The fundamental component of a laser pulse is focused through a nonlinear second harmonic (SH) crystal into ambient air. The second harmonic generated in the nonlinear crystal mixes with the fundamental pulse and provides the desired field symmetry breaking (the phase between the ω and 2ω fields is controlled by the crystal position along the optical axis). The plasma channel generated close to the lens focus emits THz radiation, which is then filtered out by a set of longpass filters (in most cases, high-resistivity silicon wafers)
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