Researchers from Changchun Institute of Optics, Fine Mechanics and Physics led by Prof. LI Wei have established a universal approach to realizing ultra-broadband directional thermal emitters based on effective medium theory (EMT).

In this study, directional thermal emitters with an ultra-broadband (5-30 μm) or arbitrary discrete bands (such as the two atmospheric windows) can be realized. Furthermore, they showed various designs of broadband directional thermal emitters that exhibit strong (emissivity>0.8) directional (75-85°) emission over an ultra-broad spectral range (5-30 μm). The study was published in Nanophotonics entitled “Ultra-broadband directional thermal emission”.

Thermal emission is ubiquitous and typically near-isotropic. However, surplus emission in unwanted directions can result in the waste of energy and low efficiency. Therefore, directional control of thermal emission over its broad wavelength range is a fundamental challenge. Lately, emitters based on gradient epsilon-near-zero (ENZ) materials (absorbing medium) on top of a metal reflector (Figure 1A) have been proposed to overcome this constraint. However, the bandwidth is still inherently limited due to the availability of ENZ materials covering a broad bandwidth and additional undesired omnidirectional modes in multilayer stacking with increased thickness. 

In this study, the research group developed a general approach to achieving broadband thermal emission (BDTE) based on EMT. Here, they defined a figure of merit (FOM) to describe directionality for the directional emission (Figure 1C-D). To enable strong directional thermal emission over an ultra-broad wavelength range, it is desired to regulate the permittivity throughout the intended wavelength range. Here, they utilized multiphase composite metamaterials based on effective medium theory (EMT) to flatten the permittivity of the absorbing medium in the high FOM area over the entire thermal emission wavelength range. 

For common conducting materials, there is a narrow wavelength range of BDTE in the short-wavelength range. The permittivity of such material is negative in the long-wavelength region. To expand the BDTE bandwidth, they diluted this material with dielectrics (positive permittivity) to form metamaterials as the absorbing medium based on EMT. After the dilution, a large bandwidth of strong BDTE can be obtained as mentioned above (Figure 1C-D). The target spectral regions can be shifted by adjusting the volume fractions of the materials.

By applying the theory to different combinations of materials and nanostructures, directional thermal emitters with an ultrabroad band (5-30 μm) or arbitrary discrete bands (the two atmospheric windows) have been realized with only two or three materials. Furthermore, they showed various designs of broadband directional thermal emitters that exhibit strong (emissivity>0.8) directional (75-85°) thermal emission over 5-30 μm (Figure 2).

The results indicate new opportunities for the comprehensive capture and utilization of light and heat energy, such as information encryption, energy collection and utilization, thermal camouflaging, and infrared detection. Moreover, achieving ENZ behavior in a suitable waveband introduces novel opportunities to investigate the extraordinary physical phenomena of ENZ materials, such as electromagnetic tunneling, nonlinear enhancement, optical nonreciprocity, and Casimir interaction.

This paper, entitled " Ultra-broadband directional thermal emission", is published in Nanophotonics. Congratulations to Qiuyu Wang and all co-authors!

Paper link：https://doi.org/10.1515/nanoph-2023-0742

Figure 1: Design principle of metamaterials for broadband directional thermal emission.

Figure 2: Multiple designs of ultra-broadband directional thermal emitter (material 1: conducting material; material 2: dielectric; material 3: air).