An orchestra without a conductor, where the musicians start playing in sync thanks to the shared acoustics of the hall. That is one way to picture superfluorescence: a multitude of excited nanocrystals, each of which on its own glows at random, suddenly synchronize and produce a single powerful, short burst. 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 together 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: 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 burst, a shortening of the rise time as the pump increased, and damped oscillations after the peak, known as 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 ordinary lasing.
Previously it was unclear how the spatial geometry of excitation affects the competition between superfluorescence and amplified spontaneous emission. Now it has been shown that shortening the excited stripe switches the dominant process from amplified spontaneous emission to superfluorescence. This opens a path to low-threshold coherent light sources based on colloidal nanocrystals that operate at room temperature. The orchestra has started to play in sync, and the conductor turned out to be the geometry of the stage itself.
The Burnham–Chiao rings observed in the experiment are named after the physicists who predicted them in 1973 as intensity oscillations after a superfluorescence burst. These oscillations arise from the exchange of energy between the radiation and the collective polarization, similar to the damped oscillations of a pendulum.