Intense THz source based on BNA organic crystal pumped at Ti:sapphire wavelength

Abstract

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...

Efficient Broadband Terahertz Generation from Organic Crystal BNA Using Near Infrared Pump

“…However, the main spectral contents are in the 1-5 THz range. This is much wider than the spectrum obtained with Ti:Sa 800 nm pump which previously reported to extend up to 3 THz with spectral peak around 1 THz [16]. This finding is a direct reflection of the phase matching conditions of BNA [18].…”

Yellow‐Colored Electro‐Optic Crystals as Intense Terahertz Wave Sources

“…The chemical structures of the new series of yellow‐colored electro‐optic salt crystals are shown in Figure . The previously reported yellow‐colored organic THz crystals 2‐cyclooctylamino‐5‐nitropyridine (COANP) and BNA have a remarkably different chemical structure compared with benchmark organic electro‐optic crystals. COANP and BNA crystals are nonionic crystals that consist of molecules with one aromatic ring in the push–pull π‐conjugated bridge, while most benchmark organic electro‐optic crystals (ionic or nonionic) are based on chromophores that consist of two aromatic rings, with electron‐donor and ‐acceptor groups at the opposite end of each ring .…”

“…Recently, yellow‐colored electro‐optic crystals, in addition to red‐ or orange‐colored crystals, have shown a good potential for THz wave generation by employing the optical rectification method . For instance, the yellow N ‐benzyl‐2‐methyl‐4‐nitroaniline (BNA) crystal exhibits high optical‐to‐THz conversion efficiency, 0.26%, when pumped at 800 nm with a pulse fluence of 5.3 mJ cm −2 and duration of 50 fs …”

The SwissFEL Experimental Laser facility

“…About 98% of the energy available for the diagnostics branch will be used to drive the THz source. It is based on optical rectification with a tilted pulse front (TPF) in LiNbO 3 (Hebling et al, 2002), optimized for high output energy (Stepanov et al, 2008) or alternatively on BNA (Shalaby et al, 2016). We expect from LiNbO 3 , with a typical conversion efficiency of 0.1%, a THz energy of about 10 mJ and a central frequency of about 0.3-0.5 THz.…”

“…† OH1, DSTMS and DAST show frequency components up to 10 THz(Shalaby & Hauri, 2015;Vicario et al, 2015).…”