The present paper theoretically demonstrates coherent thermal emission in the infrared region by exciting magnetic polaritons between metallic gratings and an opaque metallic film, separated by a dielectric spacer. The coupling of the metallic strips and the film induces a magnetic response that is characterized by a negative permeability and positive permittivity. On the other hand, the metallic film intrinsically exhibits a negative permittivity and positive permeability in the near infrared. This artificial structure is equivalent to a pair of single-negative materials. By exciting surface magnetic polaritons, large emissivity peaks can be achieved at the resonance frequencies and are almost independent of the emission angle. The resonance frequency of the magnetic response can be predicted by an analogy to an inductor and capacitor circuit. The proposed structure can be easily constructed using micro/nanofabrication.

Passive radiative cooling functions by reflecting the solar spectrum and emitting infrared waves in broadband or selectively. However, cooling enclosed spaces that trap heat by greenhouse effect remains a challenge. We present a Janus emitter (JET) consisting of an Ag–polydimethylsiloxane layer on micropatterned quartz substrate. The induced spoof surface plasmon polariton helps overcome inherent emissivity loss of the polymer and creates near-ideal selective and broadband emission on the separate sides. This design results in not only remarkable surface cooling when the JET is attached with either side facing outwards but also space cooling when used as an enclosure wall. Thus, the JET can passively mitigate the greenhouse effect in enclosures while offering surface cooling performance comparable to conventional radiative coolers.

The present study reports a novel concept of a direct solar thermal collector that harnesses the localized surface plasmon of metallic nanoparticles suspended in water. At the plasmon resonance frequency, the absorption and scattering from the nanoparticle can be greatly enhanced via the coupling of the incident radiation with the collective motion of electrons in metal. However, the surface plasmon induces strong absorption with a sharp peak due to its resonant nature, which is not desirable for broad-band solar absorption. In order to achieve the broad-band absorption, we propose a direct solar thermal collector that has four types of gold-nanoshell particles blended in the aquatic solution. Numerical simulations based on the Monte Carlo algorithm and finite element analysis have shown that the use of blended plasmonic nanofluids can significantly enhance the solar collector efficiency with an extremely low particle concentration (e.g., approximately 70% for a 0.05% particle volume fraction). The low particle concentration ensures that nanoparticles do not significantly alter the flow characteristics of nanofluids inside the solar collector. The results obtained from this study will facilitate the development of highly efficient solar thermal collectors using plasmonic nanofluids.

Coherent thermal emission from surface relief gratings holds promise for spectral and directional control of thermal radiation but is limited to transverse magnetic waves, which can excite surface plasmon or phonon polaritons in the grating structure. We show in this letter that a coherent thermal source can be constructed with a thin polar material coated on a one-dimensional photonic crystal. The excitation of surface waves at the interface of the coated layer and the photonic crystal results in highly spectral and directional emission in the infrared for both the transverse electric wave and the transverse magnetic wave.