ULTRAFAST LASERS: Femtosecond pump-probe reveals exciton formation

Researchers at Washington State University (Pullman, WA) and Los Alamos National Laboratory (Los Alamos, NM) are using ultrafast laser pulses to investigate the minute details of charge conduction as it occurs in semiconductor and optoelectronic materials. The motion of electrons in solids is normally described in terms of quantum states that extend throughout a crystal and that are referred to as Bloch waves. But to comprehend the workings of increasingly important semiconductor materials and related properties the descriptive terms must now be expanded to include the dynamics of localized electronic states and the effects of localized photoinduced states upon optical characteristics and transport properties. These material properties include metastable defects in amorphous silicon, DX centers in gallium arsenide (GaAs) and related semiconductors, and the localized excitations in organic light-emitting devices.

The Washington State and Los Alamos researchers have set about exploring these localized excitations by looking at the interplay between electronic and vibrational degrees of freedom. They are studying the formation of self-trapped excitons within the quasi-one-dimensional structure of an inorganic polymer based on alternating platinum and bromide ions.

"Low-dimensional materials, and especially quasi-one-dimensional materials, are ideal systems for studying these electron-lattice interactions," says Susan Dexheimer, assistant professor of physics and materials science at Washington State and lead author in a recently published paper on the team's work.¹ "The reduced dimensionality can lead to strong electron-phonon interactions, and the linear structure of the materials simplifies the dynamical configuration space, in that the dominant motion is expected to occur along the linear axis."

The observations were conducted using a femtosecond pump-probe technique, in which 35-fs pulses from a Ti:sapphire laser, centered at 800 nm with a 1-kHz repetition rate, excited the optical intervalence-charge-transfer (IVCT) transition of a metal-halide complex (with an onset wavelength of 800 nm and peak of 550 nm for the platinum bromide ethylenediamine complex in this experiment). The probe portion of the experiment was carried out using either a signal split off from the pump pulse or a broadband femtosecond continuum generated in a 2-mm-thick sapphire plate.

Following the IVCT transition excitation, the formation of a self-trapped exciton on a 200- to 300-fs time scale was accompanied by a strongly damped, low-frequency wave packet modulation (~110 cm^-1). The researchers also observed coherent oscillations at the ground-state vibrational frequency along with harmonics.

"What's interesting about this particular experiment is that it was done in the limiting case where our [35-fs] pulses are short compared to the characteristic [185-fs] vibrational periods of the material," Dexheimer said. "That allows you to actually time resolve the lattice motion. If you hit something with a short impulse it will ring. You can actually see vibrational oscillation that corresponds semiclassically to vibrational ringing of the system, following an impulsive excitation."

Resolving structural change in a material at the atomic level is also quite different from traditional observations of thermally activated processes, she said. "The process that we're looking at is an electronic process that is directly driven by the lattice motions, and the structural rearrangement takes place essentially as fast as is physically possible."