Chiral Plasmonic Nanohelix: Circular Dichroism

Nanohelix Geometry

Particles on helix (N) 14

Helix pitch (nm/turn) 55

Incident wavelength λ 530 nm

Extinction (arb. units)

A(LCP)

0.000

A(RCP)

0.000

CD = Aₗ−Aᵣ

0.000

Dissymmetry g-factor

0.000

Handedness

Left

How it works

A gold nanoparticle helix — the kind assembled on DNA-origami templates in real chiroptical experiments — is modeled here as N plasmonic spheres, each a driven Lorentzian dipole oscillator with polarizability

α(λ) = α₀ / [(1 − x²) − iΓx],   x = λ₀/λ

resonant near λ₀ ≈ 530 nm (a small Au nanosphere's localized surface plasmon resonance, Γ its damping). Illuminated along the helix axis by left- or right-circularly-polarized light, particle i sees

E_inc,i = E₀(x̂ ± iŷ) e^(ikz_i),   k = 2π/λ

The particles also couple to each other through their near-field dipole radiation — this is a discrete-dipole approximation (DDA), solved self-consistently by relaxation:

p_i = α(λ)[E_inc,i + Σ_j≠i κ₀(a/R_ij)³ (3n̂(n̂·p_j) − p_j) e^(ikR_ij)]
σ_ext(λ) ∝ Σ_i Im[E_inc,i* · p_i]      (optical theorem)
CD(λ) = A_LCP(λ) − A_RCP(λ),   g = 2·CD / (A_LCP + A_RCP)

Handedness flips the sign of the azimuthal step between particles, mirroring the whole structure — flip it and the CD signal flips sign, exactly as a real enantiomeric nanohelix does.
N / pitch set the geometric arrangement — how tightly the near-field–coupled dipoles wind around the propagation axis, which controls how strongly the structure discriminates LCP from RCP.
λ sweeps the driving wavelength across the plasmon resonance; CD peaks near where the coupling-induced splitting is largest, not exactly at λ₀.
Sphere glow along the helix shows the relative magnitude of each particle's induced dipole |p_i| for whichever polarization is being animated — the chiral near-field coupling makes this pattern asymmetric between LCP and RCP.

This near-field-coupled-dipole picture is the standard simplified mechanism used to explain plasmon-induced circular dichroism in chiral metal nanostructures (helices, gammadions, DNA-templated nanoparticle assemblies): geometric chirality plus retarded dipole–dipole coupling, not molecular optical activity.

Gold nanoparticle dipole Incident field vector

CD(λ), 450–700 nm

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