Two-photon spectroscopy reveals carrier dynamics

Researchers at the US Department of Energy Lawrence Berkeley National Laboratory (LBNL; Berkeley, CA) and the department of chemistry at the University of California, Berkeley, have used femtosecond, two-photon photo emission spectroscopy to study the movements of an electron between a metal and a nonmetal. Such studies may improve understanding of carrier dynamics in optoelectronic devices such as organic light-emitting diodes.

Currently, the area of electron localization between dissimilar materials remains largely unexplored, according to Nien-Hui Ge, lead author of a recently published paper on the group`s work.¹ The research techniques, developed by the Berkeley team under the leadership of Charles Harris, are expected to be applicable to studying a wide range of phenomena, including two-dimensional magnetism, high-temperature superconductivity, and electrical conductivity. "These self-trapped carriers and the associated lattice relaxation affect a wide range of phenomena, such as photochemical defect formation, atomic desorption from solid surfaces, and various properties of high-temperature superconducting oxides," Ge wrote.

Electrons in metals usually behave like free electrons, whereas electrons in dielectric materials tend to become localized as polarons in self-induced potentials as a result of strong carrier-lattice interactions. The lattice deformation in which the electron traps itself is caused by small shifts in the positions of positively charged atomic nuclei around the negatively charged electron, Harris said.

The Berkeley researchers used a combination of angle-resolved two-photon photoemission and femtosecond-laser techniques to observe the dynamic electron transition from a delocalized to a localized state at the metal-dielectric interface (see image on p. 31). The light source for the "pump and probe" portion of the experiment was a tunable, femtosecond Ti:sapphire laser (Coherent; Santa Clara, CA). A self-modelocked oscillator was used to seed a 225-kHz regenerative amplifier producing 150-fs pulses at 800 nm. The 800-nm beam was split in an optical parametric amplifier into two beams. One beam was focused onto a sapphire crystal to generate a white-light continuum. The other beam was frequency-doubled using a beta barium borate (BBO) crystal. The white-light continuum and frequency-doubled light source were then mixed in another BBO crystal to generate a parametrically amplified pulse. The output beam was tunable over the range of 470 to 700 nm and was further compressed to yield 80-fs pulses at 600 nm.

The tunable visible output of the optical parametric amplifier was frequency-doubled to provide an ultraviolet second harmonic that was used to "pump" electrons out of a silver surface into an interface with an overlying coating of alkane molecules. The visible output of the optical parametric amplifier was also used to "probe" the excited electron out of the interface and into the vacuum, where electron kinetic energy at various angles in the relaxation process was measured by time-of-flight analysis.

Within a few hundred femtoseconds after pump pulse excitation, the interfacial electrons were observed to self-trap as polarons. They then tunneled through the adlayer potential barrier and decayed back to the metal within picoseconds. "The ability to both time- and angle-resolve the dynamics of electrons at interfaces allows a quantitative determination of the relaxation energies and lattice displacements associated with the small-polaron self-trapping process," Harris said.

REFERENCE

1. N-H. Ge et al., Science 279(9), 202 (January 9, 1998).