2 μm pumped optical rectification in LiNbO₃ for >2% laser-to-0.4 THz conversion

Optical rectification (OR) in lithium niobate (LN) crystals, pumped by near-infrared lasers with tilted pulse front, has emerged as a widely adopted approach for generating broadband terahertz (THz) pulses in the frequency range of 0.1-1 THz. However, the laser to THz conversion efficiency remains a critical bottleneck, typically limited to a few parts per thousand despite extensive optimization efforts. Here, we demonstrate that increasing the pump laser wavelength can effectively enhance the OR efficiency. Numerical simulations incorporating material absorption identify approximately 2 μm as the optimal pump wavelength for the LN-based OR system. Experimentally, by pumping a cryogenically cooled LN crystal with a 2 μm laser, we achieve a conversion efficiency of 2.12%, which represents a 3.9-fold improvement over the efficiency obtained using an 800 nm pump laser under identical crystal conditions. Electro-optic sampling measurements reveal that the generated THz pulse exhibits a single-cycle duration with a central frequency of 0.4 THz. Our findings establish 2 μm pumping as a promising strategy for developing efficient THz sources for various applications.