[Paper Review] Broadband enhanced chirality with tunable response in hybrid plasmonic helical metamaterials

Ufuk Kılıç, Matthew Hilfiker+5 more|arXiv (Cornell University)|Jan 27, 2021

Metamaterials and Metasurfaces Applications64 references51 citations

TL;DR

This paper presents broadband, tunable, and ultrabroadband chiroptical response in hybrid plasmonic helical metamaterials fabricated via glancing-angle deposition (GLAD). By integrating dielectric (Si) and plasmonic (Ag) helical subsegments, the authors achieve the highest reported Kuhn’s dissymmetry factor (g-factor) of up to 1.98 in transmission—approaching the theoretical limit of 2—across the visible to ultraviolet spectrum, with tunability via geometric parameters and material composition.

ABSTRACT

Designing broadband enhanced chirality is of strong interest to the emerging fields of chiral chemistry and sensing, or to control the spin orbital momentum of photons in recently introduced nanophotonic chiral quantum and classical optical applications. However, chiral light-matter interactions have an extremely weak nature, are difficult to be controlled and enhanced, and cannot be made tunable or broadband. In addition, planar ultrathin nanophotonic structures to achieve strong, broadband, and tunable chirality at the technologically important visible to ultraviolet spectrum still remain elusive. Here, we tackle these important problems by experimentally demonstrating and theoretically verifying spectrally tunable, extremely large, and broadband chiroptical response by nanohelical metamaterials. The reported new designs of all-dielectric and dielectric-metallic (hybrid) plasmonic metamaterials permit the largest and broadest ever measured chiral Kuhn dissymmetry factor achieved by a large-scale nanophotonic structure. In addition, the strong circular dichroism of the presented bottom-up fabricated optical metamaterials can be tuned by varying their dimensions and proportions between their dielectric and plasmonic helical subsections. The currently demonstrated ultrathin optical metamaterials are expected to provide a substantial boost to the developing field of chiroptics leading to significantly enhanced and broadband chiral light-matter interactions at the nanoscale.

Motivation & Objective

• To overcome the inherent weakness and narrowband nature of chiral light-matter interactions in natural chiral materials and conventional plasmonic structures.
• To develop planar, ultrathin, and large-scale nanophotonic metamaterials capable of strong, broadband, and tunable chirality in the visible and ultraviolet spectrum.
• To enable practical on-chip integration of chiral nanophotonic devices by using a bottom-up, mask-free fabrication method (GLAD) with tunable material composition.
• To achieve experimentally measurable, high-g-factor chiroptical responses that surpass previous limits in both magnitude and spectral bandwidth.

Proposed method

• Employed glancing-angle deposition (GLAD) to fabricate 3D helical metamaterials with controlled, sequential deposition of dielectric (Si) and plasmonic (Ag) materials, enabling hybrid plasmonic-dielectric helices.
• Used generalized Mueller matrix spectroscopic ellipsometry to measure both transmitted and reflected circularly polarized light, enabling accurate computation of the Kuhn’s dissymmetry factor (g-factor) via Equation 2 in Methods.
• Applied finite element method (FEM) simulations to model near-field chiral distributions and validate the physical origin of the enhanced chiral response.
• Engineered the metamaterials with varying numbers of helical turns (1–4) and proportions of Si/Ag subsegments to tune the chiral response across the visible and UV spectrum.
• Calculated transmission-based (𝑔𝑇𝑟𝑎𝑛𝑠) and reflection-based (𝑔𝑅𝑒𝑓) g-factors using Mueller matrix elements (Equations S9 and S10), with 𝑔𝑇𝑟𝑎𝑛𝑠 = 2(𝑀14𝑇𝑟𝑎𝑛𝑠 / 𝑀11𝑇𝑟𝑎𝑛𝑠) and similarly for reflection.
• Measured and compared transmittance, reflectance, and absorbance for left- and right-handed circularly polarized light to confirm broadband chiral response.

Experimental results

Research questions

• RQ1 Can broadband, high-g-factor chiral response be achieved in ultrathin, planar nanophotonic structures at visible and ultraviolet frequencies?
• RQ2 Can the chiroptical response of helical metamaterials be spectrally tuned by varying geometric and material parameters?
• RQ3 Does the hybrid plasmonic-dielectric design enhance the chiral response beyond what is achievable with all-dielectric or purely plasmonic helices?
• RQ4 Can the Kuhn’s dissymmetry factor (g-factor) be experimentally measured with high accuracy using ellipsometry, and does it exceed previous limits in the visible-UV range?
• RQ5 Is the chiral response stronger in transmission or reflection, and can the metamaterial function as an efficient chiral optical filter?

Key findings

• The hybrid plasmonic Si-Ag helical metamaterials achieved a transmission-based Kuhn’s dissymmetry factor (𝑔𝑇𝑟𝑎𝑛𝑠) of up to 1.98 at 3.5 eV, approaching the theoretical maximum of ±2, representing the highest reported g-factor in the visible-UV range.
• The all-dielectric Si helical metamaterial with four turns exhibited a g-factor of 1.98 in transmission, confirming the broadband and strong chiral response across a wide spectral range.
• The chiral response was tunable: increasing the number of helical turns introduced a new dip in the g-factor spectrum due to localized surface plasmon resonances in the Ag subsegments.
• The transmission-based g-factor was significantly stronger than the reflection-based g-factor, indicating that the metamaterials function as efficient chiral filters only in transmission.
• The plasmonic hybrid structure showed higher reflection-based g-factor than the all-dielectric structure due to increased refractive index mismatch from silver, but transmission remained dominant for chiral filtering.
• Finite element method (FEM) simulations confirmed the presence of strong chiral near-field distributions and validated the experimental results, linking the enhanced response to collective photonic band effects and localized plasmon resonances.