Scanning filters based on surface plasmons would be used as the adjustable wavelength-selective (band-pass) devices in a class of proposed miniature spectrometers. These filters would occupy much less space and weigh much less than do the diffraction gratings and other wavelength-selective optics of conventional spectrometers; as a result, the proposed spectrometers would be correspondingly smaller and less massive.
In a recently developed tunable band-pass filter [NASA Tech Briefs, Volume 22, No. 8 (August 1998), page 18a] based on surface plasmons, the pass wavelength band is adjusted by changing the airgap between two prisms. The filters in the proposed spectrometers would implement an alternative approach to tuning. A filter of this type would include two glass plates separated by a narrow gap filled with air or another dielectric with a low index of refraction. The facing surfaces of the plates would be grooved to form microprisms and coated with thin metal films. In a typical example, the dielectric would be air and the microprisms would be made of BK-7 (or equivalent) glass, formed for an angle of incidence of 41.5 °C, and coated with silver to a thickness of 40 nm.
The microprism plates would be tilted slightly to make the gap thickness vary from one edge to the other. The assembly of plates would be mounted on a scanning mechanism below a spectrometer slit, so that by use of the mechanism, the thickness of the gap illuminated through the slit could be varied. Inasmuch as the wavelength of peak transmission would depend on the thickness of the gap, scanning by use of the mechanism would thus result in variation of the pass wavelength band. In the example cited above, scanning from a gap 300 nm thick to a gap 5,000 nm thick would shift the wavelength of peak transmission from 400 to 1,600 nm.
This work was done by Yu Wang and Bedabrata Pain of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category. NPO-20179
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Scanning Surface-Plasmon Filters for Miniature Spectometers
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