Differential Phase Shift Detection System for High Sensitivity High Resolution Optical Sensing of Nanostructured Plasmonic Thin-Films

This paper reports on an optoelectronic detection system capable to measure very small values/variations of the optical absorption coefficient of plasmonic metal nanoparticles thin-films, potentially useful for sensing and biosensing application in agri-food and medicine. The proposed solution has been implemented by using commercial devices and instrumentations, employing few optical elements and reduced electronics to be suitable for the fabrication of future photonic integrated circuits. The optical absorption coefficient and its variations are measured by a differential phase-shift detection technique that allows for continuous monitoring of the laser beam power passing through the plasmonic film. The system is based on the detection of the light-induced changes of the internal capacitance of two Si photodiodes operating in photovoltaic mode biased through a sinusoidal voltage signal with an amplitude of ± 50 mV. The system potentiality was tested for the determination of catechin, a polyphenol with high antioxidant and nutraceutical properties. Experiments demonstrate that the developed solution has a phase shift sensitivity of 4.6 degrees/nW in terms of laser power variations, corresponding to a detection resolution of about 2.2 pW, and a phase shift sensitivity of 0.8 degrees/µM in terms of the concentration of catechin, corresponding to a detection resolution of about 12.5 nM.