Interdigital concept in photonic sensors based on an array of lossy mode resonances

Ismel Dominguez,Jesus M Corres,Ignacio R Matias,Ignacio Del Villar,Omar Fuentes

doi:10.1038/s41598-021-92765-0

Ismel Dominguez, Jesus M Corres + Show 3 more

Open Access

https://doi.org/10.1038/s41598-021-92765-0

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Abstract

Multi-parameter detection is key in the domain of sensors. Here it is demonstrated that an indium tin oxide (ITO) nanocoating can be used to generate multiple lossy mode resonances (LMRs) in the optical spectrum. To achieve this, a nanocoating with a gradient in thickness is generated on the surface of a planar waveguide, permitting broadening of the LMR because the position of an LMR in the optical spectrum is directly related to the nanocoating thickness. The nanocoating with a gradient in thickness contributes multiple LMRs, each one centred at a different wavelength. With a further etching or deposition using a mask, a pattern of deposited and non-deposited regions can be created, resulting in isolation of the LMRs by preventing LMR overlap. This enables tracking of each central wavelength separately, which can be tuned through control of the gradient or nanocoating pattern. The array of LMR-based sensors is a photonics analogue to the interdigital concept in electronics, enabling multiple resonances to be used for multiparameter sensing.

Highlights

Thin-film, and the phase matching of this mode with the modes guided in the coverslip is what causes an increase in the losses of these modes and, an absorption band in the transmission spectrum[5]
This lower sensitivity for higher order lossy mode resonances (LMRs) is due to the effective index of the mode guided in the thin film[6]; for higher order modes the effective index of the mode guided in the thin film is less sensitive to parameters such as thin film thickness and surrounding medium refractive index, which is related to the presence a thicker thin-film required for generating higher order LMRs
Each LMR corresponds to a different lossy mode order, which leads to quite different sensitivities that drastically decrease as a function of the mode order

Summary

Introduction

Thin-film, and the phase matching of this mode with the modes guided in the coverslip is what causes an increase in the losses of these modes and, an absorption band in the transmission spectrum[5]. To optimise sensitivity, the difference in refractive index must be minimised through the choice of the substrate[8], the surrounding medium[23], or both Another interesting property is the capability of generating multiple LMRs in the optical spectrum. The separation between the LMRs is difficult to control because the wavelengths of the LMRs depend mainly on the three parameters that control the sensitivity, i.e., the refractive index, thickness of the thin-film, and the surrounding medium refractive index, which are fixed in each application the LMR generating structure is used This issue can be solved by exploiting two more properties of LMR-generating structures: the simplicity in tuning the position of the resonance, just by controlling the thickness of the coating[8], and the capability of obtaining the LMR in a broad spectrum if the material satisfies the conditions for generating LMRs in that wavelength range[24]. This technique permits, with a single deposition, the generation of a structure on a planar waveguide that, subdivided in discrete sections, sets the basis for the translation of the concept of interdigitated electrodes, from the point of view of the structure, to the domain of photonic sensors

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