THz-wave generation perpendicular to the output surface is achieved by setting of the incident angles of pump and idler beams to the crystal surface as shown in Fig.1(a). For perpendicular emission at 1.5 THz, the incident angle of the idler wave is set at 65º, i.e., the phase match angle, δ_1.5THz, at 1.5 THz generation, and the pump beam is set at 64.3º, i.e., δ_1.5THz - θ_1.5THz, as shown in Fig. 1a. These angles exceed the critical angle (θ_crit. = 27.7º) of the interface between the MgO:LiNbO₃ crystal (n_p,i≈2.15) and air (n = 1). So, the idler and pump waves are totally reflected at the surface. By varying the pump incident angle from 64.6 to 63.5°, the THz-wave frequency tuning in the range 0.8–3 THz is obtained; the THz-wave generation direction changes by about 15.9º to the crystal surface.

The experimental setup of our surface-emitted TPO is illustrated schematically in Fig. 1b. Two rectangular and one trapezoidal MgO:LiNbO₃ crystals were used for the surface-emitted TPO, and two rectangular crystals were placed on both sides of the trapezoidal crystal. Two rectangular crystals were cut into 4 ´ 50 ´ 5 mm (a ´ b ´ c-axis). The pump laser was a multi-longitudinal mode Q-switched Nd:YAG laser (1.064 μm, 25 ns, 50 Hz, TEM₀₀). The idler generated in the crystal and the pump waves were totally reflected at the THz-wave output surface of the trapezoidal crystal. The polarization of the generated idler and THz wave are parallel to that of pump beam. The idler and pump beams interacted in a 109-mm-long crystal in a symmetric-geometry resonator, consisting of two flat cavity mirrors for idler wave resonance. These cavity mirrors, M₁ and M₂, were specially fabricated with high transmittance at 1.064 mm and a reflectivity of >90% above 1.068 mm. These high-performance mirrors were key to the setup. The resonant mirrors were placed so that the incident angle of the resonant idler to the output surface was 65º, satisfying the condition for perpendicular generation of 1.5 THz. The surface-emitted TPO was fixed on a rotating stage, and continuous frequency tuning was achieved by varying the phase-matching angle between the resonated idler and pump beams by rotating the stage.

THz-wave output energy at a pump input of 20.3 mJ/pulse. The THz-wave frequency is tuned by rotating the stage. Wide range tunability from 0.8 to 2.74 THz was observed for a pump energy of 20.3 mJ/pulse.  The measured maximum THz-wave output was 104 pJ/pulse at 1.46 THz. The minimum sensitivity of the Si bolometer is about 10 fJ/pulse. The Si bolometer became saturated at 5 pJ/pulse for the pulsed THz-wave, so we used calibrated thin metalized Mylar film was used as a THz-wave attenuator.

To evaluate the beam quality, we measured the THz-wave beam profile using two-dimensional scanning with a 400 μm diameter pinhole. The THz wave that passed through a pinhole was measured by a Si bolometer. Figure 3a shows the far-field two-dimensional THz-wave pulse energy distribution at a distance of 22 mm from the output surface.

This asymmetricity is caused by the elliptical radiation cross-section. We estimated the width of the radiation cross-section based on the diffraction theory. The measured far-field divergence angles at 1.5 THz were 3.15 and 7.73º in the x and y directions, respectively. The far-field divergence angle, θ_div., increased in inverse proportion to the width of the radiation cross section, which is θ_div.≈λ/W when, θ_div<<1, where λ is THz-wave wavelength and W is the width of the emission cross section on the output surface. The larger divergence angle along the y direction was caused by the smaller emission cross section width in the y direction on the output surface.

To evaluate the THz beam quality, we measured the beam quality factor (M²) from the optical axis (z) dependence of the focused beam spot size. For this measurement, the knife-edge method was used to measure the beam spot size. The THz wave emitted from the surface-emitted TPO was collimated using a 30-mm-focal length cylindrical lens in the vertical (y) direction and was focused using a 30-mm-focal length Tsurupica lens (plastic lens for THz use). The beam spot sizes were measured between the two 30-mm-focal length lenses along the optical propagation direction. The result is shown in Fig. 3 (b). The measured beam waist sizes were 607 and 410 μm in the horizontal (x) and vertical (y) directions, respectively. From the theoretical calculation using the measured waist size, the estimated beam quality factor (M²) was 1.15 and 1.25 for the horizontal and vertical directions, respectively. The excellent beam quality of the THz-wave from the surface-emitted TPO allows focusing the beam to very small areas, and thus, this THz-wave source is practical for many applications that use THz waves, such as imaging.