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Generation and detection of picosecond far-infrared radiation

We discuss the generation of ultrashort far infrared (FIR) pulses using external switching of the output of a FIR superradiant cell pumped by a transversely excited atmospheric CO2 laser. The external switching mechanism consists of either one or two semiconducting wafers that have been optically excited with the second harmonic of a synchronized modelocked YAG:Nd3+ laser. The output of our FIR superradiant cell, a 50 ns FIR pulse is incident at Brewsters angle on a semiconducting wafer. On optical excitation above the band gap the (FIR) transmitting semiconductor takes on a metallic reflectance and the FIR transmission of the wafer is switched. If silicon is used as the switching medium, the long relaxation time requires that two wafers are used, one to turn on the pulse, and the second to turn it off. Using a pair of intrinsic silicon wafers the FIr pulse width can be adjusted with the ultimate limit being of the order of the width of the picosecond optical laser.