**1. Introduction**

322 Advanced Photonic Sciences

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The first optical microscope was made by Zacharias Janssen in the late sixteenth century. Robert Hooke used his own microscope to observe bio-samples and draw these structures on the micrographia in the seventeenth century. August Köhler, in 1893, developed an illumination technique, allowing for even sample lighting and setting the corner stone for modern light microscopy. In 1930s, Zeiss laboratory invented the upright microscope that is a prototype of modern optical microscopy. Such earlier effort to extend human vision in examining tiny objects is however limited by wave diffraction, as pointed out by Helmholtz and Abbe [1]. Namely, in visible wavelength range, optical resolution is about 250 nm. Such resolution limit certainly does not satisfy the need for current development and progress of nanotechnology, in which objects within 100 nm portray unique properties and functions that are not predictable by direct extrapolation from their macro-counterparts. It is thus crucial to extend the optical resolution below the Abbe's limit if we want to further the role of optical microscopy in future technology advance.

Although both electron microscopy and scanning probe microscopy easily meet the resolution requirement for nanotechnology, optical microscopy provides two unique characteristics that are not possible by them. First, optical microscopy is a noninvasive characterization method and can therefore examine objects in ambient environment or through transparent condensed media. Second, the energy resolving power of an optical probe surpasses the other two microscopic techniques, greatly enhancing its species identification ability in conjunction with various spectroscopic probes. These two distinctive capabilities are promised to more clearly unravel the novel relationship between the size/shape of a nano-object and its property which is essential to the advancement of nanotechnology, if the spatial resolution of optical microscopy can reach sub-10 nanometer scale.

A dramatic improvement of optical resolution has become possible by the invention of aperture-type scanning near-field optical microscopy (SNOM) [2-5]. Viewing through a tiny pinhole, firstly proposed by Synge [6] in 1928, spurs a series of attempts to realize such

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

**2. Theory of** *s***-SNOM** 

Nanometer Probe for Surface Electromagnetic Field and Local Dielectric Trait 325

The field enhancement of a metallic tapered tip apex could be up to 1000-fold higher than the field of an incident optical wave that impinges onto it in an optimum condition for electromagnetic resonance [16]. As a consequence, the electromagnetic interaction between the tip and the incident field can be greatly altered owing to the presence of a near-by surface within the near-field enhancement zone, conferring an enhanced scattered radiation to the far field. Understanding the electromagnetic interaction taking place between the tip apex and the sample surface entails the detailed structure of the tip and solving the electromagnetic field in vicinity of the tip-surface composite system. Nonetheless, a simple model that can reveal the fundamental nature of such electromagnetic interaction would assist in understanding the limits of *s*-SNOM and in furthering its advancement and generalization. The first simplification is that only the response of the tip to the incident optical wave coming from the region around the tip apex is considered, whose dimension is much smaller than the wavelength of the incident wave, such that phase retardation is negligible over such region and thus a quasi-static treatment is justified [17-19]. Secondly, in spite that many multipole fields could be produced from the interaction between the incident wave and the tip apex of finite size and of complicated shape, only the dipole field from a sphere that approximates the tip apex is considered for it dominates the far-field radiation characteristics from the tip-surface system. Lastly, only a flat sample surface is considered for extracting the far-field scattered radiation, though a corrugated surface is expected to modify the outcome. The influence of the surface structure will be discussed later.

Fig. 2. Schematic of quasi-electrostatic dipole model illustrating the electromagnetic

interaction happening in *s*-SNOM of side-illuminating geometry.

concept. In 1984, two groups have simultaneously demonstrated it in the visible wavelength range [7-8]. Light through a sub-wavelength diameter aperture, which is evanescent in character, illuminates a sample within its near-field range. The optical response thus recorded while the sample is scanned laterally constructs a near-field optical image scanning nearfield optical microscope (SNOM). Its resolution depends on the diameter of aperture. Although breaking the Abbe's resolution limit, this new type of optical microscope holds three restrictions. The first one is that the resolution cannot be better than 50 nm, caused by the finite penetration into the optical field-confinement metal structure that defines the aperture. The second one is the trade-off between spatial resolution and collected signal intensity the smaller the aperture is, the smaller the optical signal is collected [9]. The third one is that the throughput of the aperture is highly wavelength dependent, thus distorting the recorded optical spectrum. The aforementioned restrictions thus restrain the practice of aperture-type SNOM from the applications in nanophotonics and nanotechnology in general.

In contrast to the aperture-type SNOM, Wickramasinghe and coworkers in 1994 demonstrated an apertureless SNOM with a 10-nm resolution on the basis of sensing the dipole-dipole coupling between a tip end and a sample [10]. Later, Keilmann's team furthermore extended this approach to extract both amplitude and phase of such electromagnetic coupling simultaneously, concocting a comprehensive scattering-type SNOM (*s*-SNOM) [11-15], as depicted in Fig. 1. The collimated optical radiation impinges onto the site where the apex of an atomic force microscopic tip (AFM probe) is in close proximity of sample surface (enclosed by green dotted ellipse). Right-handed image of the figure 1 shows a blow-up view of the induced localized field at the tip apex. The restrictions of aperture-type SNOM are unleashed in such new-generation SNOM. Firstly, the resolution of s-SNOM is only limited by the tip radius. Secondly, the enhanced local field at the tip apex, owing to plasmon resonance, intensifies the electromagnetic interaction at the tipsample system and thus boosts up the resulted scattered radiation for detection.

Fig. 1. Schematic view of scattering-type scanning near-field optical microscope.

In this chapter, we delineate the fundamental theory of *s*-SNOM (Section 2) and show its experimental setup (Section 3). Section 4 describes issues relevant to extracting near-field images. Its spatial resolution is demonstrated and discussed in Section 5. We show that *s*-SNOM is a powerful tool to unravel local optical properties in nanocomposite systems in Section 6. Furthermore in Section 7, we illustrate that it is possible to extract both amplitude and phase of surface plasmon polariton. The two cases above exemplify the great potential of *s*-SNOM to explore plasmonics and metamaterials, as well as nano-structured composite systems. Finally, Section 8 concludes this chapter.
