Making Light Pulses Breathe
Stacking glass and air helps prevent beam collapse.
Motivated in part by ideas proposed for clouds of ultracold atoms, researchers at California Institute of Technology in Pasadena and at the University of Massachusetts Amherst sent femtosecond laser pulses through a stack of glass slides and intervening air gaps without the beam collapsing on itself. Instead, the beam shrank and expanded in width, seeming to breathe in and out as it passed through the stack.
Passing a laser beam through a stack of alternating glass slides and air gaps enables management of the beam’s nonlinearity. The blue curves show experimental results (for the beam width) as a function of propagation distance. Stars indicate propagation in air; triangles, propagation in glass; circles, propagation in alternating slidesair. Only the short propagation distances are shown for propagation in glass prior to the formation of plasma. The dash-dotted lines show the corresponding model results. z = propagation distance.
This demonstration of nonlinearity management — which had been theoretically predicted but not experimentally accomplished — ultimately could lead to a number of uses, according to the institute’s Demetri Psaltis, an electrical engineering professor and research team leader. “Applications could be found in lithography or sensing and possibly optical switching,” he said.
Most materials are self-focusing when hit with femtosecond laser pulses. This behavior occurs because the refractive index increases with beam intensity, although the effect is insignificant for low intensities. The increase causes the beam to focus, which further boosts the intensity and therefore drives up the refractive index, resulting in a runaway situation. In practice, this positive feedback is halted when the intensity gets high enough to create plasma in the material. As a result, femtosecond beams are not stable in uniform self-focusing media.
In their work, the investigators exploited the fact that the feedback effect is negligible in air. The proof-of-principle setup used a stack of one to nine 1-mm-thick glass slides separated by 1-mm air gaps. They sent 160-fs pulses at 800 nm from a seed laser from Coherent Inc. and an amplifier from Spectra-Physics through a lens, so that it diverged before entering the slide-air stack.
The beam, which was 43 μm in diameter at the entry point, converged because of the self-focusing of the first glass slide, diverged again when it passed through the air gap and converged once more at the next glass slide. The result was a beam that oscillated around an average width of 30 μm through the first three glass slides. After that, the beam diverged, although more slowly than it would have in air alone.
The researchers measured the effect using a CCD camera from JAI Pulnix of San Jose, Calif., and compared their results with those derived from theory. “This setting provides perhaps the best agreement between experimental results and model predictions that I have seen in my own work,” said the university’s Panayotis G. Kevrekidis.
In an application, the width could be manipulated by varying the width of the air gap between slides. This could facilitate optical switching, for instance.
With the basic concept proved, the researchers are working on ways to overcome various issues. For example, the slide-air stack suffered an overall loss of some 63 percent of the incoming beam intensity when passing through eight slides. The scientists, therefore, are exploring ways to model and minimize those losses. They have tried, among other optimizations, slides coated with an antireflection film, achieving promising results as reported in a recently submitted paper.
It also is possible that self-defocusing substances could be inserted in the stack in a manner analogous to the use of positive and negative dispersion materials in a dispersion-managed fiber. However, institute researcher Martin Centurion noted that currently self-defocusing materials have significant problems.
“In general, they have high absorption, which reduces transmission, and are not readily available,” he said.
Physics Review Letters, Vol. 97, 033903, 2006.
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