Thermal radiation describes the emission of electromagnetic waves from hot objects. Although the basics of thermal radiation have been well understood for more than a century, engineering of thermal radiation is an active research field, in part because of applications to energy harvesting, lighting, and thermoregulation. The rapidly evolving research base sits at the intersection of materials science, photonics, and thermal physics. In eight research papers and one opinion paper, this feature issue of Optical Materials Express advances the multidisciplinary field of engineering of thermal light.

Every object radiates energy in the form of electromagnetic waves, typically referred to as thermal emission or thermal radiation. The radiation comes from the thermally excited movement of charged particles inside materials. Thermal emission from an ideal emitter (i.e., a “blackbody”) is described by Planck’s law [1]. For non-blackbody thermal emitters, their emission power is the product of the Planck blackbody radiation spectrum and their emissivity, with the emissivity being equal to the emitter’s optical absorptivity, as described by Kirchhoff’s law [2]. Blackbody radiation is broadband, unpolarized, and is emitted in all directions; however, the relationship between the absorptivity and emissivity of an object provides a route to alter the bandwidth, polarization, and directionality of thermal emission with respect to standard blackbody radiation. During the past two decades, there has been a flourishing of research on engineering of thermal light via nanophotonic approaches and/or materials engineering, which has enabled many applications, including energy harvesting, lighting, thermoregulation, and imaging [3], [4].

This feature issue focuses on optical materials and devices for narrowband, directional, polarized, and dynamically reconfigurable thermal emission. Nine papers (two invited and seven contributed) were accepted for publication, with eight research papers and one opinion paper. Two papers are related to advanced mid-infrared light sources, focusing on controlled spectral bandwidth and angular distribution [5], as well as the polarization state [6]. Two papers are about thermal management, with one focusing on radiative cooling [7] and one on thermal homeostasis [8]. Three papers are related to the topic of active modulation of thermal emission, including one study on the dynamics of phase-change materials for mid-infrared applications [9], one study on magnetic-field-tunable thermal emitters [10], and one study for optimizing the efficiency of fast modulated incandescent light sources [11]. One paper discusses the use of metalenses for infrared imaging [12], and then the feature issue is concluded with an opinion paper about thermal emission from materials with broken symmetries [13]. These nine papers represent a wide range of topics and the exciting potential of thermal-emission engineering.

Two papers of this feature issue are about controlling the bandwidth, directionality, and polarization of thermal emission. In [5], Blanchard et al. introduced and experimentally demonstrated a metallo-dielectric metasurface that controls the central emission wavelength, the spectral bandwidth, the peak emissivity value, as well as the angular distribution of thermal radiation. The emitter, comprising silicon, silica, and platinum, can sustain high temperatures. Besides bandwidth and directionality, the polarization of thermal emission can also be controlled. However, emission of circularly polarized thermal light has remained a challenge. In [6], Wang et al. theoretically explored the emission of high-purity circularly polarized thermal light from a planar slab made of magnetic Weyl semimetals (MWS). For a representative MWS, it is numerically shown that strong circularly polarized thermal emission can be realized over 6 to 14 µm with reasonably large emissivity (> 0.3) and a large coverage of the emission solid angle (75%). The circularly polarized thermal emission arises from the strong infrared gyrotropy or nonreciprocity of magnetic Weyl semimetals.

Thermal management is an important application of thermal-emission engineering. In [7], Zhang et al. used artificial neural networks to identify the optimal preparation conditions of porous polymer coatings for daytime radiative cooling. They experimentally demonstrated near-unity solar reflectance and mid-IR emittance (defined as the weighted average emissivity by the blackbody radiation at room temperature over 8 to 13 µm), resulting in cooling below the ambient temperature by up to 6 °C during daytime. They also extended their experiments to extreme environmental conditions that simulate space applications. Beyond engineering static reflectance and emissivity, engineering the dynamics and temperature-dependence of the emissivity can be used to enable thermal regulation. In an invited research paper [8], Shrewsbury et al. leveraged the phase transition of vanadium dioxide (VO2) to propose a lithography-free design for thermal homeostasis (i.e., maintaining a temperature that is insensitive to changes in the environment). Via numerical simulations, their multilayer thin film device demonstrated substantial tunability in total emitted thermal-emission power between temperatures where VO2 was in insulating and metallic phases, thus obtaining good temperature-stabilization performance.

Three papers in our feature issue are related to the modulation of thermal emission. In [9], Hafermann et al. studied how ion irradiation can be used to modify disorder in crystalline Ge2Sb2Te5 (GST) thin films. GST is a phase-change material that features large changes in its electrical and optical properties, which can be used for modulation of thermal emission. In this work, the thresholds of Ar+ -ion irradiation at room temperature for complete amorphization for two different types of crystalline GST (rock-salt and hexagonal) were experimentally identified and a recovery time scale of seconds or less were found for the defects in GST at room temperature. In [10], Caratenuto et al. designed a tunable metamaterial thermal emitter for the terahertz region. The tunability is made possible by including indium antimonide in their emitter design, whose optical properties change by applying a DC magnetic field. The authors numerically demonstrated narrowband thermal emission with a peak value close to one at a wavelength of ∼55 µm, with the bandwidth and central wavelength tunable via an external magnetic field. In [11], Nguyen et al. investigated the trade-off between fast modulation and efficiency of incandescent sources. Faster modulation of temperature can be realized in nanoscale emitters that are in contact with a cold substrate, but heat conduction to the substrate is detrimental to the efficiency of the thermal light source. To investigate this issue, Nguyen et al. developed a theoretical model based on the local Kirchhoff law and the derivation of the spatial and temporal dependence of the temperature field to model the efficiency of a modulated incandescent source. Their findings show that several parameters can be adjusted to boost the efficiency at high modulation frequencies.

In [12], Huang et al. demonstrated meta-optical elements that enable imaging in the long-wave infrared (LWIR) under ambient thermal radiation. Even with strongly chromatic meta-optics, they showed that it is possible to image in ambient light, including objects with different temperatures. Because meta-optical elements may be thin and lightweight compared to their classical optics counterparts, this technology may be relevant for wearable imaging devices and imagers in flight, such as on drones or satellites. The special feature issue concludes with an invited opinion paper about thermal emission from materials that break inversion symmetry and/or time-reversal symmetry [13]. In this paper, Pajovic et al. compared radiative transport phenomena in systems such as Weyl semimetals, oxide perovskites, alkali-metal chalcogenides, and narrow-bandgap semiconductors under strain or external magnetic fields. These interesting phenomena of thermal radiation in symmetry-breaking systems could lead to new methods of thermal management and object manipulation at short length scales.

The field of thermal-emission engineering relies on advances in materials and new ways to tailor material properties. Motivated by its relevance to many energy applications, this field is expected to further grow. It is the hope of the editorial team that this feature issue will inspire new fundamental research and applications of the engineering of thermal light. Finally, the editorial team would like to thank all the authors and anonymous reviewers for making this feature issue possible, and the Optical Materials Express journal staff for their support.

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5. C. Blanchard, L. Wokszvzyk, C. Jamois, J.-L. Leclercq, C. Chevalier, L. Ferrier, P. Viktorovitch, I. Moldovan-Doyen, F. Marquier, J. J. Greffet, and X. Letartre, “Metallo-dielectric metasurfaces for thermal emission with controlled spectral bandwidth and angular aperture,” Opt. Mater. Express 12(1), 1–12 (2022) [CrossRef]  

6. Y. Wang, C. Khandekar, X. Gao, T. Li, D. Jiao, and Z. Jacob, “Broadband circularly polarized thermal radiation from magnetic Weyl semimetals,” Opt. Mater. Express 11(11), 3880 (2021) [CrossRef]  

7. D. Fan, H. Zhang, J. Huang, and P. Tie, “Inverse design, fabrication, and tolerance to extreme environments of radiative cooling coating,” Opt. Mater. Express 11(11), 3706–3716 (2021) [CrossRef]  

8. B. K. Shrewsbury, A. M. Morsy, and M. L. Povinelli, “Multilayer planar structure for optimized passive thermal homeostasis,” Opt. Mater. Express 12(4), 1442–1449 (2022). [CrossRef]  

9. M. Hafermann, R. Schock, C. Wan, J. Rensberg, M. A. Kats, and C. Ronning, “Fast recovery of ion-irradiation-induced defects in Ge2Sb2Te5 thin films at room temperature,” Opt. Mater. Express 11(10), 3535–3545 (2021) [CrossRef]  

10. A. Caratenuto, F. Chen, Y. Tian, M. Antezza, G. Xiao, and Y. Zheng, “Magnetic field-induced emissivity tuning of InSb-based metamaterials in the terahertz frequency regime,” Opt. Mater. Express 11(9), 3141–3153 (2021) [CrossRef]  

11. A. Nguyen and J.-J. Greffet, “Efficiency optimization of mid-infrared incandescent sources with time-varying temperature,” Opt. Mater. Express 12(1), 225–239 (2022) [CrossRef]  

12. L. Huang, Z. Coppens, K. Hallman, Z. Han, K. F. Bohringer, N. Akozbek, A. Raman, and A. Majumdar, “Long wavelength infrared imaging under ambient thermal radiation via an all-silicon metalens,” Opt. Mater. Express 11(9), 2907–2914 (2021) [CrossRef]  

13. S. Pajovic, Y. Tsurimaki, X. Qian, X. Qian, and S. V. Boriskina, “Radiative heat and momentum transfer from materials with broken symmetries: opinion,” Opt. Mater. Express 11(9), 3125–3131 (2021) [CrossRef]