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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 18 — Aug. 27, 2012
  • pp: 20783–20789
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Terahertz substance imaging by waveform shaping

Minwoo Yi, Hyosub Kim, Kyong Hwan Jin, Jong Chul Ye, and Jaewook Ahn  »View Author Affiliations


Optics Express, Vol. 20, Issue 18, pp. 20783-20789 (2012)
http://dx.doi.org/10.1364/OE.20.020783


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Abstract

Terahertz pulse shaping technique is used to adaptively design terahertz waveforms of enhanced spectral correlation to particular materials among a given set of materials. In a proof-of-principle experiment performed with a two-dimensional image target consisted of meta-materials of distinctive resonance frequencies, the as-designed waveforms are used to demonstrate terahertz substance imaging. It is hoped that this material-specific terahertz waveforms may enable single- or few-shot terahertz material classification when being used in conjunction with terahertz power measurement.

© 2012 OSA

1. Introduction

Recent advances in science and technology of terahertz (THz) frequency wave have made impactful contributions to a variety of research areas including physics, chemistry, material science, and electric engineering [1

1. Y.-S. Lee, Principles of Terahertz Science and Technology (Springer, USA, 2009).

]. Because of the spectral richness of many materials in THz frequency range, THz time-domain spectroscopy (THz-TDS) has become one of the essential analyzing tools in material science nowadays [2

2. D. Grischkowsky, S. Keiding, M. van Exter, and Ch. Fattinger, “Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors,” J. Opt. Soc. Am. B 7, 2006 (1990). [CrossRef]

], and its applications are particularly promising in biological and medical imaging, nondestructive inspection, and hazardous material classification to list [3

3. B. Ferguson and X.-C. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1, 26–33 (2002). [CrossRef]

5

5. K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Express 11, 2549–2554 (2003). [CrossRef] [PubMed]

]. Although THz-TDS is a powerful technique of high-resolution spectroscopy, its temporal profile measurement of electric fields is often time-consuming. However, some THz-TDS applications, for example, material identification may not require a full measurement of the THz spectral response function of an unknown material, but instead comparing its spectral correlations to an archived THz spectral database of a classified materials may be enough. In that regards, transmissive THz power detection with a series of pre-determined THz waveforms coupled to the material database can simply identify the unknown material. Therefore, one could consider adaptive pulse-shaping technique to design a set of THz waveforms of material-specific spectral correlations and use those shaped THz pulses for substance imaging, which is the subject of the present paper.

As for the pulse-shaping, there has been much interest in optical frequency domain, in part because of its applications in telecommunication and signal processing [6

6. A. M. Weiner, J. P. Heritage, and J. A. Salehi, “Encoding and decoding of femtosecond pulses,” Opt. Lett. 13, 300–302 (1988). [CrossRef] [PubMed]

], and in part because of its potential use as a new optical means toward selective control of electronic and vibrational response of materials [7

7. R. Bartels, S. Backus, E. Zeek, L. Misoguti, G. Vdovin, I. P. Christov, M. M. Murnane, and H. C. Kapteyn, “Shaped pulse optimization of coherent emission of high-harmonic soft X-rays,” Nature 406, 164–166 (2000). [CrossRef] [PubMed]

10

10. J. Lim, H.-g. Lee, S. Lee, and J. Ahn, “Quantum control in two-dimensional Fourier transform spectroscopy,” Phys. Rev. A 84, 013425 (2011). [CrossRef]

]. Of particular relevance in the context of the present paper is the coherent control spectroscopy introduced by Ogilvie and co-workers, where the nonlinear nature of two-photon fluorescence provided by shaped optical fields has been exploited for selective substance microscopy of biological materials [11

11. J. P. Ogilvie, D. Débarre, X. Solinas, J.-L. Martin, E. Beaurepaire, and M. Joffre, “Use of coherent control for selective two-photon fluorescence microscopy in live organisms,” Opt. Express 14, 759–766 (2006). [CrossRef] [PubMed]

]. In THz frequency region, several methods have been developed for the generation of arbitrary THz waveforms, most of which are based on THz emission by spatially and/or temporally shaped optical pulses [12

12. J. Ahn, A. Efimov, R. Averitt, and A. Taylor, “Terahertz waveform synthesis via optical rectification of shaped ultrafast laser pulses,” Opt. Express 11, 2486–2496 (2003). [CrossRef] [PubMed]

18

18. C. D’Amico, M. Tondusson, J. Degert, and E. Freysz, “Tuning and focusing THz pulses by shaping the pump laser beam profile in a nonlinear crystal,” Opt. Express 17, 592–597 (2009). [CrossRef]

], or tunable THz waveform generation from quasi-phase matching process [19

19. Y.-S. Lee, N. Amer, and W. C. Hurlbut, “Terahertz pulse shaping via optical rectification in poled lithium niobate,” Appl. Phys. Lett. 82, 170–172 (2003). [CrossRef]

], or air-plasma breakdown phenomena [20

20. M. Yamaguchi and J. Das, “Terahertz wave generation in nitrogen gas using shaped optical pulses,” J. Opt. Soc. Am. B 26, A90–A94 (2009). [CrossRef]

]. However, there are few practical imaging methodologies which explicitly take the full advantage of pulse-shaping capability.

In this paper, we consider THz waveform shaping for the purpose of material classification. For this, we use a set of meta-materials of distinctive spectral responses, and THz waveforms are adaptively designed to match their spectral responses to the materials, respectively. With the prepared THz waveforms then we carry out a proof-of-principle experiment of THz substance imaging of a two-dimensional target constructed with those meta-materials.

2. Sample preparation

For the test of the material classification based on THz pulse-shaping, we used meta-material samples of noticeable spectral feature in THz frequency range. We chose electrically resonant meta-materials (ETMs), in which magnetic response is suppressed by symmetry and thus purely electrically resonant response can be constructed [21

21. W. J. Padilla, M. T. Aronsson, C. Highstrete, M. Lee, A. J. Taylor, and R. D. Averitt, “Electrically resonant terahertz metamaterials: Theoretical and experimental investigations,” Phys. Rev. B 75, 041102(R) (2007). [CrossRef]

]. In Figs. 1(a) and 1(b), the design of the two ETMs (ETM1 and ETM2) are shown. The norm of the electric field at resonance frequency is displaced, where the red regions in the gaps indicate strong local field enhancement. As the capacitance C of ETM1 is about two-times larger than that of ETM2, while their inductance L’s are about the same, the resonance frequency, which is given by ωo=1/LC for an RLC circuit, of ETM1 is red-shifted compared to that of ETM2.

Fig. 1 Meta-material samples ETM1 and ETM2. (a, b) Electric field norms plotted at their resonance frequencies. (c, d) Microscope images of the fabricated samples. (e, f) Measured transmitted amplitude spectra of unshaped THz pulses. (Dotted lines indicate the calculated resonance frequencies.)

The fabrication of the designed ETMs was carried out by using a conventional photolithographic method. Gold stripes of 150-nm thickness were arranged in a planar array geometry on a 500-μm-thick silicon substrate with a 15-nm-thick titanium adhesion layer. The fabricated ETMs are shown in Figs. 1(c) and 1(d). The density of each ETM was chosen so that the same amount of the power for an unshaped THz pulse transmitted through the samples. The lattice constants of ETM1 and ETM2 were chosen 54 and 50 μm, respectively. In Figs. 1(e) and 1(f), the measurements of the transmitted amplitude spectra are shown, where the resonance frequencies of ETM1 and ETM2 are 0.47 and 0.77 THz, respectively.

3. Experimental procedure

Experimental demonstration was performed in a generic THz-TDS with a pulse-shaping add-on. The experimental setup is schematically shown in Fig. 2(a). Ultrafast optical pulses were produced from a Ti:sapphire mode-locked laser oscillator, and pulse-shaped by a spatial light modulator (SLM) placed in the Fourier plane of a 4- f geometry Martinez zero-dispersion stretcher [22

22. A. M. Weiner, “Femtosecond pulse shaping using spatial light modulators,” Rev. Sci. Instrum. 71, 1929–1960 (2000). [CrossRef]

]. The SLM consisted of 128 liquid crystal pixels of individual phase modulation for the wavelength range of 830–870 nm. For the spectral arrangement of the optical pulses, a pair of holographic grating of 1800 lines/mm groove density was used as well as a pair of cylindrical lens of focal length of 150 mm [12

12. J. Ahn, A. Efimov, R. Averitt, and A. Taylor, “Terahertz waveform synthesis via optical rectification of shaped ultrafast laser pulses,” Opt. Express 11, 2486–2496 (2003). [CrossRef] [PubMed]

]. Then, the shaped optical pulses were focused on to a photoconductive antenna to generate shaped THz pulses. The generated THz radiation pulses were detected via optical gating method with a 2-mm-thick 〈110〉 ZnTe crystal in the far-field region, where the temporal profile measurement of the THz pulses was carried out. For the THz transmitted power measurement, we used temporal integration of the square of the temporal electric field measurement. It is noted that a direct THz power detection using a low-temperature bolometer, as shown in Fig. 2(a), could replace the THz-TDS of our experiment.

Fig. 2 (a) Schematic experimental diagram of THz pulse shaping with a spatial light modulator. (b) Adaptive feed-back loop implemented for the THz pulse profile optimization.

THz pulses were shaped with an adaptive feed-back algorithm, where the optimization target function was the spectral correlation difference C1 defined as
C1({E(ω)})=(|t1(ω)E(ω)|2|t2(ω)E(ω)|2)dω,
(1)
where E(ω) is the amplitude spectrum of the tested THz pulse and t1,2(ω) are the amplitude transmission coefficients [23

23. E. Hecht, Optics4th ed. (Addison Wesley, 2002).

] of ETM1 and ETM2, respectively. So, a shaped THz waveform E(ω) which finds the biggest positive (negative) C1({E(ω)}) can be used to detect the first (the second) among the two materials. Likewise, this method can be generalized for a larger set of material substances than two: Finding the ith material among M different materials can be achieved by finding the biggest value for the correlation function, for example, Ci({E(ω)}) which can be defined as
Ci({E(ω)})=2|ti(ω)E(ω)|2dωj=1M|tj(ω)E(ω)|2dω,
(2)
where ti(ω) is the amplitude transmission coefficient [23

23. E. Hecht, Optics4th ed. (Addison Wesley, 2002).

] of the ith material.

4. Results and discussion

Fig. 3 (a,d) Measured correlation difference C1 vs. the iteration number for the recognition of ETM1 and ETM2 substances, respectively. Measurement of the final THz waveforms (b,e) and their THz spectra (c,f).

Finally, we performed the substance imaging experiment with the adaptively achieved THz waveforms. The imaging target shown in Fig. 4(a) consisted of the two different THz meta-materials, where the left half square was filled with ETM1 and the right half with ETM2. Figure 4(b) shows the THz power imaging result with an unshaped THz pulse, which has little image contrast between the two ETM regions. On the other hand, the results shown in Figs. 4(c) and 4(d) were obtained by utilizing the adaptively found THz waveforms in Figs. 3(b) and 3(e), respectively, and the highly contrasting THz power images in Fig. 4(c) for the biggest positive C1 and Fig. 4(d) for the biggest negative C1, respectively, are clearly demonstrated. Therefore, the two ETM regions unresolved by the initial unshaped THz pulse are clearly distinguished as a result of evolutionary THz pulse shaping. Figs. 4(e) and 4(f) show the measured THz power along the lateral X-positions marked with dashed lines in Figs. 4(c) and 4(d), which results indicate that the relative contrast, defined by Pcontrast = (P1P2) / (P1 + P2), where P1 and P2 are the transmitted THz powers in the ETM1 and ETM2 regions, respectively, is enhanced by a factor of 9.5 compared to the initial power fluctuation level. Therefore, the shaped THz pulse shaping system demonstrates enhanced THz substance contrast imaging as a result of selective attenuation sensitivity of the shaped THz pulses coupled with the material responses.

Fig. 4 (a) An imaging target consists of two different THz meta-material regions, where the left half square is filled with ETM1 of 0.5 THz resonance frequency and the right half with the other with 0.8 THz. (b–d) Measured THz power images with three different THz pulses: (b) the near zero, (c) the biggest positive, and (d) the biggest negative correlation-difference phase-mask arrays, respectively. (e,f) Measured THz power (red) along the lateral X-positions marked with dashed lines in (c,d), respectively, where the reference measurement (blue) is from (b).

5. Conclusions

In summary, this work has demonstrated that THz material substance imaging is achieved through a THz pulse-shaping method. THz pulse shaping has provided frequency-modulated THz pulses and material specific absorption change. During the procedure of evolutionary algorithm, the increase or decrease of spectral correlation difference has made it possible to distinguish between the given materials. In the imaging experiments, the shaped THz pulse imaging system has successfully identified the localized substance distribution. The observed image contrast enhancement of 9.5 times is achieved compared to the initial power fluctuation level of unshaped THz pulses. It is hoped that this method could be extended to frequency-dependent functional biological and medical imaging, and nondestructive inspection in THz frequency region.

Acknowledgments

This work was supported in part by Basic Science Research Programs [2009-0090843, 2010-0013899, 2009-0083512] and in part by the World Class Institute Program [WCI 2011-001] through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology of Korea.

References and links

1.

Y.-S. Lee, Principles of Terahertz Science and Technology (Springer, USA, 2009).

2.

D. Grischkowsky, S. Keiding, M. van Exter, and Ch. Fattinger, “Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors,” J. Opt. Soc. Am. B 7, 2006 (1990). [CrossRef]

3.

B. Ferguson and X.-C. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1, 26–33 (2002). [CrossRef]

4.

M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photon. 1, 97105 (2007). [CrossRef]

5.

K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Express 11, 2549–2554 (2003). [CrossRef] [PubMed]

6.

A. M. Weiner, J. P. Heritage, and J. A. Salehi, “Encoding and decoding of femtosecond pulses,” Opt. Lett. 13, 300–302 (1988). [CrossRef] [PubMed]

7.

R. Bartels, S. Backus, E. Zeek, L. Misoguti, G. Vdovin, I. P. Christov, M. M. Murnane, and H. C. Kapteyn, “Shaped pulse optimization of coherent emission of high-harmonic soft X-rays,” Nature 406, 164–166 (2000). [CrossRef] [PubMed]

8.

D. Meshulach and Y. Silberberg, “Coherent quantum control of two-photon transitions by a femtosecond laser pulse,” Nature 396, 239–242 (1998). [CrossRef]

9.

S. Lee, J. Lim, J. Ahn, V. Hakobyan, and S. Guerin, “Strong-field two-level two-photon transition by phase shaping,” Phys. Rev. A 82, 023408 (2010). [CrossRef]

10.

J. Lim, H.-g. Lee, S. Lee, and J. Ahn, “Quantum control in two-dimensional Fourier transform spectroscopy,” Phys. Rev. A 84, 013425 (2011). [CrossRef]

11.

J. P. Ogilvie, D. Débarre, X. Solinas, J.-L. Martin, E. Beaurepaire, and M. Joffre, “Use of coherent control for selective two-photon fluorescence microscopy in live organisms,” Opt. Express 14, 759–766 (2006). [CrossRef] [PubMed]

12.

J. Ahn, A. Efimov, R. Averitt, and A. Taylor, “Terahertz waveform synthesis via optical rectification of shaped ultrafast laser pulses,” Opt. Express 11, 2486–2496 (2003). [CrossRef] [PubMed]

13.

K. K. Kohli, A. Vaupel, S. Chatterjee, and W. W. Rühle, “Adaptive shaping of THz-pulses generated in <110> ZnTe crystals,” J. Opt. Soc. Am. B 26, A74–A78 (2009). [CrossRef]

14.

S. Vidal, J. Degert, J. Oberlè, and E. Freysz, “Femtosecond optical pulse shaping for tunable terahertz pulse generation,” J. Opt. Soc. Am. B 27, 1044–1050 (2010). [CrossRef]

15.

M. Yi, K. Hee Lee, I. Maeng, J.-H. Son, R. D. Averitt, and J. Ahn, “Tailoring the spectra of terahertz emission from CdTe and ZnTe electro-optic crystals,” Jpn. J. Appl. Phys. 47, 202–204 (2008). [CrossRef]

16.

Y. Liu, S.-G. Park, and A. M. Weiner, “Terahertz waveform synthesis via optical pulse shaping,” IEEE J. Sel. Top. Quantum Electron. 2, 709–719 (1996). [CrossRef]

17.

J. Y. Sohn, Y. H. Ahn, D. J. Park, E. Oh, and D. S. Kim, “Tunable terahertz generation using femtosecond pulse shaping,” Appl. Phys. Lett. 81, 13–15 (2002). [CrossRef]

18.

C. D’Amico, M. Tondusson, J. Degert, and E. Freysz, “Tuning and focusing THz pulses by shaping the pump laser beam profile in a nonlinear crystal,” Opt. Express 17, 592–597 (2009). [CrossRef]

19.

Y.-S. Lee, N. Amer, and W. C. Hurlbut, “Terahertz pulse shaping via optical rectification in poled lithium niobate,” Appl. Phys. Lett. 82, 170–172 (2003). [CrossRef]

20.

M. Yamaguchi and J. Das, “Terahertz wave generation in nitrogen gas using shaped optical pulses,” J. Opt. Soc. Am. B 26, A90–A94 (2009). [CrossRef]

21.

W. J. Padilla, M. T. Aronsson, C. Highstrete, M. Lee, A. J. Taylor, and R. D. Averitt, “Electrically resonant terahertz metamaterials: Theoretical and experimental investigations,” Phys. Rev. B 75, 041102(R) (2007). [CrossRef]

22.

A. M. Weiner, “Femtosecond pulse shaping using spatial light modulators,” Rev. Sci. Instrum. 71, 1929–1960 (2000). [CrossRef]

23.

E. Hecht, Optics4th ed. (Addison Wesley, 2002).

24.

M. Mitchell, An Introduction to Genetic Algorithms (MIT Press, 1996).

OCIS Codes
(320.0320) Ultrafast optics : Ultrafast optics
(320.5540) Ultrafast optics : Pulse shaping
(160.3918) Materials : Metamaterials
(110.6795) Imaging systems : Terahertz imaging

ToC Category:
Ultrafast Optics

History
Original Manuscript: May 4, 2012
Revised Manuscript: July 31, 2012
Manuscript Accepted: August 20, 2012
Published: August 24, 2012

Citation
Minwoo Yi, Hyosub Kim, Kyong Hwan Jin, Jong Chul Ye, and Jaewook Ahn, "Terahertz substance imaging by waveform shaping," Opt. Express 20, 20783-20789 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-18-20783


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References

  1. Y.-S. Lee, Principles of Terahertz Science and Technology (Springer, USA, 2009).
  2. D. Grischkowsky, S. Keiding, M. van Exter, and Ch. Fattinger, “Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors,” J. Opt. Soc. Am. B7, 2006 (1990). [CrossRef]
  3. B. Ferguson and X.-C. Zhang, “Materials for terahertz science and technology,” Nat. Mater.1, 26–33 (2002). [CrossRef]
  4. M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photon.1, 97105 (2007). [CrossRef]
  5. K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Express11, 2549–2554 (2003). [CrossRef] [PubMed]
  6. A. M. Weiner, J. P. Heritage, and J. A. Salehi, “Encoding and decoding of femtosecond pulses,” Opt. Lett.13, 300–302 (1988). [CrossRef] [PubMed]
  7. R. Bartels, S. Backus, E. Zeek, L. Misoguti, G. Vdovin, I. P. Christov, M. M. Murnane, and H. C. Kapteyn, “Shaped pulse optimization of coherent emission of high-harmonic soft X-rays,” Nature406, 164–166 (2000). [CrossRef] [PubMed]
  8. D. Meshulach and Y. Silberberg, “Coherent quantum control of two-photon transitions by a femtosecond laser pulse,” Nature396, 239–242 (1998). [CrossRef]
  9. S. Lee, J. Lim, J. Ahn, V. Hakobyan, and S. Guerin, “Strong-field two-level two-photon transition by phase shaping,” Phys. Rev. A82, 023408 (2010). [CrossRef]
  10. J. Lim, H.-g. Lee, S. Lee, and J. Ahn, “Quantum control in two-dimensional Fourier transform spectroscopy,” Phys. Rev. A84, 013425 (2011). [CrossRef]
  11. J. P. Ogilvie, D. Débarre, X. Solinas, J.-L. Martin, E. Beaurepaire, and M. Joffre, “Use of coherent control for selective two-photon fluorescence microscopy in live organisms,” Opt. Express14, 759–766 (2006). [CrossRef] [PubMed]
  12. J. Ahn, A. Efimov, R. Averitt, and A. Taylor, “Terahertz waveform synthesis via optical rectification of shaped ultrafast laser pulses,” Opt. Express11, 2486–2496 (2003). [CrossRef] [PubMed]
  13. K. K. Kohli, A. Vaupel, S. Chatterjee, and W. W. Rühle, “Adaptive shaping of THz-pulses generated in <110> ZnTe crystals,” J. Opt. Soc. Am. B26, A74–A78 (2009). [CrossRef]
  14. S. Vidal, J. Degert, J. Oberlè, and E. Freysz, “Femtosecond optical pulse shaping for tunable terahertz pulse generation,” J. Opt. Soc. Am. B27, 1044–1050 (2010). [CrossRef]
  15. M. Yi, K. Hee Lee, I. Maeng, J.-H. Son, R. D. Averitt, and J. Ahn, “Tailoring the spectra of terahertz emission from CdTe and ZnTe electro-optic crystals,” Jpn. J. Appl. Phys.47, 202–204 (2008). [CrossRef]
  16. Y. Liu, S.-G. Park, and A. M. Weiner, “Terahertz waveform synthesis via optical pulse shaping,” IEEE J. Sel. Top. Quantum Electron.2, 709–719 (1996). [CrossRef]
  17. J. Y. Sohn, Y. H. Ahn, D. J. Park, E. Oh, and D. S. Kim, “Tunable terahertz generation using femtosecond pulse shaping,” Appl. Phys. Lett.81, 13–15 (2002). [CrossRef]
  18. C. D’Amico, M. Tondusson, J. Degert, and E. Freysz, “Tuning and focusing THz pulses by shaping the pump laser beam profile in a nonlinear crystal,” Opt. Express17, 592–597 (2009). [CrossRef]
  19. Y.-S. Lee, N. Amer, and W. C. Hurlbut, “Terahertz pulse shaping via optical rectification in poled lithium niobate,” Appl. Phys. Lett.82, 170–172 (2003). [CrossRef]
  20. M. Yamaguchi and J. Das, “Terahertz wave generation in nitrogen gas using shaped optical pulses,” J. Opt. Soc. Am. B26, A90–A94 (2009). [CrossRef]
  21. W. J. Padilla, M. T. Aronsson, C. Highstrete, M. Lee, A. J. Taylor, and R. D. Averitt, “Electrically resonant terahertz metamaterials: Theoretical and experimental investigations,” Phys. Rev. B75, 041102(R) (2007). [CrossRef]
  22. A. M. Weiner, “Femtosecond pulse shaping using spatial light modulators,” Rev. Sci. Instrum.71, 1929–1960 (2000). [CrossRef]
  23. E. Hecht, Optics4th ed. (Addison Wesley, 2002).
  24. M. Mitchell, An Introduction to Genetic Algorithms (MIT Press, 1996).

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