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17 Jun 2002
This year's European Conference on Optical Communication (ECOC) saw Japanese, US and European scientists discuss the latest advances in photonic crystal structures.
Photonic crystals - or photonic bandgap materials - are periodic dielectric structures that contain a bandgap. The bandgap restricts the propagation of certain light frequencies which means that these materials can be used to control light in ways that conventional optics cannot.
Since their introduction 10 years ago, these materials have been attracting more and more attention from scientific communities. This year's 27th European Conference on Optical Communication (ECOC) devoted an entire symposium to photonic crystals.
Susumu Noda of Japan's Kyoto University kicked off the session with his work on semiconductor three-dimensional (3D) and two-dimensional (2D) photonic crystals. Using a technique called wafer fusion to stack a criss-cross array of III-V semiconductor strips, Noda's group has developed 3D near-infrared photonic crystals. Recent landmarks include a 90°-bend waveguide; the introduction of a light-emitter to a 3D crystal; and a single defect cavity design.
Noda also emphasized the importance of 2D photonic crystals in making high-power single-mode surface-emitting lasers and ultra-small fiber-optic devices. By adding single defects to photonic crystal slabs Kyoto researchers have created devices that trap and emit photons of different wavelengths into free space. Noda believes that these devices could be used to drop photons into an optical fiber.
Willem Vos of the Van Der Waals-Zeeman Institute at the University of Amsterdam in the Netherlands is tackling 3D photonic crystals differently. Taking advantage of the self-organizing nature of colloidal crystals, Vos and his team have made ordered air void structures surrounded by high-refractive materials such as titania.
Vos expects his crystals to find use in miniature light emitting diodes and lasers, all optical transistors and optical circuits.
German researchers at the University of Ulm are investigating the applications of photonic crystals. Ulm researcher Heiko Unold and colleagues have etched hexagonal arrays of air holes onto 980 nm vertical-cavity surface-emitting lasers (VCSELs) to fabricate photonic crystal surface emitting lasers (PCSELs).
By changing the hole diameters, Unold says his team can alter the power and singlemode range of the PCSELs. "Investigating the optical guiding properties has led to [the development] of design rules," he said. "Precise tailoring of the mode behavior [of the PCSELs] is now feasible."
Scientists at the Ecole Polytechnique in Palaiseau, France believe that photonic crystals are crucial to ultra-compact optoelectronic integration. Henri Benisty and colleagues have made 3D perforated waveguides based on conventional GaAs and InP substrates as well as novel membrane systems.
The team has made progress towards integrated optical guides, coupled guides and cavities and is now investigating ways to reduce bend losses. Benisty believes that his work opens up new applications such as etched laser mirrors. "A more ambitious program is to revisit the whole lasing cavity with photonic crystal concepts," he added.
Moving on to fibers, UK scientists are looking at a novel application for photonic crystal fiber (PCF) preforms. Philip Russell and colleagues of the University of Bath are attempting to merge light with sound to produce acousto-optic devices based on PCFs.
The team has observed acoustic coupling between defects in the core of a preform. "By trapping light and sound in a very small space we force them to converse," said Russell. "Since a photonic crystal fiber is a scaled-down preform we expect to find the same acoustic effects at higher frequencies in PCF."
US-based Corning researcher James Fajardo summed up developments in PCFs for communications applications. Fajardo and colleagues have fabricated three types of fiber: low loss effective-index crystal fibers; high linearity/low dispersion air-clad core fibers; and photonic bandgap fibers (PBGFs) that guide light through an air core. Fajardo believes that PBGFs are the most "unique".
Citing PBGFs as key to achieving true light guidance in low index cores, the team believes that these fibers will offer properties that are yet to be discovered in standard fibers. They also highlight however that the future of these fibers has to focus on the practicality of the technology.
"The promise that crystal fibers will one day replace conventional fibers will be tested as the realities of large-scale manufacturing are confronted," concluded Fajardo.
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