Abstract
We report an ultra-sensitive refractive index (RI) sensor employing phase detection in a guided mode resonance (GMR) structure. By incorporating the GMR structure in to a Mach-Zehnder Interferometer, we measured the phase of GMR signal by calculating the amount of fringe shift. Since the phase of GMR signal varies rapidly around the resonance wavelength, the interference fringe pattern it forms with the reference signal becomes very sensitive to the surrounding RI change. The sensitivity comes out to be 0.608π phase shift per 10−4 RI change in water medium which is more than 100 times higher than the other reported GMR based phase detection method. In our setup, we can achieve a minimum phase shift of (1.94 × 10−3) π that corresponds to a RI change of 3.43 × 10−7, outperforming any of reported optical sensors and making it useful to detect RI changes in gaseous medium as well. We have developed a theoretical model to numerically estimate the phase shift of the GMR signal that predicts the experimental results very well. Our phase detection method comes out to be much more sensitive than the conventional GMR sensors based on wavelength or angle resolved scanning methods.
Highlights
Guided Mode Resonance (GMR) is a resonance phenomenon where a diffracted order of grating excites a guided mode of a waveguide that again couples out of the waveguide and interferes with the zeroth order reflected light
Kuo et al.[12] have used the heterodyne interferometer configuration to measure the phase change of the guided mode resonance (GMR) signal with respect to a phase reference signal generated by a function generator
The aforementioned methods to detect the phase of the GMR signals are not straight forward and experimentally quite complex to be applicable for biosensors
Summary
Guided Mode Resonance (GMR) is a resonance phenomenon where a diffracted order of grating excites a guided mode of a waveguide that again couples out of the waveguide and interferes with the zeroth order reflected light. The wavelength shift can be directly related to the change in RI due to addition of these biolayers and can be properly calibrated for detecting different biological samples Apart from this wavelength scanning method; angular-resolved method that involves scanning through different incidence angles[5] enables sensing capability. Kuo et al.[12] have used the heterodyne interferometer configuration to measure the phase change of the GMR signal (taken as the resultant of s- & p- polarized light) with respect to a phase reference signal generated by a function generator They have plotted the phase curve in terms of the incident angle and rotated an analyzer to tune this phase curve for higher sensitivity. This phase detection method turns out to be a very simple and efficient way to measure the phase of a GMR signal that can be applied to RI sensing and bio-sensing
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