THz generation from DFG in a noncollinear phase-matched GaP crystal pumped with a compact diode-pumped two-frequency LiYF₄:Nd laser

Abstract

Pumping with a nanosecond two-frequency compact diode-pumped LiYF₄:Nd laser emitting at 1047 and 1053 nm, a THz wave has been produced in a GaP crystal, attributed to the difference frequency generation process $\frac{1}{{{\lambda _3}}} - \frac{1}{{{\lambda _2}}}=\frac{1}{{{\lambda _{{\text{TH}}z}}}}$. In a first set-up, the two pumping beams had a tunable angular separation provided by a sapphire prism. The phase matching angle of the noncollinear interaction has been determined to be 0.202° external angle and the refractive index 3.377 at 1.6 THz. The angular acceptance was rather broad, due to the fact that the thickness of the nonlinear crystal was only 3 mm. With a second set-up using a beam displacer constituted by YVO₄ plates of different thicknesses, smaller angular separations were explored under other directions of polarizations of the pump beams. Because at the three wavelengths involved in this study the velocity of the nonlinear polarization wave inside GaP is higher than the THz wave phase velocity, a Cerenkov phase matching is possible. The plane-wave calculations show that the THz radiation under this process exits from the output face of the crystal, as it is experimentally observed.