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We report on efficient generation of ultra-broadband terahertz (THz) waves via optical rectification in a novel nonlinear organic crystal with acentric core structure, i.e. 2-(4-hydroxystyryl)-1-methylquinolinium 4-methylbenzenesulfonate (OHQ-T), which possesses an ideal molecular structure leading to a maximized nonlinear optical response for near-infrared-pumped THz wave generation. By systematic studies on wavelength-dependent phase-matching conditions in OHQ-T crystals of different thicknesses we are able to generate coherent THz waves with a high peak-to-peak electric field amplitude of up to 650 kV/cm and an upper cut-off frequency beyond 10 THz. High optical-to-THz conversion efficiency of 0.31% is achieved by efficient index matching with a selective pumping at 1300 nm.
© 2016 Optical Society of America
1. Introduction
Terahertz (THz) electromagnetic wave turned out to be an efficient light source for a number of applications in medical imaging, material inspection, telecommunications, and time-domain spectroscopy. Its peculiar properties in the specific region between microwave and infrared spectral ranges, for instance, the non-destructive high transparency of most dielectric materials and the distinguishable capability of specific label-free molecules [1–3] can further expand the applicability of THz waves. Recently, the availability of intense single-cycle THz pulses opened a new possibility for intensive researches on THz nonlinear optics [4–6], ultrafast THz nonlinear dynamics by resonant modes [7,8], and super-resolution THz near-field imaging [9], which would reveal the unexplored fields related to not only the fundamental interaction mechanisms of intense THz radiation with matters, but also new schemes for resonant and non-resonant control of molecules and materials. To pave the way for the next horizon of THz science, ultrashort (<1 ps pulse duration), intense (few hundreds kV/cm electric field amplitude) and ultra-broadband (0.1 to 10 THz frequencies) coherent THz sources are required.
For intense and broadband THz wave generation by employing optical rectification, there are several issues to be considered between the optical pump and generated THz waves. For instance, phase and group velocity matching, dispersion in THz emitter crystals, transparency of interacting waves, and optical nonlinearity [10,11] are crucial parameters to be optimized. It is not a trivial task to generate intense THz waves possessing broad spectral bandwidth in well-known inorganic electro-optic crystals such as ZnTe, GaP, GaAs and GaSe because of their low nonlinear optical susceptibility and relatively narrow spectral acceptance for effective index matching between the optical and THz waves [12]. Recently, a prism-cut LiNbO3 crystal with pulse-front tilting technique took a center stage of intense THz wave generation [13], but the emitted THz spectrum was mostly distributed in the low THz frequency region of 1-2 THz because of strong absorption at higher frequencies [14].
Recently, nonlinear optical organic crystals have been proposed as promising materials for generating and detecting broadband THz waves delivering high electric fields, because they possess much larger optical nonlinearities and relatively low dielectric constants than those of inorganic crystals and their electro-optic coefficients and refractive indices are nearly frequency-independent in the THz region [15]. Furthermore, nonlinear organic crystals provide excellent optical-to-THz energy conversion efficiency at room temperature and possibilities for generation of diffraction-limited beam profiles without sophisticated pumping and pulse shaping techniques [16]. Most of the efforts in the past THz researches on nonlinear organic crystals, however, have conducted to crystal growth and comparison of characteristics with previously reported crystals because of the lack of high figure of merits and high-quality single crystals [17]. In our previous study [18], we reported the development of OHQ-T crystals and showed its potential applicability for generation of THz waves in comparison with a well-known ZnTe crystal. However, in-depth analysis and experimental investigation on walk-off or coherence length and optimal pump wavelength were not yet carried out. In the present work we performed systematic analysis and required experiments with OHQ-T crystals for efficient generation of ultra-broadband THz waves with high electric field amplitudes. Refractive indices in the optical and THz regions were measured to find efficient phase-matching condition. As expected, THz wave characteristics generated in OHQ-T crystals are strongly affected by pump-wavelength-dependent phase-matching condition, absorption behavior, and coherent length. As-grown OHQ-T crystal with an optimal thickness of 0.78 mm pumping at λpump = 1300 nm produces ultra-broadband THz waves with an upper cut-off frequency beyond 10 THz and a peak-to-peak THz electric field amplitude of 650 kV/cm.
2. Optical properties of OHQ-T crystal
The designed OHQ-T crystal consists of the OHQ (2-(4-hydroxystyryl)-1-methylquinolinium) cation and the tosylate (4-methylbenzene sulfonate) anion. The chemical structure of OHQ-T and the as-grown crystal image are shown in Fig. 1.
OHQ-T powder was synthesized by condensation reaction with a similar manner described in Ref. 18 and OHQ-T single crystals were subsequently grown by solution growth method in the polar solvent mixture of methanol and acetonitrile. The macroscopic electro-optic response of OHQ-T crystal is quite large with an effective hyperpolarizability tensor βiiieff of 121 × 10−30 esu. This value is in the same order of magnitude compared with those of the benchmark ionic DAST (161 × 10−30 esu) and the non-ionic phenolic polyene OH1 (63 × 10−30 esu) crystals [18,19]. Such large electro-optic response is resulted from the highest possible order parameter with cos3θp = 0.9996 (1.0 for ideal case), where θp indicates the misaligned angle between the main charge transfer direction of the polar molecules and the main-charge transfer axis of the crystal [20]. In our previous study [18], we mainly investigated linear optical properties of as-grown OHQ-T crystals in the optical and THz regions and confirmed its availability for broadband photonic applications. In particular, a relatively small THz absorption coefficient (< 10 mm−1) of OHQ-T crystal in a wide THz frequency range except a region around 1.5 THz due to strong absorption of existing phonon modes reveals that this crystal is a promising candidate for gapless broadband THz wave generation. In the present work, we performed in-depth analysis and observation in terms of ultra-broadband phase-matching properties of OHQ-T crystals. Figure 2(a) shows the optical and THz refractive indices measured along the polar axis of OHQ-T crystals with error bars. For the experimental reliability, we used OHQ-T samples of two different thicknesses (0.24-mm and 0.63-mm thick crystals) for the measurement of the refractive indices.
Fig. 2 (a) Optical group index ng (red circles and line) and THz phase index nTHz (blue circles and line) measured by auto-correlation and THz time-domain spectroscopy methods along the polar axis of as-grown 0.24-mm and 0.63-mm thick OHQ-T crystals. (b) Two-dimensional contour of the effective coherence length in OHQ-T crystal.
The optical group index ng of OHQ-T crystal was measured by employing auto-correlation method based on noncollinear second harmonic generation [21], while the THz phase index nTHz was estimated for the frequency range of 0.5-7 THz by THz time-domain spectroscopy (THz-TDS). The measured values of the THz phase indices are distributed approximately from 2.2 to 2.4 and matched well to the near-infrared (NIR) group index values between 1200 nm and 1600 nm. Phase-matching characteristics between the optical excitation and generated THz pulses inside the nonlinear medium are generally determined by the dispersion property as shown in Fig. 2(a). To identify the strong dependency of the generated THz bandwidth on λpump, we calculated the effective coherence length for optical rectification, which is given by [22]
where Lc,eff is the effective coherence length, is the phase-mismatching factor, c is the speed of light in vacuum, nTHz(ν) and ng(λ) are refractive indices in the THz and NIR ranges, respectively. This calculation was done up to 7 THz based on the measured refractive indices depicted in Fig. 2(a). The two-dimensional contour plot in Fig. 2(b) shows Lc,eff of OHQ-T crystal as a function of the optical wavelength and the THz frequency. This contour visualizes directly broadband phase-matching characteristics and enables us to determine the optimal thickness of OHQ-T crystal and λpump for efficient broadband THz wave generation. OHQ-T crystal was well phase-matched over the wide THz frequency range where the optimal λpump is located around 1300 nm. In the shorter pump wavelength region than 1100 nm, the bandwidth of the generated THz spectrum and the effective coherence length of OHQ-T crystal decreases drastically. As λpump increases, the phase-matchable range becomes wider and covers quite broad THz spectrum, but the effective coherence length from 2 to 6 THz decreases again when λpump is longer than 1400 nm. As a consequence, it is clearly seen that ultra-broadband THz waves can be efficiently generated in OHQ-T crystal with 1300-nm pumping.3. THz wave generation in OHQ-T crystals
To verify the characteristics of THz wave generation depending on the phase-matching condition as predicted from the above calculation based on the measured refractive indices, required experiments were performed by utilizing as-grown OHQ-T single crystals with different thicknesses from 0.2 to 1.8 mm. A 1-kHz Ti:sapphire regenerative amplifier (Spitfire Ace, Spectra Physics) and an optical parametric amplifier system (TOPAS Prime-F, Spectra Physics) delivering 100-150 fs pulses at 700-1600 nm were used as the pumping light source. The pump pulses entering OHQ-T crystals were linearly polarized parallel to the polar axis of the crystal. Generated THz waves were then guided and tightly focused by using four 90° off-axis parabolic gold mirrors onto the detection crystal. The experimental setup was purged by dry air to suppress absorption of water vapors in atmosphere. Temporal traces of the generated THz electric fields were recorded by electro-optic (EO) sampling in a 50-μm thick GaP crystal as a function of time delay between the THz and optical probe pulses. Note that a high purity <110> GaP crystal with a thickness of <100 μm was used because it shows quasi-flat response below 10 THz [12,23]. To calculate the THz electric field amplitude, we measured the THz electric field-induced phase retardation of the ultrashort optical probe pulse in the EO crystal. The intensity parameter △I/I measured by a balanced photo-detector was calibrated to the THz electric field amplitude using the relation of where λ0 = 800 nm is the wavelength of the probe pulse, △I is the intensity difference in two polarized components at the balanced photodetector, I is the sum of two components, n0 = 3.2 is the refractive index of GaP at 800 nm, r41 = 0.88 pm/V is the EO coefficient, and L is the crystal thickness. The Fresnel transmission coefficient of GaP is tGaP = 0.46 [24,25]. All THz spectra were converted from the measured time-trace of the THz electric fields by fast Fourier transform (FFT).
Figure 3(a) shows the peak-to-peak electric field Eptp of the THz waves as a function of the thickness of OHQ-T crystal, generated at five different λpump. Here, the average powers of the incident pump pulses at each λpump were maintained to 10 mW for a fair comparison under identical condition of THz wave generation. Dramatic changes of the generated Eptp from 3.82 to 148 kV/cm were clearly visible depending on the phase-matching condition and the effective coherence length. As the thickness of OHQ-T crystal increases up to the corresponding coherence length for each wavelength, highest THz output power was achieved by the constructive interference of the THz pulses propagating in the nonlinear crystal in each case. When the thickness of crystal exceeded the walk-off or coherence length, the accumulated phase velocity mismatching factor between the optical pump and THz pulses within the crystal leads to a remarkable decrease of conversion efficiency. The maximum electric field amplitudes, when pumped at λpump = 800 and 1100 nm, were achieved only in a very thin (0.35 mm) OHQ-T crystal because of short Lc,eff. We observed that the phase-matchable thickness of OHQ-T crystal increases, as λpump increases up to a certain wavelength region. The maximum Eptp was achieved with 1300-nm pumping due to longer Lc,eff. These results agree well with our theoretical calculations shown in Fig. 2(b).
Fig. 3 (a) THz peak-to-peak E-field amplitude generated in OHQ-T crystals with 10-mW pumping in dependence of λpump and crystal thickness. (b) Two-dimensional contour plot of the THz ETHz(t) for 0.78-mm thick OHQ-T crystal. The differences between optical group index and THz phase index in the 0.5-7 THz range vs. λpump (red circles: experimental results, dash-dotted wine line: calculated results) are also depicted. The inter-distance between two horizontal dotted red lines means the full width at half maximum Δτenv of the THz electric field envelope. (c) Two-dimensional contour plot of the THz spectral amplitude ETHz(v). Five red circles and dot lines indicate upper cut-off frequencies νcut-off for different λpump.
A two-dimensional contour plot of the THz electric field ETHz(t) obtained in 0.78-mm thick OHQ-T crystal, where this crystal thickness turned out to be optimal for pumping at λpump = 1300 nm and was therefore further investigated subsequently, is shown as a function of λpump in Fig. 3(b) to elucidate the correlation between ETHz(t) and λpump. The distance between two horizontal dotted lines indicates the full width at half maximum Δτenv of the THz electric field envelope, which slightly varied around 0.8 ps. Although the generation of shorter THz pulses is expected for pumping at longer wavelengths with a constant pulse width due to increased spectral acceptance, the contour plot does not clearly show this tendency. The reason is that the pump pulses used in the present experiment became slightly broader when increasing the pump wavelength. Accordingly, the generated THz pulses were a bit broader at longer pump wavelengths. The experimental results and the Sellmeier Eq. (-)based calculation of the index matching factor are also shown as a function of λpump. The amplitude of the generated ETHz(t) reaches a maximum for 1300-nm pumping due to better phase matching.
Figure 3(c) shows the corresponding two-dimensional contour plot of the THz spectral amplitude ETHz(v) in the spectral range between 0.1 THz and 10 THz. As a result of increasing the pumping wavelength, high frequency components of the THz waves begin to emerge due to satisfied phase-matching condition and also low absorption throughout this frequency range. The spectral vacancy around 1.5 THz was caused by the absorption of phonon modes related to the tosylate anion of OHQ-T molecule. Similar phenomena were also reported for the well-known DAST crystal, the same type of representative ionic organic crystal [26]. We observed distinct changes (from 3.6 to 8.5 THz) of upper cut-off frequency νcut-off, which is defined as the frequency in high frequency region at which the THz electric field amplitude ETHz(v) becomes larger than the average amplitude of noise level. From our results on THz wave generation depending on λpump and crystal thickness, it is clearly seen that efficient generation of ultra-broadband THz waves was realized by optimized index matching. The magnitude of the THz electric field amplitude and the optical-to-THz conversion efficiency were measured by the EO sampling method and a calibrated pyroelectric THz detector (THz5B-MT, Gentec-EO Inc.), respectively. The results shown in Fig. 3 are summarized in Table 1 for comparison.
Table 1. Summary of Results on THz Wave Generation in 0.78-mm thick OHQ-T Crystal with Pumping at Different Wavelengths
Figure 4(a) shows Eptp and νcut-off, generated in 0.78-mm thick OHQ-T crystal with pumping at 1300 nm. As the incident pump power increases, THz Eptp increases but νcut-off is nearly saturated at ~12 THz above 60-mW pumping. A maximum THz output power of 0.37 mW and a maximum optical-to-THz energy conversion efficiency of 0.31% and a photon conversion efficiency of >400% calculated for a center frequency of 3 THz with cascaded χ(2) effect [27] were achieved by efficient phase matching in OHQ-T crystal. Typical diameters of the focused beams were measured to be about 1 mm to 2 mm depending on the frequency and the spectral bandwidth of the generated THz waves. The THz time-trace with the highest Eptp of 650 kV/cm and the corresponding ultra-broadband THz spectra with νcut-off beyond 10 THz were achieved with 120-mW pumping as shown in Fig. 4(c).
Fig. 4 Efficient broadband THz wave generation in 0.78-mm thick OHQ-T crystals with pumping at 1300 nm. (a) Peak-to-peak THz electric field amplitude (red circles) and upper cut-off frequencies (blue triangles). (b) THz average output power (black circles) and optical-to-THz conversion efficiencies (red triangles) vs. incident pump power. (c) Time-trace of THz ETHz(t) and the corresponding FFT spectrum (inset) generated with 120-mW pumping.
4. Conclusions
In conclusion, OHQ-T, an organic crystal with large nonlinear optical response, was proposed as a promising candidate for efficient ultra-broadband THz wave generation. As-grown OHQ-T crystal exhibits small absorption coefficient in the optical and THz frequency regions and large nonlinearity due to suitable crystal packing structure. To confirm optimal phase-matching condition, our theoretical analysis on characteristics of the THz waves emitted from the crystal was experimentally investigated. High peak-to-peak electric field amplitude of 650 kV/cm, extremely broad THz spectrum beyond 10 THz, and average output power of 0.37 mW with 0.31% optical-to-THz conversion efficiency were achieved under the optimal phase-matching condition. These outstanding properties of the OHQ-T crystal as a THz emitter can be further improved by up-scaling crystal aperture size and pump energy as well as by applying shorter pump pulses. The presented results can be applied for advanced THz science and technology including ultra-broadband THz time-resolved nonlinear spectroscopy and high-resolution THz imaging.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) Grants funded by the Korean Government (MSIP and Ministry of Education) (2011-0017494, WCI 2011-001, 2013R1A2A2A01007232, 2014R1A5A1009799, and 2009-0093826). This work was also partially supported by the Center for Advanced Meta-Materials (CAMM) funded by Korea Government (MSIP) as Global Frontier Project (CAMM-2014M3A6B3063709).
References and links
1. M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics 1(2), 97–105 (2007). [CrossRef]
2. B. Ferguson and X.-C. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1(1), 26–33 (2002). [CrossRef] [PubMed]
3. M. Nagel, P. H. Bolivar, M. Brucherseifer, H. Kurz, A. Bosserhoff, and R. Büttner, “Integrated THz technology for label-free genetic diagnostics,” Appl. Phys. Lett. 80(1), 154–156 (2002). [CrossRef]
4. P. Gaal, K. Reimann, M. Woerner, T. Elsaesser, R. Hey, and K. H. Ploog, “Nonlinear terahertz response of -type GaAs,” Phys. Rev. Lett. 96(18), 187402 (2006). [CrossRef] [PubMed]
5. E. Freysz and J. Degert, “Terahertz Kerr effect,” Nat. Photonics 4(3), 131–132 (2010). [CrossRef]
6. D. Turchinovich, J. M. Hvam, and M. C. Hoffmann, “Self-phase modulation of a single-cycle terahertz pulse by nonlinear free-carrier response in a semiconductor,” Phys. Rev. B 85(20), 201304 (2012). [CrossRef]
7. T. Kampfrath, K. Tanaka, and K. A. Nelson, “Resonant and nonresonant control over matter and light by intense terahertz transients,” Nat. Photonics 7(9), 680–690 (2013). [CrossRef]
8. M. Jewariya, M. Nagai, and K. Tanaka, “Ladder climbing on the anharmonic intermolecular potential in an amino acid microcrystal via an intense monocycle terahertz pulse,” Phys. Rev. Lett. 105(20), 203003 (2010). [CrossRef] [PubMed]
9. F. Blanchard, A. Doi, T. Tanaka, and K. Tanaka, “Real-time, subwavelength terahertz imaging,” Annu. Rev. Mater. Res. 43(1), 237–259 (2013). [CrossRef]
10. A. M. Sinyukov and L. M. Hayden, “Generation and detection of terahertz radiation with multilayered electro-optic polymer films,” Opt. Lett. 27(1), 55–57 (2002). [CrossRef] [PubMed]
11. F. D. J. Brunner, A. Schneider, and P. Günter, “Velocity-matched terahertz generation by optical rectification in an organic nonlinear optical crystal using a Ti:sapphire laser,” Appl. Phys. Lett. 94(6), 061119 (2009). [CrossRef]
12. B. Wu, L. Cao, Q. Fu, P. Tan, and Y. Xiong, “Comparison of the detection performance of three nonlinear crystals for the electro-optic sampling of a FEL-THz source,” in Proceedings of the 5th International Particle Accelerator Conference, (European Physical Society, 2014), pp. 2891–2893.
13. H. Hirori, A. Doi, F. Blanchard, and K. Tanaka, “Single-cycle terahertz pulses with amplitudes exceeding 1 MV/cm generated by optical rectification in LiNbO3,” Appl. Phys. Lett. 98(9), 091106 (2011). [CrossRef]
14. L. Pálfalvi, J. Hebling, J. Kuhl, A. Peter, and K. Polgar, “Temperature dependence of the absorption and refraction of Mg-doped congruent and stoichiometric LiNbO3 in the THz range,” J. Appl. Phys. 97(12), 123505 (2005). [CrossRef]
15. A. Schneider, M. Neis, M. Stillhart, B. Ruiz, R. U. A. Khan, and P. Günter, “Generation of terahertz pulses through optical rectification in organic DAST crystals: theory and experiment,” J. Opt. Soc. Am. B 23(9), 1822–1835 (2006). [CrossRef]
16. C. Vicario, B. Monoszlai, and C. P. Hauri, “GV/m single-cycle terahertz fields from a laser-driven large-size partitioned organic crystal,” Phys. Rev. Lett. 112(21), 213901 (2014). [CrossRef] [PubMed]
17. P.-J. Kim, J.-H. Jeong, M. Jazbinsek, S.-B. Choi, I.-H. Baek, J.-T. Kim, F. Rotermund, H. Yun, Y. S. Lee, P. Guenter, and O.-P. Kwon, “Highly efficient organic THz generator pumped at near infrared: Quinolinium single crystals,” Adv. Funct. Mater. 22(1), 200–209 (2012). [CrossRef]
18. S.-H. Lee, B.-J. Kang, J.-S. Kim, B.-W. Yoo, J.-H. Jeong, K.-H. Lee, M. Jazbinsek, J. W. Kim, H. Yun, J. Kim, Y. S. Lee, F. Rotermund, and O.-P. Kwon, “New acentric core structure for organic electro-optic crystals optimal for efficient optical-to-THz conversion,” Adv. Opt. Mater. 3(6), 756–762 (2015). [CrossRef]
19. J.-H. Jeong, B.-J. Kang, J.-S. Kim, M. Jazbinsek, S.-H. Lee, S.-C. Lee, I.-H. Baek, H. Yun, J. Kim, Y. S. Lee, J.-H. Lee, J. H. Kim, F. Rotermund, and O.-P. Kwon, “High-power broadband organic THz generator,” Sci. Rep. 3, 3200 (2013). [CrossRef] [PubMed]
20. O.-P. Kwon, S. J. Kwon, M. Jazbinsek, F. D. J. Brunner, J. I. Seo, C. Hunziker, A. Schneider, H. Yun, Y. S. Lee, and P. Günter, “Organic phenolic configurationally locked polyene single crystals for electro-optic and terahertz wave applications,” Adv. Funct. Mater. 18(20), 3242–3250 (2008). [CrossRef]
21. A. Schneider, F. D. J. Brunner, and P. Gunter, “Determination of the refractive index over a wide wavelength range through time-delay measurements of femtosecond pulse,” Opt. Commun. 275(2), 354–358 (2007). [CrossRef]
22. F. D. J. Brunner, S.-H. Lee, O.-P. Kwon, and T. Feurer, “THz generation by optical rectification of near-infrared laser pulses in the organic nonlinear optical crystal HMQ-TMS,” Opt. Mater. Express 4(8), 1586–1592 (2014). [CrossRef]
23. S. Casalbuoni, H. Schlarb, B. Schmidt, P. Schmüser, B. Steffen, and A. Winter, “Numerical studies on the electro-optic detection of femtosecond electron bunches,” Phys. Rev. Spec. Top. Accel. Beams 11(7), 072802 (2008). [CrossRef]
24. B. J. Kang, I. H. Baek, J. H. Jeong, J. S. Kim, S. H. Lee, O.-P. Kwon, and F. Rotermund, “Characteristics of efficient few-cycle terahertz radiation generated in as-grown nonlinear organic single crystals,” Curr. Appl. Phys. 14(3), 403–406 (2014). [CrossRef]
25. M. Jewariya, M. Nagai, and K. Tanaka, “Enhancement of terahertz wave generation by cascaded χ(2) processes in LiNbO3,” J. Opt. Soc. Am. B 26(9), A101–A106 (2009). [CrossRef]
26. M. Stillhart, A. Schneider, and P. Günter, “Optical properties of 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate crystals at terahertz frequencies,” J. Opt. Soc. Am. B 25(11), 1914–1919 (2008). [CrossRef]
27. S.-W. Huang, E. Granados, W. R. Huang, K.-H. Hong, L. E. Zapata, and F. X. Kärtner, “High conversion efficiency, high energy terahertz pulses by optical rectification in cryogenically cooled lithium niobate,” Opt. Lett. 38(5), 796–798 (2013). [CrossRef] [PubMed]
