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| 80.4 - Summer 2007 | ||||||||||
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New Lasers: Sparking Theoretical Progress
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By JOHANNES HIRN
What the heck is a laser anyway? Why don’t we understand it after 50 years?
Applied Physics Professor A. Douglas Stone begins his seminar, commanding the attention of a dozing audience. Now, your sleep may not be disturbed by the impossibility of predicting laser output power from first principles, but to Stone, “the approximations in the standard theory of lasers are an embarrassment.”
With his collaborators, Stone has developed a more complete and predictive theory. Next year, in-house experiments on non-conventional lasers will test this new theory further. The experiments will be led by Professor Hui Cao -the discoverer of random lasers, now at Northwestern University- who will be joining the Yale faculty this coming January.
Change doesn’t always come from withinStone is in fact more of a condensed matter physicist than an optics specialist. He therefore encounters extra hurdles when telling optics researchers that “nobody knows how to predict the output of a laser.”
To back up this claim, Stone pulls out quotes from textbooks, such as the ubiquitous understatement: “we will not discuss this question in more detail here.”
In fact, his background in condensed matter has proved of great help to Stone, whose work on lasers borrows techniques from his previous studies of electrons in cavities. But why did the standard theory need an overhaul in the first place?
A laser without output is not a laserThe standard theory of lasers is “oversimplified beyond necessity,” to Stone’s mind: it views a laser as a closed cavity. The downside is a closed cavity does not let light out, so this approximation precludes computation of the output power. Among other drawbacks, lasing wavelengths are not predicted accurately and spatial inhomogeneities inside the cavity are not taken into account.
These flaws have been corrected in a new mathematical framework developed by Stone, Dr. Hakan E. Türeci of ETH Zürich, Switzerland (a former graduate student of Stone’s), as well as students Braxton Collier and Li Ge. In their framework, it becomes possible to predict the output of a laser. This should facilitate the design of new laser cavities. But the new theory even applies to lasers without a cavity: random lasers.
Random lasers: from stars to Yale...Doing away with the cavity of a laser was a bold step. Instead of costly mirrors, Vladilen S. Letokhov suggested in 1968 that tiny particles could scatter the light in zigzags through the gain medium.
He also showed this phenomenon to occur naturally in gas clouds surrounding stars, leading to light amplification. A similar amplification was first demonstrated in the laboratory in 1994 by Prof. Nabil Lawandy, then at Brown University.
To really obtain the first random laser, which brought Cao many awards, resonant feedback was necessary. This occurs when there are enough scatterers in the gain medium.
Put simply, some of the photons are scattered along closed paths, effectively finding a cavity by themselves. (Reality is in fact more complicated, and involves chaotic trajectories, see Box 2.)
According to Stone, “random lasers are the ideal testing ground for the new theory.” This time, it is not only the power and wavelength that are unknown, but also the direction of the outgoing beam. Fortunately, random lasers can be tested in configurations more controllable than stellar clouds. Even better, ease of fabrication is the main advantage of random lasers: random lasing was first observed by Cao and collaborators in 1999 in zinc oxyde powder (0.1 micron diameter particles).
...and beyondStone already has predictions about the experiments Cao will perform when she joins Yale this January. Hopefully, their common work will bring random lasers one step closer to practical applications.
Many such uses have already been proposed, such as cancer diagnosis. Lasing from a dye injected in a patient would be stronger in damaged tissues (such as a tumor), than in healthy ones, making tumors distinguishable. This is because damaged tissues contain more inhomogeneities, which are the basis for random lasing.
Also, the unique wavelengths of a given random laser would act as a signature. Yet another possible application would be screen displays: because random lasers emit at various angles, the displays should be readable from any position (see Boxed Text 2).
Although we have yet to see which of these applications will make it to the market, random lasers have an edge over conventional lasers: because they do not require a cavity, they are cheaper to manufacture.
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