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ScienceWeek
MATERIALS SCIENCE: ON PHOTONIC CRYSTALS
The following points are made by M. Qi et al (Nature 2004 429:538):
1) Photonic crystals(1-3) offer unprecedented opportunities for miniaturization and integration of optical devices. They also exhibit a variety of new physical phenomena, including suppression or enhancement of spontaneous emission, low-threshold lasing, and quantum information processing(4). Various techniques for the fabrication of three-dimensional (3D) photonic crystals -- such as silicon micromachining(5), wafer fusion bonding, holographic lithography, self-assembly, angled-etching, micromanipulation, glancing-angle deposition, and auto-cloning, -- have been proposed and demonstrated with different levels of success. However, a critical step towards the fabrication of functional 3D devices, that is, the incorporation of microcavities or waveguides in a controllable way, has not been achieved at optical wavelengths.
2) The photonic crystal (PhC) of relevance here is a layered structure that allows arbitrarily designed defects to be introduced in any layer. The layers are an alternating stack of two complementary types of 2D-periodic photonic-crystal slabs: dielectric rods in air and air holes in dielectric. The "woodpile" structure (without intentional defects) is apparently the only other such layered PhC successfully fabricated and measured at optical wavelengths(5). An advantage of the present structure is that the high symmetry of each layer enables future photonic devices to be realized by modifying only one layer. Moreover, defect states in this 3D PhC closely emulate their counterparts in 2D. This allows a straightforward upgrade to 3D for most components and devices designed and analyzed in 2D. Furthermore, the existence of two types of PhC slabs in this 3D crystal offers unprecedented polarization control. Finally, another potential advantage is a large photonic bandgap (21% of the mid-gap frequency for circular holes and 25% for hexagonal holes, versus 17% for the woodpile structure).
3) In summary: The authors present the fabrication of 3D photonic crystals that are particularly suited for optical device integration using a lithographic layer-by-layer approach. Point-defect microcavities are introduced during the fabrication process and optical measurements show they have resonant signatures around telecommunications wavelengths (1.3-1.5 microns) Measurements of reflectance and transmittance at near-infrared are in good agreement with numerical simulations.
References (abridged):
1. Yablonovitch, E. Inhibited spontaneous emission in solid-state physics and electronics. Phys. Rev. Lett. 58, 2059-2062 (1987)
2. John, S. Strong localization of photons in certain disordered dielectric superlattices. Phys. Rev. Lett. 58, 2486-2489 (1987)
3. Joannopoulos, J. D., Meade, R. D. & Winn, J. N. Photonic Crystals (Princeton Press, Princeton, New Jersey, 1995)
4. Joannopoulos, J. D., Villeneuve, P. R. & Fan, S. Photonic crystals: putting a new twist on light. Nature 386, 143-149 (1997)
5. Fleming, J. G. & Lin, S. Y. Three-dimensional photonic crystal with a stop band from 1.35 to 1.95 µm. Opt. Lett. 24, 49-51 (1999)
Nature http://www.nature.com/nature
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APPLIED PHYSICS: ON PHOTONIC CRYSTAL FIBERS
Notes by ScienceWeek:
The term "photonic crystal" refers in general to a crystal lattice in which diffraction of electromagnetic radiation occurs with visible light. An example of such a lattice is a material consisting of a periodic array of drilled microscopic holes.
The following points are made by Jonathan C. Knight (Nature 2003 424:847):
1) Could optical fibers as we now know them become obsolete? It seems an unlikely proposition, and yet a new generation of optical fibers currently being developed could outperform conventional fibers for many applications. Modern optical fibers were one of the major technological successes of the 20th century, and they have become the pre-eminent method of communicating information. This technology has developed at an incredible pace, from the first low-loss (less than 20 dB/km) single-mode waveguides in 1970 to being key components of our sophisticated global telecommunication network.
2) Modern optical fibers, which transmit information in the form of short optical pulses over long distances at exceptionally high speeds, have become an integral part of life in the information age. There are non-telecom applications for fibers, too, in beam delivery for medicine, machining and diagnostics, sensing and a host of other fields. And yet, amazingly, the basic physics of optical fibers has remained unchanged since the 19th century. This, in part, may be a direct result of the fast initial implementation of fibre optic technology. The system was established early on, and all future developments were incremental improvements to individual components.
3) State-of-the-art optical fibers represent a careful trade-off between optical losses, optical nonlinearity, group–velocity dispersion, and polarization effects. Some of these effects (e.g. optical losses) are inherent in the raw material used to make the fibers, which is usually synthetically produced silica, SiO2. Others (nonlinearity and dispersion) are strongly affected by the material's properties but can be influenced by fibre design. Other properties such as polarization-mode dispersion result from imperfections in the fabrication processes. After 30 years of intensive research, incremental steps have refined the capabilities of the system and the fabrication technology nearly as far as they can go.
4) Researchers and engineers in several laboratories around the world are working on ways to revolutionize fibre-optic design and performance. Since the 1980s, optical physicists have recognized that the ability to structure materials on the scale of the optical wavelength, a fraction of a micrometer or less, will allow the development of new optical materials known as "photonic crystals". Photonic crystals rely on regular morphological microstructure incorporated into the material to radically alter its optical properties. Many research groups are investigating these materials in two and three dimensional and planar geometries, and a few researchers are using them to form new types of optical fibre. Such fibers are known as "photonic crystal fibers" (PCFs), as they rely on the unusual properties of photonic crystals to deliver previously unimaginable performance from an optical fibre waveguide.
5) In summary: Photonic crystal fibers have wavelength-scale morphological microstructure running down their length. This structure enables light to be controlled within the fibre in ways not previously possible or even imaginable. Our understanding of what an optical fibre is and what it does is changing because of the development of this new technology, and a broad range of applications based on these principles is being developed.(1-5)
References (abridged):
1. Birks, T. A., Knight, J. C. and Russell, P. St J. Endlessly single-mode photonic crystal fiber. Opt. Lett. 22, 961-963 (1997)
2. Birks, T. A., Roberts, P. J., Russell, P. St.J., Atkin, D. M. & Shepherd, T. J. Full 2-D photonic bandgaps in silica/air structures. Elect. Lett. 31, 1941-1942 (1995)
3. Yeh, P., Yariv, A. & Marom, E. Theory of Bragg fiber. J. Opt. Soc. Am. 68, 1196-1201 (1978)
4. Knight, J. C., Birks, T. A., Russell, P. St J. & Atkin, D. M. All-silica single-mode optical fiber with photonic crystal cladding Opt. Lett. 21, 1547-1549 (1996)
5. Kaiser, P. & Astle, H. W. Low loss single material fibers made from pure fused silica. Bell Syst. Tech. J. 53, 1021-1039 (1974)
Nature http://www.nature.com/nature
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ON PHOTONIC CRYSTALS AND BAND GAPS
The following points are made by John D. Joannopoulos (Nature 2001 414:257):
1) Photonic crystals have emerged as a potentially powerful platform for making light do things previously impossible. For example, optical light could be guided through air rather than through optical fibers, which would reduce losses. Photonic devices, from high-speed switches to low-power microlasers, would be useful ingredients in fiber-optics communications and in future approaches to high-speed optical computing. The essential idea is to design periodic structures that affect the behavior of photons in much same way that crystalline semiconductors affect the properties of electrons.
2) Central to this idea is the formation of a photonic "band gap" -- a range of frequencies for which light is forbidden to exist within the bulk of the photonic crystal. The presence of a band gap depends on a particular periodic structure within the crystal, but whereas the periodic arrangement of atoms occurs naturally in semiconductors, photonic crystals need to be fabricated artificially.
3) The challenge of fabricating photonic band-gap structures is enormous, since the lattice constant of the photonic crystal (the size of the periodic unit cell) must be comparable to the wavelength of the light passing through the crystal. Optical communications systems, for example, operate in the near-infrared, which means that the photonic crystal-lattice constant must have dimensions of approximately a micron. Although this is some 2000 times larger than the lattice constant of atomic crystals, it is still over 100 times smaller than the average diameter of a human hair. At such micron length scales, controlling the fabrication of intricate structures is far from straightforward.
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