Voltage-tunable optical band-pass filters based on surface plasmon waves have been proposed. These filters would function at both visible and infrared wavelengths. Whereas liquid-crystal tunable optical filters now on the market exhibit efficiencies of 20 percent or less, theoretical calculations predict that the efficiencies of the proposed tunable surface-plasmon filters could exceed 60 percent in some cases.
Figure 1 schematically illustrates one of two types of the proposed filters. A thin film of a suitable electro-optical material (for example, a liquid crystal) would be sandwiched between two high-index-of-refraction prisms coated with thin metal films at the prism/electro-optical-film interfaces. If p-polarized white light were to impinge on this device at a certain angle (denoted the surface-plasmon angle), then the energies of some of the incident photons would be converted into collective motions of free electrons in the upper metal film. Because of the thinness of the electro-optical film, the optical field would penetrate the film and excite the same collective motion of electrons in the lower metal film. As a result, light would be transmitted in the sense that it would be re-radiated from the bottom. Only the photons at the surface-plasmon resonance frequency could generate surface plasmon waves and could thereby be coupled through the thickness to contribute to the transmitted light; consequently, the transmitted light would be colored.
The surface-plasmon resonance frequency would depend on the indices of refraction of both the metal film and the liquid crystal or other electro-optical material. If a voltage were applied to control the index of refraction of the electro-optical material, then the voltage would control surface-plasmon resonance frequency and thus the spectrum of the transmitted light.
For example, theoretical calculations were performed for a device like that of Figure 1 comprising TiO2 prisms with 45° angles, silver films 35 nm thick, and a 150-nm-thick electro-optical film made of a recently developed liquid crystal, the index of refraction of which can be made to shift as much as 0.5. According to the calculations, with no voltage applied to the silver films, the device would exhibit peak transmission at a wavelength of 450 nm (blue), with an efficiency of 62 percent. With enough voltage applied to shift the index of refraction by 0.5, the peak of the transmission spectrum would be shifted to 650 nm (red), and the efficiency would be 70 percent.
If a metal other than silver were used, the device could be made to work in infrared light. For example, if the silver films in the device described in the preceding paragraph were replaced with potassium films, then the wavelength of peak transmission could be made to range from 1,050 to 1,700 nm.
A proposed tunable surface-plasmon optical filter of the second type would also include prisms partly coated with thin metal films (see Figure 2), but there would be no electro-optical film with a voltage applied to control its index of refraction. Instead, an airgap would be left between the metal films, and the distance between the prisms would be varied to vary the airgap and thereby vary the surface-plasmon resonance frequency. A practical device of this type could be made from sheets of microprisms, with piezoelectric spacers for varying the airgap.
This work was done by Yu Wang of Caltech forNASA's Jet Propulsion Laboratory. For further information,access the Technical Support Package (TSP) free on-line at www.techbriefs.comunder the Physical Sciences category.
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