arXiv subjects

Ramon Paniagua-Dominguez

Publications and source records attributed to Ramon Paniagua-Dominguez.

24 records · Page 2Linked to original sources

arXiv2019

Super-resonances in microspheres: extreme effects in field localization

We reveal the existence of optical super-resonance modes supported by dielectric microspheres. These modes,with field-intensity enhancement factors on the order of 10^4-10^5, can be directly obtained from analytical calculations. In contrast to usual optical resonances, which are related to the poles of the electric and magnetic scattering amplitudes, super-resonance modes are related to the poles of the internal field coefficients, obtained for specific values of the size parameter. We also reveal the connection of these super-resonances in the generation of magnetic nanojets and of giant magnetic fields in particles with high refractive index.

physics.optics↗

arXiv2018

Lasing action in active dielectric nanoantenna arrays

Directional lasing, with a low threshold and high quality factor, in active dielectric nanoantenna arrays is demonstrated. This is achieved through a leaky resonance excited in coupled gallium arsenide (GaAs) nanopillars. The leaky resonance is formed by partially breaking a bound state in the continuum (BIC) generated by the collective, vertical electric dipole resonances excited in the nanopillars for sub-diffractive arrays. By opening an unprotected, diffractive channel along one of the periods of the array one can control the directionality of the emitted light without sacrificing the high Q associated with the BIC mode, thus achieving directional lasing. A quality factor Q = 2750 is achieved at a controlled angle of emission of ~ 3 degrees with respect to the normal of the array with a pumping fluence as low as 10 uJ/cm^2. We demonstrate the possibility to control the lasing directivity and wavelength by changing the geometrical parameters of the nanoantenna array, and by tuning the gain spectrum of GaAs with temperature. Lasing action is demonstrated at different wavelengths and emission at different angles, which can be as large as 25 degrees to the normal. The obtained results provide guidelines for achieving surface emitting laser devices based on active dielectric nanoantennas that are compact and highly transparent.

physics.optics↗

arXiv2018

A Metalens with Near-Unity Numerical Aperture

The numerical aperture (NA) of a lens determines its ability to focus light and its resolving capability. Having a large NA is a very desirable quality for applications requiring small light-matter interaction volumes or large angular collections. Traditionally, a large NA lens based on light refraction requires precision bulk optics that ends up being expensive and is thus also a specialty item. In contrast, metasurfaces allow the lens designer to circumvent those issues producing high NA lenses in an ultra-flat fashion. However, so far, these have been limited to numerical apertures on the same order of traditional optical components, with experimentally reported values of NA <0.9. Here we demonstrate, both numerically and experimentally, a new approach that results in a diffraction limited flat lens with a near-unity numerical aperture (NA>0.99) and sub-wavelength thickness (~λ/3), operating with unpolarized light at 715 nm. To demonstrate its imaging capability, the designed lens is applied in a confocal configuration to map color centers in sub-diffractive diamond nanocrystals. This work, based on diffractive elements able to efficiently bend light at angles as large as 82°, represents a step beyond traditional optical elements and existing flat optics, circumventing the efficiency drop associated to the standard, phase mapping approach.

physics.optics↗

arXiv2016

Suppression of scattering for small dielectric particles: an anapole mode and invisibility

We reveal that an isotropic homogeneous subwavelength particle with a high refractive index can produce ultra-weak total scattering due to vanishing contribution of the electric dipole moment. This effect can be explained with the help of the Fano resonance and scattering efficiency associated with the excitation of an anapole mode. The latter is a nonradiative mode emerging from destructive interference of electric and toroidal dipole moments, and it can be employed for a design of highly transparent optical materials.

physics.optics↗

arXiv2015

Generalized Brewster-Kerker effect in dielectric metasurfaces

Polarization is one of the key properties defining the state of light. It was discovered in the early 19th century by Brewster, among others, while studying light reflected from materials at different angles. These studies led to the first polarizers, based on Brewster's effect. One of the most active trends in photonics now is devoted to the study of miniaturized, sub-wavelength devices exhibiting similar, or even improved, functionalities compared to those achieved with bulk optical elements. In the present work, it is theoretically predicted that a properly designed all-dielectric metasurface exhibits a generalized Brewster effect potentially for any angle, wavelength and polarization of choice. The effect is experimentally demonstrated for an array of silicon nanodisks at visible wavelengths. The underlying physics of this effect can be understood in terms of the suppressed scattering at certain angles that results from the interference between the electric and magnetic dipole resonances excited in the nanoparticles, predicted by Kerker in early 80s. This reveals deep connection of Kerker's and Brewster's legacies and opens doors for Brewster phenomenon to new applications in photonics, which are not bonded to a specific polarization or angle of incidence.

physics.optics↗

arXiv2014

Optimum Forward Light Scattering by Spherical and Spheroidal Dielectric Nanoparticles with High Refractive Index

High-refractive index dielectric nanoparticles may exhibit strong directional forward light scattering at visible and near-infrared wavelengths due to interference of simultaneously excited electric and magnetic dipole resonances. For a spherical high-index dielectric, the so-called first Kerker's condition can be realized, at which the backward scattering practically vanishes for some combination of refractive index and particle size. However, Kerker's condition for spherical particles is only possible at the tail of the scattering resonances, when the particle scatters light weakly. Here we demonstrate that significantly higher forward scattering can be realized if spheroidal particles are considered instead. For each value of refractive index exists an optimum shape of the particle, which produces minimum backscattering efficiency together with maximum forward scattering. This effect is achieved due to the overlapping of magnetic and electric dipole resonances of the spheroidal particle at the resonance frequency. It permits the design of very efficient, low-loss optical nanoantennas.

physics.optics↗