Generation of low-frequency components due to phase-amplitude modulation of subcycle far-infrared pulses in near-field diffraction

We observe carrier phase dependent generation of low-frequency spectral components in the near-field zone of a subwavelength aperture illuminated by subcycle far-infrared (terahertz) pulses. Diffraction induced pi/2-phase shift of the pulse carrier results in the spectral intensity variation of the low-frequency components. The effect appears only if the pulse duration is shorter than the period of the carrier oscillation. (C) 2002 American Institute of Physics.