6 August/September 2002 NEWS

Photonic Crystals By E. Yablonovitch Photonic crystals are the tures though not full photonic some of the first calculations on (Given their long wavelength they electromagnetic analog of semicon- bandgaps. A complete bandgap thin-film 2-dimensional slab should be called “electromagnetic ductor crystals. They are artificial seems to have eluded nature—it photonic crystals. Such thin films crystals” rather than “photonic crystal structures that do for seems to require too much refrac- were not thought to be useful for crystals”) For example a cellular electromagnetic waves what semi- tive index contrast. Nevertheless, trapping light, since they are com- telephone often uses radio waves conductor crystals do for an incomplete bandgap can still be pletely open, top and bottom. that are 35cm long in free space. electron waves. In today’s world, very useful. Novel forms of Nonetheless, they are intriguing in The corresponding electromag- electronic semiconductors are the synthetic opal can be self-as- that they could be easily patterned netic crystal consisting of multiple basis for the micro-electronic, tele- sembled in titanium dioxide by standard integrated circuit periods would have to be even communications, and computer particles, the white pigment used production methods. When one of larger than that and not very prac- industries. We are just now begin- in paint and to make printer paper the holes is left plugged up, the re- tical for carrying around. Here the ning to understand the exciting white. Coherent scattering of light sult is a “donor” cavity, a local common electrical circuit of induc- potential of their electromagnetic can give more whiteness for less electromagnetic mode in a region tors and capacitors (“LC-circuit”) cousins for tomorrow’s world. titanium dioxide. One day we may with an otherwise forbidden rescues us. An LC-circuit can Figure 1: The first photonic crystal The powerful analogy between find photonic crystals all around bandgap. confine an electromagnetic wave was formed by drilling three intersect- photonic and semiconductor crys- ing arrays of holes into a block of ce- us on painted walls and in the Surprisingly, these 2-dimen- to a small volume and arrays of LC tals has unleashed the collective ramic material. Each array is angled stacks of documents that clutter sional cavities can be very effective circuits can behave as photonic scientific imagination of many 35o into the plane, producing a struc- our work desks! for trapping light, in spite of being crystals, controlling long electro- creative physicists, engendering a ture now called Yablonovite. The pat- While a perfect 3-dimensional open top and bottom. Indeed O. magnetic waves, even though the profusion of synthetic electromag- tern of 6mm-diameter holes blocks ra- structure is needed to block all Painter, R.K. Lee, A. Scherer, A. whole array can be smaller than netic crystal structures. These dio waves from 13 to 16 GHz. waves in all directions, we have Yariv, J.D. O’Brien, P.D. Dapkus, and one free space wavelength. usually have an electromagnetic learned that 2-dimensional I. Kim at Cal Tech and USC have This simple concept has led to a bandgap, a band of frequencies in photonic crystals might be even recently fabricated the tiniest series of innovative new ideas in which electromagnetic waves are the problems that faced theo- more valuable. Two-dimensional lasers ever from them. These 2-di- electromagnetics. For example, forbidden. Various 2-dimen- rists. Electromagnetic waves are photonic crystals come in many mensional photonic crystal thin using arrays of LC-circuits, David sional and 3-dimensional photonic vectors like electric fields. It forms, since there is considerable films can be readily patterned into Smith, Willie J. Padilla, D. C. Vier, crystal structures have now been therefore took time for theorists freedom in handling the 3rd dimen- optical circuits that would repre- S. C. Nemat-Nasser and Shelley conceived for application in high to retool their band structure sion. If the 3rd dimension is sent the ultimate limit of Schultz of UCSD have created the capacity optical fibers, color pig- computer programs to accept stretched out long and narrow, optoelectronic miniaturization. first “left handed” materials, in ments, and especially vector waves. Several groups that photonic crystals provide a new Many researchers believe that which the group velocity and phase nano-photonic integrated circuits undertook this task, including M. method for confining light in opti- these types of photonic crystal velocity are opposite! that might be included in standard Leung of Polytechnic University, cal communications fibers, as first integrated circuits stand ready to Meanwhile, M.C.K. Wiltshire, J.B. microchips. and K.M. Ho, C.T. Chan and C.M. introduced by J. C. Knight, J. extend the integrated circuit Pendry, I.R. Young, D.J. Larkman, D.J. Soukoulis of Iowa State Univer- Broeng, T. A. Birks, and Philip St. J. revolution into the domain of high Gilderdale, and J.V. Hajnal of Impe- A Little History sity, began to make valuable rial College have used LC In electronic semiconductor predictions. The Iowa State electromagnetic bandgap arrays for crystals, electron waves scatter group discovered that the dia- manipulating the radio magnetic off the layers or rows of atoms. mond structure would indeed fields used in medical magnetic reso- Bumping into periodic row after produce a real bandgap. Dia- nance imaging and D. Sievenpiper, periodic row of atoms, the back- mond structure is a form of Z. Lijun, R.F.J. Broas, N.G. scattering is reinforced if the face-centered-cubic (fcc) in Alexopolous, and E. Yablonovitch electron wavelength matches the which two atoms, instead of one, have used LC resonator arrays for spacing of successive layers. are inscribed into each unit cell. controlling radio antennas. Venturing off in different direc- The form of diamond structure It appears likely that these tions, the electron waves meet that was most effective, giving the circuit concepts can be extended other layers of atoms. No matter widest photonic bandgap, right back up to optical frequen- which direction they go, they just consisted of only the dielectric Figure 2: Left: the cladding of several hundred capillary tubes confines light cies, where they emerge as to the central hole which is about 15 microns in diameter. Right: the pattern can’t get through if their wave- rods (“valence bonds”) between of colors shows that the optical confinement of a bandgap fiber depends so-called “plasmons”, the optical lengths roughly match the layer the atoms, which were allowed strongly on wavelength. frequency currents that can flow spacings. The result is the cel- to shrink simply to points. Images courtesy of University Bath, UK. on metallic surfaces. Such ultra- ebrated forbidden bandgap of There was also the question of Russell of Bath University. Normally bandwidth optical signals, follow- miniature LC circuit arrays, smaller electronic semiconductors like whether the required refractive light is trapped in optical fibers by ing the same miniaturization than an optical wavelength, may silicon. index might be unattainable in real total internal reflection in a high trajectory as conventional elec- eventually represent the ultimate While it took thousands of years materials, but the calculations refractive index region at the core tronic integrated circuits. end point of photonic crystal min- of metallurgy and materials science showed that a refractive index of of the fiber. In contrast, bandgap From the ultimate miniaturiza- iaturization. to discover and bring to perfection as little as 1.87 was enough in a confinement allows the core to tion of tiny optical waves, we go to Eli Yablonovitch is professor of electronic semiconductor crystals, diamond structure. As there are have a lower refractive index, in- macroscopic radio waves. Can the electrical engineering at the Univer- photonic crystals are in principle many optical materials available deed to consist of an empty hole. concept of an electromagnetic sity of California, Los Angeles. He is a more accessible. Since electromag- with refractive indices of up to 3.5, These “holey” fibers allow new free- bandgap be useful for radio waves? Fellow of the APS. netic waves appear equally well at it seemed feasible that photonic dom in fiber design that can be all wavelengths from giant radio bandgaps could be successfully valuable even when photonic waves to tiny gamma rays, artificial made from real existing materials. bandgap confinement is absent. It electromagnetic crystal structures But theoretical searches for is predicted that holey fibers may APS E-Board Passes Resolution can be made with any convenient photonic bandgaps in fcc struc- carry up to 100 times the informa- on Perpetual Motion Machines row spacing and size. tures were at first elusive. Initially, tion of conventional telecom- Only human imagination limits only a pseudo-gap emerged be- munications fibers, potentially with The APS Executive Board ap- tronic newsletter “What’s New,” the crystal design and structure— tween the 2nd and 3rd bands but such low losses that optical ampli- proved a resolution at its June 2002 reported that the US Patent Of- we are no longer restricted to real eventually, at a little higher fre- fiers and repeaters would be meeting in Annapolis, MD, affirm- fice has received several patent material crystals that grow in na- quency, a bandgap emerged2 unnecessary. ing the fraudulent nature of claims applications for perpetual mo- ture. Yet initially there was no between the 8th and 9th bands in A photonic crystal is often most of perpetual motion machines. tion machines during the first six assurance that any particular fcc structures. Later, contrary to functional when an artificial defect The resolution was deemed months of this year alone. [Park’s design would actually produce a all expectations, H.S. Sozuer, J.W. is introduced, similar to doping in necessary because of a recent in- 2000 book, Voodoo Science, de- forbidden photonic bandgap. Ulti- Haus, and R. Inguva found that a a conventional electronic semicon- crease in patent applications for voted considerable space to the mately the search for the first bandgap, albeit a small one, could ductor. Added dielectric material such devices. Robert Park, APS phenomenon of such devices electromagnetic bandgap crystal exist even in a simple cubic is equivalent to “donor” doping, Director of Public Information throughout history.] The text of would take four years, and involve “scaffold” structure. and deleted dielectric material is and author of the weekly elec- the APS resolution follows. the participation of numerous equivalent to “acceptor” doping. experimentalists and theorists who Some Real Life Photonic An example of this is the thin The Executive Board of the American Physical Society is con- had no idea in advance whether a Crystals film 2-dimensional hexagonal-ar- cerned that in this period of unprecedented scientific advance, mis- true photonic bandgap could ever Nature already makes photonic ray photonic crystal. John guided or fraudulent claims of perpetual motion machines and other even exist. crystals, in the sparkling gem opal, Joannopoulos, Shanhui Fan and sources of unlimited free energy are proliferating. Such devices di- rectly violate the most fundamental laws of nature, laws that have The absence of early empiri- and in the colors of butterfly wings. Pierre Villeneuve of MIT and E. F. guided the scientific progress that is transforming our world. cal success was compounded by These have photonic band struc- Schubert of Boston University did