Fig. 1. (a) Experimental setup for systematic characterization of the terahertz emission spectrum depends on the pump polarization, the emission position, as well as the input spectrum. The grating density is of 1500 lines/mm. The focal length of the imaging lens between the grating is 85 mm; HWP, half wave plate; SC, sheet copper; LN, lithium niobate crystal; OAP, 90° off-axis parabolic mirror; 1/4 WP, quarter-wave plate. (b) Photo of the lithium niobate crystal used in our experiment as well as its axis illustration.

Fig. 2. (a) Normalized infrared (IR) input spectra for different GVD, and (b) their corresponding output terahertz spectra. (c) The detected terahertz signal as a function of the different GVD values of the pump pulses; (d) terahertz bandwidth and terahertz central frequency as functions of GVD values of the pump pulses; (e) calculated chirped pulse duration as a function of the different GVD values of the pump pulses.

Fig. 3. (a) Normalized output terahertz spectra for different rotation angles of the IR polarizer between the grating and the imaging lens. Zero degree means the pump laser is vertically polarized in the laboratory coordinate. (b) Normalized terahertz output spectra for the typical complementary angles, respectively. (c) Polarization angle dependence of the terahertz central frequency and spectral width. (d) Recorded terahertz signal as a function of the rotation angle of the IR polarizer.

Fig. 4. (a) Terahertz signal as a function of the knife-edge position. (b) Detected terahertz temporal waveforms in knife-edge measurement directly recorded after the crystal emission plane, and (c) their corresponding normalized Fourier transform spectra.