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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 17 — Aug. 17, 2009
  • pp: 14832–14838
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Optimized terahertz-wave generation

Katsuhiko Miyamoto, Seigo Ohno, Masazumi Fujiwara, Hiroaki Minamide, Hideki Hashimoto, and Hiromasa Ito  »View Author Affiliations


Optics Express, Vol. 17, Issue 17, pp. 14832-14838 (2009)
http://dx.doi.org/10.1364/OE.17.014832


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Abstract

An effective and widely tunable means of producing terahertz (THz) radiation using difference-frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated by optimizing the pump wavelengths. We calculated the (wideband) refractive index and coherence length mapping of BNA to establish the optimum phase-matching condition of the DFG configuration. To satisfy the phase-matching conditions over the range 0.1–20 THz, both pump wavelengths were varied from 780 to 950 nm, and the optical wavelength dependency of the THz-wave output was clearly observed. We expanded the range of THz-wave generation and increased the power output to ten times that obtained using previous methods.

© 2009 OSA

1. Introduction

Terahertz (THz) technology has recently undergone considerable development [1

1. A. R. Sanchez and X. C. Zhang, “Terahertz Science and Technology Trends,” IEEE J. Sel. Top. Quantum Electron. 14(2), 260–269 (2008).

], and THz radiation has attracted a great deal of interest for a growing number of applications, including biomedical imaging, security, medicine, art conservation, and nondestructive testing [2

2. T. Dekorsky, V. A. Yakovlev, W. Seidel, M. Helm, and F. Keilman, “Infrared-phonon-polariton resonance of the nonlinear suspectibility in GaAs,” Phys. Rev. Lett. 90, 05508 (2003).

6

6. S. Ohno, A. Hamano, K. Miyamoto, C. Suzuki, and H. Ito, “Surface mapping of carrier density in a GaN wafer using a frequency-agile THz source,” J. Eur. Opt. Soc. Rapid Publ. 4, 09012 (2009).

]. A monochromatic THz-wave source [7

7. K. Kawase, M. Sato, T. Taniuchi, and H. Ito, “Coherent tunable THz-wave generation from LiNbO3 with monolithic grating coupler,” Appl. Phys. Lett. 68(18), 2483–2485 (1996).

10

10. T. Tanabe, K. Suto, J. Nishizawa, K. Saito, and T. Kimura, “Tunable terahertz wave generation in the 3- to 7-THz region from GaP,” Appl. Phys. Lett. 83(2), 237–239 (2003).

] using a nonlinear optical (NLO) process was employed in developing many of these applications, but the advancement of broadband sources and sources that possess some degree of frequency agility is desirable.

We previously demonstrated an ultra-broadband capability using difference-frequency generation (DFG) in an organic crystal 4-dimethylamino-N-methyl-4-stilbavolium tosylate (DAST) [11

11. K. Kawase, M. Mizuno, S. Sohma, H. Takahashi, T. Taniuchi, Y. Urata, S. Wada, H. Tashiro, and H. Ito, “Difference-frequency terahertz-wave generation from 4-dimethylamino-N-methyl-4-stilbazolium-tosylate by use of an electronically tuned Ti:sapphire laser,” Opt. Lett. 24(15), 1065–1067 (1999).

15

15. K. Suizu, K. Miyamoto, T. Yamashita, and H. Ito, “High-power terahertz-wave generation using DAST crystal and detection using mid-infrared powermeter,” Opt. Lett. 32(19), 2885–2887 (2007). [PubMed]

]. The sources described in Refs. 14 and 15 had the important characteristic of frequency-agile operation. The tuning ranges of monochromatic THz-wave sources using organic materials were much wider than those of inorganic materials, and the range of the DAST-DFG THz-wave source was ultra-wideband (1.5–40 THz). Even so, we developed another wideband THz-wave source using the organic solid N-benzyl-2-methyl-4-nitroaniline (BNA) [16

16. K. Miyamoto, H. Minamide, M. Fujiwara, H. Hashimoto, and H. Ito, “Widely tunable terahertz-wave generation using an N-benzyl-2-methyl-4-nitroaniline crystal,” Opt. Lett. 33(3), 252–254 (2008). [PubMed]

]. BNA, which was invented by Hashimoto [17

17. H. Hashimoto, Y. Okada, H. Fujimura, M. Morioka, O. Sugihara, N. Okamoto, and R. Matsushima, “Second-Harmonic Generation from Single Crystal of N-substituted 4-Nitroanilines,” Jpn. J. Appl. Phys. 36(Part 1, No. 11), 6754–6760 (1997).

19

19. M. Fujiwara, K. Yanagi, M. Maruyama, M. Sugisaki, K. Kuroyanagi, H. Takahashi, S. Aoshima, Y. Tsuchiya, A. Gall, and H. Hashimoto, “Second Order Nonlinear Optical Properties of the Single Crystal of N-Benzyl 2-methyl-4-nitroaniline: Anomalous Enhancement of the d333 Component and Its Possible Origin,” Jpn. J. Appl. Phys. 45(11), 8676–8685 (2006).

], is a promising material for wideband, efficient, and high-power THz-wave generation because of its large second-order optical nonlinearity. The BNA-DFG monochromatic THz-wave was tunable over the range 0.1–15 THz, which is a different frequency spectrum compared to that of DAST-DFG.

2. Properties of N-benzyl-2-methyl-4-nitroaniline (BNA) crystals

The NLO coefficient d33 of a BNA crystal is 234 pm/V, the largest ever reported among yellow-colored NLO materials [20

20. M. Fujiwara, M. Maruyama, M. Sugisaki, H. Takahashi, S. Aoshima, R. J. Cogdell, and H. Hashimoto, “Determination of the d-Tensor Components of a Single Crystal of N-Benzyl-2-methyl-4-nitroaniline,” Jpn. J. Appl. Phys. 46(No. 4A), 1528–1530 (2007).

]. We generated the wideband monochromatic THz-wave in the BNA crystal using a type-0 collinear phase-matching configuration (whereby the polarization of all waves involved in the nonlinear wavelength conversion is parallel to the c-axis) by maintaining the crystal angle [16

16. K. Miyamoto, H. Minamide, M. Fujiwara, H. Hashimoto, and H. Ito, “Widely tunable terahertz-wave generation using an N-benzyl-2-methyl-4-nitroaniline crystal,” Opt. Lett. 33(3), 252–254 (2008). [PubMed]

]. The refractive index is very important in determining the phase-matching condition for THz-wave generation. We estimated the refractive index in the range 0.1–22 THz based on the absorption coefficient [21

21. H. Ito, K. Miyamoto, and H. Minamide, “Ultra-broadband, frequency-agile THz-wave generator and its applications,” in Advanced Solid-State Photonics, (Optical Society of America, 2008), WD1.

], which was determined from the FTIR transmittance spectrum (see Figs. 1
Fig. 1 Transmittance spectrum of the BNA crystal. The crystal thickness was 0.3 mm, and the FTIR polarization response was controlled using a wire-grid polarizer parallel to the [001] direction (c-axis).
and 2
Fig. 2 The measured absorption coefficient, with the red line showing the results of spectral simulations using a Lorentz-type oscillator model.
, which show the transmittance spectrum and absorption coefficient, respectively). The thickness of the crystal, which was polished and thinned, was 0.3 mm. BNA crystals have another favorable characteristic in that they are not hygroscopic and are thus stable during water-based mechanical processing. The dots in Fig. 2 show the experimental absorption coefficient measured by FTIR spectroscopy, with the red dots showing the results of spectral simulations using a Lorentz-type oscillator model. Several absorption peaks are observed and the maximum absorption coefficient is around 15 THz.

Using this wideband refractive index data, one can more accurately estimate the coherence length distribution shown in Fig. 4
Fig. 4 Coherence-length distribution of BNA-DFG with a type-0 phase-matching configuration.
. The horizontal axis shows the THz-wave frequency; the vertical axis is the pump wavelength λ1 (< λ2). The red level shows the magnitude of the coherence length, which is greater than 1 mm. We achieved collinear phase-matching for the type-0 configuration because the nopt and nTHz are almost the same. A noteworthy point, revealed in coherence length calculations, arises in considering the phase-matching condition in BNA crystal for high-efficiency frequency conversion. The between 700 and 1100nm band is very significant pump tuning range for THz generation on BNA-DFG. This range will be enabled to efficient and widely tunable THz generation, concurrently using the independently controlled dual pump wavelength suitable for the long coherence length at each THz frequency.

3. Widely tunable THz-wave sources using BNA crystals

An independently controlled dual pump source is necessary for THz-wave generation using BNA-DFG. A double-crystal KTiOPO4 (KTP) optical parametric oscillator (OPO) was used to excite the BNA-DFG, utilizing an experimental setup almost identical to that originally developed for DAST-DFG [14

14. H. Ito, K. Suizu, T. Yamashita, A. Nawahara, and T. Sato, “Random frequency accessible broad tunable terahertz-wave source using phase-matched 4-dimethylamino-N-methyl-4-stilbazolium tosylate crystal,” Jpn. J. Appl. Phys. 46(11), 7321–7324 (2007).

,15

15. K. Suizu, K. Miyamoto, T. Yamashita, and H. Ito, “High-power terahertz-wave generation using DAST crystal and detection using mid-infrared powermeter,” Opt. Lett. 32(19), 2885–2887 (2007). [PubMed]

]. The resonator that was used differs, however, and a schematic diagram of the coherent system is shown in Fig. 5
Fig. 5 Widely tunable monochromatic BNA DFG THz-wave source
.

The vertically polarized output of KTP OPO was focused onto the BNA crystal with a beam diameter of ~2 mm [full-width at half-maximum (FWHM)] using an f = 250-mm lens (T = 90% at 900 nm). The BNA crystal had an 8-mm cross section and 1-mm thickness. The pump beam and stray light were excluded from the detector using a Yoshinaga filter and a black polyethylene filter. Since the response time of the galvano scanners is less than 1 ms, any required frequency pair for THz-DFG was accessible within 1 ms; i.e., frequency-agile operation [23

23. K. Miyamoto and H. Ito, “Wavelength-agile mid-infrared (5-10 microm) generation using a galvano-controlled KTiOPO4 optical parametric oscillator,” Opt. Lett. 32(3), 274–276 (2007). [PubMed]

] was possible.

4. Results and discussion

Figure 6
Fig. 6 The pump-wavelength dependence mapping of BNA-DFG.
shows a two-dimensional map of the dependence of the output power on the pump wavelengths. The horizontal and vertical axes represent the frequency and pump wavelength λ1, respectively. The control range of λ1 and λ2 was 780–950 nm, corresponding to the THz frequency range from 0.1 to 20 THz. The mapping resolution was 200 × 200 pixels, and averaging over five measurements was used. The signal acquisition time was ~33 minutes, which could detect from pulse to pulse using synchronized control with laser rep. rate 100Hz. The incident energy was 3 mJ/pulse at dual pump-wavelengths (λ1 = 887 nm and λ2 = 902 nm). The intensity has been normalized to the maximum at each frequency. The red level shows the magnitude of the output THz energy at each frequency. The small dark stripe at the center of Fig. 6 shows that the λ1 output power was extremely strong and efficient compared to the flat operation of a KTP crystal angle for beam normal incident angle, and therefore, the THz output energy was higher than in other areas. It is not suitable for this research purpose; the obtained data have been cut out of this area. The other black area does not represent THz generation to limited to dual pump-wavelength tuning.

The effective THz generation is possible by selecting the proper pair of wavelengths of the two pump beams, λ1 and λ2. This proper selection is based on the very accurate data of the coherence length (or refractive index) of the organic NLO crystal used. Unfortunately, such accurate values of the refractive index are not easily obtained because of the difficulty in mechanical processing of organic crystals due to its fragility. Nevertheless the proper pair of the wavelengths can be experimentally determined by our frequency-agile technique. Our frequency-agile technique can potentially be applied to other NLO crystals for their effective THz-wave generation.

We determined the optimum pump-wavelength at each frequency using pump-wavelength dependence mapping in Fig. 6. One can make a lookup table of output power at various pairs of pump wavelengths. Figure 7
Fig. 7 THz-wave output spectrum of the optimum phase-matched BNA-DFG. The red line is dual wavelength-controlled, and the black line is λ1-fixed.
shows the maximum THz-wave output spectrum using this optimum lookup table with independently controlled λ1 and λ2. This THz-wave generation efficiency increases compared to the fixed wavelength λ1 at 812 nm. The dependency of the THz-wave output on the wavelength λ1 was clearly observed. The maximum output power obtained was about ten times at 11.6THz compared to when λ1 was fixed. The sensitivity of the bolometer used in this experiment is 142 pJ/pulse for 1 V output at 1.4 THz [24

24. T. Ikari, R. Guo, H. Minamide, and H. Ito, “Enhancement of Output Energy and Efficiency for A Terahertz-wave Parametric Oscillator Using A Surface-Emitted Configration,” Rev. Laser Eng. 37, 370–373 (2009).

]. It has the frequency dependency due to filters and window transmission characteristics. The output characteristics shown in Fig. 7 include this frequency dependency. The tuning range was 0.1–20 THz. This is of great interest because the BNA-DFG might play a key role as radiation source linking the sub-THz and THz regions. This source has the potential for higher output power and wideband THz generation, using the dual pump wavelength source with the tunability from about 700 to 1100 nm.

5. Conclusion

We have demonstrated that the optimum phase-matching condition of BNA crystal for the THz wave generation can be obtained by controlling the wavelengths of the DFG dual pump sources. The coherence lengths calculated from the experimentally determined refractive indexes have provided us with rough estimation of the proper set of the two pump wavelengths λ1 and λ2. On the basis of this rough estimation, we have obtained the pump-wavelength dependence mapping of BNA-DFG by scanning both λ1 and λ2, independently, which tells us the optimum set of λ1 and λ2 for efficient THz generation. In fact, we have simultaneously realized 10 times enhancement of the output power and wider frequency range up to 20THz with the optimized wavelengths. This frequency-agile technique with the two independently controlled pump sources is a key to the highly efficient and widely tunable THz-wave generation.

Acknowledgment

We thank C. Takyu and T. Shoji, who performed the dielectric coating and polishing of various optical components. This work was partly supported by a Grant-in-Aid for Scientific Research (A) (No. 19206009) from the Japan Society for the Promotion of Science (JSPS).

References and links

1.

A. R. Sanchez and X. C. Zhang, “Terahertz Science and Technology Trends,” IEEE J. Sel. Top. Quantum Electron. 14(2), 260–269 (2008).

2.

T. Dekorsky, V. A. Yakovlev, W. Seidel, M. Helm, and F. Keilman, “Infrared-phonon-polariton resonance of the nonlinear suspectibility in GaAs,” Phys. Rev. Lett. 90, 05508 (2003).

3.

H. Yoneyama, M. Yamashita, S. Kasai, K. Kawase, H. Ito, and T. Ouchi, “Membrane device for holding biomolecle samples for terahertz spectroscopy,” Opt. Commun. 281(7), 1909–1913 (2008).

4.

K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Exp., 11, 2549–2554 (2003), http://www.opticsinfobase.org/abstract.cfm?URI=oe-11-20-2549.

5.

K. Fukunaga, Y. Ogawa, S. Hayashi, and I. Hosako, “Terahertz spectroscopy for art conservation,” IEICE Electron. Express 4(8), 258–263 (2007).

6.

S. Ohno, A. Hamano, K. Miyamoto, C. Suzuki, and H. Ito, “Surface mapping of carrier density in a GaN wafer using a frequency-agile THz source,” J. Eur. Opt. Soc. Rapid Publ. 4, 09012 (2009).

7.

K. Kawase, M. Sato, T. Taniuchi, and H. Ito, “Coherent tunable THz-wave generation from LiNbO3 with monolithic grating coupler,” Appl. Phys. Lett. 68(18), 2483–2485 (1996).

8.

G. D. Boyd, T. J. Bridges, C. K. N. Patel, and E. Buehler, “Phase-matched submillimeter wave generation by difference-frequency mixing in ZnGeP2,” Appl. Phys. Lett. 21(11), 553–555 (1972).

9.

W. Shi, Y. J. Ding, N. Fernelius, and K. Vodopyanov, “Efficient, tunable, and coherent 0.18-5.27-THz source based on GaSe crystal,” Opt. Lett. 27(16), 1454–1456 (2002).

10.

T. Tanabe, K. Suto, J. Nishizawa, K. Saito, and T. Kimura, “Tunable terahertz wave generation in the 3- to 7-THz region from GaP,” Appl. Phys. Lett. 83(2), 237–239 (2003).

11.

K. Kawase, M. Mizuno, S. Sohma, H. Takahashi, T. Taniuchi, Y. Urata, S. Wada, H. Tashiro, and H. Ito, “Difference-frequency terahertz-wave generation from 4-dimethylamino-N-methyl-4-stilbazolium-tosylate by use of an electronically tuned Ti:sapphire laser,” Opt. Lett. 24(15), 1065–1067 (1999).

12.

K. Kawase, T. Hatanaka, H. Takahashi, K. Nakamura, T. Taniuchi, and H. Ito, “Tunable terahertz-wave generation from DAST crystal by dual signal-wave parametric oscillation of periodically poled lithium niobate,” Opt. Lett. 25(23), 1714–1716 (2000).

13.

T. Taniuchi, J. Shikata, and H. Ito, “Tunable THz-wave generation in DAST crystal with dual-wavelength KTP optical parametric oscillator,” Electron. Lett. 36(16), 1414–1416 (2000).

14.

H. Ito, K. Suizu, T. Yamashita, A. Nawahara, and T. Sato, “Random frequency accessible broad tunable terahertz-wave source using phase-matched 4-dimethylamino-N-methyl-4-stilbazolium tosylate crystal,” Jpn. J. Appl. Phys. 46(11), 7321–7324 (2007).

15.

K. Suizu, K. Miyamoto, T. Yamashita, and H. Ito, “High-power terahertz-wave generation using DAST crystal and detection using mid-infrared powermeter,” Opt. Lett. 32(19), 2885–2887 (2007). [PubMed]

16.

K. Miyamoto, H. Minamide, M. Fujiwara, H. Hashimoto, and H. Ito, “Widely tunable terahertz-wave generation using an N-benzyl-2-methyl-4-nitroaniline crystal,” Opt. Lett. 33(3), 252–254 (2008). [PubMed]

17.

H. Hashimoto, Y. Okada, H. Fujimura, M. Morioka, O. Sugihara, N. Okamoto, and R. Matsushima, “Second-Harmonic Generation from Single Crystal of N-substituted 4-Nitroanilines,” Jpn. J. Appl. Phys. 36(Part 1, No. 11), 6754–6760 (1997).

18.

H. Hashimoto, H. Takahashi, T. Yamada, K. Kuroyanagi, and T. Kobayashi, “Characteristics of the terahertz radiation from single crystals of N-substituted 2-methyl-4-nitroaniline,” J. Phys. Condens. Matter 13(23), L529–L537 (2001).

19.

M. Fujiwara, K. Yanagi, M. Maruyama, M. Sugisaki, K. Kuroyanagi, H. Takahashi, S. Aoshima, Y. Tsuchiya, A. Gall, and H. Hashimoto, “Second Order Nonlinear Optical Properties of the Single Crystal of N-Benzyl 2-methyl-4-nitroaniline: Anomalous Enhancement of the d333 Component and Its Possible Origin,” Jpn. J. Appl. Phys. 45(11), 8676–8685 (2006).

20.

M. Fujiwara, M. Maruyama, M. Sugisaki, H. Takahashi, S. Aoshima, R. J. Cogdell, and H. Hashimoto, “Determination of the d-Tensor Components of a Single Crystal of N-Benzyl-2-methyl-4-nitroaniline,” Jpn. J. Appl. Phys. 46(No. 4A), 1528–1530 (2007).

21.

H. Ito, K. Miyamoto, and H. Minamide, “Ultra-broadband, frequency-agile THz-wave generator and its applications,” in Advanced Solid-State Photonics, (Optical Society of America, 2008), WD1.

22.

K. Kuroyanagi, M. Fujiwara, H. Hashimoto, H. Takahashi, S. Aoshima, and Y. Tsuchiya, “Determination of Refractive Indices and Absorption Coefficients of Highly Purified N-Benzyl-2-methyl-4-nitroaniline Crystal in Terahertz Frequency Regime,” Jpn. J. Appl. Phys. 45(29), L761–L764 (2006).

23.

K. Miyamoto and H. Ito, “Wavelength-agile mid-infrared (5-10 microm) generation using a galvano-controlled KTiOPO4 optical parametric oscillator,” Opt. Lett. 32(3), 274–276 (2007). [PubMed]

24.

T. Ikari, R. Guo, H. Minamide, and H. Ito, “Enhancement of Output Energy and Efficiency for A Terahertz-wave Parametric Oscillator Using A Surface-Emitted Configration,” Rev. Laser Eng. 37, 370–373 (2009).

OCIS Codes
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(190.4410) Nonlinear optics : Nonlinear optics, parametric processes
(190.4970) Nonlinear optics : Parametric oscillators and amplifiers

ToC Category:
Nonlinear Optics

History
Original Manuscript: June 19, 2009
Revised Manuscript: July 16, 2009
Manuscript Accepted: July 19, 2009
Published: August 6, 2009

Citation
Katsuhiko Miyamoto, Seigo Ohno, Masazumi Fujiwara, Hiroaki Minamide, Hideki Hashimoto, and Hiromasa Ito, "Optimized terahertz-wave generation using BNA-DFG," Opt. Express 17, 14832-14838 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-17-14832


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References

  1. A. R. Sanchez and X. C. Zhang, “Terahertz Science and Technology Trends,” IEEE J. Sel. Top. Quantum Electron. 14(2), 260–269 (2008).
  2. T. Dekorsky, V. A. Yakovlev, W. Seidel, M. Helm, and F. Keilman, “Infrared-phonon-polariton resonance of the nonlinear suspectibility in GaAs,” Phys. Rev. Lett. 90, 05508 (2003).
  3. H. Yoneyama, M. Yamashita, S. Kasai, K. Kawase, H. Ito, and T. Ouchi, “Membrane device for holding biomolecle samples for terahertz spectroscopy,” Opt. Commun. 281(7), 1909–1913 (2008).
  4. K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Exp. 11, 2549–2554 (2003), http://www.opticsinfobase.org/abstract.cfm?URI=oe-11-20-2549 .
  5. K. Fukunaga, Y. Ogawa, S. Hayashi, and I. Hosako, “Terahertz spectroscopy for art conservation,” IEICE Electron. Express 4(8), 258–263 (2007).
  6. S. Ohno, A. Hamano, K. Miyamoto, C. Suzuki, and H. Ito, “Surface mapping of carrier density in a GaN wafer using a frequency-agile THz source,” J. Eur. Opt. Soc. Rapid Publ. 4, 09012 (2009).
  7. K. Kawase, M. Sato, T. Taniuchi, and H. Ito, “Coherent tunable THz-wave generation from LiNbO3 with monolithic grating coupler,” Appl. Phys. Lett. 68(18), 2483–2485 (1996).
  8. G. D. Boyd, T. J. Bridges, C. K. N. Patel, and E. Buehler, “Phase-matched submillimeter wave generation by difference-frequency mixing in ZnGeP2,” Appl. Phys. Lett. 21(11), 553–555 (1972).
  9. W. Shi, Y. J. Ding, N. Fernelius, and K. Vodopyanov, “Efficient, tunable, and coherent 0.18-5.27-THz source based on GaSe crystal,” Opt. Lett. 27(16), 1454–1456 (2002).
  10. T. Tanabe, K. Suto, J. Nishizawa, K. Saito, and T. Kimura, “Tunable terahertz wave generation in the 3- to 7-THz region from GaP,” Appl. Phys. Lett. 83(2), 237–239 (2003).
  11. K. Kawase, M. Mizuno, S. Sohma, H. Takahashi, T. Taniuchi, Y. Urata, S. Wada, H. Tashiro, and H. Ito, “Difference-frequency terahertz-wave generation from 4-dimethylamino-N-methyl-4-stilbazolium-tosylate by use of an electronically tuned Ti:sapphire laser,” Opt. Lett. 24(15), 1065–1067 (1999).
  12. K. Kawase, T. Hatanaka, H. Takahashi, K. Nakamura, T. Taniuchi, and H. Ito, “Tunable terahertz-wave generation from DAST crystal by dual signal-wave parametric oscillation of periodically poled lithium niobate,” Opt. Lett. 25(23), 1714–1716 (2000).
  13. T. Taniuchi, J. Shikata, and H. Ito, “Tunable THz-wave generation in DAST crystal with dual-wavelength KTP optical parametric oscillator,” Electron. Lett. 36(16), 1414–1416 (2000).
  14. H. Ito, K. Suizu, T. Yamashita, A. Nawahara, and T. Sato, “Random frequency accessible broad tunable terahertz-wave source using phase-matched 4-dimethylamino-N-methyl-4-stilbazolium tosylate crystal,” Jpn. J. Appl. Phys. 46(11), 7321–7324 (2007).
  15. K. Suizu, K. Miyamoto, T. Yamashita, and H. Ito, “High-power terahertz-wave generation using DAST crystal and detection using mid-infrared powermeter,” Opt. Lett. 32(19), 2885–2887 (2007). [PubMed]
  16. K. Miyamoto, H. Minamide, M. Fujiwara, H. Hashimoto, and H. Ito, “Widely tunable terahertz-wave generation using an N-benzyl-2-methyl-4-nitroaniline crystal,” Opt. Lett. 33(3), 252–254 (2008). [PubMed]
  17. H. Hashimoto, Y. Okada, H. Fujimura, M. Morioka, O. Sugihara, N. Okamoto, and R. Matsushima, “Second-Harmonic Generation from Single Crystal of N-substituted 4-Nitroanilines,” Jpn. J. Appl. Phys. 36(Part 1, No. 11), 6754–6760 (1997).
  18. H. Hashimoto, H. Takahashi, T. Yamada, K. Kuroyanagi, and T. Kobayashi, “Characteristics of the terahertz radiation from single crystals of N-substituted 2-methyl-4-nitroaniline,” J. Phys. Condens. Matter 13(23), L529–L537 (2001).
  19. M. Fujiwara, K. Yanagi, M. Maruyama, M. Sugisaki, K. Kuroyanagi, H. Takahashi, S. Aoshima, Y. Tsuchiya, A. Gall, and H. Hashimoto, “Second Order Nonlinear Optical Properties of the Single Crystal of N-Benzyl 2-methyl-4-nitroaniline: Anomalous Enhancement of the d333 Component and Its Possible Origin,” Jpn. J. Appl. Phys. 45(11), 8676–8685 (2006).
  20. M. Fujiwara, M. Maruyama, M. Sugisaki, H. Takahashi, S. Aoshima, R. J. Cogdell, and H. Hashimoto, “Determination of the d-Tensor Components of a Single Crystal of N-Benzyl-2-methyl-4-nitroaniline,” Jpn. J. Appl. Phys. 46(No. 4A), 1528–1530 (2007).
  21. H. Ito, K. Miyamoto, and H. Minamide, “Ultra-broadband, frequency-agile THz-wave generator and its applications,” in Advanced Solid-State Photonics, (Optical Society of America, 2008), WD1.
  22. K. Kuroyanagi, M. Fujiwara, H. Hashimoto, H. Takahashi, S. Aoshima, and Y. Tsuchiya, “Determination of Refractive Indices and Absorption Coefficients of Highly Purified N-Benzyl-2-methyl-4-nitroaniline Crystal in Terahertz Frequency Regime,” Jpn. J. Appl. Phys. 45(29), L761–L764 (2006).
  23. K. Miyamoto and H. Ito, “Wavelength-agile mid-infrared (5-10 microm) generation using a galvano-controlled KTiOPO4 optical parametric oscillator,” Opt. Lett. 32(3), 274–276 (2007). [PubMed]
  24. T. Ikari, R. Guo, H. Minamide, and H. Ito, “Enhancement of Output Energy and Efficiency for A Terahertz-wave Parametric Oscillator Using A Surface-Emitted Configration,” Rev. Laser Eng. 37, 370–373 (2009).

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