Growth of Ultrathin and Highly Efficient Organic Nonlinear Optical

Jan 10, 2013 - Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan ... Present address: Department of Physic...
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Growth of Ultrathin and Highly Efficient Organic Nonlinear Optical Crystal 4′-Dimethylamino‑N‑methyl-4-Stilbazolium p‑Chlorobenzenesulfonate for Enhanced Terahertz Efficiency at Higher Frequencies S. Brahadeeswaran,*,† Y. Takahashi,† M. Yoshimura,† M. Tani,‡ S. Okada,§ S. Nashima,⊥ Y. Mori,† M. Hangyo,# H. Ito,∥ and T. Sasaki† †

Division of Electrical, Electronic and Information Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Research Centre for Development of Far-Infrared Region, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan § Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa-shi, Yamagata 992-8510, Japan ⊥ Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto Sumiyoshi-ku, Osaka-shi 558-8585, Japan # Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan ∥ RIKEN, 519-1399, Aramaki-Aoba, Aoba-ku, Sendai 980-0845, Japan ‡

ABSTRACT: Growth of an ultrathin (thickness as low as 23 μm) organic nonlinear optical crystal of 4′-dimethylamino-n-methyl-4-stilbazolium p-chlorobenzenesulfonate (DASC) has been achieved by the slope nucleation method for generating THz waves with relatively higher efficiencies especially at higher frequencies. The propensity of lower solubility of this material in methanol solvent as compared to 4′-dimethylamino-n-methyl-4-stilbazolium tosylate (DAST) was favorably exploited to obtain these thin crystals. The DASC crystals thus obtained were studied for the generation of THz waves, and their efficiency was compared with DAST and zinc telluride (ZnTe) single crystals. The enhancement THz efficiency observed in thinner DASC could be attributed to better velocity matching and weaker absorption as compared to thicker crystals of the same material. and nonlinear optical coefficient d14 ∼ 95 pm/V] is currently the most widely used crystalline material for the generation and detection of THz frequencies because of its good phase matching property when used with 800 nm lasers, such as the mode-locked Ti: sapphire laser.9,6 It has been shown that organic crystals such as 4-N,Ndimethylamino-4′-N′-methyl stilbazolium tosylate (DAST), 7,10,11 2-(α-methylbenzyl-amino)-5-nitropyridine (MBANP),12 and N-benzyl-2-methyl-4-nitroaniline (BNA)13 could be effectively used for efficient generation and detection of THz wave with a high signal-to-noise ratio owing to their excellent NLO susceptibilities and EO coefficients. Among the listed crystals, DAST is widely studied as it has been proved to generate highly efficient THz waves7,10,11 and to detect THz frequencies using the EOS method14 due to the highest NLO susceptibilities (for e.g., χ 111 = 1230 ± 130 pm/V at 800 nm) and EO coefficients (for e.g., r111 = 77 ± 8 pm/V at 800 nm) reported so far. In general, a typical NLO crystal used for the generation of THz is expected to possess excellent NLO coefficient, possibilities of velocity-matching [wherein the group velocity of the femtosecond laser (vg) and the phase velocity of the generated THz waves (vTHz) are equal] for the case of OR process using a femtosecond laser,15 reasonable

I

n recent years extensive research has been carried out on the generation and detection of terahertz (THz) frequencies. Important molecules such as proteins, pharmaceuticals, explosives, narcotic drugs, etc., which are of interest to the scientific community, exhibit their characteristic fingerprints in this region of the electromagnetic spectrum. However, this THz region is hitherto less explored due to the lack of a reliable source for generation and sensor for detection,1 and its significance has already been realized in different fields.2 Various methods such as photoconductive switching,3 free electron laser (FEL),4 optical rectification (OR),5,6 difference frequency generation (DFG),7,8 etc. of generating THz waves have been developed with different ranges of THz frequencies. Among the methods mentioned above, both DFG and OR necessitate highly efficient nonlinear optical (NLO) single crystals with the ability of phase matching so that high output power of THz frequencies can be produced. In particular, the OR method, where two optical waves with angular frequency ω interact with each other in a non-centrosymmetric crystal to generate a dc polarization through the nonlinear susceptibility χ (2) (Ω = 0; ω, −ω), where Ω is a dc polarization term, is a desirable choice as this produces THz waves with high signalto-noise ratios and with very broad bandwidth in the THz range used with femtosecond laser excitations. The detection of the THz frequency is efficiently done by the electro-optic sampling (EOS) method, which also requires an NLO crystal. Inorganic zinc telluride (ZnTe) [EO coefficient r41 ∼ 4 pm/V © 2013 American Chemical Society

Received: May 4, 2012 Revised: January 9, 2013 Published: January 10, 2013 415

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thinner (thickness in terms of coherence length lc) NLO crystals could be used for this purpose to achieve broadband THz emission. However, preparing thin samples from organic single crystals is a more tedious process as compared to their inorganic counterparts since they are soft and fragile and often get damaged while reducing their thickness. Encouragingly, it has been shown that as-grown organic NLO crystals with high efficiency, such as DAST, could be directly utilized for the generation of efficient THz emission.20 Hence, it is proposed here that employing as-grown, ultrathin and high NLO coefficient organic NLO crystals could be an alternative strategy to realize broadband and efficient generation (orders of magnitude higher than commercially available ZnTe thin crystals) of THz waves. However, growth of high quality DAST is a difficult task, and various attempts have been made to accomplish this,26−28 and successful growth of high quality DAST crystals has been achieved by Ruiz et al.29 We have recently shown that twin-free DAST crystals can be grown by slope nucleation method (which facilitates controlled nucleation27,28) coupled with isothermal solvent evaporation (SNMSE) through solutions of lower supersaturation and that the asgrown thin crystals exhibited excellent surface roughness30 and enhanced bulk homogeneity, when examined through THz waves.31 The DAST crystals obtained by this method had the dimensions of (a × b × c) in the range of (3−7) × 3−7 × 0.2− 0.8 mm3 and the growth rates were about 0.05−0.20 mm/day. DAST crystals thinner than the reported thickness range along the c-axis (i.e., 0.2−0.8 mm) could also be obtained either by processing the grown crystal (t = 60 μm)21 or by growing thin films (t = 80 μm).32 In the present work, we report our maneuver with the growth of a relatively new, ultrathin and highly efficient NLO crystal, namely, 4′-dimethylamino-N-methyl-4-stilbazolium pchlorobenzenesulfonate (DASC), for the efficient generation of THz waves by SNM-SE through solutions of lower supersaturation. We also show that these crystals could be useful in enhancing THz efficiency especially at higher frequencies, which is highly desired to realize their THz applications in this region also. DASC is an isomorphous form of DAST and, in many aspects (including lattice parameters), compared closely with this well-studied crystal. The chemical structure of DASC is shown along with that of DAST in Figure 1. This crystal has

transparency in the frequency region of interest, and high laser damage threshold. The coherence length lc, i.e., the distance over which the velocity mismatch between the optical and THz waves can be tolerated,16 is an important factor considered for the efficient THz generation and is given as9 lc = π /|Δk| = πc /{Ω | n THz − ng |}

(1)

where Δk = kopt(ω − Ω) − kopt(ω) + kTHz (Ω) = Ω [nTHz(Ω) − ng(ω)] /c = 0, ng is optical group index and nTHz is THz index. Crystal thickness and phase matching related studies have been reported for the NLO single crystals such as ZnTe (for THz generation17 and detection18,19) and DAST (for THz generation11 and detection20,21). Wider phase-matching bandwidth, i.e., approximate phase matching for all frequencies, for the ultrashort pulses is a desirable but difficult task to achieve as they possess many wavelengths.22 The phasemismatch limits the amplitude and bandwidth of the THz generation, whereas the phonon absorption that is associated with the lattice resonance effect in an NLO crystal leads to absorption of the THz radiation. In addition, large dispersion of the THz index that exists near any absorption band makes the effective phase matching region narrow.21 However, a relatively wider phase matching bandwidth has already been achieved using a very thin NLO crystal ZnTe (a material with relatively lower NLO efficiency) since the difference between the optical group velocity and the terahertz phase velocity (i.e., group velocity mismatch, GVM = 1/vTHz − 1/vg) in the thin crystal is expected to cause a smaller phase difference and thus limiting the amount of destructive interference.15,17 Moreover, studies on organic NLO single crystalline films of methyl nitroaniline (MNA) (NLO coefficient d12 = 38 pm/V in its bulk crystalline form23) in their ultrathin forms have shown that the phase matching became irrelevant (non phase-matched condition) and, as a result, that the ultra broadband second harmonic generation (SHG) could be observed.24 Hence, it could be stated that if a thin crystal of highly efficient organic NLO materials could be used, it would be a desirable approach to achieve a broad phase-matching bandwidth with reasonably higher efficiency of THz waves. Moreover, a thinner crystal detects higher THz frequencies better than a thicker crystal, albeit with reduced detector sensitivity as compared to that of a thicker crystal.16 Thickness of crystals (typically the morphology) can be controlled by parameters such as degree of supersaturation, pH of the solution, growth temperature, certain catalytic reactions, etc. For instance, thickness control of 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile (phenolic polyene OH1) crystals using metal acetate additives has recently been reported by Choi et al.25 as a method of producing crystals with varying thicknesses. This approach has been utilized to optimize the conversion efficiency according to the velocity-matching condition and absorption at either different pump optical wavelengths or desired terahertz frequency ranges. For DAST it has been shown theoretically and experimentally that the thick crystals (>1 mm) exhibit higher THz power below 3 THz11 and that thin c-cut and polished crystal (t = 60 μm) exhibited, as expected, extended THz emission and reduced absorption in THz waves, due to the multiple phonon bands caused by the weak intermolecular interaction.21 From the aforementioned aspects it could be understood that there has been incessant quest for suitable materials and methods for the efficient generation of THz waves and that

Figure 1. Molecular schemes of DAST and DASC.

been used to generate THz frequencies in the range of 2−19 THz by the DFG method, and it exhibited the highest THz energy of 520 nJ for 11.3 THz33 and its mixed form with DAST has also been reported.34 One striking feature which differentiates the DASC from DAST is its relatively lower solubility (0.27 times) in methanol solvent.35 The reason for this lower solubility could be attributed to the salvation of anion moiety in methanol solvent.36 This means that the growth of DASC crystal would take place at a relatively lower equilibrium concentration and supersaturation as compared to that of DAST. This propensity of DASC material could be favorably 416

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numbers of crystals nucleated on the grooves in the Teflon slope, as a consequence of controlled nucleation, as compared to those available in the bottom of the Teflon flask. The crystals grown in the bottom of the flask were separated from the highquality crystals grown on the slope using a high-density liquid called Fluorinert.28 This liquid, when filled just above the Teflon slope, can make the poor-quality DASC crystals float on the Fluorinert−DASC solution interface. Thus, this process leaves crystals that are attached to the grooves only in the Teflon slope. Since the growth and harvesting temperatures are almost the same, additional nucleations on the surface of the DASC crystals, usually observed due to the evaporation of solvent when these temperatures are significantly different, were not observed. The morphology of the DASC crystals obtained by the SNM-SE method was closely related to that of DAST, and their dimensions were, as expected, in the range of 1.5−4.5 × 1.4− 3.5 × 0.023−0.120 mm3 and two typical crystals are shown in Figure 4. It is important to mention here that the crystals were

utilized for growing ultrathin crystals when the aforementioned and well established SNM-SE method is adopted. In this method, a Teflon slope with parallel grooves in uniform intervals is kept inside the DASC solution with a specific slanting angle. This modification facilitates a considerable number of nucleations to slide through the Teflon slope and settle in the grooves which in turn act as seeds for further growth.30 The Teflon flask used for the growth of DASC crystals by the evaporation method is schematically shown in the picture (Figure 2c), and the flask used for SNM (Figure 2a,b) also is given for comparison.

Figure 2. Schematic of Teflon growth flasks used for SNM (a, b) by solution cooling and (c) for solvent evaporation (SNM-SE).

Saturated solutions of DASC were prepared with three different concentrations 5 g/L, 6 g/L, and 7 g/L in methanol solvent and kept in Teflon flasks. These flasks were housed in a water bath whose temperature is controlled to the accuracy of ±0.01 °C and were heated to 60 °C in about 6 h and were kept at this temperature for 12 h. The solutions were then cooled to 36 °C, and the Teflon caps were replaced with a Teflon disc which has a hole of diameter 1 mm. The bath temperature was then reduced to 20 °C, and the solution was allowed to evaporate. The heating program employed for the growth of DASC crystals is shown in Figure 3. More details regarding the

Figure 4. Typical DASC crystals with dimensions (a) 4.5 × 3.5 × 0.104 mm3, (b) 1.4 × 1.4 × 0.023 mm3 grown by the SNM-SE method.

too thin to handle, and the thinnest crystal that could be separated safely was about 23 μm and, hence, that sufficient care must be taken to handle these crystals. These DASC crystals were initially studied for their optical quality under transmission mode and surface quality under reflection mode using a Keyence digital microscope (model VHX-100). These observations facilitated the selection of DASC crystals without any visible inclusions. The optical transmission spectrum from 500 nm to 2 μm recorded for the DASC crystal of dimension 3.5 × 3.0 × 0.261 mm3 is shown in Figure 5. From the figure it can be seen that the as-grown DASC crystal exhibited transmission of about 80% and that these crystals could be directly used for NLO applications and subsequently for THz applications. High quality crystals selected carefully with varying thicknesses (from 23 to 114 μm) along the c-axis

Figure 3. Heating program employed for the growth of DASC crystals.

growth aspects can be found elsewhere.30 The grown crystals were separated for further analysis such as the growth morphology, dimensions, optical transmission, and THz generation. Controlled nucleation on the Teflon plate and subsequent growth were observed for the case of DASC crystals (as like DAST), and we seldom noticed twinned DASC crystals. The nucleations were observed after 5, 3, and 2 days for the solutions at 20 °C with concentrations 5 g/L, 6 g/L, and 7 g/L respectively. Also, we have observed that the nucleation control was relatively better for the DASC solution with 6 g/L concentration and that the growth duration was typically 14 days for this concentration. There were relatively more

Figure 5. Transmission spectrum of DASC crystal. 417

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([001]) were studied for their THz properties. The c-cut DAST crystal of thickness about 200 μm and the ZnTe ([110]) crystal of 10 μm thick (in order to achieve broadband THz emission) have also been studied for comparison. The experimental setup used for obtaining THz emission efficiency is shown in Figure 6. Mode-locked Ti: Sapphire pulsed laser with center

Figure 7. Amplitude spectra of THz waves generated from DASC crystals of various thicknesses (t = 55, 81, 84, and 113 μm) with the ordinate in the logarithmic scale.

indicates that the coherence length lc is smaller than 55 μm, and the THz emission efficiency is limited by the crystal thickness. It is worthwhile to mention here that the material and THz properties of DASC crystal are, as stated earlier, similar to those of DAST crystals. As far as the coherence length is concerned, it has been shown for the DAST crystals that it depends on the pump wavelengths used for THz generation and that for the pump wavelength of 800 nm the coherence length could be around a few hundreds of micrometers in the frequency range of 1−8 THz and about 10 μm at higher frequencies (>12 THz).38,39 These results which are obtained for DAST crystals could be seen to corroborate the results shown in Figure 7 for the DASC crystals. The amplitude spectra of THz emission from the DAST and ZnTe crystals have also been shown along with the thicker DASC crystal (t = 113 μm) separately for comparison in Figure 8 in the range of 0−30 THz. The spectral dips seen on the

Figure 6. Experimental setup used for the generation and detection of THz waves in DASC crystals.

wavelength of 810 nm and pulse width of 15 fs at 75 MHz repetition rate was employed for this purpose, and the experiments were carried out under identical conditions. The laser was polarized in such a way that it is parallel to the a-axis of the DASC crystals since the NLO coefficient along d11 is observed to be maximum, similar to that of DAST. A photoconductive dipole antenna made with low-temperaturegrown GaAs was used as the THz sampling detector, which was triggered with probe laser pulses separated from the pump laser pulses. The probe laser and THz waves were both focused on the photoconductive antenna from the air side of the device to avoid THz absorption in the GaAs substrate,37 and the incident probe laser power was about 20 mW. The photoconductive signal was detected by a lock-in amplifier, which differentiated the THz signal from the background synchronized with pump beam modulation by an optical chopper. The THz time-domain waveforms were obtained by scanning the optical delay in the pump beam path. The THz emission experiment was performed in a usual ambient condition at room temperature and atmospheric pressure. The THz spectroscopy for evaluation of DAST and DASC crystals was performed in an airtight chamber and using dry air with its dew-point at −60 °C to remove the influence of the water vapor evaporation. During these experiments it was found that the DASC could be safely used up to 50 mW incident laser power (i.e., without any damage), and the beam focusing using an off-axis parabolic mirror (f = 75 mm) was adjusted to about 100 μm that corresponds to the peak intensity of about 0.5 GW/cm2. The amplitude spectra obtained by Fourier transformation of THz time-domain waveforms for the DASC crystals of thicknesses 55, 81, 84, and 113 μm are shown in Figure 7. From the figure it can be observed that the DASC crystals could generate THz frequencies in the range of 10−33 THz (wideband) and that there are no significant changes in the intensities at the generated frequencies for the crystal thicknesses studied. However, the thinner DASC (t = 55 μm) exhibits relatively higher THz efficiency as compared to that of other thicker DASC crystals in the frequency range studied. This result

Figure 8. Amplitude spectra of THz waves generated from ZnTe (t = 10 μm), DASC (t = 110 μm), and DAST (t = 200 μm) crystals.

DAST and DASC spectra are actually due to optical phonon resonances of the respective crystals. From the figure it can be seen that there are slight shifts in the absorption of THz frequencies between DAST and DASC crystals. This phenomenon has already been reported in ref 33, and it has been stated that DAST and DASC can complement each other in the given frequency range.34 It can also be observed from Figure 8 that DASC has generated THz frequencies with reasonably higher efficiency than that of relatively thicker DAST (t = 200 μm). This observation agreed well with the reported results of Matsukawa et al.36 for THz emission experiment using DASC and DAST crystals using relatively wider pump pulse widths (70−200 fs) in the observed THz 418

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emission frequency range below 7 THz. When compared with ultrathin inorganic NLO crystal ZnTe (t = 10 μm), the DASC exhibited significantly enhanced efficiency (about 80 times in the integrated spectral power). It is to be mentioned that thin ZnTe crystal has already been employed to achieve broadband THz emission, but with low conversion efficiency19,40,41 due to its low NLO efficiency. The thinnest DASC crystal (t = 23 μm) was also studied for its THz generation efficiency after it was mounted with great care and was compared with the relatively thicker DASC (t = 104 μm) and thinnest ZnTe (t = 10 μm). Figure 9a,b shows the results obtained for the thicker and thinnest DASC and thinnest ZnTe crystals in time and frequency domains, respectively. From Figure 9a, it is evident that even the thinnest ZnTe crystal exhibited relatively weaker peaks in the range studied as compared to that of thick and thin DASC

crystals, and this trend is clearly visible in the corresponding frequency domain also (Figure 9b) in the range of 0−40 THz. Although both thicker and thinnest DASC crystals showed similar broad bandwidth up to about 35 THz and multiple phonon absorption at same frequencies, their strengths at different frequency ranges were strikingly different. When all these generated THz waves were checked in the linear scale on the abscissa and with a reduced frequency range of 0−15 THz (Figure 9c), it became evident that thicker DASC crystal showed higher THz efficiency up to about 3 THz, and the thinnest DASC exhibited enhanced efficiency at frequencies higher than 3 THz with a strength in the range between about 1.5 and 5 times as compared to the thicker DASC and that the thinnest ZnTe had the lowest THz efficiency. The reason for the crossover in the strength of THz waves at 3 THz is attributed to the change in the coherence length with THz frequency. Even if the dispersion in the NLO crystal is small, the coherence length lc is scaled with the inverse of THz frequency Ω as indicated by eq 1. Thus, the coherence length lc increases as the frequency decreases and the thicker NLO crystal becomes preferable for THz emission. It is to be noted that the bandwidth of an electro-optic crystal for THz generation and detection is determined by the optical phonon resonances in that crystal. The enhancement in the THz emission efficiency for the thinnest DASC crystal could be attributed to relatively broader THz peaks of constructive interferences due to velocity matching as compared to the thicker crystals15 and weaker absorption due to the less propagation distance for THz waves in the thinner crystal: The intensity decay of THz wave in the absorptive crystal is described by the Lambert−Beer law, exp(−α z) where α is the absorption coefficient at a specific frequency (at an optical phonon band in this case) and z is the propagation distance in the crystal. This kind of observation has also been reported for the thin (t = 60 μm) DAST crystal.21 However, this thin DAST crystal plate was a processed sample from a commercially purchased crystal, whereas the thinner (t = 23 μm) DASC crystal employed in our present study was an as-grown sample, which did not necessitate such a tedious sample preparation. Hence, from the present results it could be understood that asgrown ultrathin DASC crystal with thickness as low as 23 μm could generate THz waves with enhanced strength at higher frequencies and, consequently, these as-grown thinner DASC crystals could be effectively used to realize their applications in this region. Since the thinner crystals detect higher frequencies better than the thicker crystals of the same material,16 the enhanced THz efficiency of the thinner DASC at the higher region also is expected to possess enhanced detectability for the THz frequencies in this region when it is used as the EO sampling detector. As the EOS detection depends on the refractive index values at THz frequencies, herein we present the refractive indices (n) and extinction coefficients (κ) of DASC and DAST crystals along the a-axis and b-axis measured in a frequency range below 3 THz by THz time-domain spectroscopy (Figure 10). The measurement was done by inserting the crystals in the focused THz beam path shown in Figure 4 and detecting the transmitted THz waveform from each crystal. At frequencies higher than 3 THz it was difficult to extract reliable data because of poor signal-to-noise ratios. At a frequency range from 1 to 1.2 THz it was also difficult to extract reliable data for DAST due to the strong absorption around 1.1 THz (Note that only thick crystals were available for DAST). From Figure 10 it

Figure 9. (a) Time-domain waveforms of the THz pulses obtained for the ZnTe (t = 10 μm), DASC (for t = 104 and 23 μm). (b) Amplitude spectra obtained by Fourier transformation of the time-domain waveforms of the THz pulses shown in Figure 9a. (The amplitude in abscissa is given in logarithmic scale.) (c) Amplitude spectra of the THz waves shown in Figure 9b are shown for a limited frequency range of 0−15 THz (with the ordinate in the linear scale). 419

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obtained for DAST and ZnTe single crystals. The results revealed that the DASC crystals, in general, exhibited enhanced THz frequencies in the region studied and, in particular, that the DASC crystals with a thickness as low as 23 μm were found to exhibit significantly higher THz efficiency in the higher frequency region. This enhancement could be attributed to relatively broader THz peaks of constructive interferences due to velocity matching as compared to the thicker crystals and weaker absorption due to the less propagation distance for THz waves in the thinner crystal. Hence these ultrathin DASC crystals could be very useful for realizing applications in this region and for better detectability of THz frequencies. As the lowest thickness of the DASC crystal that we could obtain was only about 23 μm (slightly higher than the estimated coherence length of 10 μm for higher THz frequencies), it would be interesting to study further the effect of thickness of DASC crystals on the efficiency of THz frequencies around the coherence length also. The refractive indices measured for DASC along the a- and b-axes up to 3 THz have shown that they could detect THz waves in this region except certain frequencies wherein characteristic absorptions are observed.

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Figure 10. The refractive indices (n) and extinction coefficients (κ) of DASC and DAST crystals along with (a) the a-axis and (b) b-axis measured in a frequency range 0−3 THz by THz-TDS.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Mobile: +919442317559. Present address: Department of Physics, Anna University, BIT Campus - Tiruchirappalli, 620024 India.

could also be observed that the refractive index values vary from 2.8 to 3.3 for a-axis and 1.6 to 2.2 for b-axis in the frequency range of 0−3 THz. These values could be used to calculate the THz intensity loss. The maximum value of the THz intensity loss due to the reflection at the air−crystal interface is estimated to be around 29% and 14% for a- and b-axes respectively. Other losses which are caused by absorption and scattering are also expected to be low since many of the DASC crystals employed here had thicknesses less than 100 μm. Along the a-axis, the extinction coefficient of DASC shows a peak at 1.04 THz, similar to DAST. Except the strong absorption band at 1.04 THz, DASC has a good transparency in the observed frequency region as well as DAST. The refractive index of DASC along the a-axis is larger than DAST in this frequency range. This might be attributed to the larger molecular polarization caused by the chlorine (Cl) atom attached to the end of DASC molecule (Figure 1), whereas a methyl-group is attached in DAST. It is expected, if DASC has a similar dispersion property in the optical region as that of DAST, the refractive index mismatch will be reduced by using lasers with longer-wavelengths, such as 1.55 μm, and thus the THz emission efficiency will be improved. The refractive indices and extinction coefficients along the baxis of DASC and DAST are shown in Figure 10b. The DASC and DAST show three absorption bands but at slightly different frequencies: 0.76, 1.0, and 1.5 THz for DASC, and 0.84, 1.1, and 1.5 THz for DAST. The refractive index of DASC is larger than that of DAST in frequencies higher than 1.6 THz, while below 0.7 THz their values are almost the same. These complex refractive index data along the b-axis might be useful when we try to use nonlinear optical coefficients other than d111, such as d122 (∼96 pm/V for SHG of 1318 nm).42 In conclusion, we have shown that the propensity of the lower solubility of the DASC material in methanol solvent could be favorably utilized to obtain ultrathin and high quality DASC crystals. Many such as-grown crystals were verified for their THz generation efficiencies and were compared with that

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS One of the authors (S.B.) thanks DST-SERC of Government of India for the award of Fast Track Young Scientist Scheme (SR/ FTP/PS-53/2007 Dated 22-08-2008).

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REFERENCES

(1) Ferguson, B.; Zhang, X.-C. Nat. Mater. 2002, 1, 26−33. (2) Tonouchi, M. Nat. Photonics 2007, 1, 97−105. (3) Auston, D. H.; Cheung, K. P.; Valdmanis, J. A.; Kleinman, D. A. Phys. Rev. Lett. 1984, 53, 1555−1558. (4) Williams, G. P. Rev. Sci. Instrum. 2002, 73, 1461−1463. (5) Zhang, X.-C.; Jin, Y.; Ma, X. F. Appl. Phys. Lett. 1992, 61, 2764− 2766. (6) Leitenstorfer, A.; Hunsche, S.; Shah, J.; Nuss, M. C.; Knox, W. H. Phys. Rev. B 1999, 61, 16642−16652. (7) Kawase, K.; Mizuno, M.; Sohma, S.; Takahashi, H.; Taniuchi, T.; Uratta, Y.; Wada, S.; Tashiro, H.; Ito, H. Opt. Lett. 1999, 24, 1065− 1067. (8) Ito, H.; Suizu, K.; Yamashita, T.; Nawahara, A.; Sato, T. Jpn. J. Appl. Phys. 2007, 46, 7321−7324. (9) Nahata, A.; Weiling, A. S.; Heinz, T. F. Appl. Phys. Lett. 1996, 69, 2321−2323. (10) Schneider, A.; I. Biaggio, I.; G̈ unter, P. Opt. Commun. 2003, 224, 337−341. (11) Taniuchi, T.; Okada, S.; Nakanishi, H. J. Appl. Phys. 2004, 95, 5984−5988. (12) Carey, J. J.; Bailey, R. T.; Pugh, D.; Sherwood, J. N.; Cruickshank, F. R.; Wynne, K. Appl. Phys. Lett. 2002, 81, 4335−4337. (13) Miyamoto, K.; Minamide, H.; Fujiwara, M.; Hashimoto, H.; Ito, H. Opt. Lett. 2008, 33, 252−254. (14) Han, P. Y.; Tani, M.; Pan, F.; Zhang, X. -C. Opt. Lett. 2000, 25, 675−677. (15) Schneider, A.; Biaggio, I.; Gunter, P. Opt. Commun. 2003, 224, 337−341. 420

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Crystal Growth & Design

Communication

(16) Dexheimer, S. L. Terahertz Spectroscopy: Principles and Applications; CRC Press, Boca Raton, FL, 2008; p 58. (17) van der Valk, N. C. J.; Planken, P. C. M.; Buijserd, A. N.; Bakker, H. J. J. Opt. Soc. Am. 2005, B 22, 1714−1718. (18) Gallot, G.; Zhang, J.; McGowan, R. W.; Jeon, T. I.; Grischkowsky, D. Appl. Phys. Lett. 1999, 74, 3450−3452. (19) Schall, M.; Jepsen, P. U. Appl. Phys. Lett. 2000, 77, 2801−2803. (20) Schneider, A., Ph.D. Thesis Generation and detection of terahertz pulses in organic crystals. Swiss Federal Institute of Technology: Zurich, Switzerland, 2005. (21) Zheng, X.; McLaughlin, C. V.; Cunningham, P.; Hayden, L. M. J. Nanoelectron. Optoelectron. 2007, 2, 58−76. (22) Trebino, R.; O’shea, P.; Kimmel, M.; Gu., X. Opt. Photonics News 2001, June, 22−25. (23) Levine, B. F.; Bethea, C. G.; Thurmond, C. D.; Lynch, R. T.; Bernstein, J. L. J. Appl. Phys. 1979, 50, 2523−2527. (24) White, J. O.; Hulin, D.; Joffre, M.; Migus, A.; Antonetti, A.; Toussaere, E.; Hierle, R.; Zyss, J. Appl. Phys. Lett. 1994, 64, 264−266. (25) Choi, E.-Y.; Seo, J.-Y.; Jazbinsek, M.; Kwon, O.-P. Cryst. Growth Des. 2009, 9, 4269−4272. (26) Pan, F.; Wong, M. S.; Bosshard, C.; Gunter, P. Adv. Mater. 1996, 8, 592−595. (27) Mori, Y.; Takahashi, Y.; Iwai, T.; Yoshimura, M.; Yap, Y. K.; Sasaki, T. Jpn. J. Appl. Phys. 2000, 39, L1006−1008. (28) Brahadeeswaran, S.; Onduka, S.; Takagi, M.; Takahashi, Y.; Adachi, H.; Yoshimura, M.; Mori, Y.; Sasaki, T. J. Cryst. Growth 2006, 292, 441−444. (29) Ruiz, B.; Jazbinsek, M.; Gunter, P. Cryst. Growth Des 2008, 8, 4173−4184. (30) Brahadeeswaran, S.; Onduka, S.; Takagi, M.; Takahashi, Y.; Adachi, H.; Kamimura, T.; Yoshimura, M.; Mori, Y.; Yoshida, K.; Sasaki, T. Cryst. Growth Des. 2006, 6, 2463−2468. (31) Yoshimura, M.; Takagi, M.; Takahashi, Y.; Onduka, S.; Brahadeeswaran, S.; Mori, Y.; Sasaki, T.; Suzuki, M.; Tonouchi, M.; Nakanishi, H. International Workshop on Terahertz Technology (TeraTech’05), 16−18 November 2005, Osaka, Japan. (32) Laveant, P.; Medrano, C.; Ruiz, B.; Gunter, P. Chimia 2003, 57, 349−351. (33) Glavcheva, Z.; Umezawa, H.; Mineno, Y.; Odani, T.; Okada, S.; Ikeda, S.; Taniuchi, T.; Nakanishi, H. Jpn. J. Appl. Phys. 2005, 44, 5231−5235. (34) Matsukawa, T.; Mineno, Y.; Odani, T.; Okada, S.; Taniuchi, T.; Nakanishi, H. J. Cryst. Growth 2007, 299, 344−348. (35) Mineno, Y.; Okada, S.; Glavcheva, Z.; Umezawa, H.; Odani, T.; Ikeda, S.; Taniuchi, T.; Nakanishi, H. Nonlinear Opt., Quantum Opt. 2005, 34, 53−56. (36) Matsukawa, T.; Yoshimura, M.; Takahashi, Y.; Takemoto, Y.; Takeya, K.; Kawayama, I.; Okada, S.; Tonouchi, M.; Kitaoka, Y.; Mori, Y.; Sasaki, T. Jpn. J. Appl. Phys. 2010, 49, 075502−1−6. (37) Kono, S.; Tani, M.; Gu, P.; Sakai, K. Appl. Phys. Lett. 2000, 77, 4104−4106. (38) Pan, F.; Knöpfle, G.; Bosshard, C.; Follonier, S.; Spreiter, R.; Wong, M. S.; Günter, P. Appl. Phys. Lett. 1996, 69, 13−15. (39) Walther, M.; Jensby, K.; Keiding, S. R.; Takahashi, H.; Ito, H. Opt. Lett. 2000, 25, 911−913. (40) Liu, K.; Xu, J.; Zhang, X.-C Appl. Phys. Lett. 2004, 85, 863−865. (41) Huber, R.; Brodschelm, A.; Tauser, F.; Leitenstorfer, A. Appl. Phys. Lett. 2000, 76, 3191−3193. (42) Meier, U.; Bösch, M.; Bosshard, Ch.; Pan, F.; Günter, P. J. App. Phys. 1998, 83, 3486−3489.

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dx.doi.org/10.1021/cg300606g | Cryst. Growth Des. 2013, 13, 415−421