Electronic 'Crowd Behavior' Revealed In Semiconductors

Like crowds of people, microscopic particles can act in concert under the right conditions. By exposing crowd behavior at the atomic scale, scientists discover new states and properties of matter. Now, ultrafast lasers have revealed a previously unseen type of collective electronic behavior in semiconductors, which may help in the design of optoelectronic devices.

The work at JILA, a joint venture of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder, is described in a new paper in the Proceedings of the National Academy of Sciences.*

Design of optoelectronic devices, like the semiconductor diode lasers used in telecommunications, currently involves a lot of trial and error. A designer trying to use basic theory to calculate the characteristics of a new diode laser will be off by a significant amount because of subtle interactions in the semiconductor that could not be detected until recently.

To shed light on these interactions, the JILA team used a highly sensitive and increasingly popular method of manipulating laser light energy and phase (the point in time when a single light wave begins) to reveal the collective behavior of electronic particles that shift the phase of any deflected light. Their work is an adaptation of a technique that was developed years ago by other researchers to probe correlations between spinning nuclei as an indicator of molecular structure (and led to a Nobel prize).

In the latest JILA experiments, a sample made of thin layers of gallium arsenide was hit with a continuous series of three near-infrared laser pulses lasting just 100 femtoseconds each. Trillions of electronic structures called excitons were formed. Excitons are large, fluffy particles consisting of excited electrons and the "holes" they left behind as they jumped to higher-energy vibration patterns.

By tinkering with the laser tuning--the frequency and orientation of the electric field--and analyzing how the semiconductor altered the intensity and phase of the light, the researchers identified a subtle coupling between pairs of excitons with different energy levels, or electron masses. The experimental data matched advanced theoretical calculations of the electronic properties of semiconductors, confirming the importance of the collective exciton behavior--and dramatically demonstrated the superiority of those calculations over simpler models of semiconductor behavior (see graphic).

The work may help researchers better predict optoelectronic device characteristics, not only the magnitude of the emissions signals but also the phase, which is especially significant in optics.

Authors of the paper include a NIST collaborator and theorists from Philipps University in Marburg, Germany. The JILA research is supported in part by the U.S. Department of Energy.

* T. Zhang, I. Kuznetsova, T. Meier, X. Li, R.P. Mirin, P. Thomas and S.T. Cundiff. Polarization-dependent optical two-dimensional Fourier transform spectroscopy of semiconductors. Proceedings of the National Academy of Sciences. Scheduled to be posted on-line July 6.