Growth, transmission, Raman spectrum and THz generation of DAST crystal

Abstract

The organic crystal 4-N,N-dimethylamino-4-N-methyl stilbazolium tosylate (DAST) was obtained by seed crystal method. We measured a maximum optical transmittance of 81%. Difference frequency generation (DFG) with reflecting double-pass optical parametric oscillator (OPO) was applied to generate terahertz (THz) wave. Tunable THz waves in the broad range of 0.51–19.69 THz were achieved. The maximum pulse energy was 267.7 nJ at 4.11 THz, and the laser-to-THz conversion efficiency of 1.04 × 10⁻⁴ was obtained. Raman spectrum measurement also indicates the consistence of the Raman modes and the THz tuning curve from DAST crystal. The theoretical THz transmittance values are calculated and compared with the measured values. The variation tendency of the measured transmittance with THz frequency is found to be in agreement with that of the theoretical ones.

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1. Introduction

The efficient generation of terahertz (THz) radiation has attracted considerable interest due to its various applications.¹ Nonlinear optical materials are commonly used for THz generation. Inorganic crystals (such as ZnTe, LiNbO₃, GaSe,² and GaAs) are common materials for THz generation, with well-known properties and well-developed growth techniques. However, their main disadvantage is the limited spectral range generated and the low efficiency. Recently, organic crystals gained a lot of attention because of the advanced crystal production technology and success in their integration in science applications.^3–6 Organic crystal-based sources can be broadband or monochromatic. Recent reports on broadband sources demonstrated very high generation efficiency (∼2%) and an extremely intense peak field of 83 MV cm⁻¹—presently the most intense THz source ever.¹ Nevertheless, narrowband THz sources are still very inefficient (0.001%).

The organic crystal 4-N,N-dimethylamino-4-N-methyl stilbazolium tosylate (DAST) possesses large second-order nonlinear optical and electro-optical coefficients and low dielectric constant, which make it one of the most promising materials for THz generation¹ and detection.^7,8 Due to the low quality and small dimensions of DAST crystals, THz wave generation using the crystals is still in its infancy. Slope nucleation method and seed crystal method have been developed for growing DAST crystals.^9–11 In addition, difference frequency generation (DFG) has been used to generate THz waves from DAST crystals.^12,13 Taniuchi et al.¹⁴ obtained THz waves ranging from 1.5 to 6.5 THz using thin DAST crystal. They also reported a widely tunable and high-frequency THz wave generation from 2 to 20 THz in DAST crystal.¹³ Adachi et al.¹⁵ demonstrated THz pulse radiation of up to 30 THz by pumping a 1 mm-thick DAST crystal. However, the laser-to-THz conversion efficiency is still low using DFG, which has so far limited DFG applications.^3–6 In this work, we report on the high-efficiency growth of organic crystal DAST for DFG applications.

Based on our previous work,^16–19 high-quality DAST crystal was obtained by seed crystal method. In this paper, the transmission spectrum and the Raman spectrum of the DAST crystal were characterized. THz wave with widely tunable frequency was generated by DFG. The THz wave was detected by a Si bolometer cooled by liquid helium. The laser-to-THz energy conversion efficiency in the crystal was calculated and compared with the previous work and showed significant enhancement. The theoretical transmittance of the sample was also calculated and compared with the measured transmittance.

2. Experimental

2.1 Synthesis

The raw materials of DAST crystal were synthesized by the condensation of 4-N,N-dimethylamino-benzaldehyde and 4-methyl-N-methyl pyridinium tosylate. 4-Methyl-N-methyl pyridinium tosylate used in the above reaction was prepared from 4-picoline and methyltoluenesulfonate. During synthesis, piperidine was used as the catalyst. The final products were kept in a vacuum system at 110 °C for 2 h to prevent absorption of water from the atmosphere. The purity of the products was further improved by several successive recrystallizations from methanol.

2.2 Crystal growth

We investigated DAST crystal growth by slope nucleation method (SNM) coupled with seed-crystal method (SCM). SNM is used to prepare seed crystal here, and SCM is employed for large-size, single-crystal growth.

As for SNM, the slope inclination angle and the temperature cooling rate are the two important parameters. In our growth system, a Teflon plate with slope angle of about 45 °C was placed in the growth vessel, and the prepared solutions were preheated to 3 °C above the saturation temperature for 8 h to ensure the homogeneous concentration. The cooling rate of the solution temperature was set at 0.2 °C per day until several visible crystals appeared, and then the cooling rate was adjusted to 0.1 °C per day. To prepare seed crystals, different concentrations of solutions were prepared. Tiny DAST crystals with natural morphology were obtained from the spontaneous crystallization. The tiny crystals with regular shape and high quality were selected using an optical polarizing microscope and used as seed crystals.

During the entire crystal growth, the quality of seed crystals and the stability and temperature-reducing program can all influence the crystal quality. The schematic diagram of the crystal growth system in our experiment is shown in Fig. 1. In this growth procedure, DAST-methanol growth solutions were prepared in 3 g/100 mL concentration, and the temperature of the solution was kept at equilibrium temperature for 10 h. Seed crystal was fixed on the seed rod, then placed into the solution, and the temperature was lowered with a cooling rate of 0.1 °C per day. When the seed crystal began to grow, the cooling rate was also reduced to ensure the crystal quality.

Fig. 1 Schematic drawing of the crystal growth system: 1 – silicone plug, 2 – thermometer, 3 – growth cell, 4 – teflon cover, 5 – growth solution, 6 – seed crystal, 7 – Teflon seed holder, 8 – water bath.

2.3 Characterization

Our sample is as-grown DAST (not polished) crystal with a thickness of 0.5 mm and aperture of 3 mm. We first measured its optical transmission where the incident light and (001) faces should be orthogonal to each other. The crystal transmission, ranging from the visible to the near-IR region, was measured using a UV-vis-NIR spectrometer (Cary 500, Varian, USA). Raman spectrum of the crystal was measured by a Renishaw Laser Micro-Analysis System. Then, we used it to generate THz waves. Our laser system consists of a frequency-doubled Nd:YAG laser with an injection seeder used for the pump source (line width: 90 MHz). The optical parametric oscillator (OPO) was developed using two KTiOPO₄ (KTP) crystals. A detailed schematic diagram is shown in Fig. 2. The resulting 1.3–1.6 μm light was extracted with a dichroic mirror and focused on the DAST crystal with a lens.

Fig. 2 Schematic diagram of the experimental arrangement for THz wave generation.²⁰ The inset is the photo of packaged DAST crystal used for testing.

3. Results and discussion

3.1 Growth technique

During the entire growth process, changing the pH value of the growth solution and adding some agents may change the growth pattern of organic–ionic crystals. Ramaclus et al. found that oleic acid could inhibit growth on the (010) direction by directly interacting with DAST.²¹ In the previous work, we reported that the addition of toluenesulfonic acid (PTSA) to the solution could reduce the growth rate and lead to the irregular shape of DAST crystal.²² In the present work, we just use the pH value of the growth solution itself, and the value is about 5.4 as tested by a pH meter.

In the present work, we used natural cooling method and added heat sources at the same time to control the cooling rates. This method needs a certain range of temperature, the metastable zone, within which the seed crystals can grow. However, the metastable zone is usually narrow, and much of the solute remains in the solution when the crystals stop growing. Therefore, a relatively large volume, e.g. 500–1000 mL or more, of growth solution is needed to allow for the depletion of solute. Here, 500 mL solutions were used. In addition, the large volume also ensures the solution's relative homogeneity and stability.

Supersaturation is the driving force for crystal growth, and it is very important for crystal quality. Supersaturation can be achieved by lowering the temperature of a solution. In order to grow high-quality crystal, the growth rate should be linear to ensure crystals without growth lines, other inclusions, etc. Therefore, nonlinear cooling rate is adopted to ensure linear growth. Particularly, when the crystals grow, the cooling rate should be increased.²³

In conclusion, temperature fluctuation and supersaturation fluctuation will be caused by the improper control of the growth process in the solution growth method. These fluctuations will lead to the generation of crystal defects, such as inclusions and growth lines.²⁴ Therefore, the light scattering inside the crystal will be caused, and the laser-to-THz conversation efficiency will be reduced. To avoid the fluctuations, a stirrer motor was also employed.

3.2 Optical transmission

Fig. 3 shows the optical transmission spectrum of the DAST crystal, which shows a peak transmission of 81%. This high transmission is a result of the high quality of the crystal and reduced levels of impurities. The photograph of the as-grown crystal at 50× magnification in Fig. 3 also indicates its surface quality. The absorption edge occurs at nearly 600 nm, which is consistent with previous reports.^19,25 It is well known that the crystal growth technique has important influence on materials' quality. Growth conditions, such as pH value of the growth solution, additions, and cooling methods, etc., also have great effect on the optical properties of the crystals. It is noteworthy to mention that Sun et al.²⁵ showed improved transmission (>80%) when the crystal is grown in the presence of activated carbon. Therefore, in order to obtain even higher-quality DAST crystals, activated carbon can be used in combination with the present improved growth procedure.

Fig. 3 Optical transmission spectrum of the DAST crystal.

3.3 THz generation

During the THz generation, the two optical wavelengths generated from the OPO were collinearly polarized. The principal axis (a-axis) of DAST crystal was set to be parallel to the optical polarization. The residual optical radiation was blocked by a black polyethylene filter. The THz radiation was detected by a Si bolometer cooled by liquid helium.

The THz output energy as a function of the THz frequency with the as-grown DAST crystal is shown in Fig. 4. Continuously and widely tunable THz waves are generated from 0.51–19.69 THz for the sample under the dual-wavelength pump energy of 5.17 mJ. The maximum single-pulse energy was 267.7 nJ at 4.11 THz, and the conversion efficiency was 1.04 × 10⁻⁴. The output THz energy depends on the phase-matching conditions of DFG and the absorption of the crystal. In a molecular crystal with N atoms per primitive cell and Z primitive cells per unit cell, each atom possesses three degrees of freedom, and therefore, a total of 3NZ vibrational modes are possible in a molecule. The lattice wave numbers for DAST crystal (N = 55, Z = 4) are in 3 acoustic modes (one longitudinal wave, two transverse waves) and 3N − 3 optic modes (including longitudinal and transverse waves).²⁶ The study on the DAST crystal absorption of THz wave indicated that the decrease below 2 THz (e.g. at 1.0 THz) is attributed to the strong absorption in the crystal, which corresponds to the resonance of the transverse optical phonon in DAST.²⁷ The absorption at 3 THz was caused by the non-resonance absorption of the crystal.²⁸ The frequency dips at 5, 7.2, 8.4, 14, 15 and 17 THz are mainly due to the intrinsic absorption of DAST crystal.²⁹ At the frequency above 19 THz, the detection efficiency of the Si-Bolometer falls off, and the energy decreases at higher frequencies.

Fig. 4 THz output energy as a function of the THz frequency obtained from DAST crystal.

Table 1 lists the reports on THz generation in DAST crystals by various research groups using the DFG method. The tuning range, maximum output energy, and conversion efficiency were all highly improved in the present study. The maximum output energy was 267.7 nJ at 4.11 THz, which was higher than that of the maximum output energy (110 nJ at 19 THz) obtained from DAST crystal by Taniuchi et al. The conversion efficiency in the present work was about 10–1000 times higher than other reported results.^17,30,31

Table 1 List of reports on THz generation in DAST crystals through DFG

Thickness	Tuning range	The maximum output energy	Conversion efficiency	Ref.
0.5 mm	2–20 THz	110 nJ at 19 THz	1.2 × 10^−5	T. Taniuchi et al.^30
0.6 mm	1–10 THz	0.1 fJ at 1.5 THz	1.0 × 10^−7	L. Kun et al.^31
1 mm	1.16–16.71 THz	27.4 nJ at 3.8 THz	1.4 × 10^−5	B. Teng et al.^17
0.5 mm	0.51–19.69 THz	267.7 nJ at 4.11 THz	1.04 × 10^−4	Present work

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3.4 Raman spectrum

Fig. 5 shows the measurement of Raman spectrum along the polar a-axis. The Raman modes at 407, 500, 572, 730 and 777 cm⁻¹ were equal to 12.2, 15, 17.2, 21.9 and 23.3 THz, respectively (100 cm⁻¹ = 3 THz).²⁷ These positions of Raman peaks were consistent with the previous work.³⁰ The Raman modes near 12, 15, and 17 THz corresponded to the three minimum values of the relative tuning curve (in Fig. 4).

Fig. 5 Raman spectrum of DAST crystal for light polarized along the polar a-axis.

3.5 THz transmission

We measured the transmission spectrum of the a-axis of 0.5 mm-thick DAST crystal in the range of 0.1–3.5 THz using terahertz time domain spectroscopy (TDS) (TAS7500SP, Advantest Corporation). From Fig. 6 (black line), we could see that the maximum THz transmittance of DAST crystal is below 20%, which is obviously lower than the optical transmittance (∼80%). Strong absorption bands were observed at around 1.1 THz and 3.0 THz, which were consistent with the results in Fig. 4.

Fig. 6 The THz transmittance for the a-axis of DAST crystal using a tunable THz source. Blue line: calculated values; black line: measured values.

We also calculated the theoretical transmittance (Fig. 6, blue line) and compared them with the experimental values. The theoretical transmittance is given by the equation T(ν) = I(ν)/I₀(ν), where I₀ and I are the intensities of the incident and transmission light, respectively, and are given by:

where α is the absorption coefficient, L is the length of the sample, T₁ and T₂ are the interface transmissivity of the sample and the mediums before and after it, respectively, and n₁ and n₂ are the index of refraction of the sample and the mediums before and after it, respectively. In the experiment, the mediums means the atmosphere. The values of α, n₁ and n₂ can be obtained according to ref. 32.

Comparing the results of theoretical and measured transmittance of the sample, we could see that the tendencies of transmittance as a function of THz frequency between the calculated and experimental values are consistent with each other. Absorption bands also exist at around 1 THz, which demonstrates the intrinsic absorption of the DAST crystal. Therefore, to make up the shortage, researchers have designed and prepared some new organic nonlinear optical (NLO) materials to generate THz waves covering the spectral range of 0.5–3 THz.³³ Compared with the calculated transmissivity, several transmittance peaks measured below 1 THz are found, which are due to the interference of reflected waves from the front and back surfaces of the crystal. In addition, as for TDS, the measurement error is larger at around zero THz. In the range of 0.1–3.5 THz, the experimental transmittance is lower than the calculated ones, mainly attributed to the excess loss caused by the interior impurity of the crystal. We believe that the optical and THz transmissivity will be higher after optimizing the crystal growth procedures; thus the conversion efficiency will be improved further.

4. Conclusions

DAST crystal with high optical transmission (81%) was obtained by seed crystal method. The present study demonstrates that it is possible to develop high-quality DAST crystals by optimizing the growth parameters. DFG was applied for THz wave generation, and the reflecting bi-pass optical parametric oscillator pumping organic DAST crystal could increase the pump pulse's utilization efficiency. Therefore, widely tunable THz wave ranging from 0.51–19.69 THz was generated. The maximum single-pulse energy was 267.7 nJ at 4.11 THz, and the high laser-to-THz conversion efficiency of 1.04 × 10⁻⁴ was obtained. Raman spectrum further demonstrated the position of absorption peaks as a function of THz frequency for the sample. The theoretical values of the THz transmittance as a function of THz frequency in the range between 0.1–3.5 THz were calculated and compared with the measured transmittance for the a-axis of the sample. The variation tendency of the calculated transmittance is in agreement with that of the measured values generated in the DAST crystal sample. Both results show absorption bands at around 1 THz and the THz transmittance of 20% near 2 THz.

Acknowledgements

The authors are grateful to Dr M. Shalaby (Swiss FEL, Paul Scherrer Institute/OVGA) and Dr P. X. Liu (Nankai University, China) for reviewing. This work was supported by Program for National Natural Science Foundation of China (No. 51402159 and No. 51172111) and a Project of Shandong Province Higher Educational Science and Technology Program (No. J14LA17), China Postdoctoral Science Foundation (No. 2015M57073), the Qingdao Postdoctoral Application Research Project (No. 2015116) and Innovation Project of China Electronics Technology Group Corporation No. 46 Research Institute (No. CJ20140401).

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