September 01, 2005
Modeling of photonic crystals at NSF supercomputing centers, now partners in the TeraGrid, over several years has led the way to a major advance in laser surgery, exemplifying how computational simulations no longer take a back seat in driving scientific discovery.
In November 2004, a woman in North Carolina with potentially suffocating growths in her larynx and trachea had them removed by a high-power laser -- and went home the same day. This condition had never before been treated without anesthesia and operating-room surgery. Six years earlier, physicists at MIT used supercomputers to learn something no one knew about mirrors.
These two seemingly separate events indeed are linked. A new laser technology, developed from a startling insight into the physics of light, may have saved the woman's life and, at the least, promises huge savings in the treatment of her disease -- recurrent respiratory papillomatosis -- one that affects tens of thousands of people in the United States alone.
It's a success, furthermore, that exemplifies how supercomputing is no longer merely a supporting character, but with increasing frequency plays a lead role in scientific discovery. In 1998, John Joannopoulos and his team of researchers at MIT discovered what has come to be called a "perfect mirror." Their "eureka!" moment came not in the laboratory or with pencil and paper working out of mathematical theory; it happened because a computational model produced results no one expected.
For the past decade, Joannopoulos and his team have pushed forward new understanding of "photonic crystals" -- fascinating materials, crafted from layers of silicon, which have unprecedented ability to trap, guide and control light. While he works closely with a laboratory team, headed by MIT professor Yoel Fink, to fabricate these challenging materials, a key to this work driving forward has been computational simulations that predict -- successfully and precisely -- how photonic crystals will work in advance of actually making them. "Computation," said Joannopoulos, "has played a dominant role in the study of photonic crystals."
The Perfect Mirror
It may be the most significant advance in mirror technology, said the New York Times, since Narcissus fell in love with his own image in a pool of water. The perfect mirror is so called because it reflects light at any angle with virtually no loss of energy. As a result, it makes possible a number of applications in optical technology, the most significant to date being flexible optical fiber that can transmit the high-powered CO2 lasers used in endoscopic surgery.
Until Joannopoulos' team's 1998 finding, reported with a paper in Science, mirrors were understood to come in two basic flavors, both with inherent limitations. Everyone who looks in the bathroom mirror for signs of life in the morning knows about metallic mirrors. They work all too well for seeing your own face, but they don't work to make optical fiber because a large portion of the light leaks away, absorbed by the metal, rather than reflected.
For optical fiber and other applications where energy loss matters, the choice has been mirrors made from dielectrics -- materials that don't conduct electricity well. Dielectrics generally don't reflect light well either, but scientists have found ways to alternate thin dielectric layers of different reflective properties to achieve reflection without energy loss. The drawback has been that these dielectric mirrors reflect light only from certain angles, and their application depends on being able to use light at a limited range of angles and frequencies.
This limitation was thought to be a law of nature, like gravity - no way to get around it -- until 1998, when Joannopoulos and company noticed anomalous results from a computational model of a photonic crystal mirror they were running at the San Diego Supercomputer Center. The light seemed to reflect at a much larger angle than was thought possible. "We saw some interesting results in the computation," he said. "Then came the theory to explain the computation, and then came a real experiment making something like this and testing it."
The result: a multi-layered dielectric mirror that reflects light from all angles without energy loss. Within a few years, the perfect mirror proved to be the solution for delivering a high-powered laser via flexible optical fiber.
Open Wide for a High-Power Laser
Fiber optics to transmit visible light, based on conventional dielectric mirror technology, has been around for years. These silica-based fibers have a light-carrying core with an index-of-refraction higher than the surrounding material. This layered approach traps light within the inner core -- called "total internal reflection." It works well for visible light, but high-power lasers -- such as CO2 lasers used in endoscopic surgery -- will melt conventional optical fiber.
Joannopoulos and Fink realized that the perfect mirror offered a potential solution for high-power transmission. With further computations and pioneering laboratory work, the team developed a hollow-core fiber -- essentially a dielectric perfect mirror rolled up into a tube -- designed in such a way, based on photonics, to transmit high-power lasers.
To take this idea beyond the laboratory into useful applications, in 2000, Joannopoulos and Fink helped form OmniGuide Communications, a company dedicated to developing and marketing the new hollow-core fiber. Further computations over the next few years -- in San Diego, Illinois and Pittsburgh -- explored other fundamental issues and phenomena of this new class of cylindrical photonic-crystal fiber.
In endoscopic surgery, the lack of a fiber for high-power transmission has meant that the laser had to be delivered to a patient via an apparatus with an articulated arm and large handpiece -- which has precluded using these precise lasers for many minimally invasive procedures. For this reason, the surgery to treat RRP required dislocating the patient's jaw and general anesthesia, so that the laser could be brought close enough to the affected area.
A test case for OmniGuide's hollow-core fiber presented itself last year. In serious cases of RRP, the surgery often must be repeated to keep the breathing passage open. Dr. Jamie Koufman, director of the Center for Voice and Swallowing Disorders of Wake Forest University Baptist Medical Center, had a woman RRP patient who had undergone several previous RRP surgeries, but once again had developed near-total obstruction of the larynx and trachea.
Koufman obtained FDA approval to use the prototype fiber. She used a numbing topical spray in the throat and trachea, requiring no anesthesia, and with a CO2 laser delivered via an OmniGuide fiber cleared the RRP growths. The patient, who went home that day, is doing fine.
"Unsedated, laryngeal laser surgery with the OmniGuide fiber is a dream come true for me as an endoscopic surgeon," said Koufman. "The patient loved it because it was easy for her." Typical cost of RRP operating-room surgery with general anesthesia is $25,000. With expected FDA approval, the new procedure promises large cost savings nationally.
"These novel optical fibers, based on photonic crystals offer a new approach for medical lasers, making it possible to guide a CO2 laser beam, which can cut tissue with high precision, into a patient's body through a very small incision," said Joannopoulos. "It will likely prove itself useful for many procedures."
Computational science has come a long way over the past 20 years," he added. "Even well known equations can have remarkable unexpected consequences that we would never learn about without these powerful computational engines, such as LeMieux (PSC's terascale system). This is just one advance that highlights how these machines are invaluable tools of discovery."
Michael Schneider is a senior science writer at the Pittsburgh Supercomputing Center.
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