M. Pinar Mengüç

Near-field radiative heat transfer enhancement via surface phonon polaritons coupling in thin films

We investigate near-field thermal radiation between a nanometric film and a bulk SiC using fluctuational electrodynamics. Results show a narrow spectral band enhancement of the radiative flux for nanometric emitters due to coupling of surface phonon polaritons inside the film. For a 10nm thick SiC emitter, the total radiative flux is 2.2 times larger than for a bulk emitter. The total radiative flux is increased by a factor of 3.3 if a dielectric is coated with a 10nm SiC film due to a splitting of the resonant frequency into two distinct ones, which has practical interests for near-field thermophotovoltaic devices.

Spectral tuning of near-field radiative heat flux between two thin silicon carbide films

Spectral distributions of radiative heat flux between two thin silicon carbide films separated by sub-wavelength distances in vacuum are analysed. An analytical expression for the near-field flux between two layers of finite thicknesses in terms of film reflection and transmission coefficients is derived for the first time. The resulting equation clearly shows the resonant modes of thermal emission, absorption and the cross-coupling of surface phonon-polaritons (SPhPs) between the layers. When the films are of the same thickness, the resonant frequencies maximizing near-field thermal emission almost match those of absorption. The small discrepancies, due to SPhP coupling between the films, lead to loss of spectral coherence affecting mostly the low frequency mode. The flux profiles also show that splitting of the resonance into two distinct frequencies happens when the ratio thickness of the film over the separation gap is less than unity. When the thickness of one film increases relative to the other, spectral distributions of flux are significantly altered due to an important mismatch between the resonant frequencies of high emission and absorption. This modification of the near-field flux is mostly due to weaker SPhP coupling within the layer of increasing thickness. Based on an asymptotic analysis of the dispersion relation, an approximate approach is proposed to predict the resonant modes maximizing the flux between two films, which can be potentially extended to multiple thin layers. The outcome of this work would allow tailoring near-field radiative heat transfer, and can eventually be used to design customized nanostructures for energy harvesting applications.

Solution of the inverse radiation problem for inhomogeneous and anisotropically scattering media using a Monte Carlo technique

An analysis is presented for the solution of the inverse radiation problem using a Monte Carlo technique. For inhomogeneous planar media, the profile of the single scattering albedo is obtained from the inverse analysis. For homogeneous, anisotropically scattering media, the single scattering albedo and the asymmetry factor are recovered. A step phase function approximation is used to account for the anisotropic scattering in the medium. The confidence bounds on the estimated parameters for errors in the input data are evaluated. The results show that the medium properties can be recovered with high accuracy even if there is up to 10% error in the input data. The primary advantage of the Monte Carlo method is that a single direct solution yields the coefficients of a multivariate polynomial for each set of observation data, which are then used to obtain the medium properties by a non-linear least-square minimization technique.

Local density of electromagnetic states within a nanometric gap formed between two thin films supporting surface phonon polaritons

We present a detailed physical analysis of the near-field thermal radiation spectrum emitted by a silicon carbide (SiC) film when another nonemitting SiC layer is brought in close proximity. This is accomplished via the calculation of the local density of electromagnetic states (LDOS) within the gap formed between the two thin films. An analytical expression for the LDOS is derived, showing explicitly that (i) surface phonon polariton (SPhP) coupling between the layers leads to four resonant modes, and (ii) near-field thermal radiation emission is enhanced due to the presence of the nonemitting film. We study the impact of the interfilm separation gap, the distance where the fields are calculated, and the thickness of the nonemitting layer on the spectral distribution of the LDOS. Results show that for an interfilm gap of 10 nm, the near-field spectrum emitted around the SPhP resonance can increase more than an order of magnitude as compared to a single emitting thin layer. Interfilm SPhP coupling also in...

Forward and inverse analysis of single and multiple scattering of collimated radiation in an axisymmetric system

Abstract In this work, scattering of a collimated light source incident on a single/multiple scattering axisymmetric medium is studied and the extent to which analysis can be used to recover the effective radiative properties of the medium is determined. For this purpose, a He-Ne laser nephelometer is designed and the angular scattered intensity distributions measured in the single and multiple scattering regimes for mono- and polydispersed suspensions of particles. Analytical expressions are derived for the angular intensity distribution, accounting for up to two successive scatters. The experiments show very good agreement with the theoretical calculations. Also, an inverse analysis is presented to determine the phase function coefficients of particles in situ using the experimental results. The first two coefficients of the Legendre expansion of the phase function are recovered for both ymono- and polydispersions within 10% of the actual value for experiments conducted at low optical thicknesses ( τ

COMPARISON OF MONTE CARLO TECHNIQUES TO PREDICT THE PROPAGATION OF A COLLIMATED BEAM IN PARTICIPATING MEDIA

Monte Carlo (MC) techniques are the most versatile approaches in solving the integrodifferential radiative transfer equation (RTE). They are based on numerical simulation of propagation of radiant energy within absorbing, emitting, and scattering media [1-4]. MC simulations can easily be applied to multidimensional, nonhomogenous, highly forward-scattering media with time-dependent boundary conditions, where other techniques are almost impossible to implement. On the down side, statistical errors associated with these techniques can be significant if the number of photons accounted for in the simulation is not sufficiently large, yet the computational penalty increases considerably with increasing number of photons. Here, we consider three different MC approaches in solving the RTE in a planeparallel, absorbing and isotropically scattering medium subjected to a collimated light source. The collimated light source is assumed to be an impulse function impinged instantaneously on the upper boundary of the medium. The strength of the light source is not important in the simulations, since all the computed quantities will be normalized accordingly. Time-dependent as well as steady-state cases are considered. Three FORTRAN codes were developed to predict radiative reflectance and transmittance. They are compared in terms of speed of convergence and statistical accuracy.

Characterizing field emission from individual carbon nanotubes at small distances

This article demonstrates the characterization of field emission from individual carbon nanotubes (CNTs) attached to a tungsten tip, when the separation distance s between the anode and tip of the CNT (cathode) is less than 15μm. The separation distance is adjusted with a nanopositioning stage after establishing a datum by detecting the anode surface with the CNT tip. Our separation distance s differs by the height h of the CNT from the distance d that is often measured between the planar anode and the planar substrate of an emitting cathode. Consequently, the electric field at the tip of the CNT is modeled by F=λV∕s, where λ is our field amplification factor, rather than by F=γV∕d, where γ is the more conventional field enhancement factor. Twenty-four sets of current-voltage I(V) data were measured from an individual multiwall CNT at separation distances s between 1.4 and 13.5μm. A nonlinear curve-fitting algorithm extracted Fowler-Nordheim (FN) parameters from each set of I(V) data, rather than conventi...

Monte Carlo methods in radiative transfer and electron-beam processing

Monte Carlo methods (MCMs) are the most versatile approaches in solving the integro-differential equations. They are statistical in nature and can be easily adapted for simulation of the propagation of ensembles of quantum particles within absorbing, emitting, and scattering media. In this paper, we use MCM for the solution of the Boltzmann transport equation, which is the governing equation for both radiative transfer and electron-beam processing. We briefly outline the methodology for the solution of MCMs, and discuss the similarities and differences between the two different application areas. The focus of this paper is primarily on the treatment of different scattering phase functions.

An investigation of dependent/independent scattering regimes using a discrete dipole approximation

Abstract Dependent scattering/independent scattering regimes are investigated in two different particle systems using a discrete dipole approximation for modeling the interaction of electromagnetic waves with the matter. In the first case, absorption and scattering by two small Rayleigh spheres, separated at an arbitrary distance, are discussed and compared against the Lorenz-Mie theory predictions for single spheres. After that, small agglomerates of up to 12 spheres are investigated. Results show that the mutual interaction of two spheres does not affect absorption when the ratio of their distance to the radius, c = d/a , is greater than 3. Contrary, an analogous criterion for independent scattering depends on the individual sphere size parameter, x s such that it is valid only if c ≥ 2/ x s . It is shown that an agglomerate formed of N individual spheres can be approximated by an effective sphere if x e ≤ 0.2. For x e ≈ 2, agglomerates scatter similar to N independent particles. These limits bracket the true radiative properties of the agglomerates for 0.2 ≤ x e ≤ 2, where dependent effects cannot be neglected. Additionally, an experimental methodology is suggested to qualitatively identify the process of agglomeration of a number of individual particles even when changes in the number density of particles are unknown.

Surface waves and atomic force microscope probe-particle near-field coupling: discrete dipole approximation with surface interaction

Evanescent waves on a surface form due to the collective motion of charges within the medium. They do not carry any energy away from the surface and decay exponentially as a function of the distance. However, if there is any object within the evanescent field, electromagnetic energy within the medium is tunneled away and either absorbed or scattered. In this case, the absorption is localized, and potentially it can be used for selective diagnosis or nanopatterning applications. On the other hand, scattering of evanescent waves can be employed for characterization of nanoscale structures and particles on the surface. In this paper we present a numerical methodology to study the physics of such absorption and scattering mechanisms. We developed a MATLAB implementation of discrete dipole approximation with surface interaction (DDA-SI) in combination with evanescent wave illumination to investigate the near-field coupling between particles on the surface and a probe. This method can be used to explore the effects of a number of physical, geometrical, and material properties for problems involving nanostructures on or in the proximity of a substrate under arbitrary illumination.