Terahertz time-domain spectroscopy THz-TDS

Nishizawa, S., Tsumura, N., Kitahara, H., Wada Takeda, M. and Kojima, S. (2002) New application of terahertz time-domain spectroscopy (THz-TDS) to the phonon-polariton observation on ferroelectric crystals. Phys. Med. Biol., 47, 3771-3776. 

Recent efforts to generate THz radiation have been essentially based on the optical rectification effect in ferroelectrics or the generation of transient photogenerated currents in semiconductors, both with femtosecond pulses some variants of these approaches, also termed terahertz time-domain spectroscopy (THz-TDS), have produced very promising results and open the way to the development of compact coherent sources that emit pulses in the THz-region with durations close to the single terahertz optical cycle. Many technical problems, however, still remain before these schemes can be implemented in reliable devices for tunable THz-radiation. 

Whereas CM in bulk materials is usually determined in photocurrent device measurements, that is, by collecting the carriers, CM in QDs is studied by (optical) spectroscopic measurements, in which the orbital occupation of the QDs is probed on ultrafast (picosecond) timescales. Hence, the commonly used experimental procedures to determine CM in QDs (ultrafast spectroscopy) and in bulk (device measurements) are rather different. While time-resolved optical and IR spectroscopies are ideally suited to probe carrier populations in colloidal QDs, " light of terahertz (THz) frequencies interacts strongly with free carriers in the bulk material and allows the direct characterization of carrier density and mobility. From THz-time domain spectroscopy (TDS) experiments, one can quantitatively assess the number of photogenerated carriers in bulk semiconductors picoseconds after the light is absorbed. Additionally, as a result of the contact-free nature of the THz probe, it is possible to determine the CM factor in isolated samples of bulk semiconductors without the need to apply contacts, which is necessary in the device measurements. For these reasons, THz-TDS experiments have been employed to quantify CM in bulk PbSe and PbS on ultrafast timescales " in order to make a bulk-QD comparison in the context of the CM controversy. The CM factor in bulk PbS and PbSe was determined for excitation with various photon energies from the UV to the IR. 

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