We report on high energy terahertz pulses by optical rectification (OR) in the organic crystal N-benzyl-2-methyl-4-nitroaniline (BNA) directly pumped by a conventional Ti:Sapphire (Ti:Sa) amplifier. The simple scheme provides an optical to terahertz conversion efficiency of 0.25% when pumped by a collimated laser pulses with duration of 50 fs and central wavelength of 800nm. The generated radiation spans frequencies between 0.2 and 3 THz. We measured the damage threshold as well as the dependency of the conversion efficiency on the pump fluence, pump wavelength, and pulse duration.High field terahertz (0.1-10 THz) generation is a topic of numerous ongoing researches due to its wide range of science applications [1][2][3][4][5][6][7][8][9][10][11]. Presently, optical rectification (OR) in nonlinear crystals is the preferred generation technique as it allows the realization of the most intense THz source. The figures of merit of a high-field THz source are the pump-to-THz conversion efficiency, THz spectrum, focusability and the applicable pump wavelength. The wide use of Ti:Sa laser technology in ultrafast THz spectroscopy has inspired the development of a THz emitter which can be pumped directly by those powerful lasers. Presently, for the most intense THz sources Ti:Sapphire needs to be converted to the short wavelength midinfrared spectral range to provide efficient phase-matching [11][12][13][14].In this letter, we report on THz generation in the organic crystal BNA which is pumped at the Ti:Sa 800 nm wavelength. The conversion efficiency and spectral density offered by BNA surpasses other organic crystals pumped at conventional Ti:Sa wavelength in the range of 0.1-3 THz by a factor 10-100. It thus represents a valuable option to the widely used Ti:Sa pumped Lithium Niobate (LN) crystal as the generation scheme is simpler and does not require pulse front tilting.Among all nonlinear crystals, LN [10] and organic crystals (DAST, DSTMS, OH1) [11][12][13][14][15] provide most efficient OR. The typical conversion efficiency for LN pumped at 800 nm [10] and 1030 nm reaches 0.1-0.2 % that can thus be further increased by a factor of three by cryo-cooling the LN [15,16]. However, the reported conversion efficiency is still debated as other research groups [15,16] who repeated the experiment obtained conversion efficiency smaller by an order of magnitude than ref. [17,18]. Due to the notorious focusing issues of LN, which are partly caused by the asymmetric beam quality and the complex geometry, the maximum reported THz field strength has not surpassed 1 MV/cm [10]. LN-based sources emit in the low-THz range (0.1-3 THz). Until now, this range has been difficult to access by organic crystals.Organic crystals offer excellent phase-matching for OR in a simple pump beam configuration and superior THz focusability. The high 2-3% conversion efficiency is achieved for a pump wavelength between 1.2-1.5 µm [11,13]. Such a scheme allows for extreme focusing peak fields reaching up to 83 MV/cm [11]. Yet, the main drawback of or...
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