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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 14 — Jul. 6, 2009
  • pp: 11543–11549
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Broadband and high power terahertz pulse generation beyond excitation bandwidth limitation via χ(2) cascaded processes in LiNbO3

Masaya Nagai, Mukesh Jewariya, Yuki Ichikawa, Hideyuki Ohtake, Toshiharu Sugiura, Yuzuru Uehara, and Koichiro Tanaka  »View Author Affiliations


Optics Express, Vol. 17, Issue 14, pp. 11543-11549 (2009)
http://dx.doi.org/10.1364/OE.17.011543


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Abstract

We proposed a novel THz generation technique beyond the limitation of the input optical pulse width, based on phase modulation via cascaded χ(2) process. When intense THz electric field generated by optical rectification lies in electro-optic (EO) crystal, emitted THz field gives phase modulation to the optical excitation pulse. The phase modulation causes excitation pulse narrowing and consequently gives rise to the enhancement of conversion efficiency and THz wave bandwidth broadening. We experimentally realize this generation technique with high χ(2) EO crystal LiNbO3 and with subpicosecond pulse from Yb-doped fiber laser. It opens new concept of THz technologies.

© 2009 Optical Society of America

1. Introductions

Intense monocycle terahertz (THz) electric field generation techniques allow us to perform time-domain spectroscopy and are attractive for various applications such as material characterization, sensing, imaging, telecommunication, and high-frequency devices [1

1. K. Sakai, Terahertz Optoelectronics (Springer, 2005). [CrossRef]

,2

2. M. Tonouchi, “Cutting-edge terahertz technology,” Nature Photonics , 1, (2), 97–105 (2007). [CrossRef]

]. Recent advances in the ultrafast pulse techniques, which are based on Ti: sapphire laser, are decisive for these broadband THz technologies. This is because the bandwidth of monocycle THz pulse has been determined directly by the excitation pulse duration [3

3. Y. R. Shen, The principles of Nonlinear Optics (Wiley Interscience,1984).

5

5. A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss, and W. H. Knox, “Detectors and sources for ultrabroadband electro-optic sampling: Experiment and theory,” Appl. Phys. Lett. 74, 1516 (1999). [CrossRef]

]. Here, we proposed a novel THz generation technique beyond the limitation of the input optical pulse width. This generation technique is based on non-resonant χ (2) processes. Figure 1 shows the schematic diagram of our THz wave generation technique. When intense THz electric field generated by optical rectification lies in EO crystal, emitted THz field gives phase modulation to the optical excitation pulse. Optical pulse shape can be easily modulated in EO crystal by intense THz pulse. These simultaneous optical rectification, phase modulation processes and cascaded χ (2) processes, are equivalent to effective χ (3) process. Therefore these processes can give rise to the narrowing of optical pulse during its propagation inside the crystal, which leads to further efficient and broadband THz wave generation beyond the limitation of the input optical pulse width.

Fig. 1. Schematic diagram of THz generation technique beyond limitation of the input optical pulse width. Simultaneous optical rectification (OR) and optical pulse phase modulation (PM) processes bring to the narrowing of both optical and THz pulses.

2. Experimental setup

We used a Yb-doped fiber laser (IMRA America Inc. Co, D1000, 1040 nm, 100 kHz, output power of 1 W). Linear dispersion of laser pulse is compensated with a grating pair. The pulse duration of it, 0.60 ps, is carefully evaluated from self-correlation with pulse shape assumption of sech2 function. The optical pulses split into two beams for THz generation and detection. We control wavefront and angular frequency dispersion of excitation beam with 1800/mm grating and two lenses, and focus on the stochiometric LiNbO3 prism with 1.5 % Mg doping with spot size of 0.5 mm. One surface is cut with an angle of 62° with anti-reflection coating for near infrared beam. The generated THz pulses are guided with two off-axis parabolic mirrors and emitter’s image is transferred with half size to the surface of the (110)-oriented 1mm-thick CdTe crystal [9

9. A. Syouji, S. Saito, K. Sakai, M. Nagai, K. Tanaka, H. Ohtake, T. Bessho, T. Sugiura, T. Hirosumi, and M. Yoshida, “Evaluation of a terahertz wave spectrum and construction of a terahertz wave-sensing system using a Yb-doped fiber laser,” J. Opt. Soc. Am. B 24(8), 2006–2012 (2007). [CrossRef]

]. The sampling beam is focused on the same EO crystal as a sampling pulse, and the birefringence of the sampling pulse modulated by the electric field of THz pulses is measured using a quarter wave plate, Wolston prism, and two balanced Si detectors. For high-S/N measurement, we chop the pump beam at 1 kHz and modulated signal is extracted with lock-in amplifier. Absolute value of electric field can be easily calibrated from induced phase modulation θ with the formula of sin θ=2π n opt 3 r 41 tE THz L/λ [9

9. A. Syouji, S. Saito, K. Sakai, M. Nagai, K. Tanaka, H. Ohtake, T. Bessho, T. Sugiura, T. Hirosumi, and M. Yoshida, “Evaluation of a terahertz wave spectrum and construction of a terahertz wave-sensing system using a Yb-doped fiber laser,” J. Opt. Soc. Am. B 24(8), 2006–2012 (2007). [CrossRef]

], where n opt=2.6, r 41=6.8 pm/V [4

4. A. Yariv, Quantum Electronics, 3ed ed. (John Wiley & Sons Inc, 1989).

], and t’=2/(n THz -1) are the optical index at λ=1.04 µm, EO coefficient, THz Fresnel coefficient in CdTe, respectively. By varying the time delay between a THz pulse and a sampling pulse, the electric field amplitude of THz pulses can be detected as a function of time.

3. Experiments

Let us consider these phenomena in frequency domain. THz wave ω THz is generated by optical rectification, and optical beam ω is also modulated via both differential frequency ω - ω THz and sum frequency ω + ω THz generations. Consequently, the spectral bandwidth of optical pulse gets broadened, which allows the generation of higher frequency THz components. Figure 3(a) shows the power spectra of emitted THz wave at different excitation densities. Gray curves show the power spectra as shown in Fig 1(a) and we obtain calibrated spectra (bold curves) from these by dividing Frourier transformation spectrum of 0.6 ps pulse. Weighted center frequency of the power spectrum is 0.42 THz at low input power, but it extensively shifts to 1.25 THz at the maximum input power. Our experimental results imply that THz components at high frequency are strongly enhanced. Figure 3 (b) shows the optical spectra of transmitted excitation pulses from LiNbO3. We collect whole transmitted excitation beam into 30 cm spectrometer and detect the spectra with cooled InGaAs linear image sensor. Weighted center wavelength of excitation beam is 288.6 THz at low excitation power. However, the spectral shifts towards longer wavelength proceeds above 2 µJ, and center wavelength lies at 288.2 THz at the maximum power of 8.5µJ, which corresponds to a shift of 0.4 THz frequency. These spectral changes are consistent with temporal behaviors shown in Figs. 2(b) and (c).

Fig. 2. (a) Temporal profile of emitted THz radiation at different excitation powers 0.95, 3.7, and 8.5 µJ. We evaluate net profile (colored curves) with the assumption of 0.60 ps sampling pulse duration from EO signal (gray curves). (b) Input power dependence of interval between maximum and minimum electric field times. c Input power dependence of the maximum electric field calibrated form EO signal. Inset shows the scheme of experimental setup.

Fig. 3. (a) Power spectra of THz radiation from LiNbO3 at different excitation powers. (b) Optical spectra of transmitted excitation pulses through LiNbO3 at corresponding excitation densities. These spectra are normalized by their area.

We also measured spatial distribution of THz wave on the surface of LiNbO3 crystal. We controlled the position of emitter’s image on the detector by moving one parabolic mirror horizontally. Figure 4 shows the power density (upper) and interval of maximum and minimum times (lower) of THz electric field emitted from different positions of LiNbO3 surface. Power density is evaluated from the time integration of measured E 2, and normalized by the square of input power. At low input power, spatial distribution of the power density should be independent of it. We set the origin at peak power density position at 0.8 µJ input power. At this position THz wave is the most efficiently generated via optical rectification, where one edge of excitation beam approaches to the surface of prism. By increasing input power, emitted THz electric field is enhanced at the location of d=0.6 mm. This shift implies that long propagation of excitation beam with intense THz electric field brings in a strong phase modulation of excitation pulse. Coincidently, spatial modulation of excitation pulse may occur and increases the excitation power density and causes the enhancement in THz wave generation efficiency. Similar results have been reported in LiNbO3 using Ti: sapphire laser [22

22. A. G. Stepanov, J. Hebling, and J. Kuhl, “THz generation via optical rectification with ultrashort laser pulse focused to a line,” Appl. Phys. B 81, 23–26 (2005) [CrossRef]

]. However, Figure 4 shows the that enhancement of THz wave generation coincides with the time interval of narrowing of THz profile. Considering that the envelop shape of excitation pulse determines the THz pulse profile, this time interval narrowing directly shows the optical pulse narrowing caused by cascaded χ (2) process. Both modulations bring in the enhancement of THz wave generation efficiency.

Fig. 4. The position dependence of THz radiation on the surface of LiNbO3. Upper shows the output power densities of THz radiation, which is normalized by the square of input power. Lower shows corresponding time intervals between maximum and minimum electric field locations. We set the original where power density is the maximum at 0.8 µJ. We measured THz electric fields as shown in Figs. 1 and 2(a) at the position of d=0.6 mm.

Large spectral broadening of THz pulse clearly shows the narrowing of excitation pulse width in Fig. 3(a). Nevertheless, spectral modulation of excitation beam is not so large in Fig. 3(b). This is due to that the excitation laser pulse has higher-order dispersion, which cannot be compensated with a grating pair. Bandwidth of spectral limited pulse with 0.60 ps duration is 0.53 THz in spite of spectral width 1.8 THz of our laser pulse. Of course the profile of THz wave is simply determined by optical pulse duration, and is independent of the pulse dispersion [3

3. Y. R. Shen, The principles of Nonlinear Optics (Wiley Interscience,1984).

]. If this higher order dispersion is compensated via cascaded χ (2) process, small spectral modulation with envelop shape of excitation pulse becomes narrow. This mechanism is not clear now. Looking back on the reports of pulse compression via second-harmonic generation process [23

23. X. Liu, L. J. Qian, and F. Wise, “High-energy pulse compression by use of negative phase shifts produced by the cascade χ(2): χ (2) nonlinearity,” Opt. Lett. 24, 1777–1779 (1999). [CrossRef]

], we believe that pulse compression via cascaded χ (2) process should occurs with broadband THz wave generation even using spectral limited excitation pulse.

4. Summary

We have experimentally demonstrated broadband THz wave generation beyond the excitation pulse limitation with extensive phase modulation via cascaded χ (2) process. Our result indicates that subpicosecond excitation pulse, which seems rather unfavorable in THz technologies, suffices monocycle THz wave generation with the bandwidth above several THz frequencies. Yb-doped fiber laser we used here is attractive in the viewpoint of compact, high-power and high-efficiency excitation light source. Releasing pulse-width limitation of laser enables compact and convenient light souse of intense THz pulse. This opens novel THz sensing based on nonlinear spectroscopy, THz EO devices, and real-time large-aperture imaging.

Acknowledgements

References and links

1.

K. Sakai, Terahertz Optoelectronics (Springer, 2005). [CrossRef]

2.

M. Tonouchi, “Cutting-edge terahertz technology,” Nature Photonics , 1, (2), 97–105 (2007). [CrossRef]

3.

Y. R. Shen, The principles of Nonlinear Optics (Wiley Interscience,1984).

4.

A. Yariv, Quantum Electronics, 3ed ed. (John Wiley & Sons Inc, 1989).

5.

A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss, and W. H. Knox, “Detectors and sources for ultrabroadband electro-optic sampling: Experiment and theory,” Appl. Phys. Lett. 74, 1516 (1999). [CrossRef]

6.

A. Rice, Y. Jin, X. F. Ma, X.-C. Zhang, D. Bliss, J. Larkin, and M. Alexander, “Terahertz optical rectification from 110 zinc-blende crystals,” Appl. Phys. Lett. 64, 1324–1326 (1994). [CrossRef]

7.

Q. Wu and X.-C. Zhang, “Ultrafast electro-optic field sensors,” Appl. Phys. Lett. 681604–1606 (1996). [CrossRef]

8.

M. Nagai, K. Tanaka, H. Ohtake, T. Bessho, T. Sugiura, T. Hirosumi, and M. Yoshida, “Generation and detection of terahertz radiation by electro-optical process in GaAs using 1.56µm fiber laser pulses,” Appl. Phys. Lett. 85, 3974–3976 (2004). [CrossRef]

9.

A. Syouji, S. Saito, K. Sakai, M. Nagai, K. Tanaka, H. Ohtake, T. Bessho, T. Sugiura, T. Hirosumi, and M. Yoshida, “Evaluation of a terahertz wave spectrum and construction of a terahertz wave-sensing system using a Yb-doped fiber laser,” J. Opt. Soc. Am. B 24(8), 2006–2012 (2007). [CrossRef]

10.

F. Blanchard, L. Razzari, H. C. Bandulet, G. Sharma, R. Morandotti, J. C. Kieffer, T. Ozaki, M. Reid, H. F. Tiedje, H. K. Haugen, and F. A. Hegmann, “Generation of 1.5 µJ single-cycle terahertz pulses by optical rectification from a large aperture ZnTe crystal,” Opt. Express 15(20), 13212–13220 (2007). [CrossRef]

11.

K. Kawase, H. Minamide, K. Imai, J. Shikata, and H. Ito, “Injection-seeded terahertz-wave parametric generator with wide tenability,” Appl. Phys. Lett. 80(2), 195–197 (2002). [CrossRef]

12.

J. Hebling, G. Almási, and Z. I. Kozma, “Velocity matching by pulse front tilting for largearea THz-pulse generation,” Opt. Express 10, (21) 1161–1166 (2002). [PubMed]

13.

M. C. Hoffmann, K.-L. Yeh, J. Hebling, and K. A. Nelson, “Efficient terahertz generation by optical rectification at 1035 nm,” Opt. Express 15(18), 11706 (2007). [CrossRef]

14.

K.-L. Yeh, M. C. Hoffmann, J. Hebling, and K. A. Nelson, “Generation of 10µJ ultrashort terahertz pulses by optical rectification,” Appl. Phys. Lett. 90, 171121 (2007). [CrossRef]

15.

J. Hebling, K.-L. Yeh, M. C. Hoffmann, B. Bartal, and K. A. Nelson, “Generation of high-power terahertz pulses by tilted-pulse-front excitation and their application possibilities,” J. Opt. Soc. Am. B 25 (7), 6–19 (2008). [CrossRef]

16.

A. Stepanov, J. Kuhl, I. Kozma, E. Riedle, G. Almási, and J. Hebling, “Scaling up the energy of THz pulses created by optical rectification,” Opt. Express 13(15), 5762–5768 (2005). [CrossRef]

17.

A. G. Stepanov, A. A. Mel’nikov, V. O. Kompanets, and S. V. Chekalin, “Spectral Modification of Femtosecond Laser Pulses in the Process of Highly Efficient Generation of Terahertz Radiation via Optical Rectification,” JETP Lett. 85(5), 279–282 (2007).

18.

T. Hattori and K. Takeuchi “Simulation study on cascaded terahertz pulse generation in electro-optic crystals,” Opt. Express 15(13), 8076 (2007). [CrossRef]

19.

E. Matsubara, T. Sekikawa, and M. Yamashita, “Generation of ultrashort optical pulses using multiple coherent anti-Stokes Raman scattering in a crystal at room temperature,” Appl. Phys. Lett. 92, 071104 (2008). [CrossRef]

20.

M. C. Hoffmann, K.-L. Yeh, H. Y. Hwang, T. S. Sosnowski, B. S. Prall, J. Hebling, and K. A. Nelson, “Fiber laser pumped high average power single-cycle terahertz pulse source,” Appl. Phys. Lett. 93, 141107 (2008). [CrossRef]

21.

N. Zhavoronkov and G. Korn, “Generation of Single Intense Short Optical Pulses by Ultrafast Molecular Phase Modulation,” Phys. Rev. Lett. 88, 203901 (2002). [CrossRef] [PubMed]

22.

A. G. Stepanov, J. Hebling, and J. Kuhl, “THz generation via optical rectification with ultrashort laser pulse focused to a line,” Appl. Phys. B 81, 23–26 (2005) [CrossRef]

23.

X. Liu, L. J. Qian, and F. Wise, “High-energy pulse compression by use of negative phase shifts produced by the cascade χ(2): χ (2) nonlinearity,” Opt. Lett. 24, 1777–1779 (1999). [CrossRef]

OCIS Codes
(190.4410) Nonlinear optics : Nonlinear optics, parametric processes

ToC Category:
Nonlinear Optics

History
Original Manuscript: April 28, 2009
Revised Manuscript: June 10, 2009
Manuscript Accepted: June 11, 2009
Published: June 25, 2009

Citation
Masaya Nagai, Mukesh Jewariya, Yuki Ichikawa, Hideyuki Ohtake, Toshiharu Sugiura, Yuzuru Uehara, and Koichiro Tanaka, "Broadband and high power terahertz pulse generation beyond excitation bandwidth limitation via χ(2) cascaded processes in LiNbO3," Opt. Express 17, 11543-11549 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-14-11543


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References

  1. K. Sakai, Terahertz Optoelectronics (Springer, 2005). [CrossRef]
  2. M. Tonouchi, "Cutting-edge terahertz technology," Nat. Photonics 1(2), 97-105 (2007). [CrossRef]
  3. Y. R. Shen, The Principles of Nonlinear Optics (Wiley Interscience, 1984).
  4. A. Yariv, Quantum Electronics, 3rd ed. (John Wiley & Sons Inc, 1989).
  5. A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss, and W. H. Knox, "Detectors and sources for ultrabroadband electro-optic sampling: Experiment and theory," Appl. Phys. Lett. 74, 1516 (1999). [CrossRef]
  6. A. Rice, Y. Jin, X. F. Ma, X.-C. Zhang, D. Bliss, J. Larkin, and M. Alexander, "Terahertz optical rectification from 110 zinc-blende crystals," Appl. Phys. Lett. 64, 1324-1326 (1994). [CrossRef]
  7. Q. Wu, and X.-C. Zhang, "Ultrafast electro-optic field sensors," Appl. Phys. Lett. 68, 1604-1606 (1996). [CrossRef]
  8. M. Nagai, K. Tanaka, H. Ohtake, T. Bessho, T. Sugiura, T. Hirosumi, and M. Yoshida, "Generation and detection of terahertz radiation by electro-optical process in GaAs using 1.56μm fiber laser pulses," Appl. Phys. Lett. 85, 3974-3976 (2004). [CrossRef]
  9. A. Syouji, S. Saito, K. Sakai, M. Nagai, K. Tanaka, H. Ohtake,T. Bessho, T. Sugiura, T. Hirosumi, and M. Yoshida, "Evaluation of a terahertz wave spectrum and construction of a terahertz wave-sensing system using a Yb-doped fiber laser," J. Opt. Soc. Am. B 24(8), 2006-2012 (2007). [CrossRef]
  10. F. Blanchard, L. Razzari,H. C. Bandulet, G. Sharma, R. Morandotti, J. C. Kieffer, T. Ozaki, M. Reid, H. F. Tiedje, H. K. Haugen, and F. A. Hegmann, "Generation of 1.5 μJ single-cycle terahertz pulses by optical rectification from a large aperture ZnTe crystal," Opt. Express 15(20), 13212-13220 (2007). [CrossRef]
  11. K. Kawase, H. Minamide, K. Imai, J. Shikata, and H. Ito, "Injection-seeded terahertz-wave parametric generator with wide tenability," Appl. Phys. Lett. 80(2), 195-197 (2002). [CrossRef]
  12. J. Hebling, G. Almási and Z. I. Kozma, "Velocity matching by pulse front tilting for largearea THz-pulse generation," Opt. Express 10, (21) 1161-1166 (2002). [PubMed]
  13. M. C. Hoffmann, K.-L. Yeh, J. Hebling, and K. A. Nelson, "Efficient terahertz generation by optical rectification at 1035 nm," Opt. Express 15(18), 11706 (2007). [CrossRef]
  14. K.-L. Yeh, M. C. Hoffmann, J. Hebling, and K. A. Nelson, "Generation of 10μJ ultrashort terahertz pulses by optical rectification," Appl. Phys. Lett. 90, 171121 (2007). [CrossRef]
  15. J. Hebling, K.-L. Yeh, M. C. Hoffmann, B. Bartal, and K. A. Nelson, "Generation of high-power terahertz pulses by tilted-pulse-front excitation and their application possibilities," J. Opt. Soc. Am. B 25(7), 6-19 (2008). [CrossRef]
  16. A. Stepanov, J. Kuhl, I. Kozma, E. Riedle, G. Almási, and J. Hebling, "Scaling up the energy of THz pulses created by optical rectification," Opt. Express 13(15), 5762-5768 (2005). [CrossRef]
  17. A. G. Stepanov, A. A. Mel’nikov, V. O. Kompanets, and S. V. Chekalin, "Spectral Modification of Femtosecond Laser Pulses in the Process of Highly Efficient Generation of Terahertz Radiation via Optical Rectification," JETP Lett. 85(5), 279-282 (2007).
  18. T. Hattori and K. Takeuchi "Simulation study on cascaded terahertz pulse generation in electro-optic crystals," Opt. Express 15(13), 8076 (2007). [CrossRef]
  19. E. Matsubara, T. Sekikawa, and M. Yamashita, "Generation of ultrashort optical pulses using multiple coherent anti-Stokes Raman scattering in a crystal at room temperature," Appl. Phys. Lett. 92, 071104 (2008). [CrossRef]
  20. M. C. Hoffmann, K.-L. Yeh, H. Y. Hwang, T. S. Sosnowski, B. S. Prall, J. Hebling, and K. A. Nelson, "Fiber laser pumped high average power single-cycle terahertz pulse source," Appl. Phys. Lett. 93, 141107 (2008). [CrossRef]
  21. N. Zhavoronkov, and G. Korn, "Generation of Single Intense Short Optical Pulses by Ultrafast Molecular Phase Modulation," Phys. Rev. Lett. 88, 203901 (2002). [CrossRef] [PubMed]
  22. A. G. Stepanov, J. Hebling, and J. Kuhl, "THz generation via optical rectification with ultrashort laser pulse focused to a line," Appl. Phys. B 81, 23-26 (2005) [CrossRef]
  23. X. Liu, L. J. Qian, and F. Wise, ‘‘High-energy pulse compression by use of negative phase shifts produced by the cascade χ(2): χ (2) nonlinearity,’’Opt. Lett. 24, 1777-1779 (1999). [CrossRef]

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