Reflective flat-panel color display devices based on surface plasmons are undergoing development. Heretofore, no reflective flat-panel color display devices have been available. The active matrix liquid-crystal devices now used to provide flat-panel color displays must be lit internally, are power-hungry (typically consuming about 80 percent of the power of a laptop computer), and cannot be read in bright ambient light. In contrast, the surface-plasmon display devices would be operated without internal lighting, would consume much less power, and would be readable in bright ambient light (including sunlight).
This development is based on voltage-induced color-selective absorption of light in surface plasmons: When a surface-plasmon wave is excited at a metal/liquid-crystal interface, the absorption spectrum of surface-plasmon resonance can be shifted across the visible range by altering a voltage applied to the liquid crystal. This effect can be exploited to make a tunable notch filter -- more specifically, a filter that absorbs most of the incident light in a voltage-adjustable wavelength range (the "notch") and reflects most of the incident light outside that range. If incident white light can be reflected from two tunable notch filters in succession and if the absorption wavelength range of each filter can be made to span about 2/3 of the visible spectrum, then by suitable choice of the notch wavelengths, the resulting display can be made to appear black (most of the incident light absorbed), white (most of the incident light reflected), or any primary color at a selectable level of brightness.
The problem then becomes one of how to implement a voltage-tunable notch filter and to combine a number of such filters into an array of pixels to construct a flat-panel display device. Figure 1 illustrates such a filter, which includes two high-index-of-refraction prisms for coupling, plus four silver films interspersed with three liquid-crystal layers. Surface-plasmon waves are excited at the six liquid-crystal/metal-film interfaces, and the layers are made sufficiently thin that the surface-plasmon waves are coupled together. When a voltage is applied between the two outermost silver films, the device acts electrically like three capacitors in series, and the electric field affects the indices of refraction of the liquid-crystal layers, causing a wavelength shift of the absorption spectrum. A theoretical calculation shows that this filter would exhibit the desired notch characteristic, that with no voltage applied, it would absorb primarily in blue-green light, and that its absorption wavelength region could be shifted to red or beyond by applying increasing voltage.
Figure 2 shows part of a proposed flat-panel display device, wherein each pixel would contain two notch filters. The prisms -- now microscopic to fit the pixels -- would be molded into sheets of high-index-of-refraction plastic. A polarizer sheet would be mounted on the front surface to select p-polarized light. After polarization, the incident light would be reflected from one notch filter, then from the other notch filter. By choice of the voltages applied to the two notch filters in each pixel, one could obtain a desired color combination as described above.
This work was done by Yu Wang of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Surface-plasmon reflective flat-panel color diplays
(reference NPO20244) is currently available for download from the TSP library.
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