arXiv subjects

Michele Tamagnone

Publications and source records attributed to Michele Tamagnone.

At least 19 recordsLinked to original sources

arXiv2026

Topologically Protected Polaritonic Bound State in the Continuum

Bound states in the continuum (BICs) have emerged as powerful tools to realize ultra-high-Q resonances in nanophotonics. While previous implementations have primarily relied on dielectric metasurfaces, their optical confinement remains fundamentally limited by diffraction. In this work, we theoretically and numerically demonstrate and experimentally validate the existence of topologically protected phonon-polaritonic BICs in periodic arrays of cylindrical nanoresonators composed of isotopically enriched hexagonal boron nitride (h11BN), which support two restrahlen bands (lower (type-I) and upper (type II)), with the present work focusing on the lower Reststrahlen band (RB-1). Owing to the uniaxial anisotropy of hBN and the rotational symmetry of the structure, these systems support topologically symmetry-protected BICs at the Γ-point, where radiative losses are suppressed. The total quality factor is ultimately bounded by the intrinsic phonon damping of h11BN, enabling high-Q polaritonic modes with minimal radiation leakage. When cylindrical symmetry is broken via angular tilting of incident light away from normal incidence, these BICs transition into quasi-BICs (q-BICs), with strong field confinement and tunable radiation leakage. Their topological features enable robust control over mode lifetimes and confinement, paving the way towards scalable polaritonic platforms for mid-infrared optoelectronics, sensing and quantum nanophotonics.

physics.optics↗

arXiv2026

A scalable platform for nanometer-scale quantum confinement

Overcoming the limitations of current nanofabrication techniques to achieve nanoscale feature sizes is essential for achieving new regimes of light-matter interactions at extreme frequencies and length scales. Here, we demonstrate a scalable nanofabrication platform capable of producing in-plane feature sizes down to 1.75 nm, pushing the boundaries of current top-down nanofabrication techniques. Using precise thickness control of atomic layer deposition (ALD) and employing widely spaced oxide nanofins, we transform conventional ALD into a surface structuring method that produces nanolaminates with sub-10 nm periodicities over large areas. The resulting nanostructures can be used as a one-dimensional gate array to control charge carriers in two-dimensional materials. As an initial demonstration, we integrate the platform with graphene and perform electron transport measurements. In the presence of the gate array enabled by the nanolaminate, we observe satellite Dirac peaks consistent with band-structure modulation, suggestive of quantum-confinement effects. Our platform paves the way for exploring previously inaccessible regimes of nanoscale light-matter interactions, holding significant promise for applications in short wavelength optics, electronics, and polaritonics.

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

Topologically protected four-dimensional optical singularities

Optical singularities play a major role in modern optics and are frequently deployed in structured light, super-resolution microscopy, and holography. While phase singularities are uniquely defined as locations of undefined phase, polarization singularities studied thus far are either partial, i.e., bright points of well-defined polarization, or unstable for small field perturbations. We demonstrate for the first time a complete, topologically protected polarization singularity; it is located in the 4D space spanned by the three spatial dimensions and the wavelength and is created in the focus of a cascaded metasurface-lens system. The field Jacobian plays a key role in the design of such higher-dimensional singularities, which can be extended to multidimensional wave phenomena, and pave the way to novel applications in topological photonics and precision sensing.

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↗

arXiv2019

Semiconductor ring laser frequency combs induced by phase turbulence

Semiconductor ring lasers are miniaturized devices that operate on clockwise and counterclockwise modes. These modes are not coupled in the absence of intracavity reflectors, which prevents the formation of a standing wave in the cavity and, consequently, of a population inversion grating. This should inhibit the onset of multimode emission driven by spatial hole burning. Here we show that, despite this notion, ring quantum cascade lasers inherently operate in phase-locked multimode states, that switch on and off as the pumping level is progressively increased. By rewriting the master equation of lasers with fast gain media in the form of the complex Ginzburg-Landau equation, we show that ring frequency combs stem from a phase instability---a phenomenon also known in superconductors and Bose-Einstein condensates. The instability is due to coupling of the amplitude and phase modulation of the optical field in a semiconductor laser, which plays the role of a Kerr nonlinearity, highlighting a connection between laser and microresonator frequency combs.

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↗

arXiv2019

Laser radio transmitter

Since the days of Hertz, radio transmitters have evolved from rudimentary circuits emitting around 50 MHz to modern ubiquitous Wi-Fi devices operating at gigahertz radio bands. As wireless data traffic continues to increase there is a need for new communication technologies capable of high-frequency operation for high-speed data transfer. Here we give a proof of concept of a new compact radio frequency transmitter based on a semiconductor laser frequency comb. In this laser, the beating among the coherent modes oscillating inside the cavity generates a radio frequency current, which couples to the electrodes of the device. We show that redesigning the top contact of the laser allows one to exploit the internal oscillatory current to drive an integrated dipole antenna, which radiates into free space. In addition, direct modulation of the laser current permits encoding a signal in the radiated radio frequency carrier. Working in the opposite direction, the antenna can receive an external radio frequency signal, couple it to the active region and injection lock the laser. These results pave the way to new applications and functionality in optical frequency combs, such as wireless radio communication and wireless synchronization to a reference source.

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

Graphene Reflectarray Metasurface for Terahertz Beam Steering and Phase Modulation

We report a THz reflectarray metasurface which uses graphene as active element to achieve beam steering, shaping and broadband phase modulation. This is based on the creation of a voltage controlled reconfigurable phase hologram, which can impart different reflection angles and phases to an incident beam, replacing bulky and fragile rotating mirrors used for terahertz imaging. This can also find applications in other regions of the electromagnetic spectrum, paving the way to versatile optical devices including light radars, adaptive optics, electro-optical modulators and screens.

physics.app-ph↗

arXiv2017

Magnetoplasmonic Enhancement of Faraday Rotation in Patterned Graphene Metasurfaces

Faraday rotation is a fundamental property present in all non-reciprocal optical elements. In the THz range, graphene displays strong Faraday rotation; unfortunately, it is limited to frequencies below the cyclotron resonance. Here we show experimentally that in specifically design metasurfaces, magneto-plasmons can be used to circumvent this limitation. We find excellent agreement between theory and experiment and provide new physical insights and predictions on these phenomena. Finally, we demonstrate strong tuneability in these metasurfaces using electric and magnetic field biasing.

cond-mat.mes-hall↗

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↗

arXiv2017

Guided Modes of Anisotropic van der Waals Materials Investigated by Near-Field Scanning Optical Microscopy

Guided modes in anisotropic two-dimensional van der Waals materials are experimentally investigated and their refractive indices in visible wavelengths are extracted. Our method involves near-field scanning optical microscopy of waveguide (transverse electric) and surface plasmon polariton (transverse magnetic) modes in h-BN/SiO2/Si and Ag/h-BN stacks, respectively. We determine the dispersion of these modes and use this relationship to extract anisotropic refractive indices of h-BN flakes. In the wavelength interval 550-700 nm, the in-plane and out-of-plane refractive indices are in the range 1.98-2.12 and 1.45-2.12, respectively. Our approach of using near-field scanning optical microscopy allows for direct study of interaction between light and two-dimensional van der Waals materials and heterostructures.

physics.optics↗

arXiv2017

Mechanical detection and imaging of hyperbolic phonon polaritons in hexagonal Boron Nitride

Mid-infrared nano-imaging and spectroscopy of two-dimensional (2D) materials have been limited so far to scattering-type Scanning Near-field Optical Microscopy (s-NSOM) experiments where light from the sample is scattered by a metallic-coated Atomic Force Microscope (AFM) tip interacting with the material at the nanoscale. These experiments have recently allowed imaging of plasmon polaritons in graphene as well as hyperbolic phonon polaritons (HP2) in hexagonal Boron Nitride (hBN). Here we show that the high mechanical sensitivity of an AFM cantilever can be exploited for imaging hyperbolic phonon polaritons in hBN. In our imaging process, the lattice vibrations of hBN micrometer-sized flakes are locally enhanced by the launched phonon polaritons. These enhanced vibrations are coupled to the AFM tip in contact to the sample surface and recorded during scanning. Imaging resolution better than λ/20 is showed, comparable to the best resolution in s-NSOM. Importantly, this detection mechanism is free from light background and it is in fact the first photon-less detection of phonon polaritons.

cond-mat.mtrl-sci↗

arXiv2014

Gate-controlled mid-infrared light bending with aperiodic graphene nanoribbons array

Graphene plasmonic nanostructures enable subwavelength confinement of electromagnetic energy from the mid-infrared down to the terahertz frequencies. By exploiting the spectrally varying light scattering phase at vicinity of the resonant frequency of the plasmonic nanostructure, it is possible to control the angle of reflection of an incoming light beam. We demonstrate, through full-wave electromagnetic simulations based on Maxwell equations, the electrical control of the angle of reflection of a mid-infrared light beam by using an aperiodic array of graphene nanoribbons, whose widths are engineered to produce a spatially varying reflection phase profile that allows for the construction of a far-field collimated beam towards a predefined direction.

physics.optics↗

arXiv2014

Nonlocal Electromagnetic Response of Graphene Nanostructures

Nonlocal electromagnetic effects of graphene arise from its naturally dispersive dielectric response. We present semi-analytical solutions of nonlocal Maxwell's equations for graphene nano-ribbons array with features around 100 nm, where we found prominent departures from its local response. Interestingly, the nonlocal corrections are stronger for light polarization parallel to the ribbons, which manifests as additional broadening of the Drude peak. For the perpendicular polarization case, nonlocal effects lead to blue-shifts of the plasmon peaks. These manifestations provide a physical measure of nonlocal effects, and we quantify their dependence on ribbon width, doping and wavelength.

physics.optics↗

arXiv2014

Reconfigurable THz Plasmonic Antenna Concept Using a Graphene Stack

The concept and analysis of a Terahertz (THz) frequency-reconfigurable antenna using graphene are presented. The antenna exploits dipole-like plasmonic resonances that can be frequency-tuned on large range via the electric field effect in a graphene stack. In addition to efficient dynamic control, the proposed approach allows high miniaturization and good direct matching with continuous wave THz sources. A qualitative model is used to explain the excellent impedance stability under reconfiguration. These initial results are very promising for future all-graphene THz transceivers and sensors. Keywords: Reconfigurable antenna, Graphene, Plasmons, Terahertz, frequency-tuning.

physics.optics↗