Optical sensing and determination of complex reflection coefficients of plasmonic structures using transmission interferometric plasmonic sensor

The combination of interferometry and plasmonic structure, which consists of gold nanoparticle layer, sputter coated silicon oxide spacer layer, and aluminum mirror layer, was studied in transmission mode for biosensing and refractive index sensing applications. Because of the interferometric nature of the system, the information of the reflection amplitude and phase of the plasmonic layer can be deduced from one spectrum. The modulation amplitude in the transmission spectrum, caused by the interference between the plasmonic particle layer and the mirror layer, increases upon the refractive index increase around the plasmonic particles due to their coherent backscattering property. Our proposed evaluation method requires only two light sources with different wavelengths for a stable self-referenced signal, which can be easily and precisely tuned by a transparent spacer layer thickness. Unlike the standard localized surface plasmon sensors, where a sharp resonance peak is essential, a broad band plasmon resonance is accepted in this method. This leads to large fabrication tolerance of the plasmonic structures. We investigated bulk and adsorption layer sensitivities both experimentally and by simulation. The highest sensitivity wavelength corresponded to the resonance of the plasmonic particles, but useful signals are produced in a much broader spectral range. Analysis of a single transmission spectrum allowed us to access the wavelength-dependent complex reflection coefficient of the plasmonic particle layer, which confirmed the reflection amplitude increase in the plasmonic particle layer upon molecular adsorption.