Steve Cundiff and his group investigate the interaction of ultrashort laser pulses with semiconductors. They probe the fundamental physical processes associated with the interaction of light with these materials. Their goal is to foster the design of better optoelectronic semiconductor-based devices. The study of exciton scattering in semiconductor quantum wells is part of this research. Excitons are very large, fluffy, and strongly interactive particles formed when an electron, excited by a photon into the conduction band of a semiconductor, binds with the positively charged hole it left behind in the valence band. Quantum wells are thin layers between two confining energy barriers. The layers are thin enough that the motion perpendicular to the layer is quantized.

Cundiff's group studies coherence in direct-gap semiconductors such as GaAs with pump-probe, transient-four-wave mixing, and pulse propagation experiments. Most matter that has been resonantly excited by coherent laser light will remember the phase of the incident light for a period of time known as the dephasing time. For many types of matter, the dephasing time is tens of nanoseconds or longer. However, in a typical semiconductor, the dephasing time is usually less than a picosecond. His group has shown that this unusual response to optical excitation arises from carriers that are in extended states (i.e., their wave functions are spread over many lattice sites, not just a single atom). As a result, excitons interact strongly with each other, exhibiting interactive behavior not present in isolated atoms and molecules. This behavior is responsible for the dramatic difference in dephasing times between semiconductors and other materials.

The researchers have recently developed an improved technique that reveals previously hidden electronic interactions in semiconductors. By measuring the correlation between the phase of incident light and that of the emitted light, it is possible to observe coupling among excitons. The technique provides a unique and powerful probe into the underlying many-body physics. In related research, the Cundiff group is developing experiments to compare the many-body interactions of excitons in semiconductors with the interactions of excitons present in dense atomic vapors.

Cundiff's group is exploring optical techniques to better understand electron spin coherence. (Electron spin is a weak magnetic energy state, characterized as "spin up" and "spin down.") Spin coherence depends on the density of optically excited electrons, which are initially spin polarized. The researchers have discovered that the time it takes for spin coherence to be lost decreases at high excitation density because of increases in spin-spin scattering among electrons. They are currently studying factors, including varying amounts of doping, that influence the decrease in spin coherence time. Eventually, they want to determine the feasibility of using the spin degree of freedom of electrons in semiconductors to improve "spintronic" devices, which are currently quite primitive. (Spintronic devices exploit the quantum propensity of electrons to spin as well as making use of their charge state.)

Capitalizing on his understanding of the interaction of light with semiconductors, Cundiff and his group are now using these materials to investigate properties of optical fibers, integrated circuits, and mode-locked lasers. They recently developed a solid-state laser technique that should work well for measuring the carrier-envelope phase of direct currents.

The researchers are also investigating second harmonic generation, a nonlinear optical process typically used to combine photons interacting with a nonlinear material to form new photons with twice the energy. They want to know whether this process can be used to probe symmetry properties in complex materials such as high-temperature superconductors.