Optical properties and far field radiation of periodic nanostructures fed by an optical waveguide for applications in fluorescence and Raman scattering

Abstract

Various systems based on nanostructures built on optical waveguides have recently appeared in literature, since this configuration guarantees an efficient optical feeding to nano-elements and/or the possibility to manipulate guided signals. In this work, we present the analysis of the optical scattering properties of such type of structures, more specifically a periodic array of Au nano-cylinders or nano-domes fabricated upon an ion-exchanged waveguide, an integrated system considered for fluoroscopy and Raman spectroscopy. Absorption, scattering and extinction were calculated by means of Finite Difference Time Domain (FDTD) simulations and Scattering Theory. We took advantage of some particular adaptations to apply the classical scattering theory to this context, and used it to extract the far-field radiation pattern. We show that the rectangular periodicity allows to obtain highly collimated beams and negligible irradiation in a wide angle range, which enables the possibility to steer away the undesirable excitation signals from the detectors, avoiding a mixing with emitted photons from molecules. Our analysis suggests that, in the context of this setup and applications, the modeling of the nanostructures, which aims at the tuning of the plasmonic enhancement of the emission, can be relatively separated from the definition of spatial filtering through the periodic array. Our results add new considerations to the list of potentialities of this configuration for the development of efficient integrated surface enhanced spectroscopic setups.

Enhancing the Scattering Induced by Gold Periodic Arrays Over Optical Waveguides Through Indium Tin Oxide Buffer Layers

“…However, attention must be paid on the array's period and on the working wavelength, as in some cases ITO may be detrimental and harm the coupling efficiency. In case of sensing applications where the Au array is required to act as substrate for chemical functionalization, such as in the field of integrated scatteringinduced fluorescence [9], ITO buffer layers may represent a double sensitivity enhancer: the demonstrated increase in optical coupling pairs with the increase in overall surface for chemical functionalization, being ITO easy to functionalize (besides gold) with a broad spectrum of chemical species [42], [43], [44]. Furthermore, a thin-film ITO coating through physical vapor deposition can represent a low-cost, simple and fast enhancement/fine-tuning tool in the case of off-chip optical routing, as optimizing the amount of deflected optical power may represent a time-costly re-design of the patterns, or a technological challenge if critical features need to be readjusted in the fabrication phase.…”

“…1D periodic diffraction gratings were chosen as, both in optical communications and optical sensing, they represent the simplest and most common tool for redirecting light out of the optical waveguide to secondary paths [2]. 2D periodic nanocylinders arrays were chosen as they have been already studied for light scattering and diffraction, and as tools for biosensing applications [36], [9]. Taking into consideration all the parameters of the system, such as working wavelength, refractive index and shape of the materials, Au structures having periods around 350 nm and 250 nm are capable of redirecting light at about 90°angle with respect to the guided light flow direction, and at about 120°angle with respect to the guided light flow direction, respectively [2], [37].…”

“…Taking into consideration all the parameters of the system, such as working wavelength, refractive index and shape of the materials, Au structures having periods around 350 nm and 250 nm are capable of redirecting light at about 90°angle with respect to the guided light flow direction, and at about 120°angle with respect to the guided light flow direction, respectively [2], [37]. Moreover, Au nanocylinders with a pitch of 250 nm and a diameter of 100 nm have been proven to optimize focus, collimation and directionality of the diffracted beams in integrated optics, enhancement of fluorescence and biosensing applications with similar configurations [9], [38].…”

“…The selected wavelength range of the light excitation spans from 490 nm to 590 nm, with 2 nm steps (50 steps). The input light mode is TM fundamental, as it has been proved to better interact with periodic arrays of gold nanostructures in this specific configuration [9]. Maxwell's equations for the electromagnetic field were implemented in a Finite-Difference Time-Domain (FDTD) model, using Ansys Lumerical as simulation software.…”

“…In integrated photonics, planar nanogratings coupled with optical waveguides exploit the diffraction phenomenon for routing portions of the guided light in and out of the optical chip [4], [5], [6], [7]. In plasmonics, light interaction with 2D metal periodic nanoarrays is used for surface plasmon resonance (SPR) and induces phenomena such as fluorescence, dichroism, and Raman scattering [8], [9], [10]. Such structures are implemented for applications spanning from signal treatment and routing, to biomolecular sensing, at times aided by chemical functionalization [11], [12], [13], [14].…”