Nanoscale optics

Optical measurements at the nanometer scale require a light source with an illumination
spot in the nanometer range. For visible-light frequencies, where the wavelength
is a few hundred nanometers, conventional optical microscopy fails because the resolution is restricted to half the wavelength of the used light. To overcome this problem, the light must be localized in a spot with a diameter much smaller than the wavelength of the light. Ideally,
the spot should have nanometer-scale dimensions. This can be done by applying small apertures.

The price for this high resolution is that the character of the light changes drastically when it propagates through the aperture.

The localization of the light waves results in the formation of evanescent waves, which have an imaginary wave number and decay exponentially in space (in contrast to conventional light waves, which propagate freely). The intensity of an evanescent wave thus decays rapidly as the distance from the aperture increases. Therefore, the aperture has to be close to the object, often
only a fraction of the wavelength away. This is the regime of near-field optics.


Applications of optical microscopy are generally limited by the standard resolution limit set by the wavelength of visible light. The invention of near-field scanning optical microscopy
(NSOM) first enabled this limit to be overcome, opening up many systems, from physics to biology, to investigation by optical microscopy. NSOM offered greatly improved spatial resolution compared with conventional optical microscopy, and the use of tunable excitation sources allowed basic spectroscopic information to be obtained.


NSOM techniques have many applications in solid state physics, where substantial efforts are made to design electronic devices with features on the nanometer scale.

Optics in the Nano-World
S. W. Koch and A. KnorrScience 21 September 2001 293: 2217-2218
[DOI: 10.1126/science.1065119