An ultrafast laser has revealed the existence of a new semiconductor quasiparticle, a microscopic, liquid-like, but short-lived, droplet of electrons and holes clustered together that can be created inside solid materials and have predictable behavior. The new particle is more complex than the simplest example of a quasiparticle, an exciton, which is simply a single electron and a single hole paired up through electrostatic forces.
Writing in the journal Nature, Steven Cundiff of the University of Colorado, in Boulder, Colorado, USA, and colleagues there and at Philipps-University Marburg, Germany, have identified an unpaired arrangement of electrons and holes, which they refer to as a “dropleton”. The entity has well-ordered energy levels but also has some of the characteristics of a liquid, such as forming ripples. The liquid-like behavior is manifest only below a finite size above which the association between electrons and holes disappears [Almand-Hunter, et al. Nature 506 (2014) 471–475].
“Electron–hole droplets are known in semiconductors, but they usually contain thousands to millions of electrons and holes,” explains Cundiff. “Here we are talking about droplets with around five electrons and five holes.” The quasiparticle revealed by Cundiff and colleagues exists for a mere 25 picoseconds but that is long enough for the team to investigate how light interacts with such novel forms of matter and highly correlated states of matter. Cundiff concedes that nobody is going to build a quantum droplet widget. “Nevertheless, this does have indirect benefits in terms of improving our understanding of how electrons interact in various situations, including in optoelectronic devices,” he suggests.
The new quasiparticle was created by exciting a gallium-arsenide semiconductor with an ultrafast red laser, pulsing 100 million times per second. “The pulses generate electrons and holes, and properly correlated photons directly generate dropletons above well-defined threshold intensities, which makes dropletons extremely sensitive to quantum-optical fluctuations of the light,” Marburg's Mackillo Kira told Materials Today.
The Colorado team's experimental data on energy levels of individual droplet rings pairs well with the theoretical many-body calculations carried out by the Marburg team showing that each energy level within the quantum droplet could be accessed by tweaking the quantum-optical correlations of laser pulses to match dropleton's particle correlations.
“We are particularly interested to study the quantum emission properties of dropletons,” Marburg's Kira told us. “In this connection, it is interesting to study how long and well quantum-optical fluctuations are stored to and retrieved from dropletons. This information helps us to understand how to entangle photons with large many-body systems with the intention to develop solid-state-based quantum-information protocols. Since dropletons are also relatively large, we intend to image them directly, which should complement the time resolved spectral information we have collected so far.”