An orchestra without a conductor, where the musicians start playing in time thanks to the hall's shared acoustics. That's one way to picture superfluorescence: a multitude of excited nanocrystals, each of which on its own emits light randomly, suddenly synchronize and produce a single powerful, short flash. Previously, this could only be seen at very low temperatures, because heat threw the "musicians" out of tune. Now physicists have found a way to make them play in harmony at room temperature, and the key turned out to be not temperature but the shape of the light spot.

The experiment used a film of spherical CdS/CdSe/CdS quantum wells: these are nanocrystals a few nanometers in size, inside which charge carriers are confined as if in a tiny room. When a short laser pulse was directed at the film, it began to glow. But the character of the glow depended on how long a stripe was illuminated. If the stripe was long (3 mm), the light was amplified as it passed through the medium, like a voice in a long corridor, but it remained incoherent. If the stripe was shortened to 2.1 mm, signs of superfluorescence appeared: a delay before the flash, a shortening of the rise time as pumping increased, and damped oscillations after the peak, the Burnham-Chiao rings.

The main result: the superfluorescence threshold under stripe pumping turned out to be about 30 times lower than under point pumping. This means that much less energy is needed to trigger collective emission if you excite not a point but a short stripe. Why does this happen? The optical field propagating along the stripe helps the nanocrystals "feel" each other and align their phases, even when the gain is insufficient for a conventional laser effect. The orchestra played in harmony, and the conductor turned out to be the very geometry of the stage.