ABSTRACT
Terahertz (THz) frequencies are particularly sensitive to crystal structures, and thus can be used to examine small concentrations of defects within crystals, such as impurity molecules. In contrast to direct observation, the influence of the defect/impurity is evident in the response of the crystal to THz frequencies. THz waves are electromagnetic waves with frequencies that between those of microwaves and infrared (IR) light. Initially, developments of THz wave applications were slow pace due to a lack of availability of a good light source. A monochromatic, coherent THz wave can be generated according to the principle of difference frequency generation of IR laser beams in a GaP crystal. Monochromatic CW-THz emission has been achieved using low-temperature-grown gallium arsenide as a photoconductive switch. Pharmaceutical development is one area in which THz spectroscopy can play an important role, as this technique is especially sensitive to quality degradation, e.g., during the manufacturing process and with respect to the shelf life.
