Broad-band Mach-Zehnder Interferometry as a detection principle for label-free biochemical sensing Kitsara M , Raptis I, Misiakos K, Makarona E Institute of Microelectronics NCSR “Demokritos” Aghia Paraskevi Attikis, 15310 Greece elmak@imel.demokritos.gr Abstract—In this work the Broad-Band Mach-Zehnder Interferometry (BB-MZI) is presented as an alternative operation principle for optical biochemical sensors that will allow for ultra-sensitive, label-free, multi-analyte detection schemes. Simulation studies were performed for a Si-based MZI so as to explore its sensing capabilities in terms of the transmission output spectrum. The presented simulation results concern biochemical applications such as aqueous glucose solutions and hypothetical thin protein adlayers adsorbed on the sensing arm. I. INTRODUCTION Integrated optical biosensors are commonly based on the use of planar waveguides, the coupling efficiency of which depends on the medium surrounding the waveguide. Specifically the overall optical transmission coefficient depends on the effective refractive index sensed within the range of the evanescent field of the waveguided modes. Biomolecular reactions that take place onto the waveguide surface change the effective refractive index, and this is indicated as a change in the photon flux monitored at the end of the waveguide. The most attractive characteristic of the evanescent field based detection is that it is non-invasive and allows for real-time, multi-analyte monitoring. The detection principles invented and commonly applied in the optical biosensors and biosensor arrays, are based either on labeling (enzymes, fluorescent or chemoluminescent molecules) or on optical signal changes (label-free sensors). The majority of the optical label-free approaches - such as Reflectometric Interference Spectroscopy (RIfS) [1], Surface Plasmon Resonance (SPR) [2], Optical Ring Resonators [3], Mach-Zehnder Interferometry (MZI) [4,5] - are based on the monitoring of changes of the optical properties of the areas where the chemical or biomolecular interactions are taking place. Among them, integrated Mach-Zehnder Interferometers (MZIs) have been recognized as a powerful tool for highly sensitive, label-free, real-time detection of analytes. According to the bibliography, a refractive index resolution of ~5×10 -8 can be obtained [4]. In the present work a thorough theoretical study of a Si- based MZI that can be operated with broad-band lights sources emitting in the visible range (450nm-750nm) is presented. The scope of this work is to prove that Broad- Band Mach-Zehnder Interferometry (BB-MZI) not only can be used as the operating principle of miniaturized optical sensors, but also offers increased sensitivity, versatility of application and reduced manufacturing and operating cost compared to standard MZIs. II. BROAD-BAND MACH-ZEHNDER INTERFEROMETRY In a conventional MZI [4,5] sensor a single wavelength (SW-MZI) coherent beam is split via a beam splitter (Y- junction) into two arms, one used as a reference and one as the sensing element. The difference of the refractive indices of the evanescent fields between the sensing and reference arms caused by the analyte absorption/adsorption/binding onto the sensing arm are recorded as intensity changes at the output waveguide after the recombination of the two beams at a second Y-junction. The MZI device offers very high sensitivities in terms of refractive index changes [6], limited mainly by the sensing arm length, while it can be fabricated on Si wafers with standard microelectronic processes [7]. However, the input light used has been so far monochromatic for ease of implementation and data analysis, rendering the use of external laser light sources imperative. In mathematical terms, the output signal of a SW-MZI (I T ) operating at wavelength ζ is given by: )) ( cos( 2 t I I I I I R S R S T ϕ Δ ⋅ + + = (1) where I S and I R are the light intensities in the sensor and reference arms, respectively. 1-4244-2581-5/08/$20.00 ©2008 IEEE 934 IEEE SENSORS 2008 Conference