Abstract

We show measurements and calculations of the terahertz (THz) near field of a metal tip with a specially formed, semicircular apex that allows us to identify the separate contributions of the tip apex and shaft to the measured signal. We find that when the tip–crystal distance is not modulated the measured near-field signal is overwhelmed by contributions from the tip shaft, resulting in a relatively large THz spot size. When the tip–crystal distance is modulated, with subsequent lock-in detection at the modulation frequency, only the near-field distribution of the semicircular apex is observed, resulting in a much smaller THz spot size and thus improved spatial resolution.

© 2004 Optical Society of America

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References

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  1. N. C. J. van der Valk and P. C. M. Planken, Appl. Phys. Lett. 81, 1558 (2002).
    [CrossRef]
  2. K. Wang, A. Barkan, and D. M. Mittleman, Appl. Phys. Lett. 84, 305 (2004).
    [CrossRef]
  3. H.-T. Chen, R. Kersting, and G. C. Cho, Appl. Phys. Lett. 83, 3009 (2003).
    [CrossRef]
  4. B. Knoll and F. Keilmann, Opt. Commun. 182, 321 (2000).
    [CrossRef]
  5. R. Hillenbrand, T. Taubner, and F. Keilmann, Nature 418, 159 (2002).
    [CrossRef] [PubMed]
  6. R. Fikri, T. Grosges, and D. Barchiesi, Opt. Lett. 28, 2147 (2003).
    [CrossRef] [PubMed]
  7. C. A. Balanis, Antenna Theory, Analysis and Design, 2nd ed. (Wiley, New York, 1997).
  8. P. C. M. Planken and N. C. J. van der Valk are preparing a manuscript called “Measurement and calculation of the near field of a terahertz apertureless scanning optical microscope” for submission to J. Opt. Soc. Am. B.

2004

K. Wang, A. Barkan, and D. M. Mittleman, Appl. Phys. Lett. 84, 305 (2004).
[CrossRef]

2003

H.-T. Chen, R. Kersting, and G. C. Cho, Appl. Phys. Lett. 83, 3009 (2003).
[CrossRef]

R. Fikri, T. Grosges, and D. Barchiesi, Opt. Lett. 28, 2147 (2003).
[CrossRef] [PubMed]

2002

N. C. J. van der Valk and P. C. M. Planken, Appl. Phys. Lett. 81, 1558 (2002).
[CrossRef]

R. Hillenbrand, T. Taubner, and F. Keilmann, Nature 418, 159 (2002).
[CrossRef] [PubMed]

2000

B. Knoll and F. Keilmann, Opt. Commun. 182, 321 (2000).
[CrossRef]

Balanis, C. A.

C. A. Balanis, Antenna Theory, Analysis and Design, 2nd ed. (Wiley, New York, 1997).

Barchiesi, D.

Barkan, A.

K. Wang, A. Barkan, and D. M. Mittleman, Appl. Phys. Lett. 84, 305 (2004).
[CrossRef]

Chen, H.-T.

H.-T. Chen, R. Kersting, and G. C. Cho, Appl. Phys. Lett. 83, 3009 (2003).
[CrossRef]

Cho, G. C.

H.-T. Chen, R. Kersting, and G. C. Cho, Appl. Phys. Lett. 83, 3009 (2003).
[CrossRef]

Fikri, R.

Grosges, T.

Hillenbrand, R.

R. Hillenbrand, T. Taubner, and F. Keilmann, Nature 418, 159 (2002).
[CrossRef] [PubMed]

Keilmann, F.

R. Hillenbrand, T. Taubner, and F. Keilmann, Nature 418, 159 (2002).
[CrossRef] [PubMed]

B. Knoll and F. Keilmann, Opt. Commun. 182, 321 (2000).
[CrossRef]

Kersting, R.

H.-T. Chen, R. Kersting, and G. C. Cho, Appl. Phys. Lett. 83, 3009 (2003).
[CrossRef]

Knoll, B.

B. Knoll and F. Keilmann, Opt. Commun. 182, 321 (2000).
[CrossRef]

Mittleman, D. M.

K. Wang, A. Barkan, and D. M. Mittleman, Appl. Phys. Lett. 84, 305 (2004).
[CrossRef]

Planken, P. C. M.

P. C. M. Planken and N. C. J. van der Valk are preparing a manuscript called “Measurement and calculation of the near field of a terahertz apertureless scanning optical microscope” for submission to J. Opt. Soc. Am. B.

Planken, P. C. M.

N. C. J. van der Valk and P. C. M. Planken, Appl. Phys. Lett. 81, 1558 (2002).
[CrossRef]

Taubner, T.

R. Hillenbrand, T. Taubner, and F. Keilmann, Nature 418, 159 (2002).
[CrossRef] [PubMed]

van der Valk, N. C. J.

N. C. J. van der Valk and P. C. M. Planken, Appl. Phys. Lett. 81, 1558 (2002).
[CrossRef]

van der Valk, N. C. J.

P. C. M. Planken and N. C. J. van der Valk are preparing a manuscript called “Measurement and calculation of the near field of a terahertz apertureless scanning optical microscope” for submission to J. Opt. Soc. Am. B.

Wang, K.

K. Wang, A. Barkan, and D. M. Mittleman, Appl. Phys. Lett. 84, 305 (2004).
[CrossRef]

Appl. Phys. Lett.

N. C. J. van der Valk and P. C. M. Planken, Appl. Phys. Lett. 81, 1558 (2002).
[CrossRef]

K. Wang, A. Barkan, and D. M. Mittleman, Appl. Phys. Lett. 84, 305 (2004).
[CrossRef]

H.-T. Chen, R. Kersting, and G. C. Cho, Appl. Phys. Lett. 83, 3009 (2003).
[CrossRef]

J. Opt. Soc. Am. B

P. C. M. Planken and N. C. J. van der Valk are preparing a manuscript called “Measurement and calculation of the near field of a terahertz apertureless scanning optical microscope” for submission to J. Opt. Soc. Am. B.

Nature

R. Hillenbrand, T. Taubner, and F. Keilmann, Nature 418, 159 (2002).
[CrossRef] [PubMed]

Opt. Commun.

B. Knoll and F. Keilmann, Opt. Commun. 182, 321 (2000).
[CrossRef]

Opt. Lett.

Other

C. A. Balanis, Antenna Theory, Analysis and Design, 2nd ed. (Wiley, New York, 1997).

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Figures (4)

Fig. 1
Fig. 1

Schematic of the experimental setup: A copper tip (right) is held close to a (100)-oriented GaP crystal. The tip is mounted on a piezo to allow modulation of the tip–crystal separation.

Fig. 2
Fig. 2

(a) Integrated THz intensity measured while raster scanning the metal tip across the surface of the GaP crystal. (b) Integrated THz intensity measured while raster scanning the metal tip across the surface of the GaP crystal, while simultaneously modulating the tip–crystal separation. In (a) and (b) red indicates maximum intensity and dark blue indicates zero intensity. The red double arrow in the inset shows the direction x of the horizontal part of the semicircular apex. (c), (d) THz electric fields as a function of time, measured on either side of the zero-intensity line (see text), showing a π phase difference.

Fig. 3
Fig. 3

(a) Diagram of a vertical wire antenna above a GaP crystal. Ra, Rb, and R are the respective distances from the bottom, the middle, and the top of the antenna to the observation point p=x,z in the crystal. l indicates the interaction length between the near field and the probe laser pulse. (b) Diagram of a horizontal wire antenna at position z0 above the GaP crystal, oriented along the x axis. R-a, R, and Ra are the respective distances from the left end, the middle, and the right end of the antenna to the observation point p=x,z in the crystal. In both antennas the current is assumed to be largest in the middle, and zero at the edges, following a triangular profile.

Fig. 4
Fig. 4

(a) Measured (squares) and calculated (solid curve) intensity of the THz near field as a function of separation r between the tip and the probe beam. Referring to Fig. 3, the curve is calculated for a=3.5 µm, b-a=500 µm, and l=65 µm. (b) Calculated electric field amplitude versus x of a horizontal wire antenna. a=5 µm, and z0=2 µm. The field is calculated just underneath the surface of the crystal at z=0.

Equations (2)

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Ez14π01+r1Ra+1Rb-2R,  z<0,
Ez-24π01+rz-z0×x+aR-a+x-aRa-2xR,  z<0.

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