dimethylamino-N-methyl-4-stilbazolium Tosylate Research Articles

Theoretical Exploration of Terahertz Single-Photon Detection and Imaging by Nonlinear Optical Frequency Up-Conversion

Terahertz single-photon detection and imaging have attracted full attention recently. Nonlinear optical frequency up-conversion is a promising technique that can be expected to satisfy this demand thanks to its high sensitivity and fast response. In this paper, theoretical analysis and numerical calculations based on the organic salt 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal were performed to show that the optimization of the detection of terahertz is different from that of the generation of terahertz, including the use of difference-frequency generation (DFG) technique, the larger thickness of the crystal, and especially the selection of the polarization direction of the pumping laser. For two different polarization configurations, the photons of the up-converted signal light both can be amplified compared with the number of incident terahertz photons under some optimal designs of the nonlinear frequency conversion process. Therefore, terahertz single-photon detection can be realized with single-photon detectors (SPDs) or even possibly with ordinary avalanche photodiodes (APDs). Furthermore, for terahertz single-photon imaging, the frequency up-conversion with the pumping laser polarized along b-axis of DAST crystal has a better performance, which is rarely used in terahertz generation.

Self-Raman Nd-doped vanadate laser: a pump source of organic crystal based difference frequency generation

We analyse a method of high repetition rate, efficient terahertz wave difference frequency generation (DFG) by using a self-Raman Nd-doped vanadate laser to pump 4’-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST). A semi-analytical model is introduced that combines the rate equations (for the self-Raman dual-wavelength laser) and the coupled-wave equations (for the DAST-based DFG). Calculations were performed on a diffusion-bonded Nd:YVO4-YVO4 laser with an acousto-optic Q-switch. Parameters (including transmittances at output mirror and crystal lengths) were optimized to obtain high output. Sub-milliwatt average power is predicted under current pump condition.

The Influence of Dimensional Parameters of DAST Nanocrystals on Their Linear and Nonlinear Optical Parameters

Nonlinear optical trans-4'-(dimethylamino)-N-methyl-4-stilbazolium tosylate (DATS) nanocrystals are synthesized in polymethylmethacrylate by nucleation in a drying polymer film at different solvent concentrations. Nanocrystals with sizes progressively increasing from 70 to 2500 nm are obtained. The effect of size on the nonlinear optical coefficient is studied by generation of the second harmonic of a 1064-nm Nd:YAG laser. It is shown that the nonlinear optical coefficient decreases with decreasing crystal size by no more than a factor of 1.5. The optical absorption spectra and the shape of nanocrystals are studied as functions of their size.

Влияние размерных параметров нанокристаллов DAST на их линейные и нелиейно-оптические параметры

AbstractNonlinear optical trans -4'-(dimethylamino)- N -methyl-4-stilbazolium tosylate (DATS) nanocrystals are synthesized in polymethylmethacrylate by nucleation in a drying polymer film at different solvent concentrations. Nanocrystals with sizes progressively increasing from 70 to 2500 nm are obtained. The effect of size on the nonlinear optical coefficient is studied by generation of the second harmonic of a 1064-nm Nd:YAG laser. It is shown that the nonlinear optical coefficient decreases with decreasing crystal size by no more than a factor of 1.5. The optical absorption spectra and the shape of nanocrystals are studied as functions of their size.

Theoretical Study of Organic Crystal-Based Terahertz-Wave Difference Frequency Generation and Up-Conversion Detection

Optical pumped quasi-monochromatic and tunable terahertz (THz) wave generation and detection system is investigated theoretically. Calculations are performed on an organic nonlinear medium: 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), due to the superior properties (large dynamic range and extremely wide tunability). The behavior of pulses during the nonlinear interaction is characterized. Previous experimental results are well explained. Our theoretical model provides an approach to improve the performance of the coherent THz system from two aspects. For the source part, an optimal crystal thickness that maximizes the THz output is obtained. For the detector part, a linear conversion relation (used for calibration) and the parameter dependence of responsivity are provided. It was predicted that a dynamic range of ten orders could be achieved with the DAST-based THz system. The presented model can also be applied to guide the design of THz systems with other nonlinear mediums in collinear phase-matching geometry.

Growth of Ultrathin and Highly Efficient Organic Nonlinear Optical Crystal 4′-Dimethylamino-N-methyl-4-Stilbazolium p-Chlorobenzenesulfonate for Enhanced Terahertz Efficiency at Higher Frequencies

Growth of an ultrathin (thickness as low as 23 μm) organic nonlinear optical crystal of 4′-dimethylamino-n-methyl-4-stilbazolium p-chlorobenzenesulfonate (DASC) has been achieved by the slope nucleation method for generating THz waves with relatively higher efficiencies especially at higher frequencies. The propensity of lower solubility of this material in methanol solvent as compared to 4′-dimethylamino-n-methyl-4-stilbazolium tosylate (DAST) was favorably exploited to obtain these thin crystals. The DASC crystals thus obtained were studied for the generation of THz waves, and their efficiency was compared with DAST and zinc telluride (ZnTe) single crystals. The enhancement THz efficiency observed in thinner DASC could be attributed to better velocity matching and weaker absorption as compared to thicker crystals of the same material.

Crystal Growth and Morphology Control of OH1 Organic Electrooptic Crystals

We report on the growth of large-size phenolic configurationally locked polyene OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile) organic bulk crystals. OH1 has been recently identified as very promising for electro-optic and THz-wave applications, with several advantages compared to the benchmark ionic salts such as DAST (4′-dimethylamino-N-methyl-4-stilbazolium tosylate) and DSTMS (4-N,N-dimethylamino-4′-N′-methyl-stilbazolium 2,4,6-trimethylbenzene sulfonate). We investigate the thermodynamic equilibrium conditions, the solubility, and the metastable zone of OH1/methanol system. The intermolecular hydrogen-bond interactions between the OH1 and methanol molecules impede the main supramolecular interactions between the OH1 molecules, which results in a strong delay in nucleation and provides a very large metastable-zone width. We furthermore studied the crystal morphology and the solvent effect by analyzing the relationship between the morphology and the molecular orientation at...

Post-processing of spin-coated organic thin films in solvent vapors: Vapor pressure monitoring by infrared absorption and the effect of electric fields

We have examined the effect of the post-processing of spin-coated tetracene and DAST (4′-dimethylamino-N-methyl-4-stilbazolium tosylate) films in their solvent vapor to examine the possibility of precisely controlling the vapor pressure and studying the effect of applying electric fields to their structural properties. It was demonstrated that the solvent concentration in the vapor phase can be monitored by infrared spectroscopy. The thickness uniformity of the tetracene films was improved by the formation of a mixture of the tetracene crystals with the saturated solution (“solution ultrathin film”) on the surface. Crystals with an ordered orientation were obtained by additionally applying an electric field during the vapor exposure of the DAST. The present technique for monitoring the solvent vapor pressure is also applicable for controlling the drying speed during the inkjet printing of functional organic materials.

An investigation on the growth and characterization of DAST crystals grown by two zone growth technique

Two zone growth technique (TZGT) is reported for the growth of 4′-dimethylamino- N-methyl-4-stilbazolium tosylate (DAST) crystals, being one of the best organic materials for second-order nonlinear optics. The growth rates are estimated with respect to the temperature difference ( Δ T ) between the hot zone and cold zone. The grown crystals are morphologically interpreted. The crystal quality is examined from the XRD double crystal rocking curve. The microhardness number has been estimated for the DAST crystals grown at the different Δ T's. A good optical transmission is found in the cleaved plate of grown DAST crystal. The formation of microcrystals is observed on the ( 0 0 1 ¯ ) face.

Femtosecond Laser-Induced Crystallization of 4-(Dimethylamino)-N-methyl-4-stilbazolium Tosylate

A femtosecond laser pulse was irradiated to a supersaturated methanol solution of an organic molecule, 4-(dimethylamino)-N-methyl-4-stilbazolium tosylate (DAST), and its efficient crystallization was demonstrated under concentration and temperature conditions where spontaneous crystallization does not occur. The efficiency was higher than that using a nanosecond laser, and the nucleation depended on the laser repetition rate. The results suggest that the nucleation is trigged by cluster formation, which is ascribed to femtosecond laser irradiation effects, such as shock wave generation due to the ablation of the medium and an optical pressure due to the strong electric field.

Electroabsorption in single-crystal film of a second-order optical material

In this letter, we report results of electroabsorption and resonant electro-optic measurements in single-crystal film of 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST). The electroabsorption measurement was made by recording the changes in transmission through the film as an ac field was applied along the dipole axis. The electro-optic measurement was made using the field-induced birefringence method. The measurements were made in the absorptive domain, at 633 and 488nm. The modulation (ΔT∕T) due to electroabsorption decreased as the angle between the incident polarization and the dipole axis was increased. The observed electroabsorption is due to the imaginary part of the electro-optic coefficient. The real part of the electro-optic coefficient (r11) as measured at 633nm is 770pm∕V. The imaginary parts of the electro-optic coefficients at 633 and 488nm are 104 and 259pm∕V, respectively.

Higher-order optical nonlinearities in 4 ′-dimethylamino- N-methyl-4-stilbazolium tosylate

Higher-order optical nonlinearities in an organic molecular salt, 4 ′-dimethylamino- N-methyl-4-stilbazolium tosylate (DAST), are investigated by a two-beam self-diffraction technique using a picosecond laser under single photon as well as two-photon absorption. The intensity dependence of the diffracted signals is measured. At a low concentration (1.0 × 10 −4mol/l) the ratio of χ eff (5)/ χ eff (3) is found to be 0.24 × 10 −6 cm 3/erg. A diffraction efficiencies of 1.2% and 0.02% are observed in the first and sixth-order signals, respectively.

Quadratic Optical Nonlinearities of N‐Methyl and N‐Aryl Pyridinium Salts

AbstractFour series of dyes with dimethylamino electron donor groups and N‐R‐pyridinium (R = methyl Me, phenyl Ph, 2,4‐dinitrophenyl 2,4‐DNPh, or 2‐pyrimidyl 2‐Pym) electron acceptors are studied as their hexafluorophosphate salts. The intramolecular charge‐transfer (ICT) energies (Emax) of these compounds decrease within each of the series in the order R = Me > Ph > 2,4‐DNPh > 2‐Pym, as the electron‐accepting ability of the pyridinium ring increases. Hyper‐Rayleigh scattering with femtosecond 1300 or 800 nm lasers yields fluorescence‐free first hyperpolarizabilities β, and static first hyperpolarizabilities β0[H] are obtained via the two‐state model. Dipole moment changes Δμ12 for the ICT transitions obtained from Stark spectroscopy afford β0[S] values by using β0 = 3Δμ12(μ12)2/2(Emax)2 (μ12 = transition dipole moment). The β0[S] data show that the combination of pyridyl N‐arylation with conjugation extension affords large increases in β0. The β0[H] data generally agree with this conclusion, but resonance effects may explain some apparent anomalies. X‐ray structural studies on various salts reveal that the use of tosylate anions is not a generally applicable approach to engineering noncentrosymmetric structures of pyridinium salts. However, trans‐N‐phenyl‐4‐(4‐dimethylaminophenyl‐4‐buta‐1,3‐dienyl)pyridinium hexafluorophosphate adopts the polar space group Cc, and shows a very large powder second harmonic generation efficiency from a 1907 nm laser, which is similar to that of the well‐studied material trans‐4′‐(dimethylamino)‐N‐methyl‐4‐stilbazolium tosylate (DAST).

Organic thin film crystal growth for nonlinear optics: present methods and exploratory developments

We discuss different methods to grow organic single crystalline thin films from solution. Specifically, we address exploratory developments and apply them on the highly nonlinear material DAST (4′-dimethylamino-N-methyl-4-stilbazolium tosylate) in a DAST/methanol system. We describe the nucleation and growth thermodynamics in solution using a solubility diagram and its Ostwald–Meiers metastable region. The important experimental parameters for bidimensional growth are discussed and first promising results are presented. To cite this article: S. Manetta et al., C. R. Physique 3 (2002) 449–462.

Macroscopically ordered thin films of an organic salt grown by low-pressure organic vapor-phase deposition

We demonstrate the process of low-pressure organic vapor-phase deposition (LP-OVPD) for the growth of crystalline thin films of the organic charge-transfer salt 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) on amorphous TiO 2 substrates. LP-OVPD grown films exhibit polycrystallinity with long-range structural ordering limited only by substrate size. Film surface coverage and macroscopic ordering are strongly determined by the preparation of the TiO 2 surface prior to growth. The morphology and ordering of the films are also found to depend on the deposition temperature and the composition of the precursor molecules. These results suggest that LP-OVPD films are of sufficient quality for use in optical waveguide and related photonic devices.

Single-pass thin-film electro-optic modulator based on an organic molecular salt

Single crystal films of 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) with excellent optical quality have been prepared by modified shear method. Electro-optic modulation for light propagating perpendicular to the film surface was measured at a wavelength (720 nm) where absorption is small. Very large electro-optic modulation (20%) was demonstrated in a 3 μm thick film at a low ac field (1 V/μm, at 4 kHz). Electro-optic modulation in such thin films is suitable for a wide range of applications in free-space interconnects and various modulator structures with light propagating perpendicular to the film. The magnitude of the electro-optic coefficient at 720 nm, r11∼530 pm/V, is significantly larger than that of bulk crystals.

Intense second harmonic generation and long-range structural ordering in thin films of an organic salt grown by organic vapor phase deposition

We discuss the growth of thin films of the organic salt, 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) by the novel process of organic vapor phase deposition (OVPD). These films show long-range structural ordering and very intense second harmonic generation efficiencies (SHG) significantly greater (when normalized for thickness) than that of randomly oriented pure DAST powders. This is the first demonstration of such intense SHG radiation and long-range ordering for a thin film sample of a nonlinear organic salt. Furthermore, this work suggests that OVPD represents a general technique for growing thin films of highly polar, nonlinear optical materials such as DAST.

Organic vapor phase deposition: a new method for the growth of organic thin films with large optical non-linearities

We describe a novel technique, organic vapor phase deposition, for the growth of thin films of optically non-linear organic salts. A volatile precursor of each component of the salt is carried as a vapor to a hot-wall reaction chamber by independently controlled streams of carrier gas. The components react to form a polycrystalline thin film on substrates of glass and gold. Excess reactants and reaction products are purged from the system by the carrier gas. As an example, we demonstrate the growth of polycrystalline, optically non-linear thin films of 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) with > 95% purity.

Molecular crystals and polymers for nonlinear optics

We present new experimental results on second-harmonic generation in highly conjugated organic molecules and molecular crystals recently developed in our group, as well as on electro-optic eftects in thin-film polymers. A large value of the molecular hyperpolarizability of a new hydrazone-type molecule crystallizing in a noncentrosymmetric point group is reported. Large nonlinear optical coefticients of up to 840 ± 200 pmN were measured in crystals of 4'-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST). The new experimental results are compared with theoretical calculations of limiting values of nonlinear optical coefticients. Computer modeling of crystal structures shows good agreement with experimental results. Measurements on electro-optic polymers with high thermal stability are presented.