Electro-Optically Modulated Lossy-Mode Resonance─A New Approach for Label-Free Sensing

In this work, we report studies on the impact of thin dielectric layer formation on the optical, electrochemical, and electro-optically modulated responses of an indium tin oxide (ITO)-coated optical fiber sensor. The properties of ITO, such as optical transparency, high refractive index, and electrochemical activity, make it possible to obtain a lossy-mode resonance (LMR) effect in the optical domain and simultaneously use the sensor as an electrode in the electrochemical domain. The dielectric layer has been obtained on the ITO surface with precision down to a single nanometer using the atomic layer deposition (ALD) method. It is considered a reference to forming a biological or chemical layer during label-free sensing applications of such dual-domain sensors. It has been found that the sensor responds in both optical and electrochemical domains to the formation of a coating on the ITO surface. Numerical and experimental studies have proven that there is also a strong impact of the dielectric layer on the electro-optical modulation effectiveness. Changes in ITO's refractive index and extinction coefficient at its interface with the layer are induced by the modulation of free charge carriers' density. It has been shown that changes in the thickness of the dielectric layer, down to tenths of nanometers, can be precisely monitored when modulation is applied. Such an attribute is hardly possible when standard optical measurements, i.e., without modulation are considered. The findings open new opportunities for using electro-optical modulation in label-free sensing and biosensing, especially when small biological species or their low concentrations are targeted.