Physics + Math

28
Feb

Physicists Zap a Semiconductor with a Laser, Accidentally Produce a New Quasiparticle

Recently, a team of physicists was bombarding a film of gallium arsenide with a mode-locked titanium-sapphire laser. They focused the beam into an incredibly fine dot, just 100 nm, pulsed the beam for 320 femtosectonds (320 quadrillionths of a second), and waited to see what happened. You know, just another day in the lab.

Well, waiting might be an overstatement. What the team was looking for were quasiparticles known as excitons, which they found in spades. But when they cranked the frequency of the laser pulses up to 100 million per second, they stumbled upon something they never unexpected—an entirely new quasiparticle that flashed into existence for 25 millionths of a second, then vanished. They’re calling it a quantum droplet, or dropleton.

dropleton
A rendering of dropletons

The dropleton, it turns out, is an especially bizarre species of an already curious phenomenon. It’s close relative, the exciton, is formed when photons strike a semiconductor and knock an electron loose, creating a hole where the electron used to be. (That’s essentially how solar cells work. In fact, gallium arsenide, the material used in this experiment, works as a photovoltaic.)

But rather than the hole created by the loosed electron simply existing as empty space, it exerts a weak force that reaches out to the electron. If the electron stays connected with the hole through that Coulomb force, the pair form a quasiparticle known as an exciton. The team from Philipps-University of Marburg, Germany and the Joint Institute for Lab Astrophysics at the University of Colorado had created plenty of excitons.

Then they started pulsing the laser faster and faster. Above 100 million pulses per second, the electrons and holes started pooling together, forming clumps of four, five, and six electron/holes that behaved unlike anything previously studied. They swam around each other, like H2O molecules in a drop of water, and they formed ripple rings like the disturbed surface of a pond.

Clara Moskowitz, reporting for Scientific American:

The unexpected quasiparticle got its name when the researchers realized, “It has to be a new particle, it has a small size, it has liquid properties,” [Mackillo] Kira, [one of the paper’s authors,] recalls. “Okay, let’s call it a dropleton.”

“This is new physics, not just a small detail of well-established physics,” says Glenn Solomon of the Joint Quantum Institute in Gaithersburg, Md., who was not involved in the research. “Hopefully, it will spark a variety of experiments.”

Those experiments should yield some interesting results, thanks to another unique property of the dropleton—its immense size, at least for a quasiparticle. At 182 nanometers across, it’s large enough to be observed with a microscope. That could open a new window into the world where light and matter meet.