High-frequency response and acoustic phonon contribution of the linear electro-optic effect in DAST

We present a simple method to measure the contribution from acoustic phonons to the linear electro-optic effect. A step voltage with a short rise time (600 ps) is applied to the sample, and the electro-optic response is measured in the time domain. For the organic salt 4-N, N-dimethylamino-4(?)-N(?)-methyl-stilbazolium tosylate (DAST), we obtained a negative contribution of r(a)(111) = -1.0 +/- 0.1 pm/V to the overall electro-optic coefficient of r(111) = 48 +/- 5 pm/V (lambda = 1535 nm). The electro-optic response was limited only by the rise time of the applied voltage, which corresponds to a frequency of 1 GHz.     

Cascaded contributions to degenerate four-wave mixing in an acentric organic crystal

We show both theoretically and experimentally by degenerate four-wave mixing that the combined processes of optical rectification and the linear electro-optic effect contribute to the large effective third-order susceptibilities [14chi((3))(CS(2))] of the organic salt 4-N, N^- dimethylamino-4(?) -N(?) -methylstilbazolium toluene-p- sulfonate (DAST). Furthermore, the knowledge of the cascaded contributions was used to determine absolute values of the third-order susceptibilities as well as their sign.     

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。     

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.     

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.     

Atomic-force microscope observation and molecular-scale flattening of the single-crystal surface of 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST)

The surface of an organic electro-optic crystal tends to be covered with a degenerate rough layer, which may cause light scattering or unfavorable transmission of light. We demonstrate a novel method of removing this layer and flattening the (001) surface of a 4-dimethylamino- N -methyl-4-stilbazolium tosylate (DAST) crystal on a molecular scale by applying suitable force on the tip of an atomic-force microscope (AFM). When the loading force on the AFM tip is kept near 10 nN, the DAST molecules can be removed layer by layer. This method produced a large, flat terrace of 250,000 nm(2) , and the molecular-scale flatness of this area was confirmed by AFM observation.     

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.     

非线性光学晶体4-(4-二甲基氨基苯乙烯基)甲基吡啶对甲基苯磺酸盐的制备与性能研究

采用籽晶法制备了大体积高质量的4-(4-二甲基氨基苯乙烯基)甲基吡啶对甲基苯磺酸盐(DAST)晶体，对制备条件进行了优化，对DAST晶体X射线衍射谱图中的主要峰进行了指标化.另外，还对所制备的DAST晶体的透光性、热稳定性进行了研究， 并对265℃的焙烧产物进行了X射线衍射 (XRD)和傅里叶变换红外光谱(FT-IR)测试，证明了260℃是DAST晶体的熔化相变温度， 说明采用熔融法制备DAST晶体是可行的；同时还对DAST晶体的热失重过程进行了初步探讨. 关键词： 4-(4-二甲基氨基苯乙烯基)甲基吡啶对甲基苯磺酸盐 晶体生长 X射线衍射 热稳定性     

Far-infrared second-harmonic generation and pulse characterization with the organic salt DAST

We generated the second harmonics of pulses in the far-infrared region (30-55 microm), using the organic salt dimethylamino-4-N-methylstilbazolium tosylate (DAST). We demonstrate that DAST can be used to characterize ultrashort pulses in a spectral region where no other materials are available. To illustrate the need for such characterizations, we show the effects of propagation through air on the shape of ultrashort far-infrared pulses.     

利用国内生长的有机晶体DAST作为THz源的研究

利用时域光谱技术分别测量了通过光整流法在国内新生长的有机晶体DAST和国际常用的高效能THz晶体ZnTe中产生的THz脉冲的时域光谱,也测量和分析了有机晶体DAST在THz波段的色散和吸收特性,并通过热辐射探测计(4.2 K Si-bolometer)测量了THz脉冲的绝对能量。结果表明,国内新生长的有机晶体DAST的确是一种高效率的THz产生晶体,优化生长条件后将可得到更高的产生效率。     

Thin-Film Nonlinear Optical Crystals for Intracavity Electro-Optic Modulation

Optics and Spectroscopy - Methods of preparation of single-crystal 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST) films with a thickness of several micrometers, designed...     

核磁共振波谱法表征 DAST 分子结构

DAST 晶体（即 4-N, N-二甲胺基-4’-N’-甲基-氮杂芪的对甲苯磺酸盐晶体）具有优良的二阶非线性光学性能和电光效应特性,可用于红外至太赫兹波段辐射与检测领域．为了表征 DAST 源粉分子结构及纯度,选择 CD₃OD 和 DMSO-d₆ 两种溶剂分别对 DAST 源粉进行了¹H NMR、 ¹³C NMR、 ¹H-¹H COSY、 HSQC 和 HMBC 表征．通过对谱峰的归属及分析发现, DAST 在 CD₃OD 溶液中倾向于以离子对状态存在,而在 DMSO-d₆ 溶液中倾向于以解离状态存在,并根据谱峰的变化推测了 DAST 在从 CD₃OD 溶液环境向 DMSO-d₆ 溶液环境变化时解离过程中电子云在共轭结构上重新分布的规律和骨架的变化情况．     

Femtosecond laser ablation of DAST

High rate femtosecond (fs) laser ablation of the organic salt 4-N,N-dimethylamino-4-N-methyl-stilbazolium tosylate (DAST), an organic crystal with very high optical nonlinearities has been demonstrated. The threshold fluence and the ideal fluence range for damage free ablation for the wavelengths 550, 600, and 775 nm have been determined and the quality of the produced grooves has been investigated. The threshold fluences are in the order of 10–70 mJ/cm² and the ideal fluence range for damage free ablation is ranging from 30 to 300 mJ/cm², depending on the wavelength. The optimal focussing for ablation has been investigated and first results towards the structuring of a ridge waveguide are presented. We conclude that this method is most promising for waveguide patterning of DAST surfaces for integrated optics applications.     

Highly efficient terahertz electro-optic sampling by material optimization at 1060 nm

Different electro-optic crystals (CdTe, DAST, GaAs, GaP, and ZnTe) are investigated with respect to their ability as detection crystals for terahertz pulse electro-optic sampling at a wavelength of 1060 nm. Calculations of the detection response are in good qualitative agreement with experimental results. While CdTe shows the strongest signal for applications below 1 THz, GaP exhibits a broadband spectrum. By the choice of an adequate crystal a comparable detection efficiency to electro-optic sampling at a wavelength of 800 nm can be achieved.     

Tunable terahertz-wave generation from DAST crystal by dual signal-wave parametric oscillation of periodically poled lithium niobate

The first known demonstration of tunable terahertz-wave generation by difference-frequency generation of dual signal-wave quasi-phase-matched optical parametric oscillation was performed with periodically poled LiNbO(3) (PPLN) with a series of gratings. An organic ionic salt, 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST), was used as a nonlinear crystal. A compact terahertz-wave source resulted, and changing the temperature of the PPLN permitted the wavelength to be varied from 120 to 160 mum. The wavelength could be tuned from 100 to 700 mum by proper selection of combinations of periodically poled gratings.     

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.     

SECOND-ORDER NONLINEAR OPTICAL PROPERTIES OF TITANIUM DIOXIDE NANOPARTICLES AND NANOPARTICLES-DYE COMPOSITES

Hyper-Rayleigh Scattering (HRS) technique was used to study the second-order nonlinear optical (NLO) responses of aqueous titanium dioxide (TiO₂) nanoparticles of 10nm in size, and two nanoparticles-dye composites prepared by adding rhodamine B (Rh610) or the organic tosylate salt of dimethylaminostilbazolium (DAST). Results showed that the "per particle" first hyperpolarizability β for TiO₂ nanoparticles is very large, in the range of 10²⁶ esu. With adding Rh610 and DAST, the HRS signals of the composites were further enhanced. And in TiO₂/Rh610 composite the enhancement was obviously greater than that of TiO₂/DAST composite. It has proved that non-centrosymmetry of the nanocrystal/solution interface contributes mainly to its large "per particle" β, overwhelming the nanocrystal core. So the interactions between nanoparticles surfaces and adsorbed dyes were very important for their second-order NLO responses. HRS technique provides a useful new NLO method to characterize the surface structures and microenvironment of nanoscale materials.     

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.     

电光聚合物在全光技术实现THz波产生与检测中的研究进展

研究和开发基于超短波和低能量激光系统的高振幅、宽频带的太赫兹系统一直是太赫兹应用领域的一个热点。与传统电光晶体材料相比,以电光聚合物薄膜作为太赫兹波产生和检测的最大的优势在于： 一方面电光聚合物不具有晶格结构相应的不存在声子的吸收和太赫兹折射率的散射问题,避免光谱间隙的产生;另一方面电光聚合物薄膜与已用于宽频太赫兹系统中的超薄电光晶体材料相比,更易于加工和处理。另外,可以自行设计得到低相位失配和高电光系数的电光聚合物材料,得到高效率和宽频带的太赫兹辐射源。可以预见,电光聚合物在这一领域的应用必将有宽阔的发展空间和学术研究意义。介绍电光聚合物的电光效应及二阶非线性生色团合成等相关理论问题基础上,综述了近二十多年来,电光聚合物在基于全光技术实现太赫兹波产生与检测系统中的研究进展,主要包括: 掺钛蓝宝石飞秒激光激发下,共聚型和主客体型电光聚合物实现太赫兹波产生与检测;光纤飞秒激光激发下,主客体型电光聚合物实现太赫兹波的产生与检测。     

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.