Chiral Plasmonic Metasurface

• Chiral plasmonic metasurfaces are metal-based, periodically patterned structures whose optical response varies with the handedness of circularly polarized light due to engineered symmetry breaking.
• They employ localized plasmonic resonances, diffractive coupling, and magneto-optical effects to generate significant circular dichroism and enhanced nonlinear optical responses.
• Key applications include polarization manipulation, chiral molecular sensing, and quantum photonics, though challenges such as ohmic losses and angular sensitivity persist.

A chiral plasmonic metasurface is a periodically patterned metallic surface or metal-based multilayer whose optical response depends on the handedness of incident light, so that left- and right-circularly polarized waves experience different transmission, reflection, absorption, phase, or nonlinear conversion. In the literature, chirality is realized through genuinely three-dimensional metallic meta-atoms, planar arrays whose in-plane geometry breaks mirror and inversion symmetries, extrinsically chiral nanohole lattices under oblique incidence, and hybrid plasmonic cavities or lattice modes whose resonances are polarization selective (0809.3163, Wu et al., 2021, Edet et al., 2019, 2207.14710, Aupiais et al., 2024). The subject therefore spans several related but distinct regimes: localized plasmonic resonances, magnetoelectric modes, lattice plasmon modes, quasi-bound states in the continuum, and magneto-plasmonic surface-plasmon cavities. Across these regimes, chirality is not a single geometric property but a resonance-mediated symmetry property of the full optical system.

1. Definitions, scope, and forms of chirality

In plasmonic metasurfaces, chirality may be intrinsic to the unit cell, induced by the arrangement of otherwise simple resonators, or activated by the combination of structure and illumination geometry. A planar chiral plasmonic metasurface based on a metal–dielectric–metal stack with rotated rectangular holes, for example, is chiral not because it contains a 3D helix, but because the oriented aperture array responds differently to left-circularly polarized and right-circularly polarized light in reflection (Wu et al., 2021). By contrast, arrays of three-dimensional metallic meta-atoms such as Möbius-strip-inspired resonators or cut wire–split-ring resonator combinations are intrinsically chiral at the meta-atom level and exhibit optical activity exceeding that of pseudo-planar chiral metamaterials by about an order of magnitude (0809.3163).

A second distinction is between intrinsic and extrinsic chirality. In a magneto-plasmonic nanohole metasurface based on a perforated Au film on Ce:YIG/YIG/SiO$_2$/TiN, the nanohole lattice is centrosymmetric in plane, yet the device becomes chiral in an extrinsic sense when illuminated obliquely; under normal incidence, the circular dichroism is near zero, whereas glancing-angle illumination produces an asymmetric near-field distribution and a finite far-field circular dichroism (Edet et al., 2019). Related work on planar gold metasurfaces formed by the fusion of three equilateral triangles further distinguishes structural chirality from pseudo-chirality: structural chirality arises from geometry and substrate-induced vertical asymmetry, while pseudo-chirality is activated by oblique excitation and the relative orientation of the incident beam and the asymmetric motif (Palermo et al., 1 Aug 2025).

A common misconception is that chirality in metasurfaces requires an explicitly helical three-dimensional object. The literature shows otherwise. Planar asymmetry alone can generate strong circular dichroism, but genuinely 3D metallic meta-atoms can strengthen optical activity by enabling stronger electric–electric or electric–magnetic mode coupling (Wu et al., 2021, 0809.3163). Another recurrent ambiguity concerns whether a measured handedness-dependent signal is “true circular dichroism” or a mixture of circular and linear anisotropies. Work on optically active achiral plasmonic metasurfaces with U-shaped resonators shows that the measured circular differential optical absorption can contain contributions from linear birefringence and linear dichroism, although in that system the total true optical activity accounts for more than 90% of the measured circular differential optical absorption (Nicolas et al., 2023).

2. Symmetry breaking and resonant mechanisms

The fundamental design variable of a chiral plasmonic metasurface is symmetry. Chirality appears when mirror symmetry and inversion symmetry are removed from the full optical system, or when symmetry is reduced so that one handedness couples more strongly than the other to a resonant mode. In the rectangular-hole Au/SiO$_2$/Au metamirror, the rotation angle of each rectangle relative to the lattice axes is the primary control parameter. For the studied layout, the centers of neighboring holes form a regular hexagon relation, and the circular dichroism becomes zero at $\theta = 0^\circ, 30^\circ, 60^\circ, 90^\circ$; away from those angles, symmetry is broken and the reflectance contrast can be strongly enhanced, reaching $\mathrm{CD}_{\max}=0.76$ near $\lambda=880$ nm for $P=500$ nm, $t=400$ nm, $w=285.7$ nm, and $\theta=21^\circ$ (Wu et al., 2021). The same work concludes that the period satisfies $P < \lambda$ whenever chirality appears.

Resonant mode structure determines how symmetry breaking is converted into chiroptical response. In U-shaped gold resonator arrays, the decisive modes are magnetoelectric modes in which currents flow in opposite directions in the two lateral arms. These modes generate the strongest circular dichroism signatures, and the associated near fields are locally chiral even when the far-field circular differential optical absorption is zero at normal incidence (Nicolas et al., 2023). In three-dimensional metallic meta-atoms, a simple physical rule emerges: the meta-atom should contain two resonant elements, one that couples strongly to the incident field and another that radiates an orthogonal polarization component; the two resonances must be coupled and the geometry must lack mirror symmetry (0809.3163). In practice, this can be realized by two electric dipoles or by an electric dipole coupled to a magnetic dipole, as in a cut wire–split-ring resonator geometry.

Periodic order introduces an additional mechanism: diffractive coupling and lattice plasmons. In square lattices of plasmonic quadrumers, asymmetric transmission arises because left- and right-circularly polarized light excite lattice plasmon modes with different efficiencies. The decisive quantity is the differential in-plane scattering $_2$0 of the isolated nanoparticle; asymmetric transmission is strongest when the maxima of $_2$1 are aligned along the propagation direction of a grazing diffraction order (Chaitanya et al., 2021). This diffractive mechanism differs from the non-diffractive planar-chirality mechanism because it can operate at normal incidence even when the nanoparticle possesses 4-fold rotational symmetry. In fully 3D chiral metacrystals made from ordered arrays of Pt nanohelices, the localized plasmon resonance of the helix hybridizes with diffractive orders to form chiral surface lattice resonances whose strength depends on excitation handedness (2207.14710).

Magneto-plasmonic and THz surface-plasmon systems introduce yet another resonance class. In InSb-based THz patch-cavity metasurfaces, magnetic field induces circular birefringence in the bulk magnetoplasma, and the cavity modes inherit a handedness-dependent frequency shift proportional to the integrated spin density of the mode (Aupiais et al., 2024). In this regime, chirality is not only a question of geometric handedness but also of spin–orbit-mixed surface-plasmon mode structure.

3. Representative architectures and material systems

The experimental literature covers several recurring architectures, each combining a specific plasmonic resonance with a specific symmetry-breaking strategy.

Platform	Chirality mechanism	Representative reported result
Au/SiO$_2$2/Au reflective metasurface with rotated rectangular holes (Wu et al., 2021)	Rotation-angle-controlled planar chirality in reflection	Maximum CD reaches $_2$3 near $_2$4 nm
Au nanohole array on Ce:YIG/YIG/SiO$_2$5/TiN (Edet et al., 2019)	Extrinsic chirality under oblique incidence plus magneto-optical modulation	CD modulation from $_2$6 to $_2$7 at $_2$8 nm
Silver–LiNbO$_2$9–silver reflective metasurface (Semone et al., 27 Jul 2025)	Asymmetric chiral unit cell plus qBIC-enhanced plasmonic and cavity confinement	LH-polarized SPDC emission up to $\theta = 0^\circ, 30^\circ, 60^\circ, 90^\circ$0 photon pairs/s
Periodic arrays of Pt nanohelices (2207.14710)	Intrinsic 3D chirality plus diffractive coupling to form chiral surface lattice resonances	Best sensing sensitivity $\theta = 0^\circ, 30^\circ, 60^\circ, 90^\circ$1
Refractory Mo chiral metasurface on SiO$\theta = 0^\circ, 30^\circ, 60^\circ, 90^\circ$2/Mo backplane (Yu et al., 7 Feb 2025)	Z-shaped mirror-asymmetric nanoantenna in absorptive cavity geometry	THG-CD $\theta = 0^\circ, 30^\circ, 60^\circ, 90^\circ$3 and operation under intensities above $\theta = 0^\circ, 30^\circ, 60^\circ, 90^\circ$4

The planar metal–dielectric–metal architecture is especially common because it suppresses transmission and converts chiroptical contrast into reflection or absorption contrast. In the rectangular-hole metamirror, the bottom Au layer is thick enough to block transmission, so chirality is measured in reflection (Wu et al., 2021). In the refractory Mo device, an optically thick Mo backplane similarly suppresses transmission and boosts absorption in the top chiral resonator (Yu et al., 7 Feb 2025). The silver–LiNbO$\theta = 0^\circ, 30^\circ, 60^\circ, 90^\circ$5–silver quantum metasurface is likewise reflective and ultrathin, with a continuous silver substrate acting as an opaque reflector and an x-cut lithium niobate spacer selected for its strong second-order nonlinearity, especially the $\theta = 0^\circ, 30^\circ, 60^\circ, 90^\circ$6 tensor component (Semone et al., 27 Jul 2025).

Not all platforms are purely planar. Three-dimensional metallic meta-atoms provide a foundational route to strong optical activity, exemplified by the cut wire–split-ring resonator geometry with an average polarization rotation of $\theta = 0^\circ, 30^\circ, 60^\circ, 90^\circ$7, corresponding to $\theta = 0^\circ, 30^\circ, 60^\circ, 90^\circ$8 per meta-atom layer (0809.3163). At the other end of the scale, THz patch cavities on InSb use deep-subwavelength metal patches over a thin Si$\theta = 0^\circ, 30^\circ, 60^\circ, 90^\circ$9N$\mathrm{CD}_{\max}=0.76$0 spacer and a bulk magneto-plasmonic semiconductor to realize field confinement within a depth of about $\mathrm{CD}_{\max}=0.76$1 and mode volumes around $\mathrm{CD}_{\max}=0.76$2 (Aupiais et al., 2024).

The material choice strongly conditions the operating regime. Gold and silver remain canonical visible and near-infrared plasmonic materials, but they are limited by optical loss and, under high-power pumping, by thermal fragility. Refractory molybdenum has therefore been proposed for nonlinear chiroptics because it supports plasmonic resonance in the visible–NIR, survives up to $\mathrm{CD}_{\max}=0.76$3C without obvious deformation, and tolerates intensities above $\mathrm{CD}_{\max}=0.76$4 (Yu et al., 7 Feb 2025). Magneto-optical oxides such as Ce:YIG provide a different advantage: lower optical loss than ferromagnetic metals combined with a strong magneto-optical effect, enabling active chiral modulation (Edet et al., 2019).

4. Chiroptical observables, field diagnostics, and theoretical descriptions

Circular dichroism is the canonical observable, but the literature uses several operational definitions depending on whether the device is measured in transmission, reflection, or absorption. For the reflective rectangular-hole metasurface, circular dichroism is defined as

$\mathrm{CD}_{\max}=0.76$5

where $\mathrm{CD}_{\max}=0.76$6 and $\mathrm{CD}_{\max}=0.76$7 are the reflectances under left- and right-circularly polarized incidence (Wu et al., 2021). In absorptive plasmonic devices, a common definition is

$\mathrm{CD}_{\max}=0.76$8

together with the chiral efficiency

$\mathrm{CD}_{\max}=0.76$9

as used for the refractory Mo metasurface (Yu et al., 7 Feb 2025). In reflective THz chiral surface plasmonics, the handedness-dependent observables are the ellipticity

$\lambda=880$0

and the optical rotation angle

$\lambda=880$1

where $\lambda=880$2 are circular-basis reflection coefficients and $\lambda=880$3 their phases (Aupiais et al., 2024).

Field maps are central to physical interpretation. In the rectangular-hole metamirror, the resonant condition at $\lambda=880$4 and $\lambda=880$5 nm produces strong electric-field enhancement at the four corners of the rectangles and the short side for right-circularly polarized incidence, while the field is comparatively weak for left-circularly polarized incidence; the resulting near-field imbalance explains the large reflectance circular dichroism (Wu et al., 2021). In U-shaped plasmonic resonators, near-field analysis distinguishes electric and magnetic modes and shows that the magnetic mode produces stronger and more spatially structured chirality density than the electric mode (Nicolas et al., 2023). In planar gold metasurfaces designed for enantiomer recognition, near-field scanning optical microscopy confirms polarization-selective localized plasmonic modes and dense hot-spots around sharp corners, tips, and notches (Palermo et al., 1 Aug 2025).

A widely used near-field diagnostic is the optical chirality density

$\lambda=880$6

or, in one work, an equivalent form using $\lambda=880$7 in place of $\lambda=880$8 (Nicolas et al., 2023, Edet et al., 2019, Palermo et al., 1 Aug 2025). This quantity is used to locate superchiral near-fields and to connect far-field chiroptical observables with local light–matter interaction strength. In U-shaped resonator arrays, the chirality density can exceed the value for circularly polarized plane waves, and the relation $\lambda=880$9, valid for plane waves, breaks down in the near field (Nicolas et al., 2023). In planar gold metasurfaces with chiral and pseudo-chiral response, the normalized chirality $P=500$0 exceeds $P=500$1 and even reaches above $P=500$2 at resonance (Palermo et al., 1 Aug 2025).

Theoretical descriptions range from Jones-matrix and Mueller-matrix analysis to coupled oscillators, coupled-mode methods, and non-Hermitian pole-zero formalisms. Mueller matrix polarimetry with differential decomposition has been used to separate true circular dichroism from linear birefringence, retardance, and linear dichroism; in U-shaped plasmonic metasurfaces, linear birefringence and linear dichroism are at least one order of magnitude smaller than the circular-dichroism contribution (Nicolas et al., 2023). Chiral surface lattice resonances in nanohelix arrays are described by a three-coupled-oscillator Hamiltonian that hybridizes the localized plasmon resonance with multiple diffractive orders (2207.14710). THz chiral surface plasmonics are analyzed through complex poles and zeros of the reflection coefficient, with

$P=500$3

thereby separating radiative and ohmic contributions in a non-Hermitian, dispersive system (Aupiais et al., 2024).

5. Nonlinear, quantum, and actively tunable chiral plasmonics

Nonlinear and active chiral plasmonic metasurfaces extend the subject beyond passive circular dichroism. A notable example is a silver–LiNbO$P=500$4–silver chiral reflective metasurface designed to enhance spontaneous parametric down-conversion and produce circularly polarized photon pairs. Its key ingredients are a high-$P=500$5 quasi-bound state in the continuum, plasmonic resonances for large local electric-field enhancement, and an asymmetric chiral unit-cell geometry that breaks in-plane inversion and mirror symmetries (Semone et al., 27 Jul 2025). At the main resonance near $P=500$6 nm, left-handed circularly polarized light is absorbed strongly, approaching unity absorption, while right-handed excitation is weakly absorbed; the reported circular dichroism peaks around $P=500$7. Using the standard quantum-classical correspondence between SPDC and sum-frequency generation, the authors report up to $P=500$8 photon pairs/s for LH-polarized emission and about $P=500$9 photon pairs/s for RH-polarized emission under a bandwidth of $t=400$0 nm and a pump intensity of $t=400$1 mW (Semone et al., 27 Jul 2025).

A second nonlinear branch uses refractory plasmonic molybdenum. The Mo metasurface is a periodic array of Z-shaped chiral nanoantennas on a SiO$t=400$2 spacer above an optically thick Mo backplane. For the $t=400$3 nm design, the chiral resonance is around $t=400$4 nm, where right-circularly polarized absorption approaches unity and the peak absorption reaches about $t=400$5 (Yu et al., 7 Feb 2025). The same structure shows giant nonlinear chiroptical effects: under RCP pumping, third-harmonic generation around $t=400$6 nm is strong, whereas under LCP pumping it is much weaker. The reported nonlinear metrics are $t=400$7 and a nonlinear $t=400$8-factor of about $t=400$9, both much larger than the linear circular dichroism maximum of about $w=285.7$0 (Yu et al., 7 Feb 2025). The same work observes a transition from saturated absorption to reverse saturable absorption around $w=285.7$1 mW pump power for RCP excitation and demonstrates a reflective circularly polarized light limiter.

Active chirality control can be achieved magnetically or optically. In the Ce:YIG-based magneto-plasmonic nanohole metasurface, an out-of-plane magnetic field up to $w=285.7$2 kOe continuously tunes the far-field circular dichroism from $w=285.7$3 to $w=285.7$4 at $w=285.7$5 nm under $w=285.7$6 incidence (Edet et al., 2019). The mechanism combines magneto-optical circular dichroism in Ce:YIG with magnetic-field-induced modulation of the superchiral near-field distribution. Large-area fabrication by self-assembly enables a $w=285.7$7 chiral image and a reported reflectance circular-dichroism modulation amplitude of about $w=285.7$8 in most metasurface regions (Edet et al., 2019).

An ultrafast route is optical synthesis of transient chirality in an achiral plasmonic metasurface. A double-layered Au metasurface composed of nanostripes and triangular split-ring resonators is pumped at $w=285.7$9 nm with $\theta=21^\circ$0 fs pulses, generating an inhomogeneous hot-carrier distribution that transiently breaks mirror symmetry (Kim et al., 2023). The induced chirality flips sign when the pump polarization is changed from $\theta=21^\circ$1 to $\theta=21^\circ$2, and the genuinely induced chirality component has a sub-picosecond lifetime because electron-temperature diffusion destroys the spatial asymmetry faster than electron–phonon relaxation (Kim et al., 2023). This demonstrates a single metasurface with near-perfectly invertible handedness in the visible.

6. Applications, limitations, and relation to adjacent low-loss platforms

The application space of chiral plasmonic metasurfaces is broad but internally differentiated. Simple reflective or absorptive devices serve polarization manipulation, optical communication of spin information, analytical chemistry, and biological sensing (Wu et al., 2021). Magnetoelectric resonator arrays are proposed as alternative platforms for chiral molecule detection because they generate locally chiral near-fields even when far-field circular dichroism is absent at normal incidence (Nicolas et al., 2023). Planar gold metasurfaces with structural chirality and pseudo-chirality exhibit measurable enantiospecific optical response when coated with thin left- or right-handed chiral overlayers, supporting label-free chiral sensing (Palermo et al., 1 Aug 2025). In fully 3D chiral metacrystals, circular dichroism itself becomes a sensing channel: Pt nanohelix arrays reach a best sensitivity of $\theta=21^\circ$3 when the plasmonic and photonic components of the chiral surface lattice resonance are balanced appropriately (2207.14710).

Quantum and high-field applications impose stricter constraints. The silver–LiNbO$\theta=21^\circ$4–silver qBIC metasurface is positioned as a compact and integrable quantum source of circularly polarized entangled single-photon pairs operating at room temperature with efficient free-space outcoupling (Semone et al., 27 Jul 2025). The refractory Mo platform is motivated by nonlinear sensing, nonlinear hot electron generation, XOR gates of the all-optical full-adder, and reflective optical limiting, precisely because conventional Au and Ag structures are more vulnerable to thermal deformation under strong pumping (Yu et al., 7 Feb 2025). In the THz regime, chiral surface-plasmon cavities on InSb are proposed for agile and multifunctional THz metasurfaces and for ultrastrong chiral light–matter interactions at low energy in matter (Aupiais et al., 2024).

Three limitations recur across the field. The first is ohmic loss, which remains the canonical disadvantage of plasmonic platforms. Several studies explicitly contrast plasmonic metasurfaces with low-loss dielectric alternatives on this basis (Jo et al., 2024, Tonkaev et al., 18 Aug 2025, Kumar et al., 12 May 2026). The second is interpretive ambiguity: measured circular differential optical absorption can contain linear-anisotropy artifacts, making full polarimetry and differential decomposition important whenever “true circular dichroism” is claimed (Nicolas et al., 2023). The third is geometric and angular sensitivity. In planar or pseudo-chiral systems, chirality can vanish when resonator orientation matches lattice symmetry axes, or can depend critically on oblique incidence, diffraction-order alignment, or substrate asymmetry (Wu et al., 2021, Chaitanya et al., 2021, Palermo et al., 1 Aug 2025).

Recent dielectric and 2D-material metasurfaces are therefore relevant as comparison classes rather than as plasmonic exemplars. WS$\theta=21^\circ$5-based planar chiral metasurfaces use high refractive index and relatively low absorption in the visible to pursue geometric-phase chirality without severe metallic loss (Jo et al., 2024). Free-standing silicon membrane metasurfaces demonstrate nonlinear chiral response in the absence of plasmonics and explicitly position themselves against plasmonic ohmic loss (Tonkaev et al., 18 Aug 2025). Bilayer dielectric metasurfaces with rotated $\theta=21^\circ$6-symmetric apertures show that strong circular dichroism can emerge from resonant chiral modes unlocked by symmetry breaking, again without metallic resonators (Kumar et al., 12 May 2026). This suggests a broader historical trajectory: plasmonic chiral metasurfaces established many of the key mechanisms—localized field enhancement, strong circular dichroism, extrinsic and intrinsic chirality, magnetoelectric coupling, and resonance-controlled handedness—while adjacent dielectric and hybrid platforms increasingly absorb those design principles where lower loss is required.

Within that broader trajectory, the chiral plasmonic metasurface remains a distinct class of nanophotonic system: metal-enabled, resonance-driven, symmetry-sensitive, and unusually versatile in how it translates geometric or magneto-optical asymmetry into handedness-selective far- and near-field behavior.