Towards Intense THz Spectroscopy on Water: Characterization of Optical Rectification by GaP, OH1, and DSTMS at OPA Wavelengths

Abstract

Water is the most prominent solvent. The unique properties of water are rooted in the dynamical hydrogen-bonded network. While TeraHertz (THz) radiation can probe directly the collective molecular network, several open issues remain about the interpretation of these highly anharmonic, coupled bands. In order to address this problem, we need intense THz radiation able to drive the liquid into the nonlinear response regime. Firstly, in this study, we summarize the available brilliant THz sources and compare their emission properties. Secondly, we characterize the THz emission by Gallium Phosphide (GaP), 2–{3–(4–hydroxystyryl)–5,5–dimethylcyclohex–2–enylidene}malononitrile (OH1), and 4–N,N–dimethylamino–4′–N′–methyl–stilbazolium 2,4,6–trimethylbenzenesulfonate (DSTMS) crystals pumped by an amplified near-infrared (NIR) laser with tunable wavelength. We found that both OH1 as well as DSTMS could convert NIR laser radiation between 1200 and 2500 nm into THz radiation with high efficiency (> 2 × 10−4), resulting in THz peak fields exceeding 0.1 MV/cm for modest pump excitation (~ mJ/cm2). DSTMS emits the broadest spectrum, covering the entire bandwidth of our detector from ca. 0.5 to ~7 THz, also at a laser wavelength of 2100 nm. Future improvements will require handling the photothermal damage of these delicate organic crystals, and increasing the THz frequency.

Nonlinear TeraHertz Transmission by Liquid Water at 1 THz

“…Liquid water strongly absorbs terahertz (THz) radiation [1][2][3][4][5][6][7][8][9]. Intermolecular interactions determine the unique properties of water [10,11] and can assist biochemical reactions [12,13].…”

“…Intermolecular interactions determine the unique properties of water [10,11] and can assist biochemical reactions [12,13]. In the low frequency range water displays an overlap of different intermolecular hydrogen bond and Debye-like modes [4,5]. Recent ab initio calculations described infrared (IR) and Raman spectra [14] and separated the intramolecular and intermolecular modes [15].…”

“…In order to disentangle the distinct modes, nonlinear techniques are required. This is especially challenging in the THz range due to lack of brilliant sources [4]. To this aim, a variety of nonlinear laser techniques have been used [16][17][18][19][20][21][22][23][24].…”

Attenuated Total Reflection at THz Wavelengths: Peculiarities of Total Internal Reflection and Polariscopy

“…However, for the unique spectral window at < 3 THz where table top THz sources are not available for high brilliance/intensity, synchrotron polarisation is mostly linear (∼ 90%; along the slit of the first mirror used to extract THz radiation from the synchrotron) and can be utilised for THz polariscopy, which is still at the very early stage of development. Debye rotation, network stretching, rocking and wagging librations of water molecules all are active at low-THz spectral band [29]. Spectral filters for specific spectral bands can be made using photolithography on free-standing SU8 laminates as shown above.…”

Performance Evaluation of a THz Pulsed Imaging System: Point Spread Function, Broadband THz Beam Visualization and Image Reconstruction

“…Actually, THz radiation can be produced and detected from a variety of designed THz emitters and receivers, both in continuous wave (CW) and in pulsed regime. Among THz pulsed radiation, some schemes use photoconductive antennas (PCAs) both for THz emission and detection [21,22], some others exploit THz production using inorganic and organic crystals [23,24], or by plasma in air [25] and electro-optic sampling (EOS) [26] for detection, etc.…”