A novel scheme for the optical generation and coherent detection of tunable narrow-band far-infrared radiation in free space is described. This technique involves the optical heterodyning of two linearly chirped broadband pulses to produce a quasi-sinusoidal intensity modulation at tunable terahertz frequencies. The frequency content of the narrow-band terahertz radiation produced by mixing chirped optical pulses in a nonlinear optoelectronic device such as a photoconducting dipole antenna can be tailored simply by controlling the phase modulation of the optical pulses and the delay between them. An optoelectronic terahertz beam system composed of a tunable narrow-band Hertzian dipole emitter and a synchronously gated tunable dipole detector is presented. The performance of this system as a potentially powerful tool for far-infrared pump–probe spectroscopy and the limitations produced by cubic phase modulation in the chirped optical pulses are discussed.
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