Fig. 1. (a) Experimental setup: detection with a THz camera. L: plano-convex lens (f = 300 mm); M: off-axis parabolic mirror (f = 150 mm). Inset: picture of the plasma filament (the size of the square is 1 × 1 cm²). (b) Evolution of the THz beam along the Z-axis, as defined in (a).

Fig. 2. Coherent 2D electro-optic sampling setup using ZnTe detection crystal. (a) Experiment. CH: optical chopper for dynamic subtraction; L1: plano-convex lens (f = 300 mm); CF: ceramic filter; SF: silicon filter; BS: beamsplitter; AN: analyzer; L2: objective lens (f = 50 mm). (b) Left: 2D transverse profile of the THz electric field for a given time delay between the THz and probe pulses (scale: the white bar is 5 mm long); Center: temporal evolution of the THz electric field at the position corresponding to the intersection of the white lines (left image); Right: corresponding amplitude of THz spectrum.

Fig. 3. (a) Aperture-scanning-assisted balanced detection in GaP crystal. (a) Experiment. L1: plano-convex lens (f = 300 mm); M: off-axis parabolic mirror (f = 150 mm, diameter = 40 mm); (b) Left: temporal evolution of the THz electric field; Right: corresponding amplitude of THz spectrum.

Fig. 4. Aperture-scanning-assisted balanced detection with ABCD system. (a) Experiment. L1: plano-convex lens (f = 300 mm); L2: plano-convex lens (f = 200 mm); M1: off-axis parabolic mirror (f = 100 mm, diameter = 50 mm); M2: off-axis parabolic mirror (f = 150 mm, diameter = 50 mm). (b) Left: temporal evolution of the THz electric field; Right: corresponding amplitude of THz spectrum.

Fig. 5. Coherent 2DEOS in ZnTe crystal. Evolution of THz amplitude as a function of frequency and HOA. (a) Alumina first. (b) Silicon first. Yellow color indicates a higher THz amplitude. Insets: 2D spatial distributions of the THz electric field at 0.37 THz and 1.05 THz (from Ref. [17]).

Fig. 6. Aperture-scanning-assisted balanced detection in GaP crystal. The laser probe pulse is transmitted by a hole (4 mm in diameter) drilled in the last parabolic mirror used to focus the THz pulse onto the GaP crystal. (a) Evolution of THz amplitude as a function of frequency and pinhole position. (b) Amplitude of the spectra integrated for different spectral windows.

Fig. 7. Aperture-scanning-assisted balanced detection in GaP crystal. The parabolic mirror with a hole has been replaced by a lens followed by an ITO plate. (a) Evolution of THz amplitude as a function of frequency and pinhole position. (b) Amplitude of the spectra integrated for different spectral windows.

Fig. 8. Aperture-scanning-assisted ABCD technique. The drilled mirror has been covered by an ITO plate. (a) Evolution of THz amplitude as a function of frequency and pinhole position. (b) Integrated spectra for different spectral bandwidths. (c) Amplitude of the spectra at three different positions of the pinhole, as indicated by the dashed line in (a). “Center” refers to 0 mm (spectrum at the center of the THz beam), “Ring” refers to −2.5 mm (spectrum along the emission cone emitted at high frequency, above 12 THz), “Edge” refers to −7 mm (spectrum at the edge of the emission cone).