New unifying theory of lasers advanced by physicists

Caption: An artist's rendering of a random laser that is pumped with incoherent light from the top and emits coherent light in random directions. Credit: (Courtesy of Robert Tandy & Science Magazine) Usage Restrictions: with credit given

New Haven, Conn. — Researchers at Yale and the Institute of Quantum Electronics at ETH Zurich have formulated a theory that, allows scientists to better understand and predict the properties of both conventional and non-conventional lasers, according to a recent article in Science. “The lasers that most people are familiar with emit a narrow beam of light in a fixed direction that has a well-defined wavelength and a predictable power output — like those in laser pointers, bar-code readers, surgical instruments and CD players,” said senior author A. Douglas Stone, the Carl A. Morse Professor of Applied Physics at Yale. In these conventional lasers, the light is trapped and amplified between parallel mirrors or interfaces and bounces back and forth along one dimension.

Scientists can determine what the light output will be based on the “leakiness” of the mirrors, which is usually quite small.

But, a new breed of lasers — diffusive random lasers (DRLs) — made possible by modern nanofabrication capabilities, consist of a simple aggregate of nanoparticles and have no mirrors to trap light. These lasers were pioneered by Hui Cao, now a professor of applied physics at Yale, and have been proposed for applications in environmental lighting (“laser paint”), medical imaging and displays. Until now, there has been no simple way for scientists to predict the wavelengths and intensities of the light emitted by DRLs.

Although, superficially, conventional lasers and DRLs appear to operate very differently, experimental results indicated many basic similarities, and scientists have searched for a unifying description that would apply to all lasers.

The properties of a laser are determined by measuring the lasing modes, including the pattern of light intensity within the laser, and the wavelengths of light it puts out. With conventional lasers, these modes can easily be obtained through simulations.

“For random lasers, time-dependent simulations are difficult to do, hard to interpret, and don't answer the question: ‘What is the nature of the lasing modes in a random laser,’” according to Stone. “Researchers really wanted a description similar to that for conventional lasers, but no one knew how to develop such a description.”

To create their unifying theory, the researchers derived a wholly new set of non-linear equations that fit both conventional and non-conventional lasers — such as the DRL or other nanostructured lasers. Based on these equations Stone, his former PhD student Hakan Tureci, now at ETH Zurich, and two other members of Stone’s research group, Li Ge and Stefan Rotter, created a detailed computer code that can predict all the important properties of any kind of laser from simple inputs.

“The state of laser theory after forty years was an embarrassment; it was essentially qualitative, but not predictive or quantitative,” says Stone. “We went back to the basics — and we think we have now solved that problem.”

A “Perspective” review of the theory in the same issue of Science noted, “By developing a new theory in which the main properties of a laser can be physically understood . . . they have provided a substantially broader perspective of laser physics that unifies the physical description of many possible laser structures.”

“Ultimately, we hope that our code can be used as a design tool for new classes of micro- and nano-lasers with important applications” says Stone, who also believes that eventually their theory will become part of the answer to the question: “How does a laser work"” ###

This research was funded by the National Science Foundation, the Max Kade and W. M. Keck foundations, and by the Aspen Center for Physics. Citation: Science 320: 643-646. (May 2, 2008) [DOI: 10.1126/science.1155311]. Perspective review: Science 320: 623. (May 2, 2008) [DOI: 10.1126/science.1157494]

Further supporting material is available online at www.sciencemag.org/

Contact: Janet Rettig Emanuel janet.emanuel@yale.edu 203-432-2157 Yale University

Tags: Nano or Nanotechnology and Nanotech

Brown Researchers Work Toward Ending Cartilage Loss

Brown University nanotechnology engineer Thomas Webster has published a first-ever study that shows how a surface of carbon nanotubes combined with electrical pulses could help regenerate cartilage naturally in the body.

PROVIDENCE, R.I. [Brown University] — Scientists have long wrestled with how to aid those who suffer cartilage damage and loss. One popular way is to inject an artificial gel that can imitate cartilage’s natural ability to act as the body’s shock absorber. But that solution is temporary, requiring follow-up injections.

Now Brown University nanotechnology specialist Thomas Webster has found a way to regenerate cartilage naturally by creating a synthetic surface that attracts cartilage-forming cells. These cells are then coaxed to multiply through electrical pulses. It’s the first study that has shown enhanced cartilage regeneration using this method; it appears in the current issue of the Journal of Biomedical Materials Research, Part A.

“Cartilage regeneration is a big problem,” said Webster, an associate professor in the Division of Engineering and the Department of Orthopaedics at Brown. “You don’t feel pain until significant cartilage damage has occurred and it’s bone rubbing on bone. That’s why research into how to regenerate cartilage is so important.”

Webster’s work involves carbon nanotubes, which are molecular-scale tubes of graphitic carbon that are among the stiffest and strongest fibers known and are great conductors of electrons. They are being studied intensively worldwide for a range of commercial, industrial and medical uses.

Webster and his team, including Brown researcher Dongwoo Khang and Grace Park from Purdue University, found that the tubes, due to their unique surface properties, work well for stimulating cartilage-forming cells, known scientifically as chondrocytes. The nanotube’s surface is rough; viewed under a microscope, it looks like a bumpy landscape. Yet that uneven surface closely resembles the contours of natural tissue, so cartilage cells see it as a natural environment to colonize.

“We’re tricking the body, so to speak,” Webster said. “It all goes back to the fact that the nanotubes are mimicking the natural roughness of tissues in the first place.”

Previous research has involved using a micron surface, which is smoother at the nanoscale. Webster said his team’s nanosurface works better than micron due to its roughness and because it can be shaped to fit the contours of the degenerated area, much like a Band-Aid.

The researchers also learned they could prod the cartilage cells to grow more densely by applying electrical pulses. Scientists don’t completely understand why electricity seems to trigger cartilage growth, but they think it helps calcium ions enter a cell, and calcium is known to play an integral role in growing cartilage.

The team plans to test the cartilage regeneration method procedure with animals, and if that is successful, to conduct the research on humans.

Webster’s cartilage regeneration studies parallel research he has done with bone regeneration and implants that was published last year in Nanotechnology. The principles are the same: Bone cells are more apt to adhere to a rough carbon nanotube surface than other surfaces and to colonize that surface. And tests by scientists in Japan and elsewhere have shown that electrical pulses stimulate bone cell growth.

The National Science Foundation, under the federal National Nanotechnology Initiative, funded the work.

Editors: Brown University has a fiber link television studio available for domestic and international live and taped interviews, and maintains an ISDN line for radio interviews. For more information, call (401) 863-2476.

Contact: Richard Lewis Richard_Lewis@brown.edu 401-863-3766 Brown University

Tags: Nano or Nanotechnology and Nanotech