Accurate Thermal Conductivity Measurements of Porous Thin Films by Time-Domain Thermoreflectance

Accurate measurements of the thermal conductivity (κ) of porous thin films are still limited due to challenges to deposit flat and continuous metal transducers on porous samples, a necessity for many thermal measurement techniques for nanostructures. In this paper, we introduce an approach based on time-domain thermoreflectance (TDTR) to accurately and conveniently measure κ of porous thin films by transferring a flat and smooth metal film unto porous samples as the transducer for TDTR measurements. We demonstrate our approach by measuring κ of a series of microscale holey SiO2 films with diameters of 1-3.5 μm and porosity of 13-50%. To achieve a measurement uncertainty of <12%, we ensure that the metal transducer films are sufficiently stiff and establish good thermal contact with the holey SiO2 samples. Our κ measurements agree well with calculations of κ from effective medium theory. Our approach could provide a convenient way to further investigate the thermal transport properties of porous films.

A Study on the Radiation Cooling Characteristics of Cerambycini Latreille

The severe climate and energy issues require more environmentally friendly and efficient cooling methods. Radiative cooling offers a cooling solution with significant advantages. However, current radiative cooling technologies focus primarily on seeking perfect materials to achieve complete wavelength absorption. However, numerous research studies have shown that achieving such a perfect scenario is not feasible. Here, inspired by the surface of the Cerambycini Latreille, the inherent mechanism of radiative cooling functionality in the unique structure of these hairs is revealed using effective medium theory and Finite Difference Time Domain (FDTD) optical simulation analysis. Through alkaline etching and template methods, a biomimetic radiative cooling film (BRCF) was successfully fabricated. The BRCF not only efficiently reflects solar radiation but also enhances absorption in the atmospheric window wavelength range. The radiative cooling mechanism proposed in this study and the BRCF presented here may inspire researchers to further explore the field of structural radiative cooling.

Nonlocal effective medium theory for phononic temporal metamaterials

We have developed a nonlocal effective medium theory (EMT) for phononic temporal metamaterials using the multiscale technique. Our EMT yields closed-form expressions for effective constitutive parameters and reveals these materials as reciprocal media with symmetric band dispersion. Even without spatial symmetry breaking, nonzero Willis coupling coefficients emerge with time modulation and broken time-reversal symmetry, when the nonlocal effect is taken into account. Compared to the local EMT, our nonlocal version is more accurate for calculating the bulk band at high wavenumbers and essential for understanding nonlocal effects at temporal boundaries. This nonlocal EMT can be a valuable tool for studying and designing phononic temporal metamaterials beyond the long-wavelength limit.

Topological Transitions and Surface Umklapp Scattering in Weakly Modulated Periodic Metasurfaces

Controlling and manipulating surface waves is highly beneficial for imaging applications, nanophotonic device design, and light-matter interactions. While deep-subwavelength structuring of the metal-dielectric interface can influence surface waves by forming strong effective anisotropy, it disregards important structural degrees of freedom such as the interplay between corrugation periodicity and depth and its effect on the beam transport. Here, we unlock these degrees of freedom, introducing weakly modulated metasurfaces, structured metal-dielectric surfaces beyond effective medium. We utilize groove-structuring with varying depths and periodicities to demonstrate control over the transport of surface waves, dominated by the depth-period interplay. We show unique backward focusing of surface waves driven by an umklapp process-momentum relaxation empowered by the periodic nature of the structure and discover a yet unexplored, dual-stage topological transition. Our findings can be applied to any type of guided wave, introducing a simple and versatile approach for controlling wave propagation in artificial media.

Effective medium theory and its limitations for the description of MoO3films doped with nanoparticles

The present work analyses the transmittance and reflectance spectra of molybdenum trioxide film doped by silver nanoparticles as a function of angle of incidence and wavelength. As will be seen in this work, at values of angle of incidence below 40 degrees and with volume filling fraction below 1% also, some differences between the two effective medium theories are presented. First, the volume filling fraction is limited for low values (<1%) and second the scattering amplitude cannot be ignored for these cases. The novelty of this work is that the use of the effective medium model (refractive index) shows limitations in the description of the optical properties when it was applied to thin solid films.

Thin Films of Nonlinear Metallic Amorphous Composites

We studied the nonlinear optical response of metallic amorphous composite layers in terms of a self-phase-modulated, third-order Kerr nonlinearity. A nonlinear effective medium theory was used to describe low densities of gold and iridium nanoparticles embedded in an equally nonlinear host material. The fill fraction strongly influences the effective nonlinear susceptibility of the materials, increasing it by orders of magnitude in the case of gold due to localized surface plasmonic resonances. The enhancement of the nonlinear strength in amorphous composites with respect to the bulk material has an upper limit in metallic composites as dominating absorption effects take over at higher fill factors. Both saturated and induced absorption in the thin films of amorphous composites were observed depending on the selected frequency and relative position to the resonant frequency of electron excitation in the metallic inclusions. We demonstrated the depths to which thin films are affected by nonlinear enhancement effects.

Effective Approximation Method for Nanogratings-induced Near-Field Radiative Heat Transfer

Nanoscale radiative thermal transport between a pair of metamaterial gratings is studied within this work. The effective medium theory (EMT), a traditional method to calculate the near-field radiative heat transfer (NFRHT) between nanograting structures, does not account for the surface pattern effects of nanostructures. Here, we introduce the effective approximation NFRHT method that considers the effects of surface patterns on the NFRHT. Meanwhile, we calculate the heat flux between a pair of silica (SiO₂) nanogratings with various separation distances, lateral displacements, and grating heights with respect to one another. Numerical calculations show that when compared with the EMT method, here the effective approximation method is more suitable for analyzing the NFRHT between a pair of relatively displaced nanogratings. Furthermore, it is demonstrated that compared with the result based on the EMT method, it is possible to realize an inverse heat flux trend with respect to the nanograting height between nanogratings without modifying the vacuum gap calculated by this effective approximation NFRHT method, which verifies that the NFRHT between the side faces of gratings greatly affects the NFRHT between a pair of nanogratings. By taking advantage of this effective approximation NFRHT method, the NFRHT in complex micro/nano-electromechanical devices can be accurately predicted and analyzed.

Enhanced Thermoelectric Performance by Strong Phonon Scattering at the Heterogeneous Interfaces of the Mg2Sn/Mg3Sb2 High-Content Nanocomposite

Nano approaches are practical strategies to boost the thermoelectric figure of merit due to the strong phonon scattering from the grain boundaries and nanoinclusions. Here, we have reported a strong phonon scattering at the heterogeneous interfaces of Mg₂Sn/Mg₃Sb₂ high-content nanocomposites (HCnCs). As a result, a significantly reduced lattice thermal conductivity of 1.09 W m^-1 K^-1 was observed in the equimolar Mg₂Sn/Mg₃Sb₂ HCnC, 80% lower than pure Mg₂Sn and 25% lower than pure Mg₃Sb₂. As a result, a high ZT ∼ 1.13 at 773 K was achieved in the Mg₂Sn/Mg₃Sb₂ HCnC. Furthermore, various defects, including solid solutions, nanoinclusions, and misfit dislocations, were observed in both the Mg₃Sb₂ phase and the Mg₂Sn phase through the microstructure characterization.

Extinction Coefficient Modulation of MoO3 Films Doped with Plasmonic Nanoparticles: From an Effective Medium Theory Description

This work focused on the application of the effective medium theory to describe the extinction coefficient (Qext) in molybdenum trioxide (MoO3) doped with different kinds of plasmonic nanoparticles, such as silver (Ag), gold (Au), and copper (Cu). Usually, in studies of these materials, it is normal to analyze the transmission or absorption spectra. However, the effect of this type or size of nanoparticles on the spectra is not as remarkable as the effect that is found by analyzing the Qext of MoO3. It was shown that the β-phase of MoO3 enhanced the intensity response of the Qext when compared to the α-phase of MoO3. With a nanoparticle size of 5 nm, the Ag-doped MoO3 was the configuration that presents the best response in Qext. On the other hand, Cu nanoparticles with a radius of 20 nm embedded in MoO3 was the configuration that presented intensities in Qext similar to the cases of Au and Ag nanoparticles. Therefore, implementing the effective medium theory can serve as a guide for experimental researchers for the application of these materials as an absorbing layer in photovoltaic cells.

Decay Rates of Plasmonic Elliptical Nanostructures via Effective Medium Theory

This paper investigates the spontaneous decay rate of elliptical plasmonic nanostructures. The refractive index was analyzed using the effective medium theory (EMT). Then, the polarizability, spontaneous radiative, non-radiative decay rate, and electric field enhancement factor were characterized for the targeted elliptical nanostructures at different aspect ratios. All of the optical analyses were analyzed at different distances between the excited fluorescent coupled atom and the plasmonic nanostructure (down to 100 nm). This work is promising in selecting the optimum elliptical nanostructure according to the required decay rates for optical conversion efficiency control in energy harvesting for solar cells and optical sensing applications.

Dielectric Properties of BaTiO3-Epoxy Nanocomposites in the Microwave Regime

We synthesized BaTiO₃–epoxy nanocomposites (particle size < 100 nm) with volume fractions up to 25 vol. %, whose high-frequency complex permittivity was characterized from 8.2 to 12.5 GHz. The maximum dielectric constant approaches 9.499 with an acceptable loss tangent of 0.113. The dielectric loss gradually saturates when the particle concentration is higher than 15 vol. %. This special feature is an important key to realizing high-k and low-loss nanocomposites. By comparing the theoretical predictions and the experimental data, four applicable effective-medium models are suggested. The retrieved dielectric constant (loss tangent) of 100-nm BaTiO₃ nanopowder is in the range of 50–90 (0.1–0.15) at 8.2–12.5 GHz, exhibiting weak frequency dispersion. Two multilayer microwave devices—total reflection and antireflection coatings—are designed based on the fabricated nanocomposites. Both devices show good performance and allow broadband operation.

Development of oral cancer tissue-mimicking phantom based on polyvinyl chloride plastisol and graphite for terahertz frequencies

SIGNIFICANCE

A new concept of a biotissue phantom for terahertz (THz) biomedical applications is needed for reliable and long-term usage.

AIM

We aimed to develop a new type of biotissue phantom without water content and with controllable THz optical properties by applying graphite powders into a polyvinyl chloride plastisol (PVCP) matrix and to give a numerical description to the THz optical properties of the phantoms using the Bruggeman model (BM) of the effective medium theory (EMT).

APPROACH

The THz optical properties of graphite and the PVCP matrix were measured using THz time-domain spectroscopy, which works in the frequency range from 0.1 to 1 THz. Two phantoms with 10% and 12.5% graphite were fabricated to evaluate the feasibility of describing phantoms using the EMT. The EMT then was used to determine the concentration of graphite required to mimic the THz optical properties of human cancerous and healthy oral tissue.

RESULTS

The phantom with 16.7% of graphite has the similar THz optical properties as human cancerous oral tissue in the frequency range of 0.2 to 0.7 THz. The THz optical properties of the phantom with 21.9% of graphite are close to those of human healthy oral tissue in the bandwidth from 0.6 to 0.8 THz. Both the refractive index and absorption coefficient of the samples increase with an increase of graphite concentration. The BM of the EMT was used as the numerical model to describe the THz optical properties of the phantoms. The relative error of the BM for the refractive index estimation and the absorption coefficient is up to 4% and 8%, respectively.

CONCLUSIONS

A water-free biotissue phantom that mimics the THz optical properties of human cancerous oral tissue was developed. With 21.9% of graphite, the phantom also mimics human healthy oral tissue in a narrow frequency range. The BM proved to be a suitable numerical model of the phantom.

Real-Time Analysis of Laser-Induced Plasmon Tuning in Nanoporous Glass Composite

Laser-induced structuring in nanoporous glass composites is promising for numerous emerging applications in photonics and plasmonics. Local laser irradiation activates an interplay of photo-thermo-chemical mechanisms that are extremely difficult to control. The choice of optimum laser parameters to fabricate structures with desired properties remains extremely challenging. Another challenging issue is the investigation of the properties of laser-induced buried structures. In this paper, we propose a way to control the plasmonic structures formation inside a nanoporous glass composite with doped silver/copper ions that are induced by laser irradiation. Experimental and numerical investigations both demonstrate the capacities of the procedure proving its validity and application potential. In particular, we register transmitted laser power to analyse and control the modification process. Spectral micro-analysis of the irradiated region shows a multilayer plasmonic structure inside the glass composite. Subsequently, the effective medium theory connects the measured spectral data to the numerically estimated size, concentration, and chemical composition of the secondary phase across the initial GC sample and the fabricated structure.

Tunable Thermal Transport Characteristics of Nanocomposites

We present a study of tunable thermal transport characteristics of nanocomposites by employing a combination of a full-scale semi-ab inito approach and a generalised and extended modification of the effective medium theory. Investigations are made for planar superlattices (PSLs) and nanodot superlattices (NDSLs) constructed from isotropic conductivity covalent materials Si and Ge, and NDSLs constructed from anisotropic conductivity covalent-van der Waals materials MoS 2 and WS 2 . It is found that difference in the conductivities of individual materials, period size, volume fraction of insertion, and atomic-level interface quality are the four main parameters to control phonon transport in nanocomposite structures. It is argued that the relative importance of these parameters is system dependent. The equal-layer thickness Si/Ge PSL shows a minimum in the room temperature conductivity for the period size of around 4 nm, and with a moderate amount of interface mass smudging this value lies below the conductivity of SiGe alloy.

Electronic Percolation Threshold of Self-Standing Ag-LaCoO3 Porous Electrodes for Practical Applications

Perovskite LaCoO3 materials have various applications, from selective permeable membranes and gas sensing devices to water splitting applications. However, the intrinsic electrical resistivity of the perovskite limits the applicative potential. To overcome that, Ag powder was used with LaCoO3 to obtain porous composite electrodes with enhanced conductivities. For that, a series of composite Ag-LaCoO3 powders were prepared into pellets and pre-sintered at various temperatures up to 1000 ∘C. Their structural properties and morphology were investigated by X-ray diffraction and scanning electron microscopy. The electronic transport of compacted specimens was studied by impedance spectroscopy. The results indicate that the presence of Ag acts as pre-sintering additive to obtain porous electrodes, with porosity values as high as 40% at 50 vol. % Ag. Moreover, the overall electrical resistivity of the composite electrodes varied well over four orders of magnitude. The results are discussed within the generalized Bruggeman theory for effective media comprising arbitrarily shaped metallic and semiconducting inclusions.

Directed Thermal Diffusions through Metamaterial Source Illusion with Homogeneous Natural Media

Owing to the utilization of transformation optics, many significant research and development achievements have expanded the applications of illusion devices into thermal fields. However, most of the current studies on relevant thermal illusions used to reshape the thermal fields are dependent of certain pre-designed geometric profiles with complicated conductivity configurations. In this paper, we propose a methodology for designing a new class of thermal source illusion devices for achieving directed thermal diffusions with natural homogeneous media. The employments of the space rotations in the linear transformation processes allow the directed thermal diffusions to be independent of the geometric profiles, and the utilization of natural homogeneous media improve the feasibility. Four schemes, with fewer types of homogeneous media filling the functional regions, are demonstrated in transient states. The expected performances are observed in each scheme. The related performance are analyzed by comparing the thermal distribution characteristics and the illusion effectiveness on the measured lines. The findings obtained in this paper see applications in the development of directed diffusions with minimal thermal loss, used in novel “multi-beam” thermal generation, thermal lenses, solar receivers, and waveguide.

Phase-Field Based Multiscale Modeling of Heterogeneous Solid Electrolytes: Applications to Nanoporous Li3PS4

Modeling the effective ion conductivities of heterogeneous solid electrolytes typically involves the use of a computer-generated microstructure consisting of randomly or uniformly oriented fillers in a matrix. However, the structural features of the filler/matrix interface, which critically determine the interface ion conductivity and the microstructure morphology, have not been considered during the microstructure generation. Using nanoporous β-Li₃PS₄ electrolyte as an example, we develop a phase-field model that enables generating nanoporous microstructures of different porosities and connectivity patterns based on the depth and the energy of the surface (pore/electrolyte interface), both of which are predicted through density functional theory (DFT) calculations. Room-temperature effective ion conductivities of the generated microstructures are then calculated numerically, using DFT-estimated surface Li-ion conductivity (3.14 × 10^-3 S/cm) and experimentally measured bulk Li-ion conductivity (8.93 × 10^-7 S/cm) of β-Li₃PS₄ as the inputs. We also use the generated microstructures to inform effective medium theories to rapidly predict the effective ion conductivity via analytical calculations. When porosity approaches the percolation threshold, both the numerical and analytical methods predict a significantly enhanced Li-ion conductivity (1.74 × 10^-4 S/cm) that is in good agreement with experimental data (1.64 × 10^-4 S/cm). The present phase-field based multiscale model is generally applicable to predict both the microstructure patterns and the effective properties of heterogeneous solid electrolytes.

Mie-Metamaterials-Based Thermal Emitter for Near-Field Thermophotovoltaic Systems

In this work, we theoretically analyze the performance characteristics of a near-field thermophotovoltaic system consisting a Mie-metamaterial emitter and GaSb-based photovoltaic cell at separations less than the thermal wavelength. The emitter consists of a tungsten nanoparticle-embedded thin film of SiO2 deposited on bulk tungsten. Numerical results presented here are obtained using formulae derived from dyadic Green’s function formalism and Maxwell-Garnett-Mie theory. We show that via the inclusion of tungsten nanoparticles, the thin layer of SiO2 acts like an effective medium that enhances selective radiative heat transfer for the photons above the band gap of GaSb. We analyze thermophotovoltaic (TPV) performance for various volume fractions of tungsten nanoparticles and thicknesses of SiO2.

Generalized Effective Medium Theory for Particulate Nanocomposite Materials

The thermal conductivity of particulate nanocomposites is strongly dependent on the size, shape, orientation and dispersion uniformity of the inclusions. To correctly estimate the effective thermal conductivity of the nanocomposite, all these factors should be included in the prediction model. In this paper, the formulation of a generalized effective medium theory for the determination of the effective thermal conductivity of particulate nanocomposites with multiple inclusions is presented. The formulated methodology takes into account all the factors mentioned above and can be used to model nanocomposites with multiple inclusions that are randomly oriented or aligned in a particular direction. The effect of inclusion dispersion non-uniformity is modeled using a two-scale approach. The applications of the formulated effective medium theory are demonstrated using previously published experimental and numerical results for several particulate nanocomposites.

PDielec: The calculation of infrared and terahertz absorption for powdered crystals

The Python package PDielec is described, which calculates the infrared absorption characteristics of a crystalline material supported in a non-absorbing medium. PDielec post processes solid-state quantum mechanical and molecular mechanical calculations of the phonons and dielectric response of the crystalline material. Using an effective medium method, the package calculates the internal electric field arising from different particle morphologies and calculates the resulting shift in absorption frequency and intensity arising from the coupling between a phonon and the internal field. The theory of the approach is described, followed by a description of the implementation within PDielec. Finally, a section providing several examples of its application is given. © 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.

Numerical Modeling of Sub-Wavelength Anti-Reflective Structures for Solar Module Applications

This paper reviews the current progress in mathematical modeling of anti-reflective subwavelength structures. Methods covered include effective medium theory (EMT), finite-difference time-domain (FDTD), transfer matrix method (TMM), the Fourier modal method (FMM)/rigorous coupled-wave analysis (RCWA) and the finite element method (FEM). Time-based solutions to Maxwell’s equations, such as FDTD, have the benefits of calculating reflectance for multiple wavelengths of light per simulation, but are computationally intensive. Space-discretized methods such as FDTD and FEM output field strength results over the whole geometry and are capable of modeling arbitrary shapes. Frequency-based solutions such as RCWA/FMM and FEM model one wavelength per simulation and are thus able to handle dispersion for regular geometries. Analytical approaches such as TMM are appropriate for very simple thin films. Initial disadvantages such as neglect of dispersion (FDTD), inaccuracy in TM polarization (RCWA), inability to model aperiodic gratings (RCWA), and inaccuracy with metallic materials (FDTD) have been overcome by most modern software. All rigorous numerical methods have accurately predicted the broadband reflection of ideal, graded-index anti-reflective subwavelength structures; ideal structures are tapered nanostructures with periods smaller than the wavelengths of light of interest and lengths that are at least a large portion of the wavelengths considered.