Design and analysis of a metallic waveguide with a DAST cap for continuously phase-matched terahertz difference frequency generation

We present a novel source for continuous terahertz (THz) wave generation using an organic ionic salt, 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST). THz waves are generated based on difference frequency generation (DFG) in the device. Phase matching condition and THz generation between 1.3 THz and 2.7 THz, for optical pump around 1.6 μm, are investigated. Our calculations predict that the device produces a relatively high THz output power of 11.07 μW from a 4 cm long waveguide at 2 THz.     

Widely tunable terahertz-wave generation using an N-benzyl-2-methyl-4-nitroaniline crystal

Widely tunable terahertz (THz)-wave generation using difference frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated. To our knowledge, this is the first report of THz-wave generation by BNA DFG. Large, high-quality single crystals of BNA (phi 8 mm x 30 mm) were grown using the vertical Bridgman method. The nonlinear optical (NLO) coefficient d(33) of the BNA crystal is approximately 234 pm/V, which is the largest value reported for any yellow NLO material. The collinear phase-matching condition of the type-0 configuration is satisfied using a 0.7-1 microm band pump wavelength. We generated THz waves using an organic BNA crystal; the generation range is 0.1-15 THz.     

Terahertz generation with tandem seeded optical parametric generators

A simple difference frequency generation (DFG) scheme based on two seeded optical parametric generators is presented as a tunable terahertz (THz) source. Using the nonlinear optical crystal 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST) as the DFG crystal, our system has demonstrated continuous and seamless tunable operation from 1.6 to 4.5 THz. The output bandwidth of the THz source is 2.4 GHz. The utility of the source over this spectral range is demonstrated by measuring a high-resolution transmission spectrum of water vapor in air.     

Difference-frequency terahertz-wave generation from 4-dimethylamino-N-methyl-4-stilbazolium-tosylate by use of an electronically tuned Ti:sapphire laser

Among nonlinear materials, the organic ionic salt crystal 4-dimethylamino- N -methyl-4-stilbazolium-tosylate (DAST) is known for its large nonlinearity. We generated a coherent terahertz (THz) wave, using DAST, from the difference frequency between two oscillating wavelengths of an electronically tuned Ti:sapphire laser. In LiNbO(3), LiTaO(3), KTiOPO(4), and GaP crystals, THz-wave generation was not observed under the same experimental conditions. This result proves the high efficiency of DAST crystals for generation of difference-frequency THz waves.     

基于有机吡啶盐晶体的太赫兹频率上转换探测

利用激光泵浦国产有机吡啶盐4-(4-二甲基氨基苯乙烯基)甲基吡啶对甲基苯磺酸盐(4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate,DAST)晶体,通过非线性频率上转换方法实现了室温运转的高灵敏、快响应、宽频段太赫兹探测.高效生成了近红外上转换光,采集到其脉冲包络和光谱,获得了ns量级的时间分辨率,并换算太赫兹波的频率,实现了对太赫兹信息的全面表征.与商用高莱探测器相比,上转换方法在19 THz频点的探测灵敏度高4个数量级;在可探测频率3.15—29.82 THz范围内,响应度普遍高2—3个数量级.结果表明:室温下的光泵频率上转换探测方法在时间分辨率和响应度方面远优于传统的热探测器,极大地提高了差频有源太赫兹系统的动态范围,使差频源在太赫兹波谱分析和成像等领域具有更大的应用潜力.     

Tunable terahertz generation from one CO2 laser in a GaSe crystal

Coherent terahertz pulses have been generated at a range of 236.3-1104.5 μm (0.27-1.3 THz) by one CO₂ laser with dual-wavelength output based on collinearly phase-matched different frequency generation (DFG) in a GaSe crystal. This source has the advantages of compact and simplicity for tuning. The output power of the THz pulse and phase-matching conditions were investigated. The maximum single pulse energy of 11 nJ was generated at a frequency of 1.23 THz (243.6 μm), corresponding to a peak output power 182 mW.     

Generation of widely tunable Fourier-transform-limited terahertz pulses using narrowband near-infrared laser radiation

Widely tunable, Fourier-transform-limited pulses of terahertz (THz) radiation have been generated using (i) crystals of the highly nonlinear organic salt 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST), (ii) zinc telluride (ZnTe) crystals, (iii) gallium phosphide (GaP) crystals, and (iv) low-temperature-grown gallium arsenide (LTG-GaAs) photomixers with THz spiral antennas. Outputs from two narrowband (Δν < 1 MHz, λ ∼ 800 nm) cw titanium-doped sapphire (Ti:Sa) ring lasers with a well-controlled frequency difference were shaped into pulses using acousto-optic modulators (AOM), coupled into an optical fiber, pulse amplified in Nd:YAG-pumped Ti:Sa crystals and used as optical sources to pump the THz emitters. The THz radiation was detected over a broad frequency range and its bandwidth was determined to be ∼10 MHz. The spectroscopic potential of the THz source is illustrated by the absorption spectrum of a pure rotational transition of OCS.     

Broadband terahertz light source pumped by a 1 μm picosecond laser

We obtained a frequency tunable, low-coherence, picosecond, terahertz (THz) output with a high repetition rate from a picosecond Nd:YVO₄ bounce laser in combination with tandem periodically poled stoichiometric lithium tantalate and 4′-dimethylamino-N-methyl-4-stilbazolium tosylate crystals. The frequency of the THz output was tunable in the range 2.1–7.1 THz with a linewidth of ~3.5 THz at 2.2 THz. The THz output had a maximum peak power of ~180 mW and an average power of ~0.65 μW at 3.9 THz. This system has the potential to realize ultra-high speed, THz coherence tomography.     

DAST/SiO2 multilayer structure for efficient generation of 6 THz quasi-single-cycle electromagnetic pulses

We propose a DAST/SiO(2) multilayer structure for efficient generation of near-single-cycle THz transients with average frequency around 6 THz via collinear optical rectification of 800 nm femtosecond laser pulses. The use of such a composite material allows compensation for the phase mismatch that accompanies THz generation in bulk DAST crystals. The presented calculations indicate a strong increase in the THz generation efficiency in the DAST/SiO(2) structure in comparison to the case of bulk DAST crystal.     

有机电光晶体4-(4-二甲基氨基苯乙烯基)甲基吡啶对甲基苯磺酸盐的太赫兹光谱研究

采用色散校正的密度泛函理论(dispersion-corrected density functional theory,DFT-D2)对有机电光晶体4-(4-二甲基氨基苯乙烯基)甲基吡啶对甲基苯磺酸盐(4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate,DAST)进行结构优化和太赫兹光谱计算,通过逐步提高精度进行几何优化的方法寻找DAST收敛的基态稳定结构,获得与DAST初始结构相一致的基态稳定结构.在此结构的基础上,在0—4 THz范围的太赫兹计算光谱与实验测量结果一致,说明采用DFT-D2进行优化的合理性.重要的是,首次通过计算的太赫兹光谱对DAST在0—4 THz范围的太赫兹吸收峰的振动模式进行了详细归属.结果表明:1.12 THz处的振动是DAST阴阳离子的光学声子模式,1.46 THz和1.54 THz两处的振动主要与磺酸盐有关,而2.63 THz和3.16 THz两处的振动则分别源于阳离子的扭转振动和阴离子的转动.该结果不仅很好地说明了阴阳离子分别在太赫兹响应中的贡献,而且为今后通过取代阴阳离子基团获取具有更高二阶非线性效应的DAST衍生物的新合成提供了重要的参考和指导.本文结果说明密度泛函理论在太赫兹光子学上的重要应用,对探究有机电光晶体的太赫兹响应物理原理、性能控制等具有重要的指导价值.     

ESR Spin Trapping of the Reaction Between Urate and Peroxynitrite: the Hydrogen Adduct

Under basic conditions, the hydrogen adduct of the spin traps N-t-butyl-phenylnitrone (PBN) and α-(4-pyridyl-1-oxide)-N-t-butylnitrone (POBN) is formed as a major product in the reaction between the urate anion or dianion and peroxynitrite. It shows a characteristic nine-line spectrum with an intensity pattern of (1:2:1:1:2:1:1:2:1) and hyperfine splitting constants of a(H) = 10.68 G (two magnetically equivalent protons) and a(N) = 16.64 G. The hydrogen adduct of PBN also forms in the absence of urate. In this case, its formation is proposed to follow the non-traditional “inverted spin trapping” mechanism, followed by electron transfer between spin traps and the PBN–OH (or POBN–OH) adduct or one of its decomposition products. The H adduct formation is amplified when uric acid and peroxynitrite are present. Different yields of H adducts obtained from various spin traps supported the inverted spin trapping mechanism and can be rationalized by the relative redox potential of the spin traps.     

Highly-efficient fully-fiberized mid-infrared differential frequency generation source and its application to laser spectroscopy

Widely-tunable, fully-monolithic, mid-infrared (mid-IR) deference frequency generation source (DFG) is presented. By using a custom designed fiber-pigtailed periodically poled lithium niobate (PPLN) crystal module the idler beam was generated with an efficiency of 21%/W, yielding 2.6 mW of optical output power. The proposed all-fiber configuration radically simplified the optical frequency conversion setup, making it robust and easily configurable. The usefulness of the constructed source was verified by performing simultaneous wavelength modulation spectroscopy (WMS) laser trace gas detection of methane, near 2999 cm^?1, and ethane, near 2997 cm^?1, via two independently generated, tunable idler beams.     

High-speed planar laser-induced fluorescence of the CH radical using the C^{ 2} \varSigma ^{ + } {-}X^{ 2} \varPi \left( {0,0} \right) band

The potential for kHz-rate or high-speed planar laser-induced fluorescence (PLIF) of the Methylidyne (CH) radical using its \(C^{ 2} \varSigma ^{ + } {-}X^{ 2} \varPi\) (v′ = 0, v′′ = 0) band was investigated. We show that due to its strength and the excitation wavelength (~314 nm), which is conveniently generated by a dye laser operating with a red dye, the C–X(0,0) band is a good choice for CH LIF studies wherein suppression of background scattering is not required. While interference from polycyclic aromatic hydrocarbons is small, that caused by hydroxyl (OH) can be significant. Nonetheless, the OH lines can be avoided, and we observe good CH image fidelity. Most importantly, we show that due to the favorable properties of the CH C–X band (i.e., good signal strength and convenient wavelength), it can be used for PLIF at kHz acquisition rates using a continuously pulsing laser system. This is demonstrated in laminar and turbulent CH₄-air flames with a laser system operating at 10 kHz and delivering ~0.2 mJ/pulse at 314 nm.     

Enhancement of terahertz wave difference frequency generation based on a compact walk-off compensated KTP OPO

A compact, walk-off compensated dual-wavelength KTP OPO near the degenerate point of 2.128 μm pumped by a Nd:YAG pulsed laser is employed as the pump for terahertz (THz) source based on difference frequency generation (DFG) in a GaSe crystal. Coherent THz radiation that is continuously tunable in the range of 81-1617 μm (0.186-3.7 THz) is achieved. An enhancement of 76.7% in average for the THz energies at different wavelengths is realized using the walk-off compensated KTP OPO than the common one. Using a 8 mm-long GaSe crystal, the maximum output THz pulse energy is 48.9 nJ with the peak power of 11 W, corresponding to the energy conversion efficiency of 5.4 × 10^− 6 and the photon conversion efficiency of about 0.09%.     

New spiro[benzotetraphene-fluorene] Derivatives: Synthesis and Application in Sky-Blue Fluorescent Host Materials

Blue light-emitting spiro[benzotetraphene-fluorene] (SBTF)-based host materials, 3-(1-naphthyl)-10-naphthylspiro[benzo[ij]tetraphene-7,9′-fluorene] (1), 3-(2-naphthyl)-10-naphthylspiro[benzo[ij]tetraphene-7,9′-fluorene] (2), and 3-[2-(6-phenyl)naphthyl]-10-naphthylspiro[benzo[ij]tetraphene-7,9′-fluorene] (3) were designed and prepared via multi-step Suzuki coupling reactions. Introducing various aromatic groups into SBTF core lead to a reduction in band gap and a determination of the color purity and luminescence efficiency. Typical sky-blue fluorescent organic light emitting diodes with the configuration of ITO/N,N′-di(1-naphthyl)-N,N′-bis[(4-diphenylamino)phenyl]-biphenyl-4,4′-diamie (60 nm)/N,N,N′,N′-tetra(1-biphenyl)-biphenyl-4,4′-diamine (30 nm)/host: dopant (30 nm, 5 %)/LG201 (electron transporting layer, 20 nm)/LiF/Al were developed using SBTF derivatives as a host material and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph) as a sky-blue dopant material. A device obtained from three materials doped with DSA-Ph showed color purity of 0.148 and 0.239, a luminance efficiency of 7.91 cd/A, and an external quantum efficiency >4.75 % at 5 V.     

High-efficiency Red Electrofluorescence Devices Employing a Rhenium Complex as Phosphorescent Sensitizer

Efficiencies of up to 7.81 cd/A and brightness of 8843 cd/m² are obtained based on green–yellow phosphorescent material, fac 4, 4′-bi (tert-butyl)-2, 2′-bipyridine Re(CO)₃Cl[(Bu ^t bpy)Re(CO)₃Cl] doped into 4,4′-N,N′-dicarbazole-biphenyl (CBP) host. We also fabricate high-efficiency red devices by introduction of (Bu^tbpy)Re(CO)₃Cl as a sensitizer, and the undoped ultrathin layer 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as fluorescent acceptor. Förster energy of triplet and singlet are efficient due to appropriate overlap between emission spectrum of (Bu ^t bpy)Re(CO)₃Cl and absorption spectrum of DCJTB.     

Use of the organic crystal DAST for terahertz beam applications

We report the use of the organic crystal DAST as a freely propagating electromagnetic-wave sensor at terahertz (THz) frequency. We also report the result of using a DAST crystal as a mid-IR THz emitter. Compared with the optical rectification from our best ZnTe emitter, that obtained by use of DAST demonstrates a sixfold enhancement of radiation at 7-20 THz.     

Determination of the refractive index over a wide wavelength range through time-delay measurements of femtosecond pulses

We present a simple method to measure the refractive index dispersion over a broad wavelength range (0.6-1.6 μm). In a first step, the optical group indices are obtained by measuring the time-retardation of tunable 150 fs laser pulses within a sample relative to air. The refractive index dispersion is then calculated using a Sellmeier equation that describes the measured group index dispersion. We show that our experimental data agree with previously published results to within 2 × 10⁻⁴ for a 3 mm thick sample of fused silica and to within 3 × 10⁻³ for the index n₁ of a 2 mm thick crystal of the highly dispersive and anisotropic organic crystal 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST).     

Mid-infrared trace gas detection using continuous-wave difference frequency generation in periodically poled RbTiOAsO4

A tunable mid-infrared continuous-wave (cw) spectroscopic source in the 3.4–4.5 μm region is reported, based on difference frequency generation (DFG) in a quasi-phase-matched periodically poled RbTiOAsO₄ (PPRTA) crystal, DFG power levels of 10 μW were generated at approximately 4 μm in a 20-mm long PPRTA crystal by mixing two cw single-frequency Ti:Al₂O₃ lasers operating near 713 nm and 871 nm, respectively, using a laser pump power of 300 mW. A quasi-phase-matched infrared wavelength-tuning bandwidth (FWHM) of ∼12 cm^-1 and a temperature tuning rate of 1.02 cm^-1/°C were achieved. Experimental details regarding the feasibility of trace gas detection based on absorption spectroscopy of CO₂ in ambient air using this DFG radiation source are also described. Received: 23 October 2000 / Revised version: 22 January 2001 / Published online: 27 April 2001     

非线性有机材料DAST的研究及应用

随着光通信、光信息处理技术等的快速发展,非线性光学材料越来越受到学术界与工业的关注。与无机非线性光学材料相比,有机非线性光学材料具有响应时间短、易于加工和高非线性系数等优点。其中,4-(4-二甲氨基苯乙烯基)甲基吡啶对甲苯磺酸盐(DAST)是人工设计的一种具有非中心对称性和较强极化率的有机非线性材料。大量理论和实验结果表明,DAST具有高二阶光学非线性系数、高电光系数、大双折射率差和低介电常数等特性,能够产生更快、更强的光学非线性响应,还具有优异的太赫兹波发射及吸收性能,是目前综合性能最优、应用最广、最具研究价值的非线性有机材料。近年光谱研究显示,DAST还表现出各向异性的太赫兹光谱特性。文章系统地总结了DAST晶体在太赫兹波发射、二次谐波产生、电光探测和电光调制等众多领域的国内外研究成果,还概括了我们团队近期在DAST基薄膜、太赫兹光谱、光电特性、超材料等新领域的研究进展。此外,还提出将准相位匹配法用于DAST晶体研究、利用电场或自组装膜诱导生长DAST晶体等新思路。对DAST成果的总结和梳理,将促进非线性DAST有机材料在电光调制器、频率转换器、THz探测器等重要领域的应用与理论研究,使其能够更加广泛地应用在光通信、光信息处理、军事技术等重要领域。