Blackbody Emission Research Articles

Evaluation of a ZrO2/ZrO2-aerogel one-dimensional photonic crystal as an optical filter for thermophotovoltaic applications

This paper computationally explores the optical response of various designs of one-dimensional photonic crystal optical filters comprised of alternating layers of dense zirconia and zirconia aerogel. COMSOL Multiphysics software is used to assess the optical characteristics of the filters. The performance of the filters within a thermophotovoltaic (TPV) system with a blackbody emitter and GaSb PV cell is investigated using two different methods including an ideal case wherein the emitter and PV cell are infinite parallel planes and the Monte Carlo ray-tracing method (MCM) with finite areas of its components. Results show that the optimized filter structure is comprised of two stacks of 5 bilayers with different peak positions stacked to form a single structure. When the optimized filter is used in the ideal TPV configuration, a spectral efficiency of 46%, a system efficiency of 33%, and a power density of 8.5 W/cm2 are achieved at the emitter temperature of 1800 K. According to the MCM code, a TPV system comprised of the optimized filter, an emitter, and a PV cell with equal areas of 2 x 2 cm and a 1 mm gap achieves a spectral efficiency of 45%, a system efficiency of 25% and a power density of 7.8 W/cm2 at the emitter temperature of 1800 K. Moreover, MCM results reveal that when the emitter temperature and gap length are 1800 K and 1 mm, respectively, a maximum system efficiency of 27% is realized at an optimal ratio of 0.75 between the PV cell area and the emitter area.

Theory of "Hot" Photoluminescence from Drude Metals.

We provide a complete quantitative theory for light emission from Drude metals under continuous wave illumination, based on our recently derived steady-state nonequilibrium electron distribution. We show that the electronic contribution to the emission exhibits a dependence on the emission frequency which is very similar to the energy dependence of the nonequilibrium distribution, and characterize different scenarios determining the measurable emission line shape. This enables the identification of experimentally relevant situations, where the emission lineshapes deviate significantly from predictions based on the standard theory (namely, on the photonic density of states), and enables the differentiation between cases where the emission scales with the metal object surface or with its volume. We also provide an analytic description (which is absent from the literature) of the (polynomial) dependence of the metal emission on the electric field, its dependence on the pump laser frequency, and its nontrivial exponential dependence on the electron temperature, both for the Stokes and anti-Stokes regimes. Our results imply that the emission does not originate from either Fermion statistics (due to e-e interactions), and even though one could have expected the emission to follow boson statistics due to involvement of photons (as in Planck's Black Body emission), it turns out that it deviates from that form as well. Finally, we resolve the arguments associated with the effects of electron and lattice temperatures on the emission, and which of them can be extracted from the anti-Stokes emission.

Mid-infrared radiative emission from bright hot plasmons in graphene.

Carrier excitation and decay processes in graphene are of broad interest since relaxation pathways that are not present in conventional materials are enabled by a gapless Dirac electronic band structure. Here, we report that a previously unobserved decay pathway-hot plasmon emission-results in Fermi-level-dependent mid-infrared emission in graphene. Our observations of non-thermal contributions to Fermi-level-dependent radiation are an experimental demonstration of hot plasmon emission arising from a photo-inverted carrier distribution in graphene achieved via ultrafast optical excitation. Our calculations indicate that the reported plasmon emission process can be several orders of magnitude brighter than Planckian emission mechanisms in the mid-infrared spectral range. Both the use of gold nanodisks to promote scattering and localized plasmon excitation and polarization-dependent excitation measurements provide further evidence for bright hot plasmon emission. These findings define an approach for future work on ultrafast and ultrabright graphene emission processes and mid-infrared light source applications.

Reflection Modeling of the Black Hole Binary 4U 1630–47: The Disk Density and Returning Radiation

Abstract We present the analysis of X-ray observations of the black hole binary 4U 1630−47 using relativistic reflection spectroscopy. We use archival data from the Rossi X-ray Timing Explorer, Neil Gehrels Swift Observatory, and Nuclear Spectroscopic Telescope Array observatories, taken during different outbursts of the source between 1998 and 2015. Our modeling includes two relatively new advances in modern reflection codes: high-density disks, and returning thermal disk radiation. Accretion disks around stellar-mass black holes are expected to have densities well above the standard value assumed in traditional reflection models (i.e., n e ∼ 1015 cm−3). New high-density reflection models have important implications in the determination of disk truncation (i.e., the disk inner radius). This is because one must retain self-consistency in the irradiating flux and corresponding disk ionization state, which is a function of disk density and system geometry. We find that the disk density is n e ≥ 1020 cm−3 across all spectral states. This density, combined with our constraints on the ionization state of the material, implies an irradiating flux impinging on the disk that is consistent with the expected theoretical estimates. Returning thermal disk radiation—the fraction of disk photons that bend back to the disk producing additional reflection components—is expected predominantly in the soft state. We show that returning radiation models indeed provide a better fit to the soft-state data, reinforcing previous results that show that in the soft state, the irradiating continuum may be blackbody emission from the disk itself.

A method for calculating the radiation power coefficients of black and gray bodies with dimensions commensurable with the radiated wavelengths

A technique is proposed and the calculations of the dependences of the emissivity of an abso-lute black body (BBB) on the size of the diaphragms of the emitting aperture are performed for hypothetical cases when the sizes of the diaphragms are commensurate with the emitted wavelengths, and the diaphragms are made of a dielectric opaque for radiation. The value of the cutoff wavelength  = 1.772A for the square aperture of the diaphragm was determined, where A is the side of the square and  = 1. 571D for the round hole. where D is the hole di-ameter, i.e. it is shown that the body cannot emit wavelengths λ greater than 1.772A in the case of a square hole and 1.571D in the case of a round hole. It is shown that if the “cut off” wavelengths made any significant contribution to the integral radiation of a blackbody with temperature T at standard diaphragm diameters (i.e., at diameters of much larger radiated wavelengths), then the emissivity of this body becomes less than unity and rapidly decreases when the size of the diaphragms is commensurate with . In these cases, such a body ceases to be an absolutely black body and the laws of Planck and Stefan–Boltzmann cannot be used to calculate the power of its radiation, but the technique proposed in this work can be used.

Dust moments: towards a new modeling of the galactic dust emission for CMBB-modes analysis

The characterization of the spectral energy distribution (SED) of dust emission has become a critical issue in the quest for primordialB-modes. The dust SED is often approximated by a modified black body (MBB) emission law but the extent to which this is accurate is unclear. This paper addresses this question, expanding the dust SED at the power spectrum level. The expansion is performed by means of moments around the MBB law, related to derivatives with respect to the dust spectral index. We present the mathematical formalism and apply it to simulations andPlancktotal intensity data, from 143 to 857 GHz, because no polarized data are yet available that provide the required sensitivity to perform this analysis. With simulations, we demonstrate the ability of high-order moments to account for spatial variations in MBB parameters. Neglecting these moments leads to poor fits and a bias in the recovered dust spectral index. We identify the main moments that are required to fit thePlanckdata. The comparison with simulations helps us to disentangle the respective contributions from dust and the cosmic infrared background to the high-order moments, but the simulations give an insufficient description of the actualPlanckdata. Extending our model to cosmic microwave backgroundB-mode analyses within a simplified framework, we find that ignoring the dust SED distortions, or trying to model them with a single decorrelation parameter, could lead to biases that are larger than the targeted sensitivity for the next generation of CMBB-mode experiments.

A comprehensive view of blackbody components in the X-ray spectra of GRBs

Abstract A small fraction of gamma-ray bursts (GRBs) exhibit blackbody emission in the X-ray spectra, the origin of which is debated. In order to gain a more complete understanding of this phenomenon, we present a search for blackbody components in 116 GRBs with known redshifts observed by Swift X-ray telescope (XRT). A time-resolved spectral analysis is carried out and the significance of the blackbody is assessed with respect to an absorbed power-law model. We report nine new detections and confirm the previously reported blackbody in GRB 171205A. Together with our previous results, there are a total of 19 GRBs with significant blackbody emission in a sample of 199 GRBs observed by Swift over 13 yr. The detections include one short GRB and two low-luminosity GRBs. We estimate fireball parameters from the blackbody components and note that the blackbody luminosity is correlated with the temperature and inferred Lorentz factor. There is a large spread in the properties of the blackbody components and the light curves, which points to different origins for the emission. In about a third of the GRBs, the blackbody is clearly associated with late prompt emission from the jet. The rest of the sample includes cases that are fully consistent with the expectations from a cocoon, as well cases that may be explained by high-latitude emission or more energetic cocoons. These results indicate that thermal emission is associated with all parts of the jet.

Linking Soft Excess in Ultraluminous X-Ray Sources with Optically Thick Wind Driven by Supercritical Accretion

Abstract Supercritical accretion onto compact objects may drive massive winds that are nearly spherical, optically thick, and Eddington limited. Blackbody emission from the photosphere is the direct observational signature of the wind. Here we investigate whether or not it can explain the soft emission component seen in the energy spectra of ultraluminous X-ray sources (ULXs). Based on high-quality XMM-Newton spectra of 15 ULXs, we find that the soft component can be modeled as blackbody emission with a nearly constant luminosity, and the five known pulsating ULXs (PULXs) in the sample display a blackbody luminosity among the lowest. These are consistent with the scenario that the soft emission originates from the photosphere of the optically thick wind. However, the derived blackbody luminosity for PULXs is significantly above the Eddington limit for neutron stars. A possible explanation is that a considerable fraction of the optically thick wind roots in the inner accretion flow, where the radiative flux could exceed the Eddington limit due to a reduced scattering cross-section or enhanced radiation transfer with magnetic buoyancy. Based on a wind model, the inferred mass accretion rate in these standard ULXs overlaps but is on average lower than that in luminous and very soft X-ray sources, which are also candidates with supercritical accretion. Alternatively, it cannot be ruled out that the soft emission component is a result of the hard component, e.g., via down-scattering in a cool medium, as a weak correlation may exist between them.

Superior spectral performance of decoy flares compositions via inclusion of graphite as a black body emitter

Graphite particles can offer high interfacial surface area as well as high emissivity as black body; therefore it can find wide applications in advanced decoy flares. In the current study, graphite particles of 100 µm size, were employed with different reactive metal fuels including Mg, Al, and Mg-Al alloy. Different formulations based on graphite ratio up to 8 wt % were developed via granulation with subsequent pressing. Thermal signature was measured using IR spectrometer (1-6 µm) Arc-Optics. Composite flare based on Mg-Al fuel, and graphite demonstrated superior spectral intensity; this formulation offered an increase in average intensity by 83 % to reference MTV flare. In the mean time, composite flare based on Al and graphite offered the highest relative intensity value θ of 0.6. Graphite, as an allotrope of carbon, can act as an excellent source of carbonaceous materials; that can strengthen incandescence emission. Furthermore Mg-Al reactive metal alloy can offer novel synergism as Mg could vaporize at 1000 °C offering efficient combustion process. It can be concluded that tailored flare with customized thermal signature can be developed.

Dense cores in the Seahorse infrared dark cloud: physical properties from modified blackbody fits to the far-infrared–submillimetre spectral energy distributions

Context. Infrared dark clouds (IRDCs) can be the birth sites of high-mass stars, and hence determining the physical properties of dense cores in IRDCs is useful to constrain the initial conditions and theoretical models of high-mass star formation. Aims. We aim to determine the physical properties of dense cores in the filamentary Seahorse IRDC G304.74+01.32. Methods. We used data from the Wide-field Infrared Survey Explorer (WISE), Infrared Astronomical Satellite (IRAS), and Herschel in conjuction with our previous 350 and 870 μm observations with the Submillimetre APEX Bolometer Camera (SABOCA) and Large APEX BOlometer CAmera, and constructed the far-IR to submillimetre spectral energy distributions (SEDs) of the cores. The SEDs were fitted using single or two-temperature modified blackbody emission curves to derive the dust temperatures, masses, and luminosities of the cores. Results. For the 12 analysed cores, which include two IR dark cores (no WISE counterpart), nine IR bright cores, and one H II region, the mean dust temperature of the cold (warm) component, the mass, luminosity, H2 number density, and surface density were derived to be 13.3 ± 1.4 K (47.0 ± 5.0 K), 113 ± 29 M⊙, 192 ± 94 L⊙, (4.3 ± 1.2) × 105 cm−3, and 0.77 ± 0.19 g cm−3, respectively. The H II region IRAS 13039-6108a was found to be the most luminous source in our sample ((1.1 ± 0.4) × 103 L⊙). All the cores were found to be gravitationally bound (i.e. the virial parameter αvir < 2). Two out of the nine analysed IR bright cores (22%) were found to follow an accretion luminosity track under the assumptions that the mass accretion rate is 10−5 M⊙ yr−1, the stellar mass is 10% of the parent core mass, and the radius of the central star is 5 R⊙. Most of the remaing ten cores were found to lie within 1 dex below this accretion luminosity track. Seven out of 12 of the analysed cores (58%) were found to lie above the mass-radius thresholds of high-mass star formation proposed in the literature. The surface densities of Σ > 0.4 g cm−3 derived for these seven cores also exceed the corresponding threshold for high-mass star formation. Five of the analysed cores (42%) show evidence of fragmentation into two components in the SABOCA 350 μm image. Conclusions. In addition to the H II region source IRAS 13039-6108a, some of the other cores in Seahorse also appear to be capable of giving birth to high-mass stars. The 22 μm dark core SMM 9 is likely to be the youngest source in our sample that has the potential to form a high-mass star (96 ± 23 M⊙ within a radius of ~0.1 pc). The dense core population in the Seahorse IRDC has comparable average properties to the cores in the well-studied Snake IRDC G11.11-0.12 (e.g. Tdust and L agree within a factor of ~1.8); furthermore, the Seahorse, which lies ~60 pc above the Galactic plane, appears to be a smaller (e.g. three times shorter in projection, ~100 times less massive) version of the Snake. The Seahorse core fragmentation mechanisms appear to be heterogenous, including cases of both thermal and non-thermal Jeans instability. High-resolution follow-up studies are required to address the fragmented cores’ genuine potential of forming high-mass stars.

MIRACLES: atmospheric characterization of directly imaged planets and substellar companions at 4–5 μm

The circumstellar disk of PDS 70 hosts two forming planets, which are actively accreting gas from their environment. The physical and chemical characteristics of these planets remain ambiguous due to their unusual spectral appearance compared to more evolved objects. In this work, we report the first detection of PDS 70 b in the Brα and M′ filters with VLT/NACO, a tentative detection of PDS 70 c in Brα, and a reanalysis of archival NACO L′ and SPHERE H23 and K12 imaging data. The near side of the disk is also resolved with the Brα and M′ filters, indicating that scattered light is non-negligible at these wavelengths. The spectral energy distribution (SED) of PDS 70 b is well described by blackbody emission, for which we constrain the photospheric temperature and photospheric radius to Teff = 1193 ± 20 K and R = 3.0 ± 0.2 RJ. The relatively low bolometric luminosity, log(L∕L⊙) = −3.79 ± 0.02, in combination with the large radius, is not compatible with standard structure models of fully convective objects. With predictions from such models, and adopting a recent estimate of the accretion rate, we derive a planetary mass and radius in the range of Mp ≈ 0.5–1.5 MJ and Rp ≈ 1–2.5 RJ, independently of the age and post-formation entropy of the planet. The blackbody emission, large photospheric radius, and the discrepancy between the photospheric and planetary radius suggests that infrared observations probe an extended, dusty environment around the planet, which obscures the view on its molecular composition. Therefore, the SED is expected to trace the reprocessed radiation from the interior of the planet and/or partially from the accretion shock. The photospheric radius lies deep within the Hill sphere of the planet, which implies that PDS 70 b not only accretes gas but is also continuously replenished by dust. Finally, we derive a rough upper limit on the temperature and radius of potential excess emission from a circumplanetary disk, Teff ≲ 256 K and R ≲ 245 RJ, but we do find weak evidence that the current data favors a model with a single blackbody component.

Synergy between NIR luminescence and thermal emission toward highly sensitive NIR operating emissive thermometry

There are many figures of merit, which determine suitability of luminescent thermometers for practical applications. These include thermal sensitivity, thermal accuracy as well as ease and cost effectivness of technical implementation. A novel contactless emission thermometer is proposed, which takes advantage of the coexistence of photoluminescence from Nd3+ doping ions and black body emission in transparent Nd3+ doped-oxyfluorotellurite glass host matrix. The opposite temperature dependent emission from these two phenomena, enables to achieve exceptionally high relative sensitivity SR = 8.2%/°C at 220 °C. This enables to develop new type of emissive noncontact temperature sensors.

Angular Dependence and Spatial Distribution of Jupiter's Centimeter‐Wave Thermal Emission From Juno's Microwave Radiometer

AbstractNASA's Juno spacecraft has been monitoring Jupiter in 53‐day orbits since 2016. Its six‐frequency microwave radiometer (MWR) is designed to measure black body emission from Jupiter over a range of pressures from a few tenths of a bar to several kilobars in order to retrieve details of the planet's atmospheric composition, in particular, its ammonia and water abundances. A key step toward achieving this goal is the determination of the latitudinal dependence of the nadir brightness temperature and limb darkening of Jupiter's thermal emission through a deconvolution of the measured antenna temperatures. We present a formulation of the deconvolution as an optimal estimation problem. It is demonstrated that a quadratic expression is sufficient to model the angular dependence of the thermal emission for the data set used to perform the deconvolution. Validation of the model and results from a subset of orbits favorable for MWR measurements is presented over a range of latitudes that cover up to 60° from the equator. A heuristic algorithm to mitigate the effects of nonthermal emission is also described.

Spaceborne radiance temperature standard blackbody for Chinese high-precision infrared spectrometer

As a radiance temperature standard source for the Chinese high-resolution spectrum infrared spectrometer, the National Institute of Metrology, China, developed a spaceborne radiance temperature standard blackbody (SRTSB) combined with a series of miniature phase change fixed points (MPCFPs) for temperature calibration and emissivity verification using a thermal radiation loop (TRL). The MPCFPs of the SRTSB contain H2O, Ga, Ga–Sn alloy, and phenyl salicylate. The TRL is used to measure the emissivity of the spaceborne blackbody on the basis of controlling environmental radiation. Experimental results show that the blackbody emissivity is higher than 0.998 and the temperature control stability is better than 5 mK. Temperature control uniformity at the cavity bottom and axial are better than or equal to 11 mK and 38 mK, respectively, in the working temperature range of 268.15 K to 331.15 K.. The repeatability of the MPCFPs is less than 5 mK, and the results of the continuous phase transition of the MPCFPs are in a clear temperature plateau. The expanded uncertainty of the SRTSB is lower than or equal to 0.084 K (k = 2).

Estimation of the black hole spin in LMC X-1 using AstroSat

ABSTRACT LMC X-1, a persistent, rapidly rotating, extra-galactic, black hole X-ray binary (BHXB) discovered in 1969, has always been observed in its high soft state. Unlike many other BHXBs, the black hole mass, source distance, and binary orbital inclination are well established. In this work, we report the results of simultaneous broad-band spectral studies of LMC X-1 carried out using the data from Soft X-ray Telescope and Large Area X-ray Proportional Counter aboard AstroSat as observed on 2016 November 26 and 2017 August 28. The combined spectrum was modelled with a multicolour blackbody emission (diskbb), a Gaussian along with a Comptonization component (simpl) in the energy range 0.7–30.0 keV. The spectral analysis revealed that the source was in its high soft state (Γ = 2.67$^{+0.24}_{-0.24}$ and Γ = 2.12$^{+0.19}_{-0.20}$) with a hot disc (kTin = 0.86$^{+0.01}_{-0.01}$ and kTin = 0.87$^{+0.02}_{-0.02}$). Thermal disc emission was fitted with a relativistic model (kerrbb) and spin of the black hole was estimated to be 0.93$^{+0.01}_{-0.01}$ and 0.93$^{+0.04}_{-0.03}$ (statistical errors) for the two Epochs through X-ray continuum-fitting, which agrees with the previous results.

Directing near-infrared photon transport with core@shell particles

Directing the propagation of near-infrared radiation is a major concern in improving the efficiency of solar cells and thermal insulators. A facile approach to scatter light in the near-infrared region without excessive heating is to embed compact layers with semiconductor particles. The directional scattering by semiconductor@oxide (core@shell) spherical particles (containing Si, InP, TiO2, SiO2, or ZrO2) with a total radius varying from 0.1 μm to 4.0 µm and in an insulating medium at a low volume fraction is investigated using Lorenz–Mie theory and multiscale modeling. The optical response of each layer is calculated under irradiation by the Sun or a blackbody emitter at 1180 K. Reflectance efficiency factors of up to 83.7% and 63.9% are achieved for near-infrared solar and blackbody radiation in 200 µm thick compact layers with only 1% volume fraction of bare Si particles with a radius of 0.23 µm and 0.50 µm, respectively. The maximum solar and blackbody efficiency factors of layers containing InP particles were slightly less (80.2% and 60.7% for bare particles with a radius of 0.25 µm and 0.60 µm, respectively). The addition of an oxide coating modifies the surrounding dielectric environment, which improves the solar reflectance efficiency factor to over 90%, provided it matches the scattering mode energies with the incident spectral density. The layers are spectrally sensitive and can be applied as a back or front reflector for solar devices, high temperature thermal insulators, and optical filters in gradient heat flux sensors for fire safety applications.

Planck2018 results

The study of polarized dust emission has become entwined with the analysis of the cosmic microwave background (CMB) polarization in the quest for the curl-likeB-mode polarization from primordial gravitational waves and the low-multipoleE-mode polarization associated with the reionization of the Universe. We used the newPlanckPR3 maps to characterize Galactic dust emission at high latitudes as a foreground to the CMB polarization and use end-to-end simulations to compute uncertainties and assess the statistical significance of our measurements. We presentPlanckEE,BB, andTEpower spectra of dust polarization at 353 GHz for a set of six nested high-Galactic-latitude sky regions covering from 24 to 71% of the sky. We present power-law fits to the angular power spectra, yielding evidence for statistically significant variations of the exponents over sky regions and a difference between the values for theEEandBBspectra, which for the largest sky region areαEE = −2.42 ± 0.02 andαBB = −2.54 ± 0.02, respectively. The spectra show that theTEcorrelation andE/Bpower asymmetry discovered byPlanckextend to low multipoles that were not included in earlierPlanckpolarization papers due to residual data systematics. We also report evidence for a positiveTBdust signal. Combining data fromPlanckand WMAP, we have determined the amplitudes and spectral energy distributions (SEDs) of polarized foregrounds, including the correlation between dust and synchrotron polarized emission, for the six sky regions as a function of multipole. This quantifies the challenge of the component-separation procedure that is required for measuring the low-ℓreionization CMBE-mode signal and detecting the reionization and recombination peaks of primordial CMBBmodes. The SED of polarized dust emission is fit well by a single-temperature modified black-body emission law from 353 GHz to below 70 GHz. For a dust temperature of 19.6 K, the mean dust spectral index for dust polarization isβdP= 1.53±0.02. The difference between indices for polarization and total intensity isβdP−βdI= 0.05±0.03. By fitting multi-frequency cross-spectra betweenPlanckdata at 100, 143, 217, and 353 GHz, we examine the correlation of the dust polarization maps across frequency. We find no evidence for a loss of correlation and provide lower limits to the correlation ratio that are tighter than values we derive from the correlation of the 217- and 353 GHz maps alone. If thePlancklimit on decorrelation for the largest sky region applies to the smaller sky regions observed by sub-orbital experiments, then frequency decorrelation of dust polarization might not be a problem for CMB experiments aiming at a primordialB-mode detection limit on the tensor-to-scalar ratior ≃ 0.01 at the recombination peak. However, thePlancksensitivity precludes identifying how difficult the component-separation problem will be for more ambitious experiments targeting lower limits onr.

Error analysis of on-orbit thermal emission band radiometric calibration for FY3C/MERSI

The Chinese medium resolution spectral imager (MERSI) carried on the Fengyun-3C (FY-3C) satellite can realize the high-precision global measurement of the cloud, aerosol, land surface, sea surface, and low-level water vapor. The MERSI is described conceptually in this paper, and the radiometric calibrations of its thermal emission bands by the on-board blackbody (BB) were given by two different algorithms. In order to evaluate the stability of the instrument’s performance, the measured irradiance after the dark current correction was compared with the theoretical value calculated from the BB temperature by Planck function. The results showed that the algorithm of linear fitting to the BB measurements averaged over sampling points was an optimal choice for the radiometric calibration, and that the calibration coefficients determined by this method could reflect the pixel difference while maintaining a relative stability between each frame. Using the perturbation method, the influences of the BB temperature, wavelength, and emissivity offsets on the calibration coefficients were analyzed and discussed through theoretical deduction and computer simulation for improving the calibration accuracy of the instrument.

Maximal nighttime electrical power generation via optimal radiative cooling

We present a systematic optimization of nighttime thermoelectric power generation system utilizing radiative cooling. We show that an electrical power density >2 W/m2, two orders of magnitude higher than the previously reported experimental result, is achievable using existing technologies. This system combines radiative cooling and thermoelectric power generation and operates at night when solar energy harvesting is unavailable. The thermoelectric power generator (TEG) itself covers less than 1 percent of the system footprint area when achieving this optimal power generation, showing economic feasibility. We study the influence of emissivity spectra, thermal convection, thermoelectric figure of merit and the area ratio between the TEG and the radiative cooler on the power generation performance. We optimize the thermal radiation emitter attached to the cold side and propose practical material implementation. The importance of the optimal emitter is elucidated by the gain of 153% in power density compared to regular blackbody emitters.

Broadband electroluminescence from reverse breakdown in individual suspended carbon nanotube pn-junctions

There are various mechanisms of light emission in carbon nanotubes (CNTs), which give rise to a wide range of spectral emission characteristics that provide important information regarding the underlying physical processes that lead to photon emission. Here, we report spectra obtained from individual suspended CNT dual-gate field effect transistor (FET) devices under different gate and bias conditions. By applying opposite voltages to the gate electrodes (i.e., Vg1 = −Vg2), we are able to create a pn-junction within the suspended region of the CNT. Under forward bias conditions, the spectra exhibit a peak corresponding to E11 exciton emission via thermal (i.e., blackbody) emission occurring at electrical powers around 8 µW, which corresponds to a power density of approximately 0.5 MW/cm2. On the other hand, the spectra observed under reverse bias correspond to impact ionization and avalanche emission, which occurs at electrical powers of ~10 nW and exhibits a featureless flat spectrum extending from 1,600 nm to shorter wavelengths up to 600 nm. Here, the hot electrons generated by the high electric fields (~0.5 MV/cm) are able to produce high energy photons far above the E11 (ground state) energy. It is somewhat surprising that these devices do not exhibit light emission by the annihilation of electrons and holes under forward bias, as in a light emitting diode (LED). Possible reasons for this are discussed, including Auger recombination.