[Paper Review] Color Gamut Behavior in Epsilon Near-Zero Nanocavities during Propagation of Gap Surface Plasmons

Giuseppe Emanuele Lio, Antonio Ferraro+3 more|arXiv (Cornell University)|Jan 1, 2020

Gold and Silver Nanoparticles Synthesis and ApplicationsMaterials Science109 references33 citations

TL;DR

This study demonstrates that epsilon near-zero (ENZ) metal-insulator-metal (MIM) nano-cavities with silver and tunable dielectrics (PVP, ITO, ZnO) enable extraordinary optical transmission (up to 55%) and zero reflection at resonant wavelengths, driven by gap surface plasmons (GSPs) and surface plasmon polaritons (SPPs). The systems exhibit giant enhancement of the pseudo dielectric constant's imaginary part (|ε₂| = 500) and sharp ellipsometric responses (Ψ and ∆), enabling a giant Goos-Hänchen shift and angle-dependent color tuning, ideal for tunable filters, sensors, and anti-counterfeiting devices.

ABSTRACT

This work reports on numerical and experimental results obtained in plasmonic metal–insulator nanocavities. The systems are composed of silver as metal, and different materials as insulator, namely polyvinylpyrrolidone (PVP), indium tin oxide (ITO), and zinc oxide (ZnO). The proposed nanocavities exhibit extraordinary optical effects as tunable color hue, highlighted in gamut maps, depending on incident/viewing angles, extraordinary transmission and zero reflection at resonant wavelengths, for different incident polarizations. These phenomena are related to the formation of surface plasmon polaritons (SPPs) and gap surface plasmons (GSPs) whose presence is evidenced by a remarkable sigmoidal behavior of the pseudo dielectric function, with epsilon-near-zero singularities in its real and imaginary parts. This function is directly calculated from the measured ellipsometric parameters Ψ and Δ and allows probing, in a fast and effective way, the existence of the plasmonic modes. Moreover, in presence of these singularities, the ellispometric analysis of the systems also shows a pronounced dephasing between p- and s-reflected beams that can lead to a Goos–Hänchen shift effect to be exploited for sensing applications. Thanks to their unusual optical properties, the proposed nanocavities open a wide scenario of applications in fields like tunable color filters, optics, photonics, physical security, and sensing.

Motivation & Objective

• To design and fabricate large-area, reproducible plasmonic nano-cavities using ENZ conditions in MIM and MIMI structures.
• To investigate the role of dielectric layer thickness and material (PVP, ITO, ZnO) in enabling extraordinary transmission and zero reflection.
• To exploit the resulting plasmonic resonances for enhanced Goos-Hänchen shifts and angle-dependent color tuning.
• To demonstrate the feasibility of these structures as tunable optical filters, sensors, and anti-counterfeiting components via ellipsometric characterization.

Proposed method

• Fabrication of Ag/MIM/insulator/Ag nano-cavities using DC magnetron sputtering (Ag: 20 nm, PVP/ITO/ZnO: 380/85/83 nm) and spin-coating (PVP).
• Characterization via Wase-M2000 ellipsometer across 300–900 nm with incident angles of 50°, 60°, 70°, using p- and s-polarized light.
• Numerical simulation using Numerical Ellipsometer Analysis (NEA) in COMSOL Multiphysics to fit experimental data and extract optical constants.
• Calculation of pseudo dielectric constant <ε̃> = <ε₁> − i<ε₂> from ellipsometric parameters Ψ and ∆ using the relation <ε̃> = sin²θ_inc[1 − tan²θ_inc(1−ρ)/(1+ρ)]², where ρ = tanΨ e⁻ⁱ∆.
• Analysis of the sigmoidal behavior of Ψ and ∆ to identify plasmonic resonances and quantify Goos-Hänchen shift.
• Comparison of experimental and simulated transmittance, reflectance, and ellipsometric responses to validate the model.

Experimental results

Research questions

• RQ1 How do different dielectric materials (PVP, ITO, ZnO) influence the plasmonic response and optical transmission in Ag/MIM/Ag nano-cavities?
• RQ2 To what extent can the giant enhancement of the imaginary part of the pseudo dielectric constant (|ε₂|) be achieved in MIMI structures under ENZ conditions?
• RQ3 Can the sharp variation in ellipsometric parameters Ψ and ∆ be exploited to measure and enhance the Goos-Hänchen shift in subwavelength plasmonic cavities?
• RQ4 How does the angle-dependent color tuning arise from the hybridization of SPPs and GSPs in these nano-cavities?
• RQ5 Can these nano-cavities serve as effective, tunable, and anti-counterfeitable optical components based on their unique ellipsometric fingerprint?

Key findings

• The Ag/PVP/Ag MIM nano-cavity exhibits three distinct transmission peaks at blue, green, and red wavelengths, with 30% maximum transmission and 0% reflection at resonant wavelengths.
• The Ag/ITO/Ag/MIMI structure achieves 50% transmission and 0% reflection at λ ≈ 520 nm, with a sharp dip in s-polarized reflectance indicating strong plasmonic resonance.
• The Ag/ZnO/Ag/ZnO MIMI structure demonstrates the highest performance with 55% transmission and |ε₂| = 500 at λ = 530 nm under 50° incidence, indicating a giant enhancement of the pseudo dielectric constant.
• The ellipsometric parameters Ψ and ∆ exhibit a sigmoidal response at resonance, enabling precise measurement of the giant Goos-Hänchen shift, which is angle- and material-dependent.
• The nano-cavities display strong angle-dependent color changes: green in transmission and purple in reflection for the ZnO-based MIMI, with video evidence of dynamic color tuning.
• Numerical simulations using NEA in COMSOL Multiphysics show excellent agreement with experimental data for transmittance, reflectance, and ellipsometric responses across all configurations.