Abstract

We explore the application of terahertz spectroscopic techniques for the remote determination of the water content of paper. The aim is the development of a rapid diagnostic imaging tool applicable in paper fabrication processes. THz radiation offers a high sensitivity for water, a good spatial resolution, and insensitivity to scattering at the paper surface. The advent of THz cameras makes fast large-area image detectors feasible. In this paper, we show for the case of a 0.6-THz fixed-frequency system, that the water content of paper can be determined with high accuracy. We demonstrate a quantitative (calibrated) method for determining the moisture content in paper based on extinction and phase measurements in the lower THz range with a spatial resolution in the mm-range and scanning times below two minutes.

© 2008 Optical Society of America

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  1. S. J. Hashemi and W. J. M. Douglas, “Moisture nonuniformity in drying paper: measurement and relation to process parameters,” Drying Technology 21, 329–347 (2003).
    [CrossRef]
  2. J. F. Pugh, “The infrared measurement of surface moisture in paper,” Tappi,  63, 131–134 (1980).
  3. R. Kuusela, Infrared moisture measurement of paper, board and pulp, Ph.D. Thesis, University of Kuopio, Finland, (1990).
  4. R. Boulay, R. Gagnon, D. Rochette, and J. R. Izatt, “Paper sheet moisture measurements in the far infrared,” Int. J. Infrared Millim. Waves 5, 1221–1234 (1984).
    [CrossRef]
  5. D. M. Mittleman, R. H. Jacobsen, and M. C. Nuss, “T-Ray imaging,” IEEE J. Sel. Top. Quantum Electron. 2, 679–692 (1996).
    [CrossRef]
  6. T. Yasui and T. Araki, “Sensitive measurement of water content in dry material based on low-frequency terahertz time-domain spectroscopy,” Proc. of SPIE 6024, 60240A, 69–74 (2005).
  7. C. Rønne, P.-O. Åstrand, and S. R. Keiding, “THz spectroscopy of liquid H2O and D2O,” Phys. Rev. Lett. 82, 2888–2891 (1999).
    [CrossRef]
  8. C.W. Robertson, B. Curnutte, and D Williams, “The infrared spectrum of water,” Mol. Phys. 26, 183–191 (1973).
    [CrossRef]
  9. J. Xu, K. W. Plaxco, and S. J. Allen, “Absorption spectra of liquid water and aqueous buffers between 0.3 and 3.72 THz,” J. Chem. Phys. 124, 036101 1–3 (2006).
    [CrossRef] [PubMed]
  10. J. T. Kindt and C. A. Schmuttenmaer, “Far-infrared dielectric properties of polar liquids probed by femtosecond terahertz pulse spectroscopy,” J. Phys. Chem. 100, 10373–10379 (1996).
    [CrossRef]
  11. S. Hadjiloucas, L. S. Karatzas, and W. J. Bowen, “Measurement of leaf water content using terahertz radiation,” IEEE Trans. Microwave Theory Tech. 47, 142–149 (1999).
    [CrossRef]
  12. K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3006 (2002).
    [CrossRef]
  13. N. Hasegawa, T. Löffler, M. Thomson, and H. G. Roskos, “Remote identification of protrusions and dents on surfaces by THz reflectometry with spatial beam filtering and out-of-focus detection,” Appl. Phys. Lett. 83, 3996–3998 (2003).
    [CrossRef]
  14. T. Löffler, T. Bauer, K. Siebert, H. Roskos, A. Fitzgerald, and S. Czasch, “Terahertz dark-field imaging of biomedical tissue,” Opt. Express 9, 616–621 (2001).
    [CrossRef] [PubMed]
  15. T. Löffler, T. Hahn, M. Thomson, F. Jacob, and H. G. Roskos, “Large-area electro-optic ZnTe terahertz emitters,” Opt. Express 13, 5353–5362 (2005).
    [CrossRef] [PubMed]
  16. T. Löffler, M. Kreß, M. Thomson, T. Hahn, N. Hasegawa, and H. G. Roskos, “Comparative performance of terahertz emitters in amplifier-laser-based systems,” Semicond. Sci. Technol. 20, S134–S141 (2005).
    [CrossRef]
  17. T. Löffler, T. May, C. am Weg, A. Alcin, B. Hils, and H. G. Roskos, “Continuous-wave terahertz imaging with a hybrid system,” Appl. Phys. Lett. 90, 091111 1–3 (2007).
    [CrossRef]
  18. A. Luukanen, L. Grönberg, P. Helistö, J. S. Penttilä, H. Seppä, H. Sipola, C. R. Dietlein, and E. N. Grossman, “An array of antenna-coupled superconducting microbolometers for passive indoors real-time THz imaging,” Proc. of SPIE Vol. 6212, 62120Y 1–9 (2006).
    [CrossRef]
  19. A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H.G. Roskos, “Terahertz imaging with GaAS field-effect transistors,” Electronics Lett. 44, 408–409 (2008).
    [CrossRef]
  20. A.W. M. Lee and Q. Hu, “Real-time, continuous-wave terahertz imaging by use of a microbolometer focal-plane array,” Optics Lett. 30, 2563–2565 (2005).
    [CrossRef]
  21. A. W. M. Lee, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Real-time imaging using a 4.3-THz quantum cascade laser and a 320×240 microbolometer focal-plane array,” IEEE Photon. Technol. Lett. 18, 1415–1417 (2006).
    [CrossRef]

2008 (1)

A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H.G. Roskos, “Terahertz imaging with GaAS field-effect transistors,” Electronics Lett. 44, 408–409 (2008).
[CrossRef]

2007 (1)

T. Löffler, T. May, C. am Weg, A. Alcin, B. Hils, and H. G. Roskos, “Continuous-wave terahertz imaging with a hybrid system,” Appl. Phys. Lett. 90, 091111 1–3 (2007).
[CrossRef]

2006 (3)

A. Luukanen, L. Grönberg, P. Helistö, J. S. Penttilä, H. Seppä, H. Sipola, C. R. Dietlein, and E. N. Grossman, “An array of antenna-coupled superconducting microbolometers for passive indoors real-time THz imaging,” Proc. of SPIE Vol. 6212, 62120Y 1–9 (2006).
[CrossRef]

J. Xu, K. W. Plaxco, and S. J. Allen, “Absorption spectra of liquid water and aqueous buffers between 0.3 and 3.72 THz,” J. Chem. Phys. 124, 036101 1–3 (2006).
[CrossRef] [PubMed]

A. W. M. Lee, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Real-time imaging using a 4.3-THz quantum cascade laser and a 320×240 microbolometer focal-plane array,” IEEE Photon. Technol. Lett. 18, 1415–1417 (2006).
[CrossRef]

2005 (4)

A.W. M. Lee and Q. Hu, “Real-time, continuous-wave terahertz imaging by use of a microbolometer focal-plane array,” Optics Lett. 30, 2563–2565 (2005).
[CrossRef]

T. Yasui and T. Araki, “Sensitive measurement of water content in dry material based on low-frequency terahertz time-domain spectroscopy,” Proc. of SPIE 6024, 60240A, 69–74 (2005).

T. Löffler, T. Hahn, M. Thomson, F. Jacob, and H. G. Roskos, “Large-area electro-optic ZnTe terahertz emitters,” Opt. Express 13, 5353–5362 (2005).
[CrossRef] [PubMed]

T. Löffler, M. Kreß, M. Thomson, T. Hahn, N. Hasegawa, and H. G. Roskos, “Comparative performance of terahertz emitters in amplifier-laser-based systems,” Semicond. Sci. Technol. 20, S134–S141 (2005).
[CrossRef]

2003 (2)

N. Hasegawa, T. Löffler, M. Thomson, and H. G. Roskos, “Remote identification of protrusions and dents on surfaces by THz reflectometry with spatial beam filtering and out-of-focus detection,” Appl. Phys. Lett. 83, 3996–3998 (2003).
[CrossRef]

S. J. Hashemi and W. J. M. Douglas, “Moisture nonuniformity in drying paper: measurement and relation to process parameters,” Drying Technology 21, 329–347 (2003).
[CrossRef]

2002 (1)

K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3006 (2002).
[CrossRef]

2001 (1)

1999 (2)

S. Hadjiloucas, L. S. Karatzas, and W. J. Bowen, “Measurement of leaf water content using terahertz radiation,” IEEE Trans. Microwave Theory Tech. 47, 142–149 (1999).
[CrossRef]

C. Rønne, P.-O. Åstrand, and S. R. Keiding, “THz spectroscopy of liquid H2O and D2O,” Phys. Rev. Lett. 82, 2888–2891 (1999).
[CrossRef]

1996 (2)

J. T. Kindt and C. A. Schmuttenmaer, “Far-infrared dielectric properties of polar liquids probed by femtosecond terahertz pulse spectroscopy,” J. Phys. Chem. 100, 10373–10379 (1996).
[CrossRef]

D. M. Mittleman, R. H. Jacobsen, and M. C. Nuss, “T-Ray imaging,” IEEE J. Sel. Top. Quantum Electron. 2, 679–692 (1996).
[CrossRef]

1984 (1)

R. Boulay, R. Gagnon, D. Rochette, and J. R. Izatt, “Paper sheet moisture measurements in the far infrared,” Int. J. Infrared Millim. Waves 5, 1221–1234 (1984).
[CrossRef]

1980 (1)

J. F. Pugh, “The infrared measurement of surface moisture in paper,” Tappi,  63, 131–134 (1980).

1973 (1)

C.W. Robertson, B. Curnutte, and D Williams, “The infrared spectrum of water,” Mol. Phys. 26, 183–191 (1973).
[CrossRef]

Alcin, A.

T. Löffler, T. May, C. am Weg, A. Alcin, B. Hils, and H. G. Roskos, “Continuous-wave terahertz imaging with a hybrid system,” Appl. Phys. Lett. 90, 091111 1–3 (2007).
[CrossRef]

Allen, S. J.

J. Xu, K. W. Plaxco, and S. J. Allen, “Absorption spectra of liquid water and aqueous buffers between 0.3 and 3.72 THz,” J. Chem. Phys. 124, 036101 1–3 (2006).
[CrossRef] [PubMed]

Araki, T.

T. Yasui and T. Araki, “Sensitive measurement of water content in dry material based on low-frequency terahertz time-domain spectroscopy,” Proc. of SPIE 6024, 60240A, 69–74 (2005).

Åstrand, P.-O.

C. Rønne, P.-O. Åstrand, and S. R. Keiding, “THz spectroscopy of liquid H2O and D2O,” Phys. Rev. Lett. 82, 2888–2891 (1999).
[CrossRef]

Bauer, T.

K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3006 (2002).
[CrossRef]

T. Löffler, T. Bauer, K. Siebert, H. Roskos, A. Fitzgerald, and S. Czasch, “Terahertz dark-field imaging of biomedical tissue,” Opt. Express 9, 616–621 (2001).
[CrossRef] [PubMed]

Boubanga-Tombet, S.

A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H.G. Roskos, “Terahertz imaging with GaAS field-effect transistors,” Electronics Lett. 44, 408–409 (2008).
[CrossRef]

Boulay, R.

R. Boulay, R. Gagnon, D. Rochette, and J. R. Izatt, “Paper sheet moisture measurements in the far infrared,” Int. J. Infrared Millim. Waves 5, 1221–1234 (1984).
[CrossRef]

Bowen, W. J.

S. Hadjiloucas, L. S. Karatzas, and W. J. Bowen, “Measurement of leaf water content using terahertz radiation,” IEEE Trans. Microwave Theory Tech. 47, 142–149 (1999).
[CrossRef]

Coquillat, D.

A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H.G. Roskos, “Terahertz imaging with GaAS field-effect transistors,” Electronics Lett. 44, 408–409 (2008).
[CrossRef]

Curnutte, B.

C.W. Robertson, B. Curnutte, and D Williams, “The infrared spectrum of water,” Mol. Phys. 26, 183–191 (1973).
[CrossRef]

Czasch, S.

K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3006 (2002).
[CrossRef]

T. Löffler, T. Bauer, K. Siebert, H. Roskos, A. Fitzgerald, and S. Czasch, “Terahertz dark-field imaging of biomedical tissue,” Opt. Express 9, 616–621 (2001).
[CrossRef] [PubMed]

Dietlein, C. R.

A. Luukanen, L. Grönberg, P. Helistö, J. S. Penttilä, H. Seppä, H. Sipola, C. R. Dietlein, and E. N. Grossman, “An array of antenna-coupled superconducting microbolometers for passive indoors real-time THz imaging,” Proc. of SPIE Vol. 6212, 62120Y 1–9 (2006).
[CrossRef]

Douglas, W. J. M.

S. J. Hashemi and W. J. M. Douglas, “Moisture nonuniformity in drying paper: measurement and relation to process parameters,” Drying Technology 21, 329–347 (2003).
[CrossRef]

Dyakonova, N.

A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H.G. Roskos, “Terahertz imaging with GaAS field-effect transistors,” Electronics Lett. 44, 408–409 (2008).
[CrossRef]

Fatimy, A. El

A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H.G. Roskos, “Terahertz imaging with GaAS field-effect transistors,” Electronics Lett. 44, 408–409 (2008).
[CrossRef]

Fitzgerald, A.

Gagnon, R.

R. Boulay, R. Gagnon, D. Rochette, and J. R. Izatt, “Paper sheet moisture measurements in the far infrared,” Int. J. Infrared Millim. Waves 5, 1221–1234 (1984).
[CrossRef]

Grönberg, L.

A. Luukanen, L. Grönberg, P. Helistö, J. S. Penttilä, H. Seppä, H. Sipola, C. R. Dietlein, and E. N. Grossman, “An array of antenna-coupled superconducting microbolometers for passive indoors real-time THz imaging,” Proc. of SPIE Vol. 6212, 62120Y 1–9 (2006).
[CrossRef]

Grossman, E. N.

A. Luukanen, L. Grönberg, P. Helistö, J. S. Penttilä, H. Seppä, H. Sipola, C. R. Dietlein, and E. N. Grossman, “An array of antenna-coupled superconducting microbolometers for passive indoors real-time THz imaging,” Proc. of SPIE Vol. 6212, 62120Y 1–9 (2006).
[CrossRef]

Hadjiloucas, S.

S. Hadjiloucas, L. S. Karatzas, and W. J. Bowen, “Measurement of leaf water content using terahertz radiation,” IEEE Trans. Microwave Theory Tech. 47, 142–149 (1999).
[CrossRef]

Hahn, T.

T. Löffler, M. Kreß, M. Thomson, T. Hahn, N. Hasegawa, and H. G. Roskos, “Comparative performance of terahertz emitters in amplifier-laser-based systems,” Semicond. Sci. Technol. 20, S134–S141 (2005).
[CrossRef]

T. Löffler, T. Hahn, M. Thomson, F. Jacob, and H. G. Roskos, “Large-area electro-optic ZnTe terahertz emitters,” Opt. Express 13, 5353–5362 (2005).
[CrossRef] [PubMed]

Hasegawa, N.

T. Löffler, M. Kreß, M. Thomson, T. Hahn, N. Hasegawa, and H. G. Roskos, “Comparative performance of terahertz emitters in amplifier-laser-based systems,” Semicond. Sci. Technol. 20, S134–S141 (2005).
[CrossRef]

N. Hasegawa, T. Löffler, M. Thomson, and H. G. Roskos, “Remote identification of protrusions and dents on surfaces by THz reflectometry with spatial beam filtering and out-of-focus detection,” Appl. Phys. Lett. 83, 3996–3998 (2003).
[CrossRef]

Hashemi, S. J.

S. J. Hashemi and W. J. M. Douglas, “Moisture nonuniformity in drying paper: measurement and relation to process parameters,” Drying Technology 21, 329–347 (2003).
[CrossRef]

Helistö, P.

A. Luukanen, L. Grönberg, P. Helistö, J. S. Penttilä, H. Seppä, H. Sipola, C. R. Dietlein, and E. N. Grossman, “An array of antenna-coupled superconducting microbolometers for passive indoors real-time THz imaging,” Proc. of SPIE Vol. 6212, 62120Y 1–9 (2006).
[CrossRef]

Hils, B.

T. Löffler, T. May, C. am Weg, A. Alcin, B. Hils, and H. G. Roskos, “Continuous-wave terahertz imaging with a hybrid system,” Appl. Phys. Lett. 90, 091111 1–3 (2007).
[CrossRef]

Hu, Q.

A. W. M. Lee, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Real-time imaging using a 4.3-THz quantum cascade laser and a 320×240 microbolometer focal-plane array,” IEEE Photon. Technol. Lett. 18, 1415–1417 (2006).
[CrossRef]

A.W. M. Lee and Q. Hu, “Real-time, continuous-wave terahertz imaging by use of a microbolometer focal-plane array,” Optics Lett. 30, 2563–2565 (2005).
[CrossRef]

Izatt, J. R.

R. Boulay, R. Gagnon, D. Rochette, and J. R. Izatt, “Paper sheet moisture measurements in the far infrared,” Int. J. Infrared Millim. Waves 5, 1221–1234 (1984).
[CrossRef]

Jacob, F.

Jacobsen, R. H.

D. M. Mittleman, R. H. Jacobsen, and M. C. Nuss, “T-Ray imaging,” IEEE J. Sel. Top. Quantum Electron. 2, 679–692 (1996).
[CrossRef]

Karatzas, L. S.

S. Hadjiloucas, L. S. Karatzas, and W. J. Bowen, “Measurement of leaf water content using terahertz radiation,” IEEE Trans. Microwave Theory Tech. 47, 142–149 (1999).
[CrossRef]

Keiding, S. R.

C. Rønne, P.-O. Åstrand, and S. R. Keiding, “THz spectroscopy of liquid H2O and D2O,” Phys. Rev. Lett. 82, 2888–2891 (1999).
[CrossRef]

Kindt, J. T.

J. T. Kindt and C. A. Schmuttenmaer, “Far-infrared dielectric properties of polar liquids probed by femtosecond terahertz pulse spectroscopy,” J. Phys. Chem. 100, 10373–10379 (1996).
[CrossRef]

Knap, W.

A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H.G. Roskos, “Terahertz imaging with GaAS field-effect transistors,” Electronics Lett. 44, 408–409 (2008).
[CrossRef]

Kreß, M.

T. Löffler, M. Kreß, M. Thomson, T. Hahn, N. Hasegawa, and H. G. Roskos, “Comparative performance of terahertz emitters in amplifier-laser-based systems,” Semicond. Sci. Technol. 20, S134–S141 (2005).
[CrossRef]

Kumar, S.

A. W. M. Lee, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Real-time imaging using a 4.3-THz quantum cascade laser and a 320×240 microbolometer focal-plane array,” IEEE Photon. Technol. Lett. 18, 1415–1417 (2006).
[CrossRef]

Kuusela, R.

R. Kuusela, Infrared moisture measurement of paper, board and pulp, Ph.D. Thesis, University of Kuopio, Finland, (1990).

Lee, A. W. M.

A. W. M. Lee, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Real-time imaging using a 4.3-THz quantum cascade laser and a 320×240 microbolometer focal-plane array,” IEEE Photon. Technol. Lett. 18, 1415–1417 (2006).
[CrossRef]

Lee, A.W. M.

A.W. M. Lee and Q. Hu, “Real-time, continuous-wave terahertz imaging by use of a microbolometer focal-plane array,” Optics Lett. 30, 2563–2565 (2005).
[CrossRef]

Leonhardt, R.

K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3006 (2002).
[CrossRef]

Lisauskas, A.

A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H.G. Roskos, “Terahertz imaging with GaAS field-effect transistors,” Electronics Lett. 44, 408–409 (2008).
[CrossRef]

Löffler, T.

T. Löffler, T. May, C. am Weg, A. Alcin, B. Hils, and H. G. Roskos, “Continuous-wave terahertz imaging with a hybrid system,” Appl. Phys. Lett. 90, 091111 1–3 (2007).
[CrossRef]

T. Löffler, M. Kreß, M. Thomson, T. Hahn, N. Hasegawa, and H. G. Roskos, “Comparative performance of terahertz emitters in amplifier-laser-based systems,” Semicond. Sci. Technol. 20, S134–S141 (2005).
[CrossRef]

T. Löffler, T. Hahn, M. Thomson, F. Jacob, and H. G. Roskos, “Large-area electro-optic ZnTe terahertz emitters,” Opt. Express 13, 5353–5362 (2005).
[CrossRef] [PubMed]

N. Hasegawa, T. Löffler, M. Thomson, and H. G. Roskos, “Remote identification of protrusions and dents on surfaces by THz reflectometry with spatial beam filtering and out-of-focus detection,” Appl. Phys. Lett. 83, 3996–3998 (2003).
[CrossRef]

K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3006 (2002).
[CrossRef]

T. Löffler, T. Bauer, K. Siebert, H. Roskos, A. Fitzgerald, and S. Czasch, “Terahertz dark-field imaging of biomedical tissue,” Opt. Express 9, 616–621 (2001).
[CrossRef] [PubMed]

Luukanen, A.

A. Luukanen, L. Grönberg, P. Helistö, J. S. Penttilä, H. Seppä, H. Sipola, C. R. Dietlein, and E. N. Grossman, “An array of antenna-coupled superconducting microbolometers for passive indoors real-time THz imaging,” Proc. of SPIE Vol. 6212, 62120Y 1–9 (2006).
[CrossRef]

May, T.

T. Löffler, T. May, C. am Weg, A. Alcin, B. Hils, and H. G. Roskos, “Continuous-wave terahertz imaging with a hybrid system,” Appl. Phys. Lett. 90, 091111 1–3 (2007).
[CrossRef]

Mittleman, D. M.

D. M. Mittleman, R. H. Jacobsen, and M. C. Nuss, “T-Ray imaging,” IEEE J. Sel. Top. Quantum Electron. 2, 679–692 (1996).
[CrossRef]

Nuss, M. C.

D. M. Mittleman, R. H. Jacobsen, and M. C. Nuss, “T-Ray imaging,” IEEE J. Sel. Top. Quantum Electron. 2, 679–692 (1996).
[CrossRef]

Penttilä, J. S.

A. Luukanen, L. Grönberg, P. Helistö, J. S. Penttilä, H. Seppä, H. Sipola, C. R. Dietlein, and E. N. Grossman, “An array of antenna-coupled superconducting microbolometers for passive indoors real-time THz imaging,” Proc. of SPIE Vol. 6212, 62120Y 1–9 (2006).
[CrossRef]

Plaxco, K. W.

J. Xu, K. W. Plaxco, and S. J. Allen, “Absorption spectra of liquid water and aqueous buffers between 0.3 and 3.72 THz,” J. Chem. Phys. 124, 036101 1–3 (2006).
[CrossRef] [PubMed]

Pugh, J. F.

J. F. Pugh, “The infrared measurement of surface moisture in paper,” Tappi,  63, 131–134 (1980).

Quast, H.

K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3006 (2002).
[CrossRef]

Reno, J. L.

A. W. M. Lee, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Real-time imaging using a 4.3-THz quantum cascade laser and a 320×240 microbolometer focal-plane array,” IEEE Photon. Technol. Lett. 18, 1415–1417 (2006).
[CrossRef]

Robertson, C.W.

C.W. Robertson, B. Curnutte, and D Williams, “The infrared spectrum of water,” Mol. Phys. 26, 183–191 (1973).
[CrossRef]

Rochette, D.

R. Boulay, R. Gagnon, D. Rochette, and J. R. Izatt, “Paper sheet moisture measurements in the far infrared,” Int. J. Infrared Millim. Waves 5, 1221–1234 (1984).
[CrossRef]

Rønne, C.

C. Rønne, P.-O. Åstrand, and S. R. Keiding, “THz spectroscopy of liquid H2O and D2O,” Phys. Rev. Lett. 82, 2888–2891 (1999).
[CrossRef]

Roskos, H.

Roskos, H. G.

T. Löffler, T. May, C. am Weg, A. Alcin, B. Hils, and H. G. Roskos, “Continuous-wave terahertz imaging with a hybrid system,” Appl. Phys. Lett. 90, 091111 1–3 (2007).
[CrossRef]

T. Löffler, M. Kreß, M. Thomson, T. Hahn, N. Hasegawa, and H. G. Roskos, “Comparative performance of terahertz emitters in amplifier-laser-based systems,” Semicond. Sci. Technol. 20, S134–S141 (2005).
[CrossRef]

T. Löffler, T. Hahn, M. Thomson, F. Jacob, and H. G. Roskos, “Large-area electro-optic ZnTe terahertz emitters,” Opt. Express 13, 5353–5362 (2005).
[CrossRef] [PubMed]

N. Hasegawa, T. Löffler, M. Thomson, and H. G. Roskos, “Remote identification of protrusions and dents on surfaces by THz reflectometry with spatial beam filtering and out-of-focus detection,” Appl. Phys. Lett. 83, 3996–3998 (2003).
[CrossRef]

K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3006 (2002).
[CrossRef]

Roskos, H.G.

A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H.G. Roskos, “Terahertz imaging with GaAS field-effect transistors,” Electronics Lett. 44, 408–409 (2008).
[CrossRef]

Schmuttenmaer, C. A.

J. T. Kindt and C. A. Schmuttenmaer, “Far-infrared dielectric properties of polar liquids probed by femtosecond terahertz pulse spectroscopy,” J. Phys. Chem. 100, 10373–10379 (1996).
[CrossRef]

Seppä, H.

A. Luukanen, L. Grönberg, P. Helistö, J. S. Penttilä, H. Seppä, H. Sipola, C. R. Dietlein, and E. N. Grossman, “An array of antenna-coupled superconducting microbolometers for passive indoors real-time THz imaging,” Proc. of SPIE Vol. 6212, 62120Y 1–9 (2006).
[CrossRef]

Siebert, K.

Siebert, K. J.

K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3006 (2002).
[CrossRef]

Sipola, H.

A. Luukanen, L. Grönberg, P. Helistö, J. S. Penttilä, H. Seppä, H. Sipola, C. R. Dietlein, and E. N. Grossman, “An array of antenna-coupled superconducting microbolometers for passive indoors real-time THz imaging,” Proc. of SPIE Vol. 6212, 62120Y 1–9 (2006).
[CrossRef]

Teppe, F.

A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H.G. Roskos, “Terahertz imaging with GaAS field-effect transistors,” Electronics Lett. 44, 408–409 (2008).
[CrossRef]

Thomson, M.

T. Löffler, M. Kreß, M. Thomson, T. Hahn, N. Hasegawa, and H. G. Roskos, “Comparative performance of terahertz emitters in amplifier-laser-based systems,” Semicond. Sci. Technol. 20, S134–S141 (2005).
[CrossRef]

T. Löffler, T. Hahn, M. Thomson, F. Jacob, and H. G. Roskos, “Large-area electro-optic ZnTe terahertz emitters,” Opt. Express 13, 5353–5362 (2005).
[CrossRef] [PubMed]

N. Hasegawa, T. Löffler, M. Thomson, and H. G. Roskos, “Remote identification of protrusions and dents on surfaces by THz reflectometry with spatial beam filtering and out-of-focus detection,” Appl. Phys. Lett. 83, 3996–3998 (2003).
[CrossRef]

K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3006 (2002).
[CrossRef]

von Spiegel, W.

A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H.G. Roskos, “Terahertz imaging with GaAS field-effect transistors,” Electronics Lett. 44, 408–409 (2008).
[CrossRef]

Weg, C. am

T. Löffler, T. May, C. am Weg, A. Alcin, B. Hils, and H. G. Roskos, “Continuous-wave terahertz imaging with a hybrid system,” Appl. Phys. Lett. 90, 091111 1–3 (2007).
[CrossRef]

Williams, B. S.

A. W. M. Lee, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Real-time imaging using a 4.3-THz quantum cascade laser and a 320×240 microbolometer focal-plane array,” IEEE Photon. Technol. Lett. 18, 1415–1417 (2006).
[CrossRef]

Williams, D

C.W. Robertson, B. Curnutte, and D Williams, “The infrared spectrum of water,” Mol. Phys. 26, 183–191 (1973).
[CrossRef]

Xu, J.

J. Xu, K. W. Plaxco, and S. J. Allen, “Absorption spectra of liquid water and aqueous buffers between 0.3 and 3.72 THz,” J. Chem. Phys. 124, 036101 1–3 (2006).
[CrossRef] [PubMed]

Yasui, T.

T. Yasui and T. Araki, “Sensitive measurement of water content in dry material based on low-frequency terahertz time-domain spectroscopy,” Proc. of SPIE 6024, 60240A, 69–74 (2005).

Appl. Phys. Lett. (3)

K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3006 (2002).
[CrossRef]

N. Hasegawa, T. Löffler, M. Thomson, and H. G. Roskos, “Remote identification of protrusions and dents on surfaces by THz reflectometry with spatial beam filtering and out-of-focus detection,” Appl. Phys. Lett. 83, 3996–3998 (2003).
[CrossRef]

T. Löffler, T. May, C. am Weg, A. Alcin, B. Hils, and H. G. Roskos, “Continuous-wave terahertz imaging with a hybrid system,” Appl. Phys. Lett. 90, 091111 1–3 (2007).
[CrossRef]

Drying Technology (1)

S. J. Hashemi and W. J. M. Douglas, “Moisture nonuniformity in drying paper: measurement and relation to process parameters,” Drying Technology 21, 329–347 (2003).
[CrossRef]

Electronics Lett. (1)

A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H.G. Roskos, “Terahertz imaging with GaAS field-effect transistors,” Electronics Lett. 44, 408–409 (2008).
[CrossRef]

IEEE J. Sel. Top. Quantum Electron. (1)

D. M. Mittleman, R. H. Jacobsen, and M. C. Nuss, “T-Ray imaging,” IEEE J. Sel. Top. Quantum Electron. 2, 679–692 (1996).
[CrossRef]

IEEE Photon. Technol. Lett. (1)

A. W. M. Lee, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Real-time imaging using a 4.3-THz quantum cascade laser and a 320×240 microbolometer focal-plane array,” IEEE Photon. Technol. Lett. 18, 1415–1417 (2006).
[CrossRef]

IEEE Trans. Microwave Theory Tech. (1)

S. Hadjiloucas, L. S. Karatzas, and W. J. Bowen, “Measurement of leaf water content using terahertz radiation,” IEEE Trans. Microwave Theory Tech. 47, 142–149 (1999).
[CrossRef]

Int. J. Infrared Millim. Waves (1)

R. Boulay, R. Gagnon, D. Rochette, and J. R. Izatt, “Paper sheet moisture measurements in the far infrared,” Int. J. Infrared Millim. Waves 5, 1221–1234 (1984).
[CrossRef]

J. Chem. Phys. (1)

J. Xu, K. W. Plaxco, and S. J. Allen, “Absorption spectra of liquid water and aqueous buffers between 0.3 and 3.72 THz,” J. Chem. Phys. 124, 036101 1–3 (2006).
[CrossRef] [PubMed]

J. Phys. Chem. (1)

J. T. Kindt and C. A. Schmuttenmaer, “Far-infrared dielectric properties of polar liquids probed by femtosecond terahertz pulse spectroscopy,” J. Phys. Chem. 100, 10373–10379 (1996).
[CrossRef]

Mol. Phys. (1)

C.W. Robertson, B. Curnutte, and D Williams, “The infrared spectrum of water,” Mol. Phys. 26, 183–191 (1973).
[CrossRef]

Opt. Express (2)

Optics Lett. (1)

A.W. M. Lee and Q. Hu, “Real-time, continuous-wave terahertz imaging by use of a microbolometer focal-plane array,” Optics Lett. 30, 2563–2565 (2005).
[CrossRef]

Phys. Rev. Lett. (1)

C. Rønne, P.-O. Åstrand, and S. R. Keiding, “THz spectroscopy of liquid H2O and D2O,” Phys. Rev. Lett. 82, 2888–2891 (1999).
[CrossRef]

Proc. of SPIE (1)

T. Yasui and T. Araki, “Sensitive measurement of water content in dry material based on low-frequency terahertz time-domain spectroscopy,” Proc. of SPIE 6024, 60240A, 69–74 (2005).

Proc. of SPIE Vol. (1)

A. Luukanen, L. Grönberg, P. Helistö, J. S. Penttilä, H. Seppä, H. Sipola, C. R. Dietlein, and E. N. Grossman, “An array of antenna-coupled superconducting microbolometers for passive indoors real-time THz imaging,” Proc. of SPIE Vol. 6212, 62120Y 1–9 (2006).
[CrossRef]

Semicond. Sci. Technol. (1)

T. Löffler, M. Kreß, M. Thomson, T. Hahn, N. Hasegawa, and H. G. Roskos, “Comparative performance of terahertz emitters in amplifier-laser-based systems,” Semicond. Sci. Technol. 20, S134–S141 (2005).
[CrossRef]

Tappi (1)

J. F. Pugh, “The infrared measurement of surface moisture in paper,” Tappi,  63, 131–134 (1980).

Other (1)

R. Kuusela, Infrared moisture measurement of paper, board and pulp, Ph.D. Thesis, University of Kuopio, Finland, (1990).

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Figures (4)

Fig. 1.
Fig. 1.

Evolution of the frequency-dependent relative optical-density (relative to the measurement after 66 minutes) of a moist paper sheet drying. The spectra are calculated from THz transmission transients such as those shown in the inset. For the evaluation of the spectra, the full scan range of the transients of 20 ps is taken into account and not only the time window of 7 ps presented in the inset.

Fig. 2.
Fig. 2.

Temporal evolution of the relative optical-density at 0.6 THz (×, lower panel) and of the phase change (×, upper panel, both from the TDS measurements, relative to the measurement after 66 minutes) compared to a typical drying curve of a different sample(—, gravimetrically determined with a precision scale).

Fig. 3.
Fig. 3.

Phase change (∘, upper panel) and relative optical-density (∘, lower panel) relative to the dry paper sample shown as a function of the water content in the moist sample. The linear regressions (—) linking the water content to the THz relative optical-density and phase shift, respectively, are also shown in the graphs.

Fig. 4.
Fig. 4.

Transmission image through 80g/m2 paper at 0.6 THz showing the distribution of water in terms of moisture content (absolute values of kg water per kg fiber)

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