1730 IEEE SENSORS JOURNAL, VOL. 13, NO. 5, MAY 2013 Analysis of Integrated Optofluidic Lab-on-a-Chip Sensor Based on Refractive Index and Absorbance Sensing Narayan Krishnaswamy, Member, IEEE, Talabattula Srinivas, Senior Member, IEEE , Gowravaram Mohan Rao, and Mukundan Manoj Varma Abstract— The analysis of a fully integrated optofluidic lab-on- a-chip sensor is presented in this paper. This device is comprised of collinear input and output waveguides that are separated by a microfluidic channel. When light is passed through the analyte contained in the fluidic gap, optical power loss occurs owing to absorption of light. Apart from absorption, a mode-mismatch between the input and output waveguides occurs when the light propagates through the fluidic gap. The degree of mode-mismatch and quantum of optical power loss due to absorption of light by the fluid form the basis of our analysis. This sensor can detect changes in refractive index and changes in concentration of species contained in the analyte. The sensitivity to detect minute changes depends on many parameters. The parameters that influence the sensitivity of the sensor are mode spot size, refractive index of the fluid, molar concentration of the species contained in the analyte, width of the fluidic gap, and waveguide geometry. By correlating various parameters, an optimal fluidic gap distance corresponding to a particular mode spot size that achieves the best sensitivity is determined both for refractive index and absorbance-based sensing. Index Terms— Absorbance, mode-mismatch, optofluidics, refractive index, sensitivity. I. I NTRODUCTION O NE of the challenges of lab-on-a-chip system, especially for optofluidic system is the total integration of all the fluidic and optical components into a miniaturized micro-chip so that such lab-on-a-chip sensor can be used right at the point-of-care. Optofluidics is a new branch within photonics that attempts to unify concepts from optics and microfluidics [1]. Unification of photonics and microfluidics enables us to carry out analysis of fluids including human physiolog- ical fluids through highly sensitive optical sensing devices Manuscript received October 6, 2012; revised January 18, 2013; accepted January 23, 2013. Date of publication January 29, 2013; date of current version April 2, 2013. The associate editor coordinating the review of this paper and approving it for publication was Dr. Alexander Fish. N. Krishnaswamy and G. M. Rao are with the Department of Instrumen- tation and Applied Physics, Indian Institute of Science, Bangalore 560012, India (e-mail: narayan@isu.iisc.ernet.in; gmrao@isu.iisc.ernet.in). T. Srinivas is with the Department of Electrical and Communication Engineering, Indian Institute of Science, Bangalore 560012, India (e-mail: tsrinu@ece.iisc.ernet.in). M. M. Varma is with the Centre for Nano Science and Engi- neering, Indian Institute of Science, Bangalore 560012, India (e-mail: mvarma@ece.iisc.ernet.in). 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/JSEN.2013.2243429 [2]–[5]. Some of the previous experimental works have focused on the integration of fluorescence and absorbance based detection schemes with fluidics. A common structural motif used in many of these works, is a fluidic gap separating two devices, one of which serves as the source of light and the other for collecting and detecting the light modulated by the fluidic gap. Previous authors have considered sensitivity analy- sis of integrated optical detection from the point of evanes- cent sensing [6]–[14]. In contrast we propose waveguide-gap structure details of which are shown in Figs. 1(a) and (b). The light when propagates through the input and output waveguide, the field profile remains unchanged and hence mode-spot size (mode-spot size is measured by taking the width of the field profile at the point where field drops to 1/e th of its peak value) remains the same. However when the light propagates through the bulk of the analyte in the fluidic gap the mode- spot sizes enlarges. In this article we refer to mode-mismatch as the difference between mode spot sizes of the field profiles of the light entering and exiting the fluidic gap. The fluidic gap, containing the analyte, which is the molecule/material to be sensed, modifies the coupling of light from the input to the output waveguide. This is because of the mode-mismatch. Thus mode-mismatch forms an important signal transduction and can be used for analysis of such devices. The extent of the mode-mismatch depends on the device parameters such as fluidic gap distance, refractive index of the fluid, the mode spot size and so on. From the point of view of sensor design, one is interested in choosing the input and output waveguide parameters and the fluidic gap width, which maximizes the sensitivity. The values of these parameters for optimization are different for different detection techniques and are important considerations for the sensor design. In this paper we have discussed the effect of device parameters on the sensitivity of detection using the waveguide-gap structure for high throughput micro-refractometry and absorbance sensing. In refractometric or absorbance sensing, we rely on the refractive index or absorbance change caused due to variations in the molar concentration of analyte. For instance the amount tissue glucose level in blood plasma is an important indicator for detection of diabetes. The mechanism of detection of tissue glucose levels is explained in [5]. We have considered oxy- hemoglobin HbO 2 as an illustrative example as the analyte for the analysis described here. As pointed out earlier in this section, the considerations for performance optimizations 1530-437X/$31.00 © 2013 IEEE