arXiv subjects

Antonio Ambrosio

Publications and source records attributed to Antonio Ambrosio.

At least 19 recordsLinked to original sources

arXiv2026

Ultrafast Control of Lifetime in High Q Anisotropic Plasmon Polaritons

Losses are a major roadblock in the technological implementation of surface plasmons at optical frequencies. The recent emergence of MoOCl2, a correlated van-der-Waals material with strongly anisotropic optical properties, offers new avenues to circumvent this limit. We report the far-field observation of high-Q surface plasmon polaritons in this material, arising from the anisotropic hybridisation of surface plasmons and dielectric modes. We then explore nonlinear pumping of intraband electrons to the conduction band in these structures, leading, contrary to intuition, to an abrupt increase in the lifetime of the polariton resonance, despite the injection of hot electrons. This counterintuitive phenomenon stems from the competition between photon and plasmon excitations in a lenticular polariton resonance, yielding a largely tuneable lifetime at ultrafast speeds.

physics.optics↗

arXiv2026

Intrinsic plasmon canalization in the biaxial van der Waals crystal MoOCl$_2$

Anisotropic polaritons in low-symmetry crystals allow for subwavelength confinement and directional routing of light. The most extreme form of such anisotropy arises at the topological transition between elliptical and hyperbolic dispersion, where the isofrequency contours collapse into parallel lines and polaritons propagate in a diffractionless, beam-like fashion. This canalization regime has previously been accessed through twisted heterostructures or engineered metasurfaces. Here we show that natural canalization can be achieved without any fabrication or structuring by exploiting the intrinsic elliptical-to-hyperbolic transition in the van der Waals crystal MoOCl$_2$ at room temperature. Using near-field imaging, we directly visualize plasmon-polariton canalization emerging at the low-loss Drude crossing point along the [010] crystal axis. Owing to the moderate slope of the Drude permittivity, the resulting polaritons remain highly directional across a broad spectral window. This weak dispersion also enables robust thickness-dependent tuning, and we demonstrate, both experimentally and theoretically, that the canalization wavelength can be adjusted by more than 1 μm simply by varying the flake thickness. This work brings canalized polariton propagation into the 4.5 - 6 μm range, beyond the frequency limits of phonon-polariton platforms and overlapping with important molecular vibrations, opening new opportunities for mid-IR nanophotonics and sensing.

physics.optics↗

arXiv2026

Anisotropic electron gas in a hyperbolic van der Waals material

Electron gases in low dimensional materials exhibit unconventional transport and optical phenomena due to reduced phase space, enhanced interactions, and strong sensitivity to lattice symmetry. While commonly realized in quantum confined systems and engineered heterostructures, such states are rare in naturally occurring materials. Hyperbolic materials provide a compelling alternative, as extreme lattice anisotropy can host unconventional electronic states and novel electron-phonon interactions. Here, we investigate the angle resolved polarized Raman (ARPR) response of MoOCl2, the first naturally occurring hyperbolic material whose hyperbolicity originates from a highly anisotropic electron gas. We observe pronounced polarization dependent Fano line shapes, revealing coherent coupling between phonons and an anisotropic electronic continuum. We characterize the directional response of this continuum, incorporating it into effective Raman tensors that quantitatively reproduce the ARPR measurements and capture the distinct Raman fingerprint of MoOCl2. Excitation energy and thickness dependent ARPR measurements further demonstrate a tunable quasi 1D electronic continuum with weak interlayer coupling, establishing MoOCl2 as a model system for Raman studies of electron-phonon coupling in hyperbolic materials

cond-mat.mes-hall↗

arXiv2024

Visible-frequency hyperbolic plasmon polaritons in a natural van der Waals crystal

Controlling light at subwavelength scales is one of the main challenges of nanophotonics. Leveraging hyperbolic polaritons supporting arbitrarily large wavevectors can lead to extreme light confinement, effectively overcoming the diffraction limit. Hyperbolicity was initially realized in artificial metamaterials, but their performances are limited by high losses in the metallic components. While recently discovered natural low-loss hyperbolic phonon polaritons initiated a revival in the interest for hyperbolic materials, they are confined to the mid-infrared frequency range, limiting their use for several applications. Some hyperbolic materials at visible frequencies have been studied, but they are either very lossy or only feature out-of-plane hyperbolicity. Here, we demonstrate the presence of low-loss, in-plane hyperbolic plasmon polaritons in the visible and near-infrared in thin films of $\mathrm{MoOCl}_2$, a natural van der Waals crystal. The polariton dispersion is predicted based on the framework of light propagation in biaxial media, and experimentally confirmed by real space nano imaging on exfoliated flakes. $\mathrm{MoOCl}_2$ constitutes a novel material platform for visible-range applications leveraging the unboundedness of hyperbolic modes, such as hyperlensing, Purcell factor enhancement and super-resolution imaging, without the drawbacks of metamaterials.

physics.optics↗

arXiv2024

Electromechanical response of saddle points in twisted hBN moiré superlattices

In twisted layered materials (t-LMs), an inter-layer rotation can break inversion symmetry and create an interfacial array of staggered out-of-plane polarization due to AB/BA stacking registries. This symmetry breaking can also trigger the formation of edge in-plane polarizations localized along the perimeter of AB/BA regions (i.e., saddle point domains). However, a comprehensive experimental investigation of these features is still lacking. Here, we use piezo force microscopy to probe the electromechanical behavior of twisted hexagonal boron nitride (t-hBN). For a parallel stacking alignment of t-hBN, we reveal very narrow (width ~ 20 nm) saddle point polarizations, which we also measure in the anti-parallel configuration. These localized polarizations can still be found on a multiply-stacked t-hBN structure, determining the formation of a double moiré. We also visualize a t-hBN moiré superlattice in the topography maps with atomic force microscopy, related to the strain accumulated at the saddle point domains. Our findings imply that polarizations in t-hBN do not only point in the out-of-plane direction, but also show an in-plane component, giving rise to a much more complex 3D polarization field.

cond-mat.mtrl-sci↗

arXiv2024

Sub-cycle modulation of light's Orbital Angular Momentum

The exploration of light has traditionally focused on its spatial properties, particularly its orbital angular momentum (OAM), while its temporal dynamics have remained an underexplored frontier due to the slow response times of existing modulation techniques. In this context, we introduce a method to modulate the OAM of light on a femtosecond scale by engineering a controllable space-time coupling in ultrashort pulses. By intricately linking azimuthal position with time, we implement a static, azimuthally varying wavefront transformation to dynamically alter the spatial distribution of light in a fixed transverse plane. Our experiments demonstrate self-torqued wave packets that exhibit spiraling motions and rapid temporal OAM changes down to a few femtoseconds. We further extend this concept to generate wave packets that angularly self-accelerate. We reveal that these wave packets dynamically adjust their OAM by redistributing their energy density across their spectral bandwidth, all without the influence of external forces. Owing to the unique properties of self-torque and angular acceleration, these time-varying OAM beams offer an accessible avenue for exploring light at fundamental time scales, with far-reaching implications for ultrafast spectroscopy, nano- and micro-structure manipulation, condensed matter physics, and other related areas.

physics.optics↗

arXiv2023

Bound states in the continuum and long-range coupling of polaritons in hexagonal boron nitride nanoresonators

Bound states in the continuum (BICs) garnered significant for their potential to create new types of nanophotonic devices. Most prior demonstrations were based on arrays of dielectric resonators, which cannot be miniaturized beyond the diffraction limit, reducing the applicability of BICs for advanced functions. Here, we demonstrate BICs and quasi-BICs based on high-quality factor phonon-polariton resonances in isotopically pure h11BN and how these states can be supported by periodic arrays of nanoresonators with sizes much smaller than the wavelength. We theoretically illustrate how BICs emerge from the band structure of the arrays and verify both numerically and experimentally the presence of these states and enhanced quality factor. Furthermore, we identify and characterize simultaneously quasi-BICs and bright states. Our method can be generalized to create a large number of optical states and to tune their coupling with the environment, paving the way to miniaturized nanophotonic devices with more advanced functions.

physics.optics↗

arXiv2022

Radially and azimuthally pure vortex beams from phase-amplitude metasurfaces

To exploit the full potential of the transverse spatial structure of light using the Laguerre-Gaussian basis, it is necessary to control the azimuthal and radial components of the photons. Vortex phase elements are commonly used to generate these modes of light, offering precise control over the azimuthal index but neglect the radially dependent amplitude term which defines their associated corresponding transverse profile. Here we experimentally demonstrate the generation of high purity Laguerre-Gaussian beams with a single step on-axis transformation implemented with a dielectric phase-amplitude metasurface. By vectorially structuring the input beam and projecting it onto an orthogonal polarisation basis, we can sculpt any vortex beam in phase and amplitude. We characterize the azimuthal and radial purity of the generated vortex beams, reaching a purity of 98% for a vortex beam with $\ell=50$ and $p=0$. Furthermore, we comparatively show that the purity of the generated vortex beams outperform those generated with other well-established phase-only metasurface approaches. In addition, we highlight the formation of 'ghost' orbital angular momentum orders from azimuthal gratings (analogous to ghost orders in ruled gratings), which have not been widely studied to date. Our work brings higher-order vortex beams and their unlimited potential within reach of wide adoption.

physics.optics↗

arXiv2021

Optical vortex crystals with dynamic topologies

Vortex crystals are geometric arrays of vortices found in various physics fields, owing their regular internal structure to mutual interactions within a spatially confined system. In optics, vortex crystals may form spontaneously within a nonlinear resonator but their usefulness is limited by the lack of control over their topology. On the other hand, programmable devices used in free space, like spatial light modulators, allow the design of nearly arbitrary vortex distributions but without any intrinsic dynamics. By combining non-Hermitian optics with on-demand topological transformations enabled by metasurfaces, we report a solid-state laser that generates vortex crystals with mutual interactions and actively-tunable topologies. We demonstrate 10x10 coherent vortex arrays with nonlocal coupling networks that are not limited to nearest-neighbor coupling but rather dictated by the crystal's topology. The vortex crystals exhibit sharp Bragg diffraction peaks, witnessing their coherence and high topological charge purity, which we resolve spatially over the whole lattice by introducing a parallelized analysis technique. By structuring light at the source, we enable complex transformations that allow to arbitrarily partition the orbital angular momentum inside the cavity and to heal topological charge defects, making these resonators a robust and versatile tool for advanced applications in topological optics.

physics.optics↗

arXiv2021

Roadmap on multimode light shaping

Our ability to generate new distributions of light has been remarkably enhanced in recent years. At the most fundamental level, these light patterns are obtained by ingeniously combining different electromagnetic modes. Interestingly, the modal superposition occurs in the spatial, temporal as well as spatio-temporal domain. This generalized concept of structured light is being applied across the entire spectrum of optics: generating classical and quantum states of light, harnessing linear and nonlinear light-matter interactions, and advancing applications in microscopy, spectroscopy, holography, communication, and synchronization. This Roadmap highlights the common roots of these different techniques and thus establishes links between research areas that complement each other seamlessly. We provide an overview of all these areas, their backgrounds, current research, and future developments. We highlight the power of multimodal light manipulation and want to inspire new eclectic approaches in this vibrant research community.

physics.optics↗

arXiv2020

High quality factor polariton resonators using van der Waals materials

We present high quality factor optical nanoresonators operating in the mid-IR to far-IR based on phonon polaritons in van der Waals materials. The nanoresonators are disks patterned from isotopically pure hexagonal boron nitride (isotopes 10B and 11B) and α-molybdenum trioxide. We experimentally achieved quality factors of nearly 400, the highest ever observed in nano-resonators at these wavelengths. The excited modes are deeply subwavelength, and the resonators are 10 to 30 times smaller than the exciting wavelength. These results are very promising for the realization of nano-photonics devices such as optical bio-sensors and miniature optical components such as polarizers and filters.

physics.optics↗

arXiv2020

Recent twists in twisted light: A perspective on optical vortices from dielectric metasurfaces

Optical vortices are the electromagnetic analogue of fluid vortices studied in hydrodynamics. In both cases the traveling wavefront, either made of light or fluid, is twisted like a corkscrew around its propagation axis - an analogy that inspired also the first proposition of the concept of optical vortex. Even though vortices are one of the most fundamental topological excitations in nature, they are rarely found in their electromagnetic form in natural systems, for the exception of energetic sources in astronomy, such as pulsars, quasars and black holes. Mostly optical vortices are artificially created in the laboratory by a rich variety of approaches. Here we provide our perspective on a technology that shook-up optics in the last decade - metasurfaces, planar nanostructured metamaterials - with a specific focus on its use for molding and controlling optical vortices.

physics.optics↗

arXiv2020

Arbitrary polarization conversion for pure vortex generation with a single metasurface

The purity of an optical vortex beam depends on the spread of its energy among different azimuthal and radial modes. The smaller is this spread, the higher is the vortex purity and the more efficient are its creation and detection. There are several methods to generate vortex beams with well-defined orbital angular momentum but only few exist allowing to select a pure radial mode. These typically consist of many optical elements with rather complex arrangements, including active cavity resonators. Here we show that it is possible to generate pure vortex beams using a single metasurface plate in combination with a polarizer. We generalize an existing theory of independent phase and amplitude control with birefringent nanopillars considering arbitrary input polarization states. The high purity, sizeable creation efficiency and impassable compactness make the presented approach a powerful complex amplitude modulation tool for pure vortex generation, even in the case of large topological charges.

physics.optics↗

arXiv2019

Polariton Nanophotonics using Phase Change Materials

Polaritons formed by the coupling of light and material excitations such as plasmons, phonons, or excitons enable light-matter interactions at the nanoscale beyond what is currently possible with conventional optics. Recently, significant interest has been attracted by polaritons in van der Waals materials, which could lead to applications in sensing, integrated photonic circuits and detectors. However, novel techniques are required to control the propagation of polaritons at the nanoscale and to implement the first practical devices. Here we report the experimental realization of polariton refractive and meta-optics in the mid-infrared by exploiting the properties of low-loss phonon polaritons in isotopically pure hexagonal boron nitride (hBN), which allow it to interact with the surrounding dielectric environment comprising the low-loss phase change material, Ge$_3$Sb$_2$Te$_6$ (GST). We demonstrate waveguides which confine polaritons in a 1D geometry, and refractive optical elements such as lenses and prisms for phonon polaritons in hBN, which we characterize using scanning near field optical microscopy. Furthermore, we demonstrate metalenses, which allow for polariton wavefront engineering and sub-wavelength focusing. Our method, due to its sub-diffraction and planar nature, will enable the realization of programmable miniaturized integrated optoelectronic devices, and will lay the foundation for on-demand biosensors.

physics.app-ph↗

arXiv2018

A mid-infrared biaxial hyperbolic van der Waals crystal

Hyperbolic media have attracted much attention in the photonics community, thanks to their ability to confine light to arbitrarily small volumes and to their use for super-resolution applications. The 2D counterpart of these media can be achieved with hyperbolic metasurfaces, which support in-plane hyperbolic guided modes thanks to nanopatterns which, however, pose significant fabrication challenges and limit the achievable confinement. We show that thin flakes of the van der Waals material α-MoO3 can support naturally in-plane hyperbolic polariton guided modes at mid-infrared frequencies without any patterning. This is possible because α-MoO3 is a biaxial hyperbolic crystal, with three different Restrahlen bands, each for a different crystal axis. Our findings can pave the way towards new paradigm to manipulate and confine light in planar photonic devices.

physics.optics↗

arXiv2018

Engineering Phonon Polaritons in van der Waals Heterostructures to Enhance In-Plane Optical Anisotropy

Van der Waals heterostructures assembled from layers of 2D materials have attracted considerable interest due to their novel optical and electrical properties. Here we report a scattering-type scanning near field optical microscopy study of hexagonal boron nitride on black phosphorous (h-BN/BP) heterostructures, demonstrating the first direct observation of in-plane anisotropic phonon polariton modes in vdW heterostructures. Strikingly, the measured in-plane optical anisotropy along armchair and zigzag crystal axes exceeds the ratio of refractive indices of BP in the x-y plane. We explain that this enhancement is due to the high confinement of the phonon polaritons in h-BN. We observe a maximum in-plane optical anisotropy of α_max=1.25 in the 1405-1440 cm-1 frequency spectrum. These results provide new insights on the behavior of polaritons in vdW heterostructures, and the observed anisotropy enhancement paves the way to novel nanophotonic devices and to a new way to characterize optical anisotropy in thin films.

physics.optics↗

arXiv2018

Anisotropic Surface Plasmon Polariton Generation Using Bimodal V-Antenna Based Metastructures

V-shaped nanoantennas are among the popular choices for the unit element of a metasurface, a nanostructured surface used for its ability to mold and control the wavefront of light. In general, the motivation for choosing the V-antenna as the unit element comes from its bimodal nature, where the introduction of the second mode offers extra control over the scattered wavefronts. Here, through near-field scanning optical microscopy, we study a 1D metastructure comprised of V-antennas in the context of generating asymmetric surface plasmon polariton (SPP) wavefronts. The key point is that the use of the V-antenna allows for the creation of a two-dimensional phase gradient with a single line of antennas, where the extra phase dimension offers additional control and allows for asymmetric features. Two different asymmetries are created: (1) SPP wavefronts that have different propagation directions on either side of the metastructure, and (2) SPP wavefront asymmetry through focusing: one side of the metastructure focuses SPP wavefronts, while the other side has diverging SPP wavefronts.

physics.optics↗

arXiv2017

Selective excitation and imaging of ultraslow phonon polaritons in thin hexagonal boron nitride crystals

Polaritons in 2D and van der Waals (vdW) materials have been investigated in several recent works as an innovative platform for light-matter interaction, rich of new physical phenomena.Hexagonal Boron Nitride (h-BN), in particular, is an out of plane anisotropic material (while it is in-plane isotropic) with two very strong phonon polaritons bands where the permittivity becomes negative. In the first restrahlen band (RS1, 780-830 cm-1) the relative out of plane permittivity is negative, while in the second restrahlen band (RS2, 1370-1610 cm-1) the relative in-plane permittivity is negative. Due to these optical properties, thin h-BN flakes support guided modes which have been observed experimentally both via far field and near field methods. In this work, we show how selectively excite the more confined modes in the RS1 and RS2 bands. The supported guided modes have phase and group velocities respectively tens and hundreds of times slower than the speed of light. We also show the possibility of full hyperspectral nano-imaging of modes in RS1 band by means of photo-induced force microscopy (PiFM). Moreover, a direct comparison of (PiFM) and scattering-type near-field microscopy (s-SNOM) is obtained by imaging the modes of the RS2 band with both techniques implemented on the same platform. The possibility of addressing ultraslow (ultraconfined) polaritonic modes of h-BN crystal flakes together with the possibility of optical nano-imaging in both the restrahlen bands have many innovative aspects that can lead to unprecedented schemes for strong light-matter interaction, slow and confined light.

physics.app-ph↗