Tosylate Crystal Research Articles

An ultra-broadband frequency-domain terahertz measurement system based on frequency conversion via DAST crystal with an optimized phase-matching condition

By applying the frequency conversion technique to 4-dimethylamino-N-methyl-4-stilbazolium tosylate crystal, a monochromatic terahertz (THz) measurement system, including both generation and detection, has been developed over quite a broad frequency band, from 1.85 to 30 THz. In the case of frequency upconversion detection of THz waves, for the first time, we used gratings instead of filters to tackle the tough phase-matching conditions for broadband operations. By synchronizing the rotation of two gratings to extract the frequency upconverted signal, the infrared (IR) pumping beam can be tuned freely over 300 nm with decent diffraction efficiency and sufficient isolation between the weak frequency upconversion signal and the strong IR pumping beam to be realized. Such a large tuning range has overcome the limit of commercial filters with a fixed passband, while such a high optical density value has been beyond the limit of commercial tunable filters. Consequently, the proposed frequency domain system gives the largest THz frequency band. Unlike THz time-domain spectroscopy systems in which a fs laser is applied and broadband THz pulses are applied, our system works based on a ns laser and it can function at a single THz frequency with random frequency access ability from pulse to pulse.

Real-time terahertz wave imaging by nonlinear optical frequency up-conversion in a 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate crystal

Real-time terahertz (THz) wave imaging has wide applications in areas such as security, industry, biology, medicine, pharmacy, and arts. In this letter, we report on real-time room-temperature THz imaging by nonlinear optical frequency up-conversion in organic 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate crystal. The active projection-imaging system consisted of (1) THz wave generation, (2) THz-near-infrared hybrid optics, (3) THz wave up-conversion, and (4) an InGaAs camera working at 60 frames per second. The pumping laser system consisted of two optical parametric oscillators pumped by a nano-second frequency-doubled Nd:YAG laser. THz-wave images of handmade samples at 19.3 THz were taken, and videos of a sample moving and a ruler stuck with a black polyethylene film moving were supplied online to show real-time ability. Thanks to the high speed and high responsivity of this technology, real-time THz imaging with a higher signal-to-noise ratio than a commercially available THz micro-bolometer camera was proven to be feasible. By changing the phase-matching condition, i.e., by changing the wavelength of the pumping laser, we suggest THz imaging with a narrow THz frequency band of interest in a wide range from approximately 2 to 30 THz is possible.

Bridging a few terahertz to tens of terahertz: Inspection on a cost-effective, room-temperature operated measurement system based on frequency conversion via 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate crystal

Based on experimental studies, we inspected the feasibility of a frequency conversion system, including both terahertz (THz) generation and detection, by using 4-dimethylamino-N′-methy-4′-stilbazolium tosylate crystal for wideband measurement. At 27 THz, more than five orders dynamic range in power have been obtained. Compared with typical pyroelectric detectors, it is four orders better in terms of sensitivity. Power calibration has been implemented and the minimum detectable pulse energy is 3 fJ, with a corresponding noise equivalent power of 22 pW/Hz1/2. For broadband operation, we have achieved good signal level downwards to 2.5 THz. Such a system can be an excellent complement to classical time-domain spectroscopy systems.

Pump-beam-induced optical damage depended on repetition frequency and pulse width in 4-dimethylamino-N ′-methyl-4′-stilbazolium tosylate crystal

We investigated the dependence of optical damage to an organic nonlinear optical crystal of 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate (DAST) on the repetition frequency and pulse width of the pump beam used to cause the thermal damage. For a pump beam with a pulse width of 15 ns at a wavelength of 1064 nm, the highest damage threshold of 8.0 J/cm2 was measured for repetition frequencies in the range from 10 to 40 Hz. On the other hand, DAST crystals were easily damaged under the repetition rates from 50 to 100 Hz. For 600-ps pulses, a higher damage threshold that was a factor of 11 to 28 times higher in terms of peak intensity was obtained compared with that of 15-ns pulses. In both the cases of 15-ns pulse duration and 600-ps duration, we demonstrated that the thermal effects in DAST crystals dominated the optical damage, which depended on thermal accumulation and dissipation.

Widely tunable broadband terahertz radiation generation using a configurationally locked polyene 2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene] malononitrile crystal via difference frequency generation

Widely tunable terahertz (THz) waves were successfully generated from 0.5 to 10 THz via difference frequency generation (DFG) in a configurationally locked polyene 2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene] malononitrile (OH1) crystal. Potassium titanium oxide phosphate optical parametric oscillator pumped by nanosecond Q-switched Nd:YAG laser was used to generate two waves, which were then used to irradiate OH1 crystal. The maximum energy of the generated THz wave was about 461 pJ/pulse. We investigated the dependency of generated THz energy to the excitation pump power density and OH1 crystal thickness. In addition, we compared the THz energy generated by OH1 crystal to 4-N,N-dimethylamino-4′-N′-methyl-4-stilbazolium tosylate (DAST) crystal using DFG, and we achieved 560 times higher energy using OH1 crystal than DAST crystal at around 1.1 THz.

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.

Growth of Optical Quality 4-Dimethylamino-N-Methyl-4-Stilbazolium Tosylate (DAST) Single Crystals by Novel Technique and their Characterization

Optical quality 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystals have been grown by a novel technique. In conventional growth methods, it was difficult to control the position of spontaneous nucleation, number of nuclei, cluster formation and the nucleation period. Difficulty in the nucleation and growth positioning was effectively solved by the slope nucleation method (SNM) and sizable DAST crystal was grown by two zone growth technique (TZGT) using a seed. Furthermore removal and fixing of seed crystal was difficult in the two zone growth technique. In the present study, we could obtain optical quality bulk DAST crystals by combining SNM and TZGT. The growth rates, size are estimated with respect to temperature gradient and slope shape. The structural, functional and optical analyses were carried for the grown crystals and the results are discussed in detail.

Nucleation kinetics and growth of high-quality DAST crystals with the aid of activated carbon as the adsorbent to micro-crystals in solutions

Growth of highly nonlinear optical 4- N, N-dimethylamino-4′- N′-methyl stilbazolium tosylate (DAST) crystals had been carried out with the addition of activated carbon into the methanol solutions. The metastable zone width and induction period were experimentally determined for the modified growth conditions. Nucleation kinetics and fundamental growth parameters such as surface free energy, critical radius and critical free energy change were also evaluated according to the experimental data. The observations indicated that adsorption to tiny nuclei by activated carbon favored to improve the stabilities of growth solutions. Twin-free seeds and platelets with good qualities were effectively obtained under the optimized conditions. The introduction of activated carbon into growth solutions favors the growth of bulk DAST crystals at a relatively high rate, and high quality bulk crystal with dimensions of 8.3×8.4×2.5 mm 3 was obtained under the optimized growth conditions. In addition, the obtained DAST crystals with well-developed (0 0 1) and (0 0 −1) faces facilitate the direct utilization for electro-optic and nonlinear optical applications. The grown crystals were also characterized by the micro-hardness test, optical transmission and etching investigation.

Vickers Hardness Studies on (001) and (110) Faces of DAST Crystal

4-Dimethylamino-N-methyl 4-Stilbazolium Tosylate（DAST） crystals were prepared by the slope nucleation method. The Vickers hardness of DAST crystal was studied on (001) and (110) faces. It was found that the Vickers hardness increases with increasing load and theHvon (001) face is higher than that on (110) face. The values of Meyer’s index numbernand hardness were calculated fromHv. The results showed that the DAST crystals belong to the soft material category.

Thermal, optical, mechanical and dielectric properties of nonlinear optical crystal 4-methoxy benzaldehyde-N-methyl 4-stilbazolium tosylate

We have grown organic nonlinear optical 4-methoxy benzaldehyde-N-methyl-4-stilbazolium tosylate (MBST) crystals by slow evaporation technique. The grown crystals were analyzed by powder XRD, FTIR, NMR, UV, Thermal, mechanical and dielectric measurements. Lattice parameters and crystallinity of the crystal is confirmed by the XRD studies. The FTIR and NMR studies give the details about the various functional groups present. In order to study the optical quality of the crystal, the UV–vis absorption spectrum was recorded and cut-off wavelength was determined. Melting point of the crystal is found from the differential scanning calorimetry. The powder SHG studies were done with the Kurtz powder technique and the NLO efficiency is 17.2 times greater than that of urea. The surface of the grown crystal was analyzed with etching. Laser damage threshold studies have been carried out for the crystal using a Q-switched Nd:YAG laser of 10 ns pulses.

Surface Morphology and Shape Analysis of Nonlinear Optical Crystal DAST

DAST (4-N, N-dimethylamino-4′-N′-methyl stilbazolium tosylate) crystal has been grown by slope nucleation method (SNM) at different cooling rates (0.5°С/d and 0.1°С/d). The morphology of crystal growing surfaces was observed by scanning electron microscope (SEM) and microscope. Different growth striations were found on the growth surfaces. The arrangements of crystal ions and cooling rate have a great influence on the crystal shape. The relationship between the DAST crystal molecular structure and the crystal shape was systematically studied by analyzing the growing surface morphology. Two-dimensional nucleation mechanism was found on growing faces with increasing supersaturation.

The Surface Morphology and Mechanical Behavior Studies of DAST Crystal on (001) and (110) Faces

4-dimethyla-mino-N-methyl 4-stilbazolium tosylate（DAST） crystals were prepared by the slope nucleation method (SNM). The structural and optical properties of the grown crystals were studied by employing powder XRD and FIIR. The surface morphology of DAST crystals was observed by scanning electron microscope (SEM). The Vickers hardness studies were carried out on (001) and (110) faces of grown DAST crystal. The values of Meyer’s index number n and hardness were calculated from Hv. The results showed that the DAST crystals belong to the soft material category.

Generation of mid- to far-infrared ultrashort pulses in 4-dimethylamino-N-methyl-4-stilbazolium tosylate crystal

Mid- to far-infrared (IR) ultrashort pulses were generated using the organic nonlinear optical crystal 4-dimethylamino-N-methyl-4-stilbazolium tosylate. Frequency downconversion was performed by difference frequency generation (DFG) between the signal and idler pulses from an optical parametric amplifier. By the use of a noncollinear configuration in DFG, the phase matching condition was satisfied for pulse generation in the mid-IR and far-IR ranges. At a noncollinear angle of 0.7° between the signal and idler pulses, IR pulses with spectra extending from 8 to 30 THz were generated. The pulse energy and the temporal duration were estimated to be 300 nJ and 250 fs, respectively, for a signal wavelength of 1540 nm and a noncollinear angle of 0.7°.

High-sensitivity detection of terahertz waves using nonlinear up-conversion in an organic 4-dimethylamino-N-methyl-4-stilbazolium tosylate crystal

We achieved high-sensitivity, rapid-response detection of terahertz (THz) waves using an organic nonlinear optical crystal, 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST). Nonlinear up-conversion in the crystal resulted in a shift of THz waves to near-infrared radiation at room temperature. A minimum THz-wave peak power of about 300 μW was measured at 19.2 THz by detecting the up-converted optical signal with an InGaAs-based photodetector. A noise equivalent power of about 6 nW/Hz1/2 was estimated in this experiment. Optimum conditions were obtained for THz-wave detection using a DAST crystal.

Microhardness properties of 4-methoxy benzaldehyde N-methyl 4-stilbazolium tosylate

The mechanical properties of crystals are evaluated by mechanical testing which reveals certain mechanical characteristics. The fastest and simplest type of mechanical testing is hardness measurement. The Vickers and Knoop microhardness studies have been carried out on 4-methoxy benzaldehyde N-methyl 4-stilbazolium tosylate crystals grown by a slow evaporation technique over a load range of 10–100 g. The Vickers hardness number ( H v ) and the Knoop microhardness number ( H k ) were found to increase with the increase in load. The Meyer's index number ‘ n’ was calculated from H v. The Young's modulus was calculated using the Knoop hardness values. Hardness anisotropy has been observed in accordance with the orientation of the crystal.

Optical phase detection in a 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate crystal for terahertz time domain spectroscopy system at 1.55 μm wavelength

We report the electro-optic detection of terahertz (THz) pulses in a 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate crystal (DAST) using optical phase detection technique and femtosecond optical pulses at telecom wavelength. A THz time-domain spectroscopy system based on 1.55 μm wavelength fiber laser pulses that integrates an ion-irradiated In0.53Ga0.47As photoconductive antenna as the emitter and a DAST electro-optic crystal as the detector is shown to provide a detection bandwidth of 5 THz.

Ultrabroadband terahertz generation using 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate single crystals

Ultrabroadband terahertz generation up to 200 THz has been demonstrated using a 5 fs Ti:sapphire laser and a 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) crystal. The high-frequency components beyond 100 THz are much stronger than those generated using conventional electro-optic crystals such as GaSe. A simple simulation of the difference frequency generation in the DAST crystal by considering the refractive index dispersion can reproduce the broadband generation of the terahertz wave and its chirp dependence. Because the generated terahertz wave is coherent and has a broad bandwidth, it can serve as a suitable light source for ultrabroadband terahertz time-domain spectroscopy of a material.

Frequency-agile terahertz-wave generation and detection using a nonlinear optical conversion, and their applications for imaging

Abstract We have developed widely tunable terahertz (THz)-wave sources using nonlinear optical crystals. In the 1–3 THz region, a frequency-agile THz-wave parametric oscillator in a ring-cavity configuration has been realized using MgO-doped LiNbO3 as the nonlinear crystal. An organic 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal is promising for THz-wave generation in an ultra-wide range 1–40 THz. Difference-frequency generation from DAST, pumped using a dual-wavelength KTiOPO4 (KTP) optical parametric oscillator, is demonstrated. Additionally, THz-wave detection, with high sensitivity, fast response-time and room-temperature operation, is achieved by nonlinear up-conversion. Water concentration mapping of biological tissue samples and carrier density mapping of semiconductor wafers using frequency-agile THz-wave sources are demonstrated.

Observation of Strain Effects of SrTiO3 Thin Films by Terahertz Time-Domain Spectroscopy with a 4-Dimethylamino-N-methyl-4-stilbazolium Tosylate Emitter

We measured strain effects of SrTiO3 (STO) thin films by terahertz time-domain spectroscopy (THz-TDS) with a 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal that was expected to be a broadband THz emitter. STO thin films were deposited on MgAl2O4, MgO, and LSAT substrates, and lattice constants of the films changed from the value of unstrained STO bulk because of lattice mismatches between STO and the substrates. The dielectric dispersions of the STO thin films deposited on MgO and MgAl2O4 were measured up to 6.0 THz by the broadband THz-TDS system and all the features of dielectric peaks of soft modes responsible for ferroelectricity were clearly observed. The soft-mode frequencies of the films shifted from the original value of STO bulk owing to the strain effects. In particular, the softening of the soft mode of the film on MgAl2O4, which implied that the in-plane tensile strain enhanced ferroelectric fluctuations, was of interest.

Simultaneous all-solid-state multi-wavelength lasers—a promising pump source for generating highly coherent terahertz waves

A diode-end-pumped Nd:YAG dual-wavelength laser operating at 1319 and 1338 nm is demonstrated. The maximum average output power of the quasi-continuous wave linearly polarized dual-wavelength laser is obtained to be 2.1 W at a repetition rate of 50 kHz with an output power instability of less than 0.38% and beam quality factor M2 of 1.45. Using the two lines, the highly coherent and narrow linewidth terahertz radiation of 3.23 THz can be generated in an organic 4-N, N-dimethylamino–methyl-stilbazolium tosylate (DAST) crystal. Meanwhile, the multi-wavelength red laser at 659.5, 664 and 669 nm is generated by frequency doubling and sum frequency processes in a lithium triborate (LBO) crystal. The average red laser output power is enhanced up to 1.625 W at a repetition rate of 15 kHz with an output power instability of better than 0.53% and beam quality factor M2 of 6.05. Using the three lines, it is possible to generate the multi-wavelength THz radiation of 3.3, 3.43 and 6.73 THz in an appropriate difference frequency crystal.