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

Fabio Novelli,Martina Havenith,Biswajit Guchhait

doi:10.3390/ma13061311

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.

Highlights

Terahertz (THz) radiation probes the collective intermolecular modes of hydrogen-bonded water molecules from ~0.2 to ~20 THz [1]
In the quest to find intense sources operating in the THz gap, here, we report the experimental characterization of the optical rectification (OR) in Gallium Phosphide (GaP), OH1, and DSTMS over the full range of the wavelengths emitted by an optical parametric amplifier (OPA), i.e., between 1200 and 2500 nm
We investigated the THz emission by GaP pumped by a mode-locked oscillator tunable

Summary

Introduction

Terahertz (THz) radiation probes the collective intermolecular modes of hydrogen-bonded water molecules from ~0.2 to ~20 THz [1]. These modes [1,2,3,4,5] represent the microscopic solvent coordinate for proton transfer [6], are involved in bio-reactions like drug intercalation into DNA [7,8] and, according to molecular dynamics simulations [9], are the drive of the structural rearrangements of the molecular network. While infrared spectroscopy (IR) typically probes intramolecular modes, the absorption features in the THz range can be broad [16]. Previous molecular dynamics (MD) calculations [1] indicated that

Methods

Results

Conclusion