June 16, 2003
Berkeley Lab Science Beat Berkeley Lab Science Beat
Capturing Excitons
in Transition
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A team of scientists at Lawrence Berkeley National Laboratory (Berkeley Lab), using simultaneous ultrashort flashes of near-infrared and far-infrared terahertz (trillion cycles per second) electromagnetic radiation, created and observed electron-hole gases in semiconductor quantum wells as they changed from electrical insulators to conductors and back to insulators again.

The results demonstrate that semiconductor nanostructures, whose dimensions are measured in billionths of a meter, are an ideal means for learning more about Coulomb interactions in many-body systems -- those made up of multiple interacting particles -- which lie at the heart of all our technology in electronics and chemistry.

"We have developed a unique new way to study the dynamics of excitons and unbound electron-hole pairs in semiconductors and got some surprising results," says Daniel Chemla, director of Berkeley Lab's Advanced Light Source and the physicist who led the research. "Our findings open up a whole new set of questions about Coulomb interactions in many-body systems, in particular in nanostructures."

Daniel Chemla, Mark Carnahan, and Robert Kaindl examine their experimental set-up, incorporating more than 100 mirrors. Each is computer controlled so that the entire assembly can quickly be brought into precise alignment.

Collaborating with Chemla on this experiment were Robert Kaindl and Marc Carnahan, who are with Berkeley Lab's Materials Sciences Division, and Daniel Hägele and Reinhold Lövenich, visiting researchers who have since returned to Germany. The Berkeley team members are also affiliated with the Physics Department of the University of California at Berkeley. Results of this research, which was funded by the U.S. Department of Energy's Office of Basic Energy Science, were published in the June 12, 2003 issue of the journal Nature.

In a semiconductor crystal, absorption of visible light "photoexcites" an electron into a higher energy space. The electron is moved into the conduction band, up from the valence band where it is tightly bound to the parent atom, and leaves behind a vacancy or "hole" that acts like a positively charged particle. Electron and hole -- being oppositely charged -- attract each other via the same Coulomb force that stabilizes the shapes of atoms or molecules.

"If created by a sufficiently high-energy photon, this electron-hole pair is unbound and can carry an electric current as its constituents freely move around the crystal," explains Chemla. "Under the right conditions, however, new quantum states called excitons can form, with an electron and hole bound to each other. Owing to its local charge neutrality, an exciton gas behaves as an insulator while a collection of unbound electron-hole pairs form a conducting plasma. The transition from electron-hole plasma to exciton gas is a typical metal-insulator transition."

The internal energy levels of excitons are analogous to those of a hydrogen atom, but the binding energy of excitons is about 1,000 times smaller, as is the spacing of their energy levels. Until now, experimental investigations of exciton dynamics have been indirect, as they probed excitons at photon energies much higher than their level spacing.

Departing from this script, the Berkeley Lab researchers used near-IR laser light to create the electron-hole pairs, then analyzed the results with picosecond (trillionth of a second) pulses of far-infrared, terahertz radiation. At a photon energy of about 4 meV (milli electron-volts), terahertz radiation is the ideal light for observing the internal electromagnetic transitions of excitons and unbound electron-hole gases.

"Working with semiconductors, experiments can be done in nanostructures of exceptional purity and controlled growth," says Kaindl. "Conditions including temperature, electron density, and magnetic fields can be varied over many orders of magnitude, and the dimensionality can be custom tailored so we can create Coulomb interactions of varying strengths."

Temporal snapshots of the correlated motion between electrons and holes in a semiconductor nanostructure: terahertz (THz) conductivity sharpens up on a picosecond time scale as excitons take shape.  

Kaindl explains that "our experiment, in accessing the response to terahertz radiation, gives us information about whether the constituents in the electron-hole soup have paired up into an insulating state -- such that their mutual strong attractive binding into excitons makes them virtually immune to the small terahertz electric field -- or, alternatively, whether they are still loosely associated in unbound states such that the external terahertz field can induce an electric current. By using ultrashort pulses, we can obtain a new view on the dynamic evolution of the many particle system between these two extreme situations."

Through the combination of two-dimensional confinement in nanosized quantum wells of undoped gallium arsenide, and tunable near-infrared photoexcitation light from a titanium-doped sapphire laser, the Berkeley Lab researchers were able to create, with a great degree of control, a plasma of electron and hole pairs.

They then probed this plasma with picosecond pulses of terahertz radiation. Generating both the pump and the probe light from the same source enabled them to control very precisely the timing of events and access the transition dynamics with excellent accuracy.

"Insulators and conductors live in two different worlds of experimental apparatus, so usually scientists are looking at one or the other but not both," said Chemla. "Ours is a unique experimental set-up and allows us to study Coulomb interactions without the traditional need for an electrical contact."

The experimental set-up, which was configured by Kaindl, features an elaborate maze of about one hundred mirrors and other optics, precisely positioned to deliver both the pump and the probe infrared pulses at a desired length and frequency. Using picosecond pulse lengths, the researchers were able to obtain a series of snapshot images showing the dynamic evolution of the Coulomb interactions they generated.

Says Kaindl, "What we observed -- unexpectedly -- is a Coulomb correlation enhancement at zero time delay after the excitation pulse has just created a plasma of unbound electron-hole pairs. While the overall formation of excitons takes place over a period of several hundred picoseconds, this enhancement appears almost instantaneously and is related to the extremely fast Coulomb interactions between the vast number of constituents of the electron-hole plasma."

Adds Chemla, "In a normal metal, the density of charged particles is so large that the Coulomb interaction is effectively screened. The transient phase we observed seems to be something like a mixture of bound and unbound electron-hole pairs. For now we are calling it 'strange metal,' but we don't know what it really is yet."

While they expect their results will keep the theorists busy trying to offer an explanation of this intermediate state during the formation of excitons, the Berkeley researchers believe that the responses to terahertz radiation they've demonstrated should pave the way for future studies of Coulomb interactions in multibody systems that have remained elusive to visible and near-infrared optics.