Abstract

We report on the implementation of a confocal microscopy system based on a 2.9 THz quantum cascade laser source. Lateral and axial resolutions better than 70 μm and 400 μm, respectively, are achieved, with a large contrast enhancement compared to the non-confocal arrangement. The capability of resolving overlapping objects lying on different longitudinal planes is also clearly demonstrated.

© 2012 Optical Society of America

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  1. M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics 1, 97–105 (2007)
    [CrossRef]
  2. Y. Lee, Principles of Terahertz Science and Technology (Springer, 2009).
  3. W. Chan, J. Deibel, and D. Mittleman, “Imaging with terahertz radiation,” Rep. Prog. Phys. 70, 1325–1379 (2007).
    [CrossRef]
  4. A. Fitzgerald, V. Wallace, M. Jimenez-Linan, L. Bobrow, R. Pye, A. Purushotham, and D. Arnone, “Terahertz pulsed imaging of human breast tumors,” Radiology 239, 533–540 (2006).
    [CrossRef] [PubMed]
  5. B. Hu and M. Nuss, “Imaging with terahertz waves,” Opt. Lett. 20, 1716–1718 (1995).
    [CrossRef] [PubMed]
  6. B. Ferguson and X. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1, 26–33 (2002).
    [CrossRef]
  7. D. Arnone, C. Ciesla, and M. Pepper, “Terahertz imaging comes into view,” Phys. World 4, 35–40 (2000).
  8. D. Mittleman, R. Jacobsen, and M. Nuss, “T-ray imaging,” IEEE J. Sel. Top. Quantum Electron. 2, 679–692 (1996).
    [CrossRef]
  9. D. Mittleman, M. Gupta, R. Neelamani, R. Baraniuk, J. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B-Lasers O. 68, 1085–1094 (1999).
    [CrossRef]
  10. V. Wallace, E. MacPherson, J. Zeitler, and C. Reid, “Three-dimensional imaging of optically opaque materials using nonionizing terahertz radiation,” J. Opt. Soc. Am. A 25, 3120–3133 (2008).
    [CrossRef]
  11. J. Zeitler, P. Taday, D. Newnham, M. Pepper, K. Gordon, and T. Rades, “Terahertz pulsed spectroscopy and imaging in the pharmaceutical setting-a review,” J. Pharm. Pharmacol. 59, 209–223 (2007).
    [CrossRef] [PubMed]
  12. J. Zeitler, Y. Shen, C. Baker, P. Taday, M. Pepper, and T. Rades, “Analysis of coating structures and interfaces in solid oral dosage forms by three dimensional terahertz pulsed imaging,” J. Pharm. Sci. 96, 330–340 (2007).
    [CrossRef]
  13. I. Sinka, S. Burch, J. Tweed, and J. Cunningham, “Measurement of density variations in tablets using x-ray computed tomography,” Int. J. Pharm. 271, 215–224 (2004).
    [CrossRef] [PubMed]
  14. M. Minsky, “Memoir on inventing the confocal scanning microscope,” Scanning 10, 128–138 (1988).
    [CrossRef]
  15. T. Corle and G. Kino, Confocal Scanning Optical Microscopy and Related Imaging Systems (Academic Press, 1996).
  16. R. Webb, “Confocal optical microscopy,” Rep. Prog.Phys. 59, 427–471 (1996).
    [CrossRef]
  17. N. Zinovev and A. Andrianov, “Confocal terahertz imaging,” Appl. Phys. Lett. 95, 011114 (2009).
    [CrossRef]
  18. M. Salhi, I. Pupeza, and M. Koch, “Confocal thz laser microscope,” J. Infrared Millim. Te. 31, 358–366 (2010).
  19. R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156–159 (2002).
    [CrossRef] [PubMed]
  20. R. Degl’Innocenti, M. Montinaro, J. Xu, V. Piazza, P. Pingue, A. Tredicucci, F. Beltram, H. E. Beere, and D. A. Ritchie, “Differential Near-Field Scanning Optical Microscopy with THz quantum cascade laser sources,” Opt. Express 17, 23785–23792 (2009).
    [CrossRef]
  21. T. Losco, J. Xu, R. Green, A. Tredicucci, H. E. Beere, and D. A. Ritchie, “Thz quantum cascade designs for optimized injection,” Physica E 40, 2207 – 2209 (2008).
    [CrossRef]
  22. A. Siegman, M. Sasnett, and T. Johnston, “Choice of clip levels for beam width measurements using knife-edge techniques,” IEEE J. Quantum Elect. 27, 1098–1104 (1991).
    [CrossRef]
  23. G. Xu, R. Colombelli, S. P. Khanna, A. Belarouci, X. Letartre, L. Li, E. H. Linfield, A. G. Davies, H. E. Beere, and D. A. Ritchie, “Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures,” Nature Commun. 3, 952 (2012).
    [CrossRef]

2012 (1)

G. Xu, R. Colombelli, S. P. Khanna, A. Belarouci, X. Letartre, L. Li, E. H. Linfield, A. G. Davies, H. E. Beere, and D. A. Ritchie, “Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures,” Nature Commun. 3, 952 (2012).
[CrossRef]

2010 (1)

M. Salhi, I. Pupeza, and M. Koch, “Confocal thz laser microscope,” J. Infrared Millim. Te. 31, 358–366 (2010).

2009 (2)

2008 (2)

V. Wallace, E. MacPherson, J. Zeitler, and C. Reid, “Three-dimensional imaging of optically opaque materials using nonionizing terahertz radiation,” J. Opt. Soc. Am. A 25, 3120–3133 (2008).
[CrossRef]

T. Losco, J. Xu, R. Green, A. Tredicucci, H. E. Beere, and D. A. Ritchie, “Thz quantum cascade designs for optimized injection,” Physica E 40, 2207 – 2209 (2008).
[CrossRef]

2007 (4)

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

W. Chan, J. Deibel, and D. Mittleman, “Imaging with terahertz radiation,” Rep. Prog. Phys. 70, 1325–1379 (2007).
[CrossRef]

J. Zeitler, P. Taday, D. Newnham, M. Pepper, K. Gordon, and T. Rades, “Terahertz pulsed spectroscopy and imaging in the pharmaceutical setting-a review,” J. Pharm. Pharmacol. 59, 209–223 (2007).
[CrossRef] [PubMed]

J. Zeitler, Y. Shen, C. Baker, P. Taday, M. Pepper, and T. Rades, “Analysis of coating structures and interfaces in solid oral dosage forms by three dimensional terahertz pulsed imaging,” J. Pharm. Sci. 96, 330–340 (2007).
[CrossRef]

2006 (1)

A. Fitzgerald, V. Wallace, M. Jimenez-Linan, L. Bobrow, R. Pye, A. Purushotham, and D. Arnone, “Terahertz pulsed imaging of human breast tumors,” Radiology 239, 533–540 (2006).
[CrossRef] [PubMed]

2004 (1)

I. Sinka, S. Burch, J. Tweed, and J. Cunningham, “Measurement of density variations in tablets using x-ray computed tomography,” Int. J. Pharm. 271, 215–224 (2004).
[CrossRef] [PubMed]

2002 (2)

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

R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156–159 (2002).
[CrossRef] [PubMed]

2000 (1)

D. Arnone, C. Ciesla, and M. Pepper, “Terahertz imaging comes into view,” Phys. World 4, 35–40 (2000).

1999 (1)

D. Mittleman, M. Gupta, R. Neelamani, R. Baraniuk, J. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B-Lasers O. 68, 1085–1094 (1999).
[CrossRef]

1996 (2)

R. Webb, “Confocal optical microscopy,” Rep. Prog.Phys. 59, 427–471 (1996).
[CrossRef]

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

1995 (1)

1991 (1)

A. Siegman, M. Sasnett, and T. Johnston, “Choice of clip levels for beam width measurements using knife-edge techniques,” IEEE J. Quantum Elect. 27, 1098–1104 (1991).
[CrossRef]

1988 (1)

M. Minsky, “Memoir on inventing the confocal scanning microscope,” Scanning 10, 128–138 (1988).
[CrossRef]

Andrianov, A.

N. Zinovev and A. Andrianov, “Confocal terahertz imaging,” Appl. Phys. Lett. 95, 011114 (2009).
[CrossRef]

Arnone, D.

A. Fitzgerald, V. Wallace, M. Jimenez-Linan, L. Bobrow, R. Pye, A. Purushotham, and D. Arnone, “Terahertz pulsed imaging of human breast tumors,” Radiology 239, 533–540 (2006).
[CrossRef] [PubMed]

D. Arnone, C. Ciesla, and M. Pepper, “Terahertz imaging comes into view,” Phys. World 4, 35–40 (2000).

Baker, C.

J. Zeitler, Y. Shen, C. Baker, P. Taday, M. Pepper, and T. Rades, “Analysis of coating structures and interfaces in solid oral dosage forms by three dimensional terahertz pulsed imaging,” J. Pharm. Sci. 96, 330–340 (2007).
[CrossRef]

Baraniuk, R.

D. Mittleman, M. Gupta, R. Neelamani, R. Baraniuk, J. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B-Lasers O. 68, 1085–1094 (1999).
[CrossRef]

Beere, H.

R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156–159 (2002).
[CrossRef] [PubMed]

Beere, H. E.

G. Xu, R. Colombelli, S. P. Khanna, A. Belarouci, X. Letartre, L. Li, E. H. Linfield, A. G. Davies, H. E. Beere, and D. A. Ritchie, “Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures,” Nature Commun. 3, 952 (2012).
[CrossRef]

R. Degl’Innocenti, M. Montinaro, J. Xu, V. Piazza, P. Pingue, A. Tredicucci, F. Beltram, H. E. Beere, and D. A. Ritchie, “Differential Near-Field Scanning Optical Microscopy with THz quantum cascade laser sources,” Opt. Express 17, 23785–23792 (2009).
[CrossRef]

T. Losco, J. Xu, R. Green, A. Tredicucci, H. E. Beere, and D. A. Ritchie, “Thz quantum cascade designs for optimized injection,” Physica E 40, 2207 – 2209 (2008).
[CrossRef]

Belarouci, A.

G. Xu, R. Colombelli, S. P. Khanna, A. Belarouci, X. Letartre, L. Li, E. H. Linfield, A. G. Davies, H. E. Beere, and D. A. Ritchie, “Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures,” Nature Commun. 3, 952 (2012).
[CrossRef]

Beltram, F.

R. Degl’Innocenti, M. Montinaro, J. Xu, V. Piazza, P. Pingue, A. Tredicucci, F. Beltram, H. E. Beere, and D. A. Ritchie, “Differential Near-Field Scanning Optical Microscopy with THz quantum cascade laser sources,” Opt. Express 17, 23785–23792 (2009).
[CrossRef]

R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156–159 (2002).
[CrossRef] [PubMed]

Bobrow, L.

A. Fitzgerald, V. Wallace, M. Jimenez-Linan, L. Bobrow, R. Pye, A. Purushotham, and D. Arnone, “Terahertz pulsed imaging of human breast tumors,” Radiology 239, 533–540 (2006).
[CrossRef] [PubMed]

Burch, S.

I. Sinka, S. Burch, J. Tweed, and J. Cunningham, “Measurement of density variations in tablets using x-ray computed tomography,” Int. J. Pharm. 271, 215–224 (2004).
[CrossRef] [PubMed]

Chan, W.

W. Chan, J. Deibel, and D. Mittleman, “Imaging with terahertz radiation,” Rep. Prog. Phys. 70, 1325–1379 (2007).
[CrossRef]

Ciesla, C.

D. Arnone, C. Ciesla, and M. Pepper, “Terahertz imaging comes into view,” Phys. World 4, 35–40 (2000).

Colombelli, R.

G. Xu, R. Colombelli, S. P. Khanna, A. Belarouci, X. Letartre, L. Li, E. H. Linfield, A. G. Davies, H. E. Beere, and D. A. Ritchie, “Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures,” Nature Commun. 3, 952 (2012).
[CrossRef]

Corle, T.

T. Corle and G. Kino, Confocal Scanning Optical Microscopy and Related Imaging Systems (Academic Press, 1996).

Cunningham, J.

I. Sinka, S. Burch, J. Tweed, and J. Cunningham, “Measurement of density variations in tablets using x-ray computed tomography,” Int. J. Pharm. 271, 215–224 (2004).
[CrossRef] [PubMed]

Davies, A.

R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156–159 (2002).
[CrossRef] [PubMed]

Davies, A. G.

G. Xu, R. Colombelli, S. P. Khanna, A. Belarouci, X. Letartre, L. Li, E. H. Linfield, A. G. Davies, H. E. Beere, and D. A. Ritchie, “Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures,” Nature Commun. 3, 952 (2012).
[CrossRef]

Degl’Innocenti, R.

Deibel, J.

W. Chan, J. Deibel, and D. Mittleman, “Imaging with terahertz radiation,” Rep. Prog. Phys. 70, 1325–1379 (2007).
[CrossRef]

Ferguson, B.

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

Fitzgerald, A.

A. Fitzgerald, V. Wallace, M. Jimenez-Linan, L. Bobrow, R. Pye, A. Purushotham, and D. Arnone, “Terahertz pulsed imaging of human breast tumors,” Radiology 239, 533–540 (2006).
[CrossRef] [PubMed]

Gordon, K.

J. Zeitler, P. Taday, D. Newnham, M. Pepper, K. Gordon, and T. Rades, “Terahertz pulsed spectroscopy and imaging in the pharmaceutical setting-a review,” J. Pharm. Pharmacol. 59, 209–223 (2007).
[CrossRef] [PubMed]

Green, R.

T. Losco, J. Xu, R. Green, A. Tredicucci, H. E. Beere, and D. A. Ritchie, “Thz quantum cascade designs for optimized injection,” Physica E 40, 2207 – 2209 (2008).
[CrossRef]

Gupta, M.

D. Mittleman, M. Gupta, R. Neelamani, R. Baraniuk, J. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B-Lasers O. 68, 1085–1094 (1999).
[CrossRef]

Hu, B.

Iotti, R.

R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156–159 (2002).
[CrossRef] [PubMed]

Jacobsen, R.

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

Jimenez-Linan, M.

A. Fitzgerald, V. Wallace, M. Jimenez-Linan, L. Bobrow, R. Pye, A. Purushotham, and D. Arnone, “Terahertz pulsed imaging of human breast tumors,” Radiology 239, 533–540 (2006).
[CrossRef] [PubMed]

Johnston, T.

A. Siegman, M. Sasnett, and T. Johnston, “Choice of clip levels for beam width measurements using knife-edge techniques,” IEEE J. Quantum Elect. 27, 1098–1104 (1991).
[CrossRef]

Khanna, S. P.

G. Xu, R. Colombelli, S. P. Khanna, A. Belarouci, X. Letartre, L. Li, E. H. Linfield, A. G. Davies, H. E. Beere, and D. A. Ritchie, “Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures,” Nature Commun. 3, 952 (2012).
[CrossRef]

Kino, G.

T. Corle and G. Kino, Confocal Scanning Optical Microscopy and Related Imaging Systems (Academic Press, 1996).

Koch, M.

M. Salhi, I. Pupeza, and M. Koch, “Confocal thz laser microscope,” J. Infrared Millim. Te. 31, 358–366 (2010).

D. Mittleman, M. Gupta, R. Neelamani, R. Baraniuk, J. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B-Lasers O. 68, 1085–1094 (1999).
[CrossRef]

Köhler, R.

R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156–159 (2002).
[CrossRef] [PubMed]

Lee, Y.

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

Letartre, X.

G. Xu, R. Colombelli, S. P. Khanna, A. Belarouci, X. Letartre, L. Li, E. H. Linfield, A. G. Davies, H. E. Beere, and D. A. Ritchie, “Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures,” Nature Commun. 3, 952 (2012).
[CrossRef]

Li, L.

G. Xu, R. Colombelli, S. P. Khanna, A. Belarouci, X. Letartre, L. Li, E. H. Linfield, A. G. Davies, H. E. Beere, and D. A. Ritchie, “Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures,” Nature Commun. 3, 952 (2012).
[CrossRef]

Linfield, E.

R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156–159 (2002).
[CrossRef] [PubMed]

Linfield, E. H.

G. Xu, R. Colombelli, S. P. Khanna, A. Belarouci, X. Letartre, L. Li, E. H. Linfield, A. G. Davies, H. E. Beere, and D. A. Ritchie, “Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures,” Nature Commun. 3, 952 (2012).
[CrossRef]

Losco, T.

T. Losco, J. Xu, R. Green, A. Tredicucci, H. E. Beere, and D. A. Ritchie, “Thz quantum cascade designs for optimized injection,” Physica E 40, 2207 – 2209 (2008).
[CrossRef]

MacPherson, E.

Minsky, M.

M. Minsky, “Memoir on inventing the confocal scanning microscope,” Scanning 10, 128–138 (1988).
[CrossRef]

Mittleman, D.

W. Chan, J. Deibel, and D. Mittleman, “Imaging with terahertz radiation,” Rep. Prog. Phys. 70, 1325–1379 (2007).
[CrossRef]

D. Mittleman, M. Gupta, R. Neelamani, R. Baraniuk, J. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B-Lasers O. 68, 1085–1094 (1999).
[CrossRef]

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

Montinaro, M.

Neelamani, R.

D. Mittleman, M. Gupta, R. Neelamani, R. Baraniuk, J. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B-Lasers O. 68, 1085–1094 (1999).
[CrossRef]

Newnham, D.

J. Zeitler, P. Taday, D. Newnham, M. Pepper, K. Gordon, and T. Rades, “Terahertz pulsed spectroscopy and imaging in the pharmaceutical setting-a review,” J. Pharm. Pharmacol. 59, 209–223 (2007).
[CrossRef] [PubMed]

Nuss, M.

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

B. Hu and M. Nuss, “Imaging with terahertz waves,” Opt. Lett. 20, 1716–1718 (1995).
[CrossRef] [PubMed]

Pepper, M.

J. Zeitler, P. Taday, D. Newnham, M. Pepper, K. Gordon, and T. Rades, “Terahertz pulsed spectroscopy and imaging in the pharmaceutical setting-a review,” J. Pharm. Pharmacol. 59, 209–223 (2007).
[CrossRef] [PubMed]

J. Zeitler, Y. Shen, C. Baker, P. Taday, M. Pepper, and T. Rades, “Analysis of coating structures and interfaces in solid oral dosage forms by three dimensional terahertz pulsed imaging,” J. Pharm. Sci. 96, 330–340 (2007).
[CrossRef]

D. Arnone, C. Ciesla, and M. Pepper, “Terahertz imaging comes into view,” Phys. World 4, 35–40 (2000).

Piazza, V.

Pingue, P.

Pupeza, I.

M. Salhi, I. Pupeza, and M. Koch, “Confocal thz laser microscope,” J. Infrared Millim. Te. 31, 358–366 (2010).

Purushotham, A.

A. Fitzgerald, V. Wallace, M. Jimenez-Linan, L. Bobrow, R. Pye, A. Purushotham, and D. Arnone, “Terahertz pulsed imaging of human breast tumors,” Radiology 239, 533–540 (2006).
[CrossRef] [PubMed]

Pye, R.

A. Fitzgerald, V. Wallace, M. Jimenez-Linan, L. Bobrow, R. Pye, A. Purushotham, and D. Arnone, “Terahertz pulsed imaging of human breast tumors,” Radiology 239, 533–540 (2006).
[CrossRef] [PubMed]

Rades, T.

J. Zeitler, Y. Shen, C. Baker, P. Taday, M. Pepper, and T. Rades, “Analysis of coating structures and interfaces in solid oral dosage forms by three dimensional terahertz pulsed imaging,” J. Pharm. Sci. 96, 330–340 (2007).
[CrossRef]

J. Zeitler, P. Taday, D. Newnham, M. Pepper, K. Gordon, and T. Rades, “Terahertz pulsed spectroscopy and imaging in the pharmaceutical setting-a review,” J. Pharm. Pharmacol. 59, 209–223 (2007).
[CrossRef] [PubMed]

Reid, C.

Ritchie, D.

R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156–159 (2002).
[CrossRef] [PubMed]

Ritchie, D. A.

G. Xu, R. Colombelli, S. P. Khanna, A. Belarouci, X. Letartre, L. Li, E. H. Linfield, A. G. Davies, H. E. Beere, and D. A. Ritchie, “Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures,” Nature Commun. 3, 952 (2012).
[CrossRef]

R. Degl’Innocenti, M. Montinaro, J. Xu, V. Piazza, P. Pingue, A. Tredicucci, F. Beltram, H. E. Beere, and D. A. Ritchie, “Differential Near-Field Scanning Optical Microscopy with THz quantum cascade laser sources,” Opt. Express 17, 23785–23792 (2009).
[CrossRef]

T. Losco, J. Xu, R. Green, A. Tredicucci, H. E. Beere, and D. A. Ritchie, “Thz quantum cascade designs for optimized injection,” Physica E 40, 2207 – 2209 (2008).
[CrossRef]

Rossi, F.

R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156–159 (2002).
[CrossRef] [PubMed]

Rudd, J.

D. Mittleman, M. Gupta, R. Neelamani, R. Baraniuk, J. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B-Lasers O. 68, 1085–1094 (1999).
[CrossRef]

Salhi, M.

M. Salhi, I. Pupeza, and M. Koch, “Confocal thz laser microscope,” J. Infrared Millim. Te. 31, 358–366 (2010).

Sasnett, M.

A. Siegman, M. Sasnett, and T. Johnston, “Choice of clip levels for beam width measurements using knife-edge techniques,” IEEE J. Quantum Elect. 27, 1098–1104 (1991).
[CrossRef]

Shen, Y.

J. Zeitler, Y. Shen, C. Baker, P. Taday, M. Pepper, and T. Rades, “Analysis of coating structures and interfaces in solid oral dosage forms by three dimensional terahertz pulsed imaging,” J. Pharm. Sci. 96, 330–340 (2007).
[CrossRef]

Siegman, A.

A. Siegman, M. Sasnett, and T. Johnston, “Choice of clip levels for beam width measurements using knife-edge techniques,” IEEE J. Quantum Elect. 27, 1098–1104 (1991).
[CrossRef]

Sinka, I.

I. Sinka, S. Burch, J. Tweed, and J. Cunningham, “Measurement of density variations in tablets using x-ray computed tomography,” Int. J. Pharm. 271, 215–224 (2004).
[CrossRef] [PubMed]

Taday, P.

J. Zeitler, P. Taday, D. Newnham, M. Pepper, K. Gordon, and T. Rades, “Terahertz pulsed spectroscopy and imaging in the pharmaceutical setting-a review,” J. Pharm. Pharmacol. 59, 209–223 (2007).
[CrossRef] [PubMed]

J. Zeitler, Y. Shen, C. Baker, P. Taday, M. Pepper, and T. Rades, “Analysis of coating structures and interfaces in solid oral dosage forms by three dimensional terahertz pulsed imaging,” J. Pharm. Sci. 96, 330–340 (2007).
[CrossRef]

Tonouchi, M.

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

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V. Wallace, E. MacPherson, J. Zeitler, and C. Reid, “Three-dimensional imaging of optically opaque materials using nonionizing terahertz radiation,” J. Opt. Soc. Am. A 25, 3120–3133 (2008).
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A. Fitzgerald, V. Wallace, M. Jimenez-Linan, L. Bobrow, R. Pye, A. Purushotham, and D. Arnone, “Terahertz pulsed imaging of human breast tumors,” Radiology 239, 533–540 (2006).
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M. Salhi, I. Pupeza, and M. Koch, “Confocal thz laser microscope,” J. Infrared Millim. Te. 31, 358–366 (2010).

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[CrossRef] [PubMed]

J. Pharm. Sci. (1)

J. Zeitler, Y. Shen, C. Baker, P. Taday, M. Pepper, and T. Rades, “Analysis of coating structures and interfaces in solid oral dosage forms by three dimensional terahertz pulsed imaging,” J. Pharm. Sci. 96, 330–340 (2007).
[CrossRef]

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

Nat. Photonics (1)

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

Nature (1)

R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156–159 (2002).
[CrossRef] [PubMed]

Nature Commun. (1)

G. Xu, R. Colombelli, S. P. Khanna, A. Belarouci, X. Letartre, L. Li, E. H. Linfield, A. G. Davies, H. E. Beere, and D. A. Ritchie, “Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures,” Nature Commun. 3, 952 (2012).
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[CrossRef]

Radiology (1)

A. Fitzgerald, V. Wallace, M. Jimenez-Linan, L. Bobrow, R. Pye, A. Purushotham, and D. Arnone, “Terahertz pulsed imaging of human breast tumors,” Radiology 239, 533–540 (2006).
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W. Chan, J. Deibel, and D. Mittleman, “Imaging with terahertz radiation,” Rep. Prog. Phys. 70, 1325–1379 (2007).
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Figures (5)

Fig. 1
Fig. 1

Schematics of the final setup of the microscope. The lenses inserted are of different types: three Picarin lenses (L1, L2, L4 with NA ∼ 0.447), two Silicon lenses (S1, S2 with NA ∼ 0.707), a TPX lens (TPX with 0.447 numerical aperture) and two parabolic mirrors (M2 with NA ∼ 0.447 and M4 with NA ∼ 0.167 for a better coupling with the Si bolometer entrance window). The illustration highlights the differences among the various employed optics.

Fig. 2
Fig. 2

Far field profile of the laser beam (a) at the output of the QCL (without filtering) and (b) after passing through the 200 μm pinhole: the measurements were performed by means of a pyroelectric detector mounted on a x–y moving stage at a distance of ∼ 2 cm; vertical (c) and horizontal (d) cross sections of the beam profile after the aperture are reported: fitting gaussian curves are shown in red. (e) Knife-edge scanning of the beam in the sample zone for the focal plane: the minimum measured waist is ∼ 132 μm

Fig. 3
Fig. 3

Two-dimensional (2D) scan of the disk with two aluminum stripes, 400 μm and 100 μm wide respectively. The disk is oriented in order to keep the Al lines parallel to the x axis and perpendicular to the z axis. Pictures were captured with and without the 300 μm pinhole inserted in the setup ((a) and (b) respectively): each is 200x30 points. (c) Line scan in the focal plane: the 400 μm and 100 μm stripes correspond to the peaks indicated by the arrows.

Fig. 4
Fig. 4

(a) Schematics of the experimental arrangement employed for the confocal test of the optical system: two overlapping planes are involved in the measurements: the disks are separated by a distance of 1.5 mm and the whole sample is moved in the axial direction simulating the displacement of focus. ((b) to (d)) THz images of the object recorded in three positions along the optical axis: panel (a) shows the nearest plane with respect to the QCL, the other images were taken at steps of about 500 μm. The 300 μm pinhole was inserted. The colormap is chosen in order to demonstrate that the images from distant planes lay on different intensity levels. In the background some fringes are clearly visible: they are due to the circular surface roughness of the machined polyethylene substrate, which is not completely smooth. ((e) to (g)). The same measurements without confocal pinhole: note that here the resolution capability of the system is worse and it is not able to distinguish the fringes. The measured contrast between squares on the two planes is different in the two situations: Cconfocal = 0.84 while Cnon–confocal = 0.27.

Fig. 5
Fig. 5

(a) A photograph of a fresh leaf. (b) Confocal THz image of the same leaf with intensity levels referring to a wide chromatic range colormap, in order to highlight the veins. The measurements covered an area 1.8 mm x 3.2 mm and 90x160 pixels were recorded. (c) The rectangular white frame in panel (b) is imaged again with more points (200x200 pixels), in order to obtain a high resolution close-up of the leaf: the great amount of details shows the noteworthy capabilities of the instrument. (d,e) Photographs of two paper sheets which were subsequently superimposed at a distance of about 1.2 mm. Letters were written using a pencil. The left sheet is the nearest, along the optical axis, to the QCL. THz images of the paper sheets are shown in panels (f) and (g) with, respectively, the front and back plane in focus (120x100 pixels, linear colormap).

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