DOI: 10.1021/cg9009493
Large-Size Pyrrolidine-Based Polyene Single Crystals Suitable for Terahertz Wave Generation
2009, Vol. 9 5003–5005
Ji-Youn Seo,† Soo-Bong Choi,‡ Mojca Jazbinsek,§ Fabian Rotermund,‡ Peter G€ unter,§ and O-Pil Kwon*,† †
Department of Molecular Science and Technology and ‡Division of Energy Systems Research, Ajou University, Suwon 443-749, Korea, and §Nonlinear Optics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland Received August 11, 2009; Revised Manuscript Received October 7, 2009
ABSTRACT: We report on the growth of large pyrrolidine-based polyene single crystals from methyl ethyl ketone (MEK) solutions for nonlinear optical applications. The polyene crystals grown exhibit a suitable morphology and surface roughness and good optical quality. We successfully demonstrated THz wave generation with as-grown polyene crystals. Although the crystal cut was not optimized and phase-matching conditions were not fulfilled, we achieved a similar or higher THz generation efficiency compared to the well-known inorganic ZnTe emitter crystals.
*Corresponding author. Tel: þ82 31 219 2462; fax: þ82 31 219 1610; e-mail:
[email protected].
Here, we report on the growth of large size MH2 single crystals grown in methyl ethyl ketone (MEK) solution. The MH2 crystals grown exhibit a much more suitable morphology for optical applications. With as-grown, unpolished MH2 crystals we successfully demonstrated THz wave generation with a similar or higher THz generation efficiency as obtained with the commonly used inorganic ZnTe emitter crystals. The chemical structure of MH2 is shown in Figure 1a. The MH2 material was synthesized according to the literature.13 Acentric MH2 crystals with a monoclinic space-group symmetry Cc show a weak hydrogen-bonded network of CN 3 3 3 H-C bonds with N 3 3 3 H distances of about 2.7 A˚.13 The MH2 crystals are soluble in polar solvents such as MEK, acetone, acetonitrile, and dimethylformamide, but are insoluble in water. After considering the parameters for solution crystal growth such as solubility, crystal shape and size, we decided to use polar MEK solvents in order to modify the morphology of crystals. The solubility of MH2 is about 1.04 g/100 g MEK at 40 C, which is in many cases high enough for the relatively fast growth rate from solution, as well as for obtaining relatively large crystals. Single crystals of MH2 were grown from MEK solution by the slow evaporation method with spontaneous nucleation at 40 C. The MH2 crystals grown from MEK solution exhibit an identical crystal structure as those grown from acetonitrile solution. Therefore, the grown crystal structure does not include MEK molecules and the crystals do not form a hydrated-phase polymorph with a centrosymmetric molecular arrangement.13 The MH2 crystals grown from MEK solutions are thick needle-shaped with well-formed crystallographic surfaces, while those grown in acetonitrile have an arrowhead shape with round surfaces (see Figure 1b,c). The axis of the needles is along the crystallographic c-axis and the side facets are (110) and (110), as illustrated in Figure 1c and determined by single-crystal X-ray analysis. An example of a crystal grown in MEK solution with a size of about 6 5 5 mm3 is shown in Figure 1c. As-grown MH2 crystals show good optical quality with flat surfaces that also exhibit a relatively small density of surface growth steps, which leads to a good optical transmission. Figure 2 shows the crystal-packing diagram projected along the c-axis, that is, the needle axis of crystals grown in MEK. The dotted lines present the boundaries of the as-grown crystal. The solid vector presents the direction of the polar axis projected to the ab-crystallographic plane. We can see that the orientation of the faces is not optimal for nonlinear optical applications such as THz-wave generation, since the faces are not parallel to the polar
r 2009 American Chemical Society
Published on Web 10/23/2009
Ultrashort electromagnetic pulses in the range of 0.1-10 THz, so-called “T-rays” have become important for nondestructive sensing and imaging of various chemical- and biomaterials, spectroscopy, and other interdisciplinary applications.1-3 Among various THz-wave generation methods,1-3 optical rectification of femtosecond optical pulses using nonlinear optical crystals, for example, inorganic ZnTe crystals, is commonly used for generation of broad-bandwidth THz waves with a high signalto-noise ratio.4 Compared to their inorganic counterparts, organic electro-optic crystals exhibit a large second-order optical nonlinearity and low dielectric constants and can be grown relatively easy.5,6 Up to now THz-wave generation schemes in organic crystals4,5,7-11 are most often performed in ionic stilbazolium salts, N,N-dimethylamino-N0 -methylstilbazolium p-toluenesulfonate (DAST)4,9,10 and 4-N,N-dimethylamino-40 N0 -methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS).11 Ionic stilbazolium salts DAST and DSTMS are interesting because of their high generation efficiency, but have some limitations due to their strong THz absorption band at around 1 THz and solubility in water. 10,11 Recently, nonionic configurationally locked polyene (CLP) crystals have been developed,12,13 which are insoluble in water and also exhibit a high second-order optical nonlinearity. For example, MH2 (2-(5-methyl-3-(4-(pyrrolidin-1-yl)styryl)cyclohex-2-enylidene)malononitrile, see Figure 1a)13 crystals having a pyrrolidine electron donor exhibit a large macroscopic nonlinearity with a similar powder second harmonic generation (SHG) efficiency at 1.9 μm as the benchmark ionic DAST, and are therefore promising for various nonlinear optical applications. Large single crystals of MH2 with a maximal side length of up to 1 cm have been grown by spontaneous nucleation in acetonitrile solution by a slow evaporation technique, as shown in Figure 1b.13 For optical applications, including THz wave applications, high optical quality and flat crystal surfaces with parallel input/output faces are essential. However, even with relatively large crystals available, the morphology of the MH2 crystals grown in acetonitirile solution is not very suitable for nonlinear optical applications; arrowhead shape crystals with nonflat rough surfaces and nontrivial crystallographic orientation have been usually obtained (see Figure 1b). Such crystals need very complicated and time-consuming cutting, orienting, and polishing procedures to obtain samples useful for optics.
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Figure 3. Experimental setup for the generation of THz pulses by optical rectification.
Figure 1. (a) Chemical structure of MH2 molecule. Photograph of MH2 crystal grown in (b) acetonitrile13 and (c) methyl ethyl ketone (MEK) solution by the slow evaporation method.
Figure 2. Crystal packing diagram projected along the c-axis, which is the needle axis. The dotted lines present the boundaries of the as-grown crystal. The solid vector indicates the direction of the polar axis projected to the ab-crystallographic plane. Carbon atoms in the non-π-conjugated part of the cyclohexene ring and hydrogen atoms are omitted for clarity.
axis; the angle between the polar axis and the normal to the (110) face is about 50. However, as demonstrated below for light incident normal to the (110) face, the effective nonlinear optical susceptibility is still large enough for THz generation. We have demonstrated THz wave generation using unpolished as-grown MH2 crystals by optical rectification. Figure 3 shows a schematic illustration of the experimental setup for generating and detecting the THz pulses. A Ti:Sapphire regenerative amplifier (Coherent Inc., Legend) delivers 1 mJ pulses with duration of 170 fs at a wavelength of 836 nm with a 1 kHz repetition rate. To generate and detect THz pulses, the optical pump beam was split into two branches by a 9:1 beam splitter. The 90% portion was
used for pumping the MH2 crystal and generating THz waves, while the 10% portion was used as probe signal for the electrooptic sampling in a ZnTe detection crystal; both the pump and the probe beam were p-polarized. The fundamental pump beam after the MH2 crystal was filtered out by inserting a Si wafer. The THz pulses were collimated by using an off-axis parabolic mirror and the focused THz beam diameter was about 1.5 mm. This focused THz beam was characterized subsequently by an electrooptic sampling (EOS) technique in a 1-mm-thick Æ110æ-cut ZnTe crystal. The EOS of ultrashort THz waves by using a ZnTe crystal enables measuring not only the intensity of the wave, but also its amplitude and phase. The amplitude of THz field is given by δI 2λ ETHz ¼ ð1Þ ΔI πln3 r41 where ETHz is the amplitude of the generated THz wave, δI is the photodiode intensity modulation measured due to the THz field in the detection crystal, ΔI is the maximum intensity difference between the balanced photodiodes, l is the thickness, r41 the electro-optic coefficient, and n is the refractive index of the detection crystal (ZnTe). In our experimental conditions, r41 = 4.1 pm/V, l = 1 mm, n = 2.85. An as-grown unpolished MH2 single crystal with a thickness of 2.6 mm was used. The generated few-cycle THz pulses from the MH2 crystal were compared to the pulses from a 2-mm-thick ZnTe crystals under identical experimental conditions. Though the phase matching condition was not fulfilled perfectly at this wavelength, we maximized the THz conversion efficiency by rotating the MH2 crystal by about 15 degrees around the (110) surface normal from the orientation with the c-axis parallel to the pump light polarization. Even though considerable two-photon absorption occurs in MH2, no surface or bulk damages in the crystal could be observed for single pulse energies up to 250 μJ. Figure 4a shows the time traces of the measured THz field amplitude generated in the MH2 crystal and the reference ZnTe crystal. The organic MH2 crystal exhibits δI/ΔI of ∼2% corresponding to a peak-to-peak value of ∼1120 V/cm at the pump energy of 250 μJ, which is slightly higher than the THz field emitted by a 2-mm-thick Æ110æ-cut ZnTe crystal (∼1060 V/cm) under the identical experimental conditions. Note that under our experimental conditions the ZnTe crystal operates close to its phase-matching parameters, whereas for the MH2 crystals the phase-matching optimization has not been considered yet. The spectra of the generated THz fields are shown in Figure 4b, which were obtained by fast Fourier transform (FFT) of the THz time traces of Figure 4a. The central frequency was 0.69 and 0.73 THz in case of MH2 and ZnTe, respectively. The narrow absorption lines in both spectra are due to the ambient water vapor; the air humidity during the experiment was about 30%. The upper
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The MH2 crystals grown exhibit a suitable morphology and surface roughness and good optical quality for THz-wave applications. We have successfully demonstrated THz-wave generation using unpolished as-grown MH2 crystals with a similar or higher THz generation efficiency compared to the commonly used inorganic ZnTe emitter crystals. Acknowledgment. This work has been supported by the National Research Foundation of Korea Grant (NRF) funded by the Korea government (MEST) (Nos. 2009-0071457 and R112008-095-01000-0) and Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0093826). The authors highly appreciate the technical support by Korea Advanced Nano Fab Center (KANC), especially Dr. Kyong Ho Park.
References
Figure 4. Terahertz pulses generated in single crystals of organic MH2 (thickness 2.6 mm, solid line, (110) plane) and inorganic ZnTe (thickness 2 mm, dotted line, (110) plane) at a pump laser wavelength of 836 nm with the pulse duration of 170 fs. (a) Time-domain waveforms of the terahertz electric field ETHz(t). (b) Their frequency spectra ETHz(ν).
bandwidth limit of the generated THz spectra at about 2 THz is determined by the pump laser pulse duration. No broad absorption dip in the THz spectrum of MH2 is observed in the 1-THz range, as the case of organic stilbazolium salts such as DAST and DSTMS that exhibit a strong phonon absorption in this range. Furthermore, we have not observed considerable changes in the generated THz spectral amplitude and bandwidth after polishing the as-grown MH2 crystals. The results of the THz generation efficiency of organic MH2 crystals compared to inorganic standard ZnTe crystals demonstrate that MH2 crystals grown in MEK solution have a very high potential for THz applications. The conditions for the THz-wave generation in MH2 crystals are not fully optimized; the generation efficiency can still be enhanced by adjusting the phase matching conditions and crystal thickness. Compared to inorganic crystals, organic MH2 crystals can be grown relatively easily using commonly available glassware by a simple solution growth method. In addition, the as-grown MH2 crystals having flat parallel surfaces with good optical quality can be directly used for THz generation and other nonlinear optical applications without complicated polishing procedures. In summary, we have reported on the growth of large size single MH2 crystals in MEK solution by the slow evaporation method.
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