Time-Domain Terahertz Spectroscopy
Y. Gao, Z. Chen, T. Drake, and M. F. DeCamp, Half-Cycle-Pulse TeraHertz emission from an ultrafast laser-plasma in a solid target, Optics Lett. 33, 2776 (2008). [URL].
Until recently, researchers did not extensively explore the material interactions occurring in the Terahertz spectral region—the wavelengths that lie between 30 µm and 1 mm—in part because they lacked reliable sources of Terahertz radiation. However, pressure to develop new Terahertz sources arose from two dramatically different groups—ultrafast time-domain spectroscopists who wanted to work with longer wavelengths, and long-wavelength radio astronomers who wanted to work with shorter wavelengths.
Much of the recent interest in Terahertz radiation stems from its ability to penetrate deep into many organic materials without the damage associated with ionizing radiation such as X-rays (albeit without the spatial resolution). Also, because terahertz radiation is readily absorbed by water, it can be used to distinguish between materials with varying water content—for example, fat versus lean meat. These properties lend themselves to applications in process and quality control as well as biomedical imaging. Tests are currently under way to determine whether Terahertz tomographic imaging can be used in cancer screening or screening of passengers for explosives at airports.
Terahertz radiation in the wavelength range of .020-1 mm and pulsed lengths as fast as 30fs are produced in DeCamp Lab. While these wavelengths are relatively long, it is the spectral region of low frequency molecular vibrations and rotations, making this a useful range for molecular spectroscopy. Producing pulsed Terahertz radiation can easily accomplished by rapidaly accelerating charged particles. This is typcially accomplished via ultrafast excitation of semiconductors or the rapid generation of a dense electron plasma.
DeCamp Lab has recently produced and measured coherent Terahertz radiation from a dense laser-driven plasma in a copper target. This radiation is a direct measurement of the electron dynamics within the plasma, providing a novel method to performing laser-plasma diagnostics.