Broadband highly efficient source of collimated terahertz radiation driven by an ultrafast optical oscillator

We demonstrate experimentally an efficient terahertz emitter that consists of a 20 µm thick layer of LiNbO₃ clamped between a fused silica substrate and a Si semicone. A focused laser beam from an ultrafast optical oscillator propagates in the LiNbO₃ layer and emits a Cherenkov cone of terahertz radiation to the Si semicone. The radiation is totally internally reflected by the semicone's convex surface and escapes the semicone through its base as a collimated beam. By pumping the emitter with 7 nJ Ti:sapphire laser pulses, terahertz radiation with the spectrum as wide as ∼6 THz was generated with a record high efficiency of 0.06% for oscillators.