Near-field thermal radiation may play significant role in the enhancement of energy harvesting and radiative cooling by new types of designer materials, which in turn can be crucial in the development of future devices. In this work, we present a case study to explore near- to far-field thermal emission and radiative flux from a thin polar SiC film coated by different size and shape nanoparticles. The same geometry with nano-particles is also considered as a layered medium, which is analyzed using Effective Medium Theory (EMT). A significant enhancement of emission, particularly at the far infrared, is observed when nanoparticles are placed on the surface of a SiC film with certain periodicities, which shows potential use of these structures for radiative cooling applications. Yet, these enhancements are not observed when the EMT approach is adapted, which is questioned for its accuracy of predicting near-to-far field transition regime of radiation transfer from corrugated surfaces.
© 2015 Optical Society of America
With the ever increasing human population and with their insatiable thirst for energy, the world is facing a double challenge of climbing energy demand and a threat for energy security. Along with energy efficiency measures, the development of new ideas and devices for energy harvesting from all kinds of high temperature sources are more pressing than before. Both of these areas can benefit potential solutions from new and innovative ideas, particularly with the use of new materials, and different chemical and/or physical (geometric) configurations. Nanotechnology-based techniques have been investigated widely in the past decade with the intention to alter the development of metamaterials  and the next generation of solar cells . The possibility of enhancing near-field thermal radiation by orders of magnitude from the known blackbody radiation limit has also been reported repeatedly in literature [3–8]. This enhancement can be used for the development of the next generation of thermophotovoltaic cells for energy harvesting [1, 2]. Similar developments in new designer materials are likely to enhance emission from surfaces at far-infrared, and consequently allow the cooling of surfaces radiatively [9–13]. It is, therefore, important to explore and understand the fundamentals behind the radiative emission from such surfaces, whether they are thin films, sandwiched structures or coated with different shape and size nanostructures or nanoparticles (NPs). In order to explore the effect of nanostructures on emission characteristics of surfaces, reliable computational techniques are needed. Such computational tools would be crucial when working with arbitrary, sub-wavelength structures as analytical solutions for these structures in anisotropic, dispersive mediums might not be easily found.
In , we have presented that the results found for the near-field radiative transfer (NFRT) calculations via Finite Difference Time Domain (FDTD) method for perfectly flat, parallel, thin SiC films supporting surface phonon polaritons. These films were separated by a vacuum nano-gap, forming a sandwich-structure. The predictions by the FDTD approach showed a good agreement with the analytical results presented in , giving confidence in the robustness of the numerical model. We then extended the problem by introducing corrugated surfaces and studied the effects of shape, size and periodicity of nanostructures on the near-field thermal emission . The simulations suggested that the magnitude of LDOS (Local Density of States, which is related to the radiative flux through the absolute temperature multiplier) increases with an increase in the periodicity of the nano-gratings, when the distance between the gratings is much smaller than the wavelength corresponding to that is obtained from Wien’s law . In another recent study, in which we used a 2D geometry to evaluate the impact of arbitrary shape nano-gratings, the simulations indicated that the greatest impact on the enhancement of both LDOS and the radiative flux profiles were obtained with rectangular structures when compared against those obtained for elliptical or triangular NPs of the same sizes .
In this study, we present the effects of different shape and size NPs on near- to far-field emission, absorption and the spectral radiative flux extended to far- infrared region, corresponding from the wavelengths of 400 nm to 100 μm. We compare the results found for different sizes and shape SiC NPs placed on the surface of a thin SiC film. In addition, we show results obtained with the Effective Medium Theory (EMT), where the structures are assumed to form a thin layer over the original substrate as it will be discussed below, then we conclude that EMT’s accuracy in providing the effect of presence of nanoparticles on surfaces in calculations of emission and absorption is highly questionable.
In order to present the implementation of FDTD, we write the Maxwell equations in terms of the electrical displacement and magnetic fields,, and respectively. They are considered for Transverse Magnetic (TM) wave, in which the only non-zero component of magnetic field is, i.e. propagation along the - axis. They are written as following:14, 15].
Figure 1 shows the schematic of the computational domain of the geometry considered in this work. The configuration considered here is a single thin SiC film either perfectly flat or coated with SiC NPs on it, as shown in Fig. 2. The simulations were carried out to determine the profiles of LDOS and the radiative flux. The emitting film is assumed to have a temperature of 1000 K and a thickness of 100 nm. We first studied the LDOS profile 50 nm above the single SiC film, compared it with the values found for heights at 3, 20, 30and 100. We then considered different sized NPs of spherical, elliptical, rectangular and triangular shapes in perfect contact with the emitting film and compared the LDOS results against the initial benchmark results. Finally, we compared the latter results against those obtained using the EMT. Width and height of the NPs is shown in Fig. 2 with ‘w’ and ‘h’, respectively. The distance of NPs is shown with ‘d’ and NPs stands for nanoparticles from hereon.
3. Results and discussions
Our main objective is to identify the most important factors when tailoring the nanostructures in order to obtain enhancement or annihilation of NF thermal emission and radiative flux for different applications. For this purpose, we considered employing nanospheres of different sizes placed on the surface of emitting layer in the proposed configuration. For the first set of analyses, the NPs are assumed to have diameters of w = h = 350 nm. The scenario in which LDOS is calculated for a single thin layer SiC film and found at a distance ∆ = 100above the emitting layer is compared against the same scenario when nanospheres are sitting upon the surface of the film. These are shown in Fig. 3. We observed two orders of magnitude increase in the magnitude of LDOS after the NPs are placed on the surface. The magnitude of LDOS started to show much higher values at frequencies near the visible spectrum when compared against the single thin film case. A similar computational observation was previously reported  for near infrared spectrum. However, the effect of different sizes of NPs in a comparative study against the case of a single thin film as well as the EMT calculations over the infrared spectrum are reported the very first time.
Next, we compared the results we obtained in Fig. 3 against those obtained using EMT. Figure 4 depicts the comparison of the three scenarios in which LDOS profiles for a (a) single film, (b) a single film with added NPs to its surface and (c) LDOS calculated with EMT through volume fraction studies of NPs. We observe that solutions of conventional EMT which can be obtained through knowing the relative volume fraction and permittivity of the constituent media for NFRT problems dealing with corrugated surfaces may depend on a number of factors among which is the size of NPs and the observation point at which LDOS/heat flux profiles are obtained which effects the dimensions of the numerical problem under the study. Also, the method adopted for EMT calculations are among these factors. Our conclusions are based on the EMT studies that were performed through volume fraction studies of NPs where 2D-EMT was applied by calculating the total area (in cells) occupied by NPs. Then, we adjusted the height of emitting layer so that its own total area is increased by the area calculated for NPs; then the simulation is run without NPs. From the results, it might be concluded that the discrepancy between FDTD and EMT method predictions is due to the fact that the conventional EMT is incapable of describing systems where the change in dielectric permittivity from layer to layer is large. This usually happens when the medium is made of a metamaterial adjacent to any other medium with a much different permittivity. i.e. vacuum. In Fig. 5 we have magnified the results shown in Fig. 4 over the frequencies near visible range with the purpose of a clearer exploration of the results.
In addition, we considered an array of SiC rectangles with w = h = 200 nm on SiC emitting layer, at T = 1000 K. Results for coherent thermal emission and radiative flux at a distance ∆ = 30above the emitting layer were compared against the results based on the EMT approach, as depicted in Figs. 5 and 6. Figure 6 shows the results obtained for (a) the benchmark scenario, (b) LDOS profile in the presence of rectangular-shaped NPs, and (c) those obtained from EMT. The results are normalized to the peak of benchmark results. The radiative flux profiles from FDTD simulations were compared against those obtained from those using the EMT, as shown in Fig. 7. An enlarged version of these comparisons are given as an inset in Fig. 7. Note that near -field thermal flux could be evaluated based on LDOS calculations as explained in details in . We compared the predictions for thermal emission and the radiative flux obtained for corrugated surfaces against those for the flat surfaces, we observed a clear enhancement by several orders of magnitude at a number of spectral bands. This enhancement is due to coherent coupling between the resonant modes generated by surface phonon polaritons standing waves. We have also observed that the discrepancy between results found for LDOS and the radiative flux at the presence of nano-particles and those corresponding to the EMT calculations, for the cases studied here as well elsewhere . Therefore, we conclude that the use of the EMT approach for sub-wavelength nanostructures seems highly questionable over the infrared region, particularly near the visible region.
The results presented here showed that the presence of NPs over the surface of an emitting layer increases the magnitude of LDOS and the radiative flux by several orders of magnitude. A noticeable amount of discrepancy was also observed when the results obtained with EMT were compared against those for LDOS at the presence of the NPs. Note that the methodology presented here to determine NFRT between corrugated surfaces is much more fundamental than a simpler effective medium theory, which does not seem to be accurate at nano-resolutions. With the insight gained from these modeling efforts, we should be able to define more interesting functions for the corrugated geometries based on the near-field enhancements or decreases. While in this work we have focused on gratings of the same shape, future work would involve arbitrary combinations of NP shapes as well as studies of the surfaces where both emitting and non-emitting layers are corrugated.
This project was partially funded by TUBITAK-1001 (Grant No.109M170) and FP-7-PEOPLE-IRG-2008 (Grant No.239382 NF-RAD) at Özyegin University, Istanbul, Turkey. AD is funded by CEEE at Özyegin University.
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