970 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 51, NO. 5, OCTOBER2002 Exploitation of Enhanced Fluorescence via Cross-Coupling Principles Toward the Design of an Optical Integrated Thin-Film Sensor for Nanotechnology and Biomedical Applications G. C. Giakos, Senior Member, IEEE, K. Meehan, and M. Tuma Abstract—A novel fluorescence thin-film integrated sensor has been proposed that retains the beneficial selectivity characteris- tics typical of optical and electronic sensors, while improving the signal-to-noise ratio in a miniature geometry. The sensor can be tuned to measure a wide variety of biological species by varying its thin-film corrugation period. The optical properties of the sensor are determined, in large part, by optical cross coupling through a corrugated metal film and enhanced fluorescence. The surface plasmon to surface plasmon cross coupling was theoretically mod- eled and experimentally tested. Finally, prospective applications of this sensor in the key areas of nanotechnology and bioengineering are discussed. Index Terms—Bioengineering applications, enhanced fluores- cence, high sensitivity, nanotechnology, optical sensor, surface plasmon–surface plasmon (SP–SP) interaction. I. INTRODUCTION T WO CLASSES of surface plasmons are commonly recognized, namely surface plasmon waves (SPWs) and localized surface plasmons (LSPs) [1]–[5]. SPWs are TM-po- larized electromagnetic waves propagating laterally along a metal–dielectric interface. The electromagnetic field penetrates a material to a depth of the order of the wavelength of light. Light is only converted to a surface plasmon in a narrow resonant peak determined by the conditions matching the surface-plasmon and light wave vectors. Meanwhile, LSPs are confined to metal particles, across a metal–dielectric boundary, which are much smaller in size than the wavelength of the incident light. SPWs have been exploited for surface plasmon resonance (SPR) sensors [4]–[7], as well as for surface plasmon to sur- face plasmon (SP–SP) cross-coupling sensing applications [1], [8]–[13]. Manuscript received May 29, 2001; revised July 12, 2002. G. C. Giakos is with Imaging Devices, Sensors, and Intelligence Based Sensor Fusion Laboratory, Optical Electronics, Photonic Devices, and Optical Communication Networks Laboratory, Department of Electrical and Computer Engineering, Olson Research Center, The University of Akron, Akron, OH 44325-3904 USA (e-mail: giakos@uakron.edu). K. Meehan is with the Department of Computer Science and Electrical Engi- neering, University of West Virginia, Morgantown, WV 25606-6109 USA. M. Tuma is with NASA Glenn Research Center, Cleveland, OH 44135 USA. Digital Object Identifier 10.1109/TIM.2002.806037 SPR sensors for the study of surface layers have been demon- strated for both optically inert and optically excited layers at a metal surface. For a given wavelength of light a surface plasmon phenomenon is observed as a sharp minimum in the light re- flectance when the angle of incidence is varied. The critical angle is very sensitive to the binding of analytes to the metal surface. The particular conditions required for generation and use of surface plasmons for immunosensing applications have been achieved using metallized diffraction gratings and the met- allized surface of glass prisms. The main biosensing application of SPR so far has been in immunoassays [14]–[19]. The use of SPR has been directed toward the detection of the unlabeled immunoassay. Specifically, SPR can be utilized as the direct monitoring of antibody–antigens interactions. In this label-free biosensing technique, a biochemically active foundation layer, containing a particular antigen, is fixed on the sensor surface. A liquid analyte containing antibodies is then introduced on top of this layer. The antibody molecules will bind to the fixed layer antigens, leading to a net change in the effective refractive index measured by the sensor. For dyes with absorption bands in the visible frequency range, this technique is a highly sensitive means of providing information of the molecular layer structure. Pockrand and Benner [14] have shown that the SPR-enhanced fluorescence, emitted from a rhodamine 6-G layer on a silver layer can be coupled to a new plasmon mode at a shifted wave- length. The propagation distance for the SPW along the metal interface is related to the frequency. In the visible region, it is only of the order of 10 m, but in the infrared, distances of 3 cm are realistic. In this paper, the detection principles of an optical integrated sensor operating on surface plasmon to surface plasmon (SP–SP) [1], [8]–[13] cross coupling are presented. The unique optical properties of the proposed sensor are based on en- hanced molecular fluorescence [2]–[21] and cross coupling [1], [8]–[13]. Predicted properties of this sensor include: increased sensitivity, shielding from unwanted radiation, wavelength se- lection, and miniaturization. The key features of this sensor are the wavelength-selective cross coupling of light through a thin, corrugated, metallic film, which was theoretically modeled and experimentally tested. Theoretical and experimental results from this work will be discussed in the following sections. 0018-9456/02$17.00 © 2002 IEEE