Opt. Lett. 38, 796–798 (2013)

Intense, ultrafast terahertz (THz) pulses are required for a wide variety of applications, and there is keen interest in scaling the peak output power of THz generation schemes. Optical rectification using the nonlinear crystal lithium niobate (LiNbO3) is one promising method for generating high-peak-field THz pulses, but to date the maximum energy-conversion efficiency from pump to THz output has been less than 0.25%. Now, Shu-Wei Huang and co-workers from the USA, Spain and Germany have improved this by more than an order of magnitude. They achieved a record-high conversion efficiency of 3.8% by carefully optimizing the pump pulse duration to maximize the effective length of the nonlinear interaction for THz pulse generation. They further enhanced the efficiency by cooling their LiNbO3 crystal to 100 K to reduce THz absorption in the sample. They sent a pump laser with a central wavelength of 1,030 nm, a pulse duration of 680 fs, and a fluence of 4.7 mJ cm−1 to a prism-shaped LiNbO3 crystal doped with 6% MgO. The generated THz power increased monotonically as the temperature decreased to about 150 K. Below this temperature, the conversion process saturated. The conversion efficiency of 3.8% is not far from the maximum theoretical limit of 6% for LiNbO3 at 100 K.