Broadband nonreciprocal thermal emissivity and absorptivity

Area of Science:

• Physics
• Materials Science
• Optics

Background:

• Kirchhoff's law of thermal radiation states that absorptivity equals emissivity for a body in thermal equilibrium.
• Violating Kirchhoff's law is crucial for advancing photonic energy conversion and radiative cooling technologies.
• Previous demonstrations of spectral directional emissivity and absorptivity inequalities were limited to narrow resonances or non-infrared wavelengths.

Purpose of the Study:

• To demonstrate broadband nonreciprocal thermal emissivity and absorptivity.
• To achieve control over a broad spectral range of unequal spectral directional absorptivity and emissivity.
• To explore the potential for practical applications in photonic energy conversion and radiative cooling.

Main Methods:

• Utilized gradient epsilon-near-zero Indium Arsenide (InAs) layers with subwavelength thicknesses (50 nm and 150 nm).
• Applied an external magnetic field to induce nonreciprocity.
• Measured thermal emissivity and absorptivity within a specific infrared spectral range (12.5-16 μm).

Main Results:

• Demonstrated broadband nonreciprocal thermal emissivity and absorptivity.
• Achieved the effect in the 12.5-16 μm spectral range, overlapping with infrared transparency windows.
• Observed the phenomenon at moderate magnetic field strengths (1 Tesla).

Conclusions:

• The study successfully demonstrates broadband nonreciprocal thermal radiation, a significant advancement over previous narrow-band or non-infrared demonstrations.
• The use of gradient epsilon-near-zero InAs layers offers a promising pathway for practical applications requiring tailored thermal radiative properties.
• This work paves the way for enhanced performance in devices leveraging radiative heat transfer, such as advanced energy converters and cooling systems.