  Terahertz field-induced second harmonic generation (TFISH) is a technique for
optical detection of broad-band THz fields. We show that by placing an iris at
the interaction volume of the THz and optical fields, the TFISH signal
increases by few ten-fold in atmospheric air. The iris-assisted TFISH
amplification is characterized at varying air pressures and probe intensities
and provides an elegant platform for studying nonlinear phase-matching in the
gas phase.
