Optical Parametric Chirped-Pulse Amplification (OPCPA) is an emerging laser technology, providing a flexibility in operating wavelengths and power upscaling . In this work, we show that mid-IR OPCPA technology could overcome major challenges in enhancing intense broadband THz generation from organic crystals. Strong THz fields are presently under extensive research for its applications in particle accelerations and fundamental understanding of structural dynamics . Currently, the most efficient technique relies on optical rectification (OR) of femtosecond near-IR OPA laser pulses (1.3-1.5 μm) in highly nonlinear organic crystals . However, the technique is still limited to a few mJ of pump energies and organic crystals suffer from a relatively low damage threshold at this pump wavelength. To improve THz generation, it is therefore crucial to use alternative laser pump technologies, such as Cr-Forsterite lasers . However, the associated pump wavelength of 1.25 μm typically leads to an increase in nonlinear absorption, which decreases the crystal damage threshold and lowers the effectively emitted THz spectral contents. Here, we used a high-energy OPCPA system with 3.9 μm central wavelength (20 Hz repetition rate)  to generate THz from DSTMS organic crystal (Swiss Terahertz LLC) by OR. The emitted spectrum is broad and extends beyond 5 THz (compared to 2 THz central frequency in the case of Cr- Forsterite pump ). In terms of conversion efficiency, we did not observe saturation up to energy fluence of 180-200 mJ/cm2. This is remarkably high as compared to the typical saturation values of sub-10 mJ/cm2 from both Cr-Forsterite and near-IR OPA pump systems . We evaluated the damage threshold of DSTMS at 3.9 μm beyond 250 mJ/cm2, more than an order of magnitude higher than for alternative near-IR pump technologies. These two advantages, due to the small pump photon energies, which are much below the crystal band gap, allow overcoming the major challenge of technology-limited crystal size and address the experimental needs to high spectral contents.
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