Spectral Thermal Emission Research Articles

Spectrally selective mid-infrared absorber/emitter by controlling optical Tamm states with thin phononic film

Abstract We demonstrate the near-unity absorption of multilayer structures in the mid-infrared (MIR). The first configuration consists of a distributed Bragg reflector and a subwavelength-thin phononic film on top. Optical waves are confined within the cavity and the interface of the top layer, resulting in narrowband selective perfect absorption at the wavelength of 12.56 μm, which is of particular interest for many technological applications. Thus, highly spectral selective thermal emission can emerge from the engineered phononic-photonic structure. Tamm absorber can also be realized when the light from the phononic film side is perfectly absorbed. Reflectance of the fabricated sample is measured using a Fourier transform infrared spectrometer (FTIR) and matches the simulations well. Our findings will pave a new avenue for harness surface phonon (SPh) modes in the MIR, which could benefit applications such as radiative cooling, selective thermal emission and MIR sensing.

Spectral near-field thermal emission extraction by optical waveguides

As far-field thermal emission is limited by blackbody radiation, the significant enhancement of thermal radiation in the near-field plays a vital role in a variety of applications such as infrared sensing, radiation cooling, and thermophotovoltaics. Yet the techniques of exporting the near-field signal to the far field are still not mature, typically relying on complex instrumentation capable of being applied only to the surfaces of model systems and structures. Here we develop an efficient method of extracting near-field thermal radiation to the far field by integrating a nanoscale thermal emitter with a high-index optical waveguide. By directly depositing the emitter onto the optical waveguide, it requires no vacuum gap and enables efficient coupling of near-field thermal radiation into propagating wave-guided modes. A single-mode planar waveguide incorporated with an indium-tin oxide (ITO) film as a thermal emitter is theoretically investigated to prove the feasibility of thermal extraction. The Wiener-chaos expansion method is applied to directly calculate the thermal radiation from the emitter-waveguide system. We experimentally demonstrate the fidelity of the optical-waveguide-assisted near-field thermal extraction by measuring the emission spectrum of an ITO-coated optical fiber and comparing with theoretical predictions. Our experimental demonstration in conjunction with the direct simulation paves the way for effectively extracting near-field thermal radiation with applications in chemical sensing, infrared imaging, and near-field thermal energy management.

Tuneable Thermal Emission Using Chalcogenide Metasurface

AbstractModulation of thermal radiation is an essential element of infrared sensing and imaging, thermal infrared light sources, camouflage, and thermophotovoltaics. Recently, tuneable thermal emission of nanophotonic structures has been demonstrated. However, most of the current strategies involve controlling single spectral thermal emission in the far‐infrared region, and blue shifting their resonances to the shorter wavelength region is rarely explored. Moreover, fast modulation of multispectral thermal radiation remains challenging. In this work, the dynamic control of multispectral thermal emission from 2 to 4 µm from an ultrathin reconfigurable metasurface is experimentally presented based on Au/SiO2/Ge2Sb2Te5/Au multilayer. This metadevice contains several integrated thermal emitters of various wavelengths, each of which consists of gold (Au) squares array with different widths. A tuning of multispectral absorptivity (emissivity) can be achieved by transiting the state of Ge2Sb2Te5 from amorphous to crystalline. A heat‐transfer model is developed to demonstrate that the reversible switching of multispectral thermal emission can be achieved in just 300 ns. The experimental demonstration along with the theoretical framework lays the foundation for designing high‐speed reconfigurable multispectral thermal emitters, which, as expected, will initiate a new route to thermal engineering.

Enhancing the radiative heat dissipation from high-temperature SF6 gas plasma by using selective absorbers

Radiative cooling accomplished by tailoring the properties of spectral thermal emission is an interesting method for energy harvesting and high-efficiency passive cooling of terrestrial structures. This strategy, however, has not been extended to cool enclosed heat sources, common in engineering applications, and heat sources in high-temperature environments where radiative transfer plays a dominant role. Here we show a radiative cooling scheme for a high-temperature gaseous medium, using radiative heat extraction with selective absorbers matched to the gas-selective emission properties. We used SF6 gas plasma as a model, because this gas is used in gas circuit breakers, which require effective cooling of the hot insulating gas. Our theoretical analysis confirms that a copper photonic absorber, matched to the ultraviolet-to-near-infrared-selective emission properties of the gas, effectively extracts heat from the high-temperature gas plasma and lowers the radiative equilibrium gas temperature by up to 1270 K, exceeding both blackbody-like and metallic surfaces in practical operating conditions.

Resonant Thermal Infrared Emitters in Near- and Far-Fields

Although optical resonators are widely used for controlling and engineering thermal radiation, what has been lacking is a general theoretical framework to elucidate the thermal emission of optical resonators. We developed a general and self-consistent formalism to describe the thermal radiation from arbitrary optical resonators made by lossy and dispersive materials like metals, with the only assumption that the resonators have a single predominant resonance mode. Our formalism derives the fundamental limit of the spectral thermal emission power from an optical resonator and proves that this limit can be achieved when the mode losses to the emitter and the absorber (or far-field background) are matched, and meanwhile, the predominant resonant mode is electrically quasi-static. By efficiently calculating the resonant modes using the finite element methods, our formalism serves as a general principle of designing the arbitrary optical resonator thermal emitters with perfect or maximized emission in both nea...

Thermal emission from finite photonic crystals

We present a microscopic theory of thermal emission from finite-sized photonic crystals and show that the directional spectral emissivity and related quantities can be evaluated via standard bandstructure computations without any approximation. We then identify the physical mechanisms through which interfaces modify the potentially super-Planckian radiation flow inside infinite photonic crystals, such that thermal emission from finite-sized samples is consistent with the fundamental limits set by Planck’s law. As an application, we further demonstrate that a judicious choice of a photonic crystal’s surface termination facilitates considerable control over both the spectral and angular thermal emission properties.

Constrained profile retrieval applied to the observation mode of the Michelson Interferometer for Passive Atmospheric Sounding

To investigate the atmosphere of Earth and to detect changes in its environment, the Environmental Satellite will be launched by the European Space Agency in a polar orbit in October 2001. One of its payload instruments is a Fourier spectrometer, the Michelson Interferometer for Passive Atmospheric Sounding, designed to measure the spectral thermal emission of molecules in the atmosphere in a limb-viewing mode. The goal of this experiment is to derive operationally vertical profiles of pressure and temperature as well as of trace gases O(3), H(2)O, CH(4), N(2)O, NO(2), and HNO(3) from spectra on a global scale. A major topic in the analysis of the computational methodology for obtaining the profiles is how available a priori knowledge can be used and how this a priori knowledge affects corresponding results. Retrieval methods were compared and it was shown that an optimal estimation formalism can be used in a highly flexible way for this kind of data analysis. Beyond this, diagnostic tools, such as estimated standard deviation, vertical resolution, or degrees of freedom, have been used to characterize the results. Optimized regularization parameters have been determined, and a great effect from the choice of regularization and discretization on the results was demonstrated. In particular, we show that the optimal estimation formalism can be used to emulate purely smoothing constraints.

Noncontact nanosecond-time-resolution temperature measurement in excimer laser heating of Ni–P disk substrates

The thermal emission from a Ni–P disk substrate heated by a pulsed excimer laser is measured with nanosecond time resolution. A fast InGaAs photodetector is employed to capture the thermal emission signal. The spectral surface reflectivity is simultaneously measured in situ. The transient surface temperature is derived from the spectral thermal emission signal on the basis of Planck’s blackbody radiation intensity distribution. The experimental results and analytical solutions are compared and an important parameter involving the thermal diffusivity and conductivity in the transient temperature response of the material is evaluated.

Nanosecond-time-resolution thermal emission measurement during pulsed excimer-laser interaction with materials

A nanosecond-time-resolution pyrometer has been developed for measuring the transient surface temperature of a solid material heated by pulsed excimer-laser irradiation. Fast germanium diodes are employed to capture the transient thermal emission signals in the wavelength range between 1.2 and 1.6 pm. The surface temperature is derived from the measured spectral thermal emission. The directional spectral emissivity is determined in situ by measuring the transient directional spectral reflectivity and applying Kirchhoffs law. The experimental results are in good agreement with numerical thermal modeling predictions. The pyrometric thermal emission measurement also yields the solid/liquid interface temperature during the pulsed excimer-laser-induced melting. The relation between the measured interface superheating temperature and the interface velocity reveals the melting kinetic relation during the high-power, short-pulse laser-induced phase-change processes.

II. Spectral surface thermal emission and absorption of water

In the present paper we succeeded to complete the problem consideration and to obtain numerical results of the temperature dependence of refraction and absorption indices of water for the wavelengths from 2 to 80μ and temperatures from 0 to 75°C for every 5°C. Then these data were used in calculation of relative radiation factors and intensity of surface radiation of water for the ranges of wavelengths and temperatures considered above. The important result obtained is that a intensity of surface radiation of water within the most important short wavelength domain of the spectrum up to 11μ is completely due a refraction index of water, and not to an absorption index. Besides that, it was succeeded to find out that in the short wavelength range separately and in a separate domain of the long wavelength range of the spectrum water radiant as a black body. The wavelength boundaries in both short wavelength and long wavelength domains where this phenomenon is observed when temperature is changing from 0 to 75°C at the step of 5°C, are determined. A new pecularity of water related to its surface radiant depending of temperature is found. When water temperature is changing from 0 to −5°C absolute values: of surface intensity of radiation integral emissive factor and radiosity of water radiation have minima at temperatures of 30 to 35°C.

Thermal radiation from spherical microparticles: a new dipole model

A new model for thermal radiation from spherical microparticles is presented. It treats small particles as volume emitters instead of the usual surface emitters to take into account the effect of temperature distributions inside the particle on emission. The model describes the spectral thermal emission of the particle’s volume cells as the radiation of classical electric dipoles with an averaged orientation in an absorbing dielectric sphere. Within this dipole model of thermal radiation Kirchhoff’s law is rederived. For nonisothermal temperature distributions inside particles, the angular dependence and the magnitude of the emitted radiation are computed. To test the influence of the model on the heat transfer in microparticles, we compare computed temperature distributions inside volume- and surface-emitting particles.

Remote sensing of cloud, aerosol, and water vapor properties from the moderate resolution imaging spectrometer (MODIS)

The authors describe the status of MODIS-N and its companion instrument MODIS-T (tilt), a tiltable cross-track scanning spectrometer with 32 uniformly spaced channels between 0.410 and 0.875 mu m. They review the various methods being developed for the remote sensing of atmospheric properties using MODIS, placing primary emphasis on the principal atmospheric applications of determining the optical, microphysical, and physical properties of clouds and aerosol particles from spectral reflection and thermal emission measurements. In addition to cloud and aerosol properties, MODIS-N will be used for determining the total precipitable water vapor and atmospheric stability. The physical principles behind the determination of each of these atmospheric products are described, together with an example of their application to aircraft and/or satellite measurements. >

Measurements of the Spectral and Directional Emission From Microgrooved Silicon Surfaces

This paper reports measurements of both the spectral and specular thermal radiation emission characteristics of very regularly microconfigured grooved surfaces in a silicon substrate at 300 and 400°C. The resulting surfaces were phosphorus-doped, to assure the dominance of the emission from the material near the sample surface. The samples had groove depths H of zero for a reference, to 42 μm, and widths L = 12.6 to 14 μm. The geometry repeat distance was 22 μm, or 455 grooves per cm. The grooves correspond directly in size to the band of principle emission wavelengths λ that arises at these temperature levels. The measurements show strong spectral effects for normal emission, including highly favored frequencies, for H > λ. This suggests a cavity “organ pipe” mode of emission. Similar, though modified, effects were found in directional emission, away from the normal. There also were strong polarization effects, with the cross-groove polarization mode dominant. The spectral and specular measurements are compared with calculations of the classical kind, which tacitly assume that λ < < H = 0(L).

Polarization effects of the spectral thermal IR and visible emission from ferromagnets

Magneto-optical effects are usually investigated by reflection or transmission measurements. An alternative is the experimental study of the spectral thermal emission from a heated magnetized sample. In this paper we report on experiments and theory of two magneto-optical effects related to the spectral thermal emission from ferromagnets. The first effect was discovered by Müller, Fuchs and Kneubühl in 1977. They demonstrated that the spectral thermal emission of an axially magnetized Fe rod exhibits a partial circular polarization. We have measured this effect in the Spectral range from 0.25 to 2 eV for a set of FeCo alloys. The spectra show a maximum at low energies. With increasing photon energy the polarization decreases and changes its sign at 1.35, 1.50 and 1.65 eV for the alloys Fe 79 Co 21, Fe 55 Co 45 and Fe 31 Co 69, respectively. With a specific theoretical model we have succeeded in explaining the dependence of the effect on the wavelength and on the Co concentration. In relation to the above effect we have discovered that the thermal radiation emitted from a ferromagnetic sample magnetized parallel to its surface is partially linearly polarized. This effect is even with respect to the magnetization and of the order of 10 −3 for saturated Fe. We have determined experimentally this linear polarization as a function of the wavelength, the temperature and the external field. In the spectral range of 0.25-1.25 eV minima have been observed at 0.3, 0.45 and 0.6 eV for Fe 79 Co 21, Fe 55 Co 45 and Fe 31 Co 69, respectively, and maxima at 0.5, 0.75, 1 and 1.15eV for Fe 79 Co 21, Fe 55 Co 45 and Fe 31 Co 69, respectively. These spectra show a shift to higher energies for increasing Co concentration in analogy to the spectra of the circular polarization effect. Furthermore, we have measured the dependence of the polarization on the temperature from 600 K up to the Curie temperature where the effect vanishes. The polarization decreases almost linearly with rising temperature for Fe and Fe 31 Co 69. However, for Fe 79 Co 21 and Fe 55 Co 45 the polarization shows a sharp drop at the Curie temperature, which can be associated with the phase transition from bcc to fcc. In addition, Fe 55 Co 45 shows a hysteresis of linear polarization for rising and falling temperature. The origin is the order-disorder transition at 1000 K. Our study demonstrates that the measurement of polarization effects of the spectral thermal emission is supplementary to the standard transmission and reflection measurements and, therefore, is a valuable tool for the investigation of magneto-optical effects.

An even magneto-optic effect: Linear polarization of the spectral thermal emission from ferromagnetic iron

The thermal spectral emission from ferromagnetic iron observed at normal incidence and with the magnetization parallel to its surface is partially linearly polarized. Measurements of the new even magneto-optic effect have been performed in the spectral range from 1.8 μm to 5 μm wave-length.

Circular polarization of the spectral thermal emission from ferromagnets

An experimental study on a new magneto-optic effect, the partially circular polarization of the spectral thermal i.r. and vis emission from the ferromagnets iron, cobalt and nickel is presented. Measurements have been performed at photon energies from 0.2 to 2.0 eV and at temperatures between 600 and 1100 K. Results are correlated to parameters of the magneto-optic Kerr effect. The circular polarization is proportional to the magnetization, vanishes above the Curie temperature and changes sign at 1.3eV for iron, at 1.8eV for cobalt and at 0.3 eV for nickel. In addition, the circular polarization of the spectral thermal emission was studied at the hcp-fcc phase transition of cobalt at 700 K, where no Kerr-effect data are available. Finally, another new magneto-optic effect on the spectral thermal emission is suggested on the basis of the present investigation.

Observation of partial circular polarization of the spectral thermal emission from ferromagnetic iron

The spectral thermal emission from ferromagnetic iron is partially circular polarized. We have measured the degree of polarization in the range 0.6 μm ⩽ λ ⩽ 5.5 μm. An interpretation is given based on the principle of detailed balance (Kirchhoff's law) and within the framework of Maxwell's electrodynamic theory.

Partial circular polarization of the spectral thermal emission from ferromagnetic iron

The spectral thermal emission from a single crystal of ferromagnetic iron is partially circular polarized. The circular polarization changes sign at 1.6 eV and diminishes above the Curie temperature. This effect is related to the magneto-optic Kerr-effect.

Partial circular polarization of the spectral thermal emission from ferromagnetic iron

A partial circular polarization of the spectral thermal emission from a single crystal of ferromagnetic iron is observed. The circular polarization changes sign at a wavelength of 1 μm.

An improved measurement of the spectral thermal emission of LiF slabs in the far infrared

An improved measurement of the thickness of LiF slabs on a metallic substrate results in a quantitative agreement of the experimental angular dependence of the spectral thermal emission with the virtual mode concept of Fuchs and Kliewer.