arXiv subjects

Xavier Zambrana-Puyalto

Publications and source records attributed to Xavier Zambrana-Puyalto.

At least 19 recordsLinked to original sources

arXiv2025

Enhanced and directional light emission from two-dimensional excitons using Mie voids

Controlling light emission at the nanoscale has important applications in solid-state lighting, displays, and quantum light sources. Achieving this control requires both enhanced local electromagnetic fields to boost emission intensity and engineered radiation patterns to direct photons efficiently. Mie voids, consisting of an air cavity surrounded by a high-index semiconductor, are particularly suited for this purpose because they expose their strongest fields in an accessible region for nearby emitters while supporting resonances that shape directional emission through interference. Here, we demonstrate an all-van der Waals nanophotonic platform that couples excitons in atomically thin WS$_2$ to Mie void resonators formed in WSe$_2$. Guided by electromagnetic simulations, we identify void geometries that maximize photoluminescence through synergistic enhancement of excitation and emission processes. We also develop a two-step fabrication strategy that enables independent control of void diameter and depth, providing a route to systematically tune the optical response. Experimentally, we observe up to a 600-fold increase in photoluminescence intensity from monolayer WS$_2$ placed on individual voids compared to on an unstructured WSe$_2$, along with pronounced out-of-plane beaming of light that yields a forward-to-off-axis enhancement of 2.6 dB. Our results establish Mie voids in van der Waals semiconductors as a new platform for controlling light-matter interactions and realizing compact, directional, and efficient nanoscale light sources.

physics.optics↗

arXiv2025

Reflectivity-based refractive index measurement of van der Waals materials

We present a reflectivity-based method for measuring the in-plane refractive index of transparent van der Waals (vdW) materials. The approach enables the characterization of as small as $3 \times 3$~{\textmu}m$^2$ exfoliated flakes on a non-transmissive substrate without assuming any specific spectral shape of the refractive index. Exfoliated flakes are most commonly obtained through mechanical exfoliation, which generally produces vdW flakes with tens-of-micron lateral dimensions. As a result, conventional ellipsometry - which depends on large, uniform areas and specific spectral models - becomes challenging to apply. Our method determines the refractive index directly from the spectral position of reflectivity minima, provided the flake thickness and the substrate complex refractive index are known. We demonstrate the technique on hafnium disulfide (HfS$_2$), a vdW semiconductor with high refractive index and low absorption, retrieving its in-plane refractive index across the visible range. The results both validate previous ellipsometry measurements and establish this method as an accessible and spectral-model-free alternative for refractive index characterization of vdW materials.

physics.optics↗

arXiv2025

Photonics in Flatland: Challenges and Opportunities for Nanophotonics with 2D Semiconductors

Two-dimensional (2D) semiconductors are emerging as a versatile platform for nanophotonics, offering unprecedented tunability in optical properties through exciton resonance engineering, van der Waals heterostructuring, and external field control. These materials enable active optical modulation, single-photon emission, quantum photonics, and valleytronic functionalities, paving the way for next-generation optoelectronic and quantum photonic devices. However, key challenges remain in achieving large-area integration, maintaining excitonic coherence, and optimizing amplitude-phase modulation for efficient light manipulation. Advances in fabrication, strain engineering, and computational modelling will be crucial to overcoming these limitations. This perspective highlights recent progress in 2D semiconductor-based nanophotonics, emphasizing opportunities for scalable integration into photonics.

physics.optics↗

arXiv2025

Experimental demonstration of a beam shaping non-imaging metasurface

Light emitting diodes have superior performance over most other light sources, but the need for secondary optics to shape their illumination for specific applications yield bulky lighting products. Here, we present an approach to shaping light from incoherent sources, such as light emitting diodes, using non-imaging metasurfaces. We present a theoretical framework and a numerical tool for designing the metasurface phase, and use it to construct a proof-of-principle beam shaping metasurface. We demonstrate the optical performance of the fabricated sample, and perform numerical experiments to investigate the quality of the metasurface design. Our approach bridges the fields of non-imaging optics and metaoptics, and may enable metasurface applications for shaping coherent and incoherent light.

physics.optics↗

arXiv2025

Computational discovery of high-refractive-index van der Waals materials: The case of HfS$_2$

New high-refractive-index dielectric materials may enhance many optical technologies by enabling efficient manipulation of light in waveguides, metasurfaces, and nanoscale resonators. Van der Waals materials are particularly promising due to their excitonic response and strong in-plane polarizability. Here we combine ab initio calculations and experiments to discover new high-refractive-index materials. Our screening highlights both known and new promising optical materials, including hafnium disulfide (HfS$_2$), which shows an in-plane refractive index above 3 and large anisotropy in the visible range. We confirm these theoretical predictions through ellipsometry measurements and investigate the photonic potential of HfS$_2$ by fabricating nanodisk resonators, observing optical Mie resonances in the visible spectrum. Over the course of seven days, we observe a structural change in HfS$_2$, which we show can be mitigated by storage in either argon-rich or humidity-reduced environments. This work provides a comparative overview of high-index van der Waals materials and showcases the potential of HfS$_2$ for photonic applications in the visible spectrum.

physics.optics↗

arXiv2025

Fourier-Tailored Light-Matter Coupling in van der Waals Heterostructures

Dielectric structures can support low-absorption optical modes, which are attractive for engineering light-matter interactions with excitonic resonances in two-dimensional (2D) materials. However, the coupling strength is often limited by the electromagnetic field being confined inside the dielectric, reducing spatial overlap with the active excitonic material. Here, we demonstrate a scheme for enhanced light-matter coupling by embedding excitonic tungsten disulfide (WS$_2$) within dielectric hexagonal boron nitride (hBN), forming a van der Waals (vdW) heterostructure that optimizes the field overlap and alignment between excitons and optical waveguide modes. To tailor diffractive coupling between free-space light and the waveguide modes in the vdW heterostructure, we fabricate Fourier surfaces in the top hBN layer using thermal scanning-probe lithography and etching, producing sinusoidal topographic landscapes with nanometer precision. We observe the formation of exciton-polaritons with a Rabi splitting indicating that the system is at the onset of strong coupling. These results demonstrate the potential of Fourier-tailored vdW heterostructures for exploring advanced optoelectronic and quantum devices.

physics.optics↗

arXiv2024

Quality factor of dielectric optical resonators

We show analytically that the quality ($Q$) factor of magnetic and electric Mie modes in a lossless dielectric spherical resonator with high refractive index ($n \gg 1$) scales as $n^{2j+1}$ and $n^{2j+3}$ respectively, where $j$ denotes the multipolar order. We numerically validate these results and show that our high-$n$ analytical relation is accurate for the dipolar modes when $n>5$. For higher multipolar orders, the analytical relation becomes valid for increasingly lower $n$. We study the dependence of the $Q$ factor on absorption losses and determine a general functional form that describes the $Q$ factor for all Mie modes for any complex refractive index. Finally, we observe that this functional form predicts a multipolar-dependent singular value of optical gain which gives rise to a lasing condition with an infinite $Q$ factor.

physics.optics↗

arXiv2023

Non-imaging metasurface design for collimated beam shaping

Non-imaging optical lenses can shape the light intensity from incoherent sources to a desired target intensity profile, which is important for applications in lighting, solar light concentration, and optical beam shaping. Their surface curvatures are designed to ensure optimal transfer of energy from the light source to the target. The performance of such lenses is directly linked to their asymmetric freeform surface curvature, which is challenging to manufacture. Metasurfaces can mimic any surface curvature without additional fabrication difficulty by imparting a spatially-dependent phase delay using optical antennas. As a result, metasurfaces are uniquely suited to realize non-imaging optics, but non-imaging design principles have not yet been established for metasurfaces. Here, we take an important step in connecting non-imaging optics and metasurface optics, by presenting a phase-design method for beam shaping based on the concept of optimal transport. We establish a theoretical framework that enables a collimated beam to be redistributed by a metasurface to a desired output intensity profile. The optimal transport formulation leads to metasurface phase profiles that transmit all energy from the incident beam to the output beam, resulting in an efficient beam shaping process. Through a variety of examples, we show that our approach accommodates a diverse range of different input and output intensity profiles. Last but not least, a full field simulation of a metasurface has been done to verify our phase-design framework.

physics.optics↗

arXiv2022

On the helicity conservation for Mie optical cavities

The study of helicity in the context of light-matter interactions is an increasing area of research. However, some fundamental aspects of the helicity content of light fields inside scatterers have been overlooked. In this work, we demonstrate that the helicity of light fields inside lossless spherical cavities cannot be either conserved or sign-flipped as a result of a scattering event. The underlying reason is that the internal electric and magnetic Mie coefficients cannot simultaneously oscillate with equal amplitude and equal/opposite phase. Our analytical demonstration is fulfilled regardless of the refractive index, sphere size, and multipolar order. In addition, we show that the helicity of light fields inside lossy spheres can be conserved. This fact is in striking contrast to the behavior of the scattered field, whose helicity content cannot be conserved precisely when the sphere has losses. Finally, we show that the helicity content of internal fields can be flipped for materials with gain.

physics.optics↗

arXiv2020

Multiple Kerker anapoles in dielectric microspheres

High refractive index dielectric spheres present remarkable light-scattering properties in the spectral range dominated by dipolar modes. However, most of these properties are absent for larger spheres under plane wave illumination. Here, we propose to unravel dipolar regimes regardless of the sphere size and refractive index by illuminating with a pure dipolar field. This type of illumination ensures that the scattering response of the sphere is purely dipolar. In this scenario, we show that Kerker conditions are not only related to duality symmetry and a strong backward-to-forward asymmetric light-scattering, but also to the appearance of non-radiating sources: the so-called hybrid anapoles. Finally, we show that all the abovementioned scattering features under dipolar illumination are reproducible with an experimentally accessible tightly-focused Gaussian beam.

physics.optics↗

arXiv2020

Directional coupling of emitters into waveguides: A symmetry perspective

Recent experiments demonstrated strongly directional coupling of light into waveguide modes. We identify here the mechanisms behind this effect. We consider emitters near a waveguide, either centered on the median plane of the waveguide, or displaced from such plane. We show that, independently of the displacement, the directionality is mostly due to a mirror symmetry breaking caused by the axial character of the angular momentum of the emitted light. The sign of the angular momentum along an axis transverse to the waveguide determines the preferential coupling direction. The degree of directionality grows exponentially as the magnitude of such transverse angular momentum increases linearly. We trace this exponential dependence back to a property of the evanescent angular spectrum of the emissions. A binary and less pronounced directional coupling effect due to the chiral character of the handedness of the emission is possible when the displacement of the emitter breaks another of the mirror symmetries of the waveguide. We find a selection rule that allows or prevents the coupling of centered electric(magnetic) multipolar emissions onto the waveguide modes. We also show that the selection of a different angular momentum axis made in some experiments causes significant differences in the way in which directionality depends on angular momentum. We then use these differences to propose an experiment featuring a transverse magnetic bias that allows to aggregate the directional emissions from quantum dots on top of waveguides. Our symmetry-based results apply to any emitted multipolar order, clarify the spin-momentum locking concept, and generalize it to an exponentially-strong locking between the transverse angular momentumand the preferential coupling direction.

physics.optics↗

arXiv2019

Chemically-controlled self-assembly of hybrid plasmonic nanopores on graphene

Thanks to the spontaneous interaction between noble metals and biological scaffolds, nanomaterials with unique features can be achieved following relatively straightforward and cost-efficient synthetic procedures. Here, plasmonic silver nanorings are synthesized on a ring-like Peroxiredoxin (PRX) protein and used to assemble large arrays of functional nanostructures. The PRX protein drives the seeding growth of metal silver under wet reducing conditions, yielding nanorings with outer and inner diameters down to 28 and 3 nm, respectively. The obtained hybrid nanostructures can be deposited onto a solid-state membrane in order to prepare plasmonic nanopores. In particular, the interaction between graphene and PRX allows for the simple preparation of ordered arrays of plasmonic nanorings on a 2D-material membrane. This fabrication process can be finalized by drilling a nanometer scale pore in the middle of the ring. Fluorescence spectroscopic measurements have been used to demonstrate the plasmonic enhancement induced by the metallic ring. Finally, we support the experimental findings with some numerical simulations showing that the nanorings are endowed with a remarkable plasmonic field within the cavity. Our results represent a proof of concept of a fabrication process that could be suitable for nanopore-based technologies such as next-generation sequencing and single-molecule detection.

physics.app-ph↗

arXiv2019

Hybrid Metal-Dielectric Plasmonic Zero Mode Waveguide for Enhanced Single Molecule Detection

We fabricated hybrid metal-dielectric nanoantennas and measured their optical response at three different wavelengths. The nanostructure is fabricated on a bilayer film formed by the sequential deposition of silicon and gold on a transparent substrate. The optical characterization is done via fluorescence measurements. We characterized the fluorescence enhancement, as well as the lifetime and detection volume reduction for each wavelength. We observe that the hybrid metal-dielectric nanoantennas behave as enhanced Zero Mode Waveguides in the near-infrared spectral region. Their detection volume is such that they can perform enhanced single-molecule detection at tens of microM. However, a wavelength blue-shift of 40 nm dramatically decreases the performance of the nanoantennas. We compared their behavior with that of a golden ZMW, and we verified that the dielectric silicon layer improves the design. We interpreted the experimental observations with the help of numerical simulations. In addition, the simulations showed that the field enhancement of the structure highly depends on the incoming beam: tightly focused beams yield lower field enhancements than plane-waves.

physics.app-ph↗

arXiv2019

Switchable two-state plasmonic tweezers for dynamic manipulation of nano-objects

In this work we present a plasmonic platform capable of trapping nano-objects as small as 100 nm in two different spatial configurations. The switch between the two trapping states, localized on the tip and on the outer wall of a vertical gold nanochannel, can be activated by a variation in the focusing position of the excitation laser along the main axis of the nanotube. We show that the trapping mechanism is facilitated by both an electromagnetic and thermal action. The inner and outer trapping states are respectively characterized by a static and a dynamic behavior and their stiffness was measured by analyzing the position of the trapped specimens as a function of time. In addition, it was demonstrated that the stiffness of the static state is high enough to trap of particles as small as 40nm. These results show a simple, controllable way to generate a switchable two-state trapping regime, which could find applications as a model for the study of dynamic trapping or as mechanism for the development of nanofluidic devices.

physics.app-ph↗

arXiv2018

Site-selective functionalization of plasmonic nanopores for enhanced fluorescence and Förster Resonance Energy Transfer

In this work, we use a site-selective functionalization strategy to decorate plasmonic nanopores with one or more fluorescent dyes. Using an easy and robust fabrication method, we manage to build single plasmonic rings on top of dielectric nanotubes with different inner diameters. The modulation of the dimension of the nanopores allows us to both tailor their field confinement and their Purcell Factor in the visible spectral range. In order to investigate how the changes in geometry influence the fluorescence emission efficiency, thiol-conjugated dyes are anchored on the plasmonic ring, thus forming a functional nanopore. We study the lifetime of ATTO 520 and ATTO 590 attached in two different configurations: single dye, and FRET pair. For the single dye configuration, we observe that the lifetime of both single dyes decreases as the size of the nanopore is reduced. The smallest nanopores yield an experimental Purcell Factor of 6. For the FRET pair configuration, we measure two regimes. For large nanopore sizes, the FRET efficiency remains constant. Whereas for smaller sizes, the FRET efficiency increases from 30 up to 45% with a decrease of the nanopore size. These findings, which have been also supported by numerical simulations, may open new pathways to engineer the energy transfer in plasmonic nanopores with potential applications in photonics and biosensing, in particular in single-molecule detection towards sequencing.

physics.optics↗

arXiv2018

Tailoring multipolar Mie scattering with helicity and angular momentum

Linear scattering processes are usually described as a function of the parameters of the incident beam. The wavelength, the intensity distribution, the polarization or the phase are among them. Here, we discuss and experimentally demonstrate how the angular momentum and the helicity of light influence the light scattering of spherical particles. We measure the backscattering of a $4μm$ TiO$_2$ single particle deposited on a glass substrate. The particle is probed at different wavelengths by different beams with total angular momenta ranging from $-8$ to $+8$ units. It is observed that the spectral behavior of the particle is highly dependent on the angular momentum and helicity of the incoming beam. While some of the properties of the scattered field can be described with a simple resonator model, the scattering of high angular momentum beams requires a deeper understanding of the multipolar modes induced in the sphere. We observe that tailoring these induced multipolar modes can cause a shift and a spectral narrowing of the peaks of the scattering spectrum. Furthermore, specific combinations of helicity and angular momentum for the excitation lead to differences in the conservation of helicity by the system, which has clear consequences on the scattering pattern.

physics.optics↗

arXiv2016

Transverse multipolar light-matter couplings in evanescent waves

We present an approach to study the interaction between matter and evanescent fields. The approach is based on the decomposition of evanescent plane waves into multipoles of well-defined angular momentum transverse to both decay and propagation directions. We use the approach to identify the origin of the recently observed directional coupling of emitters into guided modes, and of the opposite Zeeman state excitation of atoms near a fiber. We explain how to rigorously quantify both effects, and show that the directionality and the difference in excitation rates grow exponentially with the multipolar order of the light-matter interaction. We also use the approach to study and maximize the transverse torque exerted by an evanescent plane wave onto a given spherical absorbing particle. The maximum occurs at the quadrupolar order of the particle, and for a particular polarization of the plane wave. All the obtained physical insights can be traced back to the two main features of the decomposition of evanescent plane waves into transverse multipolar modes: A polarization independent exponential dominance of modes with large transverse angular momentum, and a polarization controlled parity selection rule.

physics.optics↗

arXiv2016

Control and characterization of nano-structures with the symmetries of light

Light beams can be symmetric under different transformations: translations, rotations, mirror symmetries, duality transformations, etc. In this thesis, a systematic way of characterizing these symmetries is presented. Then, it is shown that light beams symmetric under different transformations can be used to control light-matter interactions at the nano-scale. Particular applications are developed, both theoretically and experimentally. Inducing a dual behaviour on a non-dual sample, the excitation of high multipolar order resonances and the measurement of circular dichroism using vortex beams are among them.

physics.optics↗