Page 219 - Principles and Applications of NanoMEMS Physics

208 Chapter 5

of, e.g., a nanowire. In a second approach, a near-field scanning optical
microscope (NSOM), which allows sub-wavelength resolution [224], is
utilized. In this section we provide the fundamental principles of operation
of the NSOM.

5.4.1 NSOM/SNOM

Near-field scanning optical microscopy (NSOM), also called scanning
near-field optical microscopy (SNOM), is a super-resolution optical
microscopy technique that enables the ability to view samples at spatial
resolutions beyond those attainable with conventional optical techniques
[224], [225]. Conventional optical techniques are limited by the diffraction
of light. This is characterized by the size of the spot to which a light beam
can be focused. The spot is part of a family of concentric rings, known as the
Airy disk pattern, and its size is defined as the distance d from the point of
highest intensity, located at the middle of the center spot, to the first node in
intensity (demarcating the beginning of the first ring), and it is given by,
λ
d = 61.0 0 , (17)
n sin θ

where λ is the free-space wavelength, n is the index of refraction on the
0
medium in which the light propagates, and θ is the angle describing the
light convergence for the focusing element [225]. With the value of the
denominator, denoted as numerical aperture (NA), for the objective, being
typically as high as 1.3-1.4, (17) is usually simplified to d = λ 2 . This is
0
taken as the distance two objects may be approached to one another other
while still being distinguishable. To circumvent this limit, Synge [226],
[227] proposed the scheme shown in Fig. 5-12.
Incident Light
Incident Light
Incident Light

Opaque Screen
Opaque Screen
Opaque Screen
Opaque Screen
Near Field
Near Field
Near Field
Near Field
Sample Surface
Sample Surface
Sample Surface
Sample Surface
Far Field
Far Field
Far Field
Far Field
Wavelength of Light
Wavelength of Light
Wavelength of Light
Wavelength of Light
Figure 5-12. Sketch of Synge’s concept for overcoming diffraction limit. (After [225].)