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Elusive photonic crystals come a step closer
In 1987, the physicist Eli Yablonovitch predicted that materials called photonic bandgap crystals (PBCs) would enable light to be handled in the way existing microcircuits handled electrical signals 1 . Since then, one- and two-dimensional cousins of PBCs have been microfabricated 2 , for which several applications have been found. Although some small PBCs have been formed by direct microfabrication 3 , a bulk 3D PBC material has been elusive, along with its potential applications — including next-generation computing technology. Writing in Nature , He et al. 4 report the growth of opal-like crystals that have the unusual structure required for PBCs: transparent micro-particles arranged in a manner akin to the carbon atoms in a diamond crystal. For a working PBC, these materials will need to be used as moulds to form Swiss-cheese-like ‘inverse opals’ that have holes where the current crystals have particles. Read the paper: Colloidal diamond . To understand the physics of materials such as PBCs and semiconductors, imagine trying to run across a furrowed field. If your stride matched the spacing between the furrows, you might find that you can run at two speeds: quickly, by skipping along the tops of the furrows; or more slowly, by letting your feet fall in the muddy troughs. Analogously, when a wave passes through a periodic medium that has alternating more- or less-dense ‘furrows’, it can propagate in two ways: with its crests on the peaks of the furrows, or with its crests falling between these peaks. In general, such a wave has two possible energies, corresponding to the two modes of propagation; it is not possible for any such wave to have an energy in the gap between these values. In a 3D crystal, the spacing of the furrows and the gap energies depend on the direction of the wave’s motion with respect to the axes of the crystal lattice. However, for certain kinds of crystal, there can be a range of wave energies, known as a bandgap, for which waves cannot propagate in any direction at all. In a silicon-crystal semiconductor, the waves are electrons, and the bandgap means that electrons of certain energies cannot exist, enabling devices such as transistors — the tiny switches that are ubiquitous in modern electronics. Quantum cascade laser lives on the edge . Yablonovitch showed theoretically that a similar bandgap phenomenon could occur for light waves, but only for a few crystal structures resembling the diamond lattice, and formed of microscopic particles made of certain transparent materials. Fortuitously, microparticles of the required size will often spontaneously arrange themselves into analogous ordered structures, termed colloidal crystals. Indeed, opals are naturally formed, fossilized colloidal crystals of silica particles, and the sparkle of opals is caused by the energy gaps described above. When light shines on an opal, some of the photons will have an energy (associated with a colour) in the gap. Such photons cannot enter the crystal, resulting in nearly 100% reflection. The gap energies (and therefore the reflected colours) depend on the direction of the incident light, giving opals their characteristic ‘fire’. Despite optimism in the 1990s that a simple method would yield diamond-like colloidal crystals, more than two decades and several innovations 5 would be required as a prelude to He and colleagues’ achievement. In a diamond lattice, every particle is connected to four equally spaced nearest neighbours. But making particles that attach to only four neighbours does not suffice to form diamond. When two such particles come together, they must also be rotated such that the other six particles they bind to are in the correct relative orientation. To achieve this feat, He et al. synthesized microscopic plastic building blocks that resemble chubby balloon animals. Each building block consists of four merged spheres in the shape of a triangular pyramid, with a recessed sticky patch in the centre of each pyramid face (Fig. 1a). When suspended in a drop of water, particles that dock together through their sticky patches are forced into the required angular configuration. These particles then spontaneously form highly ordered, stable crystals that have the long-sought diamond structure (Fig. 1b). Figure 1 Growth of opal-like crystals with a long-sought structure analogous to that of diamond. a , He et al. 4 synthesized microscopic plastic particles consisting of four merged spheres in the shape of a triangular pyramid, with a recessed sticky patch in the centre of each pyramid face. Some of these patches are highlighted in blue. b , When suspended in water, these particles dock through their sticky patches to spontaneously form opal-like ordered materials, in which the particles are arranged in a manner analogous to the atoms in a crystal. In the crystal shown, the particles mimic the arrangement of carbon atoms in diamond. Scale bars, 1 ?m. The authors have so far produced crystals containing only about 100,000 particles and weighing less than one microgram. However, scaling up their process should be straightforward. Then, all that remains to form large 3D PBCs is to chemically fill the empty space in these crystals with pure silicon or titanium dioxide (for use with infrared or visible light, respectively) and then dissolve the building blocks. One of the most exciting possible applications of PBCs is for quantum computers. In these devices, the digital bits that store values of ‘0’ or ‘1’ in a conventional computer are replaced with quantum bits (qubits) that can be both ‘0’ and ‘1’ at the same time. This replacement enables impressively faster computation of many difficult combinatorial problems that can be encountered in code-breaking. The challenge of building practical quantum computers lies in connecting many qubits together, typically using photonic signals, as well as isolating the qubits so that they do not get scrambled by interference from the outside world. The piping around of photons in a PBC microcircuit is a solution to the first problem, and 2D PBCs have already been used to build prototype quantum devices 6 . But because current quantum photonic circuits are thin 2D sheets, their performance is limited — photons can leak out and disturbances can leak in. A simple solution to both problems would be to sandwich these circuits between two slabs of 3D PBC. More generally, bulk PBCs will enable a broad range of technologies in the production of large quantum systems 7 , their controlled manipulation using light, and interfacing with conventional electronics 8 . The ultimate potential and applications of such technologies challenge our imagination. .
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