Research Overview

There are several independent but related directions in our work. THz spectroscopy is the tie that binds them all together.

Time Resolved THz Spectroscopy (TRTS)

A substantial fraction of our effort goes into TRTS, which entails photoexciting a sample with a visible excitation pulse, and then probing the subsequent dynamics with a THz pulse. The sample could be a dye/solvent system, a semiconductor wafer, sintered colloidal dye-sensitized TiO₂, or a dispersion of CdSe quantum dots. The frequency-dependent, time-resolved information that is obtained can not be acquired with any other experimental technique.

THz Time Domain Spectroscopy (THz-TDS)

THz-TDS experiments involve measuring the frequency-dependent absorption coefficient and index of refraction of a sample over the range of 0.1 THz to several THz. While these experiments do not measure the time evolution of the spectrum, they do provide important information regarding the frequency-dependent absorption coefficient and index of refraction. We use this information to better understand a variety of solvents and binary mixtures.

Molecular Dynamics (MD) Simulations

MD simulations give unprecedented microscopic information about complex systems like liquids. The positions, orientations, and velocities of the particles are known at any particular time in MD simulations of a system of interacting particles. Consequently, the configurations adopted in the simulation are known, and we can analyze structural relationships in the system. In addition, the dynamics can be assessed and compared to spectra from THz-TDS and low frequency infrared measurements. We are particularly interested in how hydrogen bonding in the system changes upon mixing, and how the structure is related to the dynamics of the system.

Intramolecular Charge Transfer

It is well known that an accelerating charge generates light (electromagnetic radiation). We have devised an experiment that captures the THz pulse emitted during rapid intramolecular charge transfer. This phenomenon was demonstrated using two different dye molecules, and in the future we will extend this method to photosynthetic and bacterial reaction centers, and perhaps even DNA.

Finite-Difference Time-Domain Pulse Propagation

There are some interesting and important considerations regarding the propagation of THz pulses in many of the experiments we perform. For example, in our TRTS experiments, the THz pulse is somewhat longer than the visible excitation pulse. This raises questions about how to deal with the situation wherein the visible pulse excites the sample as the THz pulse is propagating through it. That is, the beginning portion of the THz pulse travels through the non-photoexcited medium while the trailing portion of the pulse experiences photoexcited medium that may have very different optical properties from the non-photoexcited medium. This seemingly schizophrenic situation can be accurately treated by numerically propagating the pulses in the time domain through iterative solution of Maxwell's equations.