Abstract

The conventional way to generate intense broadband pulses in the infrared and far-infrared spectral regimes is nonlinear down-conversion of optical pulses derived from table-top laser systems. Until a few years ago, the different wave-mixing schemes gave access to segments of either the far-infrared (frequencies well below 10 THz) or the infrared (frequencies above 30 THz) regime, but not to both simultaneously. This has changed with the advent of intense sub-20-fs optical pulses which have led to a confluence of pulse generation across the barren region at 7-30 THz. Today, tunable pulses covering substantial parts of the spectrum from the far-to the mid-infrared, or even extending over the full range, are available , employing such techniques as difference-frequency mixing of two (near-)infrared pulses in nonlinear crystals [1], optical rectification in crystals [2] and nonlinear wave-mixing in laser-ionized gas plasmas [3,4]. An example for the latter case is illustrated by Fig. 1, showing an experimental scheme for the generation of ultra -broadband electromagnetic pulses by air-plasma excitation with femtosecond two-colour optical fields [4]. The resultant THz pulses exhibit a bandwidth (10 % from maximum) exceeding 100 THz. Very high field strengths of tens of MV/cm are reached.

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