3D Plasmonic Archimedean Spiral

• The 3D plasmonic Archimedean Spiral is a fully three-dimensional nanostructure based on the Archimedean spiral equation, engineered for broadband and single-handed chirality enhancement.
• It leverages advanced fabrication techniques such as FIB milling and template stripping to achieve deterministic conical geometries with precise arm spacing and vertical control.
• Its robust near-field chiroptical response and ability to trap chiral analytes make it a promising platform for ultra-sensitive sensing and quantum nanophotonic applications.

A 3D plasmonic Archimedean spiral (AS) is a fully three-dimensional nanostructure whose geometry follows the Archimedean spiral equation, typically realized with metallic materials such as gold. The AS leverages continuous radial growth and fixed inter-arm separation to enable broadband, spatially extended, and single-handed enhancement of optical chirality in the visible to near-infrared regime. This structure offers distinct advantages over conventional planar nanospirals or quasi-1D/2D spiral chains, including strong chiroptical asymmetry across a wide spectral range, the ability to trap chiral analytes or nanoparticles, and tunable near-field properties. Recent advances in fabrication protocols, notably focused ion beam (FIB) milling and template stripping, have yielded high-definition, deterministic 3D AS structures with robust conical hole-like geometries capable of supporting pronounced near-field effects and chiroptical responses (Jiang et al., 13 Aug 2025).

1. Defining Geometry and Fabrication Techniques

The canonical 3D AS consists of two spiral arms—typically right-handed—executing multiple full turns with a constant radial increment (for example, Ar = 150 nm/turn) and defined vertical separation between successive turns (e.g., 30 nm). The engineered structure spans a large base (e.g., diameter 2000 nm) and rises to a sharp apex (∼1280 nm height), incorporating thin connecting bars at fixed angular intervals to maintain stability of the suspended spiral.

The fabrication protocol integrates FIB milling with template-stripping. First, inverse cones are milled into a Si substrate, then coated with an 80 nm Au film. The patterned Au is transferred to a pre-patterned aperture substrate via PMMA-assisted template stripping. Finally, the AS features are directly milled into the suspended gold cones. This approach yields nearly collapse-free, high-definition 3D AS with deterministically controlled height, arm width, and inter-spiral gap, overcoming prior limitations in out-of-plane displacement (as with nano-kirigami methods).

2. Broadband Near-Field Chirality and Optical Properties

Simulations reveal that the 3D AS acts as a broadband plasmonic horn antenna, producing intense, spatially extended near-field chirality enhancement in the visible–NIR range (600–850 nm) (Jiang et al., 13 Aug 2025). Under circularly polarized illumination matching the spiral’s handedness (e.g., R-CPL on right-handed AS), the near-field exhibits uniformly enhanced optical chirality:

$C = \epsilon_0 \omega \, \mathrm{Im}(E^* \cdot B)$

where $E$ and $B$ are the local electric and magnetic fields, ε₀ is the vacuum permittivity, and ω the optical frequency. The enhancement factor is quantified as: $$C_{\mathrm{enh}} = \frac{C_{\mathrm{local}}}{C_{\mathrm{CPL}}}$$ Simulated values reach up to $C_{\mathrm{enh}} \approx 25$ over the functional spatial region (z = 800–1200 nm). This single-handed (unidirectional) enhancement is persistent over the operational bandwidth. Excitation with the opposite handedness leads to mixed or suppressed chirality distribution.

3. Experimental Chiroptical Characterization

Far-field characterization is performed by transmission circular dichroism spectroscopy on individual AS. A supercontinuum laser, together with controlled polarization optics, provides broadband CPL excitation. Differential transmission between L- and R-CPL is quantified via the dissymmetry (g-) factor:

$$g = \frac{T_{\mathrm{L-CPL}} - T_{\mathrm{R-CPL}}}{T_{\mathrm{L-CPL}} + T_{\mathrm{R-CPL}}} \times 2$$

Measured values of $g$ reach up to 0.42 and agree well with simulation across the entire visible–NIR range. The sign of $g$ remains consistent with the spiral’s chirality, indicating robust single-handed transmission and optical activity.

4. Mechanisms of Chirality Enhancement and Sensing Applications

The AS’s conical geometry concentrates the optical field and chirality in the “hot zone” (near the spiral apex), producing a large, spatially extended region where enantiomers experience significant chiroptical forces. This property makes the structure well-suited for ultra-sensitive chiral sensing applications, as functionalized nanoparticles or molecules can be trapped and spectroscopically interrogated in regions of maximum chirality enhancement (Jiang et al., 13 Aug 2025).

This suggests a plausible mechanism for improved selectivity and sensitivity versus planar nanospirals or discrete spiral chains, due to the volumetric extension and spectral breadth of the near-field chirality.

5. Limitations, Challenges, and Comparisons

Prior approaches, for example nano-kirigami methods, failed to provide reproducible vertical stretching and conical shaping. Realizing smooth, deterministic AS structures requires careful control over mechanical features (e.g., arm thickness, gap regularity, vertical displacement). Imperfections such as surface roughness or gap non-uniformity degrade the optical response and can shift or broaden resonant features. Discrepancies between simulated and measured transmission spectra are attributed to fabrication tolerances.

A comparison with spiral chains of plasmonic ellipsoids (Hadad et al., 2010) and other 3D chiral structures highlights several differences:

Feature	3D Plasmonic AS (Jiang et al., 13 Aug 2025)	Spiral Chain of Ellipsoids (Hadad et al., 2010)
Geometry	Continuous, conical, 3D	Discrete ellipsoids, quasi-1D chain
Chirality enhancement	Broadband, spatially extended	Multi-band, direction-dependent
Application focus	Sensing, trapping, CD	One-way waveguides, nonreciprocal optics
Tunability	via geometry and illumination	via spiral angle, field, resonance

This table summarizes some of the key distinctions rooted in structure and functional response.

6. Future Directions and Implications

A plausible implication is the integration of high-definition 3D AS structures into sensor arrays for large-scale biosensing, with optimization of arm width, spiral pitch, and apex shape to tailor the near-field distribution. Research directions include refining fabrication protocols to further minimize roughness, exploring alternative plasmonic materials, and combining AS with microfluidic environments for enhanced analyte delivery and trapping. Extending the approach to higher-order AS configurations (multiple arms, varying vertical profiles) may further increase both spectral coverage and selectivity. Experimental demonstration of ultra-sensitive enantiomer discrimination remains a pending objective.

7. Relevance to Plasmonic and Quantum Nanophotonics

The single-handed, broadband enhancement of optical chirality by 3D AS provides a robust platform for chiral quantum nanophotonic devices, owing to its capacity to generate and control spin-dependent local density of optical states (Pham et al., 2016). Its volumetric chirality distribution distinguishes it from planar or quasi-1D chiral nanostructures, offering avenues for encoding quantum information in polarization, orbital angular momentum, and spatial mode. Integration into hybrid plasmonic–photonic architectures may facilitate selective coupling to quantum emitters and advanced chiroptical logic elements.

In summary, the 3D plasmonic Archimedean spiral represents a structurally and functionally advanced nanostructure for chirality enhancement, with demonstrated broadband, spatially extended, and single-handed near-field effects. Its robust chiroptical response, enabled by deterministic conical shaping and controlled fabrication, establishes it as a promising candidate for future chiral sensing, trapping, and quantum photonic applications (Jiang et al., 13 Aug 2025).