Realization of tuneable ultrabroadband interconnection for solitonic-plasmonic synapsis by exploiting epsilon near zero conducting oxides

. This research introduces a novel highly efficient method to interconnect two metallic nanostrips that support the propagation of surface plasmon polariton (SPP) waves exploiting photorefractive soliton guide. The intricate design of the multilayer geometry enables light control diffraction at the metallic nanostrip's end and reduces its angular dispersion. Moreover, the system's on/off state can be switched by exploiting the epsilon near zero properties of the indium tin oxide (ITO) layer. The photorefractive crystal positioned between the two plasmonic waveguides enables the self-confinement of light, generating a waveguide that can be utilized by both the writing light and other wavelengths transmitted as signals. The resulting SPP waves can be efficiently recoupled in the second nanostrip, with an efficiency of around 40% across a broad range of wavelengths. This cutting-edge approach paves the way for significant advancements in the field of nanophotonics and provides a fundamental framework for the development of new, highly efficient optical interconnects in nanoscale systems. The findings of this study have implications for a wide range of applications, including nanoscale sensing, optical computing, and data communication.


Introduction
This paper discusses the use of surface plasmon polariton waves (SPP) interconnection with solitonic channels for the realization and the development of optical devices with high integration capabilities.While SPP waves are beneficial for miniaturization [1], their limited propagation length and absence of nonlinearity in metals reduce their applicability, especially in the visible wavelength range.To overcome these limitations, several waveguide architectures have been proposed, but they still lack the ability to transmit signals over long distances or produce active addressing.
On the other hand, self-written solitonic channels, have memory effects and can learn information [2], providing exceptional characteristics for a wide range of applications including nanoscale sensing, optical computing [3], and data communication.Moreover, exploiting the epsilon near zero properties of the indium tin oxide (ITO) [4] layer can also provide a further characteristic to the device by providing the switching between on/off state of both signal and writing beam.In this paper, the authors analyse the interconnection of two metallic strips acting as waveguides for SPP signals with self-written solitonic channels.By using photorefractive substrates, diffracted light can confine itself within a soliton channel, whose propagation characteristics can be controlled and modulated.

Epsilon near zero effect of ITO
As mentioned earlier, the ITO utilization in the presented work depends on several reasons.The main one is of course to provide an electrical contact to apply the bias electric field which is necessary to have the photorefractive effect [2].Since the optical properties of the ITO is dependent to the charge carrier density, we have utilized it as a mean to alter the refractive index by changing both the applied voltage magnitude and polarity.Also, charge accumulation in the ITO can provide the conditions to alter the plasma resonance frequency and make it a tuneable epsilon near zero material.Here, we have used this property of ITO to realize a switch to turn on/off, the propagation of the signal in the photorefractive material.

Soliton formation
In the presented numerical experiment, a 532 nm SPP wave was injected into a metallic nanostrip of silver as the writing beam for the solitonic channel formation.An electric bias was applied across the ITO layer to generate the screening soliton beam.The formation of the soliton channel was observed over time, and it was found that the system self-confined along a channel of photoexcited charges that corresponds to a channel of varied refractive index.To ensure a low angular dispersion of the diffracted beam (depicted in Fig. 1 C and D), and also isolation of the SPP interface from the photorefractive substrate, we have used a 500 nm ITO layer exactly below the silver strip.The accumulation of the charges in the ITO layer, provides a mean to manipulate the complex refractive index and modulate its optical absorption.Through this absorption modulation, we have realized a switch both for the formation of the solitonic channel and also signal propagation in the plasmonic and solitonic waveguides.Since the charge accumulation is an ultrafast process, such devices can offer modulation speeds in THz regime because it is only limited by the RC time constant.

Recoupling of the soliton channel into the second metallic nanostrip
The light carried inside the soliton waveguide can be recoupled into the underlying metallic nanostrip still in the form of SPP wave.The necessary condition for the light to be recoupled is to satisfy the dispersion condition of the wave vectors in Kretschmann configuration, as shown in figure 4A.By appropriately choosing the material of the insulating sublayer at the bottom of the structure, the dispersion curve of the light and of the plasma wave can be tuned so as to ensure their overlap for an extended region in the space k/k0 (fig.4a).
As you can see in figure 4B, the SPP recoupling gets an almost flat efficiency: between 40 and 42% in the range 1070-1190 nm (bandwidth more than 100nm), doubling almost the band (bandwidth of about 200 nm) when a coupling efficiency equal or higher than 35% is chosen (range 1050-1250 nm).We have represented in figure 4C the soliton waveguiding and the recoupling for different wavelengths, covering the most important telecom bands (even if the recoupling gets a lower efficiency).
This behaviour of the proposed structure opens a broad range of applications in different areas as well as in the artificial intelligence system.Thanks to these characteristics of the introduced hybrid structure, we can add an extra degree of freedom to the system by controlling the beam confinement.This is a very crucial aspect because it provides the possibility to realize the gate control in the photonic integrated circuits.These electrodes can be utilized for both modulating the signal's propagation direction and also the weights of the channels to modulate the intensity of the signal.Based on the saturation behaviour of the crystal [5], it is possible to use this type of structure as a platform to realize the processing in memory and solve the problem of the processing in memory dichotomy.The generation of a solitonic channel is certainly an excellent tool for interconnecting distant metallic waveguides together, thus lengthening the propagation of the SPP waves and allowing the realization of complex circuits.The use of a photorefractive material as a means to realize the interconnections offers a further important advantage: to provide the plasmon signals with a usable nonlinearity for active systems, able to perform intelligent signal processing.Specific circuit geometries based on soliton waveguides have already demonstrated [6] that they can learn information, memorize it and use it for fully optical recognition.The present study allows the transfer of optical knowledge in the nonlinear field and in signal processing to the plasmonic framework.

Figure 1 ,
Figure 1, (A) Diffraction of SPP light into the photorefractive medium by switching on or, (B) switching off the SPP excitation at the interface of Ag-ITO in the Kretschmann configuration by modulation of the ITO charge carrier density, C) SPP excitation by numerical port by boundary mode analysis and diffraction to the photorefractive medium when the ITO layer is not present and D) when a thin ITO layer is present between the metallic nanostrip and the photorefractive substrate.

Figure 2
Figure 2 Formation of a screening soliton from the SPP diffraction

Figure 3
Figure 3 Waveguiding and SPP recoupling at different wavelengths.A) The dispersion curves have been matched along an extended area in order to ensure an almost-constant recoupling B) over a large range of wavelengths.C) Signals at different wavelengths propagating within the solitonic waveguide and recoupled as SPP wave at the bottom metallic nanostrip.