# From Ideal to Real: Chiroptical Consequences of Asymmetries in Plasmonic Nano-Slit Metasurfaces

Obren Markovic, Wajdi Chaabani, Catherine Schwob, Mathieu Mivelle, Bruno Gallas

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Obren Markovic, Wajdi Chaabani, Catherine Schwob, Mathieu Mivelle, Bruno Gallas. From Ideal to Real: Chiroptical Consequences of Asymmetries in Plasmonic Nano-Slit Metasurfaces. ACS photonics, 2025, 12, pp.6360-6367. 10.1021/acsphotonics.5c01823. hal-05353617

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Distributed under a Creative Commons Attribution 4.0 International License
From Ideal to Real: Chiroptical Consequences
of Asymmetries in Plasmonic Nano-Slit
Metasurfaces

Obren Markovic, Wajdi Chaabani, Catherine Schwob, Mathieu Mivelle*, and
Bruno Gallas*

Sorbonne Université, CNRS, Institut des NanoSciences de Paris, INSP, F-75252 Paris,
France

E-mail: bruno.gallas@sorbonne-universite.fr, mathieu.mivelle@sorbonne-universite.fr

Abstract

Achiral plasmonic metasurfaces promise to enhance molecular chiroptical signals, but their performance is often undermined by fabrication imperfections. This study investigates the discrepancy between ideal theory and experimental reality for nano-slit arrays. Using micro-spectropolarimetry and numerical simulations on nano-slits in a gold film, we observe significant circular extinction (CE) and residual linear extinction (LE') signals, even after precise alignment. These results contradict the expected null signal from a perfectly symmetric structure, indicating systematic asymmetries. Our numerical analysis models geometric asymmetries like in-plane skews and out-of-plane tilts. We demonstrate that nanometer-scale in-plane asymmetries are the primary source of these far-field artifacts. The imperfections induce linear birefringence and dichroism, creating a large, spurious CE signal orders of magnitude greater than a true molecular signal, masking the desired measurement. Critically, the "superchiral" near-field, which would govern the sensing mechanism, remains remarkably robust against these structural
flaws. This highlights a crucial challenge: the far-field measurement is highly sensitive to fabrication errors that do not compromise the near-field sensing capability. Realizing reliable chiral biosensors therefore necessitates exceptional nanofabrication control and advanced polarimetric protocols to distinguish the faint analyte signal from the large, structure-induced background.

## Keywords

Plasmonics; Polarization; Numerical Simulations; Circular Dichroism;
Introduction

Chirality, the property of an object not being superimposable on its mirror image, is a fun-
damental principle in nature, particularly in biological systems. The distinct enantiomeric
forms of biomolecules can trigger vastly different physiological responses, making their se-
lective detection crucial in fields like drug development and molecular biology. The gold
standard for this task is Circular Dichroism (CD) spectroscopy, which measures the differ-
ential absorption of left- and right-circularly polarized light. However, the intrinsic chirop-
tical signal from most biomolecules is exceedingly weak, meaning conventional CD requires
large sample volumes and lacks the sensitivity for many modern applications. A promising
route to overcome this limitation is to enhance the light-matter interaction using resonant
photonic nanostructures. 1-4 These structures can generate highly confined and intense elec-
tromagnetic fields, or "superchiral fields", in their near-field, dramatically amplifying the
CD signal associated with nearby molecules. 5-12 While initial research focused on intrinsi-
cally chiral nanostructures, their own optical activity often interferes with and complicates
the measurement of the target molecule's signal. Several interaction mechanisms have been
proposed, whose relative contributions depend on the specific geometry under considera-
tion. 2,13 This diversity of mechanisms may account for the variety of reported signals, which
include both resonance wavelength shifts and the emergence of CD features. 6,7 To circum-
vent this, recent strategies have shifted towards using achiral plasmonic metasurfaces and
resonators. 2,8,14-19 These structures, being symmetric, produce no intrinsic CD signal. Their
advantage lies in generating a strong chiral near-field whose handedness can be controlled
externally, for instance, by switching the incident light's polarization from +45° to -45°. 19-21
We have previously shown numerically that simple nano-slits in a metal film can act as
ideal, tunable nanosources of superchiral light, providing a perfect platform for differential
measurements. 19 However, the promise of these ideal designs is challenged by the practi-
cal reality of nanofabrication. Unintended imperfections introduced during fabrication can
lead to various geometric deviations. While surface roughness is inevitable, its impact may
be mitigated through averaging over large ensembles of resonators or through non-local ef-
fects.²² In contrast, systematic geometric asymmetries that break the intended symmetry of
the nanostructures are likely to have a more detrimental effect. These later asymmetries can
induce linear birefringence and dichroism, which in turn create a false CD signal—an artifact
known as the Linear Dichroism-Linear Birefringence (LDLB) effect.²³⁻²⁷ This spurious signal
can be orders of magnitude larger than the expected molecular signal, rendering the sensor
ineffective.

The connection between near-field properties and far-field observables is often non-trivial.²⁸
A direct experimental access to the near-field distribution would therefore be highly valuable.
Scanning near-field optical microscopy (SNOM) has been proposed as a means to spatially
resolve the distribution of optical chirality in the vicinity of nanostructures.²⁰,²¹,²⁹,³⁰ How-
ever, existing SNOM-based studies have been limited to single-wavelength measurements.
This constraint implies that a systematic investigation would require tailoring the nanos-
tructure dimensions to optimize the near-field signal at the fixed operating wavelength of
the SNOM. An alternative approach, recently proposed, involves using electron energy-loss
spectroscopy (EELS) to image the spatial distribution of the polarization state in the near
field.³¹ However, this technique demands specially designed, electron-transparent samples
and still faces challenges in controlling the incident polarization.³² Consequently, numeri-
cal simulations remain a powerful and versatile tool for exploring the spectral and spatial
characteristics of near-field polarization.

In this work, we systematically investigate the link between fabrication-induced asymmetries and their chiroptical consequences. By combining experimental measurements on nano-slit arrays with a parametric numerical study, we dissect how specific, nanometer-scale shape deviations affect both the far-field optical activity and the near-field chiral density. We demonstrate that in-plane asymmetries (skewness) produce a correctable artifact, whereas out-of-plane asymmetries (wall tilt) induce a true, non-correctable CD. Our analysis quantifies the acceptable fabrication tolerances and provides a framework for understanding and
potentially mitigating measurement errors in next-generation chiral biosensors.

# METHODS

## Sample fabrication

We fabricated arrays of nano-slits in a gold film using electron-beam (e-beam) lithography. The process began by depositing a negative resist onto a glass substrate pre-coated with a conductive indium tin oxide (ITO) layer. We then patterned the resist using the e-beam and developed it to create rectangular openings. Subsequently, we deposited a 5 nm titanium (Ti) adhesion layer followed by an 80 nm gold film. A final lift-off step yielded the desired array of nano-slits. The nano-slits, with widths ranging from 50 to 80 nm and lengths from 100 to 150 nm, were arranged in a square lattice with a 300 nm period over a total area of 25×25 µm². As shown in the Scanning Electron Microscopy (SEM) images (Figure 1(a.1)), the slits exhibit some random shape irregularities, though no systematic skewness was visually apparent. For all measurements, the samples were oriented with the long axis of the nano-slits along the y-axis (Figure 1(a.2)).
<figure><img src="image_2.png" /><figcaption>Figure 1: Experimental characterization of the fabricated nano-slit arrays and their polarimetric response. (a) Scanning Electron Microscope (SEM) image of the array fabricated in an 80 nm thick gold film revealing random shape irregularities on individual slits. A schematic defines the unit cell and coordinate system. (b) Schematic of the experimental setup used to measure the polarimetric properties of the array. The image next to the Camera is an actual image of the 25 × 25 µm² area containing the nano-slits. The alignment procedure involved (c) rotating the polarization referential (angle $\phi$) while measuring the spectroscopic dependence of the linear extinction (LE') to find the optimal orientation. Even at this optimal angle (defined as $\phi = 0°$), a residual LE' signal persists, and the corresponding circular extinction (CE) spectrum, measured at this same orientation, reveals a significant non-zero signal.</figcaption></figure>
Sample charaterization

We characterized the samples’ polarimetric properties using a custom-built micro-spectro-
polarimeter (see schematic in Figures 1(b) and Figure S1). The setup uses a Xenon arc
lamp, and the white light is first filtered by a monochromator. The monochromatic light
then passes through a Polarization State Generator (PSG), which consists of a linear polarizer
and a photo-elastic modulator (PEM) operating at f = 50 kHz. The light is mildly focused
onto the sample by a ×20 strain-free microscope objective. The transmitted light is collected
by a ×60 thickness-compensated microscope objective and measured with a photomultiplier
tube. By demodulating the recorded signal at the first and second harmonics of the PEM
frequency (1f and 2f), we determined the circular extinction (CE) and the linear extinction
at ±45° (LE’), respectively. A critical alignment procedure was performed for each sample.
Instead of rotating the sample, we rotated the PSG (hence the polarization referential) by
an angle φ around the optic axis to find the orientation that minimized the LE’ signal across
the spectrum (Figure 1(c.1)). This procedure ensures that the incident linear polarization is
precisely oriented at 45° to the nano-slits’ long axis, and this optimal orientation is defined
as φ = 0° (see Figure S2). Even after this alignment, a residual LE’ signal was consistently
observed (Figure 1(c.2)). The CE signal was also measured at this optimal orientation
(Figure 1(c.3)) and remained largely unchanged by the rotation (see Figure S3). To confirm
these signals originated from the nanostructures and not from instrumental artifacts, we
performed a control measurement on a bare area of the substrate adjacent to the array. The
CE and LE’ signals measured on this large opened area were negligible (less than 5×10⁻¹
mdeg for the CE and less than 8×10⁻³ for the LE’) and an order of magnitude smaller than
those measured on the nano-slit array, confirming the signals are intrinsic to the fabricated
structures (see Figure S4).
## Numerical Simulations

To investigate the origin of the residual LE' and CE signals observed experimentally (Figures 1(c)), we performed numerical simulations. We hypothesized that these signals arose from small, systematic deviations from the intended symmetric shape of the nano-slits. Given the periodic and layered nature of our samples, we used the Fourier Modal Method (FMM), also known as Rigorous Coupled Wave Analysis (RCWA), implemented in the MATLAB SimPhotonicsFMM toolbox.33 This method is well-suited for calculating both near- and far-field polarimetric properties. The simulated geometry consisted of the unit cell shown in Figure 1(a.2), and we used standard tabulated optical constants for SiO₂, titanium, and gold.34,35 From each simulation, we first extracted the complex transmission coefficients as a 2×2 Jones matrix (J). For better comparison with our experimental results, we converted this Jones matrix into a 4×4 Mueller-Jones matrix.36 All elements of the Mueller matrix were normalized to the first element ($m_{00}$), which represents the total transmitted intensity. With this normalization, the $m_{02}$ and $m_{03}$ elements directly correspond to the experimentally measured LE' and CE values, respectively. To distinguish true chiroptical properties from measurement artifacts, we applied the differential decomposition to the full Mueller matrix.37 This powerful technique allowed us to isolate the true linear dichroism at ±45° (LD'), linear birefringence (LB), and circular dichroism (CD) of the nanostructure itself. To validate this entire procedure, we first simulated a perfectly rectangular (ideal) nano-slit. As expected for such an achiral structure, Figure 2(a) shows that the LE' map is antisymmetric around $\phi = 0°$, with a full sign reversal between $\phi = 45°$ and $\phi = -45°$ across the entire spectral range (see Figure 2(b)). Furthermore, the differential decomposition correctly yielded zero for all circular polarization effects (CD, circular birefringence), confirming the robustness of our analysis method (see Figure S5).
<figure><img src="image_3.png" /><figcaption>Figure 2: Calculated polarimetric properties for an ideal, perfectly symmetric nano-slit, establishing the theoretical baseline. (a) The calculated spectral and angular map of the far-field linear extinction (LE') shows a perfectly null signal along the structure's symmetry axes ($\phi = 0^{\circ}$ and $90^{\circ}$). (b) Line cuts taken from the map at $\phi = \pm 45^{\circ}$ show the corresponding LE' spectra. (c) The calculated map of the average near-field chirality density, C, as a function of wavelength and incident polarization angle reveals a strong, structured response. (d) Corresponding line cuts at $\phi = \pm 45^{\circ}$ demonstrate the bisignate spectral shape and the perfectly antisymmetric nature of the superchiral field, which exactly reverses its sign when switching the polarization angle.</figcaption></figure>

Beyond far-field properties, the near-field is where the actual sensing of chiral molecules occurs. It is therefore crucial to understand how shape deviations affect the local polarimetric properties, as any significant alteration could induce artifacts. The key metric for quantifying the interaction between light and chiral matter is the chirality density C, defined in vacuum as:⁵,²⁸

$$C = -\frac{\epsilon_o \omega}{2} \mathrm{Im}(\mathbf{E}^* \cdot \mathbf{B}) \qquad (1)$$

where E and B are the local electric and magnetic fields, $\epsilon_o$ is the vacuum permittivity, and $\omega$ is the light frequency. To quantify the field enhancement, we normalize C to the value generated by a circularly polarized plane wave in the absence of the nanostructure. With this normalization, $C = \pm 1$ for right- or left-circularly polarized light, and values of $|C| > 1$ signify the presence of a "superchiral" field, characteristic of plasmonic resonators. In this work, we
calculate C at the mid-plane of the nano-slit and average it over the (x,y) area. First, we
simulated an ideal, perfectly rectangular nano-slit to establish a baseline. Figure 2(c) shows
the calculated average chirality density C as a function of wavelength and the polarization
reference angle, $\phi$. The response is perfectly symmetric, with C reaching its maximum
positive and negative values at $\phi = \pm 45^\circ$ (Figure 2(d)). This bisignate spectral response
confirms that an ideal achiral nano-slit acts as a perfectly tunable source of superchiral light,
where the sign of C can be flipped simply by changing the incident polarization. These ideal-
case calculations, which show no spurious signals, stand in stark contrast to our experimental
results, where significant LE' and CE persist even after optimal alignment (Figure1(c)).
This discrepancy confirms that simple misalignment cannot explain the observed signals and
underscores the need to investigate the role of geometric imperfections.

## RESULTS and DISCUSSION

We have decomposed systematic asymmetries along all three dimensions of space to better
understand their respective contributions to the near- and far-field alterations. We have de-
fined three angular deformations: $\alpha_x$ (see Figure 3(a)), $\alpha_y$ (see Figure 3(b)) and $\alpha_z$ (see Fig-
ure 3(c)). For a better representation, angular deformations of $1^\circ$ are equivalent to distance
differences of 0.9 nm, 1.8 nm, and 0.9 nm along the directions y, x and z, respectively. To
model these sheared and tilted geometries within our RCWA framework, we discretized the
nano-slits into either shifted rectangular sub-nano-slits for in-plane asymmetries (skewness)
or a stack of thin 2D layers for out-of-plane asymmetry (wall tilt). We then approximated
the desired shape by shifting each sub-nano-slit or layer spatially relative to the others. We
used 100 sub-nano-slits in the x- and y-directions and 100 layers in the z-direction. These
numbers were chosen after confirming the convergence of the calculated results (see Figure
S10).
<figure><img src="image_4.png" /><figcaption>Figure 3: Schematics defining the angles used to model systematic geometric deformations. These include two in-plane asymmetries, represented by (a) a shear along the short axis ($α_x$) and (b) a shear along the long axis ($α_y$), as well as (c) an out-of-plane asymmetry, represented by a tilt of the slits' walls along the propagation axis ($α_z$).</figcaption></figure>

We first analyzed the far-field response for each asymmetry type, set to an angle of 1°. Figures 4(a)-(c) show the calculated LE' spectra as a function of the incident polarization reference angle $\phi$ varying between -2° and 2° about the perfect orientation (0°). A general observation is that LE' reverses its sign as $\phi$ changes sign, although for the $α_y$ shear, this reversal is clearly not perfectly symmetric (Figure 4(b)). Interestingly, the polarization referential angle $\phi$ that minimizes the LE' signal depends on the type of asymmetry. For in-plane shears ($α_x$ and $α_y$), the minimum is found at non-zero angles, specifically $\phi = -0.5°$ and $\phi = -1.0°$ respectively (Figures 4(d) and 4(e)). This occurs because these deformations break the structure's symmetry relative to the fixed square lattice, a situation not equivalent to a simple rotation of the entire array (see Figure S7).³⁸ In contrast, for the out-of-plane tilt ($α_z$), the minimum LE' remains at $\phi = 0°$ (Figure 4(f)). In all cases, a residual LE' signal persists at the optimal angle. However, LE' is 3 orders of magnitude smaller for the out-of-plane ($α_z$) deformation compared to the in-plane ones. LE' for in-plane asymmetries
is a direct consequence of a complex symmetry-breaking mechanism. When a shear such as $\alpha_y$ is applied (Figure 3(c)), the nano-slit's intrinsic shape develops a new, rotated, axis of symmetry. Crucially, this shear occurs while the slits remain part of a fixed square lattice that does not rotate. This distinction means that an in-plane shear is not equivalent to a simple rotation of the entire unit cell (see Figure S7), and as a result, the $\alpha_x$ and $\alpha_y$ shears are not interchangeable.³⁹ The overall symmetry is therefore broken with respect to both the slit's geometry and the lattice's orientation. It is this combined symmetry breaking that prevents a perfect zero of the signal and leads to the residual LE' observed at all polarization angles (Figures 4(d) and 4(e)).

Figures 4(g)-(i) show the circular extinction (CE) spectra calculated at the optimal angle $\phi$ that minimizes LE'. Consistent with our experimental measurements (see Figure S3), this CE signal is non-zero and varies little with small changes in $\phi$. The results reveal a critical distinction between defect types: the CE produced by in-plane shears is three orders of magnitude larger than the CE from the out-of-plane tilt. This strongly suggests that out-of-plane defects have a minor impact on the generation of spurious signals.
<figure><img src="image_5.png" /><figcaption>Figure 4: Far-field polarimetric effects of the three systematic asymmetry types ($\alpha_x, \alpha_y, \alpha_z$), each set to 1°. The top row (a-c) displays the spectral variations of linear extinction (LE') for alignment angles ($\phi$) varying between -2° and +2° in steps of 0.5° around the expected optimal alignment. The middle row (d-f) shows the corresponding residual LE' spectrum at the single optimal angle $\phi$, indicated in each plot, that minimizes the overall signal. The bottom row (g-i) presents the crucial result: the circular extinction (CE) spectrum measured at this optimal alignment. For the in-plane shears (g-h), a large CE signal persists.</figcaption></figure>

To identify the origin of the large CE arising from in-plane defects, we performed a differential decomposition of the Mueller matrix to isolate the intrinsic chiroptical parameters.37 While we could directly analyze the Mueller matrix of our samples, the differential decomposition relies on the assumption that the Mueller matrix describes a system whose optical response can be modeled as a continuous accumulation of infinitesimal transformations along the propagation direction. However, our sample includes a relatively thick gold layer and exhibits strong polarimetric responses, making a direct interpretation of the decomposition
challenging. To address this issue, we calculated the Mueller matrix of a nano-slit rescaled
by a factor of 1/8. This scaling preserved the unit cell's aspect ratio while producing a
polarimetric response qualitatively similar to that of the original structure (see Figure S5).
This approach enabled us to isolate the contribution from the LDLB effect, a well-known
polarimetric artifact. The result is shown as dash-dotted green lines in Figures 5(a-c). The
excellent agreement between the spectral shape of the LDLB contribution and the mea-
sured CE, combined with the very low CD, confirms that the observed signal arises entirely
from spurious effects, not from genuine chirality. The small residual discrepancies can be
attributed to higher-order polarimetric effects, substrate influence, or complex interactions
between shape and lattice symmetries (see Figure S8 and S9). These findings strongly sug-
gest that our fabricated samples contain systematic, nanoscale imperfections, even though
they remain undetectable in SEM images.
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

Figure 5: Far-field effects in circular polarization of the three systematic asymmetry types (a) $\alpha_x$, (b) $\alpha_y$ and (c) $\alpha_z$, each set to 1°, calculated with the model rescaled by a factor 1/8. The CE spectrum (blue lines) calculated for the optimal alignment, the CD (dashed orange lines) and the LDLB effect (dash-dotted green line) are plotted.

This finding has profound implications for practical applications. The spurious CE arti-
fact of ~10⁻² observed in Figures 4(g-i) is two orders of magnitude larger than the typical
CD signal from biomolecules (~$10^{-4}$) and would completely overwhelm it. Therefore, using such nanostructures for reliable chiral sensing requires both stringent control over fabrication to maintain symmetry and an advanced measurement protocol, including precise alignment and full polarimetric analysis, to correct for these artifacts. These challenges, as noted in other works, make detection at the single-resonator level particularly difficult.¹³,²³,²⁵

Importantly, the sensing mechanism itself originates from the near-field interaction between the nanostructure and the chiral medium, an interaction governed by the enhanced C within the nano-slit (see Figure S11). As established for an ideal structure (Figures 2(c-d)), a differential detection scheme (e.g., based on LE' contrast) fundamentally relies on the assumption that this near-field C is perfectly and symmetrically reversed when switching between orthogonal linear polarizations. Given that shape asymmetries generate significant far-field artifacts, a critical question arises: to what extent do these asymmetries also compromise the integrity and reversibility of the near-field chirality density? To quantify this effect, we calculated the average chirality density C inside the nano-slits at a wavelength of 755 nm as a function of the polarization referential angle, $\phi$. We then defined a metric, the "C-contrast", as the average of C over all azimuthal angles. This metric provides a direct measure of the response's overall asymmetry. The results are presented in Figure 6(a). For an ideal nano-slit, the C response is perfectly antisymmetric with respect to $\phi$, resulting in a C-contrast of exactly zero (green cross). For a sheared slit (e.g., $\alpha_x = 1°$), the response curve is very similar, but a subtle deviation, visible in the ×10 magnified hatched area, breaks the perfect symmetry. Consequently, the C-contrast for the sheared structure becomes non-zero (red cross), quantifying the degree of imperfection in the near-field's sign-reversal capability.
<figure><img src="image_9.png" /><figcaption>Figure 6: Analysis of the near-field robustness against systematic asymmetries, quantified by the C-contrast. (a) The C is plotted as a function of angle ($\phi$) at 755 nm, comparing an ideal structure (green line) to a sheared one ($\alpha_x = 1°$, red line). While the overall responses are similar, a small deviation is highlighted by the magnified difference curve (black line, ×10). The overall asymmetry is quantified by the "C-contrast" (the average of C over all angles), whose values for these two cases are shown as dots on the y-axis (note: the dot and difference for the sheared case are magnified tenfold for visibility). (b-d) The C-contrast is then plotted as a function of the deformation strength for each of the three asymmetry types: (b) $\alpha_x$, (c) $\alpha_y$, and (d) $\alpha_z$, demonstrating that the near-field error remains small.</figcaption></figure>

To further explore this effect, we varied the deformation parameters $\alpha_i$ (for $i = x, y, z$) between -1° and +1°. Figures 6(b-d) show the resulting C-contrast as a function of the asymmetry level. As expected, the C-contrast is zero for a perfect structure ($\alpha_i = 0$) and increases with the magnitude of the deformation. Notably, the C-contrast generated by the out-of-plane ($z$) asymmetry is an order of magnitude weaker than that produced by the in-plane ($x, y$) asymmetries. The symmetry of these plots is directly linked to the geometry of the deformations. For the in-plane cases (Figures 3(b) and 3(c)), changing the sign of $\alpha_x$ or $\alpha_y$ creates a mirror image of the shape, leading to a symmetric C-contrast response around $\alpha_i = 0$. In contrast, changing the sign of the out-of-plane tilt $\alpha_z$ (Figure 3(d))
results in a geometrically distinct, non-mirrored shape, leading to an asymmetric C-contrast
curve. Crucially, the maximum C-contrast values remain small: below $1.7×10^{-2}$ for in-plane
shear and $1.7×10^{-4}$ for out-of-plane tilt. These values are significantly smaller than the
overall C enhancement in the near-field (Figure 2(d)) and, more importantly, are an order of
magnitude smaller than the spurious CE artifact observed in the far-field (Figures 4(g-h)).
This demonstrates that the near-field response is much more robust to these defects than
the far-field measurement. This small C-contrast would only introduce a relative offset of a
few $10^{-3}$ in a differential far-field measurement, which is an acceptable level of error. This
behavior is likely to extend to a broad range of resonator geometries. However, in systems
where inter-element coupling plays a central role in generating chiral responses, fabrication
imperfections may become the dominant factor shaping the observed chiroptical behavior.²⁷

Conclusion

In this work, we have resolved the discrepancy between the theoretical ideal and the experi-
mental reality of using achiral plasmonic nano-slits for chiral sensing. We demonstrated that
the significant chiroptical signals observed experimentally, even after careful alignment, are
caused by nanometer-scale, systematic shape asymmetries. Our combined numerical and
polarimetric analysis reveals two key findings. First, the spurious far-field signal is domi-
nated by in-plane asymmetries, which contaminate the circular extinction measurement with
artifacts from linear birefringence and dichroism. Second, in stark contrast, the "superchi-
ral" near-field, which is responsible for the actual sensing mechanism, remains remarkably
robust against these same structural flaws. This creates a critical challenge for practical ap-
plications, as the far-field artifact can be two orders of magnitude larger than the expected
signal from chiral molecules like proteins, effectively masking the desired measurement. The
successful application of such resonant nanostructures for CD detection therefore imposes
stringent requirements. It demands not only exceptional control over the nanofabrication
process to minimize geometric defects, but also the use of advanced measurement protocols
with a very high signal-to-noise ratio to reliably distinguish the faint analyte signal from the
large, structure-induced background. Ultimately, our work provides a quantitative frame-
work for understanding fabrication-induced errors and establishes a clear roadmap for the
development of more robust and reliable plasmonic platforms for chiral biosensing.

Associated content

Additional information regarding the experimental setup used to characterize the nano-slits, the measured and calculated circular extinction as a function of orientation, the measured residual circular and linear extinctions, the relations between circular and linear extinctions and circular and linear dichroism, and the convergence of the calculated polarimetric response for skewed nano-slits, is available in the Supporting Information (SI).

Funding

This work has been carried out thanks to the support of the ANR (n° ANR-23-CE42-0019) and of the ERC (project FEMTOMAGNET).
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