Susan L. Dexheimer

Terahertz spectroscopy : principles and applications

Preface Instrumentation and Methods Terahertz Time-Domain Spectroscopy with Photoconductive Antennas R. Alan Cheville Nonlinear Optical Techniques for Terahertz Pulse Generation and Detection-Optical Rectification and Electrooptic Sampling Ingrid Wilke and Suranjana Sengupta Time-Resolved Terahertz Spectroscopy and Terahertz Emission Spectroscopy Jason B. Baxter and Charles A. Schmuttenmaer Applications in Physics and Materials Science Time-Resolved Terahertz Studies of Carrier Dynamics in Semiconductors, Superconductors, and Strongly Correlated Electron Materials Robert A. Kaindl and Richard D. Averitt Time-Resolved Terahertz Studies of Conductivity Processes in Novel Electronic Materials Jie Shan and Susan L. Dexheimer Optical Response of Semiconductor Nanostructures in Terahertz Fields Generated by Electrostatic Free-Electron Lasers Sam Carter, John Cerne, and Mark S. Sherwin Applications in Chemistry and Biomedicine Terahertz Spectroscopy of Biomolecules Edwin J. Heilweil and David F. Plusquellic Pharmaceutical and Security Applications of Terahertz Spectroscopy J. Axel Zeitler, Thomas Rades, and Philip F. Taday Index

Femtosecond dynamics of exciton localization: self-trapping from the small to the large polaron limit

We use femtosecond vibrational wavepacket techniques to time-resolve the coupled electronic and vibrational dynamics of exciton self-trapping in a series of materials in which the relative strength of the electron-phonon coupling can be compositionally tuned from the small to the large polaron limit. Transient absorption experiments are carried out in the quasi-one-dimensional halide-bridged mixed-valence transition metal linear chain complexes [Pt(en)2][Pt(en)2X2]⋅(ClO4)4 (en=ethylenediamine, C2H8N2) with X=Cl, Br and I. In each complex, we detect the formation of the self-trapped exciton through the appearance of its characteristic red-shifted optical absorption, and find that self-trapping occurs on a time scale of the order of a single vibrational period of the optical phonon mode that dominates the self-trapping dynamics. The associated optical phonon response, detected as wavepacket oscillations that modulate the exciton absorption, shows a significant softening of the optical phonon frequency compared to that of the unexcited system. The degree of softening is found to vary significantly with coupling strength, ranging from more than 40% in the strongly coupled chloride-bridged complex to less than 20% in the weakly coupled iodide-bridged complex. We relate these results to the extent of electronic delocalization by comparison with the electronic properties of the ground states of the materials and with the properties of their equilibrated self-trapped electronic states predicted by theoretical modeling.

Ultrafast Carrier Thermalization in Hydrogenated Amorphous Silicon

We present detailed studies of the initial relaxation processes of photoexcited carriers in hydrogenated amorphous silicon. We have carried out time-resolved measurements of the photoexcited carrier response in HWCVD a-Si:H thin films using a wavelength-resolved femtosecond pump-probe technique, in which an intense 35-fs pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in optical properties as a function of time delay following the pump pulse. Measurements of the transient optical absorbance were carried out as a function of the density of excited carriers, sample temperature, and probe wavelength. These studies indicate fast carrier thermalization via phonon emission on a ∼ 150 fs time scale and rapid phonon equilibration on a ∼ 230 fs time scale.

Femtosecond Far-Infrared Studies of Photoconductivity in a-Si:H and a-SiGe:H

We present femtosecond time-resolved studies of the frequency-dependent photoconductivity in the far-infrared spectral range (∼ 1 – 10 meV) in PECVD a-SiGe:H and a-Si:H thin films. The experiments are carried out using an optical pump / terahertz (THz) probe technique, in which a femtosecond pump pulse excites carriers into the extended states and a time-delayed probe pulse measures the resulting change in the far-infrared optical properties, which are directly related to the ac photoconductivity, as the carrier distribution evolves in time. We find that the frequency-dependent conductivity measured on picosecond time scales shows a strongly non-Drude behavior, with components of the response fitting to a power-law frequency dependence, reflecting processes associated with localized states.

Ultrafast dynamics of excitonic self-trapping

Summary form only given. The localization of electronic excitations via electron-lattice interactions is an important process in a wide range of condensed matter systems, and has a dramatic impact on the optical and transport properties of materials. In this work, we have studied the dynamics of the formation of localized excitations by using femtosecond coherent phonon techniques to directly time-resolve the lattice motions associated with the localization process. These experiments have been carried out on a series of materials that have allowed us to systematically vary physical properties that directly influence the localization dynamics, including the strength of the electron-phonon coupling and the local geometrical structure.

Ultrafast dynamics of excitonic self-trapping: The role of the electron-phonon interaction

The ultrafast electronic and vibrational dynamics of the exciton self-trapping process in quasi-one-dimensional systems are investigated as a function of the strength of the electron - phonon coupling using femtosecond vibrationally impulsive excitation techniques.

Observation of structural relaxation during exciton self-trapping via excited-state resonant impulsive stimulated Raman spectroscopy

We detect the change in vibrational frequency associated with the transition from a delocalized to a localized electronic state using femtosecond vibrational wavepacket techniques. The experiments are carried out in the mixed-valence linear chain material [Pt(en)2][Pt(en)2Cl2]⋅(ClO4)4 (en = ethylenediamine, C2H8N2), a quasi-one-dimensional system with strong electron-phonon coupling. Vibrational spectroscopy of the equilibrated self-trapped exciton is carried out using a multiple pulse excitation technique: an initial pump pulse creates a population of delocalized excitons that self-trap and equilibrate, and a time-delayed second pump pulse tuned to the red-shifted absorption band of the self-trapped exciton impulsively excites vibrational wavepacket oscillations at the characteristic vibrational frequencies of the equilibrated self-trapped exciton state by the resonant impulsive stimulated Raman mechanism, acting on the excited state. The measurements yield oscillations at a frequency of 160 cm(-1) corresponding to a Raman-active mode of the equilibrated self-trapped exciton with Pt-Cl stretching character. The 160 cm(-1) frequency is shifted from the previously observed wavepacket frequency of 185 cm(-1) associated with the initially generated exciton and from the 312 cm(-1) Raman-active symmetric stretching mode of the ground electronic state. We relate the frequency shifts to the changes in charge distribution and local structure that create the potential that stabilizes the self-trapped state.

Exciton localization probed via excited-state resonant impulsive stimulated Raman spectroscopy

We probe the transition from a delocalized to a localized electronic state in a quasi-one-dimensional system by the change in vibrational frequency detected by resonant impulsive Raman excitation of the excited state in a pump-pump-probe measurement.

Ultrafast Dispersive Transport in a-SiGe:H

Carrier dynamics in a-SiGe:H are studied using femtosecond optical techniques. The response reveals dispersive transport with a time-dependent mobility that varies systematically with temperature and with the degree of disorder in the material.

Coherent phonon dynamics in exciton self-trapping in the strong coupling limit

We probe the coupled electronic and lattice dynamics of exciton self-trapping in a strongly coupled quasi-one-dimensional system. The coherent phonon response reveals both optical and acoustic phonon contributions to the localization dynamics.