Recent developments in high-repetition-rate femtosecond sources of terahertz radiation show promising improvements in efficiency, suggesting that photoconductive emitters will remain important terahertz sources. The terahertz (1 THz velence 10~(12) Hz) frequency range lies between microwaves and the infrared. Many vibrational molecular excitations are detectable in the terahertz range, making it a useful band for chemical detection, with applications in industrial settings and in homeland security. Coupling a source of short pulses of terahertz radiation with electro-optic sampling enables terahertz time-domain spectroscopy, allowing for simultaneous ultrafast measurements of the real and imaginary parts of the response of a medium in the far infrared. With terahertz-emitting modelocked femtosecond lasers still not available, the easiest way to generate terahertz pulses involves using a modelocked laser at optical wavelengths. Note that a short terahertz pulse, which has a duration of less than 1 picosecond, corresponds to an electric field that tracks the intensity envelope of a subpicosecond optical pulse.
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