Soumyadipta Basu

Near-field radiative transfer based thermal rectification using doped silicon

In this letter, we have designed a near-field thermal rectifier using a film and a bulk of doped silicon, with different doping levels, separated by a vacuum gap. We examine the origin of nonlinearities in thermal rectification associated with near-field heat transfer, and investigate closely the effects of varying the vacuum gap and the film thickness on rectification. For a 10 nm thick film, rectification greater than 0.5 is achieved for vacuum gaps varying from 1 nm to 50 nm with the hot and cold temperatures of the terminals of the rectifier being 400 K and 300 K, respectively. The results obtained from this study may benefit future research in thermal management and energy harvesting.

Infrared Radiative Properties of Heavily Doped Silicon at Room Temperature

This paper describes an experimental investigation on the infrared radiative properties of heavily doped Si at room temperature. Lightly doped Si wafers were ion-implanted with either boron or phosphorus atoms, with dosages corresponding to as-implanted peak doping concentrations of 10 20 and 10 21 cm -3 ; the peak doping concentrations after annealing are 3.1 × 10 19 and 2.8 × 10 20 cm -3 , respectively. Rapid thermal annealing was performed to activate the implanted dopants. A Fourier-transform infrared spectrometer was employed to measure the transmittance and reflectance of the samples in the wavelength range from 2 μm to 20 μm. Accurate carrier mobility and ionization models were identified after carefully reviewing the available literature, and then incorporated into the Drude model to predict the dielectric function of doped Si. The radiative properties of doped Si samples were calculated by treating the doped region as multilayer thin films of different doping concentrations on a thick lightly doped Si substrate. The measured spectral transmittance and reflectance agree well with the model predictions. The knowledge gained from this study will aid future design and fabrication of doped Si microstructures as wavelength selective emitters and absorbers in the midinfrared region.

Radiation-based near-field thermal rectification with phase transition materials

The capability of manipulating heat flow has promising applications in thermal management and thermal circuits. In this Letter, we report strong thermal rectification effect based on the near-field thermal radiation between silicon dioxide (SiO2) and a phase transition material, vanadium dioxide (VO2), separated by nanometer vacuum gaps under the framework of fluctuational electrodynamics. Strong coupling of surface phonon polaritons between SiO2 and insulating VO2 leads to enhanced near-field radiative transfer, which on the other hand is suppressed when VO2 becomes metallic, resulting in thermal rectification. The rectification factor is close to 1 when vacuum gap is at 1 μm and it increases to almost 2 at sub-20-nm gaps when emitter and receiver temperatures are set to 400 and 300 K, respectively. Replacing bulk SiO2 with a thin film of several nanometers, rectification factor of 3 can be achieved when the vacuum gap is around 100 nm.

Near-Field Radiation Calculated With an Improved Dielectric Function Model for Doped Silicon

This paper describes a theoretical investigation of near-field radiative heat transfer between doped silicon surfaces separated by a vacuum gap. An improved dielectric function model for heavily doped silicon is employed. The effects of doping level, polarization, and vacuum gap width on the spectral and total radiative transfer are studied based on the fluctuational electrodynamics. It is observed that increasing the doping concentration does not necessarily enhance the energy transfer in the near-field. The energy streamline method is used to model the lateral shift of the energy pathway, which is the trace of the Poynting vectors in the vacuum gap. The local density of states near the emitter is calculated with and without the receiver. The results from this study can help improve the understanding of near-field radiation for applications such as thermophotovoltaic energy conversion, nanoscale thermal imaging, and nanothermal manufacturing.

Electric and magnetic surface polariton mediated near-field radiative heat transfer between metamaterials made of silicon carbide particles

Near-field radiative heat transfer between isotropic, dielectric-based metamaterials is analyzed. A potassium bromide host medium comprised of silicon carbide (SiC) spheres with a volume filling fraction of 0.4 is considered for the metamaterial. The relative electric permittivity and relative magnetic permeability of the metamaterial are modeled via the Clausius-Mossotti relations linking the macroscopic response of the medium with the polarizabilities of the spheres. We show for the first time that electric and magnetic surface polariton (SP) mediated near-field radiative heat transfer occurs between dielectric-based structures. Magnetic SPs, existing in TE polarization, are physically due to strong magnetic dipole resonances of the spheres. We find that spherical inclusions with radii of 1 μm (or greater) are needed in order to induce SPs in TE polarization. On the other hand, electric SPs existing in TM polarization are generated by surface modes of the spheres, and are thus almost insensitive to the size of the inclusions. We estimate that the total heat flux around SP resonance for the metamaterial comprised of SiC spheres with radii of 1 μm is about 35% greater than the flux predicted between two bulks of SiC, where only surface phonon-polaritons in TM polarization are excited. The results presented in this work show that the near-field thermal spectrum can be engineered via dielectric-based metamaterials, which is crucial in many emerging technologies, such as in nanoscale-gap thermophotovoltaic power generation.

Near-field radiative heat transfer between doped silicon nanowire arrays

In this letter, we investigate the near-field radiative heat transfer between two doped silicon nanowire arrays separated by a vacuum gap. Using effective medium theory and fluctuational electrodynamics, the radiative heat transfer is calculated for the silicon nanowire arrays with different filling fractions at different vacuum gaps. The energy transfer increases as the nanowire array becomes less dense due to enhancement in channels available for heat transfer. To further understand the impact of filling fraction to the total heat transfer, the dispersion relation of coupled surface plasmon polaritons is calculated inside the vacuum gap by considering temperature-dependent dielectric functions for the doped silicon nanowires. When the filling fraction is 0.5, the radiative heat transfer at a vacuum gap of 20 nm between the nanowire arrays is almost three times of that between two doped silicon plates. Results from this study will facilitate the application of doped silicon nanowires for energy harvestin...

Direct Measurement of Thermal Emission From a Fabry–Perot Cavity Resonator

There have been growing interests in selective control of thermal emission by using micro/nanostructures. The present study describes direct measurements of infrared thermal emission at elevated temperatures of an asymmetric Fabry–Perot resonator at variable angles for each polarization. The multilayered structure mainly contains a SiO2 optical cavity sandwiched between a thick (opaque) Au film and a thin Au film. Metallic adhesive and diffusion-barrier layers were deposited on a Si substrate before depositing the thick Au film. A dielectric protection layer was deposited atop the thin Au film to prevent oxidation at high temperatures. A SiC wafer was used as the reference to test the emittance measurement facility, which includes a heated sample holder, a blackbody source, mirror assembly, a polarizer, and a Fourier-transform infrared spectrometer with different detectors. The measured emittance spectra of the Fabry–Perot structure exhibit peak broadening and shifting as temperature increases; the mechanisms are elucidated by comparison with theoretical modeling. [DOI: 10.1115/1.4006088]

Direct and indirect methods for calculating thermal emission from layered structures with nonuniform temperatures

The determination of emissivity of layered structures is critical in many applications, such as radiation thermometry, microelectronics, radiative cooling, and energy harvesting. Two different approaches, i.e., the “indirect” and “direct” methods, are commonly used for computing the emissivity of an object. For an opaque surface at a uniform temperature, the indirect method involves calculating the spectral directional-hemispherical reflectance to deduce the spectral directional emissivity based on Kirchhoff’s law. On the other hand, a few studies have used a combination of Maxwell’s equations with the fluctuation-dissipation theorem to directly calculate the emissivity. The present study aims at unifying the direct and indirect methods for calculating the far-field thermal emission from layered structures with a nonuniform temperature distribution. Formulations for both methods are given to illustrate the equivalence between the indirect and the direct methods. Thermal emission from an asymmetric Fabry–Perot resonance cavity with a nonuniform temperature distribution is taken as an example to show how to predict the intensity, emissivity, and the brightness temperature. The local density of states, however, can only be calculated using the direct method.

Maximum near-field radiative heat transfer between thin films

In this letter, we investigate the maximum near-field radiative heat transfer achievable between two thin films. By using frequency-independent permittivities for the films, we obtain optimum values of the real (eopt′) and imaginary (eopt″) parts of the dielectric function maximizing the heat transfer for different thicknesses. We show that when the ratio D of the film thicknesses tf to the vacuum gap d is equal or less than 0.1, the maximum heat flux becomes independent of D. Based on the analysis performed in this study, it is possible to suitably choose film thicknesses maximizing near-field heat transfer at different vacuum gaps. The results obtained in this work also allow the interpretation of the physical details underlying near-field thermal radiation between films.

Penetration depth in near-field radiative heat transfer between metamaterials

In this letter, we investigate the penetration depth in near-field radiative heat transfer between metamaterials when surface polaritons are excited at both electrical and magnetic resonances. The analyses show that based on the optical properties of the metamaterial, two different penetration depths can be defined corresponding to electrical and magnetic resonances. Depending upon the scattering rate of the metamaterial, it is possible to selectively enhance or reduce the penetration depth of near-field thermal radiation at electric and magnetic resonances. The results obtained from this study will benefit applications of metamaterials in near-field energy harvesting.