arXiv subjects

R. Heydarian

Publications and source records attributed to R. Heydarian.

5 recordsLinked to original sources

arXiv2022

On subwavelength resolution granted by dielectric microparticles

In this work we report a theoretical study of the lateral resolution granted by a simple glass microcylinder. In this 2D study, we had in mind the 3D analogue -- a microsphere whose ability to form a deeply subwavelength and strongly magnified image of submicron objects has been known since 2011. Conventionally, the microscope in which such the image is observed is tuned so that to see the areas behind the microsphere. This corresponds to the location of the virtual source formed by the microsphere at a distance longer than the distance of the real source to the miscroscope. Recently, we theoretically found a new scenario of superresolution, when the virtual source is formed in the wave beam transmitted through the microsphere. However, in this work we concentrated on the case when the superresolution is achieved in the impractical imaging system, in which the microscope objective lens is replaced by a microlens located at a distance smaller than the Rayleigh range. The present paper theoretically answers an important question: which scenario of far-field nanoimaging by a microsphere grants the finest spatial at very large distances. We found that the novel scenario (corresponding to higher refractive indices) promises further enhancement of the resolution.

physics.optics↗

arXiv2021

Non-resonant subwavelength imaging by dielectric microparticles

Recently a hypothesis explaining the non-resonant mechanism of subwavelength imaging granted by a dielectric microsphere has been suggested. In accordance to the hypothesis, the far-field image of a subwavelength scatterer strongly coupled to a microsphere by near fields is offered by the scatterer polarization normal to the sphere surface. The radiation of a closely located normally oriented dipole is shaped by the microsphere so that the transmitted wave beam has a practically flat phase front. Then this beam turns out to be imaging -- keeping the subwavelength information about the dipole location. However, this mechanism of subwavelength imaging was only supposed in our previous paper. In this paper, we present a theoretical study which confirms this hypothesis and better explains the underlying physics. In several scenarios of the imaging beam evolution either a flat or a slightly diverging phase front of the hollow wave beam formed by a microsphere enables the deeply subwavelength ($0.1-0.2λ$) resolution of two dipole sources. We numerically simulate one of these scenarios -- that one in which the focusing lens is located closer than the Rayleigh diffraction length to the beam-forming microsphere and represents a microsphere itself. In our simulations we replace a 3D microsphere by a 2D "sphere" (microcylinder) so that to use an available electromagnetic solver for dielectric microparticles of very large optical sizes. The physical mechanism of the imaging does not suffer of this replacement.

physics.optics↗

arXiv2021

Spatial Fano resonance of a dielectric microsphere impinged by a Bessel beam

General concept of Fano resonance is considered so that to show the possibility of this resonance in space. Using a recently found solution for a Bessel wave beam impinging a dielectric sphere, we analyze the electromagnetic fields near a microsphere with different optical size and permittivity values. We theoretically reveal a spatial Fano resonance when a resonant mode of the sphere interferes with {an amount of } non-resonant modes. This resonance results in a giant jump of the electric field behind the sphere impinged by the first-order Bessel beam. The local minimum of the electromagnetic field turns out to be noticeably distanced from the rear edge of the microsphere. However, this is a near-field effect and we prove it. We also show that this effect can be utilized for engineering a submicron optical trap with unusual and useful properties.

physics.optics↗

arXiv2020

Subwavelength effects near a dielectric microcylinder illuminated by a diffraction-free beam

Generation of a photonic nanojet by a linearly polarized wave beam or a plane wave impinging a dielectric microcylinder implies partial conversion of the propagating waves into the evanescent ones. This conversion is manifested in nanojet waist of subwavelength effective width. However, this known near-field effect is relatively weak. We theoretically show that the incidence of a wave beam formed by two plane waves enables much stronger near-field effects: a deeply subwavelength focusing of the incident beam and a strong enhancement of the electric field on the whole cylinder surface and near it. The domination of the evanescent waves in the vicinity of the cylinder results from the destructive interference of the propagating spatial harmonics of the scattered field dictated by the incident wave beam.

physics.optics↗

arXiv2020

Role of the normal polarization in the far-field subwavelength imaging by a dielectric microsphere or microcylinder

Role of the normal polarization in the far-field subwavelength imaging granted by a dielectric microsphere or microcylinder is discussed and the hypotheses explaining this experimental fact are suggested. One of these hypotheses is confirmed by exact numerical simulations. This mechanism of the magnifying superlens operation is based on the excitation of creeping waves at a curved dielectric interface by a normally polarized dipole. The set of creeping waves after their ejection from the surface creates an imaging beam which may mimic either a Bessel beam or a Mathieu beam depending on the microparticle radius. This mechanism corresponds to the asymmetric coherent illumination.

physics.optics↗