Organic Nonlinear Optical Single-Crystalline Thin Film Grown by

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Organic nonlinear optical single-crystalline thin film grown by physical vapor deposition for terahertz generation Hirohisa Uchida, Ryo Yamazaki, Kengo Oota, Koutarou Okimura, Tsubasa Minami, Kei Takeya, and Kodo Kawase Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00388 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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Crystal Growth & Design

Organic nonlinear optical single-crystalline thin film grown by physical vapor deposition for terahertz generation Hirohisa Uchida*,†,‡ , Ryo Yamazaki‡, Kengo Oota‡, Koutarou Okimura‡, Tsubasa Minami‡, Kei Takeya*,‡, Kodo Kawase‡ †

ARKRAY Inc., Kamigyo-ku, Kyoto 602-0008, Japan

‡

Department of Electronics, Nagoya University, Nagoya, 464-8603, Japan

nonlinear optical crystals, crystal growth, physical vapor deposition, terahertz

There are a number of advantages to terahertz (THz) waves by Cherenkov phase matching using a nonlinear optical (NLO) crystal; however, a thin crystal of µm-scale must be grown to satisfy the optimal phase matching conditions. Inorganic crystals with well-developed crystal growth and processing techniques have been widely used to date; however, THz wave generation with improved efficiency in the broadband region could be achieved with a single-crystalline thin film of an organic NLO material with a large NLO coefficient.

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Here, the growth of a crystalline thin film with a size of several microns using a very simple method is reported. Physical vapor deposition is used to grow a crystal of the organic nonlinear material 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene) malononitrile (OH1). This method is much simpler than the conventional crystal growth method. Additionally, the OH1 single-crystalline thin film grown by physical vapor deposition has a better crystalline character than a bulk OH1 crystal grown by the solution method. The OH1 single crystal grown by this technique demonstrates efficient THz wave generation and low absorption of THz waves by the crystal.

1. Introduction 1.1. Nonlinear optical crystal as the source of terahertz waves Terahertz (THz) waves are electromagnetic waves between radio waves and optical waves, within the frequency range of 0.1 to 10 THz. Recent studies have shown that the unique properties of THz waves are useful in both basic and applied research. A broadband THz wave source with a high power output and efficiency is essential for inspection of automobiles and medicines, non-destructive inspection of rubber, and in new industries using THz applications; therefore, development of a high-performance excitation light source and generating devices for THz waves has been a focus of research in recent years.1,2 In particular, an optical device fabrication technique capable of reducing loss of pump beam is essential for the generation of highly efficient THz waves. THz wave generation methods that use inorganic nonlinear optical (NLO) crystals have been developed, and organic NLO crystals, which have high NLO coefficient values, also show significant potential. Bulk crystals of 4-N,N-dimethylamino-4’-N’-


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methyl-stilbazolium

tosylate

(DAST),3

2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-

enylidene) malononitrile (OH1),4 4-N, N-dimethylamino- 4’-N’-methyl-stilbazolium 2,4,6trimethylbenzenesulfonate (DSTMS),5 and N-benzyl-2-methyl-4-nitroaniline (BNA)6 have been used as highly efficient organic NLO crystals for THz wave generation, and various methods such as improving the quality of crystals, optimizing phase matching conditions, and excitation methods have been employed.7-10 In recent years, various research using the OH1 crystal has been reported, such as the growth of thin films using saturated solutions, electro-optic (EO) modulators, and THz wave generation with high electric field intensity.11-13 However, the processing of these crystals is not easy. Shape control is necessary for highly efficient THz wave generation. Therefore, it is practically a requirement that shape be controlled during crystal growth. In this study, organic NLO crystals were grown with shapes suitable for THz wave generation. 1.2. Terahertz generation using nonlinear optical crystals Generation of THz waves using organic NLO crystals has been studied based on Type 0 collinear phase matching conditions.14 In the collinear phase matching method, it is possible to ensure a high interaction length when using crystals with refractive indices suitable for optical and THz waves, and there has been significant advancement in the development of a THz source through collinear phase matching in organic NLO single crystals, such as DAST or OH1.7-10 However, even a crystal having a high NLO constant is not suitable for THz wave generation unless the phase matching condition is satisfied.15 Furthermore, when absorption in the THz wave region is due to the intermolecular modes or lattice vibrations, the THz waves generated can be absorbed by the crystal.16



To resolve this limitation, we used the Cherenkov phase matching method for THz wave generation.17-22 This is an efficient THz wave generation method that makes use of the difference between the group refractive indices in the optical region and the THz region of the crystal, which has previously generated broadband THz waves at intensities 1,000 times greater than those achievable using a photoconductive antenna (PCA).18,20 Inorganic crystals easily fulfil the Cherenkov phase matching condition (nTHz> nopt) for values of the refractive index nopt in the infrared region and nTHz in the THz region relatively,17,19 whereas these two indices are generally close to each other in organic NLO crystals. In the case of OH1, nopt in the optical region (at a wavelength of around 1 µm) is about 2.2023 and that in the THz region (1–2 THz) is 2.30.24 Therefore, the Cherenkov phase matching angle in the crystal is 17° = arccos (2.20/2.30) and the wave surfaces of generated THz waves undergo total reflection at the crystal interface and cannot be extracted from the crystal. However, based on the value of the refractive index of the clad material in the THz region (3.41) and that in the optical region of the crystal (2.20), the Cherenkov phase matching angle is 49.8° = arccos (2.20/3.41); hence, when the prism coupling method is used, THz wave generation by Cherenkov phase matching may be possible for organic NLO crystals with similar refractive index values in the optical and THz regions, even if the value of the NLO coefficient d is high.17,22


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1.3. Phase mismatching

Figure 1. (a) Phase mismatch diagram. As the beam width has a finite value, phases of terahertz (THz) waves generated from shallow locations (shown in red) and deep locations (shown in blue) of the crystal are mismatched along the Cherenkov phase matching angle. These waves mutually interfere and inhibit the generation of THz waves. As this effect is more pronounced on the high-frequency side, broadband generation is inhibited. (b) The relationship between the thickness of the crystal and the THz frequency that can be generated for a Cherenkov phase matching angle of 49.8° assumed for a crystal with a refractive index n = 2.3 in the THz region. To obtain a broadband exceeding 10 THz, a beam or crystal with a maximum width of 6 µm is required. Although the Cherenkov phase matching method has the above advantages, a limitation arises from phase mismatching due to the beam width or thickness of the crystal (Figure 1, left).25 Since the exciting beam has a beam width limitation, phase mismatching may occur depending upon the depth from the surface of the crystal with respect to the direction of propagation of the THz wave front. This relationship is expressed by the formula ndsinθ = λ/2. Here, n is the refractive index of the crystal in the THz region and d is the beam width (or thickness of the crystal), and λ is the wavelength of the generated THz wave. The phase mismatching



relationship is shown in Figure 1 (right) for a crystal with n = 2.30 and a Cherenkov phase matching angle of 49.8°. For broadband generation of 10 THz and above, a beam width of up to about 6 microns or less is required. Thus, for THz generation over a broadband, the beam width must be smaller still; however, there is a theoretical lower limit to the beam width of around several microns. To address this limitation, we have attempted to reduce the thickness of the crystals. Thus far, for inorganic crystals, we have used a lithium niobate (LN) waveguide with a thickness of several microns18,20. It should be possible to achieve higher intensity and efficiency using an organic NLO crystal with a high NLO coefficient. However, organic NLO crystals are difficult to process due to their physical and chemical properties, such as brittleness and solubility. Thus, producing a reduction in thickness by processing alone is difficult, and THz wave generation by the Cherenkov phase matching method using a crystalline thin film has not yet been achieved. In this study, we demonstrated the growth of organic NLO thin film to improve THz wave generation by the Cherenkov phase matching method. 1.4. Reducing the thickness of organic NLO crystals Organic NLO crystals are usually grown by dissolving an organic nonlinear material in a solvent.26,27 Using this method, the shape of the crystal is determined by the growing conditions, and artificial control of the shape is difficult. Furthermore, in general, thin films of organic crystals can be obtained by cutting, polishing, or etching processes, but these methods are often difficult to apply to organic NLO crystals because of the physical or chemical characteristics of these materials. A thickness of several µm is required for the Cherenkov phase matching method; hence, here we used vapor deposition for growth of a thin-film-type organic crystal. High purity is usually achieved by sublimation in organic semiconductor materials; however, sublimation or vapor deposition has not yet been widely applied to the growth of organic NLO materials for


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THz generation. Here, an OH1 thin film was grown by a physical vapor deposition method using the sublimation properties of OH1 molecules. The structural formula of the OH1 molecule is shown in Figure 2. The crystalline character of the OH1 thin film that was grown was evaluated, and we confirmed THz wave generation from the OH1 thin film.

Figure 2. The structural formula of 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene) malononitrile (OH1) molecule.

2. Experimental section 2.1 Confirmation of sublimation of OH1 crystals and thermogravimetry and differential thermal analysis experiments Thermogravimetry and differential thermal analysis (TG-DTA) experiments were carried out using a Rigaku TG8120 instrument. The measured sample was powdered OH1 with a particle size of several mm. The total sample weight was 8.8 mg. The temperature range employed was 60–260 °C, and the temperature was increased at a rate of 10 °C/min. The measurements were conducted under ordinary pressure in a nitrogen atmosphere.



2.2 Nuclear magnetic resonance (NMR) experiments for measurement of melting and decomposition temperatures of OH1 To confirm the structural changes in OH1 molecules after heat treatment, the structure was investigated using 1H proton NMR. The experimental apparatus used was a Bruker Ascend 500. The OH1 powder was heated from 160 °C to 220 °C at 10 °C intervals. The OH1 powder for NMR measurement was prepared by individual heating at each temperature in advance. The NMR spectrum of the OH1 heated at each temperature was measured at 26 °C. The solvent used for the measurement was deuterated methanol; the measurement sample used 0.7 ml of it, adjusted to a solution concentration of 1 wt%. 2.3 Method of developing OH1 single-crystalline thin film The OH1 thin film was grown by physical vapor deposition. The apparatus used for growth of the OH1 thin film is shown in Figure 3; it was equipped with a motor pump for adjusting the pressure within the furnace. The interior of the furnace was filled with argon gas to avoid oxidation during heating. The interior and exterior were made airtight using a two-way cock valve, and the pressure was kept constant during growth of the OH1 thin film. Sublimation and growth of the OH1 thin film were conducted in the same atmosphere by applying a temperature gradient across three zones (A, B, and C) using three heaters. The temperatures of the zones were set as follows: zone A, 180 °C for sublimation without thermal decomposition; zone C, 90 °C, where thermal weight loss was not confirmed; and zone B, 120 °C, which was an intermediate temperature between the temperatures of zone A and zone C, used to avoid sudden cooling. The substrate for formation of the OH1 thin film was quartz glass placed between zones A and C. The heating period for film growth was 24 hours. After heating, a crystalline film was observed to have formed on the quartz glass substrate.


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Figure 3. Schematic diagram of the physical vapor deposition process for the growth of OH1 thin film. Vapor deposition was carried out using a 600-mm-long tubular furnace. The pressure in the tubular furnace could be adjusted. The furnace was separated into three zones by temperature and filled with argon gas.

2.4 Observation of OH1 thin film Optical microscopy observations were made using a digital microscope (VH-8000; Keyence) by mounting the OH1 film on a slide glass. An ECLIPSE E600 POL (Nikon) apparatus was used for polarization microscopic observations. The homogeneity of the film was observed by placing the polarizer on only the side of the incident light. The acquired image was analyzed by observing the transmission image of the crystal with respect to the deflection, and by rotating it repeatedly through 45o with reference to the angle of the polarizer at which the crystal became darkest. 2.5 Evaluation of OH1 thin film by X-ray diffraction (XRD) An ATG-X instrument (Rigaku) equipped with a Cu target was used for X-ray diffraction measurements. The wavelength of the X-rays was 1.5418 Å. The OH1 thin film was mounted on



a slide glass and the lattice spacing of the (100) plane was measured by determining the out-ofplane measurements. Rocking curve measurements of the (200) plane of a bulk OH1 crystal and the OH1 single-crystalline thin film (ω scan) were also performed, and the crystal integrity was confirmed from the full width at half maximum of the diffraction peak. 2.6 Terahertz wave generation THz generation using the Cherenkov phase matching method with the grown OH1 thin film was confirmed using the THz-TDS technique, which measures the time domain waveform of the THz wave according to an optical time delay. Figure 4 shows a schematic diagram of the measurement system used in the experiments. Experimental details are available in previous reports.17 The excitation source was a femto-second fiber laser (IMRA femtolite HFX400) with a wavelength of 1560 nm, maximum output of 242 mW, repetition rate of 70 MHz, and pulse width of 48 fs. A PCA (dipole type; Hamamatsu Photonics) was used for detection of the THz wave. The wavelength used at the time of excitation of the PCA was 805 nm. The method for mounting the OH1 single-crystalline thin film used in the experiments is shown in Figure 5. To generate THz waves from the crystal, a silicon prism was fitted to the bc-plane of the OH1 single-crystalline thin film. Figure 5a shows the silicon prism, which was designed by optimizing the Cherenkov angle. Using θclad = arccos (nopt/nclad) = 52.1, the angle of generation of the THz wave was found to be θclad = 52.1, where nopt is the refractive index of the OH1 crystal in the optical region and nclad is the refractive index in the THz region.20 The pump beam was condensed in the OH1 single-crystalline thin film (side face on the prism side), as noted by the objective lens. The direction of polarization of the exciting light was parallel to the c-axis of the OH1 single-crystalline thin film.


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Figure 4. Schematic diagram of the experimental system for THz wave generation. Generation and detection were based on the THz-time-domain spectroscopy (TDS) principle, and the prismcoupled Cherenkov phase matching method was used for THz generation. The wavelength of the crystal excitation light was 1560 nm, the PCA was used for detection, and the wavelength of the probe beam was 805 nm.

Figure 5. Top: Arrangement of the silicon prism, crystal, and objective lens of the excitation source. The direction of the axis of the crystal is as shown in the diagram, and the direction of polarization of the exciting light is parallel to the c-axis. Bottom: Microscopic image of the OH1



single-crystalline thin film and fitted silicon prism. A PET film was inserted between these to avoid invasion of excitation light into the prism. The thin film was mounted on glass because it is brittle.

3. Results and discussion 3.1. Confirmation of sublimation of OH1 crystal by thermogravimetry and differential thermal analysis (TG-DTA)

Figure 6. Thermogravimetry and differential thermal analysis (TG–DTA) results for the OH1 crystals. The red line shows the DTA results; an endothermic peak corresponding to a phase change was observed near the melting point. The blue line shows the TG results; a large weight loss peak was observed above the melting point. However, a weight loss peak was also observed below the fusion point, most likely associated with sublimation of OH1.


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OH1 has a high NLO coefficient of d33 = 120 pm/V (at λ = 1.9 µm)28, and THz generation using a range of methods based on bulk OH1 crystals has been reported.29 Thermogravimetry (TG) and differential thermal analysis (DTA) measurements of powdered OH1 crystals were conducted in the temperature range of 60–260 °C to determine the thermal properties, and the results are shown in Figure 6. The blue line shows the results of the TG measurements and the red line indicates the results of the DTA measurements. In the DTA results, an endothermic peak was observed near the melting point of 212 °C30. Similarly, the TG results showed a large weight loss at a temperature above the melting point. From these results, a phase change in the OH1 crystals may be considered to have taken place at the melting point. In addition, the TG experiments showed a small but distinct weight loss in the region between 100 °C and 190 °C, and at atmospheric pressure. OH1 molecules do not undergo thermal decomposition at temperatures below 190 °C, and this weight change instead indicates sublimation at temperatures above 100 °C.

3.2. Assessment of the structure of OH1 by nuclear magnetic resonance imaging



Figure 7. Proton nuclear magnetic resonance (1H NMR) spectra for the OH1 crystals before and after heating. Heating was conducted at 170, 190, and 212 °C and a change in structure was observed with increasing temperature. The NMR spectrum up to 190 °C showed little variation from that obtained before heating, but changes in the peak were observed above 190 °C.

To determine the optimal temperature for sublimation, NMR measurements were undertaken for heat-treated OH1 in the temperature range 100–220 °C (see Supplementary Information), and NMR spectra for OH1 heated to 170, 190, and 212 °C are shown in Figure 7. The NMR peaks before heating were assigned as follows: 1.07 (s, 6H,–CH3), 2.55 (s, 2H, –CH2–), 2.61 (s,2H, – CH2–), 6.78 (s, 1H, –C––CH–), 6.68–6.81 (d, J¼8.7Hz, 2H, Ar–H),6.95–7.01 (d, J¼18Hz, 1H, – CH––CH–), 7.15–7.21 (d, J¼18Hz, 1H,–CH––CH–), 7.46–7.49 (d, J¼8.7Hz, 2H, Ar–H), and were in agreement with those reported earlier.29 Even after heating, the spectrum up to 190 °C clearly showed peaks in the same positions, whereas at 212 °C, a clear change in the spectrum was observed. Hence, there was almost no change in the molecular structure of OH1 up to 190 °C; thus, sublimation of OH1 is possible without breaking down its structure up to this temperature; this was therefore set as the upper limit for heating in vapor deposition.


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3.3. Crystal growth of OH1 single-crystalline thin films and their evaluation OH1 crystals were then grown by physical vapor deposition under conditions causing weight loss without thermal decomposition. Based on the temperature results discussed above, vapor deposition was conducted in a tubular furnace maintained at a temperature gradient of 90–180 °C (Figure 3).

Figure 8. Image of the grown OH1 thin film. (a) Image obtained under microscope observation. Left: top view; right: side view. (b) Scanning electron microscopy (SEM) observation image. Left: top view; right: side view. The crystalline film was several micrometers thick. (c)



Schematic diagram of the grown OH1 thin film in this study. The crystal was thin and long along the c-axis and was found to grow faster in a specific direction. (d) Results of observation of OH1 thin film by polarization microscopy. It was confirmed that when observation was performed while rotating the polarization direction of the beam transmitted through the crystal by 45° each time, the intensity of the transmitted light changed uniformly over the entire crystal. This indicates that a single crystal was grown.

Figure 8 shows scanning electron microscopy (SEM) measurement results of the OH1 thin film, indicating a flat film with rapid growth in a specific direction. The growth speed of the OH1 thin film was fastest in the c-axis, then the b-axis, then the a-axis. The SEM results showed that the length of the OH1 thin film was about 20 mm (c-axis), with a width of about 500 µm (baxis) and a thickness of about 5–10 µm (a-axis). Figure 8 shows imaging results for the OH1 thin film obtained using polarization microscopy. Since the light transmitted from the OH1 thin film changed uniformly and regularly with the rotation of the polarizer, it was confirmed that the film was a single crystal. From the results of these measurements, it was confirmed that the growth method using physical vapor deposition grew an OH1 single-crystalline thin film with a thickness of several µm, which can fulfil the phase matching condition in the broadband regime for generation of THz waves.


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Figure 9. Evaluation of the OH1 crystals using X-ray diffraction (XRD). (a) XRD peak for the OH1 bulk crystal in the bc-plane, and XRD peak for the OH1 single-crystalline thin film in the bc-plane. An almost identical diffraction signal was observed for both the bulk OH1 crystal and the OH1 single-crystalline thin film, ion. (b) X-ray rocking curve measurements (ω scan) for the bulk OH1 crystal and the OH1 single-crystalline thin film. A narrow peak was observed for the OH1 single-crystalline thin film in the rocking curve measurements.

Figure 9 shows the X-ray diffraction (XRD) pattern obtained from out-of-plane measurements in the bc-plane for an OH1 bulk crystal grown by the slow-cooling method using a methanol solution,30,31 and the OH1 single-crystalline thin film grown by physical vapor deposition. Both showed peaks for the (200) plane, the (400) plane, and the (600) plane at the same positions, but



a very small difference was observed in that the OH1 single-crystalline thin film showed a peak for the high-angle side. This indicated that vapor deposition had caused the formation of the thin film. The lattice constant for the a-axis calculated using the Bragg equation was 15.435 Å for the bulk OH1 crystal (literature value: 15.441 Å)32, whereas that for the OH1 single-crystalline thin film was 14.775 Å. The a-axis lattice constant was smaller than that for the bulk OH1 crystal, and it is therefore suggested that the lattice constants of the b-axis and c-axis are also different from the bulk OH1 crystal. More details of the evaluation of the OH1 single-crystalline thin film will be reported in future studies. The diffraction pattern of the OH1 single-crystalline thin film indicated that the structural purity was high, because there were no other diffraction peaks. A series of (200) rocking curve (ω-scan) measurements were performed to confirm the crystalline perfection of the bulk OH1 crystal and the grown OH1 single-crystalline thin film (Figure 9b). As shown in Figure 9b, the full width at half maximum for the bulk crystal was 59.0 arcsec and that for the OH1 film grown using physical vapor deposition was 30.6 arcsec. This confirmed that the growth method for the OH1 single-crystalline thin film could achieve a crystalline perfection equivalent to, or better than, that obtained using conventional crystal growth methods.


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3.4. Terahertz wave generation using the OH1 single-crystalline thin film

Figure 10. THz wave generated from the OH1 single-crystalline thin film using the Cherenkov phase matching method. Left: time domain waveform; data based on TDS measurements. A pulse width of 130 fs was achieved. Right: frequency spectrum. Time waveform was obtained using Fourier transformation. Output in the region of 5–6 THz was achieved while reducing the absorption in the crystal.

THz wave generation was conducted with the OH1 single-crystalline thin film using the Cherenkov phase matching method. THz waves of frequencies up to 6 THz were measured, and the pulse width of the time waveform was 130 fs (Figure 10). In THz tomography, this pulse width was equivalent to a sample with a thickness of 6.5 µm, for a sample of refractive index n = 3. A very small pulse width was obtained, which was believed to be due to the reduced thickness of the crystal. The direction of polarization of the incident light was parallel to the c-axis and a polarized THz wave was also generated along the same axis. OH1 has absorption peaks around 1.14 and 1.51 THz along the direction of the c-axis and this effect was also observed in the frequency spectrum (Figure 10).33 This result confirms the formation of a thin film crystal of OH1. A large absorption peak above 2.3 THz usually occurs along the c-axis of OH1;28 however,



from the crystal grown here, THz wave generation was achieved from 2 THz to 5 THz, with a reduction in generated power at high frequency. This indicates that the thickness of the crystal was small and that absorption of the THz wave by the crystal was limited; hence, the advantages of the Cherenkov method were utilized. Thus, THz waves can be generated in a broadband regime using Cherenkov phase matching, even in OH1, which is strongly absorptive in the THz region. It is notable that THz wave generation was possible using the crystal obtained by physical vapor deposition. A high-quality NLO crystal with a high NLO coefficient and reduced optical loss is essential for realization of THz wave generation by femto-second pulse excitation. From these experimental results, it was confirmed that the crystal grown by physical vapor deposition was an NLO crystal with the ability to generate THz waves. In addition, further optimization of the system was possible, including investigation of the optimal prism shape and the optimal change angle of the incident beam. The optical properties of the OH1 crystal in the THz region are still not fully understood; therefore, there is significant scope for increasing the efficiency of THz wave generation in the future.

4. Conclusion In conclusion, an organic NLO single crystal that was capable of generating THz waves was successfully developed by physical vapor deposition. OH1 can undergo sublimation at temperatures of 100 °C and above, and a crystalline thin film with a thickness of several microns was grown by physical vapor deposition using this property. XRD and polarization microscopic observations confirmed that the OH1 thin film obtained was a single crystal. The Cherenkov phase matching method using the OH1 single-crystalline thin film successfully generated THz


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waves; the pulse width and generated frequency were 130 fs and around the 6 THz region, respectively. These results demonstrate that it is possible to utilize physical vapor deposition to grow an organic NLO crystal capable of efficiently generating THz waves.

AUTHOR INFORMATION Corresponding Author * Hirohisa Uchida. [email protected], * Kei Takeya. [email protected] Author Contributions H. U. conceived the idea of the crystal growth and wrote the main manuscript text. All authors designed the experiment, H. U. and R. Y. performed the crystal growth experiments, K. O., K. O., T. M., and K. T. performed THz generation experiments, H. U. and K. T. are the corresponding authors and wrote the main manuscript text, and K. K. is the supervisor of this work. All authors reviewed the manuscript. Funding Sources Japan Science and Technology Agency (JST) and JSPS KAKENHI (Grant No. 25220606). ACKNOWLEDGMENT We acknowledge the contributions of J. Seto and H. Okano of Nagoya University for their experimental support and helpful discussion, and Prof. Tatsuo Mori of Aichi Institute of Technology for his advice related to the development of single-crystalline thin films. REFERENCES



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“For Table of Contents Use Only,”

Manuscript title: Organic nonlinear optical single-crystalline thin film grown by physical vapor deposition for terahertz generation

Author list: Hirohisa Uchida, Ryo Yamazaki, Kengo Oota, Koutarou Okimura, Tsubasa Minami, Kei Takeya, Kodo Kawase

TOC graphic:

Synopsis: A single-crystalline thin film of an organic nonlinear material 2-(3-(4hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene) malononitrile (OH1) is grown by physical vapor deposition. The crystalline character of the OH1 single-crystalline thin film is better than a bulk OH1 crystal. The OH1 single-crystalline thin film demonstrates efficient THz wave generation and low absorption of THz waves by the crystal.


Organic Nonlinear Optical Single-Crystalline Thin Film Grown by

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