Basil T. Wong

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.

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.

Nano-Scale Machining Via Electron Beam and Laser Processing

It is recently conceptualized that nano-scale machining might be achieved by coupling electron emission with radiation transfer. A laser may be used to heat a workpiece to a threshold temperature, and a nano-probe might then transfer additional energy via electron emission to remove a minute amount of material To investigate this hypothesis, a detailed numerical study is presented. The electron-beam transport is modeled using a Monte Carlo approach, and a radiation transfer model that includes Fresnel reflections is adapted to simulate laser heating. The numerical study suggests that approximately 0.5 W from a single electron-beam is sufficient to initialise local evaporation from a gold film. With the use of a laser, the required power can be halved if the film is sufficiently thin. This paper describes the details of the numerical study and establishes guidelines for such naon-scale machining processes.

Polarized radiative transfer in a particle-laden semi-transparent medium via a vector Monte Carlo method

Abstract A vector Monte Carlo method (VMCM) is developed to model the transfer of polarized radiation in optically thick, multiple scattering, particle-laden semi-transparent media. A comprehensive description of the theoretical background of the general VMC algorithm is introduced. The model is validated against reference results in the case of a plane–parallel geometry and applied to the simulation of a nephelometric experiment to derive the effective Mueller matrix of the complete system. Details and various features of the numerical model are discussed.

Depolarization of radiation by non-absorbing foams

A Monte Carlo/Ray-Tracing technique is developed to investigate the depolarization of radiation by foams simulated as layers of air-bubble-laden substrates. Angular and radial profiles of reflection and transmittance are predicted for one-dimensional media subjected to a collimated, polarized light beam. The effects of different bubble sizes, separation distance distributions between bubbles, and medium thickness are considered. Fresnel reflections at the boundaries of bubbles are accounted for using a ray-tracing approach. Calculations are performed to determine vertical and horizontal polarization components of both radial and angular profiles of reflection and transmission. It is shown that if the polarized reflection and transmission data can be obtained from carefully conducted experiments, they can be effectively used to diagnose the changes in the structure of foams.

Solution of the Boltzmann Transport Equation for Phonon Transport via the Speed-Up Transient Monte Carlo Method Using Reference Temperature

The Boltzmann transport equation can be solved by a statistical approach via the Monte Carlo method. An alternative model is proposed in this study by introducing a reference temperature, in which the simulation accounts only for phonon ensembles above the reference temperature over the whole frequency spectrum. The current study delves into several computational parameters (scaling factor, number of discretizations for integration, phonon frequency after scattering, and reference temperature) involved in the simulation and the effects of different choices of each of these parameters on the simulation results. In general, it is proposed that a scaling factor and discretization of 1,000 and a reference temperature close to the initial temperature be used in the simulations to ensure efficiency without compromising the accuracy of the results. The scattering parameters for silicon, germanium, and gallium arsenide used in the current study are also compared to those in the literature.

Thermal transport for applications in micro/nanomachining

Transport Equations.- Modeling of Transport Equations via MC Methods.- Modeling of e-Beam Transport.- Thermal Conduction Coupled with e-Beam Transport.- Two-Temperature Model Coupled with e-Beam Transport.- Thermal Conduction with Electron Flow/Ballistic Behavior.- Parallel Computations for Two-Temperature Model.- Molecular Dynamics Simulations.- Concluding Remarks.

Electronic Thermal Conduction in Thin Gold Films

In this work, electronic thermal conduction in thin gold film is modeled via the Boltzmann Transport Equation (BTE). The BTE is solved using a Monte Carlo Method (MCM). Temperature profiles for various film thicknesses are computed. Results show that the electronic thermal transport in gold is still diffusion-like at film thicknesses as small as 100 nm, implying that the Fourier law of conduction can be applied at this scale to predict the steady-state thermal heat transfer without comprising the physics. However, the Fourier law does not predict the temperature profiles accurately if the film thickness is reduced to 10 nm or below.Copyright

Field emission and electron deposition profiles as a function of carbon nanotube tip geometries

We present an analysis to explore the electron distribution within a workpiece subjected to field emission from the tip of a carbon nanotube. By calculating the field emission current density at sites on the periphery of the tip and by mapping this current density towards the surface using the trajectories followed by the electrons, we are able to determine the shape of the electron beam profile on the surface. Once this profile is obtained we can solve electron-beam transport equation by means of Monte Carlo simulation to determine the electron distribution inside the workpiece. We repeat these simulations for various applied voltages, gap distances, and for different tip shapes in order to understand the effects that these parameters may have on the distribution of the deposited electrons. These distributions are needed to investigate the field emission based nanomachining process.

Thermal energy conversion using near-field thermophotovoltaic device composed of a thin-film tungsten radiator and a thin-film silicon cell

In this paper, we proposed a novel nano-gap thermophotovoltaic (TPV) device made up of thin-films including the radiator. The optical, electrical, and thermal responses and performance of the device were assessed using coupled opto-electro-thermal numerical simulation. The device design consists of a thin-film tungsten radiator which is paired with a thin-film silicon TPV cell across a nanometric vacuum gap. Results were simulated based on experimental properties available in the current literature database. It is discovered that the maximum electrical power output of the thin-film nano-gap TPV device increases with cell temperature up to a certain threshold value due to improvements in generated photocurrent. Thin-film tungsten as a radiator is shown to improve radiative heat transfer above the bandgap compared to conventional bulk tungsten. The effect of cell thickness on responses and performance was also analysed. A 1-μm cell produces better performance over thinner thicknesses at the cost of greater cooling requirements. However, the improvements in output power offset the cooling costs, allowing for consistently favourable efficiencies. Finally, it is shown that the temperature profile in silicon thin-films under convective cooling can be approximated as uniform, simplifying the heat transport modelling process.In this paper, we proposed a novel nano-gap thermophotovoltaic (TPV) device made up of thin-films including the radiator. The optical, electrical, and thermal responses and performance of the device were assessed using coupled opto-electro-thermal numerical simulation. The device design consists of a thin-film tungsten radiator which is paired with a thin-film silicon TPV cell across a nanometric vacuum gap. Results were simulated based on experimental properties available in the current literature database. It is discovered that the maximum electrical power output of the thin-film nano-gap TPV device increases with cell temperature up to a certain threshold value due to improvements in generated photocurrent. Thin-film tungsten as a radiator is shown to improve radiative heat transfer above the bandgap compared to conventional bulk tungsten. The effect of cell thickness on responses and performance was also analysed. A 1-μm cell produces better performance over thinner thicknesses at the cost of greater ...