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Thermal Memory

A material that absorbs heat from above and emits it in whichever direction you program — and remembers that choice with no power. Inspired by Osaka Metropolitan University's July 2026 breakthrough in Laser & Photonics Reviews.

GST Phase State
AMORPHOUS
↑ Emits upward · Kirchhoff reciprocal
Write State
Brief voltage switches GST phase
Power
Pulse available to write new state
Why this breaks the rules: Kirchhoff's law of thermal radiation — a 160-year-old cornerstone of physics — says any material must absorb and emit equally in the same direction (reciprocity). This device breaks that law. The magneto-optical layer, placed inside a static magnetic field, violates time-reversal symmetry — the deep reason behind Kirchhoff's rule. When the GST layer is in its crystalline phase, the combination absorbs infrared from above but re-emits it downward. The GST retains whichever phase it was last written into, even with all power removed — the same principle used in DVD-RW discs, applied to heat control.
Mechanism
Published

Laser & Photonics Reviews, June 25, 2026 — Osaka Metropolitan University, led by Prof. Koichi Okamoto & Dr. Shunsuke Murai. The paper demonstrates the first integrated magneto-optical + phase-change thermal emitter that is both non-reciprocal and non-volatile.

What is GST?

Ge₂Sb₂Te₅ (germanium-antimony-telluride) is the phase-change material in DVD-RW and Blu-ray discs. A nanosecond current pulse melts it into an amorphous state; a slower anneal crystallises it. Each state has dramatically different optical and thermal properties and is stable for years without power.

Why It Matters

Non-reciprocal thermal emitters could underpin directional radiative cooling (emit heat to space from one face, absorb solar from the other), next-generation thermophotovoltaics, smarter infrared sensors that ignore their own thermal noise, and photonic memory cells that store data as heat flow rather than charge.

The Record

Breaking Kirchhoff's law has been demonstrated before in narrow microwave bands, but always requiring active power and complex active control. This is the first passive, non-volatile implementation — once written, the state persists for days at room temperature with zero power consumption.