Fig. 2. Schematic of the experimental setup.

A continuous-wave laser with a wavelength of 659.58 nm is reflected by a VBG (VBG₁) into the interferometer part of the setup through a zero-order half-wave plate (λ/2) controlling the polarization. It is then focused by a lens f₁ into a periodically poled 1-mm-long MgO-doped LiNbO₃ (PPLN) crystal generating signal and terahertz photons that are separated by an ITO. Signal and pump radiation are reflected at M_s directly into the crystal. The terahertz radiation passes the object twice, being reflected by a moveable mirror M_i. In the second traverse of the pump through the PPLN, additional signal and idler photons are generated. Afterward, the lens f₁ collimates the pump and signal radiation for the detection starting with filtering the pump radiation by three VBGs and spatial filters (SF). To obtain the frequency-angular spectrum, the signal radiation is focused through a transmission grating (TG) by the lens f₂ onto a sCMOS camera. The inset shows a frequency-angular spectrum for the used crystal (poling period Λ = 90 μm, pumped with 450 mW). The scattering angle corresponds to the angle after the transmission from the crystal to air.