arXiv subjects

P. Alonso-González

Publications and source records attributed to P. Alonso-González.

7 recordsLinked to original sources

arXiv2025

Unidirectional Ray Polaritons in Twisted Asymmetric Stacks

The vast repository of van der Waals (vdW) materials supporting polaritons offers numerous possibilities to tailor electromagnetic waves at the nanoscale. The development of twistoptics - the modulation of the optical properties by twisting stacks of vdW materials - enables directional propagation of phonon polaritons (PhPs) along a single spatial direction, known as canalization. Here we demonstrate a complementary type of directional propagation of polaritons by reporting the visualization of unidirectional ray polaritons (URPs). They arise naturally in twisted hyperbolic stacks with very different thicknesses of their constituents, demonstrated for homostructures of $α$-MoO$_3$ and heterostructures of $α$-MoO$_3$ and $β$-Ga$_2$O$_3$. Importantly, their ray-like propagation, characterized by large momenta and constant phase, is tunable by both the twist angle and the illumination frequency. Apart from their fundamental importance, our findings introduce twisted asymmetric stacks as efficient platforms for nanoscale directional polariton propagation, opening the door for applications in nanoimaging, (bio)-sensing or polaritonic thermal management.

physics.optics↗

arXiv2024

Canalization-based super-resolution imaging using a single van der Waals layer

Canalization is an optical phenomenon that enables unidirectional propagation of light in a natural way, i.e., without the need for predefined waveguiding designs. Predicted years ago, it was recently demonstrated using highly confined phonon polaritons (PhPs) in twisted layers of the van der Waals (vdW) crystal alpha-MoO3, offering unprecedented possibilities for controlling light-matter interactions at the nanoscale. However, despite this finding, applications based on polariton canalization have remained elusive so far, which can be explained by the complex sample fabrication of twisted stacks. In this work, we introduce a novel canalization phenomenon, arising in a single vdW thin layer (alpha-MoO3) when it is interfaced with a substrate exhibiting a given negative permittivity, that allows us to demonstrate a proof-of-concept application based on polariton canalization: super-resolution (up to ~λ0/220) nanoimaging. Importantly, we find that canalization-based imaging transcends conventional projection constraints, allowing the super-resolution images to be obtained at any desired location in the image plane. This versatility stems from the synergetic manipulation of three distinct parameters: incident frequency, rotation angle of the thin vdW layer, and thickness. These results provide valuable insights into the fundamental properties of canalization and constitute a seminal step towards multifaceted photonic applications, encompassing imaging, data transmission, and ultra-compact photonic integration.

physics.optics↗

arXiv2023

Multiple and spectrally robust photonic magic angles in reconfigurable α-MoO3 trilayers

The assembling of twisted stacks of van der Waals (vdW) materials had led to the discovery of a profusion of remarkable physical phenomena in recent years, as it provides a means to accurately control and harness electronic band structures. This has given birth to the so-called field of twistronics. An analogous concept has been developed for highly confined polaritons, or nanolight, in twisted bilayers of strongly anisotropic vdW materials, extending the field to the twistoptics realm. In this case, the emergence of a topological transition of the polaritonic dispersion at a given twist angle (photonic magic angle) results in the propagation of nanolight along one specific direction (canalization regime), holding promises for unprecedented control of the flow of energy at the nanoscale. However, there is a fundamental limitation in twistoptics that critically impedes such control: there is only one photonic magic angle (and thus canalization direction) in a twisted bilayer and it is fixed for each incident frequency. Here, we overcome this limitation by demonstrating the existence of multiple spectrally robust photonic magic angles in reconfigurable twisted vdW trilayers. As a result, we show that canalization of nanolight can be programmed at will along any desired in-plane direction in a single device, and, importantly, within broad spectral ranges of up to 70 cm-1. Our findings lay the foundation for robust and widely tunable twistoptics, opening the door for applications in nanophotonics where on-demand control of energy at the nanoscale is crucial, such as thermal management, nanoimaging or entanglement of quantum emitters.

physics.optics↗

arXiv2022

Twist-tunable Polaritonic Nanoresonators in a van der Waals Crystal

Optical nanoresonators are fundamental building blocks in a number of nanotechnology applications (e.g. in spectroscopy) due to their ability to efficiently confine light at the nanoscale. Recently, nanoresonators based on the excitation of phonon polaritons (PhPs) $-$ light coupled to lattice vibrations $-$ in polar crystals (e.g. SiC, or h-BN) have attracted much attention due to their strong field confinement, high-quality factors, and potential to enhance the photonic density of states at mid-infrared (IR) frequencies. Here, we go one step further by introducing PhPs nanoresonators that not only exhibit these extraordinary properties but also incorporate a new degree of freedom $-$ twist tuning, i.e. the possibility to be spectrally controlled by a simple rotation. To that end, we both take advantage of the low-loss in-plane hyperbolic propagation of PhPs in the van der Waals crystal $α$-MoO$_3$, and realize dielectric engineering of a pristine $α$-MoO$_3$ slab placed on top of metal ribbon grating, which preserves the high-quality of the polaritonic resonances. By simple rotating the $α$-MoO$_3$ slab in the plane (from 0 to 45$^{\circ}$), we demonstrate via far- and near-field measurements that the narrow polaritonic resonances (with quality factors Q up to 200) can be tuned in a broad range (up to 32 cm$^{-1}$, i.e up 6 ~ times its full width at half maximum, FWHM ~ 5 cm$^{-1}$). Our results open the door to the development of tunable low-loss nanotechnologies at IR frequencies with application in sensing, emission or photodetection.

physics.optics↗

arXiv2021

Planar nano-optics in anisotropic media: anomalous refraction and diffraction-free lensing of highly confined polaritons

As one of the most fundamental optical phenomena, refraction between isotropic media is characterized by light bending towards the normal to the boundary when passing from a low- to a high-refractive-index medium. However, in anisotropic media, refraction is a much more exotic phenomenon. The most general case of refraction between two anisotropic media remains unexplored, particularly in natural media and at the nanoscale. Here, we visualize and comprehensively study refraction of electromagnetic waves between two strongly anisotropic (hyperbolic) media, and, importantly, we do it with the use of polaritons confined to the nanoscale in a low-loss natural medium, alpha-MoO3. Our images show refraction of polaritons under the general case in which both the direction of propagation and the wavevector are not collinear. As they traverse planar nanoprisms tailored in alpha-MoO3, refracted polaritons exhibit non-intuitive directions of propagation, enabling us to unveil an exotic optical effect: bending-free refraction. Furthermore, we succeed in developing the first in-plane refractive hyperlens, which yields foci as small as lamdap/6, being lamdap the polariton wavelength (lamda0/50 with respect to the wavelength of light in free space). Our results set the grounds for planar nano-optics in strongly anisotropic media, with potential for unprecedented control of the flow of energy at the nanoscale.

physics.optics↗

arXiv2021

Giant optical anisotropy in transition metal dichalcogenides for next-generation photonics

Large optical anisotropy observed in a broad spectral range is of paramount importance for efficient light manipulation in countless devices. Although a giant anisotropy was recently observed in the mid-infrared wavelength range, for visible and near-infrared spectral intervals, the problem remains acute with the highest reported birefringence values of 0.8 in BaTiS3 and h-BN crystals. This inspired an intensive search for giant optical anisotropy among natural and artificial materials. Here, we demonstrate that layered transition metal dichalcogenides (TMDCs) provide an answer to this quest owing to their fundamental differences between intralayer strong covalent bonding and weak interlayer van der Walls interaction. To do this, we carried out a correlative far- and near-field characterization validated by first-principle calculations that reveals an unprecedented birefringence of 1.5 in the infrared and 3 in the visible light for MoS2. Our findings demonstrate that this outstanding anisotropy allows for tackling the diffraction limit enabling an avenue for on-chip next-generation photonics.

physics.app-ph↗

arXiv2011

Charge control in laterally coupled double quantum dots

We investigate the electronic and optical properties of InAs double quantum dots grown on GaAs (001) and laterally aligned along the [110] crystal direction. The emission spectrum has been investigated as a function of a lateral electric field applied along the quantum dot pair mutual axis. The number of confined electrons can be controlled with the external bias leading to sharp energy shifts which we use to identify the emission from neutral and charged exciton complexes. Quantum tunnelling of these electrons is proposed to explain the reversed ordering of the trion emission lines as compared to that of excitons in our system.

cond-mat.mes-hall↗