**7. Applications in nanophotonics**

336 Advanced Photonic Sciences

cooling yields polycrystalline. The optical signature within such mark is not discernible with conventional aperture-typed SNOM owing to its >50 nm resolution, but would be possible with high-resolution *s*-SNOM. The near-field image of the AgInSbTe phase-change layer of a blue-ray disc obtained by the *s*-SNOM is shown in Fig. 9(a). The recorded marks are revealed by the prominent near-field optical contrast. According to the dielectric constants of crystalline and amorphous AgInSbTe at 632.8 nm (3.63+*i*21.2 and 8.14+*i*9.35, respectively) [46], the ratio between the calculated tip-induced scattering intensities of the crystalline and amorphous phases is 1.28 that that is very close to the experimentally determined value of 1.30. This observation indicates that the dark regions along the protruding track in Fig. 9(a) are amorphous AgInSbTe created during the recording process. This inference agrees with TEM observation. The scattering intensity profile of the recorded disc shows that the marks have the minimum width of ~160 nm by measured the cross section (dash lines), while no such feature is present in the non-recorded disc, shown in Fig. 9(b). Such prominent marks are not recognizable in the corresponding topographic image. In addition, 30-nm isolated crystalline domains within the amorphous recorded mark emerge in Fig. 9(a) and also were

Fig. 9. (a) Image of the scattering intensity of third-harmonic order of BD-RE disc; (b) intensity profile along the track, marked by the dashed line in (a). Scale bar represents 200

nm.

In the sphere of nanophotonics, the energy of electromagnetic field is concentrated or controlled with some specially prepared nanostructures beyond the limit set by traditional optical theory where the optical mode density on which the principles of optics rely in the region of interest is assumed as in free space or only slightly perturbed from it. In contrast, the optical mode density in the content of nanophotonics has drastically distinct distribution that is governed by the nanostructured materials in proximity. Because of such distinct optical mode density, its understanding and therefore control are limited by itself that is beyond the traditional optical theory. Moreover, transferring its information content to our macroscopically sensible realm is also influenced greatly by such limitation, entailing unconventional experimental means to truthfully convey the optical information. The *s*-SNOM is representative of one of such many innovative techniques. According to Eq. (5), the scattering intensity induced by the tip-sample near-field interaction is also dependent on the field in the vicinity of such tip-sample complex. Specifically, the scattering radiation can be induced upon the presence of certain surface electromagnetic field. Surface plasmon wave (SPW) exemplifies such surface electromagnetic field, by which the evanescent wave propagates on the metal-dielectric or metal-vacuum interface [29, 43, 50]. In this section, the near-field characteristics of the SPW emanating from single hole or hole arrays of nanometer size made by focused ion beam on metal film are present.

The first example is single hole of 150 nm in diameter on a silver film of 200 nm in thickness. Figure 10 schematically shows the application of *s*-SNOM to such sample. In the case like that, three scattering field contributions are present in the thus obtained *s*-SNOM images. The first contribution is the outcome of the interaction between the incident wave and the tip apex that is in vicinity of the sample surface, producing the first scattering field, *E*1. The second contribution is made by the SPW induced by the interaction of the incident wave and the hole. The thus induced SPW propagates to the tip apex, producing the second scattering field, *E*2. The third contribution is made the SPW induced by the tip apex that is approached to the sample surface, which propagates to the hole, reflects back to the tip apex, and gives the third scattering field, *E*3. The wave vector, *kSPW* , this surface plasmon wave follows the following dispersion relation, *k k SPW* 0 12 1 2 [51], where 0 *k* is the wave vector of the incident wave, and 1 and 2 is the dielectric constants of vacuum and silver, respectively. The detailed derivation of the resultant scattering wave as well as the corresponding one in inverse space is given in Ref. 50 and is not repeated here. According to the derived results, the interference between *E*1 and *E*2 produces two *kSPW* -radius circles centered at 0 sin *<sup>i</sup> k* . Furthermore, the interference between *E*1 and *E*3

High-Resolution Near-Field Optical Microscopy: A Sub-10

exp *x y*

wave vector of surface plasmon wave.

,

*m n*

Nanometer Probe for Surface Electromagnetic Field and Local Dielectric Trait 339

of the fundamental nature of surface electromagnetic waves that may exist around nanostructures [53]. The *k*-space examination can also be applied to ordered hole arrays and has been delineated in Ref. 50. For such case, each hole in the array has both *E*2 and *E*<sup>3</sup> components, while only one *E*1 is attributed to the tip apex. The resultant *E*2 and *E*3 are given by the sum of individual holes. Assuming that two successive scattering events is neglected because of the propagating loss of SPW, the total scattering field in *k*-space can be similarly derived [50], conferring a *k-*space pattern of the near-field image of the nanohole array that is the product of the *k-*space pattern of single nanohole and the structure factor

*<sup>S</sup> <sup>k</sup> ik ma ik na* . The corresponding Fourier-transformed images in *k* space

obtained from the experimental data match very well with the predicted patterns [53]. One important implication emerges from this study. In the near-field study of surface plasmon polaritons with *s-*SNOM, the tip apex always acts as the plasmon inducer as well, making the resultant near-field images rather complicated. Performing Fourier analysis on the recorded near-field images facilitates the identification of the tip-induced contribution, setting a solid foundation for in-depth examination of the fundamental nature of surface plasmon polaritons.

Fig. 11. (a) Scattering amplitude image and (b) Fourier transformed image single nanohole on gold film; (c) schematic Fourier-transformed image of different contributions. The hole diameter is 150 nm and the excitation wavelength is 532 nm. The hole is marked as open black circle. The scale bar represents 1 *μ*m. *k*0, wave vector of incident optical wave; *kSPW*,

i

Fig. 10. Two scattering field contributions of surface plasmon waves generated from nanohole (a) and tip (b) during *s*-SNOM scanning of a circular hole on gold film. 0 *k* , the wave vector of the incident light wave; *SC k* , the wave vector of the scattering light wave; *kSPW* , the wave vector of the surface plasmon wave.

produces one 2 *kSPW* -radius circle centered at origin. Such prediction is confirmed with the scanned intensity amplitude image of such sample, shown in Fig. 11. Notice that the intensity amplitude image, shown in Fig. 11(a), exhibits rather complex ring-like pattern around the nanohole. After performing fast Fourier transform (FFT), three circles emerge in the *k-*space image, Fig. 11(b), and match with the theoretical prediction above, supporting the interpretation of near-field images obtained by *s-*SNOM. The schematic diagram of the image in *k*-space is shown in Fig. 11(c). To further support the interpretation of the nearfield amplitude image of single nanohole discussed above, numerical calculation based on finite-difference time-domain (FDTD) method was performed. The simulation, without considering the tip, was executed over a region of 16.8 *μ*m 12.8 *μ*m with a mesh size of 20 nm and the dielectric function of silver was taken from Ref. 52. A Gaussian beam with a 4 *μ*m waist was used to simulate the experimental condition. The calculated distribution of the field component normal to the surface in *k*-space only two *kSPW* -radius circles, because the third contribution of the scattering field is induced by the tip apex and was not considered in the calculation. By removing the large circle of the recorded near-field image and subsequently transforming it back to *r* space, the resultant *r-*space image matches almost perfectly with the calculated result [50]. This consistency thus supports the theoretical analysis of the Fourier-transformed image above. Finally, the single-nanohole study above therefore demonstrates that examining near-field images in *k* space helps to identify the origins of different surface plasmon waves, allowing for in-depth investigation

Fig. 10. Two scattering field contributions of surface plasmon waves generated from nanohole (a) and tip (b) during *s*-SNOM scanning of a circular hole on gold film. 0 *k* , the wave vector of the incident light wave; *SC k* , the wave vector of the scattering light wave;

produces one 2 *kSPW* -radius circle centered at origin. Such prediction is confirmed with the scanned intensity amplitude image of such sample, shown in Fig. 11. Notice that the intensity amplitude image, shown in Fig. 11(a), exhibits rather complex ring-like pattern around the nanohole. After performing fast Fourier transform (FFT), three circles emerge in the *k-*space image, Fig. 11(b), and match with the theoretical prediction above, supporting the interpretation of near-field images obtained by *s-*SNOM. The schematic diagram of the image in *k*-space is shown in Fig. 11(c). To further support the interpretation of the nearfield amplitude image of single nanohole discussed above, numerical calculation based on finite-difference time-domain (FDTD) method was performed. The simulation, without considering the tip, was executed over a region of 16.8 *μ*m 12.8 *μ*m with a mesh size of 20 nm and the dielectric function of silver was taken from Ref. 52. A Gaussian beam with a 4 *μ*m waist was used to simulate the experimental condition. The calculated distribution of the field component normal to the surface in *k*-space only two *kSPW* -radius circles, because the third contribution of the scattering field is induced by the tip apex and was not considered in the calculation. By removing the large circle of the recorded near-field image and subsequently transforming it back to *r* space, the resultant *r-*space image matches almost perfectly with the calculated result [50]. This consistency thus supports the theoretical analysis of the Fourier-transformed image above. Finally, the single-nanohole study above therefore demonstrates that examining near-field images in *k* space helps to identify the origins of different surface plasmon waves, allowing for in-depth investigation

*kSPW* , the wave vector of the surface plasmon wave.

of the fundamental nature of surface electromagnetic waves that may exist around nanostructures [53]. The *k*-space examination can also be applied to ordered hole arrays and has been delineated in Ref. 50. For such case, each hole in the array has both *E*2 and *E*<sup>3</sup> components, while only one *E*1 is attributed to the tip apex. The resultant *E*2 and *E*3 are given by the sum of individual holes. Assuming that two successive scattering events is neglected because of the propagating loss of SPW, the total scattering field in *k*-space can be similarly derived [50], conferring a *k-*space pattern of the near-field image of the nanohole array that is the product of the *k-*space pattern of single nanohole and the structure factor , exp *x y m n <sup>S</sup> <sup>k</sup> ik ma ik na* . The corresponding Fourier-transformed images in *k* space

obtained from the experimental data match very well with the predicted patterns [53]. One important implication emerges from this study. In the near-field study of surface plasmon polaritons with *s-*SNOM, the tip apex always acts as the plasmon inducer as well, making the resultant near-field images rather complicated. Performing Fourier analysis on the recorded near-field images facilitates the identification of the tip-induced contribution, setting a solid foundation for in-depth examination of the fundamental nature of surface plasmon polaritons.

Fig. 11. (a) Scattering amplitude image and (b) Fourier transformed image single nanohole on gold film; (c) schematic Fourier-transformed image of different contributions. The hole diameter is 150 nm and the excitation wavelength is 532 nm. The hole is marked as open black circle. The scale bar represents 1 *μ*m. *k*0, wave vector of incident optical wave; *kSPW*, wave vector of surface plasmon wave.

High-Resolution Near-Field Optical Microscopy: A Sub-10

Nanometer Probe for Surface Electromagnetic Field and Local Dielectric Trait 341

Fig. 12. Near-field intensity and phase images of a circular hole array with two excitation

(e), near-field optical intensity (f) and phase images (g) around the rim of elliptical hole arrays with *s*-polarization excitation; (h) schematic cross-sectional view of charge

exemplify these unique characteristics of this new-generation near-field microscopic technology. In particular, the extraction of local composition of the recorded marks of the phase-change layer of blue-ray recordable and erasable disks is presented. ~10-nm optical signatures within such recorded marks are identified and are correspondent to crystalline

*ex* = 632.8 nm. The yellow circles indicate the hole position. Topographic

*ex* = 532 nm; (c) Intensity and (d)

*ex*). (a) Intensity and (b) phase images for

distribution and electric force lines of a dipole oscillation.

wavelengths (

phase images for

The aforementioned ordered hole arrays made on a metallic film can exhibit enhanced or suppressed optical transmission through such corrugated films at specific wavelengths [54]. Many applications, such as color filters, sensitive biodetectors based on fluorescence on such films, etc., have been projected. It has been proposed and experimentally verified that such anomalous properties are originated from the launching of SPW upon the excitation of an incident optical wave at those particular wavelengths [55], while examining such plasmon wave in high resolution is not made possible with conventional aperture-type SNOM. Figure 12 presents a closer view of topographic, near-field optical intensity and phase images on the edge of a square circular hole array. Notice that the SPW outside the array is constructed coherently by those emitting from individual holes and propagates away from the array, shown in Figs. 12(a) and (b), as the incident wavelength meets the criterion for exciting surface plasmon wave of an ordered array:

$$\left| k\_0 \right| \sin \theta\_i \left( \cos q \hat{\mathbf{x}} + \sin q \hat{\mathbf{y}} \right) - k\_{SPW} = \frac{2\pi}{a} (m\hat{\mathbf{x}} + n\hat{\mathbf{y}}) , \left( m, n = 0, \pm 1, \pm 2, \cdots \right) ,\tag{7}$$

where is the azimuthal angle of 0 *k* on the surface. If the incident wavelength is off the excitation condition, no such coherently constructed plasmon wave emanates from the edge of the hole array, shown in Figs. 12(c) and (d). The interior magnified near-field amplitude and phase images of a square elliptic hole array are shown in Figs. 12(f) and (g). In this case, the surface component of the incident light wave, 0,// 0 0 cos ˆ *<sup>i</sup> k kk z* , is perpendicular to the long axis of the elliptic holes – ˆ *<sup>l</sup>* (i.e., 0,// <sup>ˆ</sup> *k l* ). As shown in Fig. 12(f), the regions of the localized field distribution manifest as the bright crescents located around the rim of the elliptical holes. This agrees with the expectation of the excitation of localized surface plasmon. In the corresponding phase image shown in Fig. 12(g), a 180° phase change takes place from the left crescent to the right crescent of the hole. The interpretation of these nearfield results relies on the understanding that *s*-SNOM probes the field component normal to the surface in the present polarization configuration [29]. The two experimental observations above therefore indicate the nature of strong dipole oscillating field along the short axis of the elliptical hole – *s*ˆ . This interpretation agrees with two additional facts in Fig. 12: no vertical field component is observed and the phase appears randomly inside the hole. These results thus reflect the excited localized surface plasmon in the case that 0,// <sup>ˆ</sup> *k l* . In contrast, no dipole-like oscillating feature appears in the near-field images in the case that 0,// *k s* ˆ , reflecting weak plasmon excitation. A calculation result based on finite-different-time-domain method further confirms this interpretation [56].

#### **8. Conclusions**

In this chapter, we present a high-resolution near-field microscope scattering-type SNOM (*s*-SNOM). Its basic operation principle is based on the near-field electromagnetic interaction between a scanning tip and the sample of interest. An interference-based heterodyne detection scheme is employed to extract both amplitude and phase of the extremely weak scattering radiation from the tip-sample complex. The local dielectric property of the sample – if the excitation wavelength is within the resonant excitation condition of the tip apex, and the local surface electromagnetic field – if the excitation wavelength is outside the resonance wavelength range of the tip apex, are readily extracted with this techniques. Considerations while exploiting such unique functions are discussed in detail. A few cases are presented to

The aforementioned ordered hole arrays made on a metallic film can exhibit enhanced or suppressed optical transmission through such corrugated films at specific wavelengths [54]. Many applications, such as color filters, sensitive biodetectors based on fluorescence on such films, etc., have been projected. It has been proposed and experimentally verified that such anomalous properties are originated from the launching of SPW upon the excitation of an incident optical wave at those particular wavelengths [55], while examining such plasmon wave in high resolution is not made possible with conventional aperture-type SNOM. Figure 12 presents a closer view of topographic, near-field optical intensity and phase images on the edge of a square circular hole array. Notice that the SPW outside the array is constructed coherently by those emitting from individual holes and propagates away from the array, shown in Figs. 12(a) and (b), as the incident wavelength meets the criterion for

> <sup>0</sup> <sup>2</sup> sin cos sin ˆ ˆ ˆ ˆ , , = 0, 1, 2, *<sup>i</sup> SPW m n mn a*

excitation condition, no such coherently constructed plasmon wave emanates from the edge of the hole array, shown in Figs. 12(c) and (d). The interior magnified near-field amplitude and phase images of a square elliptic hole array are shown in Figs. 12(f) and (g). In this case, the surface component of the incident light wave, 0,// 0 0 cos ˆ *<sup>i</sup> k kk z* , is perpendicular to

the localized field distribution manifest as the bright crescents located around the rim of the elliptical holes. This agrees with the expectation of the excitation of localized surface plasmon. In the corresponding phase image shown in Fig. 12(g), a 180° phase change takes place from the left crescent to the right crescent of the hole. The interpretation of these nearfield results relies on the understanding that *s*-SNOM probes the field component normal to the surface in the present polarization configuration [29]. The two experimental observations above therefore indicate the nature of strong dipole oscillating field along the short axis of the elliptical hole – *s*ˆ . This interpretation agrees with two additional facts in Fig. 12: no vertical field component is observed and the phase appears randomly inside the hole. These results thus reflect the excited localized surface plasmon in the case that 0,// <sup>ˆ</sup> *k l* . In contrast, no dipole-like oscillating feature appears in the near-field images in the case that 0,// *k s* ˆ , reflecting weak plasmon excitation. A calculation result based on

In this chapter, we present a high-resolution near-field microscope scattering-type SNOM (*s*-SNOM). Its basic operation principle is based on the near-field electromagnetic interaction between a scanning tip and the sample of interest. An interference-based heterodyne detection scheme is employed to extract both amplitude and phase of the extremely weak scattering radiation from the tip-sample complex. The local dielectric property of the sample – if the excitation wavelength is within the resonant excitation condition of the tip apex, and the local surface electromagnetic field – if the excitation wavelength is outside the resonance wavelength range of the tip apex, are readily extracted with this techniques. Considerations while exploiting such unique functions are discussed in detail. A few cases are presented to

finite-different-time-domain method further confirms this interpretation [56].

*k x yk x y* , (7)

*<sup>l</sup>* (i.e., 0,// <sup>ˆ</sup> *k l* ). As shown in Fig. 12(f), the regions of

is the azimuthal angle of 0 *k* on the surface. If the incident wavelength is off the

exciting surface plasmon wave of an ordered array:

the long axis of the elliptic holes – ˆ

where 

**8. Conclusions** 

Fig. 12. Near-field intensity and phase images of a circular hole array with two excitation wavelengths (*ex*). (a) Intensity and (b) phase images for *ex* = 532 nm; (c) Intensity and (d) phase images for *ex* = 632.8 nm. The yellow circles indicate the hole position. Topographic (e), near-field optical intensity (f) and phase images (g) around the rim of elliptical hole arrays with *s*-polarization excitation; (h) schematic cross-sectional view of charge distribution and electric force lines of a dipole oscillation.

exemplify these unique characteristics of this new-generation near-field microscopic technology. In particular, the extraction of local composition of the recorded marks of the phase-change layer of blue-ray recordable and erasable disks is presented. ~10-nm optical signatures within such recorded marks are identified and are correspondent to crystalline

High-Resolution Near-Field Optical Microscopy: A Sub-10

*ChemPhysChem*, 6, 2197 (2005).

108, 314 (2008).

(1998).

(2008).

(2008).

(2009).

2209 (2007).

*1997*, 104 (1997).

740 (2008).

1985).

*Phys.*, 46, 5813 (2007)

Verlag, Berlin, 1988).

W. Kimball, *Nano Lett.*, 5, 1399 (2005).

[54] A. Degiron and T. W. Ebbesen, *J. Opt. A: Pure Appl. Opt.*, 7, S90 (2005).

*Express*, 12, 4467 (2004).

*Lett.*, 83, 368 (2003).

*Interface Anal.*, 33, 75 (2002).

Nanometer Probe for Surface Electromagnetic Field and Local Dielectric Trait 343

[29] J. Y. Chu, T. J. Wang, Y. C. Chang, M. W. Lin, J. T. Yeh, and J. K. Wang, *Ultramicroscopy*,

[30] M. B. Raschke, L. Molina, T. Elsaesser, D. H. Kim, W. Knoll and K. Hinrichs,

[33] S. A. Maier, *Plasmonics: Fundamentals and Applications* (Springer-Verlag, Berlin, 2007). [34] H. H. Wang, C. Y. Liu, S. B. Wu, N. W. Liu, C. Y. Peng, T. H. Chan, C. F. Hsu, J. K. Wang

[35] R. Laddada, P. M. Adam, P. Royer and J. L. Bijeon, Opt. Eng., 37, 2142 (1998).

[38] D. Haeiger, J. M. Plitzko, and R. Hillenbrand, *Appl. Phys. Lett.*, 85, 4466 (2004). [39] R. Gupta, M. J. Dyer, and W. A. Weimer, *J. of Appl. Phys.*, 92, 5264 (2002).

[40] A. P. Li, F. Muller, A. Birner, K. Nielsh and U. Gosele, *J. of Appl. Phys.*, 84, 6023

[41] R. Hillenbrand, F. Keilmann, P. Hanarp, D. S. Sutherland and J. Aizpurua, *Appl. Phys.* 

[40] H. H. Wang, C. Y. Liu, S. B. Wu, N. W. Liu, C. Y. Peng, T. H. Chan, C. F. Hsu, J. K. Wang

[41] A. J. Huber, F. Keilmann, J. Wittborn, J. Aizpurua, and R. Hillenbrand, *Nano lett.*, 8, 3766

[42] U. C. Fischer, J. Heimel, H. J. Maas, M. Hartig, S. Hoeppener and H. Fuchs, *Surf.* 

[43] A. Huber, D. Kazantsev, F. Keilmann, J. Wittborn, and R. Hillenbrand, Adv. Mater. 19,

[44] A. J. Huber, F. Keilmann, J. Wittborn, J. Aizpurua, and R. Hillenbrand, *Nano lett.*, 8, 3766

[45] J. Y. Chu, S. C. Lo, S. C. Chen, Y. C. Chang, and J. K. Wang, *Appl. Phys. Lett.*, 95, 103105

[46] J. M. Bruneau, B. Bechevet, B. Valon, E. Butaud, *Optical Data Storage Topical Meeting* 

[48] A. J. G. Mank, A. E. Ton Kuiper, H. A. G. Nulens, B. Feddes, and G. Wei, *Jpn. J. Appl.* 

[49] J. H. Yoo, J. H. Lee, S. Y. Yim, S. H. Park, M. D. Ro, J. H. Kim, I. S. Park, and K. Cho, *Opt.* 

[50] Y. C. Chang, J. Y. Chu, T. J. Wang, M. W. Lin, J. T. Yeh, and J.-K. Wang, *Opt. Express*, 16,

[51] H. Raether, *Surface Plasmons on Smooth and Rough Surfaces and on Gratings* (Springer-

[52] *Handbook of optical constants of solids*, edited by E. D. Palik, (Academic, Press, New York,

[53] L. Yin, V. K. Vlasko-Vlasov, J. Pearson, J. M. Hiller, J. Hua, U. Welp, D. E. Brown, and C.

[31] Z. H. Kim, B. Liu and S. R. Leone, *J. Phys. Chem. B*, 109*,* 8503 (2005).

[32] Z. H. Kim and S. R. Leone, *Opt. Express*, 16, 1733 (2008).

and Y. L. Wang, *Adv. Mat.*, 18, 491 (2006).

[36] G. P. Wiederrecht, *Eur. Phys. J. Appl. Phys.*, 28, 3 (2004).

and Y. L. Wang, *Adv. Mat.*, 18, 491 (2006).

[47] S. K. Lin, I. C. Lin and D. P. Tsai, *Opt. Express*, 14, 4452 (2006).

[37] V.N. Konopsky, *Opt. Commun.*, 185, 83 (2000).

domains that are barely visible to high-resolution transmission electron microscopy. Furthermore, surface plasmon waves are revealed with *s*-SNOM and the effect caused by the tip on the acquired near-field images is carefully examined. The analysis in *k*-space domain successfully resolves this effect. The application of *s*-SNOM to nanomaterials and nanophotonics is still young. One can expect that its exploitation in these realms and even beyond can be furthered more so in near future.

#### **9. References**


domains that are barely visible to high-resolution transmission electron microscopy. Furthermore, surface plasmon waves are revealed with *s*-SNOM and the effect caused by the tip on the acquired near-field images is carefully examined. The analysis in *k*-space domain successfully resolves this effect. The application of *s*-SNOM to nanomaterials and nanophotonics is still young. One can expect that its exploitation in these realms and even

[1] M. Born and E. Wolf, Principle of Optics (Cambridge university Press, UK, 1999).

[2] B. Hecht, B. Sick, U. P. Wild, V. Deckert, R. Zenobi, O. J. F. Martin and D. W. Pohl, *The J.* 

[3] *Near-Field Nano/Atom Optics and Technology*, edited by M. Ohtsu (Springer-Verlag, Berlin,

[4] D. Courjon, *Near-Field Microscopy and Near-Field Optics* (Imperial College Press, London,

[8] A. Lewis, M. Isaacson, A. Harootunian and A. Muray, *Ultramicroscopy*, 13, 227 (1984).

[11] *Nano-Optics and Near-Field Optical Microscopy*, edited. by A. Zayats and D. Richard

[13] J. E. Hall, G. P. Wiederrecht, S. K. Gray, S. -H. Chang, S. Jeon, J. A. Rogers, R. Bachelot,

[14] L. Gomez, R. Bachelot, A. Bouhelier, G. P. Wiederrecht, S. H. Chang, S. K. Grey, F. Hua,

[17] S. Patane, P. G. Gucciardi, M. Labardi, and M. Allegrini, Riv. Nuovo Cimento 27, 1

[20] C. F. Bohren, and D. R. Huffman, *Absorption and Scattering of Light by Small Particles*

[18] R. Hillenbrand, B. Knoll and F. Keilmann, *Journal of Microscopy*, 202, 77 (2000).

S. Jeon, J. A. Rogers, M. E. Castro, S. Blaize, I. Stefanon, G. Lerondel, and P. Loyer, *J.* 

[10] F. Zenhausern, Y. Martin, and H. K. Wickramasinghe, *Science*, 269, 1083 (1995).

beyond can be furthered more so in near future.

*of Chem. Phys.*, 112, 7761 (2000).

[5] J. W. P. Hsu*, Mater. Sci. Engineer*., 33, 1 (2001). [6] E. H. Synge, *Philos. Mag.*, 6, 356 (1928).

[9] H. A. Bethe, *Phys. Rev*., 66, 163 (1944).

(Artech House, Boston, 2009)

*Opt. Soc. Am. B*, 23, 823 (2006).

[19] B. Knoll and F. Keilmann, *nature*, 399, 134 (1999).

[7] D. W. Pohl, W. Denk and M. Lanz, *Appl. Phys. Lett.*, 44, 651 (1984).

[12] M. B. Raschke and C. Lienau, *Appl. Phys. Lett.*, 83, 5089 (2003).

and P. Royer, *Opt. Express*, 15, 4098 (2007).

[15] F. Keilmann, *Journal of Electron Microscopy*, 53, 187 (2004). [16] M. I. Stockman, *Phys. Rev. Lett*., 93, 137404-1 (2004).

[21] C. Meixner and P. R. Antonicwcz, Phys. Rev. B 13, 3276 (1976).

[26] B. Knoll and F. Keilmann, *Opt. Commun.*, 182, 321 (2000). [27] F. Keilmann and R. Hillenbrand, *Phys. Rev. Lett*., 85, 3029 (2000).

[22] A. Cvitkovic, N. Ocelic and R. Hillenbrand, *Opt. Express*, 15, 8550 (2007). [23] F. Keilmann, and R. Hillenbrand, *Phil. Trans. R. Soc. Lond. A*, 362, 1 (2004). [24] R. Hillenbrand, M. Stark and R. Guckenberger, *Appl. Phys. Lett.*, 76, 3478 (2000).

[25] A. Bek, R. Vogelgesang and K. Kern, *Appl. Phys. Lett.*, 87, 163115 (2005).

[28] N. Ocelic, A. Huber and R. Hillenbrand, *Appl. Phys. Lett.*, 89, 101124 (2006).

**9. References** 

1998).

2003).

(2004).

(Wiley, 1998).


**14** 

*Bulgaria* 

**LIDAR Atmospheric Sensing by** 

**Metal Vapor and Nd:YAG Lasers** 

Dimitar Stoyanov, Ivan Grigorov, Georgi Kolarov,

*Institute of Electronics, Bulgarian Academy of Sciences, Sofia* 

LIDAR systems have widely been used for remote investigation of atmospheric parameters (Measures, 1984; Kovalev & Eichinger, 2004; Weitkamp, 2005). They are based on the socalled LIDAR (LIght Detection And Ranging) principle which consists in sending a laser pulse to the atmosphere and subsequent detecting of the radiation backscattered (at angle *π*) by atmospheric constituents or pollutants. As LIDAR is a time-of-flight technique, the return signal profile detected in the time domain contains range-resolved information about the atmospheric characteristics along the line of laser beam propagation. Advantages of the lidar measurement approaches, as compared to other available active techniques (e.g. radars), are the high spatial and temporal resolution, higher sensitivity and accuracy in sensing atmospheric particles, covering large observation areas, etc. These features make lidar systems powerful instruments for environmental measurements. At present, lidars find a variety of applications in different fields of the human activity. Along with the meteorology, atmospheric physics, and ecological monitoring, lidars are extensively used for volcanic and fire alerting, laser ranging, altimetry and bathymetry, lidar mapping and forestry, coastal morphology and hazards assessment in geology, as well as for many other applications in physics and astronomy, nuclear fusion, military, aviation, robotics, transportation, etc. There exists a variety of ground-based, air-borne and space-borne lidar systems distinguished by their types, schematics, regimes of operation, monitored parameters, constructions, etc. (Kovalev & Eichinger, 2004; Weitkamp, 2005). Among the most widely used systems are the one- or multi-wavelength aerosol lidars exploiting elastic

The present chapter describes the capabilities of LIDAR sensing techniques for atmospheric aerosol profiling by using elastic-scatter lidars based on metal vapor (MV) lasers, as well as on Nd:YAG lasers. First, a brief overview of the basic principles of the LIDAR remote sensing of the atmosphere is given. The single-scattering equations connecting return signal profiles, parameters of the experimental system and characteristics of the probed aerosols along the laser beam are presented in Sec.2, as well as some theoretical approaches for solving the lidar equations in the case of non-absorbing atmosphere (aerosol and molecular). General lidar schematics and methods for detection of lidar signals are also discussed. Special attention is paid to metal vapor and Nd:YAG lasers (Sec.3), and to advantages of

**1. Introduction** 

scattering of light.

Zahary Peshev and Tanja Dreischuh

