Enhancing Output Power of LN-Based THz Sources Using a Vertically Elongated Elliptical Beam

To achieve higher output power from a terahertz source based on a MgO:LiNbO₃ crystal, it is essential to increase the incident pump energy while avoiding optical damage, which generally requires expanding the diameter of pump beam. However, it has been known that expanding the beam diameter leads to a reduction in conversion efficiency. In this study, we systematically measured the dependence of terahertz output on the pump beam diameter at multiple terahertz frequencies and experimentally demonstrated that the efficiency degradation is primarily attributable to terahertz absorption losses within the MgO:LiNbO₃ crystal. To overcome this issue, we introduced a vertically elongated elliptical beam. By maintaining a constant beam diameter along the Y-axis, which governs the THz absorption path length within the crystal, while expanding the beam only in the vertical direction, higher pump energy can be injected into the crystal. Experiments using a terahertz parametric source showed that the vertically elongated elliptical beam achieved a 1.5-times higher terahertz output compared with a circular beam. Furthermore, we experimentally demonstrated that the terahertz output energy can be scaled by increasing only the Z-axis beam diameter of the pump beam. This effect is expected to become more pronounced at higher pump energies, indicating that vertically elongated elliptical beam is an effective approach for enhancing output power of terahertz sources based on MgO:LiNbO₃ crystals.