We report a metal-dielectric planar structure which provides high efficiency coupling of fluorescence at distances over 100 nm away from the metal surface. This hybrid metal-dielectric waveguide (MDW) consists of a continuous metal film coated with a dielectric layer. We observed efficient long-range coupling of Rhodamine B on top of a 130 nm layer of silica resulting in a narrow angular distribution of the emission. The high efficiency radiation through the Ag film appears to be due to coupling of the fluorophore to an optical waveguide mode with a long propagation length and a narrow resonance. The results were consistent with simulations. These multi-layer structures can be made using vapor deposition and/or spin coating and the silica surface can be used for conjugation to biomolecules and surface-selective detection. This simple hybrid metal-dielectric structures provides new opportunities for fluorescence sensing, genomics, proteomics and diagnostics.

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The interaction of fluorophores with nearby metallic structures is now an active area of research. Dielectric photonic structures offer some advantages over plasmonic structures, namely small energy losses and less quenching. We describe a dielectric one-dimensional photonic crystal (1DPC), which supports Bloch surface waves (BSWs) from 280 to 440 nm. This BSW structure is a quartz slide coated with alternating layers of SiO2 and Si3N4. We show that this structure displays BSWs and that the near-UV fluorophore, 2-aminopurine (2-AP), on the top surface of the structure couples with the BSWs. Fluorophores do not have to be inside the structure for coupling and show a narrow angular distribution, with an angular separation of wavelengths. The Bloch wave-coupled emission (BWCE) radiates through the dielectric layer. These BSW structures, with useful wavelength range for detection of intrinsic protein and cofactor fluorescence, provide opportunities for novel optical configurations for bioassays with surface-localized biomolecules and for optical imaging using the coupled emission.

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This paper describes an approach for the noninvasive microwave characterization of tumors in breast tissue. Tumors are modeled as lossy dielectric targets. Their complex natural resonances (CNR) can be extracted from the time-domain response and correlated with diagnostically useful properties. Finite-difference time-domain simulation is used to obtain the time-domain response from a tumor with a short electromagnetic pulse as an input. The normal breast tissue and tumor are modeled as dispersive media using the Debye model and CNRs are extracted using Prony's method. It is shown that the locations of the dominant CNRs are separated in the complex frequency plane as functions of the tumor dielectric properties. The technique has potential as a diagnostic tool to characterize breast lesions in conjunction with other imaging modalities such as ultrasound for detection.

We describe a hybrid metal-dielectric waveguide structures (MDWs) with numerous potential applications in the biosciences. These structures consist of a thin metal film coated with a dielectric layer. Depending on the thickness of the dielectric layer, the modes can be localized near the metal, within the dielectric, or at the top surface of the dielectric. The optical modes in a metal-dielectric waveguide can have either S (TE) or P (TM) polarization. The dielectric spacer avoids the quenching, which usually occurs for fluorophores within about 5 nm from the metal. Additionally, the resonances display a sharp angular dependence and can exhibit several hundred-fold increases in intensity (E2) at the silica-air interface relative to the incident intensity. Fluorophores placed on top of the silica layer couple efficiently with the metal, resulting in a sharp angular distribution of emission through the metal and down from the bottom of the structure. This coupling occurs over large distances to several hundred nm away from the metal and was found to be consistent with simulations of the reflectivity of the metal-dielectric waveguides. Remarkably, for some silica thicknesses, the emission is almost completely coupled through the structure with little free-space emission away from the metal-dielectric waveguide. The efficiency of fluorophore coupling is related to the quality of the resonant modes sustained by the metal-dielectric waveguide, resulting in coupling of most of the emission through the metal into the underlying glass substrates. Metal-dielectric waveguides also provide a method to resolve the emission from surface-bound fluorophores from the bulk-phase fluorophores. Metal-dielectric waveguides are simple to fabricate for large surface areas, the resonance wavelength can be adjusted by the dielectric thickness, and the silica surface is suitable for coupling to biomolecules. Metal-dielectric waveguides can have numerous applications in diagnostics and high-throughput proteomics or DNA analysis.

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