Broadband THz wave generation in organic benzothiazolium crystals at MHz repetition rates [Invited]

We present broadband terahertz wave generation based on benzothiazolium crystals at a 100 MHz repetition rate. Among various benzothiazolium crystals, we chose PMB-4TFS (2-(4-(4-(hydroxymethyl)piperidin-1-yl)styryl)-3-methylbenzothiazol-3-ium 4-(trifluorome-thyl)benzenesulfonate) due to its large macroscopic optical nonlinearity, good crystal characteristics, and suppressed molecular phonon vibrations. When pumped at the telecommunication wavelength of 1560 nm, the generated THz spectrum covers up to ∼15 THz and the corresponding THz amplitude for a 0.29 mm thick crystal at normal incidence is by more than one order of magnitude higher compared to the inorganic standard, 1.0 mm thick ZnTe generator crystal

High-power few-cycle THz generation at MHz repetition rates in an organic crystal

Ultrafast terahertz (THz) spectroscopy is a potent tool for studying the fundamental properties of matter. Limitations of current THz sources, however, preclude the technique being applied in certain advanced configurations or in the measurement of, e.g., strongly absorbing samples. In response to this problem, here we demonstrate the generation of 1.38 mW broadband THz radiation at 10 MHz repetition rate by combining the highly efficient nonlinear organic crystal HMQ-TMS with ultrafast pump pulses generated using a simple and stable external pulse compression of a high power, near-infrared (NIR) femtosecond ytterbium-doped fiber (Yb:fiber) laser. Utilizing spectral broadening in a large core, polarization maintaining photonic crystal fiber and a pair of SF11 prisms, we achieve a tenfold pulse compression of the Yb:fiber laser, yielding compressed 0.35 µJ pulses with a full-width at half maximum pulse duration of 22 fs, exerting a peak power of 13.8 MW at a repetition rate of 10 MHz. THz generation through optical rectification of the NIR pulses is explored in two distinct thicknesses of the organic crystal, leading to a maximum conversion efficiency of ∼5.5 · 10−4, an order of magnitude higher than that achieved with inorganic nonlinear crystals, e.g., gallium phosphide, for similar pump parameters. The focused THz beam has a peak on-axis field strength greater than 6.4 kV cm−1 in unpurged atmosphere. We believe that our moderately strong-field THz source is well suited to a variety of applications in ultrafast THz spectroscopy, in particular THz-enabled scattering-type near-field, and scanning tunneling spectroscopy, where multi-MHz repetition rate sources are required

Generation of high-field terahertz pulses in an HMQ-TMS organic crystal pumped by an ytterbium laser at 1030 nm

Copyright information see: https://www.osapublishing.org/submit/review/copyright_permissions.cfm#oalWe present the generation of high-peak-electric-field terahertz pulses via collinear optical rectification in a 2-(4-hydroxy-3-methoxystyryl)-1-methilquinolinium-2,4,6-trimethylbenzenesulfonate (HMQ-TMS) organic crystal. The crystal is pumped by an amplified ytterbium laser system, emitting 170-fs-long pulses centered at 1030 nm. A terahertz peak electric field greater than 200 kV/cm is obtained for 420 µJ of optical pump energy, with an energy conversion efficiency of 0.26% - about two orders of magnitude higher than in common inorganic crystals collinearly pumped by amplified femtosecond lasers. An open-aperture Z-scan measurement performed on an n-doped InGaAs thin film using such terahertz source shows a nonlinear increase in the terahertz transmission of about 2.2 times. Our findings demonstrate the potential of this terahertz generation scheme, based on ytterbium laser technology, as a simple and efficient alternative to the existing intense table-top terahertz sources. In particular, we show that it can be readily used to explore nonlinear effects at terahertz frequencies

Design strategy of highly efficient nonlinear optical orange‐colored crystals with two electron‐withdrawing groups

A new class of highly efficient nonlinear optical organic salt crystals is reported. In nonlinear optics based on organic materials, it is well known that using two electron-withdrawing groups (EWGs) onto cationic electron acceptors instead of conventional one EWG remarkably enhances microscopic optical nonlinearity for chromophores. However, the corresponding organic crystals possessing enhanced large macroscopic optical nonlinearity have not been reported yet. Herein, a design strategy is proposed for obtaining highly efficient nonlinear optical crystals based on two EWGs in cationic electron acceptors. Introducing a phenolic electron donor, promoting a head-to-tail interionic assembly, along with a two-EWG N-pyrimidinyl pyridinium electron acceptor in cationic chromophores results in a preferred non-centrosymmetric, perfectly parallel alignment of chromophores in crystal. Newly designed OPR (4-(4-hydroxystyryl)-1-(pyrimidin-2-yl)pyridinium) crystals exhibit approximately two times larger effective first hyperpolarizability than that of analogous N-alkyl OHP (4-(4-hydroxystyryl)-1-methylpyridinium) crystals based on only one EWG. OPR crystals exhibit comparable second-order optical nonlinearity to benchmark red-colored DAST (4-(4-(dimethylamino)styryl)-1-methylpyridinium 4-methylbenzenesulfonate) crystals, but a significant blue-shifted absorption resulting in orange-color crystals. Therefore, phenolic organic salt crystals using two EWGs are highly promising materials for various nonlinear optical applications

High‐density organic electro‐optic crystals for ultra‐broadband THz spectroscopy

Ultra-broadband THz photonics covering the 0.3–20 THz range provides a very attractive foundation for a wide range of basic research and industrial applications. However, the lack of ultra-broadband THz devices has yet to be overcome. In this work, high-density organic electro-optic crystals are newly developed for efficient THz wave generation in a very broad THz spectral range and are successfully used for a broadband THz time-domain spectroscopy. The new organic THz generator crystals, namely the OHP-TFS crystals, have very low void volume, high density, and are shown to cover the ultra-broadband THz spectrum up to about 15 THz, which cannot be easily accessed with the more widely used inorganic-based THz generators. In addition to the very favorable broadband properties, the generated THz electric-field amplitude at the pump wavelength of 1560 nm is about 40 times higher than that generated by a commercial inorganic THz generator (ZnTe crystal). By using the newly developed OHP-TFS as generation crystal in a compact table-top all-organic THz time-domain spectrometer based on a low-cost telecom fiber laser, the optical characteristics of a model material are successfully determined in the broad 1.5–12.5 THz range with high accuracy

MHz-repetition-rate, sub-mW, multi-octave THz wave generation in HMQ-TMS

We demonstrate the first megahertz (MHz) repetition-rate, broadband terahertz (THz) source based on optical rectification in the organic crystal HMQ-TMS driven by a femtosecond Yb:fibre laser. Pumping at 1035 nm with 30 fs pulses, we achieve few-cycle THz emission with a smooth multi-octave spectrum that extends up to 6 THz at -30 dB, with conversion efficiencies reaching 10-4 and an average output power of up to 0.38 mW. We assess the thermal damage limit of the crystal and conclude a maximum fluence of ∼1.8 mJ·cm-2 at 10 MHz with a 1/e2 pump beam diameter of 0.10 mm. We compare the performance of HMQ-TMS with the prototypical inorganic crystal gallium phosphide (GaP), yielding a tenfold electric field increase with a peak on-axis field strength of 7 kV·cm-1 and almost double the THz bandwidth. Our results further demonstrate the suitability of organic crystals in combination with fibre lasers for repetition-rate scaling of broadband, high-power THz sources for time-domain spectroscopic applications

Phonon-suppressing intermolecular adhesives : catechol-based broadband organic THz generators

Solid-state molecular phonons play a crucial role in the performance of diverse photonic and optoelectronic devices. In this work, new organic terahertz (THz) generators based on a catechol group that acts as a phonon suppressing intermolecular adhesive are developed. The catechol group is widely used in mussel-inspired mechanical adhesive chemistry. Newly designed organic electro-optic crystals consist of catechol-based nonlinear optical 4-(3,4-dihydroxystyryl)-1-methylpyridinium (DHP) cations and 4-(trifluoromethyl)benzenesulfonate anions (TFS), which both have multiple interionic interaction capability. Interestingly, compared to benchmark organic crystals for THz generators, DHP-TFS crystals concomitantly achieve top level values of the lowest void volume and the highest crystal density, resulting in an exceptionally small amplitude of solid-state molecular phonons. Simultaneously achieving small molecular phonon amplitude, large optical nonlinearity and good phase matching at infrared optical pump wavelengths, DHP-TFS crystals are capable of generating broadband THz waves of up to 16 THz with high optical-to-THz conversion efficiency; one order of magnitude higher than commercial inorganic THz generators

Optimal target blood pressure for the primary prevention of hemorrhagic stroke: a nationwide observational study

BackgroundThere are few reports on the preventative value of intensive blood pressure (BP) management for stroke, especially hemorrhagic stroke (HS), after new criteria for hypertension (HTN) were announced by the American College of Cardiology/American Heart Association in 2017.AimsThis study aimed to identify the optimal BP for the primary prevention of HS in a healthy population aged between 20 and 65 years.MethodsWe conducted a 10-year observational study on the risk of HS, subclassified as intracerebral hemorrhage (ICH) and subarachnoid hemorrhage (SAH) according to BP categories (e.g., low normal BP, high normal BP, elevated BP, stage 1 HTN, and stage 2 HTN) using the National Health Insurance Service Database.ResultsOut of 8,327,751 participants who underwent a health checkup in 2008, 949,550 were included in this study and observed from 2009 to 2018. The risk of ICH was significantly increased in men with stage 2 HTN {adjusted hazard ratio [aHR] 2.002 [95% confidence interval (CI) 1.203–3.332]} and in women with stage 1 HTN [aHR 2.021 (95% CI, 1.251–3.263)]. The risk of SAH was significantly increased in both men [aHR 1.637 (95% CI, 1.066–2.514)] and women [aHR 4.217 (95% CI, 2.648–6.715)] with stage 1 HTN. Additionally, the risk of HS was significantly increased in men with stage 2 HTN [aHR 3.034 (95% CI, 2.161–4.260)] and in women with stage 1 HTN [aHR 2.976 (95% CI, 2.222–3.986)].ConclusionTo prevent primary HS, including ICH and SAH, BP management is recommended for adults under the age of 65 years with stage 1 HTN

Organic broadband THz generators optimized for efficient near‐infrared optical pumping

New organic THz generators are designed herein by molecular engineering of the refractive index, phonon mode, and spatial asymmetry. These benzothiazolium crystals simultaneously satisfy the crucial requirements for efficient THz wave generation, including having nonlinear optical chromophores with parallel alignment that provide large optical nonlinearity; good phase matching for enhancing the THz generation efficiency in the near-infrared region; strong intermolecular interactions that provide restraining THz self-absorption; high solubility that promotes good crystal growth ability; and a plate-like crystal morphology with excellent optical quality. Consequently, the as-grown benzothiazolium crystals exhibit excellent characteristics for THz wave generation, particularly at near-infrared pump wavelengths around 1100 nm, which is very promising given the availability of femtosecond laser sources at this wavelength, where current conventional THz generators deliver relatively low optical-to-THz conversion efficiencies. Compared to a 1.0-mm-thick ZnTe crystal as an inorganic benchmark, the 0.28-mm-thick benzothiazolium crystal yields a 19 times higher peak-to-peak THz electric field with a broader spectral bandwidth (>6.5 THz) when pumped at 1140 nm. The present work provides a valuable approach toward realizing organic crystals that can be pumped by near-infrared sources for efficient THz wave generation

Review Therapeutic Applications of Nucleic Acids and Their Analogues in Toll-like Receptor Signaling

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