What are the problems with photonic propulsion
Photonic Crystals are periodic structures of the refractive index that occur or are created in principally transparent solids. affect the movement of photons (usually visible light or infrared) through diffraction and interference. Photonic crystals are not necessarily crystalline - their name comes from the analogous diffraction and reflection effects of X-rays in crystals due to their lattice constants.
The structure dimensions are equal to or greater than a quarter of the associated wavelength of the photons, i.e. they are in the range from fractions of a micrometer to several micrometers.
Photonic crystals can be distinguished from interference layers and diffraction gratings in that they
- can be three- or one-dimensional
- may be controllable under certain circumstances
Photonic crystals are also found in nature. For example, the shimmering colors on butterfly wings are created by periodic structures like those used in photonic crystals.
Photonic crystals consist of structured semiconductors, glasses or polymers and are mostly produced using processes known from microelectronics. By means of their specific structure, they force the light to propagate in the medium in the manner necessary for the component function. This not only makes it possible to guide light to dimensions that are in the order of magnitude of the wavelength, but also to filter it and reflect it in a wavelength-selective manner.
These are periodic dielectric structures, the period length of which is set in such a way that they influence the propagation of electromagnetic waves in a similar way as the periodic potential in semiconductor crystals influences the propagation of electrons. They therefore show unique optical properties, such as Bragg reflection of visible light.
In particular, analogous to the formation of the electronic band structure, a photonic band structure is created that can have areas of forbidden energy in which electromagnetic waves cannot propagate within the crystal (photonic band gaps, PBG = English: photonic band gap). In a certain way, photonic crystals can be seen as the optical analogue of electronic semiconductors, that is, as “optical semiconductors”.
Photonic crystals are used in particular in optical telecommunications. With the help of photonic crystals, for example, waveguides with very small curve radii (in the micrometer range) with low losses, more efficient solid-state lasers, extremely narrow-band optical filters, multiplexers and various other new types of optoelectronic components can be realized. For a long time, "photonic crystal" fibers (PCF, photonic-crystal fiber), commercially available. As Bragg gratings, they can, inter alia. can be used in DFB lasers.
Occurrence in nature
Photonic crystals that occur in nature include opals, bird feathers, butterfly wings.
Research history and literature
Photonic crystals were first described by Bykov in 1972:
- V. P. Bykov: Spontaneous Emission in a Periodic Structure. In: Soviet Journal of Experimental and Theoretical Physics. Volume 35, 1972, ISSN 0038-5646, pp. 269-273 (PDF [accessed November 3, 2013]).
and realized experimentally for the first time at the end of the 1970s by R. Zengerle:
- Remigius Zengerle: Propagation of light in flat periodic waveguides. Doctoral thesis, University of Stuttgart, 1979.
- R. Zengerle: Light Propagation in Singly and Doubly Periodic Planar Waveguides. In: Journal of Modern Optics. Volume 34, No. 12, 1987, pp. 1589-1617, doi: 10.1080 / 09500348714551531.
At the end of the 1980s, they were theoretically calculated independently of Eli Yablonovitch and Sajeev John with their optical properties:
- Eli Yablonovitch: Inhibited Spontaneous Emission in Solid-State Physics and Electronics. In: Physical Review Letters. Volume 58, No. 20, May 18, 1987, ISSN 0031-9007, pp. 2059-2062, doi: 10.1103 / PhysRevLett.58.2059.
- Sajeev John: Strong localization of photons in certain disordered dielectric superlattices. In: Physical Review Letters. Volume 58, No. 23, June 8, 1987, ISSN 0031-9007, pp. 2486-2489, doi: 10.1103 / PhysRevLett.58.2486.
Since then, research activity in this area has steadily increased and photonic crystals have become an active research area in which many working groups at universities and research institutes around the world are working.
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