Enhancement Of Thermal Emission Research Articles

Quantum nonlinear mixing of thermal photons to surpass the blackbody limit.

Nearly all thermal radiation phenomena involving materials with linear response can be accurately described via semi-classical theories of light. Here, we go beyond these traditional paradigms to study a nonlinear system that, as we show, requires quantum theory of damping. Specifically, we analyze thermal radiation from a resonant system containing a χ(2) nonlinear medium and supporting resonances at frequencies ω1 and ω2 ≈ 2ω1, where both resonators are driven only by intrinsic thermal fluctuations. Within our quantum formalism, we reveal new possibilities for shaping the thermal radiation. We show that the resonantly enhanced nonlinear interaction allows frequency-selective enhancement of thermal emission through upconversion, surpassing the well-known blackbody limits associated with linear media. Surprisingly, we also find that the emitted thermal light exhibits non-trivial statistics (g(2)(0) ≠ ~2) and biphoton intensity correlations (at two distinct frequencies). We highlight that these features can be observed in the near future by heating a properly designed nonlinear system, without the need for any external signal. Our work motivates new interdisciplinary inquiries combining the fields of nonlinear photonics, quantum optics and thermal science.

Влияние усиленной электрическим полем термической электронной эмиссии на температуру катода с тонкой диэлектрической пленкой в дуговом газовом разряде

A model of thermal electron emission enhanced by the electric field (thermo-field emission) from the metal cathode substrate into a thin insulating film formed on its surface is developed. A system of equations for the cathode surface temperature in the arc discharge and the electric field strength in the film, providing the required discharge current density, is formulated. It is shown that existence of the dielectric film can result in a considerable reduction of the cathode temperature in the discharge due to lower potential barrier height at the metal-insulator boundary than at the metal-discharge boundary in case of the metal cathode without the film. It is found that due to an enhancement of thermal emission of electrons into the film by the electric field generated in it, an additional decrease of the cathode temperature by about 100 K takes place.

Phonon-polaritonics: enabling powerful capabilities for infrared photonics

AbstractHere, we review the progress and most recent advances in phonon-polaritonics, an emerging and growing field that has brought about a range of powerful possibilities for mid- to far-infrared (IR) light. These extraordinary capabilities are enabled by the resonant coupling between the impinging light and the vibrations of the material lattice, known as phonon-polaritons (PhPs). These PhPs yield a characteristic optical response in certain materials, occurring within an IR spectral window known as the reststrahlen band. In particular, these materials transition in the reststrahlen band from a high-refractive-index behavior, to a near-perfect metal behavior, to a plasmonic behavior – typical of metals at optical frequencies. When anisotropic they may also possess unconventional photonic constitutive properties thought of as possible only with metamaterials. The recent surge in two-dimensional (2D) material research has also enabled PhP responses with atomically-thin materials. Such vast and extraordinary photonic responses can be utilized for a plethora of unusual effects for IR light. Examples include sub-diffraction surface wave guiding, artificial magnetism, exotic photonic dispersions, thermal emission enhancement, perfect absorption and enhanced near-field heat transfer. Finally, we discuss the tremendous potential impact of these IR functionalities for the advancement of IR sources and sensors, as well as for thermal management and THz-diagnostic imaging.

Broadband enhancement of thermal radiation.

Broadband thermal radiation sources are critical for various applications including spectroscopy and electricity generation. However, due to the difficulty in simultaneously achieving high absorptivity and low thermal mass these sources are inefficient. We show a platform that enables one to obtain enhanced emission by coupling a thermal emitter to an optical cavity. We experimentally demonstrate broadband enhancement of thermal emission between λ ~2 ̶ 4.2 μm using an inherently poor thermal emitter consisting of tens of nanometers thick SiC film with 10% emissivity (εSiC ~0.1). We measure over twofold enhancement of total emission power over the entire spectral band and threefold enhancement of thermal emission over 3 to 3.4 μm. Our platform has the potential to enable development of ideal blackbody sources operating at substantially lower heating powers.

Wavelength-Selective Three-Dimensional Thermal Emitters via Imprint Lithography and Conformal Metallization.

Metallic photonic crystals (MPCs) exhibit wavelength-selective thermal emission enhancements and are promising thermal optical devices for various applications. Here, we report a scalable fabrication strategy for MPCs suitable for high-temperature applications. Well-defined double-layer titanium dioxide (TiO2) woodpile structures are fabricated using a layer-by-layer soft-imprint method with TiO2 nanoparticle ink dispersions, and the structures are subsequently coated with high purity, conformal gold films via reactive deposition from supercritical carbon dioxide. The resulting gold-coated woodpile structures are effective MPCs and exhibit emissivity enhancements at a selective wavelength. Gold coatings deposited using a cold-wall reactor are found to be smoother and result in a greater thermal emission enhancement compared to those deposited using a hot-wall reactor.

Plasmonic Thermal Emitters for Dynamically Tunable Infrared Radiation

Periodic bimetallic microstructures using nickel and gold are fabricated on an elastomeric substrate by use of strain‐induced buckling of the metallic layers, which can be compatible with roll‐to‐roll manufacturing. The intrinsically low emissivity of gold in the midinfrared regime is selectively enhanced by the surface plasmonic resonance at three different midinfrared wavelengths, 4.5, 6.3, and 9.4 µm, respectively, which directly correspond to the structural periodicities of the metallic microstructures. As the thermal emission enhancement effect exists only for the polarization perpendicular to the orientation of the microstructures, substantially polarized thermal emission with an extinction ratio close to 3 is demonstrated. Moreover, the elastically deformed plasmonic thermal emitters demonstrate strain‐dependent emission peaks, which can be applied for future mechano–thermal sensing and dynamic thermal signature modulation.

Thermal graphene metamaterials and epsilon-near-zero high temperature plasmonics

The key feature of a thermophotovoltaic (TPV) emitter is the enhancement of thermal emission corresponding to energies just above the bandgap of the absorbing photovoltaic cell and simultaneous suppression of thermal emission below the bandgap. We show here that a single layer plasmonic coating can perform this task with high efficiency. Our key design principle involves tuning the epsilon-near-zero frequency (plasma frequency) of the metal acting as a thermal emitter to the electronic bandgap of the semiconducting cell. This approach utilizes the change in the reflectivity of a metal near its plasma frequency (epsilon-near-zero frequency) to lead to spectrally selective thermal emission, and can be adapted to large area coatings using high temperature plasmonic materials. We provide a detailed analysis of the spectral and angular performance of high temperature plasmonic coatings as TPV emitters. We show the potential of such high temperature plasmonic thermal emitter coatings (p-TECs) for narrowband near-field thermal emission. We also show the enhancement of near-surface energy density in graphene-multilayer thermal metamaterials due to a topological transition at an effective epsilon-near-zero frequency. This opens up spectrally selective thermal emission from graphene multilayers in the infrared frequency regime. Our design paves the way for the development of single layer p-TECs and graphene multilayers for spectrally selective radiative heat transfer applications.

Magnetic hyperbolic optical metamaterials

Strongly anisotropic media where the principal components of electric permittivity or magnetic permeability tensors have opposite signs are termed as hyperbolic media. Such media support propagating electromagnetic waves with extremely large wave vectors exhibiting unique optical properties. However, in all artificial and natural optical materials studied to date, the hyperbolic dispersion originates solely from the electric response. This restricts material functionality to one polarization of light and inhibits free-space impedance matching. Such restrictions can be overcome in media having components of opposite signs for both electric and magnetic tensors. Here we present the experimental demonstration of the magnetic hyperbolic dispersion in three-dimensional metamaterials. We measure metamaterial isofrequency contours and reveal the topological phase transition between the elliptic and hyperbolic dispersion. In the hyperbolic regime, we demonstrate the strong enhancement of thermal emission, which becomes directional, coherent and polarized. Our findings show the possibilities for realizing efficient impedance-matched hyperbolic media for unpolarized light.

Radiative cooling by light down conversion of InGaN light emitting diode bonded to a Si wafer

Using the recently proposed process of radiative cooling by light down conversion, we demonstrate cooling of about 5 K for InGaN light emitting diode (39 mg thermal load) that is self-heated up to 450 K and bonded to a cooler, a 15 × 15 × 4 mm3 Si wafer pumped with an above bandgap excitation from a 1.09-μm diode laser. Cooling occurs due to the enhancement of thermal emission in an initially transparent Si wafer when the overall energy of multiple (about 20) below bandgap photons escaping the wafer exceeds the energy of the single pumped photon. The cooling efficiency amounts to 93%.

Emulating one-dimensional resonantQ-matching behavior in a two-dimensional system via Fano resonances

Through detailed numerical and analytical studies, we establish that the significant enhancement of thermal emission via $Q$ matching, which has been possible in one-dimensional (1D) systems only, can be extended to two-dimensional (2D) systems by means of Fano resonances in the 2D system. In particular, we show the existence of essentially 1D behavior in a 2D system---a case of reduced dimensionality. Moreover, we show how properties of these spectra can be controlled by changing the geometrical parameters of the 2D system.

ChandraACIS Spectroscopy of N157B: A Young Composite Supernova Remnant in a Superbubble

We present a Chandra ACIS observations of N157B, a young supernova remnant (SNR) located in the 30 Doradus star formation region of the Large Magellanic Cloud. This remnant contains the most energetic pulsar known (PSR J053747.39-691020.2; Ė = 4.8 × 1038 ergs s-1), which is surrounded by a X-ray-bright nonthermal nebula that likely represents a toroidal pulsar wind terminal shock observed edge-on. Two of the eight pointlike X-ray sources detected in the observation are shown to have near-IR and optical counterparts (within 05 offsets), which are identified as massive stellar systems in the Cloud. We confirm the nonthermal nature of the comet-shaped X-ray emission feature and show that the spectral steepening of this feature away from the pulsar is quantitatively consistent with synchrotron cooling of shocked pulsar wind particles flowing downstream at a bulk velocity close to the speed of light. Around the cometary nebula we unambiguously detect a spatially resolved thermal component, which accounts for about 1/3 of the total 0.5-10 keV flux from the remnant. This thermal component is distributed among various clumps of metal-enriched plasma embedded in the low surface brightness X-ray-emitting diffuse gas. The relative metal enrichment pattern suggests that the mass of the supernova progenitor is 20 M☉. A comparison of the X-ray data with Hubble Space Telescope optical images now suggests that the explosion site is close to a dense cloud, against which a reflection shock is launched. The interaction between the reflected material and the nebula has likely produced both its cometary shape and the surrounding thermal emission enhancement. SNR N157B is apparently expanding into the hot low-density interior of the surrounding superbubble formed by the young OB association LH 99, as revealed by Spitzer mid-infrared images. This scenario naturally explains the exceptionally large sizes of both the thermal and nonthermal components, as well as the lack of an outer shell of the SNR. However, the real situation in the region is likely to be more complicated. We find that a partially round soft X-ray-emitting clump with distinct spectral properties may result from a separate oxygen-rich remnant. These results provide a rare glimpse into the SNR structure and evolution in a region of recent star formation.

Coherent Thermal Emission From Modified Periodic Multilayer Structures

Enhancement of thermal emission and control of its direction are important for applications in optoelectronics and energy conversion. A number of structures have been proposed as coherent emission sources, which exhibit a large emissivity peak within a narrow wavelength band and at a well-defined direction. A commonly used structure is the grating, in which the excited surface polaritons or surface waves are coupled with propagating waves in air, resulting in coherent emission for p polarization only. One-dimensional photonic crystals can also support surface waves and may be modified to construct coherent emission sources. The present study investigates coherent emission from a multilayer structure consisting of a SiC film coated atop a dielectric photonic crystal (PC). By exciting surface waves at the interface between SiC and the PC, coherent emission is predicted for both p and s polarizations. In addition to the excitation of surface waves, the emission from the proposed multilayer structure can be greatly enhanced by the cavity resonance mode and the Brewster mode.

Thermophotovoltaic generation with selective radiators based on tungsten surface gratings

Two-dimensional surface-relief gratings with a period of 1.0–0.2μm composed of rectangular microcavities were fabricated on single crystalline W substrates to develop spectrally selective radiators for thermophotovoltaic generation. The radiators displayed strong emission in the near-infrared region where narrow-band-gap photovoltaic cells could convert photons into electricity. The enhancement of thermal emission was attributed to the microcavity effect. Power generation tests were carried out and the W gratings showed more than two times higher generation efficiency, compared to a SiC radiator. The results showed that the microstructured W radiators behave as good selective radiator, with both high efficiency and high power density.

Enhancement of thermal emission from porous radiant burners

A theoretical study on using coatings to enhance the emittance of porous radiant burners is presented. Burners made of coated silica fibers are considered along with coating materials of silicon carbide, graphite, and platinum. The radiative properties of the fibers with and without coatings are computed using a radiative scattering theory and are used in a P-3 solution of the radiative transfer equation to predict the emittance of the burners. Graphite and platinum coatings are shown to enhance the emittance by a factor of three for a burner temperature of 1000 K, and by a factor of six for a temperature of 1500 K. It is also shown that it is possible to protect the graphite and platinum coatings from oxidation by applying a silica coating over them.