ible and near-infrared (IR) spectral region is critically important. On the other hand, in the mid-IR spectral region, surface emittance should be lowered in order to minimize the radiation loss in the form of thermal emission for high-temperature applications, such as concentrated solar tower. Therefore, wavelength-selective coatings are often applied to spectrally tune the optical property of the collector surface. [ 2 ] Recently, optical metamaterials have drawn much attention due to potential applications in thermophotovoltaic energy conversion, [ 3 ] solar cell, [ 4,5 ] solar thermal application, [ 6 ] and plasmonic sensors. [ 7 ] Regarding solar thermal applications, achieving perfect absorption in the visible and near-IR spectral region would be of great interest. For example, split-ring resonators, [ 8,9 ] 1D grating structures, [ 10,11 ] and cross-shape grating structures [ 12 ] were proposed to achieve perfect absorption. However, these studies showed only narrowband absorption; thus, their applicability as a solar thermal absorber is hardly possible. In order to achieve broadband absorption, various types of structures were proposed, such as photonic crystals, [ 13,14 ] nanowire and nanohole arrays, [ 15 ] and nanowire forests. [ 16 ] Alternatively, geometric combination of gratings can also attain a broadband absorption. For example, Yang et al. [ 17 ] investigated the resistor mounted crosswire and split-ring-resonator structure for the broadband and polarization-insensitive absorber in gigahertz frequency regime. In the visible and near-IR spectral region, Wang and Wang [ 18 ] showed that the metamaterial structure composed of two different-sized grating elements within one period on top of SiO 2 spacer and tungsten substrate can exhibit broadband absorption. In addition, saw-tooth [ 19,20 ] and pyramidal [ 21,22 ] shaped structures also demonstrated ultrabroadband absorption. Although several other techniques, such as a microencapsulation of phase change material, [ 23 ] a quasi-periodic nanocone, [ 24 ] a fi lm coupled nanoparticle, [ 25 ] and graphene nanodisc, [ 26 ] have been proposed for achieving broadband absorption, most of the structures are too complicated to be fabricated for real-world applications. In order to overcome the fabrication diffi culty, a relatively simpler type of tandem grating structure was proposed for demonstrating broadband absorption in the visible spectrum by exciting coupled surface plasmon and magnetic resonance; [ 27 ] however, experimental demonstration has not been made yet. It should be also noted that the temperature-dependent radiative properties In this work, a tandem grating solar absorber is proposed, which can be easily fabricated on a wafer scale and is thermally stable up to 800 K. The base of the solar thermal absorber consists of a tungsten substrate, SiO 2 spacer, and 2D tungsten nanohole array fi lled with SiO 2 . On top of the base structure, a 2D tungsten nanodisc array is coated with an additional SiO 2 spacer, forming the tandem grating structur...

In this work, we numerically investigate the electromagnetic resonances on two-dimensional tandem grating structures. The base of a tandem grating consists of an opaque Au substrate, a SiO(2) spacer, and a Au grating (concave type); that is, a well-known fishnet structure forming Au/SiO(2)/Au stack. A convex-type Au grating (i.e., topmost grating) is then attached on top of the base fishnet structure with or without additional SiO(2) spacer, resulting in two types of tandem grating structures. In order to calculate the spectral reflectance and local magnetic field distribution, the finite-difference time-domain method is employed. When the topmost Au grating is directly added onto the base fishnet structure, the surface plasmon and magnetic polariton in the base structure are branched out due to the geometric asymmetry with respect to the SiO(2) spacer. If additional SiO(2) spacer is added between the topmost Au grating and the base fishnet structure, new magnetic resonance modes appear due to coupling between two vertically aligned Au/SiO(2)/Au stacks. With the understanding of multiple electromagnetic resonance modes on the proposed tandem grating structures, we successfully design a broadband absorber made of Au and SiO(2) in the visible spectrum.

The direct utilization of solar radiation has been considered a promising energy source because of its abundance, sustainability, and cleanness. The conversion of solar radiation into usable heat largely depends on the absorption characteristics of a solar thermal collector. In the present study, we conducted design analysis of a wavelength-selective absorber composed of a two-dimensional Nickel grating, a thin SÍO2 film, and a Nickel substrate. Dimensions of the two-dimensional grating were determined with the Taguchi method, which optimized the spectral absorptance for both polarizations. The spectral absorptance demonstrated a broad-band plateau within the visible and the near-infrared spectral region, but it was significantly suppressed at longer wavelengths. Moreover, the absorptance plateau was nearly insensitive to the incident orientation of solar radiation. Physical mechanisms of the absorption enhancement were elucidated with the local magnetic field distribution.Eigure I(í3) depicts the baseline structure of proposed solar thermal absorber. The scheme only shows a unit cell, which is one period of a 2D Ni grating on a SÍO2 film and a Ni substrate. Journal of Heat Transfer

We report the development of scanning thermoreflectance thermometry and its application for steady and dynamic temperature measurement of a heated microcantilever. The local thermoreflectance signal of the heated microcantilever was calibrated to temperature while the cantilever was under steady and periodic heating operation. The temperature resolution of our approach is 0.6 K, and the spatial resolution is 2 μm, which are comparable to micro-Raman thermometry. However, the temporal resolution of our approach is about 10 μsec, which is significantly faster than micro-Raman thermometry. When the heated microcantilever is periodically heated with frequency up to 100 kHz, we can measure both the in-phase and out-of-phase components of the temperature oscillation. For increasing heating frequency, the measured cantilever AC temperature distribution tends to be confined in the vicinity of the heater region and becomes increasingly out of phase with the driving signal. These results compare well with finite element simulations.

No abstract