OPTICAL LIGHTGUIDES HAVING DIAMETER SMALLER THAN THE WAVELENGTH OF LIGHT

by Gerald Huth on January 18, 2006

I have written about this before but in conjunction with my recurring use of the term “nanowire” this reference may bear repeating. Recent work by Mazur’s Harvard group ( Tong et al, “Single-mode guiding properties of subwavelength silica and silicon wire waveguides” , OPTICS EXPRESS, Vol.12, No.6, 22 March 2004) found that when a fiberoptic lightguide is reduced in diameter to dimensions less than light wavelength (i.e., less than ~0.5 micron or 500 nm), instead of being transmitted through the interior of the guide, light flows around (or on the “outside of”) the guide itself. A further finding is that the more the diameter of the guide is reduced the more light flows outside.

A popularized description of this work from a nanotechnology; website:

Silica Nanowires Thinner Than The Wavelength of Light

Marrying fiber optics with nanotechnology, scientists at Harvard University have created silica wires that are far narrower than the wavelength of light yet can still guide a light beam with great precision. The wires, about a thousandth the width of a human hair, function with minimal signal loss even when their walls accommodate well under half the breadth of a single light pulse.

A team led by Harvard’s Eric Mazur and Limin Tong, a visiting professor from Zhejiang University in China, reports the work in the Dec. 18 issue of the journal Nature.

“You wouldn’t normally imagine that a baseball could pass through a garden hose, but these nanowires appear able to handle exactly that kind of wide load,” says Mazur, Harvard College Professor, Gordon McKay Professor of Applied Physics and professor of physics. “In some cases light is propagating along wires just one-third the width of its own wavelength. It’s almost as if the wire serves as a rail to guide the light rather than funneling it in the traditional sense.”

The nanowires carry light via evanescent waves that envelop the slender filaments. If two of the wires touch, light can jump directly from one to the other, something that’s not possible with conventional fiber optics.

Although as thin as 50 nanometers, the wires created by Mazur and Tong are up to two centimeters long, making them faintly visible to the naked eye. They display impressive resilience and flexibility, curling easily into light-conducting loops whose diameter is just a tiny fraction of a millimeter.

The diameter of retinal receptors approximates (or is even less than) the wavelength of visible light. The historic and extensive research in the vision science field that modeled retinal receptors as lightguides, i.e., considering that light flowed internally within the guide, did not anticipate this behavior.

It is my belief that this new result exactly validates my hypothesis..i.e., that light interaction is between and not “within” retinal receptors!

Further, the linkage between the evanescent wave nature of this light corresponding to the transverse dichroic orientation of retinal pigment further substantiates my claim.

I add a quote from the Mazur/Tong paper: “The nanowires carry light via evanescent waves that envelop the slender filaments. If two of the wires touch, light can jump directly from one to the other, something that’s not possible with conventional fiber optics.”

GCH

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