Time-Resolved THz Spectroscopy (TRTS)

Time Resolved THz Spectroscopy (TRTS) entails photoexciting a sample with a visible excitation pulse, and probing the subsequent dynamics with a THz pulse. The samples studied vary from dye/solvent systems to semiconductor wafers to sintered colloidal dye-sensitized TiO₂ to a dispersion of CdSe quantum dots. All of these experiments share the common characteristic of initiating some change in the sample with a pulsed visible laser, and following the ensuing change in the frequency-dependent far-infrared absorption coefficient and index of refraction on a sub-picosecond timescale. The THz pulses generated in our lab have bandwidth from about 2 wavenumbers to about 100 wavenumbers. It should be noted that there are no other sources of sub-picosecond far-infrared pulses -- even synchrotrons and free electron lasers (which can generate far-IR pulses) have pulse durations of several picoseconds. Therefore, THz spectroscopy is the best way to obtain dynamical information in the far-IR region of the spectrum. Click here to see a schematic drawing of the experimental apparatus.

Liquid Dynamics

Photoexcitation of a dye molecule in solution will disturb the surrounding solvent. As the solvent undergoes reorganization to accommodate the new charge distribution of the dye molecule, the spectrum of the low frequency collective solvent modes is expected to change as a function of time. Click here for more details.

GaAs

When electrons are excited from the valence band to the conduction band in GaAs, the sample changes almost instantaneously from insulating to conducting. This is the basis for the so-called Auston switch, which is the most rapid way to throw a switch. Because mobile electrons in the conduction band absorb far-IR light much more strongly than the immobile electrons in the valence band, we can quantitatively determine the transient photoconductivity with sub-picosecond temporal resolution based on the characteristics of the transient THz absorption. Click here for more details.

Dye-Sensitized Titanium Dioxide

A novel type of solar cell based on dye-sensitized TiO₂ was demonstrated by Gratzel and coworkers in 1991. In a nutshell, this type of solar cell relies on the absorption of sunlight by a dye molecule which absorbs strongly in the visible region of the spectrum, where the solar light intensity is highest, and injects its excited state electron into the TiO₂ substrate (which itself only absorbs light with wavelengths shorter than 400 nm, i.e., ultraviolet light). We have carried out TRTS experiments on dye-sensitized TiO₂ samples, but not on complete working cells. A significant advantage of our technique over other techniques is that we can actually study the conductivity of the electrons on a sub-picosecond timescale after they have been injected into the TiO₂ substrate with our non-contact electrical probe. That is, we obtain their mobility while photoexcited in addition to measuring how long they remain photoexcited. Click here for more details.

Quantum Dots

Recently, methods have become available to synthesize very small semiconductor quantum dots with diameters ranging from less than 1 to greater than 10 nm using wet chemical methods. These materials show a tremendous range of properties as a function of size. For example, the bandgap increases as the dot size decreases, leading to absorption and emission spectra that are tunable. Our interest lies in characterizing the transient photoconductivity of these materials using TRTS. When an object is very small, one wonders if it is even appropriate to think in terms of bulk phenomena, and our experiments will test these ideas. Click here for more details.