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  • Photonic Crystals

Photonics Spectra
Jun 2002
A Growth Industry

Daniel C. McCarthy, Senior Editor

Photonic crystals are edging toward commercial application, thanks to advances in design, materials and manufacturing.
Photonic bandgap crystals serve as a microcosm of the photonics industry: Although microscopic in structure, their versatility in manipulating photons could have a huge and pervasive impact on nearly all optical components and instrumentation, enabling new generations of tunable filters, low-loss waveguides, microlasers, all-optical switches and integrated circuits.

The photonic bandgap these crystals create has uses similar to an electronic bandgap, particularly if the crystal structure has a carefully engineered flaw. The effect such flaws have on photons is analogous to the effect dopants have on electrical current in a semiconductor: They open a well-defined path, allowing bandgap photons to travel through the crystal. One might argue that similar applications can be achieved through total internal reflection — say, in a waveguide. But total internal reflection is inherently limited by scattering losses at tight bends or small pointlike cavities. A bandgap crystal, however, traps light so completely that it will direct photons around sharp bends with little loss.

The lattice structure of a bandgap crystal is on a scale with optical wavelengths and is based on alternating materials with high and low dielectric constants. On a larger scale, similar periodic structures can form bandgaps for microwaves, radio waves and even sound waves.

Whatever the scale, the principle underlying how these crystals function is familiar to anyone who understands how a multiple-layer thin-film filter uses interference to reflect specific wavelengths. Basically, the lattice periodicity is spaced so that bandgap wavelengths destructively interfere, thereby preventing their passage through the crystal. Constructive interference enhances wavelengths outside the bandgap and allows them to propagate through the crystal with little attenuation.

Thin-film filters and fiber Bragg gratings fit the definition of one-dimensional photonic crystal structures. But they provide only a one-dimensional bandgap that moves with a change in the angle of incidence or other properties of the light. A multilayer film, for example, shows a bandgap only for wavelengths propagating normal to the crystal. These types of structures are simple enough to occur in nature. Hence, abalone shells’ iridescence derives from the angular dependence of the bandgap frequency.

Although a partial bandgap in one dimension is enough for filters and such, most of the potential and, therefore, most of the interest in photonic crystals stem from two- and three-dimensional structures. Like one-dimensional crystals, higher-order structures can filter selective wavelengths, but in a like number of dimensions.

Trial and error

About 11 years ago, three-dimensional photonic bandgap crystals existed only in theory. Like simpler structures, such as thin-film filters, they promised to block photons within a certain wavelength band from propagating through the crystal lattice. Unlike one- and two-dimensional crystals, however, these structures would create a bandgap regardless of the direction the photons traveled or their coherence, polarization or angle of incidence. The eventual application of such structures was even more theoretical, but the main thrust was to do for photons what semiconductors had done for electrons, and to progress from there.

Producing a structure that demonstrated a functional optical bandgap proved difficult, requiring years of trial and error. The earliest successes emerged in 1991 and borrowed from diamond’s tetrahedral lattice. Shortly thereafter, face-centered cubic lattices of airholes in dielectric appeared. These are actually extensions of the diamond lattice and, although they have a smaller bandgap, they can be self-assembled. A third structure, modeled after cubic scaffolding, produced an even smaller bandgap.

Self-assembled crystal fabrication has been limited to spherical colloids and, therefore, to face-centered cubic lattices. But new colloids are emerging with different shapes — including rods, ellipsoids and polygons — which could enable lattices with wider bandgaps. Courtesy of the University of Washington.

Since then, researchers have modified the lattice materials, components and shapes of photonic crystals, but the underlying structures don’t stray far from the original three. Although new designs ease fabrication or improve the bandgap, they still combine elements from both diamond tetrahedral and face-centered cubic lattices.

Three-dimensional bandgap structures can produce a complete bandgap, which offers plenty of application possibilities, particularly if the light can be coaxed through a flaw in the crystal. By combining such devices to control the propagation of light in predictable ways, researchers could develop photonic integrated circuits, or if constructed from photosensitive dielectrics, a crystal’s bandgap could be altered by incoming wavelengths, thereby blocking or routing alternate wavelengths — making a photonic switch.

“You can guide light in a waveguide or trap it in a cavity, but the next step is to combine them into devices,” said Steven Johnson, a researcher at Massachusetts Institute of Technology in Cambridge. “We have all these designs for things like channel drop filters, but they haven’t been fabricated into three-dimensional systems except in the most primitive sense.”

State-of-the-art designs for photonic crystal structures still borrow their geometry from the decade-old diamond lattice configuration. The layers of the structure shown here alternate dielectric rods in air and airholes in dielectric. Courtesy of Massachusetts Institute of Technology.

Hence, integrating bandgap crystals into a working device is a major goal, but Johnson said the research community is still refining the essential building blocks.

“I don’t see any showstoppers,” he said, in regard to integrated devices based on photonic bandgap crystals. “It’s more a question of putting in the hours at this point. There are still some unknowns.”

Among those unknowns are how to fabricate three-dimensional crystal structures in volume and how to invest them with functional defects.

Stacked logs and marbles

It is no surprise that semiconductor fabrication techniques would apply to the photonic analogue for semiconductor devices. About five years ago, Shawn-Yu Lin at Sandia National Laboratories in Albuquerque, N.M., used standard lithography techniques to create a crystal structure that resembled a stack of logs. A derivative of the face-centered cubic lattice, Lin’s crystal lattice comprised four layers, each with alternating rods and spaces laid in parallel fashion.

He deposited, patterned and etched the first layer as parallel silicon dioxide rods. He then filled the gaps with polycrystal line silicon and polished the surface. After depositing a second layer of rods turned 90°, he applied the polymer again. Rods in the third layer ran parallel to rods in the first, but they were located over the spaces in layer one. After a third polymer application, Lin laid the fourth layer of rods, then etched away the polycrystalline silicon, leaving a silica-and-air crystal lattice that had a complete three-dimensional bandgap for wavelengths between 10 and 14.5 μm.

In the May 2 issue of Nature, Lin described a similar structure that he made from tungsten, which showed a large bandgap from 8 to 20 μm.

Besides being well-established and understood, microlithography can produce complex features with enough accuracy to engineer precise and functional defects in the crystal lattice. But few academic laboratories have a lithographic stepper able to produce nanoscale defects. Consequently, a number of researchers working independently have explored self-assembly methods based on colloidal suspensions.

Microlithography can fabricate crystal lattices with complete three-dimensional bandgaps, but it is both expensive in small volumes and not given to creating three-dimensional defects. Courtesy of Sandia National Laboratories.

Early demonstrations showed that nanospheres suspended in a meniscus align into orderly stacks as evaporation moves the meniscus across a substrate. The stacked spheres serve as the mold for a photonic crystal lattice: After a high-refractive-index material is injected between the nanospheres, the spheres are chemically etched away.

This fabrication method is simpler, faster and cheaper than lithography. And three-dimensional crystals are formed directly on a substrate, which could prove handy in integrated optics applications. However, if and when crystals become a growth business, so to speak, lithography’s manufacturing benefits may win out.

“If this was something that Intel was going to do, I think they would figure out how to do it with lithography,” said Paul Braun, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign. “But from a research perspective, or in cases where you need them in lower volumes, self-assembly makes more sense.”

What’s the use of a three-dimensional bandgap if you can’t write defects in three dimensions? One approach borrows the scanning method from multiphoton microscopy to inscribe a three-dimensional defect through a bandgap crystal. Courtesy of the University of Illinois.

Thanks in part to work by David Norris and Yuri Vlasov at NEC’s Research Institute in Princeton, N.J., self-assembly methods overcame early problems with unwanted defects and particle sizes limited to 400-nm diameters. The small size, in particular, was a problem because it prevented bandgaps in the potentially lucrative telecommunications window above 800 nm. Now that unwanted flaws in the lattice are better controlled, the next challenge is how to produce intentional defects to create waveguides and other devices.

Three-dimensional defects

Multiphoton polymerization is one approach, as demonstrated by Braun and collaborators Wonmok Lee and Stephanie Pruzinsky. The team injected a photosensitive monomer into the interstices of a colloidal template and used a microscope objective to scan the focal point of an infrared laser through the template’s interior. The intense light at the focal point polymerized the monomer, allowing them to create three-dimensional waveguides in the crystal. Defects demonstrated had minimum widths of 1.5 μm, edge resolution better than 100 nm and cut sharp curves with 500-nm radii.

Like Lin’s stack-of-logs structure, self-assembled crystals define the size of their bandgap by the contrast between the material components’ refractive indices — the greater the contrast, the stronger the bandgap. Most self-assembled crystals aim for a contrast of at least three; that is, a refractive index of 1 for air and an index of 3 for the lattice material. But the multiphoton-generated defects in the University of Illinois crystal are made of polymer, which has a refractive index of about 2.5. So the next step for Braun and his collaborators is to find a way to etch out the polymer, leaving an air guide embedded in the matrix.

That’s a challenge, because before the polymer is etched out, a crystal lattice must be formed by depositing a high-refractive material within the colloid template, and the polymer cannot easily withstand the high temperature of most deposition processes. Braun said his group may have solved this problem, but he refrained from revealing how until the solution undergoes peer review.

It is possible, however, that he will borrow from earlier work in which he deposited selenium in colloidal templates. Selenium has a low surface tension, which allows it to easily penetrate the template, a high refractive index and, most importantly, a lower melting point (217 °C).

Another limitation of self-assembled crystals, as they stand today, is that they are stuck with a face-centered cubic structure based on spheres. Forming tetrahedral lattices would deliver much larger bandgaps, but such lattices require more complex colloidal structures that must be oriented as well as organized within a structure. Spheres, unfortunately, do not cooperate well and tend to collapse together like “marbles in a cup,” Norris explained. But chemists at the University of Washington in Seattle are working on a solution.

Led by Younan Xia, they innovated colloids with different shapes, including rods, ellipsoids and polygons. Oddly, the most complex colloids, the polygons, are also the simplest to form because they are essentially extensions of commonly available nanospheres.

For example, Xia created polystyrene tetrahedrons by confining an aqueous dispersion of monodisperse polymer spheres in a cell. By slowly draining the liquid through the cell, the dispersion left spherical colloids in notches along the cell wall. By controlling the ratio between the dimensions of these holes and the diameter of the beads, he could produce well-defined nonspherical clusters with consistent dimensions ranging from 150 nm to 5 μm.

The next step is to find a way to organize these building blocks into a bandgap structure. One approach that Xia is considering borrows from self-assembly methods and orients the colloids with either a magnetic or an electrical field. Whatever the approach, he said, “It’s going to be much more challenging than spheres.”

Holey logic

The relative ease in calculating in-plane bandgaps helps explain why most of the early theoretical work was done on three-dimensional photonic crystals. But, as Philip Russell pointed out, “A photonic crystal doesn’t have to have a photonic bandgap to be useful. The bandgap is just one feature.”

A case in point, as demonstrated by Russell and a handful of other scientists, is photonic crystal fiber, which comprises a one- or two-dimensional bandgap structure with a cavity that is kilometers in length — or depth, depending on how you look at it. The cavity serves as the fiber core.

Russell, a professor at Bath University in the UK and chief technology officer for Blaze Photonics, also in Bath, pioneered the development of hollow-core fibers. Fibers with similar configurations have sprung up at Corning Inc. in Corning, N.Y., and OmniGuide Communications in Cambridge, Mass.

Only constructive scattering from a photonic bandgap structure can guide light down hollow-core fibers, such as those facing left. Other photonic crystal fibers use a modified form of total internal reflection to guide light along a silica core. Whether there is silica or air in the core, photonic crystal fibers could take the ability of optical fiber to the extreme.

Short of a vacuum, the material density in the core of these fibers is as low as it gets. That low density translates into some potentially revolutionary fiber specifications, including radically higher power densities abetted by reduced nonlinearities and scattering. These features are simply not possible with conventional fiber technology.

“If you’re going to guide light in a hollow core, you have to do it with a photonic bandgap,” observed James West, a senior research scientist at Corning. Unlike the prevalent highly refractive silica core, hollow-core fibers guide light through a low-refractive material — air; instead of total internal reflection, they use a bandgap structure to confine light by constructively scattering it back into the core.

Confusing matters, both Blaze Photonics and Crystal Fibre in Birkerod, Denmark, market photonic crystal fibers that use a modified form of total internal reflection to guide light along a silica core. Although some argue that this essentially makes these fibers pumped-up versions of conventional fibers, the photonic crystal structure enables tailoring of dispersion or polarization attributes beyond what established technology can easily provide.

Different companies configure in different ways the bandgap structure of their fiber, but all are typified by promising claims. OmniGuide, for instance, says its concentric circle structure will provide losses in the range of 0.01 dB/km — 20 times less than conventional silica fibers — and a 10,000x reduction of nonlinear effects.

“That means that, where you could propagate 1 W of power for a half kilometer in other fibers, ours will propagate the same watt for 5000 km,” said Uri Kolodny, director of marketing.

There is, however, a significant gap between the promise and what’s currently possible.

“Hollow core [photonic crystal fiber] has extremely low Rayleigh scattering — though the best losses so far achieved are on the order of 0.3 dB/m,” Russell said. “Other effects dominate over Rayleigh scattering, leakage from the core being the dominant one.”

According to Russell’s figure, basic math would suggest that the state of the art for hollow-core fiber still suffers a 300-dB/km loss, well beyond what an operating fiber network can accept. An exact determination will have to wait because, thus far, loss has been calculated on fibers measuring in tens of meters, not thousands.

The reason for that — and for the loss itself — is linked mainly to process control issues in the fibers’ manufacture, which cannot maintain consistency in the photonic bandgap and core structure in long strands. Structural variations can be in-plane, axial or a combination.

All of the crystal fiber makers interviewed agreed that the manufacturing process shouldn’t be too different from that for standard fiber. The most notable divergence is in the preform.

“The idea here, of course, is to reinvent as little as possible,” Kolodny said. “So, just like in silica or two-dimensional bandgap fiber processes, the intelligence is built into the fiber preform.”

OmniGuide’s fiber is based on a one-dimensional bandgap structure — concentric circles — which lends itself to established lay-down methods for preform fabrication.

That process, however, doesn’t work for fibers with two-dimensional bandgap structures. There, the prevailing method for making fiber preforms is to stack and draw. In other words, glass rods and tubes are arranged, or stacked, into a macrocosm of the fiber structure. Consequently, West said, there’s probably a little more cost at the front end for making the preform.

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