt to an incident beam of light. The presentation attempts to relate these physical properties to optical properties such as spectral reflectance, angle of spread and perceived color of the beetles.
Effective medium theory is applied to nanoengineered inorganic
coatings in which a primary chiral medium (A) threads through a
second medium (B) that may be isotropic, birefringent or chiral.
In general A and B are represented by thin locally-birefringent
layers with axes that rotate through the thickness of the
coating. Local properties of the complex medium are represented
by a Herpin birefringent period (A/2)B(A/2), with effective
principal refractive indices and with thickness much less than
the chiral period.
The effect of changing the fraction fB of material B is
considered. With simplicity of fabrication in mind, the special
case in which A and B are identical apart from thickness and
azimuthal orientation is developed. Then, by maintaining a
constant optical thickness for the Herpin period, the circular
Bragg resonant wavelength of the primary medium can be retained.
As an example of the potential use of threaded-chiral materials,
handed resonant reflectors for elliptically-polarized light are
considered.
The optical properties of narrow-band Fabry-Perot filters for circulalry polarized light are surveyed theoretically. Here the general filter is considered to be formed by two chiral reflectors each of N turns, one with an abrupt twist of ξ and separated by an isotropic spacer of refractive index n and thickness d. The spacerless filter with d = 0, ξ = π/2 which may be regarded as a special case, is known to suffer from severe limitation of finesse and purity of polarization as N becomes large. However from previous work, there is reason to anticipate that general chiral supercavities with virtually unlimited finesse may be possible. In the presentation performance maps of the degree of polarization and the finesse confirm that chiral supercavities are possible, and conditions on n, d, and ξ are explored.
A new global approach, called 'Generalized Ellipsometry', is now capable to characterize the optical and structural properties of general anisotropic layered systems, including absorption, and can be applied, in general, to determine the linear response tensor elements for wavelengths from the far IR to the deep UV. This technique enables new insights into physical phenomena of layered anisotropic mediums, and can provide precise structural and optical data of novel compound materials. Experimental results are presented for stibnite single crystals as example for an arbitrary biaxial absorbing material, a wurtzite GaN thin film with uniaxial anisotropy grown on sapphire, a spontaneously atomically ordered III-V semiconductor alloy thin film, and a sculptured titanium dioxide film with symmetrically dielectric tensor properties.
The serial bideposition (SBD) technique is used to produce biaxial thin films with large linear birefringence and chiral (handed) media with large circular birefringence. Thin film wave plates and chiral reflectors of SBD silicon that have been fabricated show promise for applications in the wavelength range 800-2200nm. In particular we have deposited quarter-wave plates of metric thickness less than 1micrometers for the 800nm wavelength used in CD players, and half-wave plates of metric thickness about 3micrometers for the 1550nm optical communications wavelength. An observation of the Bragg resonance in a silicon chiral film in the 600-800nm wavelength range where the absorption is high but the linear birefringence is approximately equals 0.35, suggests possible applications even at the 633 HeNe wavelength. Polarizing elements that we have designed and fabricated for use with circularly polarized light in the visible and near infrared spectral regions are described. These include a two-layer Fabry-Perot filter that uses a structural phase discontinuity instead of a physical spacer layer to define the wavelength of a narrow spectral hole at the center of the Bragg dip. Further strategies, also based on phase discontinuities, are explored for adapting other filter designs from isotropic thin film filter theory to the circular case.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.