Paul D Cunningham

THz Time-Domain & Time-Resolved THz spectrocopy of Conductive Materials

In THz time-domain spectroscopy (TDS) a THz pulse is propogated through a sample, recorded in the time-domain, and compared to the transmission through a reference. Through Fourier analysis one can extract the refractive index and extinction coefficient, the complex dielectric function, or equivolently the complex frequency-dependent conductivity. The conductivity can be modeled to determine the mechanism of conductivity, the carrier scattering time or mobility, and the plasma frequency or carrier density.
In Time-resolved THz spectroscopy (TRST), i.e. Optical-pump THz-probe (OPTP) spectroscopy, a THz pulse is propagated through a photo-excited sample. The THz waveform transmitted through the excited sample is recorded in the time-domain and compared to the transmission through the unexcited sample. Through Fouier analysis, the full frequency-dependent complex photo-induced conductivity can be obtained. Modeling of the photo-induced conductivity yields the mobility and carrier density, i.e. the photon-to-carrier yield. Control of the delay between the optical excitation, i.e. pump, and the THz pulse, i.e. probe, allows you to map out the time evolution of the photoexcited state.

Conductivity dynamics in GaAs

We found that TRTS is capable of examining the onset of photoconductivity in material systems. Here, high energy, 400 nm (3.1eV), excitation promotes electrons to low-mobility satallite valleys. The onset of the change in THz transmission is due to carriers thermalizing through phonon-scattering back to the high-mobility gamma-valley. We could also resolve carrier trapping by arsenide clusters in low-temperature grown (LT-) GaAs.

The THz bandwidth typically spans the scattering rates for most semiconductors. This provides an excellent opportunity to determine the carrier mobility and density via a non-contact AC probe. We found that the complex conductivity in GaAs is well described by the Drude model, which describes a gas of free and independent electrons whose momenta are rendomized upon scattering events.

Charge transport in P3HT

I studied the room- and low- temperature photoconductivity of P3HT. At low temperature there is an increase in the photoconductivity (mobility) due to decreased torsoinal disorder which increases the effective conjugation length. The faster recombination dynamics are consistent with inhibited interchain hopping at low temperature. I found that TRTS is capable of probing nanoscale charge transport to uncover the intrinsic carrier mobility of a disordered system. This places an upper limit on the thin film transistor mobility only realizable through ideal, single-crystalline, order. I also compared charge transport resulting from exciting the singlet and triplet states and found surprising similary in the dynamics.

Charge transfer in P3HT/PCBM bulkheterojunction blends

I examined ultrafast charge transfer from P3HT to PCBM in bulkheterojunction blends. I found that even the low energy triplet state (excited with infrared light) undergoes charge transfer, and infact does so faster (660 fs) than seen for high energy UV excitation of the singlet state (2 ps). This demonstrates that large differences in energy levels are not required for efficient exciton dissociation, contrary to popular thought, and places an upper limit on the exciton binding energy of 750 meV.

Time evolution of photoconductivity in metallated polymers

We found a distinct evolution from dispersive transport to a purely excitonic response, similar to the response seen in MEH-PPV, but in contrast to the response seen in crystalline and polycrystalline systems. This property may be common to all amorphous systems and may point to a link between the degree of disorder in a system and the degree of charge carrier localization.

Temperature dependence of conductivity in high-temperaure super conductors

We found Drude-like conductivity, with a shape characteristic of a large scattering rate, at room temperature. However, as the temperature decreases the scattering rate also decreases. Near the critical temperature of ~100K, the imaginary conductivity sharply increases and the conductivity shows signs of following the two-fluid model, where the conductivity is determined by weighted fractions of normal state electrons and superconducting cooper-pairs. The superconducting fraction has an imaginary componant that is inversely proportional to frequency. Optical excitation breaks up the cooper-pairs into short lived quasiparticles, that re-pair over picosecond time scales.