For more than 160 years, engineers have designed around a rule that Gustav Kirchhoff set down in 1860: a surface that absorbs infrared radiation well from a given direction must emit it equally well in that same direction. That symmetry, a consequence of the Lorentz reciprocity built into Maxwell's equations, made it hard to treat the entering and the leaving of heat as two things that could be controlled separately.
A group led by Professor Koichi Okamoto and Dr. Shunsuke Murai at Osaka Metropolitan University's Graduate School of Engineering has now shown a way around it. Writing in Laser & Photonics Reviews, the international team describes a layered device that can absorb thermal energy from one direction while radiating it in another — and, unusually, does so at angles close to straight-on, where real equipment actually operates.
Two materials, one trick
The device pairs two well-studied ingredients. A waveguide layer of indium arsenide, a semiconductor whose electrons respond to a magnetic field, breaks the usual symmetry: placed in a field oriented across the direction of travel — the so-called Voigt configuration — it makes radiation moving one way couple differently from radiation moving the other. On top sits a nanoscale grating of GST, the germanium-antimony-telluride compound familiar from rewritable discs. GST can be flipped between a disordered and an ordered structural state by a brief pulse of heat, and it holds that state with no power applied.
Earlier attempts at directional heat control worked only at steep angles of 60 to 70 degrees, where absorption and emission are both weak. By engineering the grating so that near-vertical light is funnelled into guided modes, the team reported a directional contrast of about 0.90 at just three degrees from vertical under a moderate magnetic field. Switching the GST between its two states turns the effect on or off and latches it in place.
"We made heat radiation behave in a 'smarter' way," Murai said. The GST layer effectively gives the device a memory, so a chosen thermal configuration survives a loss of power — a shortcoming that had dogged earlier designs.
The researchers frame the work as an early step toward compact components that manage heat with the precision electronics bring to electric current. Okamoto said the goal is devices for "smarter infrared sensors, more efficient energy systems, and new types of photonic memory that store information using light and heat instead of electrical charges." Possible uses range from rooftop cooling panels to satellite sensors and infrared emitters.