DAST Crystal Research Articles

High-resolution time-of-flight terahertz tomography using a femtosecond fiber laser

High-resolution tomographic imaging is demonstrated using a reflection-type terahertz time-domain spectroscopy. To realize a practical system for general use, a robust all-fiber laser was used as the pump light source. Broadband terahertz waves were generated with the combination of optical pulses compressed to 17 fs using optical fibers and a DAST crystal. Using deconvolution signal processing, the wideband spectrum of the generated terahertz waves provided high-axial resolution leading to successful imaging of a multilayered structure containing a 2-microm-thin GaAs layer. To our knowledge, this is the first demonstration of terahertz tomographic imaging of such a thin layer.

Investigation of Technology and Property of NLO Crystals and Nanocomposites Based on Dast (4-Dimethylamino-N-Methyl-Stilbazolium Tosylate)

We study the technology and property of NLO materials based on DAST (4-dimethylamino-N-methyl-stilbazolium tosylate): namely, a monocrystal, nanocrystals, and polymeric nanocomposites. These materials are very promising because their high nonlinear coefficients higher than that of traditional LiNbO3. We have considered the methods of preparation and the modifications of DAST nanocrystals under the synthesis conditions in solutions. The modifications of DAST crystals are determined by the type of solvents, precipitation conditions, and the used surfactant. The obtained nanocrystals are mixed with polymers, and homogeneous transparent films are fabricated.

Methods of generating superbroadband terahertz pulses with femtosecond lasers

This paper presents experimental results and an analysis of methods of generating terahertz (THz) radiation, using femtosecond laser sources: generation by photoconductive semiconductor antennas, nonlinear-optical generation of the difference frequency or optical rectification, and generation using optical breakdown in gases under the action of femtosecond pulses. An undoped semiconductor crystal of indium arsenide (InAs) located in a magnetic field, an electrooptic ZnTe crystal, an organic DAST crystal, and an optical spark in air were used as generators of THz radiation

Crystal Growth of DAST

We explore the growth of the highly nonlinear optical trans-4′-(dimethylamino)-N-methyl-4-stilbazolium tosylate) (DAST) crystals from methanol solutions. We determined the key thermodynamic quantities of the DAST/methanol system, which allowed us to optimize the conditions of the crystal growth and to manipulate the size and morphology of the obtained crystals. We developed confined spontaneous nucleation (CSN), a new technique to control the nucleation and produce good quality seeds and platelets for nonlinear optical applications such as the generation of THz waves. We propose the total excess of Gibbs free energy per volume of the solution as a new parameter (ΔG/Vsol) that fundamentally affects the growth rate from solution instead of the commonly used supersaturation. This parameter involves also concentration and temperature dependence and is therefore applicable to a large range of growth conditions. A model for the growth dynamics of DAST crystals based on the mass balance equations and the measure...

Generation and detection of broadband coherent terahertz radiation using 17-fs ultrashort pulse fiber laser

We describe the generation and detection of broadband terahertz radiation using an all-fiber laser. Optical pulses from a mode-locked fiber laser oscillator are compressed using nonlinear and dispersion effects induced in optical fibers, and 17-fs optical pulses with 170-kW peak power are generated at the wavelength region around 1.5 microm. By injecting these pulses into an organic crystal DAST (4-N, N-dimethylamino-4'-N"-methyl-stilbazolium tosylate), broadband terahertz field is radiated at 0.1-25 THz. The frequency region exceeding 20 THz is achieved with a fiber laser for the first time.

Wideband 15THz response using organic electro-optic polymer emitter-sensor pairs at telecommunication wavelengths

Poled polymer films are employed in terahertz emission and detection using telecommunication wavelength femtosecond pulses. Films 10–15μm thick with electro-optic coefficients as high as 160pm∕V at 1300nm are used to generate and sense subpicosecond pulses with continuous bandwidth up to 15THz. Terahertz emission from a poled polymer film is compared to terahertz emission from the organic crystal DAST. The use of a poled polymer as the terahertz sensor allows the identification of DAST phonons at 1.1, 3.0, 5.3, 8.5, and 12.5THz.

Development of DAST crystals with high damage tolerance

We carried out the damage test of DAST crystals due to identify the causes of laser damages. DAST laser damages were classified into two regions; surface and bulk. The smooth surface tended to suppress the surface damage. As for the bulk, crystal defects such as twin boundary and narrow-line defect (NLD) were the main causes of bulk damages. Twin-free and NLD-free crystal showed high damage tolerance (over 2.8GW/cm2). Growing in the low-supersaturated solution and introducing the solution stirring were effective to obtain DAST crystals with high damage tolerance.

Suppression of Narrow-Line Defects in Organic 4-Dimethylamino-N-methyl-4-stilbazolium Tosylate Crystals through Uninterrupted Solution Stirring during Growth Process

We investigated narrow-line defects (NLDs) as a possible cause of the degradation of the bulk laser-induced-damage tolerance of organic 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystals, which are used for high-power terahertz (THz) application. NLDs are bulk defects and can be classified as planar defects along a-axis. Since the NLDs were considered to be caused by instabilities in the solution environment during the crystal growth, we introduced solution stirring throughout the growth process. Our experimental results showed that solution stirring was effective in decreasing the number of NLDs in DAST crystals in comparison with the conventional growth process. We also discuss the mechanism of NLD formation and the effect of solution stirring from the results of chemical-etching experiments.

Growth and characterization of a new chelating agent added 4-dimethylamino- N-methyl-4-stilbazolium tosylate (DAST) single crystals

A new chelating agent diethylenetriaminepentaacetic acid (DTPA) is employed for the first time to investigate its effect on the metastable zone width, crystal growth and characterization of DAST single crystals. The width of DAST in methanol is measured with and without adding DTPA and EDTA. The presence of DTPA in the DAST solutions enhances the metastable zone width as compared to the EDTA added solutions. The pure and the chelating agents added DAST crystals are grown by the slow cooling method and their crystal size distributions are presented. The addition of the chelating agent DTPA significantly suppresses spurious nucleation and cluster formation in the DAST solution, so that fewer, but larger crystals are easily obtained. The crystalline powder of the grown crystals is examined by X-ray diffraction. The Fourier transform infrared (FTIR) and UV–vis–NIR spectroscopic analyses have been done for the DAST samples. The hardness of the grown pure, DTPA and EDTA added DAST crystals is measured on the (0 0 1) face under different loads.

Synthesis, Crystal Structure, and Second-Order Nonlinear Optical Properties of New Stilbazolium Salts

Two derivatives of the highly nonlinear optical crystal DAST (4-N,N-dimethylamino-4‘-N‘-methyl-stilbazolium tosylate) with m-trifluoromethylbenzene-sulfonate (DSMFS) and p-trifluoromethylbenzenesulfonate (DSPFS) replacing the p-toluenesulfonate of DAST have been synthesized and characterized. X-ray crystallographic analyses revealed that the crystal structure of DSPFS is monoclinic Cc and very similar to that of DAST, whereas DSMFS possesses a centrosymmetric structure with two molecules with different orientations on one site. A Kurtz powder test has shown that DSPFS exhibits a large second-order optical nonlinearity 3 orders of magnitude greater than that of the urea standard.

Surface defects and mechanical hardness of rapidly grown DAST crystals

DAST crystals were grown by the two-zone growth technique and the crystal surfaces were analyzed with help of optical and scanning electron microscopic results. Owing to a high supersaturation in the growth zone, growth striations were observed on the {0 0 1} smooth surfaces (vicinal), while macrosteps were observed on the {0 0 1¯} rough surfaces of the grown crystals. Hillocks occurred on the F-surface, because new layers nucleated before the lateral growth of the deeper layers were completed. The microhardness of the grown crystals was measured on the (0 0 1) and (1 1 0) faces under different loads.

Twin-free and High-Quality DAST Crystals − Effected through Solutions of Lower Supersaturation Coupled with Isothermal Solvent Evaporation

Growth of twin-free and high-quality 4-N,N-dimethylamino-4‘-N‘-methyl stilbazolium tosylate (DAST) crystals, with a surface roughness in the range of 1.7−3.8 nm (<4 nm) for as-grown crystals, has been achieved using solutions of lower supersaturation coupled with isothermal solvent evaporation. In addition, these efficient organic−ionic nonlinear optical (NLO) crystals possessed well-developed 001 and 001 faces that facilitate direct utilization of these crystals for electrooptic (EO) and terahertz (THz) applications. Since this modified growth method necessitates a relatively lower amount (about 46%) of the expensive starting material as compared to that required for routine methods, it is also proven to be cost-effective.

Generation of terahertz pulses through optical rectification in organic DAST crystals: theory and experiment

We present a combined theoretical and experimental investigation of the generation of few-cycle terahertz (THz) pulses via the nonlinear effect of optical rectification and of their coherent detection via electro-optic sampling. The effects of dispersive velocity matching, absorption of the optical and the THz waves, crystal thickness, pulse diameter, pump pulse duration, and two-photon absorption are discussed. The theoretical calculations are compared with the measured spectra of THz pulses that have been generated and detected in crystals of the highly nonlinear organic salt 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST). The results are found to be in agreement with the theory. By the selection of the optical pump wavelength between 700 and 1600 nm, we achieved several maxima of the overall generation and detection efficiency in the spectral range between 0.4 and 6.7 THz, with an optimum at 2 THz generated with 1500 nm laser pulses.

Growth of high-quality DAST crystals for THz applications

Growth of high-quality 4- N, N-dimethylamino-4′- N′-methyl stilbazolium tosylate (DAST) crystal by slope nucleation method with controlled nucleation has been achieved when the DAST solution was subjected to a few successive thermal cycles. The ionic conductivities measured for the stirred and unstirred pre-crystalline solutions showed that the persisting cluster formation in the unstirred solution might have facilitated controlled nucleation. These efficient and high-quality organic-ionic nonlinear optical crystals can be directly utilized for electro-optic and terahertz applications.

Organic nonlinear optical DAST crystals for electro-optic measurement and terahertz wave generation

We review the recent developments of crystal growth, electro-optic (EO) measurements and terahertz (THz) wave generation of organic nonlinear optical (NLO) 4-dimethylamino- N-methyl-4-stilbazolium tosylate (DAST). DAST crystals are expected to apply high-performance electric-field sensors, THz wave generators, and so on. We had developed a new technique called the slope nucleation method (SNM) for the growth of high-quality DAST crystals. Using SNM, we could effectively grow high-quality DAST crystals as compared to conventional spontaneous nucleation method. The EO sensitivity of DAST crystals depended on the crystal thickness and exhibited an increase of more than 45-fold that of the commercial KTiOPO 4 (KTP) crystals. The high-quality DAST crystals could also generate a coherent, widely tunable THz wave in the range of 2–31.5 THz by using the difference frequency generation (DFG) based on the KTP-optical parametric oscillator (OPO). The authors also demonstrated the efficient crystallization by irradiating femtosecond laser pulse into the supersaturated solution. The laser irradiation increased the probability of nucleation. Aging the solution prior to laser irradiation could enhance the probability of nucleation. These results indicate that the laser irradiation could promote DAST nucleation in the supersaturated solution.

High efficiency generation and detection of terahertz pulses using laser pulses at telecommunication wavelengths

We report on the generation and detection of few-cycle THz pulses in the organic crystal DAST (4-N,N-dimethylamino-4'-N'-methyl stilbazolium tosylate) using the telecommunication wavelength 1.5 mum. A continuous spectrum from 1.3 to 4.8 THz was generated with a very high conversion efficiency (E(THz,max) ~ 50 kV/cm at an optical pump pulse energy of 25 muJ in a 0.6 mm thick crystal) using the high nonlinear optical susceptibility of DAST (chi (2)111 = 490 pm/V at lambda = 1535 nm) and velocity-matching observed in this material. Using a frequency-doubled probe beam at lambda = 750 nm, nearly velocity-matched detection can also be realized in ZnTe crystals, resulting in a continuous spectrum from 1.3 to 4 THz.

GROWTH OF 4-(4-DIMETHYLAMINOSTYRYL)-1-METHYLPYRIDINIUM TOSYLATE (DAST) MIXED SINGLE CRYSTAL BY BRIDGMAN–STOCKBARGER METHOD

4-(4-dimethylaminostyryl)-1-methylpyridinium tosylate (DAST) is a very promising organic nonlinear optical (NLO) material for future optoelectronic and nonlinear optical applications, though it cannot be grown from molten state. Hence, a derivative of DAST was synthesized and mixed with DAST. The melting point of the mixed crystal decreased drastically, making it possible to grow a single crystal from molten state. In the present investigation, a mixed single crystal of DAST has been grown from molten state by vertical Bridgman–Stockbarger method.

Restoration of defects generated on the DAST crystal surface by scanning with AFM cantilever tip

We have developed a simple method to restore the defects and holes generated on the (001) surface of 4-(4-Dimethylaminostyryl)-1-methylpyridinium tosylate (DAST) crystal by scanning the crystal with a cantilever tip of an atomic force microscope (AFM). The change in the surface morphology upon repeated scan process was followed by AFM observation at appropriate intervals. By adjustment of applied force on the cantilever, molecular-scale flatness on the crystal surface was restored. We proposed the following mechanism for the restoration: AFM tip carries the small fragment of DAST near step-edge and fills the holes or defects on the surface. After several times scanning of the AFM tip with the fragment, the defect-free surface was restored on the DAST crystal by filling the fragment in satisfying the ionic interaction around the holes.

Widely tunable terahertz-wave generation in an organic crystal and its spectroscopic application

Terahertz (THz)-wave generation using difference-frequency mixing in an organic 4-N, N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) crystal was investigated theoretically and experimentally. We developed a dual-wavelength optical parametric oscillator (OPO) with two KTiOPO4 crystals as the input light source. This oscillator has a tunable range from 1300–1450 nm and is pumped by a diode-pumped, frequency-doubled, Q-switched Nd:YAG laser with a pulse duration of 10 ns. Widely tunable THz waves ranging from 2 to 20 THz were generated from DAST crystals with 0.5- and 1-mm thickness using the OPO. THz output energies of 82 nJ (peak power of 10.3 W) at 11.6 THz and 110 nJ (peak power of 13.8 W) at 19 THz were obtained with a 1-mm-thick DAST crystal. Using a tunable THz-wave system consisting of the THz source and a pyroelectric detector, THz spectroscopic data for a polytetrafluoroethylene sheet were obtained and were comparable to data obtained from a conventional far-infrared spectrometer.

Widely-tunable THz-wave generation in 2–20 THz range from DAST crystal by nonlinear difference frequency mixing

Widely-tunable terahertz (THz)-wave generation from 2 to 20 THz can be achieved in DAST crystal by collinear mixing of dual wavelengths in the range 1300–1450 nm. Output energy of 12 nJ/pulse at 4.4 THz and 152 nJ/pulse at 19 THz was obtained at pump energy of 0.6 mJ/pulse.