Tunable picosecond THz-wave generation based on trapezoidal MgO:LiNbO_3 crystal in novel pentagram-shaped pump-enhancement cavity

摘要

It is well known that lithium niobate (LiNbO_3) has excellent characteristics for efficient tunable/broadband THz-wave generation. Over the last few years, we have investigated novel THz-wave sources based on MgO-doped LiNbO_3 (MgO:LiNbO_3), and have succeeded in developing a tunable picosecond THz-wave source by using a novel pentagram-shaped pump-enhancement cavity. One of the limiting factors in efficient THz-wave generation is the strong absorption by MgO:LiNbO_3 in THz-wave region. To overcome this problem, we employ a surface-emitted configuration which consists of a trapezoidal MgO:LiNbO_3 crystal and a pump-enhancement cavity folded in the shape of a pentagram. The pentagram-shaped cavity is designed for the noncollinear dual resonance of both pump and one of the down-converted waves. As a result, 1.5-ps pump pulses from a mode-locked Tirsapphire laser operating at 780 ntn allow tunable THz-wave generation via parametric down-conversion resulting from stimulated phonon-polariton scattering in the MgO:LiNbO_3 crystal. By slightly translating the position of one of the cavity mirrors, we experimentally find that the THz-wave frequency is tunable in the range from 0.1 to 3.5 THz with the average output power of dozens of nanowatts. The maximum THz-wave average power is up to 40 nW around 2 THz at the pump power of 800 mW, which is several times higher than the THz-wave output generated by using rectangular MgO:LiNbO_3 crystals for Si-prism-coupled configuration under the same pump condition.