# Deterministic Fabrication of Plasmonic Nanostructures on Optical Nanofibers via Blurred Electron Beam Deposition

Antonio Balena, Marianna d'Amato, Muhammad Fayyaz Kashif, Chengjie Ding, Lucien Belzane, Gaia de Marzo, Hanna Le Jeannic, Massimo de Vittorio, Ferruccio Pisanello, Alberto Bramati

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Antonio Balena, Marianna d'Amato, Muhammad Fayyaz Kashif, Chengjie Ding, Lucien Belzane, et al. Deterministic Fabrication of Plasmonic Nanostructures on Optical Nanofibers via Blurred Electron Beam Deposition. Advanced Science, 2025, 12 (38), pp.14404-14409. 10.1002/advs.202507004. hal-05369241

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# Deterministic Fabrication of Plasmonic Nanostructures on Optical Nanofibers via Blurred Electron Beam Deposition

Antonio Balena,* Marianna D'Amato, Muhammad Fayyaz Kashif, Chengjie Ding, Lucien Belzane, Gaia De Marzo, Hanna Le Jeannic, Massimo De Vittorio, Ferruccio Pisanello, and Alberto Bramati

This study introduces a novel method for the deterministic fabrication of metallic nanostructures with controlled geometry and composition on suspended, single-mode tapered optical nanofibers (TNFs) using a tailored Blurred Electron Beam Induced Deposition (BEBID) technique. TNFs, owing to their subwavelength diameters and intense evanescent fields, offer a unique platform for enhanced light-matter interactions at the nanoscale. However, their mechanical fragility has thus far hindered the integration of plasmonic structures using conventional high-energy deposition methods. BEBID addresses this limitation by deliberately defocusing the electron beam to reduce local mechanical stress, minimize vibration, and prevent fiber damage during deposition, thereby enabling the one-step growth of platinum nanopillars with sub-20 nm spatial precision and high structural fidelity directly on suspended TNFs. The fabricated structures are characterized using SEM, EDX, and their optical properties are investigated through broadband scattering spectra and polarization-resolved measurements, showing strong agreement with Finite-Difference Time-Domain (FDTD) simulations. Numerical modeling further reveals that ordered arrays of nanopillars can shape and direct the scattered field along the fiber axis, enabling directional emission. This work establishes BEBID as a versatile nanofabrication approach for functional photonic architectures on fragile substrates, with direct applications in quantum photonics, nano-optics, and on-fiber plasmonic sensing.

## 1. Introduction

For decades, the emergence of localized surface plasmon resonances (LSPRs) resulting from the interaction of electromagnetic fields with subwavelength-sized metal structures has been extensively studied.[1] These phenomena have been exploited in a wide range of applications, including sensing and biosensing,[2,3] neuroscience for controlling neural activity,[4-6] optical tweezers and micro/nano manipulation,[7,8] and quantum photonics, thanks to the demonstration of single-photon emission enhancement by plasmonic nanocavities.[9-11] Recently, integrating plasmonic nanostructures with multimode fiber optics has gained significant attention, opening new opportunities such as spatially-resolved[12] and wide-volume[13,14] surface-enhanced Raman scattering (SERS), light shaping of emission and collection,[15] high sensitivity sensing[16] and biosensing.[17] and actuation.[18] While these advances highlight the promise of combining plasmonic structures with optical fibers for scientific and technological innovation, they are mostly focused on systems where

A. Balena, M. D'Amato, C. Ding, L. Belzane, H. Le Jeannic, A. Bramati
Laboratoire Kastler Brossel
Sorbonne Université
CNRS, ENS-PSL Research University, Collège de France
4 Place Jussieu, Paris 75005, France
E-mail: antonio.balena@lkb.upmc.fr

M. F. Kashif
Università degli Studi di Napoli Federico II - Department of Electrical Engineering and Information Technologies
Via Claudio, 21, Napoli 80125, Italy
M. F. Kashif, M. De Vittorio, F. Pisanello
Istituto Italiano di Tecnologia - Center for Biomolecular Nanotechnologies
Via E. Barsanti, 14, Arnesano (LE) 73010, Italy
G. De Marzo, M. De Vittorio
Technical University of Denmark - Department of Health Technology
Drug Delivery and Sensing
IDUN Section
Bygning, Ørstedts Pl. 345C, Kongens Lyngby 2800, Denmark

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/advs.202507004

© 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/advs.202507004
thousands of guided or radiative modes interact with thou-
sands of plasmonic nanostructures. Such configurations gen-
erate broadband responses and prevent studying or controlling
light-matter interactions at the single-nanostructure level.

To push applications toward the quantum regime, it would
be essential to achieve controlled interaction between a single
mode and a single structure, an interaction regime that remains
largely unexplored due to the lack of fabrication technologies
able to nucleate single plasmonic nanostructures within single-
mode photonic systems deterministically. In this context, high-
transmission single-mode tapered optical nanofibers (TNFs)[¹⁹]
– optical fibers mechanically tapered to sub-wavelength diame-
ters – represent a promising photonic platform. They support a
highly controlled and deterministically defined intense evanes-
cent field near the fiber surface, ideal for strong interactions
with the external environment or nanoparticles deposited on
them,[²⁰⁻²²] as well as for quick and stable connection with fiber-
based platforms. Integrating LSPR onto TNFs, therefore, holds
great potential both for (i) applications, where the combination
of plasmon-enhanced scattering and absorption with the strong
evanescent field can enhance LSPR-based sensing or detection of
refractive index changes, and (ii) fundamental studies in nonlin-
ear optics,[²³,²⁴] cavity quantum electrodynamics,[²⁵,²⁶] and single
photon emission enhancement[²⁷] that may benefit from the ex-
treme confinement of the electromagnetic energy.

Despite the potential, currently available technologies only al-
low these investigations to be conducted on non-deterministic
coupling conditions, and deterministic plasmonic integration on
TNFs has remained a significant challenge. Sugawara et al.[²⁸]
and Shafi et al.[²⁹] recently achieved integration of gold nanorods
and single photon emitters on TNFs with stochastic coupling
conditions, showing how LSPR-mediated single-photon emis-
sion enhancement is significantly influenced by the nanostruc-
tures' geometry and orientation on the emission diagram of
the quantum emitters. From a fabrication standpoint, most ap-
proaches rely on self-assembly, which lacks precision in nanopar-
ticle's placement, spacing, and orientation, all critical aspects for
exploiting LSPRs effectively. In addition, being inherently dielec-
tric and nanometric in diameter, TNFs are often suspended over
a few centimeters, making them highly sensitive to mechani-
cal stress and damage during fabrication. This has been a ma-
jor roadblock to the deterministic integration of functional plas-
monic nanostructures on TNFs. Conventional techniques such
as high-energy electron beam lithography (EBL) and chemical
vapor deposition exert excessive mechanical or thermal stress,
leading to fiber deformation or breakage. An alternative approach
uses thin films that cover a large part of the TNF nanometric sec-
tion to exploit surface plasmon resonances (SPR), but the inter-
action region is extended along all the nanofiber, thus lacking the
spatial localization provided by discrete plasmonic elements such
nanopillar or nanoantennae.

All these observations underscore the need for methods that
can deterministically nucleate LSPR directly onto the TNFs
without mechanical, optical, or thermal alterations, and with
nanometer precision. Addressing this fabrication bottleneck is
the primary motivation for this work. To this aim, we present
a novel method for fabricating controlled-geometry platinum
(Pt) nanopillars and nanoantennae deterministically placed onto
TNFs with sub-20 nm resolution, achieved using an uncon-

ventional adaptation of Electron Beam Induced Deposition (EBID).[30,31] Unlike multi-step lithographic approaches, EBID is a direct-write technique in which nanostructures are grown in situ via the local decomposition of a gas-phase precursor by an electron beam. This process enables the simultaneous deposition and shaping of the structure in a single step, with the final geometry and material composition determined by beam parameters and precursor exposure conditions.[32,33] The TNF's narrowest section was suspended, kept under axial traction, and exposed under the SEM beam, while an organometallic platinum precursor was introduced into the chamber. In contrast to the established Focused-EBID (FEBID), our approach deliberately exploits beam defocusing to reduce the local pressure exerted by the electron beam on the suspended, nanometric, and dielectric TNF, which could be otherwise put in vibration by a tightly focused electron beam, hindering reliable fabrication. We refer to this method as *blurred* EBID (BEBID). This technique enabled the nucleation of one or multiple nanostructures on a TNF, which were optically characterized by recording the broadband spectral dispersion of scattered light and by polarization measurements. The results are in satisfactory agreement with Finite-Difference Time-Domain (FDTD) numerical simulations and confirm the efficacy of the BEBID technique. Numerical simulations also demonstrate the impact of ordered arrays of Pt nanopillars on TNFs, predicting increased overlap between the scattered electromagnetic field and the guided modes of the fiber. We anticipate that this approach can be extended to enable the fabrication of increasingly complex nanostructures for plasmonic light manipulation in TNFs, opening new avenues for TNFs' application across various scientific fields, including sensing and quantum photonics, but also to build functional plasmonic platforms on fragile photonic systems.

## 2. Results

In a subwavelength diameter TNF, the boundary conditions of guided light are influenced by the large refractive index contrast between the nanofiber and the surrounding air,[³⁴] in contrast to conventional optical fibers. Consequently, a significant evanescent field extending beyond the nanofiber's boundaries is observed, extinguishing into the surrounding air over a few hundred nanometers, with a substantial portion of the light propagating at the fiber's surface. The electromagnetic field around the TNF is determined by factors such as the propagation mode, the wavelength of the guided light, and the geometry and refractive index of the TNF itself and the surrounding environment.[³⁵] The Supplementary Material and Figures S1–S4 (Supporting Information) report the detailed design of the TNFs used in this work, while the details on the fabrication of the TNFs are reported in the "Materials and Methods" section, and Figures S5–S9 (Supporting Information).

Here we strive at nucleating single LSPR on single-mode TNFs where only the fundamental mode $HE_{11}$ propagates, a particularly challenging task due to the sub 400 nm diameter of the TNFs, their purely dielectric nature, and their suspended geometry. Many conventional nanofabrication strategies are unsuitable to achieve this aim in a deterministic fashion. For instance, the diameter of TNFs is below the diffraction limit of imaging systems typically used in photolithography, while immersion into
viscous photoresist or EBL resist would likely cause the fiber to break. Although top-down approaches, such as Focused Ion Beam Milling[36] (FIBM) using both gallium ions[37] and helium ions[38] or also laser ablation by femtosecond pulses,[39] have been successfully used to pattern TNFs, to the best of our knowledge, no bottom-up approaches nor additive, direct-write approaches for deterministic fabrication of metallic nanostructures on TNFs have been reported so far.

To overcome these limitations, we have chosen EBID, which allows the precise deposition of nanostructures by using an electron beam to locally dissociate a gaseous precursor material on the surface of the substrate. TNFs were loaded into a dual configuration Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) system (FEI Helios NanoLab 600i) equipped with a three-channel gas injection system (GIS). The TNF was suspended with a controlled axial traction applied, while exposed to the electron beam (Figure 1A). The platinum precursor used was $MeCpPtMe_3$ locally injected through the nozzle at the upper side of the sample. A specific aspect of the fabrication process was the deliberate blurring of the electron beam, achieved by controlled defocusing during exposure, to minimize the mechanical stress exerted on the fiber under the high electron flux of standard FEBID (Figure 1B). We refer to this technique as BEBID (Figure 1C). While typically considered detrimental to nanofabrication, this beam blurring enabled successful patterning on these specific substrates while maintaining the composition of the nanostructure unchanged.[40] The beam blurring is controlled by the parameter *b*, which determines the diameter of the beam at the sample plane and was adjusted to match the base diameter of the plasmonic structure to be nucleated (a nanopillar), resulting in a defocusing of approximately 3 µm below the sample surface. The tilted SEM micrograph in Figure 1D shows a comparison of a successful BEBID single pillar nucleation (100 nm base diameter, 300 nm height) on a TNF with a 187.5 nm radius of curvature, compared with a FEBID process with the same nominal parameters. To further illustrate the versatility of the BEBID process on suspended TNFs, we performed the fabrication of a custom pattern spelling the letters “LKB”, the acronym of our laboratory (Laboratoire Kastler Brossel). The pattern, shown in Figure 1E, which has a width of 200 nm, a length of 500 nm, and a linewidth of 50 nm, demonstrates the ability of BEBID to reproducibly deposit complex, non-symmetric geometries directly onto the nanofiber surface. While this structure has no specific optical function, it showcases the adaptability of the technique for arbitrary nanoscale patterning.

Since EBID-deposited structures often contain significant carbon contamination due to incomplete dissociation of the precursor molecules, we investigated the composition of BEBID-deposited materials to optimize their plasmonic responses. Figure 2A reports test depositions at different beam currents $I_{beam}$, at an accelerating voltage of $V_{acc} = 1$ kV, since exposing the TNF at higher $V_{acc}$ could compromise its integrity. Energy-dispersive X-ray spectroscopy (EDX, spectra in Figure 2B) allowed us to characterize the relative fill factor ($f_{Pt}$) of platinum to carbon as a function of $I_{beam}$. The results are plotted in Figure 2C, suggesting that the composition of the structures can be tuned by adjusting the fabrication parameters and allowing the selection of an optimal balance between satisfying the low acceleration voltage requirements imposed by the substrate and maximizing the

amount of Pt in the nanostructure. By setting $I_{beam} = 86$ pA and $V_{acc} = 1$ kV, we achieved a $f_{Pt} = 0.46$. Subsequently, an empirical approach was used to determine the optimal dwell time for fabrication on TNFs. Using a target geometry of a 500 nm high pillar with a base diameter of 100 nm, a series of pillars were fabricated with dwell times $\tau_D$ varying from 0.5 µs to 1 µs (Figure 2D). The dwell time that gives the best match between the target geometry and the actual fabricated geometry was experimentally determined at $\tau_D = 0.55$ µs by SEM inspection, as reported in Figure 2E. Interestingly, the nanopillar height decreases as the dwell time increases, a trend that may appear counterintuitive. This behavior could be explained by a transition from a regime where the precursor supply on the TNF surface is sufficient for efficient deposition, at short dwell times, to a regime at longer dwell times where the electron beam depletes precursor molecules faster than they can be replenished, leading to reduced net growth. In the literature, these regimes are referred to as the reaction-rate-limited (RRL) regime and the mass-transport-limited (MTL) regime, respectively.[41,42] While this phenomenon could explain a slower growth rate at longer dwell times, the observed shortening suggests the co-participation of additional effects leading to partial material removal, such as electron beam-induced etching or local heating-induced removal.[43] On suspended nanofibers, where surface diffusion is limited and given their thermal sensitivity and suspended geometry, these effects may be even more pronounced.

To evaluate the repeatability of the process, SEM inspection was performed on multiple fabricated structures. In particular, Figure 2F displays the profiles extracted from a series of six pillars fabricated onto the same TNF, used to assess intra-sample variability. This process yielded an average pillar diameter of $d_{avg} = 98.2 \pm 0.9$ nm (mean value $\pm$ standard deviation), corresponding to a relative error of approximately 1%, thereby demonstrating remarkable fabrication repeatability. Figure 2G shows the profiles extracted from six pillars fabricated on six different TNFs. The profiles are used in this case to assess the inter-sample variability. In this case, the extracted average diameter of $d_{avg} = 104 \pm 9$ nm, with a relative error of around 8%.

The higher standard deviation observed in the inter-sample analysis is attributed not to limitations of the fabrication process itself, but rather to uncontrolled variability introduced during manual preparation of the TNF. We hypothesize that these differences may arise from slight variations in axial strain applied during the mounting process, which affect the mechanical stability of the TNF under electron beam exposure. Inconsistent strain can lead to subtle vibrations, especially during prolonged exposure to the beam, potentially broadening the resulting nanostructures. Specifically, the manual gluing of the nanofibers onto the sample holder may lead to non-uniform mechanical tension across samples, which in turn affects the final stability of the fiber during electron beam exposure. This suggests that the observed differences are extrinsic to the technique and could be minimized through improved fiber mounting procedures.

Furthermore, to systematically investigate the effect of the beam blurring parameter “b” on the resulting nanopillar geometry, we fabricated two series of structures on suspended TNFs at a fiber section of 500 nm, while keeping all other deposition parameters constant. As shown in Figure 3A, for a target diameter $d_{target}$ of 100 nm and height of 500 nm, increasing *b* from 0
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Blurred Electron Beam-Induced Deposition. A) Sketch of the EBID fabrication of nanostructure on TNFs. During the fabrication, the fiber is suspended, and the nanometric waist region is focused under the electron beam. B) Schematized description of the FEBID process. The beam is tightly focused, and the focal spot is moved along a spiral trajectory over the sample surface. C) Schematized description of the BEBID process. The focal spot is moved along the y-axis so that the resulting blurring “b” is usually set at the diameter of the desired nanopillar. D) SEM micrograph of two pillars fabricated on the same TNF following the same process except for the beam blurring. The top pillar is realized through FEBID and results in an undefined shape, the bottom one is deposited through BEBID, and a pillar shape with a base diameter of 100 nm and a height of 300 nm is obtained. The scale bar is 300 nm. To enhance visibility, the nanofiber is rotated on the xy-plane and tilted by 52° with respect to the z-axis. E) SEM image showing the fabrication of the letters “LKB” (Laboratoire Kastler Brossel) on a suspended TNF using the BEBID approach. This demonstration highlights the flexibility and spatial precision of the technique for creating custom, arbitrary patterns on nanofibers. The scale bar is 400 nm.</figcaption></figure>

to 100 nm in 10 nm steps leads to progressively more defined pillar shapes. A second series with a $d_{target}$ = 150 nm is presented in Figure 3B, with *b* varied from 0 to 150 nm in 25 nm steps. The quantitative analysis in Figure 3C, D reveals a clear dependence of both diameter and height on the normalized blurring $b/d_{target}$. In particular, a threshold behavior is observed: for $b < d_{target}/2$ (red-shaded region), the structures appear underde-

veloped or poorly shaped. This may be attributed to the beam scanning trajectory during the deposition. When the blurring is smaller than approximately half the desired feature size, the system segments the exposure into multiple scan points to approximate the geometry, introducing beam movement during fabrication. On suspended TNFs, such scanning can result in minor vibrations or non-uniform growth, degrading structural quality.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Fabrication of BEBID Nanostructures. A) SEM micrograph of the sample fabricated for EDX characterization. A series of 800×800×500 nm³ cubes at the center of SiO₂ pads were obtained from Focused Ion Beam Milling through a 50 nm-thick gold layer and consecutive EBID. The scale bar is 3 µm. B) EDX spectra measured from the cubes in Panel C. The insets show the spectral regions corresponding to (top) the Pt Mα x-ray line and (bottom) the C Kα x-ray line. C) Fill factor $f_{Pt}$ of Pt to the total volume of Pt+C extracted from the spectra in Panel D. D) Tilted SEM micrograph of a series of pillars fabricated on a TNF by setting different dwell time values. The scale bar is 500 nm. E) Height of the pillars measured by SEM inspection VS dwell time. The horizontal dashed line indicates the target height $h = 500$ nm, while the vertical one indicates the corresponding dwell time of 0.55 µs. Grayscale profiles of different pillars fabricated onto the same TNF to evaluate the intra-sample repeatability of the method to obtain a target diameter of 100 nm. G) Grayscale profiles of different pillars fabricated onto the same TNF to evaluate the inter-sample repeatability of the method to obtain a target diameter of 100 nm.</figcaption></figure>

In contrast, when $b \ge d_{target}/2$, the beam is wide enough to cover the target area with a single static exposure, minimizing vibrations and enabling more uniform, vertical growth. Beyond this threshold, both diameter and height increase approximately linearly with *b*, and the target height is typically reached at slightly smaller blurring values than the base diameter, as particularly visible in the 100 nm series.

Finally, the SEM micrograph in Figure 3E demonstrates an example of how the technique can achieve the successful fabrication of single bottom-up nanopillars with a high aspect ratio on a TNF with a curvature radius smaller than the nanostructure height. Using the same technique, multiple structures can be fabricated sequentially, with well-controlled nanogaps separating them down to a few tens of nanometers, as the nanoantenna shown in Figure 3F.

Post-fabrication, the nanostructures-decorated TNFs underwent a plasma-oxygen exposure to cleanse the surface by removing potential dust particles (see Figure S10, Supporting Information), which could impair optimal coupling with the fabricated nanostructures.[44] Additionally, the plasma-oxygen treatment ex-

erted a “purifying” effect on the composition of EBID metallic nanostructures, reducing the carbon content while enhancing the metal content, as reported in previous studies.[45,46] This effect was further characterized on test structures, where a diameter reduction of approximately 10 nm and a height reduction of about 25 nm was observed by SEM inspection (see Figure S11, Supporting Information). Since the volume reduction is indeed related to the decrease in the dielectric component of the nanostructures, an increase in the metal percentage from 47.5% to about 65% was calculated, given the fabrication parameters identified above and an initial nanostructure height of $h = 525$ nm and diameter of $d = 110$ nm.

The optical interaction between the evanescent field guided by the TNF and a target structure comprising a nanopillar with a base diameter of $d = 100$ nm and a circular cross-section (hereafter referred to as a “monomer”) was characterized and compared to a nanoantenna configuration, consisting of two nanopillars separated by a $g = 20$ nm nanogap (hereafter referred to as a “dimer”). This observation is the basis for numerical modeling of the optical properties of the nanostructures since it warrants the
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Characterization of the effect of the beam blurring parameter “b” on the geometry of BEBID-fabricated nanopillars. A) SEM image of a series of nanopillars fabricated on a suspended TNF with a target diameter of 100 nm and target height of 500 nm. The blurring parameter “b” was varied from 0 to 100 nm in 10 nm steps. B) SEM image of a second set of pillars with a target diameter of 150 nm and height of 500 nm, with “b” varied from 0 to 150 nm in 25 nm steps. C) Measured pillar base diameters as a function of normalized blurring b/d<sub>target</sub>, for both diameter targets. D) Measured pillar heights as a function of normalized blurring. In both (C) and (D), red-shaded areas indicate under-blurring regimes in which the pillar shape is poorly defined or fails to reach the desired geometry. Scale bars in (A) and (B) are 500 nm. E) SEM micrograph of a nanopillar with a base diameter of 100 nm and a height of 500 nm fabricated on a d = 360 nm TNF via BEBID. The scale bar is 500 nm. F) SEM micrograph of a nanoantenna constituted by two identical nanopillars with the same geometry shown in panel C. The scale bar is 500 nm. The inset shows the same nanostructure seen from the top, showing the nanogap separating the two pillars. The scale bar is 250 nm.</figcaption></figure>

calculation of the effective refractive index of the Pt-C composite by Bruggemann's effective medium approximation, for which the relative permittivity $\epsilon_{eff}$ of the composite results: [47]

$$ \epsilon_{eff} = \frac{1}{2} \left\{ -[\epsilon_{Pt} (f_C - 2f_{Pt}) + \epsilon_C (f_{Pt} - 2f_C)] \pm \sqrt{[\epsilon_{Pt} (f_C - 2f_{Pt}) + \epsilon_C (f_{Pt} - 2f_C)] + 8\epsilon_C \epsilon_{Pt}} \right\} \quad (1) $$

where $\epsilon_{Pt}$ and $\epsilon_C$ are the permittivities of Pt and C, respectively, and $f_{Pt}$ and $f_C = 1 - f_{Pt}$ are the volume fractions of the two materials, and where the condition that drives the choice of the sign for the square root term is $Im\{\epsilon_{eff}\} \ge 0$. Figure S12 (Supporting Information) reports the calculated $\epsilon_{eff}(\lambda = 785$ nm) for $f_{Pt} \epsilon$ [0.6, 1].

The spectral position of the resonance peak for scattering was measured experimentally on both monomer and dimer nanostructures. A supercontinuum laser was coupled to one end of the TNF, and the scattered light from the nanostructures was collected by an objective lens and directed to a spectrometer. As shown in the spectra in **Figure 4A**, the dimer exhibits a redshift of 50 nm relative to the monomer, with the peak shifting from $\lambda = 700$ nm to $\lambda = 750$ nm. To validate the experimental data, the scattering spectra of the monomer and dimer were also simulated numerically using a Finite Difference Time Domain model (FDTD), as illustrated in Figure 4B. The monomer spectrum reveals a peak in the near-infrared at approximately $\lambda = 720$ nm, while the dimer spectrum displays a redshift of 47 nm, with a peak at $\lambda = 767$ nm, aligning well with the experimental data. The redshift can be attributed to the shaping effect on LSPR in the
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Optical characterization. A) Normalized experimental broadband extinction spectra of the monomer (blue) and the dimer (red). Dashed lines are meant as a guide for the eyes. B) Normalized numerical extinction spectra of the monomer (blue) and the dimer (red). C) Sketch of the measurement configuration used for the dependence of the scattering intensity from the input linear polarization characterization. D) Normalized polar plots of the scattering from a monomer (blue data points and curves) and a dimer (red data points and curves) measured for the configuration in panel C. Circles are the experimental data, the dashed line is the fit over the experimental data, and the continuous line is the numerical curve. The arrows labeled “a” and “b” refer to the Malus’ law-like formula used for data fitting. E) Sketch of the measurement configuration used for estimating the Dol P of the scattered light. F) Normalized polar plots of the scattering from a monomer (blue data points and curves) and a dimer (red data points and curves) measured for configuration in panel E. Circles are the experimental data, the dashed line is the fit over the experimental data, and the continuous line is the numerical curve.</figcaption></figure>
case of two closely packed, identical nanostructures. A small interpilla distance induces a significant enhancement of the electromagnetic field in the gap region between the nanostructures. Additionally, a reduced interpilla distance fosters intense mutual interaction, leading to coupling between modes of different orders. As the interpilla distance decreases, higher-order resonances emerge. The primary consequence of this interaction is the redshift of resonance in the dimer case, which manifests as a hybridized mode resulting from the convolution of multiple modes into a single and broad band. Indeed, both the numerical and experimental spectra show spectral broadening between the monomer and dimer, in addition to the redshift. In the experimental case, this broadening is observed as an increase from $FWHM_{monomer}$ = 76 nm to $FWHM_{dimer}$ = 97 nm. Numerically, the broadening is also evident, though exact estimation is less straightforward.

We hypothesize that the broader spectra observed in simulations may be attributed to the use of a spectrally flat source injected directly into the nanometric section of the TNF, close to the nanostructures, bypassing the wavelength-dependent transmission and filtering effects of the full taper profile. In contrast, in the experiment, the spectrum of the supercontinuum laser injected into the fiber, endowed from the beginning with a given spectral dispersion, passes through the tapered section of the TNF, whose profile, optimized for adiabatic transmission in a range around a wavelength of 785 nm, acts as a filter for wavelengths outside that range. Replicating the full experimental response would require modeling the entire tapered nanofiber, including its centimeter-scale length and wavelength-dependent modal evolution, an extremely demanding task given the required nanometric resolution over macroscopic distances. Our simplified model, focusing on the nanostructure interaction with the guided mode, captures the key spectral features and trends. A more complete simulation remains a compelling avenue for future work.

Thereafter, two experimental configurations were employed to investigate the polarization dependence of the scattering properties of the two distinct types of TNF-coupled BEBID nanostructures. The first configuration (Figure 4C) was used to measure the dependence of scattered light intensity on the input polarization, while the second configuration (Figure 4E) was utilized to determine the polarization state of the light scattered by the nanostructures when the input beam polarization was fixed. Both configurations have been numerically simulated using an FDTD model. As sketched in Figure S13 (Supporting Information), a laser with a wavelength of 785 nm (Toptica Photonics), close to the resonance peak of the dimer, was directed through a half-wave plate (HWP) before being coupled into one end of the TNF. This latter was maintained in an unbent state to prevent the introduction of asymmetric stress on its cross-section, thereby preserving the polarization of the propagating light by minimizing birefringence.

Scattering from the nanostructure was monitored using an sCMOS (Scientific Complementary Metal-Oxide Semiconductor) camera while the HWP continuously rotated to determine the angle at which the maximum scattering intensity is detected, which is defined as the zero degree of rotation. Starting from this reference angle, the scattering intensity is recorded by the spectrometer, with the HWP then being rotated by $\pi/36$ rad (5°) between

successive acquisitions, therefore rotating the input linear polarization of an angle of $\theta_{in} = 2\theta_{in,HWP} = \pi/18$. The extracted values are presented in Figure 4D, where blue and red represent the nanopillar monomer and dimer, respectively. The normalized scattered intensity ($I(\theta_{in})/I_{max,IN}$) = $I(\theta_{in})/I(0)$) exhibits a strong dependence on the input polarization for both the monomer and dimer and follows a Malus' law-like cosine square function $I(\theta_{in})/I(0) = a \cdot \cos^2(\theta_{in}) + b$ (but in this case, the solution is represented by the relative scattered light intensity, [48]) a quantifies the intensity difference between the minimum and maximum, and b is a non-polarization-dependent factor that quantifies the non-polarized component of light scattered by the structure. This behavior can be attributed to the intensity distribution of the evanescent field propagating along the surface of the TNF. The coupling efficiency between the TNF and the plasmonic structure is determined by the intensity of the evanescent field at the fiber surface where it interacts with the structure. Due to the asymmetric distribution of the electromagnetic field, the coupling efficiency varies accordingly, reaching its maximum when light in input is polarized along the y-axis and its minimum along the x-axis. This is confirmed numerically (continuous line), where both the structures show similar behavior for the normalized scattered intensity, with $b_{monomer,IN}$ = 0.045 and $b_{dimer,IN}$ = 0.05. The extracted experimental values are in good agreement with the numerical results for the monomer, which shows $b_{monomer,IN}$ = 0.095, while the dimer shows a higher $b_{dimer,IN}$ = 0.21, even if with a clear dependence from the input polarization.

With reference to the configuration depicted in Figure 4E, to optimize the coupling with the nanostructures, a 785 nm beam linearly polarized along the y-axis was coupled into the TNF at the entrance upon passing through WP, set at the angle that previously gave $I_{max,IN}$. The scattered light then passed through a collection half-wave plate and a polarizer, transmitting light polarized along the z-axis, ensuring that the detector captured most of the scattered light. The measured polarization of the scattered light from the BEBID nanostructures and the numerical data are presented in Figure 4F. While both structures show evident polarization of the scattered signal, the emergence of the hybridized modes in the dimer gap results in a stronger polarization, showing a numerical $b_{dimer,OUT}$ = 0.03, and an experimental $b_{dimer,OUT}$ = 0.055, while the monomer shows a numerical $b_{monomer,OUT}$ = 0.15 and an experimental $b_{monomer,OUT}$ = 0.22. The measurements indicate that the scattered signal is predominantly linearly polarized along the z-axis, corresponding to the TNF's longitudinal axis. At the same time, the component along the x-axis contributes minimally to the overall scattered intensity, while the contribution along the y-axis is not accessible during the measurement. Given that linearly polarized light along the y-axis was coupled into the TNF, the z- and y-components of the electric field were primarily coupled to the plasmonic nanostructure. However, the y-component is not observable from the top of the nanopillar, resulting in the scattered signal in the xz-plane being polarized along the z-axis. The linear polarization state of the scattered signal can be quantified using the Degree of Linear Polarization (DoLP), defined as:

$$DoLP = \frac{I_{max,OUT} - I_{min,OUT}}{I_{max,OUT} + I_{min,OUT}} \qquad (2)$$
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Normalized scattered far field intensity (|*E*|^2) on a 2π hemisphere enclosing the nanopillars. A) Definition of the coordinates system. B–G) Polar maps of the far-field intensity distribution as the number of nano-pillars increases from 1 nanopillar to 6 nanopillars.</figcaption></figure>

where *I*<sub>max,OUT</sub> and *I*<sub>min,OUT</sub> represent, respectively, the maximum electric field intensity, obtained for a polarization aligned along the z-axis on the zx-plane, and the minimum intensity, obtained for a linear polarization perpendicular to the z-axis. From the extracted data, *DoLP* can be calculated: the monomer on the TNF exhibits a *DoLP* of 0.65, whereas the dimer on the TNF shows a higher *DoLP* of 0.92, both polarized along the z-axis. To better assess how the plasmonic nanostructures interact with the guided mode beyond localized scattering, we performed numerical simulations to estimate the fraction of optical power that is transmitted, scattered, or absorbed by the nanostructures. These simulations were conducted for both monomer and dimer configurations, using a linearly polarized *HE*<sub>11</sub> mode at λ = 785 nm and varying the Pt content of the structures from 60% to 100%. The results, shown in Figure S14 (Supporting Information), indicate that while the majority of power remains guided, a significant portion is redistributed into scattering and absorption channels. As expected, the nanoantenna configuration exhibits a stronger scattering contribution than the single nanopillar, hinting at its increased optical cross-section.

Finally, to investigate the potential for ordered nanopillar ar-
rays to control the direction of the scattered electromagnetic
fields, we calculated the scattered far-field intensity (|*E*|^2) on a
2π hemisphere enclosing progressively increasing numbers of
nanopillars, from one to six. Each structure was excited by the

fundamental mode of the TNF (λ = 785 nm) propagating
through the fiber. **Figure 5** displays the scattered far-field inten-
sity on standard spherical coordinates, with each configuration
normalized to its maximum intensity. In Figure 5A, the coordi-
nate system is defined: the polar angle θ extends from the axis
normal to the TNF surface, aligned with the nanopillars' axis
(the inner circles correspond to θ values from 0° to 90°). The az-
imuthal angle φ rotates counterclockwise on the surface plane
(φ = 90° is taken as the forward-propagating direction).

With a single nanopillar (Figure 5B), scattering occurs across all azimuthal directions, and regions with over 70% of the maximum intensity span approximately 300° around the nanopillar, with a central orientation in the forward-propagating direction. When two nanopillars are arranged in a nanoantenna configuration (Figure 5C), the high-intensity scattering range above 70% of the maximum narrows to less than 240°. It shows a substantial reduction in the back-propagating component (between 240° and 300° azimuth). This effect becomes more pronounced as the number of pillars increases, as shown in Figure 5D–G, where the azimuthal distribution of the scattered field narrows into a forward-directed lobe. By the six-nanopillar configuration, this range reduces to approximately 90° in the forward-propagating direction. Numerical simulations of an increased number of pillars (Figure S15A–C, Supporting Information for 7, 9, and 10 pillars) show that increasing the number of pillars in the array
does not significantly reduce the azimuthal angle range, while the arising of a second scattering lobe is observed. The second lobe is normal to the fiber, so it is expected to reduce the overall portion of light redirected toward the fiber collection direction. These results demonstrate that the far-field scattering directivity is strongly influenced by the number of nanopillars in the array, with increased forward-directional control as the array size grows. Furthermore, they indicate that the far-field emission direction can be tuned by adjusting the geometric arrangement of the nanopillar array.[49]

### 3. Discussion and Conclusion

In this work, we introduced and validated Blurred Electron Beam Induced Deposition (BEBID) as a novel approach for the deterministic fabrication of plasmonic nanostructures directly onto suspended, single-mode TNFs. Building on conventional EBID, BEBID leverages controlled defocusing of the electron beam to minimize mechanical stress on the nanometric dielectric fiber. This enables the single-step deposition of platinum nanopillars with sub-20 nm spatial precision and high structural fidelity. Unlike conventional FEBID, where feature size is tied to beam spot size, BEBID allows tuning by controlling the extent of the beam blurring. For TNFs in this work, beam blurring below 50 nm proved insufficient to alleviate beam-induced stress on the fiber. Future advances in TNF handling and packaging are expected to overcome this limitation, broadening the applicability of BEBID to controlled nanofabrication on fragile substrates and opening new pathways for robust, high-precision plasmonic architectures beyond the limitations of conventional technologies.

By optimizing beam current, dwell time, and post-deposition plasma treatment, we achieved precise control over nanostructure geometry and composition. The resulting structures exhibited excellent intra-sample diameter uniformity (below 1%) and moderate inter-sample variation (around 8%). EDX and SEM inspection confirmed that parameter optimization and post-processing plasma oxygen purification yielded platinum-rich structures (up to ≈65% Pt by volume) with reduced carbon content, enhancing their plasmonic properties. Optical characterization of single nanopillars (monomers) and coupled dimer structures on the TNF revealed polarization-dependent scattering responses and a pronounced redshift in the extinction spectra of dimers, attributed to hybridized plasmonic modes in the nanogap. Experimental measurements, including polarization-resolved scattering maps and broadband extinction spectra, closely matched FDTD simulations, confirming both the resonance tuning and the polarization properties of the scattered field. Simulations further predicted that increasing the number of nanopillars in ordered arrays narrows the far-field angular scattering distribution, progressively enhancing forward directivity along the fiber axis.

Following this proof-of-concept demonstration, several important directions emerge, warranting future studies to explore open questions raised by this work. While the influence of beam parameters on nanostructure composition is qualitatively clear, a systematic study of the composition variation mapping the interplay between accelerating voltage, beam current, and dwell time is needed to establish robust fabrication protocols. Additionally, although the versatility to switch deposited material,

which can be modified by selecting a different precursor, is certainly an advantage, appropriate “purification” techniques, equivalent to plasma oxygen exposure for the precursor used in this work, will need to be investigated for each case. Purification of structures deposited via EBID is a topic widely addressed by the community,[50] and elevated purification levels have been observed for multiple metals, using (and not limited to) oxygen[51] or water vapor[52] exposure simultaneously to the EBID process, or post-exposure to an electron beam[53] or a laser beam.[54] In the context of coupling BEBID-fabricated nanostructures with solid-state single-photon emitters to improve single-photon emission properties, precise relative positioning becomes a central challenge. Advanced approaches such as atomic force microscopy pick-and-place[55,56] could offer precise positioning of the emitter near pre-fabricated nanostructures. Alternatively, fabricating the plasmonic structure around a pre-positioned emitter using BEBID would exploit the technique's deterministic capabilities, but would require careful evaluation of potential damage to emitter properties during growth. Looking ahead, the ability of BEBID to precisely control nanopillar placement, spacing, and geometry suggests the possibility of exploring non-periodic arrays, including Yagi-Uda nanoantennae,[57,58] which will increase the redirecting effect of the electromagnetic field, emitter-coupled configurations, or even hybrid arrays with dynamically optical properties. Altogether, BEBID emerges as a powerful and versatile platform for engineering light-matter interactions at the nanoscale, with promising applications for quantum photonics, directional emission control, and ultra-sensitive plasmonic sensing.

### 4. Experimental Section

**TNF preparation:** The TNFs were manufactured from a commercial optical fiber (Thorlabs 780HP) by the "heat-and-pull" method described in Ref.,[19] with a custom setup reported in Figure S5 (Supporting Information), designed to achieve high-transmission nanoscale fiber diameters. This setup includes an oxyhydrogen flame capable of heating fused silica to its softening point (1585 °C) and two translation stages, each equipped with a clamp to secure and pull the fiber ends. The flame was generated by a stoichiometric hydrogen-oxygen mixture, with hydrogen produced via water electrolysis. Precise flame control was achieved through two mass flow controllers, providing stable and reproducible heating conditions. During the pulling process, a motorized stage (Thorlabs PT1-Z9) precisely positions the flame below the fiber. Initially, the flame heats the fiber to its softening point. The stage lifts the flame just before pulling ends to prevent excessive heating, ensuring a smooth tapering process.

The tapering process was designed to satisfy the adiabatic criterion,[59] which prevents light from coupling from the fundamental mode $HE_{11}$ to higher-order modes. Using a MATLAB script, trajectories for the translation stages were calculated based on the desired taper profile, adiabatic criterion (both reported in Figure S6, Supporting Information), and target nanofiber diameter.

Throughout this process, a 785 nm laser (Thorlabs S1FC780) is transmitted through the fiber, with transmission monitored by a photodiode. This output was normalized to the initial transmission to ensure consistent light throughput. To verify the diameter reduction in real-time, a camera with a long-working distance objective provides high-magnification monitoring. The transmission curve recorded during the pulling process (Figure S7, Supporting Information) shows that the fiber maintained 99.7% of the signal after fabrication. Statistics of the fabricated fiber diameter and transmission were provided in Table S1 (Supporting Information). Maintaining a dust-free environment was essential to prevent particulate adhesion, which can reduce transmission.
To enable stable nanostructure fabrication using an SEM, a custom fiber
holder was designed for mounting the TNFs in the machine (Figure S8,
Supporting Information). However, due to manual assembly, strong vi-
brations were observed during SEM scanning. To mitigate this issue, the
TNF was stretched an additional length ΔL after the pulling process, and
before mounting it on the holder. Empirical observations showed that by
increasing ΔL between 75 µm and 125 µm, the vibrations during electron
beam scanning became negligible, while further increasing ΔL would in-
crease the risk of breaking the fiber. This tensioning allowed for stable SEM
imaging of the fiber at the magnification required for precise fabrication,
as demonstrated in Figure S9 (Supporting Information). After tensioning,
the fibers were glued to the holder and stored in a clean environment.

**Blurred EBID:** Nanostructure fabrication was performed using a dual-beam Focused Ion Beam/Scanning Electron Microscope (FIB/SEM), FEI Helios NanoLab 600i, equipped with a gas injection system (GIS). Electrons were generated through a Schottky field emission gun. For the different fabrication used in the paper, the accelerating voltage was set at 1 kV, 3 kV, 5 kV, and 10 kV, while beam currents of 5.4 pA, 23 pA, 86 pA, and 340 pA were used. Higher current values negatively impacted the TNF's structural integrity. For the final fabrication of nanopillars on the TNF, a setting of 1 kV and 86 pA was used for both imaging and deposition.

Achieving precise focus on the TNF was essential for fabrication accu-
racy. To achieve optimal focus before fabrication, Adjusting imaging con-
ditions were recommend on an alignment sample (or on the fiber holder if
no alignment sample was available) to minimize exposure of the nanomet-
ric TNF segment. Prolonged exposure, especially at high voltages or beam
currents, can cause damage, including fiber rupture. After focusing, the
nanometric section of the TNF was positioned under the electron beam at
high magnification, ensuring the fiber's diameter occupied approximately
one-fifth of the field of view's width.

Fabrication commenced once the chamber reached a target vacuum level of ≈5 × 10⁻⁷ mbar. The GIS needle was then introduced in the chamber, above the TNF, allowing the Pt precursor MeCpPtMe₃ (Trimethyl [(1,2,3,4,5-ETA.)-1 Methyl 2, 4-Cyclopentadien-1-YL] Platinum) to flow. After initiating the gas flow, the chamber pressure was allowed to stabilize at 0.8–1.2 × 10⁻⁵ mbar, maintaining the precursor temperature at ≈49 °C to reduce variations in the growth rate.

Upon completion, to allow the vacuum was paused to return to the level
measured before opening the GIS, preventing unintended deposition from
residual precursor gas exposure to the SEM beam.

**EDX Analysis:** Energy Dispersive X-Ray Spectroscopy measurements were performed on a test sample using a Thermo Scientific Phenom Pro G6 Desktop SEM equipped with an EDX detector. The test sample consisted of a 170 µm-thick borosilicate glass cover (Ted Pella), coated by a 100 nm 99.99% pure gold (Kurt J. Lesker) layer deposited by electron beam evaporation (Thermoionics Laboratory Inc.). The gold layer was necessary to perform subsequent SEM imaging and fabrication. The test sample was processed with a FEI Helios Nanolab 600i Scanning Electron Microscope / Focused Ion Beam dual beam system. Firstly, a 5 × 4 matrix of 3 × 3 µm² squares was milled with FIB milling (30 kV, 0.41 nA) for a depth of ≈1 µm to remove the gold and reach the underneath glass. Subsequently, EBID was used to realize a series of 5 cubes of 500 × 500 × 500 nm³, each at the center of a previously milled area, using different electron beam current and an accelerating voltage of 1 kV. EDX spectra were acquired using a 15 kV electron beam in fixed point modality and an exposure time of 30 s, and were analyzed with the Thermo Scientific Phenom ProSuite Software to extract the atomic percentage of Pt relative to C.

**Optical Characterization:** For the optical characterization, the nanostructure-decorated TNFs' ends were cleaved and connected to two temporary FC/PC connectors (Thorlabs BFT1 terminator and B30126C3 connector) and mounted in a custom-made sealed Plexiglas box to prevent dust deposition during the measurements. Light scattered from the structures was collected through a broadband optical window (Thorlabs WG41010R). The optical characterization setup comprised two light injection branches (one for broadband light injection and one for polarization-resolved monochromatic light injection) and a common detection path. For the broadband light injection branch, a supercontinuum laser (NKT Photonics SuperK Compact, fiber coupled) was intensity-

filtered by a variable neutral density filter (Thorlabs NDC-100C-4 M),
collimated, and focused on the entrance of a short single-mode patch
cord, made from the same fiber used for TNFs, through an FC/PC triplet
fiber collimator (Thorlabs TC12FC-780). For the polarization-resolved
monochromatic light injection branch, a $\lambda$ = 785 nm (Toptica Pho-
tonics DL PRO 780 S) laser passed through a half-wave plate (Thorlabs
WPHSM05-780 mounted in Thorlabs RSP1x15 indexing rotation mount)
and a variable neutral density filter before being coupled to the distal
end of the same short patch cord through another similar collimator.
The short patch cord was then connected to one end of the TNF. In the
common detection path, light scattered by the structures and passed
through the optical window was collected by an aspheric lens (Thorlabs
A220TM-B) mounted on a 3-axis piezoelectric stage (Piezosystem Jena
TRITOR 102 T-405-00). In the broadband measurements, the collected
signal was sent toward a moveable 50:50 beam splitter that temporarily
sent a fraction of it to a sCMOS camera (Hamamatsu Orca Flash 4)
to image the structure. By removing the beam splitter, the signal was
entirely sent toward a plano-convex lens (Thorlabs LA1131-B), which
then focused it at the end facet of a multimode 0.22 NA, 200 µm core
diameter patch cord (Thorlabs M25L05), used to route the signal towards
a spectrometer (Princeton Instruments Acton Series SP2500i, equipped
with a Pixis 400 Camera) for spectral recording. In the polarization
dependence measurements, the detection path was slightly modified by
adding a half-wave plate and a polarizer (Thorlabs LPVIS100-MP2) with
its T-axis aligned with the z-direction, after the objective lens.

**Numerical Simulations:** A Finite Difference Time Domain model (FDTD, Ansys Lumerical Solutions) was developed to simulate the nanos-structure and assess both scattering spectra and electromagnetic field po-larization under the influence of the TNF's fundamental mode propaga-tion. To accurately represent the fabricated structure, the TNF was mod-eled as a silica (SiO₂) cylinder with a refractive index $n_{TNF}$ = 1.46, while the nanopillars were represented as rounded-edge cylinders to approximate their physical shape. The carbon-platinum composite nanopillars' dielec-tric function and optical properties were modeled using Bruggemann's ef-fective medium approximation over the relevant optical wavelength range. The surrounding medium was defined as free space with a refractive index of $n$ = 1.

The simulation domain was defined as a large rectangular prism with
perfectly matched layer (PML) boundary conditions in all directions, care-
fully sized to prevent interaction between the evanescent fields and the
PML boundaries. A mesh override was applied to the TNF and nanopil-
lars for higher precision, while the rest of the domain employs Lumerical's
built-in nonuniform mesh algorithm with a mesh accuracy setting of 4. To
compute the mode profile for the TNF, the Ansys MODE eigensolver was
used, and the resulting guided mode was injected into the TNF using a
mode source in the FDTD model. This approach ensures accurate mode
propagation and interaction with the nanopillars in the simulated environ-
ment.

**Supporting Information**

Supporting Information is available from the Wiley Online Library or from
the author.

**Acknowledgements**

A.Ba. and M.D.A. contributed equally to this work and were co-first
authors. M.D.V., F.P., and A. Br. contributed equally to this work and
were co-last authors. A.Ba. acknowledges funding from the European
Union's Horizon 2020 research and innovation program under the Marie
Skłodowska-Curie grant agreement (#101106602). A.Ba., M.D.A., M.F.K.,
C.D., F.P., M.D.V., and A.Br. also acknowledge funding from the European
Union's Horizon 2020 research and innovation program under a grant
agreement (#828972). M.D.V., and F.P. acknowledge funding from the Eu-
ropean Union's Horizon Europe under a grant agreement (#101125498).
This work was supported by the Plan France 2030 through the project
ANR-22-PETQ-0013 and by the French Agence Nationale de la Recherche through the ANR-JCJC2023 program. A.Br. was a member of the Institut Universitaire de France (IUF).

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data supporting the findings of this study will be made available at https://doi.org/10.5281/zenodo.15773422, or from the corresponding author upon reasonable request.

## Keywords

electron beam induced deposition, nanofabrication, optical nanofibers, plasmonics

Received: April 17, 2025

Revised: July 2, 2025

Published online: July 11, 2025

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