JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 12, JUNE 15, 2012 1863 Model and Analysis of a High Sensitivity Resonant Optical Read-Out Approach Suitable for Cantilever Sensor Arrays Gino Putrino, Adrian Keating, Senior Member, IEEE, Mariusz Martyniuk, Lorenzo Faraone, Senior Member, IEEE, and John Dell, Member, IEEE Abstract—We investigate an optically resonant cavity which is created between a reflecting micro-cantilever and a diffraction grating etched into a silicon waveguide. Changes in cavity reso- nance, induced by small deflections of the micro-cantilever result in large changes in an optical signal transmitted through the waveguide. An analytical model can predict the cantilever position for maximum and minimum transmission and is confirmed by three-dimensional finite difference time domain (FDTD) simu- lations. This approach can be used to accurately determine the position of a micro-cantilever with a predicted optimal shot noise limited deflection noise density of 4.1 fm/ Hz. Index Terms—Biological sensing and sensors, diffraction grat- ings, interference, MEMS, MOEMS. I. INTRODUCTION M ICRO-CANTILEVER sensors are readily integrated into an array using low-cost, mass-production fab- rication techniques developed for micro-electromechanical systems (MEMS), and can facilitate simultaneous multi-ana- lyte chemical sensing [1]–[3]. MEMS-based microstructures are extremely sensitive elements, demonstrating mass detection limits as low as g in controlled, laboratory conditions [4], [5]. If the top surface of the micro-cantilever is functional- ized to preferentially adsorb specific molecules, an extremely sensitive and selective sensor can be fabricated. Readout technologies for micro-cantilever sensors include the use of light reflected from the cantilever tip to a distant quadrant detector [1], electrical sensing (piezoresistive, piezo- electric, capacitive, Lorentz force/emf sensing and tunneling current techniques), and optical sensing based on optical inter- ference either in an interferometer or the use of diffraction from an optical grating formed by a line of cantilevers. This latter configuration is often described as an array in the literature, but is still effectively a sensor for a single analyte [4], [6], [7]. Manuscript received November 20, 2011; revised January 31, 2012, March 04, 2012; accepted March 09, 2012. Date of publication March 14, 2012; date of current version April 09, 2012. This work is supported in part by the Commonwealth of Australia under the Australia-India Strategic Research Fund, (ST030019), and in part by the Australian Research Council. The authors are with the Microelectronics Research Group, Department of Electrical and Electronic Engineering, University of Western Australia, Perth, WA 3182, Australia (e-mail: putrig01@student.uwa.edu.au). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2012.2190973 The typical deflection noise density (DND) for micro-can- tilever sensor systems that use quadrant detectors such as those found in atomic force microscopes (AFMs) is in the range of 100–1000 fm/ Hz, although laboratory values of 17 fm/ Hz have been achieved [8], [9]. The lowest shot noise limited DND previously reported was 6 fm/ Hz for a readout using an op- tical resonance approach [10]. A significant drawback of these AFM-based readout approaches is that none of them is compat- ible with the passive, non-electrical readout of compact, large arrays of individually and uniquely functionalized can- tilever sensors. Recently, a number of approaches have been proposed that do have the ability to read compact very large arrays. Ap- proaches that rely on the sensing cantilever also being an optical waveguide have reported theoretical shot noise limited DND of 1880 fm/ Hz [11]. Using a differential detection method, cantilever-as-waveguide techniques have yielded a shot noise limited minimum detectable deflection (MDD) as low as 54 pm [12]. Another approach uses the dielectric properties of the cantilever to perturb the evanescent field of a diffraction grating approximately 400 nm below the cantilever [13]. An MDD of 170 pm has been reported for this method. Photonic microharp sensors have potential for very large arrays [14] although the problem of coupling to each sensing microharp in an array is non-trivial. The microharp technique leverages an optically resonant approach, such that the DND is limited only by thermal-mechanical noise, making it extremely sensitive. For the microbridges reported this DND is quoted as being in the tens of fm/ Hz [14]. The approach proposed and presented in this paper is schematically shown in Fig. 1. The cantilever is suspended over a waveguide with a diffraction grating directly under the cantilever and forms an optically resonant cavity, and is essentially different from evanescent field perturbation [13]. In the proposed device, the optical electric-field phase-based interactions between light reflected from the underside of the micro-cantilever and the optical fields exiting and coupling back into the waveguide via the diffraction grating affect the intensity of the light transmitted from the input waveguide on one side of the grating to the output waveguide on the other side of the grating. The resultant effect on transmitted light intensity is a highly sensitive function of the separation between the grating and the cantilever. Consequently, cantilever motion results in the intensity modulation of light transmitted through the waveguide. This technique can readily be extended to the 0733-8724/$31.00 © 2012 IEEE