Electro-optically modulated lossy-mode resonance

Abstract

Sensitivity, selectivity, reliability, and measurement range of a sensor are vital parameters for its wide applications. Fast growing number of various detection systems seems to justify worldwide efforts to enhance one or some of the parameters. Therefore, as one of the possible solutions, multi-domain sensing schemes have been proposed. This means that the sensor is interrogated simultaneously in, e.g., optical and electrochemical domains. An opportunity to combine the domains within a single sensor is given by optically transparent and electrochemically active transparent conductive oxides (TCOs), such as indium tin oxide (ITO). This work aims to bring understanding of electro-optically modulated lossy-mode resonance (LMR) effect observed for ITO-coated optical fiber sensors. Experimental research supported by numerical modeling allowed for identification of the film properties responsible for performance in both domains, as well as interactions between them. It has been found that charge carrier density in the semiconducting ITO determines the efficiency of the electrochemical processes and the LMR properties. The carrier density boosts electrochemical activity but reduces capability of electro-optical modulation of the LMR. It has also been shown that the carrier density can be tuned by pressure during magnetron sputtering of ITO target. Thus, the pressure can be chosen as a parameter for optimization of electro-optical modulation of the LMR, as well as optical and electrochemical responses of the device, especially when it comes to label-free sensing and biosensing.

Keywords: electro-optical modulation; label-free sensing; lossy-mode resonance; magnetron sputtering; optical fiber sensor; transparent conductive oxides (TCOs).

Figure 1:

Schematic representation of (A) experimental setup for combined optical and EC analysis with the investigated ITO-coated optical fiber structure and part of its cross-section. In (B) electrically equivalent model of the optical structure is schematically shown, where a monolayer of SiO₂ was introduced on ITO to represent oxidation of the film surface.

Figure 2:

E-induced evolution of accumulation layer where (A) and (B) shows mean charge carrier density and thickness of the layer, respectively, for initial bulk carrier density N reaching 3e19, 8e19, and 3e20 cm⁻³.

Figure 3:

E-induced evolution of an averaged (A) refractive index and (B) extinction coefficient in the accumulation layer. The results are shown for N reaching of 3e19 cm⁻³ where accumulation layer of ∼20 nm appears for E < −0.5 V (see Figure 2).

Figure 4:

(A) E-induced evolution of ITO-LMR spectral response when charge carrier density reaches 3e19 cm⁻³ and (B) LMR wavelength shift with E when N ranges from 3e19 cm⁻³ to 3e20 cm⁻³.

Figure 5:

Evolution of ITO-LMR sample properties with p. Examples of similar LMR spectral patterns in response to n _ext varying from 1 up to 1.42 RIU received at different ITO deposition p and t are shown in (A) and (B). In (C) is shown relative shift of the λ _R with n _ext for selected sets of p and t. The λ _R0 is the λ _R at n _ext = 1.3330 RIU for each set of p and t. In (D) is shown relation between resistance (R) of 25-mm-long active part of ITO-LMR structure and the p.

Figure 6:

(A) EC and (B) optical responses received for ITO-LMR used as a working electrode, when various ITO deposition p are considered. The analysis was performed in 0.1 M PBS and 1mM 1,1′-ferrocenedimethanol as a redox probe. Scan rate was set to 20 mV/s.