Time-resolved THz spectroscopy: Charge transfer, solvent dynamics, and photoconductivity

Time-resolved THz spectroscopy (TRTS) has been applied to study the sub-picosecond (sub-ps) to nanosecond carrier dynamics in bulk semiconductors, and semiconductor nanoparticles (NPs), as well as to study the low-frequency, collective solvent dynamics in chloroform and toluene. In a TRTS experiment the evolution of the far-infrared spectrum after a visible excitation event is measured with a temporal resolution of ∼200 fs. One of the many attractive features of THz spectroscopy is it provides a non-contact electrical probe, because, the FIR spectrum is directly related to the complex conductivity. Materials whose photoconductivity changes rapidly and/or whose morphologies make attaching wires difficult will benefit from this new spectroscopic probe. There are significant challenges in interpreting TRTS data on the sub-ps timescale, and finite-difference time-domain (FDTD) calculations have proven extremely valuable in simulating and understanding TRTS experiments.; We observed a mobility that evolves on a 1–2 ps timescale in bulk GaAs and low-temperature grown GaAs (LT-GaAs). The time-dependent mobility arises from carrier cooling that occurs immediately following photoexcitation. Our results have provided an experimentally determined time-dependent mobility with sub-ps temporal resolution for the first time in these materials. We find that the mobility in LT-GaAs is smaller than in regular GaAs. The mobility for both materials conforms to a simple variant of the Drude model.; We observed a marked size-dependent mobility and carrier dynamics in CdSe NPs. The mobility is characterized by three groups, diameters larger than the bulk carrier mean free path l_f, diameters smaller than l_f but larger than the Bohr exciton radius a_B, and diameters smaller than a_B. The size-dependent dynamics show that ballistic transport occurs to the surface of the NPs where the carriers undergo trapping.; A new spectroscopic technique, charge transfer induced electromagnetic generation (CTIEG), was developed to study rapid intramolecular charge transfer events that occur on the timescale of 0.1 to 10 ps. CTIEG allows one to determine the direction of charge transfer in solution and the detailed charge-transfer dynamics are contained in the time-dependent emitted field.