778 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 9, MAY 1, 2012 Surface Loading Sensitivity Characterization of a Resonant Planar Optical Waveguide Stack Rohit Goswami, Joshua R. Nightingale, Joseph A. Duperre III, Min S. Lim, Jeremy Michael Dawson, Aaron Timperman, Dimitris Korakakis, and Lawrence A. Hornak, Senior Member, IEEE Abstract— Detuning of a coupled planar waveguide pair through surface loading of the top waveguide that is in contact with the analyte serves as a transducer for a Stacked Planar Affinity-Regulated Resonant Optical Waveguide (SPARROW) sensor. Here we investigate the surface loading detection sensitivity of a SPARROW transducer composed of a stack of two planar alumina waveguides grown using ion beam assisted e-beam deposition. Using a sucrose analyte solution introduced to the stack surface in a poly-di-methyl-siloxane (PDMS) microfluidic channel, the change in optical output power through the coupled waveguide pair as a function of sucrose analyte solution concentration was experimentally determined. Based on the analyte in the evanescent field penetration depth volume, the effective minimum detectable surface loading of the stack was determined to be 20 pg/mm 2 with a bulk index sensitivity of 5.6 × 10 -4 refractive index units (RIU) for this stack configuration and experimental setup. Index Terms— Evanescent wave sensors, optical sensitivity analysis, planar waveguide, thin film devices. I. I NTRODUCTION E VANESCENT wave-based sensors have gained attention due to their potential for high sensitivity and flexibility in integrated waveguide design for enhancing detection and interaction between analyte and transducer [1]. Advances in the selectivity of fiber-optic, planar waveguide and stacked thin film coupled waveguide sensors have been reported [2]. Selectivity is typically achieved either by surface affinity Manuscript received September 26, 2011; revised January 24, 2012; accepted February 11, 2012. Date of publication February 16, 2012; date of current version April 11, 2012. This work was supported in part by the ONR under Grant N00014-03-1-0815, in part by the NSF RII under Grant EPS 0554328, in part by the NSF IPA Independent Research and Development Program, and in part by the WVU Research Corporation and WV EPSCoR Office. R. Goswami, J. R. Nightingale, J. A. Duperre III, J. M. Dawson, and D. Korakakis are with the Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26505 USA (e-mail: rgoswami@mix.wvu.edu; joshnightingale@gmail.com; joseph.duperre@gmail.com; jeremy.dawson@mail.wvu.edu; Dimitris.Korakakis@mail.wvu.edu). M. S. Lim is with Slippery Rock University, Slippery Rock, PA 16057 USA (e-mail: min.lim@sru.edu). A. Timperman is with the Engineer Research and Development Center, Construction Engineering Research Laboratory, Champaign, IL 61826 USA (e-mail: atimperm@wvu.edu). L. A. Hornak is with the National Science Foundation, Arlington, VA 22230 USA, and also with the Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26505 USA (e-mail: lahornak@mail.wvu.edu). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2012.2188384 Fig. 1. Transducer structure with a prism and PDMS microchannel on top. A CCD camera is placed after the microchannel to image laser power scattered from the bottom waveguide. regulation using selective surface binding or specific optical or chemical properties of the target analyte. The Stacked Planar Affinity Regulated Resonant Optical Waveguide (SPARROW) structure has been investigated as an evanescent wave-based sensing structure due to its potential for both reduced fab- rication complexity and high sensitivity [3]. The underlying transducer structure utilizes two vertically stacked aluminum oxide planar, single mode optical waveguides separated by a lower refractive index silicon dioxide layer as diagrammed in Figure 1. The top waveguide is in contact with an aqueous analyte over an interaction length defined by the width of the microfluidic channel bonded to the waveguide stack. Over this length, the waveguides are designed to have the same propagation constant and be resonantly matched when the analyte layer has the refractive index of DI water (1.333). The interaction length is chosen such that optical power initially coupled into the lower guide outside the channel region fully transfers to the upper waveguide in contact with the analyte an even number of times within the channel, ultimately arriving in the bottom guide by the end of the channel transit. Changes in the effective refractive index of the analyte layer within the evanescent field penetration depth will “detune” the waveguides over the analyte interaction length in the channel, resulting in a change in the power detected at either guide’s output. Transducer sensitivity is determined by the evanescent field penetration and the interaction length with the analyte. Sensor transducer selectivity is achieved via affinity regulation of the top waveguide surface using selective surface binding appropriate to the target analyte.In this letter 1041–1135/$31.00 © 2012 IEEE