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Nanophotonics or Nano-optics is the study of the behavior of light on the nanometer scale. It is considered as a branch of optical engineering which deals with optics, or the interaction of light with particles or substances, at deeply subwavelength length scales. Technologies in the realm of nano-optics include near-field scanning optical microscopy (NSOM), photoassisted scanning tunnelling microscopy, and surface plasmon optics. Traditional microscopy makes use of diffractive elements to focus light tightly in order to increase resolution. But because of the diffraction limit (also known as the Rayleigh Criterion), propagating light may be focused to a spot with a minimum diameter of roughly half the wavelength of the light. Thus, even with diffraction-limited confocal microscopy, the maximum resolution obtainable is on the order of a couple of hundred nanometers. The scientific and industrial communities are becoming more interested in the characterization of materials and phenomena on the scale of a few nanometers, so alternative techniques must be utilized. Scanning Probe Microscopy (SPM) makes use of a “probe”, (usually either a tiny aperture or super-sharp tip), which either locally excites a sample or transmits local information from a sample to be collected and analyzed. The ability to fabricate devices in nanoscale that has been developed recently provided the catalyst for this area of study.
The study of nanophotonics involves two broad themes 1) studying the novel properties of light at the nanometer scale 2) enabling highly power efficient devices for engineering applications.
The study has the potential to revolutionize the telecommunications industry by providing low power, high speed, interference-free devices such as electrooptic and all-optical switches on a chip.
Components of a nanophotonic system
The term typically refers to phenomena of ultraviolet, visible and near IR light, with a wavelength of approximately 300 to 1200 nanometers.
The interaction of light with these nanoscale features leads to confinement of the electromagnetic field to the surface or tip of the nanostructure resulting in a region referred to as the optical near field. This effect is to some extent analogous to a lightning rod, where the field concentrates at the tip. In this region, the field may need to adjust to the topography of the nanostructure (see boundary conditions of Maxwell's equations). This means that the electromagnetic field will be dependent on the size and shape of the nanostructure that the light is interacting with.
This optical near field can also be described as a surface bound optical oscillation which can vary on length scale of tens or hundreds of nanometers – a length scale smaller than the wavelength of the incoming light. This can provide higher spatial resolution beyond the limitations imposed by the law of diffraction in conventional far-field microscopy. The technique derived from this effect is known as near-field microscopy, and opens up many new possibilities for imaging and spectroscopy on the nanoscale. A novel embodiment which has picometer resolution in the vertical plane above the waveguide surface is dual polarisation interferometry.
Novel optical properties of materials can result from their extremely small size. A typical example of this type of effect is the color change associated with colloidal gold. In contrast to bulk gold, known for its yellow color, gold particles of 10 to 100 nm in size exhibit a rich red color. The critical size where these and related effects take place are correlated with the mean free path of the conduction electrons of the metal.
In addition to these extrinsic size effects that determine a material's optical response to incoming light, the intrinsic properties of the material can change. These size effects occur as particles become even smaller. At this stage some of the intrinsic electronic properties of the medium itself change. One example of this phenomenon is in semiconductor nanostructures where the extremely small particle size confines the quantum mechanical wavefunction, leading to discrete optical transitions, e.g., fluorescence colors that depend on the size of the particle. The changing bandgap of the semiconductor is the reason for this color change. This effect, however, since not directly correlated with optical wavelength, is not unanimously included when referring to nano-optics.
In March 2010, S. Assefa et al. of IBM reported invention of ultra-fast and noise-free nanophotonic avalanche photodetectors which are poised to bring about the exaflop light circuit era. "We are now working on integrating all of our devices onto a microprocessor alongside transistors". "The Avalanche Photodetector achievement, which is the last in a series of prior reports from IBM Research, is the last piece of the puzzle that completes the development of the “nanophotonics toolbox” of devices necessary to build the on-chip interconnects". "With optical communications embedded into the processor chips, the prospect of building power-efficient computer systems with performance at the Exaflop level might not be a very distant future.”
- Assefa, Solomon; Xia, Fengnian; Vlasov, Yurii A. (2010). "Reinventing germanium avalanche photodetector for nanophotonic on-chip optical interconnects". Nature 464 (7285): 80–4. Bibcode:2010Natur.464...80A. doi:10.1038/nature08813. PMID 20203606.
- "Research Discovery By Ethiopian Scientist At IBM at Tadias Magazine". Tadias.com. Retrieved 2010-03-15.
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- "Avalanche photodetector breaks speed record". physicsworld.com. Retrieved 2010-03-15.
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