Terahertz Probes Reveal Excitonic Enhancement

Anne L. Fischer

While terahertz technology hovers between the fields of high-frequency electronics and photonics, researchers from Lawrence Berkeley National Laboratory in Berkeley, Calif., recently pushed its limits to study transient conducting and insulating phases in a gas of oppositely charged electrons and holes.

They studied many-body interactions of these electronic quasiparticles, focusing particularly on fast transitions between different phases. Electron-hole pairs can exist as unbound charges that conduct electric currents, or they can bind into insulating pairs called excitons.

The pathways between these extremes are difficult to see in the near-visible, where usually only a subset of the excitons with vanishing center-of-mass momentum is detected. Here, transitions between internal exciton levels in the far-infrared offer an alternative, detecting even excitons traveling at large momenta. So the researchers chose a terahertz probe to investigate the dynamic interplay between excitons and unbound electron-hole pairs.

The terahertz conductivity of a quasi-2-D electron-hole gas sharpens up on a picosecond timescale while excitons with well-defined internal transitions take shape. Along with a loss of low-frequency conductivity, this indicates the increasingly correlated motion as insulating excitons form out of the initially unbound, conducting carrier gas.

Robert A. Kaindl described the experiments as fundamental to understanding electron and hole interactions in semiconductors. Changes in a temporally pulsed terahertz field were detected after it had traversed an optically excited semiconductor. The researchers quantum-confined the photo-excited quasiparticles within GaAs quantum-well nanolayers and used a tunable near-infrared pump pulse with a wavelength of around 800 nm to excite the electron-hole pairs. Then they used a terahertz probe, employing free-space electro-optic sampling to measure its electric field directly in the time domain. According to Kaindl, this scheme allowed them to extract the optical response functions and their transient changes.

Although electro-optic sampling has been used for a number of years, Kaindl explained that the Berkeley setup is unique because of the combination of a self-constructed optical pump terahertz probe with a commercial 250-kHz Ti:sapphire amplifier system. He indicated that the setup offered sensitivity because of the relatively high repetition rate, yet still enough power to excite a Using the ultrafast-terahertz-probe technique, the researchers were able to track the formation of an insulating exciton gas on a 100-ps timescale. They found an unexpected quasi-instantaneous excitonic enhancement and tested the conditions under which exciton populations prevail.

Kaindl believes that this fundamental research will benefit the understanding of ultrafast dynamics in semiconductors as well as many-particle interactions in correlated-electron systems. The group will continue to explore the high excitation density limit and the phase diagrams of excitons.