Measurement of broadband nonreciprocal thermal radiation

Until recently, differences in the spectral directional emissivity and absorptivity have been either narrowband [1] or shown at wavelengths well beyond the infrared regime [2]. We report nonreciprocal, broadband (12.5 mm – 16 mm) thermal radiation from gradient epsilon-near-zero (ENZ), degenerately-doped (n = 1.5 – 4.5 x 10¹⁸ cm^-3) InAs layers of subwavelength thicknesses (50 nm and 150 nm) that support a broadband Berreman mode. We measure both the spectral directional absorptivity and emissivity of the structure when a moderate transverse magnetic field (1 T) is applied and observe an opposite magnetic-field-dependent tuning of the two values across a wide angular range (5° < q < 75°). The broadband and nonreciprocal effect both rely on the ENZ condition of the individual InAs layers, meaning that the spectral window of the nonreciprocal radiation is tunable by the carrier concentrations of the InAs layers. The directionality of the emitted broadband radiation is determined by the thicknesses of the constituent layers. Using an angle-resolved thermal emission spectroscopy (ARTES) setup, we are able to experimentally compare the directional dependence of the magnetic field effect on the emissivity for varying thickness samples.



[1] K. J. Shayegan, S. Biswas, B. Zhao, S. Fan, and H. A. Atwater, “Direct observation of the violation of Kirchhoff’s law of thermal radiation,” Nat. Photon. (s41566-023-01261-6), 1-16 (2023).

[2] Y. Hadad, J. C. Soric, and A. Alu, “Breaking temporal symmetries for emission and absorption,” PNAS 113(13), 3471-3475 (2016).