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Inside semiconductors, a strange phenomenon emerges

Using high-intensity ultrafast laser beams, scientists have discovered liquid-like behavior inside solid semiconductors.

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Baxley/JILA
This is an artist's conception of the microscopic "quantum droplet" discovered by JILA physicists in a gallium-arsenide semiconductor excited by an ultra-fast red laser pulse. Each droplet consists of electrons and holes (representing absent electrons) arranged in a liquid-like pattern of rings. The surrounding area is plasma. The discovery adds to understanding of how electrons interact in optoelectronic devices.

Inside the semiconductors of many common electronics lurks a bizarre quantum phenomenon. 

It exists firmly inside solid matter, but it behaves as though it were a liquid, rippling and undulating. It's not exactly a particle, but a 'quasiparticle,' a collection of individual particles that act as though they were one.  

According to a paper titled "Quantum droplets of electrons and holes" published in the Feb. 27 issue of Nature, this quasiparticle, which the authors have dubbed 'dropleton,'  emerges from the condensation of smaller particles within the semiconductor. It has a lifetime of just 25 trillionths of a second.

But what, exactly, is it made of?

As the name of the paper suggests, these 'droplets' are made up of electrons and holes. 

Steven Cundiff, a physicist at JILA,  joint physics institute of the University of Colorado at Boulder and the National Institute of Standards and Technology and an author of the paper, compared the so-called holes to water bubbles. Holes are spots within a semiconductor that lack electrons, he says.

A semiconductor consists of two bands – a conduction band and a valence band. The electrons are present in the valence band and the conduction band is empty. But when light is shone on semiconductors, electrons jump from the valence band to the conductor band, leaving holes in the former.

The pairing (brought about by electrostatic attraction) that takes place between electrons and the holes form "excitons." 

The researchers discovered the dropleton after they passed high-intensity red laser beam emitting about 100 million pulses per second through a gallium-arsenide semiconductor, such as the kind found in LEDs and in some cellphones. The pulses initially formed excitons, but with an increase in the intensity of the light, even more electron-hole pairs were formed, resulting in the formation of this "new stable configuration of charged particles," Dr. Cundiff told the Monitor. 

The particles, which carry no electric charge, are so small that they behave according to the violently counterintuitive rules of quantum mechanics.

Unlike molecules within solids, the electrons and holes within the dropleton do not stay put; instead they float within the boundaries of the semiconductor.

But they are also different from particles inside liquids, in that they have a size limit, beyond which electron-hole pairs cease to exist.

In addition to understanding how light interacts with semiconductors, this study will also help researchers learn more about how electrons interact with solids.

"Regarding practical benefits, nobody is going to build a quantum droplet widget. But this does have indirect benefits in terms of improving our understanding of how electrons interact in various situations, including in optoelectronic devices," Cundiff  said in a press release. 

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