Figure 1. Experimental setup for sub-wavelength resolution THz imaging.

A repetition rate of 1 kHz, central wavelength of 800 nm and duration of 50 fs Ti: sapphire laser pulse with energy of 1 mJ/pulse was focused by a f = 30 cm lens in air. The focused pump beam passed a 0.1-mm-thick Type I β-barium borate (BBO) crystal, creating plasma at the focus. THz emission from this plasma filament is utilized as the probe of THz imaging and measured by a standard EOS (electric-optic sampling) setup. The object to be imaged is a PCB (printed circuit board) plate with multiple through-holes drilled on the plate, inserted in the middle of the filament. An additional ceramic plate was put in front of the PCB plate to terminate the laser beam and exclude the laser ablation on the sample.