An integrated thin-film device has been designed to constitute the sensory element of a miniature instrument for measuring the concentration of an atomic or molecular species that exhibits fluorescence at specified excitation and emission wavelengths. The device combines a fluorescence-emission method with a detection method to detect fluorescence emitted by those atoms or molecules of interest that are situated within a short distance of one surface of the device. Applications could be as diverse as medical and biological immunoassay and monitoring pollutant gases in engine exhausts.
The device comprises multiple corrugated thin film layers (see figure). The corrugations are characterized by a peak-to-valley depth of about 500 Å and a spatial period of about 1 µm. The function of the corrugations is explained below.
The layer in contact with the atmosphere or other medium that contains the fluorescent material to be detected is a dielectric waveguide with a thickness of the order of 2,000 Å. The thickness is chosen so that the waveguide supports the desired electromagnetic-field modes at the specified fluorescence excitation and emission wavelengths. The electric-field profiles of these modes are evanescent; that is, outside the waveguide layer, the electric-field amplitudes decrease monotonically with distance from the layer.
Optical excitation is supplied by radiation from an external source to fluorescent atoms or molecules near the surface. The resulting emission from the fluorescent atoms or molecules is coupled into the waveguide via evanescent wave modes; thus, the device opens a radiative-decay channel for fluorescence.
Below the dielectric waveguide is a metal film. Below the metal film is a dielectric buffer layer made of a material different from that of the waveguide layer. Among the electromagnetic-field modes supported by the thin-film geometry are surface plasmons, supported at the dielectric/metal interface. The amplitudes of these modes decay with increasing depth into the metal. The thickness of the metal layer is about 500 Å - enough to render the metal opaque to photons but not enough to prevent overlap of plasmon fields on opposite surfaces.
In this asymmetric dielectric/metal/dielectric configuration, the corrugation enables matching of the momenta of surface plasmons that have equal wavelength and are localized on opposite sides of the metal film. As a result, over a narrow range of wavelengths, there is cross-coupling between surface plasmons on opposite sides, with consequent generation of lower-surface plasmons. The narrow range of wavelengths is determined by the corrugation periodicity and can be chosen by design to contain the fluorescence emission wavelength of interest. Thus, optical energy at the fluorescence emission wavelength is transmitted through the film, which otherwise remains opaque to photons and thus to most background light.
Below the buffer layer is a sensing layer, which can be a semiconductor p/n junction or other electronic device that exhibits a measurable change in voltage, current, or resistance when it absorbs energy from plasmons. The buffer dielectric layer is thin enough to allow penetration of the surface plasmon electric fields into the sensing layer. Optionally, one can dispense with the buffer dielectric layer, in which case the sensing layer also serves partly as the lower dielectric layer.
This work was done by Margaret L. Tuma of Lewis Research Centerand Russell W. Gruhlke of Ohio Aerospace Institute. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.comunder the Physical Sciences category, or circle no. 157 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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