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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 7 — Mar. 31, 2008
  • pp: 4731–4738
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THz emission Microscopy with sub-wavelength broadband source

Romain Lecaque, Samuel Grésillon, and Claude Boccara  »View Author Affiliations


Optics Express, Vol. 16, Issue 7, pp. 4731-4738 (2008)
http://dx.doi.org/10.1364/OE.16.004731


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Abstract

A versatile THz/IR near field microscope is demonstrated. Collecting the scattered light from a THz in-situ subwavelength source, this microscope provides images with resolution better than λ 10 . The physical origin of the contrast is explained by a Mie scattering diffraction model. Owing to the classical nature of this microscope working in the near field, resolution of THz/IR images is improved using deconvolution process.

© 2008 Optical Society of America

1. Introduction

In this article a concept of aperture mode microscope working in the THz domain is presented, allowing sub-wavelength resolution better than λ/10. The originality lies in an in-situ THz/IR generation based on optical rectification. The concept was introduce earlier [18

18. T. Kiwa, M. Tonouchi, M. Yamashita, and K. Kawase, “Laser terahertz-emission microscope for inspecting electrical faults in integrated circuits,” Opt. Lett. 28, 2058–2060 (2003). [CrossRef] [PubMed]

] without good control of the THz source. Instead, the THz/IR microscope presented here uses a perfectly control broadband source, allowing spectroscopic and polarisation studies[19

19. R. Lecaque, S. Grésillon, N. Barbey, R. Peretti, J.-C. Rivoal, and A. C. Boccara, “THz near-field optical imaging by a local source,” Opt. Commun. 262, 125–127 (2006). [CrossRef]

]. With a simple diffraction model we show on dielectric samples that this microscope works like a classical microscope in scattering configuration. Taking advantage of known deconvolution technique we are able to improve the resolution down to λ/50.

2. Experimental setup

The experimental setup is an original illumination mode based SNOM arrangement[19

19. R. Lecaque, S. Grésillon, N. Barbey, R. Peretti, J.-C. Rivoal, and A. C. Boccara, “THz near-field optical imaging by a local source,” Opt. Commun. 262, 125–127 (2006). [CrossRef]

]. A cw Nd:YVO4 laser pumped mode-locked Ti:sapphire laser (Coherent MIRA) is used as the source of near-infrared (NIR) optical femtosecond pulses. The MIRA laser produces an output pulse energy p≥10 nJ with a repetition rate of 76 MHz and a pulse duration Δτ≤150 fs at a wavelength around 800 nm. The laser output is mechanically chopped and focused with a ×10 microscope objective (N.A.=0.2) in a 200 µm thick nonlinear ZnTe crystal, cf. Fig. 1. THz and IR pulses are generated by optical rectification in the crystal. Collection of the emitted broadband source is performed with a ×50 mirror objective, i.e. Cassegrain objective (N.A.=0.4). Bolometer or HgCdTe detectors are used according to the desired spectral range. The availability of both detectors allows us to realize alternatively THz and IR images. When using the bolometer, THz images are obtained in the [150–1500 µm] wavelength range[19

19. R. Lecaque, S. Grésillon, N. Barbey, R. Peretti, J.-C. Rivoal, and A. C. Boccara, “THz near-field optical imaging by a local source,” Opt. Commun. 262, 125–127 (2006). [CrossRef]

]. When using the HgCdTe detector IR images are obtained in the [8–15 µm] wavelength range. A lock-in amplifier with a 200 ms integration time extracts the modulated signal from the two detectors. Samples are deposited directly on the nonlinear cristal. This constitutes an originality of the system with respect to[18

18. T. Kiwa, M. Tonouchi, M. Yamashita, and K. Kawase, “Laser terahertz-emission microscope for inspecting electrical faults in integrated circuits,” Opt. Lett. 28, 2058–2060 (2003). [CrossRef] [PubMed]

] because here the non-linear crystal gives a perfect control over the efficiency of the THz source and allows better physical interpretation of the images.

Fig. 1. Near-field microscopy set-up for THz and IR radiation.

3. Characterization without samples

The abilities of our microscope depend on the quality and the homogeneity of the crystal, because images are obtained by a scan of the crystal with ultra precision motorized linear stages (1 nm resolution). Characterization of the imaging system without sample on the bare crystal provides a good estimation of the performance of the set-up, in order to understand the influence of the sample in the imaging process.

Fig. 2. Knife edge THz image with normalized intensity on a bare surface of ZnTe crystal. (a) 100×100µm 2 image of the edge of the ZnTe crystal. (b) profile taken from a line parallel to the displacement of the crystal, as drawn in image (a). The profil gives access to the size of the THz sub-wavelength source.

Figure 2(a) represents a knife edge experiment on the bare ZnTe crystal. The edge of the crystal is moved from bottom right to top left and THz signal is recorded. This image gives a good approximation of the size of the THz spot (see profile in Fig. 2(b)). For this image, the spot focalization waist is 30 µm, value given with the 10–90% criteria in Fig. 2(b). As expected, the waist of the THz beam is bigger than the waist wNIR of the near infra-red laser focused on the crystal, estimated with a CCD camera (wCCDNIR<10µm) and by calculation with gaussian beam propagation and focalization (wCalNIR~3µm) [20

20. O. Svelto, Principles of lasers (Plenum Press, 1989).

]. In Fig. 2(b) The signal to noise ratio is bigger than 50. For incident optical power Plaser≤60mW the noise has a poissonian statistic. Here the incident optical power refers to the average optical power of the NIR laser before focalization. However saturation of the THz power and deterioration of the quality of the THz signal has been detected at high NIR power (Plaser>60 mW). Therefore all the following images are obtained for incident power Plaser<50 mW.

Furthermore, the resolution of our microscope depends on the focus quality of the near-infrared pump laser into the ZnTe crystal. In the limit of subwavelength focusing, i.e. focusing spot with a diameter smaller than the average THz wavelength λTHz≃300 µm, it has been shown [21

21. G. L. Dakovski, B. Kubera, and J. Shan, “Localized terahertz generation via optical rectification in ZnTe,” J. Opt. Soc. Am. B 22, 1667–1670 (2005). [CrossRef]

] that when diffraction and optical rectification are combined the intensity of generated THz is independent on the size of the focusing area. This is indeed perfectly valid for very thin ZnTe crystals (width below 20 µm), as demonstrated recently by Yan et al. on a three-layer system[22

22. W. Yan, Y.-P. Yang, H. Chen, and L. Wang, “Terahertz electric field in a three-layer system produced by parallel dipoles with a Gaussian spatial profile,” Phys. Rev. B 75, 085323 (2007). [CrossRef]

]. However with thicker crystals other non linear effects become important, like two-photon absorption, second harmonic generation or third order non linear effects. They decrease the emitted power of THz for strong focusing, i.e. in the THz subwavelength limit. This decrease in the emitted power with crystal thickness have been observed by Zhang et al. with 500 µm thick GaAs crystals[23

23. J. Z. Xu and X. C. Zhang, “Optical rectification in an area with a diameter comparable to or smaller than the center wavelength of terahertz radiation,” Opt. Lett. 27, 1067–1069 (2002). [CrossRef]

] and by Gaivoronskii et al. with ZnTe crystals[24

24. V. Y. Gaivoronskii, M. M. Nazarov, D. A. Sapozhinkov, E. V. Shepelyavyi, S. A. Shkel’nyuk, A. P. Shkurinov, and A. V. Shuvaev, “Competition between linear and nonlinear processes during generation of pulsed terahertz radiation in a ZnTe crystal,” Quantum Electron. 35, 407–414 (2005). [CrossRef]

]. But this decrease has still not been quantitatively demonstrated.

For THz generation, when the diameter of the beam inside the crystal is smaller than 100 µm and higher than 30 µm, no bulk crystal damage occurs and only two photon absorption diminishes the THz emitted power. For IR generation, smaller focusing area can be obtained (typically with diameter smaller than 10 µm). However destruction of the crystal surface may appear. This destruction of the crystal decreases the amount of generated IR and consequently limits the signal to noise ratio.

4. THz microscopy

In order to characterize a microscope, it is common to measure or calculate the point spread function (PSF)[25

25. E. H. K. Stezler, “Contrast, resolution, pixelisation, dynamic range and signal-to-noise ratio: fundamental limits to resolution in fluorescence light microscopy,” J. Microsc. 189, 15–24 (1998). [CrossRef]

]. PSF is a function describing how a point source is imaged by an optical instrument. Experimentally, measure of PSF is implemented with a point like object. This model object has a distribution whose spatial contrast is like a Dirac distribution, i.e. the size of the model object must be smaller than the wavelength used to illuminate the sample. When an image of a point object is realized, both diffraction and optical aberration are taken into account. The final image is a transfert function of the microscope. It gives an indication of the degradation of an object by the process of image formation in the microscope. Except for the non linear origin of the local source of IR/THz, the emission microscope described here behaves as a linear imaging system. It is then possible to determine the PSF of our microscope. Estimation of the PSF is made by imaging sub-wavelength size dielectric beads.

4.1. In the THz range

For the THz domain, 30 µm diameter silicon dioxide beads have been imaged. Figure 3(a) shows one of them, with better focusing of the local source than in Fig. 2(b). Full width at half maximum (FWHM) of the imaged sphere is nearly 40 µm. It proves that our microscope is able to overcome the classical Abbe diffraction limit, and that the size of the THz local source which sets the resolution is at most equal to 30 µm, i.e. λ/10 (convolution of a THz gaussian beam with the 30 µm diameter bead). The resolution is there limited by the size of the focusing area. Unfortunately it was in this case impossible to focus below the value of 30 µm without losing too much in signal to noise ratio. There is a compromise to find for each image between the THz emitted power and the resolution. In order to get the best compromise between resolution and signal, the size of the focusing area at the surface of the crystal is measured by a knife edge experiment before each image (see figure 2(b)).

The THz emission microscope increases the amount of collected scattered light over the direct transmitted light. This is demonstrated in the following section where the physical origin of the contrast is explained. It leads to a positive contrast on the bead. The maximum to mean signal ratio is therefore a crucial parameter. For comparison, the negative contrast observed with metal wire in [19

19. R. Lecaque, S. Grésillon, N. Barbey, R. Peretti, J.-C. Rivoal, and A. C. Boccara, “THz near-field optical imaging by a local source,” Opt. Commun. 262, 125–127 (2006). [CrossRef]

] is obtained with a configuration where the transmitted light is not blocked by the secondary mirror of the Cassegrain objective.

From an applicative point of view, PSF is used to realize deconvolution of more complex images, and allows better resolution of THz images.We used the deconvolution algorithm from the software imageJ and a Wiener filter [26

26. R. C. Gonzales and R. E. Woods, Digital Image Processing (Prentice Hall, 2002).

]. In the case of a linear deterioration of images, when the deterioration is spatially homogeneous, measured image g(x,y) can be written as:

g(x,y)=[h*f](x,y)+n(x,y)

where * is the product of convolution, f is the real image, h is the PSF of the linear imaging system and n represent the additional noise. When there is no additional noise (n=0), it is possible to retrieve G by Fourier transfomation G=F·H where G (resp. F, H) is the Fourier transform of image g (resp. f, h). H is the linear transfer function which transforms the real image f into the deteriorated image g. If H can be inverted, F=H -1·G. Then return to the spatial image is obtained by inverse Fourier transform.With some restrictions on the nature of the noise, these calculations are also applied when n≠0.

This improvement is shown with THz image of human eyebrows on Fig. 4. The image is performed with 30 µm FWHM local source, as with Fig. 2(b). Resolution has been increased by a factor of two, and sets to 15 µm. Resolution can be further improved but with a strong reduction in the signal to noise ratio.

Fig. 3. (a) THz image of a 30 µm silicon dioxide bead. This image was obtained with the bolometer. (b) IR image of a 10 µm silicon dioxyde bead. This image was obtained with HgCdTe detector.
Fig. 4. (a) THz image of a human eyebrow laid on the nonlinear ZnTe crystal. (b) THz image of the same human eyebrow, with deconvolution process, showing resolution improvement.

4.2. in the IR range

In the IR domain, we realized images of silicon dioxide spheres of 5 µm diameter. The obtained images give a good approximation of the resolution of our IR microscope. But unfortunately it does not give the exact PSF because the mean IR wavelength and the size of the bead are of the same order. The bead cannot be considered as subwavelength anymore. Figure 3(b) shows an IR image obtained on a bead. Here also the contrast is positive, i.e. there is much more signal collected when the bead is on the optical axis of the IR beam. Moreover, like in the THz range, the microscope is working in the IR range according to a scattering configuration. Full Width at Half Maximum of the imaged sphere is nearly 5 µm and is entirely compatible with the real size of the sphere. Signal to noise ratio matches with the one obtained for the characterization of the microscope without samples. On the image 3(b), the Houston resolution criteria [27

27. W. V. Houston, “The fine structure and the wave-lengths of the balmer lines,” The Astrophysical Journal 64, 81–92 (1926). [CrossRef]

] gives a resolution of 5 µm. Nearly sub-wavelength resolution( λ2 , where λ is between 8 and 15 µm) is reached without mechanic probes.

5. Mechanisms involved in the imaging process

As already mentioned, contrast with images of dielectric samples is positive. Moreover, the contrast on dielectrics increases with the illumination frequencies. In the IR domain, the peak level of the silicon dioxyde bead is equal to 10 times the signal far away from the bead on the bare crystal (what is later called ‘the mean signal’) rIRexp≃10, while in the THz domain this ratio becomes rTHzexp≃3. In fact, the imaging contrast of the microscope comes first from the scattering of the subwavelength source by the sample, and then scattering combined with the collection angle of the microscope. More precisely, we explain this behavior with a model of the scattering of the beads based on Mie theory. The model provides an estimation of the ratio rIRth and rTHzth of maximum to mean signal, obtained while imaging a sub-wavelength size silicon dioxide bead in the IR and THz domain respectively. In the two domains, the signal is estimated by the radiation of a sub-wavelength electromagnetic source using Mie scattering theory[28

28. C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (J.Wiley & Sons, 1983).

]. Mie theory is clearly justified in the THz domain by the order of magnitude of the average wavelength, the size of the source and the distance of observation. Experimentally, the collection angles are contained between 15° and 30° due to the numerical aperture of the Cassegrain objective used for the collection. Therefore in the angular emission scattering diagram only part of the emitted signal is collected by the microscope. In Fig. 5 this collected signal is shown by the dashed areas, and gives an evaluation of the maximum signal. Without bead, it gives an evaluation of the mean signal measured by our microscope.

In the IR domain however we found rIRexp=10 and rIRth=4. The increase of the IR signal compare to THz is clearly reproduced. But experimental and theoretical values do not match because the dipolar emission hypothesis (λd, d being the diameter of the scattering particle) is not fulfilled in the IR domain (λIR≃10 µm, d=5 µm in the IR).

6. Conclusion

A near-field THz/IR microscope in illumination mode is presented where a local source is scanned below dielectric samples. This instrument works in a broad wavelength range, from 0.2 THz to 40 THz. Because there is no need for nanometric contact between the probe and the sample, this microscope is easier to use than the metal tip based SNOM. Thanks to in-situ THz generation and image formation, better resolution and higher contrast than usual microscopes techniques are obtained. Resolution of less than λ/10 has been reached, and better resolution can be achieved using thinner non linear crystal, leading to a more localized local source. Moreover the microscope gives a straightforward imaging contrast explained with a diffraction based model. This scattering configuration microscope is advantageous when imaging samples with weak THz/IR contrast. Furthermore improvement of the resolution of THz/IR images can be obtained by known microscopy techniques, like de-convolution process. Extension of this resolution to micrometer size objects, like biological cells or defaults in semiconductor or metal structures with coupling with local THz spectroscopy is underway.

Fig. 5. Scattering diagrams computed using Mie scattering for sub-wavelength size SiO 2 beads in the THz (a) and IR (b) domain. The dashed areas correspond to the emitted angles collected by the Cassegrain objective.

Acknowledgment

The authors acknowledge support from centre national de la recherche scientifique CNRS (“Credits d’Equipement 2003”) and thank Jean-Claude Rivoal and Yannick De Wilde for valuable discussions and comments.

References and links

1.

E. A. Ash and G. Nicholls, “Super-resolution aperture scanning microscope,” Nature (London) 237(5357), 510–512 (1972). [CrossRef] [PubMed]

2.

S. Grésillon, S. Ducourtieux, A. Lahrech, L. Aigouy, J.C. Rivoal, and A. C. Boccara, “Nanometer scale apertureless near-field microscopy,” Appl. Surf. Sci. 164, 118–123 (2000). [CrossRef]

3.

S. Hunsche, M. Koch, I. Brener, and M.C. Nuss, “THz near-field imaging,” Opt. Commun. 150, 22–25 (1998). [CrossRef]

4.

O. Mitrofanov, I. Brener, T. Harel, J. D. Wynn, L. N. Pfeiffer, K. W. West, and J. Federici, “Terahertz near-field microscopy based on a collection mode detector,” Appl. Phys. Lett. 2005, 3496–3498 (2000). [CrossRef]

5.

F. Keilmann, B. Knoll, and A. Kramer, “Long-wave-infrared near-field microscopy,” Phys. Status Solidi B. 215, 849–854 (1999). [CrossRef]

6.

Y. De Wilde, F. Formanek, R. Carminatti, B. Gralak, P.-A. Lemoine, K. Joulain, J.-P. Mulet, Y. Chen, and J.-J. Greffet, “Thermal radiation scanning tunneling microscopy,” Nature (London) 444, 740–743 (2006). [CrossRef] [PubMed]

7.

D. Courjon, editor. Near Field Microscopy and Near Field Optics (Imperial College Press, 2003).

8.

E. H. Synge, “A suggested method for extending the microscopic resolution into the ultramicroscopic region,” Philos. Mag. 6, 356–362 (1928).

9.

E. Betzig, J. K. Trautmann, T. D. Harris, J. S. Weiner, and R. L. Kostelak, “Breaking the diffraction barrier: optical microscopy on a nanometer scale,” Science 251, 1468–1470 (1991). [CrossRef] [PubMed]

10.

N. Klein, P. Lahl, U. Poppe, F. Kadlec, and P. Kuzel, “A novel metal-dielectric antenna for terahertz near-field imaging,” J. Appl. Phys. 98, 014910 (2005). [CrossRef]

11.

L.-J. Chen, H.-W. Chen, T.-F. Kao, J.-Y. Lu, and C.-K. Sun, “Low-loss subwavelength plastic fiber for terahertz waveguiding,” Opt. Lett. 31, 308–310 (2006). [CrossRef] [PubMed]

12.

Q. Chen, Z. Jiang, G. X. Xu, and X.-C. Zhang, “Near-field terahertz imaging with a dynamic aperture,” Opt. Lett. 25, 1122–1124 (2000). [CrossRef]

13.

D. Simanovskii, D. Palanker, K. Cohn, and T. Smith, “Transient optical elements: application to near-field microscopy,” J. Microsc. 210, 307–310 (2003). [CrossRef] [PubMed]

14.

B. Gompf, N. Gebert, H. Heer, and M. Dressel, “Polarization contrast terahertz-near-field imaging of anisotropic conductors,” Appl. Phys. Lett. 90, 082104 (2007). [CrossRef]

15.

H. T. Chen, R. Kersting, and G. C. Cho, “Terahertz imaging with nanometer resolution,” Appl. Phys. Lett. 83, 3009–3011 (2003). [CrossRef]

16.

A. Dazzi, R. Prazeres, F. Glotin, and J. M. Ortega, “Local infrared microspectroscopy with subwavelength spatial resolution with an atomic force microscope tip used as a photothermal sensor,” Opt. Lett. 30, 2388–2390 (2005). [CrossRef] [PubMed]

17.

A. J. L. Adam, N. C. J. Van der Valk, and P. C. M. Planken, “Measurement and calculation of the near field of a terahertz apertureless scanning optical microscope,” J. Opt. Soc. Am. B 24, 1080–1090 (2007). [CrossRef]

18.

T. Kiwa, M. Tonouchi, M. Yamashita, and K. Kawase, “Laser terahertz-emission microscope for inspecting electrical faults in integrated circuits,” Opt. Lett. 28, 2058–2060 (2003). [CrossRef] [PubMed]

19.

R. Lecaque, S. Grésillon, N. Barbey, R. Peretti, J.-C. Rivoal, and A. C. Boccara, “THz near-field optical imaging by a local source,” Opt. Commun. 262, 125–127 (2006). [CrossRef]

20.

O. Svelto, Principles of lasers (Plenum Press, 1989).

21.

G. L. Dakovski, B. Kubera, and J. Shan, “Localized terahertz generation via optical rectification in ZnTe,” J. Opt. Soc. Am. B 22, 1667–1670 (2005). [CrossRef]

22.

W. Yan, Y.-P. Yang, H. Chen, and L. Wang, “Terahertz electric field in a three-layer system produced by parallel dipoles with a Gaussian spatial profile,” Phys. Rev. B 75, 085323 (2007). [CrossRef]

23.

J. Z. Xu and X. C. Zhang, “Optical rectification in an area with a diameter comparable to or smaller than the center wavelength of terahertz radiation,” Opt. Lett. 27, 1067–1069 (2002). [CrossRef]

24.

V. Y. Gaivoronskii, M. M. Nazarov, D. A. Sapozhinkov, E. V. Shepelyavyi, S. A. Shkel’nyuk, A. P. Shkurinov, and A. V. Shuvaev, “Competition between linear and nonlinear processes during generation of pulsed terahertz radiation in a ZnTe crystal,” Quantum Electron. 35, 407–414 (2005). [CrossRef]

25.

E. H. K. Stezler, “Contrast, resolution, pixelisation, dynamic range and signal-to-noise ratio: fundamental limits to resolution in fluorescence light microscopy,” J. Microsc. 189, 15–24 (1998). [CrossRef]

26.

R. C. Gonzales and R. E. Woods, Digital Image Processing (Prentice Hall, 2002).

27.

W. V. Houston, “The fine structure and the wave-lengths of the balmer lines,” The Astrophysical Journal 64, 81–92 (1926). [CrossRef]

28.

C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (J.Wiley & Sons, 1983).

29.

E. D. Palik, editor. Handbook of Optical Constant of Solids (Academic Press, 1998).

OCIS Codes
(110.0180) Imaging systems : Microscopy
(180.4243) Microscopy : Near-field microscopy
(110.6795) Imaging systems : Terahertz imaging

ToC Category:
Microscopy

History
Original Manuscript: August 23, 2007
Revised Manuscript: October 25, 2007
Manuscript Accepted: October 26, 2007
Published: March 24, 2008

Virtual Issues
Vol. 3, Iss. 4 Virtual Journal for Biomedical Optics

Citation
Romain Lecaque, Samuel Grésillon, and Claude Boccara, "THz emission Microscopy with sub-wavelength broadband source," Opt. Express 16, 4731-4738 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-7-4731


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References

  1. E. A. Ash and G. Nicholls, "Super-resolution aperture scanning microscope," Nature (London) 237,510-512 (1972). [CrossRef] [PubMed]
  2. S. Grésillon, S. Ducourtieux, A. Lahrech, L. Aigouy, J. C. Rivoal, and A. C. Boccara, "Nanometer scale apertureless near-field microscopy," Appl. Surf. Sci. 164, 118-123 (2000). [CrossRef]
  3. S. Hunsche, M. Koch, I. Brener, and M.C. Nuss, "THz near-field imaging," Opt. Commun. 150, 22-25 (1998). [CrossRef]
  4. O. Mitrofanov, I. Brener, T. Harel, J. D. Wynn, L. N. Pfeiffer, K. W. West, J. Federici, "Terahertz near-field microscopy based on a collection mode detector," Appl. Phys. Lett. 2005, 3496-3498 (2000). [CrossRef]
  5. F. Keilmann, B. Knoll, and A. Kramer, "Long-wave-infrared near-field microscopy," Phys. Status Solidi B. 215, 849-854 (1999). [CrossRef]
  6. Y. De Wilde, F. Formanek, R. Carminatti, B. Gralak, P.-A. Lemoine, K. Joulain, J.-P. Mulet, Y. Chen, and J.-J. Greffet, "Thermal radiation scanning tunneling microscopy," Nature (London) 444, 740-743 (2006). [CrossRef] [PubMed]
  7. D. Courjon, editor. Near Field Microscopy and Near Field Optics (Imperial College Press, 2003).
  8. E. H. Synge, "A suggested method for extending the microscopic resolution into the ultramicroscopic region," Philos. Mag. 6, 356-362 (1928).
  9. E. Betzig, J. K. Trautmann, T. D. Harris, J. S. Weiner, and R. L. Kostelak, "Breaking the diffraction barrier : optical microscopy on a nanometer scale," Science 251, 1468-1470 (1991). [CrossRef] [PubMed]
  10. N. Klein, P. Lahl, U. Poppe, F. Kadlec, and P. Kuzel, "A novel metal-dielectric antenna for terahertz near-field imaging," J. Appl. Phys. 98, 014910 (2005). [CrossRef]
  11. L.-J. Chen, H.-W. Chen, T.-F. Kao, J.-Y. Lu, and C.-K. Sun, "Low-loss subwavelength plastic fiber for terahertz waveguiding," Opt. Lett. 31, 308-310 (2006). [CrossRef] [PubMed]
  12. Q. Chen, Z. Jiang, G. X. Xu, and X.-C. Zhang, "Near-field terahertz imaging with a dynamic aperture," Opt. Lett. 25, 1122-1124 (2000). [CrossRef]
  13. D. Simanovskii, D. Palanker, K. Cohn, and T. Smith, "Transient optical elements: application to near-field microscopy," J. Microsc. 210, 307-310 (2003). [CrossRef] [PubMed]
  14. B. Gompf, N. Gebert, H. Heer, and M. Dressel, "Polarization contrast terahertz-near-field imaging of anisotropic conductors," Appl. Phys. Lett. 90, 082104 (2007). [CrossRef]
  15. H. T. Chen, R. Kersting, and G. C. Cho, "Terahertz imaging with nanometer resolution," Appl. Phys. Lett. 83, 3009-3011 (2003). [CrossRef]
  16. A. Dazzi, R. Prazeres, F. Glotin, and J. M. Ortega, "Local infrared microspectroscopy with subwavelength spatial resolution with an atomic force microscope tip used as a photothermal sensor," Opt. Lett. 30, 2388-2390 (2005). [CrossRef] [PubMed]
  17. A. J. L. Adam, N. C. J. Van der Valk, and P. C. M. Planken, "Measurement and calculation of the near field of a terahertz apertureless scanning optical microscope," J. Opt. Soc. Am. B 24, 1080-1090 (2007). [CrossRef]
  18. T. Kiwa, M. Tonouchi, M. Yamashita, and K. Kawase, "Laser terahertz-emission microscope for inspecting electrical faults in integrated circuits," Opt. Lett. 28, 2058-2060 (2003). [CrossRef] [PubMed]
  19. R. Lecaque, S. Grésillon, N. Barbey, R. Peretti, J.-C. Rivoal, and A. C. Boccara, "THz near-field optical imaging by a local source," Opt. Commun. 262, 125-127 (2006). [CrossRef]
  20. O. Svelto, Principles of lasers (Plenum Press, 1989).
  21. G. L. Dakovski, B. Kubera, and J. Shan, "Localized terahertz generation via optical rectification in ZnTe," J. Opt. Soc. Am. B 22, 1667-1670 (2005). [CrossRef]
  22. W. Yan, Y.-P. Yang, H. Chen, and L. Wang, "Terahertz electric field in a three-layer system produced by parallel dipoles with a Gaussian spatial profile," Phys. Rev. B 75, 085323 (2007). [CrossRef]
  23. J. Z. Xu and X. C. Zhang, "Optical rectification in an area with a diameter comparable to or smaller than the center wavelength of terahertz radiation," Opt. Lett. 27, 1067-1069 (2002). [CrossRef]
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