Non-technical abstract: Quasiparticles are collections of particles, such as atoms or electrons in a solid, that act together as if they were a single composite particle. Our understanding of many of the materials that are important in modern day devices depends on the idea of quasiparticles and we have a good understanding of the behavior of quasiparticles once they have been created. However, our understanding of the process of creation of quasiparticles, which plays a critical role in determining the properties of a wide range of materials, is much more limited. The polaron is one example of a quasiparticle, formed from localized charge carriers, which has a profound impact on the conductivity, optical properties and energy transport in modern semiconductor materials. This project will utilize a variety of experimental probes to study the dynamics of quasiparticle formation and the influence of electronic, vibrational, and structural dynamics. This project supports the work of two Ph.D. students and an undergraduate student. The combination of basic research involving fundamental issues in condensed matter physics with practical issues in experiment development provides an excellent training ground; students working in ultrafast spectroscopy of electronic materials are well-prepared to enter positions in academia, government laboratories, and high-tech industry, given their background in both basic and applied work.
Localized quasiparticle states play a critical role in determining the properties of a wide range of materials: polaron formation has a profound impact on the charge transport properties of electronic materials, and formation of self-trapped excitons, or exciton-polarons, dramatically changes optical properties and energy transport mechanisms. While the equilibrium properties of quasiparticles are well-established in many systems, a full understanding of the dynamics of the process of quasiparticle formation has yet to be achieved. In addition to its fundamental significance, such knowledge promises a means to understand, and thereby exploit, the fast electronic and optical response of materials. This project is comprised of systematic studies of the coupled electronic and vibrational dynamics inherent to quasiparticle formation. The studies are carried out in quasi-one-dimensional materials in which the relative strengths of the electron-electron and electron-phonon interactions that drive the localization dynamics can be systematically tuned via their composition. The studies use a combination of femtosecond time-resolved techniques that are sensitive to electronic, vibrational, and structural dynamics, allowing observation of changes in the electronic charge distribution and local structure as the system evolves to form the quasiparticle state, and the role of specific vibrational modes in creating the final stabilized structure. Experimental studies are complemented by theoretical modeling.
