My watch list  

Flashes of brilliance

Roots of superfluorescent bursts from quantum wells


Kono Laboratory/Rice University

Rice University scientists detected superfluorescent bursts from a solid-state stack of quantum wells without a magnetic field. The discovery could lead to ultrahigh-speed optoelectronic devices for telecommunications.

Spontaneous bursts of light from a solid block illuminate the unusual way interacting quantum particles behave when they are driven far from equilibrium. The discovery by Rice University scientists of a way to trigger these flashes may lead to new telecommunications equipment and other devices that transmit signals at picosecond speeds.

The Rice University lab of Junichiro Kono found the flashes, which last trillionths of a second, change color as they pulse from within a solid-state block. The researchers said the phenomenon can be understood as a combination of two previously known many-body concepts: superfluorescence, as seen in atomic and molecular systems, and Fermi-edge singularities, a process known to occur in metals.

The team previously reported the first observation of superfluorescence in a solid-state system by strongly exciting semiconductor quantum wells in high magnetic fields. The new process – Fermi-edge superfluorescence – does not require them to use powerful magnets. That opens up the possibility of making compact semiconductor devices to produce picosecond pulses of light.

The results by Rice, Florida State University and Texas A&M University researchers were reported in Nature’s online journal, Scientific Reports.

The semiconducting quantum wells at the center of the experiment contain particles – in this case, a dense collection of electrons and holes – and confine them to wiggle only within the two dimensions allowed by the tiny, stacked wells, where they are subject to strong Coulomb interactions.

Previous experiments by Rice and Florida State showed the ability to create superfluorescent bursts from a stack of quantum wells excited by a laser in extreme cold and under the influence of a strong magnetic field, both of which further quenched the electrons’ motions and made an atom-like system. The basic features were essentially the same as those known for superfluorescence in atomic systems.

That was a first, but mysteries remained, especially in results obtained at low or zero magnetic fields. Kono said the team didn’t understand at the time why the wavelength of the burst changed over its 100-picosecond span. Now they do. The team included co-lead authors Timothy Noe, a Rice postdoctoral researcher, and Ji-Hee Kim, a former Rice postdoctoral researcher and now a research professor at Sungkyunkwan University in the Republic of Korea.

In the new results, the researchers not only described the mechanism by which the light’s wavelength evolves during the event (as a Fermi-edge singularity), but also managed to record it without having to travel to the National High Magnetic Field Laboratory at Florida State.

Kono said superfluorescence is a well-known many-body, or cooperative, phenomenon in atomic physics. Many-body theory gives physicists a way to understand how large numbers of interacting particles like molecules, atoms and electrons behave collectively. Superfluorescence is one example of how atoms under tight controls collaborate when triggered by an external source of energy. However, electrons and holes in semiconductors are charged particles, so they interact more strongly than atoms or molecules do.

The quantum well, as before, consisted of stacked blocks of an indium gallium arsenide compound separated by barriers of gallium arsenide. “It’s a unique, solid-state environment where many-body effects completely dominate the dynamics of the system,” Kono said.

“When a strong magnetic field is applied, electrons and holes are fully quantized – that is, constrained in their range of motion — just like electrons in atoms,” he said. “So the essential physics in the presence of a high magnetic field is quite similar to that in atomic gases. But as we decrease and eventually eliminate the magnetic field, we’re entering a regime atomic physics cannot access, where continua of electronic states, or bands, exist.”

The Kono team’s goal was to keep the particles as dense as possible at liquid helium temperatures (about -450 degrees Fahrenheit) so that their quantum states were obvious, or “quantum degenerate,” which happens when the so-called Fermi energy is much larger than the thermal energy. When pumped by a strong laser, these quantum degenerate particles gathered energy and released it as light at the Fermi edge: the energy level of the most energetic particles in the system. As the electrons and holes combined to release photons, the edge shifted to lower-energy particles and triggered more reactions until the sequence played out.

The researchers found the emitted light shifted toward the higher red wavelengths as the burst progressed.

“What’s cool about this is that we have a material, we excite it with a 150-femtosecond pulse, wait for 100 picoseconds, and all of a sudden a picosecond pulse comes out. It’s a long delay,” Kono said. “This may lead to a new method for producing picosecond pulses from a solid. We saw something essentially the same previously, but it required high magnetic fields, so there was no practical application. But now the present work demonstrates that we don’t need a magnet.”


Facts, background information, dossiers
  • Florida State University
  • Texas A&M University
  • Rice University
  • magnetic fields
  • gallium arsenide
More about Rice University
  • News

    Smaller is better for nanotube analysis

    In a great example of "less is more," Rice University scientists have developed a powerful method to analyze carbon nanotubes in solution. The researchers' variance spectroscopy technique zooms in on small regions in dilute nanotube solutions to take quick spectral snapshots. By analyzing t ... more

    Rice researchers demo solar water-splitting technology

    Rice University researchers have demonstrated an efficient new way to capture the energy from sunlight and convert it into clean, renewable energy by splitting water molecules. The technology relies on a configuration of light-activated gold nanoparticles that harvest sunlight and transfer ... more

    For 2-D boron, it's all about that base

    Scientists have theoretically determined that the properties of atom-thick sheets of boron depend on where those atoms land. Calculation of the atom-by-atom energies involved in creating a sheet of boron revealed that the metal substrate - the surface upon which two-dimensional materials ar ... more

More about Florida State University
  • News

    Nanomaterial self-assembly imaged in real time

    A team of researchers from UC San Diego, Florida State University and Pacific Northwest National Laboratories has for the first time visualized the growth of 'nanoscale' chemical complexes in real time, demonstrating that processes in liquids at the scale of one-billionth of a meter can be ... more

    Not candy crush: The nature of candy sculpture

    A team of scientists has identified the complex process by which materials are shaped and ultimately dissolved by surrounding water currents. The study, conducted by researchers at NYU's Courant Institute of Mathematical Sciences and Florida State University, appears in the Journal of Fluid ... more

    Evidence mounts for quantum criticality theory

    A new study by a team of physicists at Rice University, Zhejiang University, Los Alamos National Laboratory, Florida State University and the Max Planck Institute adds to the growing body of evidence supporting a theory that strange electronic behaviors - including high-temperature supercon ... more

More about Texas A&M University
Your browser is not current. Microsoft Internet Explorer 6.0 does not support some functions on Chemie.DE