1. Introduction

The development of coherent terahertz (THz) radiation sources is indispensable to enhance THz-wave research and to create applications for industrial markets [1,2]. Although several THz radiation sources have been developed, they have their own advantages and disadvantages such as power, frequency tunability, spectral linewidth, compactness and usability [3–6]. Among these, THz radiation sources that are based on the nonlinear optical process of difference frequency generation (DFG) are promising technique to generate high-power monochromatic coherent THz radiation with ultra-broadband frequency tunability.

The nonlinear optical properties of several crystals such as LiNbO₃ [7], GaP [8], 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) [9], and 4-dimethylamino-N-methyl-4-stilbazolium p-chlorobenzensulfonate (DASC) [10] have been exploited for THz-wave generation. Among them, organic materials owning loosely bound π electrons are more attractive as nonlinear optical crystals because of their large nonlinear susceptibilities. Moreover, monochromatic high-power THz-wave radiation with agile, random, and ultra-wideband frequency tunability from 1.5 to 40 THz was demonstrated via DFG in organic DAST crystals [11–15]. However, the DAST-DFG THz source has several intensity dips in its spectrum, which are caused by phonon vibration resonances. As a complementary use to fill the intensity dips of DAST-DFG, other organic materials with different phonon modes have gained attention for ultra-wideband THz wave generation.

Recently, another excellent organic material N-Benzyl-2-methyl-4-nitroaniline (BNA) for THz generation was developed by Hashimoto [16]. BNA is a derivative of 2-methyl-4-nitroaniline (MNA), which was well studied once as a nonlinear organic material [17]. One attractive property of BNA is chemical stability without deliquescent property. Thus, BNA can be easily polished and processed with conventional methods—a task that is very difficult with DAST.

An important problem with respect to organic materials is how to grow high-quality and large-size single crystals. In general, the molecular mass of organic materials is quite large and their structure is complex compared with inorganic materials. In addition, binding forces between organic molecules such as the Van der Waals force and hydrogen bonding are relatively weak. Therefore, it is generally difficult to grow high-quality large-size single organic crystals. Thus, optimizing their growth is still an important challenge.

Single-crystal BNA was first obtained from the melt phase via the Bridgman method. Its crystal structure and basic physical properties have been investigated [18–20]. Its second-order nonlinear coefficient (d₃₃) is 234 pm/V at 1064 nm, which is roughly ten times larger than that of conventional inorganic nonlinear materials. A tenfold increase in nonlinear susceptibility leads to a hundredfold increase in conversion efficiency in a collinear phase-matched DFG process. By using BNA, ultra-wideband DFG THz generation was first demonstrated by Miyamoto et al. [21,22]. Although the Bridgman method is well-proven technique in inorganic crystals growth, it may be not suitable for high quality crystal growth of organic materials. Because temperature gradient in the melt is necessary during the crystallization process with the Bridgman method, organic crystals, which have relative weak intermolecular bindings compared with inorganic crystals, grown by the Bridgman method tend to have lattice defects and distortions due to mechanical and thermal stresses. To improve the quality of organic BNA crystals, we newly applied a solution growth method.

In this paper, we report on single crystals of BNA grown in an ethanol solution by the slow-cooling method and discuss the crystal quality of the BNA crystals.

2. High-quality BNA crystal grown by solution method

We first searched for a suitable solvent for BNA solution growth. The solvent is very important because it strongly affects not only the crystal quality but also the morphology. Frequently-used organic solvents for growing organic compounds have been summarized in the Cambridge crystal structure database [23]. We chose ethanol, methanol, hexane, ethyl acetate, acetone, acetonitrile, and toluene as candidate solvents from the database and investigated the solubilities of BNA ingredients in these solvents. Each solvent was mixed with 300 mg of BNA ingredients at room-temperature; 20 min later, remained BNA ingredients were weighed by filtrating each solution. The room-temperature solubilities are summarized in Table 1 .

Table 1. Comparison of Solubility of Several Organic Solvents at R.T. and 60°C for BNA Ingredient

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Solvents in which the BNA ingredients are extremely soluble are not suitable for growing high optical quality single crystals because of the excessively strong interactions between the BNA ingredients and the solvent. Solvents in which BNA ingredients have low solubility are also obviously inappropriate for crystal growth in solution. The room-temperature solubility of BNA ingredients in ethanol and methanol is moderate, but it can be high at the slightly elevated temperature of around 60°C. This feature is favorable for solution growth of BNA crystals via the slow-cooling scheme. We tried to grow single BNA crystals from both ethanol and methanol. However, we couldn’t obtain bulk BNA crystal but tiny needle-shaped BNA crystal from the methanol solution. This must be the crystal habit difference depending on the solvents. Finally, we chose ethanol as the optimum solvent because the tiny needle-shaped BNA is obviously unsuitable for application. In this study, we report the optimization of slow-cooling ethanol-solution growth of BNA crystals.

BNA ingredients were organically synthesized according to the procedure described in [16]. Furthermore, with the use of ethanol, the BNA ingredients were highly purified by several recrystallization processes to exclude impurities. This purification procedure is important because impurities strongly affect not only the optical quality of the crystals but also the growth speed and stability. After several recrystallization processes, highly purified poly-crystal BNA can be obtained, as shown in Fig. 1(a) .

 

Fig. 1 (a) BNA polycrystal ingredients highly purified by several recrystallization process in ethanol, (b) BNA seed crystals for ethanol solution growth, (c) BNA crystals under the growth in ethanol solution, (d) typical BNA single crystal grown from ethanol solution.

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Poly crystal BNA was solved by ethanol at 60°C with the proper concentration of 0.3 g/ml. This BNA ethanol solution was cooled down extremely slowly with rate of 0.5°C/day until it reached the supersaturation state to give rise to some spontaneous BNA nuclei. Figure 1(b) shows some of the spontaneously generated BNA nuclei, from which the best-quality nucleus without visible defects was selected as a seed crystal for further growth. The BNA seed crystal was glued to a glass plate and immersed in saturated supernatant BNA ethanol solution. To further grow the BNA seed crystals, the saturated supernatant BNA solution was cooled from roughly 15°C to 0°C at the rate of 1°C/day. After two weeks, BNA bulk crystals were obtained, as show in Fig. 1(c).

In this process, the key factors to grow high-quality large-size BNA single crystals are concentration, temperature of the saturated supernatant solution, and cooling rate. At present, despite some difficulties growing organic crystals, we obtained single BNA crystals with typically 5x10x3 mm³. As shown in Fig. 1(d), BNA crystals consist of transparent sections and sections with delta-shaped defects. These defects may be due to a difference in growth speed in the a- and c-axis directions. To perfect the technique of growing BNA single crystals without defects further studies are being conducted.

Since the crystal structure of BNA crystals grown by the Bridgman method has already been analyzed well [18], we briefly checked the structure of BNA crystal grown from ethanol solution by using X-ray diffraction. As a result, same crystal structure with class of orthorhombic, point symmetry of mm2, lattice parameters of a, b, c axis are 7.3, 21.4, 8.1 Å, respectively is confirmed. Figure 2(a) shows the X-ray diffraction results of an ω-2θ measurement on (010) plane made with an X-ray wavelength of 1.540 Å. For this measurement, we used a transparent part of a BNA crystal grown from ethanol solution shown in Fig. 1(d). Several higher-order diffraction peaks were successfully observed and their mirror indices were assigned by using the Bragg-diffraction law for orthorhombic systems and the b-axis lattice parameter of 21.4 Å. These six diffraction peaks can be well fitted with the following transformed Bragg-diffraction law with a slope of b (see Fig. 2(b)).

 

Fig. 2 (a) X-ray diffraction result of an ω-2θ measurement on (010) plane. (b) Experimentally assigned mirror index k as a function of (2/λ)sinθ. Data are well fitted with slope of b which indicates lattice parameter of b-axis 21.4 Å.

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We also examined the quality of BNA crystals grown in ethanol solution by using X-ray diffraction analysis. The blue line in Fig. 3(a) shows the x-ray rocking curve measured for the (040) diffraction shown in Fig. 2(a). The vertical axis gives the normalized diffracted x-ray count and the horizontal axis gives the displacement of incident X-ray angle from the legitimate incident angle, which just fits Bragg’s diffraction condition. According to x-ray diffraction theory, the linewidth of the rocking curve reflects the orientation regularity of the lattice planes. More regular orientation without lattice distortions and defects results in a narrower linewidth [24]. The rocking-curve linewidth for BNA crystals grown in ethanol solution was 30 arcsec, which is significantly reduced by one-third compared with that for Bridgman-grown BNA crystals (red broken line). This result provides strong evidence that the quality of BNA crystals was improved by using ethanol solution method rather than by using the Bridgman method.

 

Fig. 3 (a) Comparison of rocking curve of BNA grown from Brdigman and ethanol solution method. (b) Comparison of peak power density where crystal damage was observed.

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We also investigated the crystal-laser-damage threshold for two BNA crystals: one grown by the Bridgman method and the other from ethanol solution. Wavelength of the pump laser was 1064 nm with pulse duration of 10 ns and a repetition rate of 100 Hz. Sample BNA crystals were exposed to the pump beam for 1 min. After this, we observed the surface state of the sample BNA crystal with an optical microscope to check the presence or absence of damage. We repeated this test with increasing peak power every 2 MW/cm² power density step. Figure 3(b) shows damage-threshold levels for each BNA crystal. Here three samples for each growth method were used and their averaged damage-threshold values are given in Fig. 3(b). We find a significant improvement in the damage-threshold level for BNA crystals grown from ethanol, which is directly related to an improvement in crystal quality. High-quality crystals have decent molecular structure with stronger molecules bindings and fewer impurities. Therefore, they can endure stronger electric fields of pump laser and may diffuse accumulated heat throughout the crystal bulk via well-coupled phonon vibrations.

3. Effective thickness of BNA for THz generation

In the ethanol solution method, growth along the b-axis direction is the most difficult of the three directions and the maximum b-axis length for BNA made from ethanol solution is currently 3 mm. The b-axis direction corresponds to the propagation direction of the pump laser beams and the generated THz radiation under the collinear phase-matching conditions of DFG. The optimum BNA crystal length is determined by the balance of emission gain and absorption loss because organic materials are not transparent to THz radiation. Prospective THz radiation spectrum including absorption loss effect under collinear phase-matched DFG is calculated theoretically and given in Fig. 4 . The vertical and horizontal axes mean crystal length in the b-axis direction and THz frequency, respectively. Here, perfect phase matching condition was assumed to simplify the calculation and second-order nonlinear susceptibility of 234 pm/V at 1064 nm and absorption coefficients at THz frequency region were used evaluated in [20,21], respectively. According to the calculation result, thinner crystals are better than thicker ones to generate THz wave effectively because of low absorption loss. However, extremely thin crystals less than one hundred μm are very difficult to obtain and treat for practical use. Besides, this result must be changed if the second-order nonlinear susceptibility of BNA has dependence on the pump wavelength. Therefore, establishment of steady growth method of high quality single BNA crystals with thickness from roughly 100 μm to several mm is important.

 

Fig. 4 Calculation result of prospective THz-wave intensity via BNA-DFG under the consideration of perfect phase matching and absorption effect.

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4. Ultra-widely tunable THz generation from solution-grown BNA

In order to confirm the ultra-wideband THz-wave radiation from BNA grown from ethanol solution, DFG experiment was conducted. In a previous report, two pump wavelengths for DFG could be tuned from 750 to 950 nm by using a dual KTP OPO [21]. In this study, to more efficiently generate THz waves, tunability of two pump wavelengths were extended up to 1000 nm by using a new dual KTP OPO. Other parts of the DFG system were essentially the same as the experimental system described in [20].

We investigated THz-intensity dependence on the whole possible combination of dual pump wavelengths λ1 and λ2 by independently controlling the two KTP angles [13]. Here, the dual-pump beam energy per pulse and the diameter were about 2 mJ and 2 mm, respectively and the BNA with 1 mm thickness was employed. From the results, best combination of λ1 and λ2 which satisfy the phase matching condition and gives the highest output intensity at each THz frequency can be obtained. Figure 5 shows the DFG-THz spectrum from the solution-grown BNA obtained under the best combination of λ1 and λ2. We also demonstrated ultra-wideband THz generation up to 20 THz from the solution-grown BNA. In addition, higher THz-output power can be achieved owing to improvements in crystal quality and the extension of the pump wavelength range compared with our previous experiment. Further optimization of BNA-DFG may be still possible by extending the dual-pump wavelength region further above 1000 nm.

 

Fig. 5 THz-wave output spectrum via DFG from BNA grown from ethanol solution. Here, two pump wavelengths were independently controlled properly to generate highest power at each THz frequency.

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5. Conclusion

We tried to crystallize organic single BNA crystals by means of solution growth method to enhance crystal quality. By optimizing several growth conditions in ethanol solution, single BNA crystals with large size and high quality could be successfully obtained. In comparison to the Bridgman method, both 1/3-reduction of line width of rocking curve and 1.4 times enhancement in damage threshold clearly indicate significant improvement of the crystal quality, which allows us to demonstrate ultra-widely tunable THz generation via DFG with higher conversion efficiency.