Photonic crystals promise an exquisite control of light that could dramatically shrink the scale and costs of fiber-optic communications devices. The 'Holy Grail' of a photonic bandgap (PBG) laser that out-performs its conventional counterparts in directionality, spectral narrowness and power consumption (and starts working instantly thanks to the absence of a threshold) could also be on the horizon. For proof that photonics has arrived, look no further than Intel's 'direct Ge-on-Si' detector. High yield and bandwidth mark micro-photonics as the next killer technology both for communication and ICs.
The idea of photonic crystals emerged in the late 1980s simultaneously, with the work of Eli Yablonovitch at UCLA and Sajeev John at the University of Toronto. Yablonovitch proposed the construction of 3D, ordered crystal structures with lattice parameters on the order of a certain electromagnetic radiation, for example light [Phys. Rev. Lett. 58 (1987) 2059]. These structures have a similar influence on the propagation of light as atomic crystals have on electrons. John described the quantum-optical effects that 'carefully prepared dielectric superlattices' could have on photons [Phys. Rev. Lett. 58 (1987) 2486]. Crystals on the length scale of the wavelength of visible light could allow spectacular quantum phenomena to take place.
Yablonovitch created the first working photonic crystal in 1991 [Phys. Rev. Lett. 67 (1991) 2295] with a full photonic bandgap for radar waves, dubbed 'Yablonovite'. But to influence shorter wavelengths, such as microwaves, one has to build micrometer-scale structures;.The current limit for these mechanical systems is insufficient to reach the critical wavelength of 1.55 p.m for optical systems. To move towards even shorter wavelengths, it becomes difficult to create ordered structures.
Since 1991, many new designs and fabrication techniques have been tried. Although none have been successful for the visible region, the 'etched woodpile' structure by Susuma Neda at Kyoto University [Phys. B 279 (2000) 142] and so-called inverted opals fabricated and studied by Vos, myself and colleagues at the University of Amsterdam [Phys. Rev. Lett. 83 (1999) 2730; Science 281 (1998) 802] have come close. Based on the natural sedimentation of colloids into three-dimensional light-reflecting opal structures, the Dutch group's airsphere crystals are one of the most promising photonic structures. These 3D macroporous crystals, consisting of spherical air voids in titania (TiO2) with radii 120-1000 nm, are suitable for the optical spectrum.
The recent plethora of small businesses created to exploit photonic crystals signals a huge step forward in this field, 15 years after its initial conception. Using photonics to improve co-axial fibers is a particular focus. John Joannopoulos' company (www.omni-guide.com) is applying the omni-directional mirror for visible light, created by his group and co-workers at MIT, for just this purpose. The multi-layer dielectric mirror would allow light to be transmitted without loss - and without using amplifiers - or around sharp bends. Danish company Crystal Fibre (www.crystal-fibre.com), Blaze Photonics (www.blazephotonics.com), and researchers at Southampton's Photonic Crystal Technologies are working on an optical-fiber with a PBG structure inside: the air voids arranged in a 1D crystal pattern allow optical signals to travel much faster and with lower losses compared to glass.
Sajeev John's start-up Keralight will look at the practicality of his latest discovery -'square spiral microfabrication architecture' using glancing angle deposition (GLAD) methods [Science 292 (2001) 1133], which is capable of creating controllable large-scale designs with a robust photonic bandgap for frequencies in the visible. Other possibilities include inserting an optically active material, e.g. a nematic-crystal (consisting of nano-sized regions that align when an electric field is applied), inside the voids of a photonic crystal. Depending on the alignment, the material would become either transparent or opaque to light - in other words, a fast optical switch. And a spin-off company started by Yablonovitch, Ethertronics, aims to have a photonic crystal antenna technology for mobile phones ready this summer.
The way forward from concept to real PBG material will require a move towards complex 3D refractive index engineering, but photonics is undoubtedly here to stay. Although a fully, 3D photonic crystal in the visible range has not been reported yet, I expect this will happen soon. In the coming years, photonics research will definitely change the perspective of optical signal processing and communication.