Metamaterial Breaks Thermal Symmetry, Enables One-Way Heat Emission

Researchers have found that a metamaterial, a stack of InGaAs semiconductor layers, can emit significantly more mid-infrared radiation than it absorbs. When this sample was heated (~540 K) in a 5-tesla magnetic field, it exhibited a record nonreciprocity of 0.43 (about twice the previous best). In other words, it strongly violates Kirchhoff's law and forces heat to flow one way. This demonstration of strong nonreciprocal thermal emission could enable devices like one-way thermal diodes and improve technologies like solar thermophotovoltaics and heat management.

A Metamaterial Breakthrough

According to the published study, the new device is made from five ultra-thin layers of a semiconductor called indium gallium arsenide, each 440 nanometers thick. The layers were gradually doped with more electrons as they went deeper and were placed on a silicon base. The researchers then heated the material to about 512°F and applied a strong magnetic field of 5 teslas. Under these conditions, the material emitted 43% more infrared light in one direction than it absorbed—a strong sign of nonreciprocity. This effect was about twice as strong as in earlier studies and worked across many angles and infrared wavelengths (13 to 23 microns).

By providing a one-way flow of heat, the metamaterial would serve as a thermal transistor or diode. It could enhance solar thermophotovoltaics by sending waste heat to energy-harvesting cells and aid in controlling heat in sensing and electronics. It has potential implications for energy harvesting, thermal control, and new heat devices

Challenging Thermal Symmetry

Kirchhoff's law of thermal radiation (1860) states that at thermal equilibrium, a material's emissivity equals its absorptivity at each wavelength and angle. Practically, this reciprocity means a surface that strongly emits infrared will absorb it equally well.

Breaking this symmetry requires violating time-reversal symmetry, such as by applying a magnetic field to a magneto-optical material. For example, a 2023 study showed that a single layer of indium arsenide (InAs) in a ~1 T magnetic field could produce nonreciprocal thermal emission. However, that effect was extremely weak and worked only at specific wavelengths and angles. Till now, magneto-optical designs have achieved only tiny emission–absorption imbalances under very restrictive conditions. The new achievement demonstrates that man-made materials can produce one-way thermal emitters.