Efficient Vortex Generation in Subwavelength Epsilon-Near-Zero Slabs

Abstract

We show that a homogeneous and isotropic slab, illuminated by a circularly polarized beam with no topological charge, produces vortices of order two in the opposite circularly polarized components of the reflected and transmitted fields, as a consequence of the difference between transverse magnetic and transverse electric dynamics. In the epsilon-near-zero regime, we find that vortex generation is remarkably efficient in sub-wavelength thick slabs up to the paraxial regime. This physically stems from the fact that a vacuum paraxial field can excite a nonparaxial field inside an epsilon-near-zero slab since it hosts slowly varying fields over physically large portion of the bulk. Our theoretical predictions indicate that epsilon-near-zero media hold great potential as nanophotonic elements for manipulating the angular momentum of the radiation, since they are available without resorting to complicated micro/nano fabrication processes and can operate even at very small (ultraviolet) wavelengths.Spin-orbit interaction (SOI) of light is a very important research topic since it provides a tool for manipulating the spatial degrees of freedom of the radiation by acting on its circular polarization state [1]. A remarkable SOI effect is the generation of optical vortices from circularly polarized beams, a process accompanied by spin to orbital angular momentum conversion. Standard procedures to achieve vortex generation are focusing by high-numerical aperture lens [2,3], scattering by small particles [3], propagation along the optical axis of a uniaxial crystal [4,5] and propagation through semiconductor microcavities [6]. Similar SOI effects involving Bessel beams have been considered in uniaxial crystals [7] and at reflection and transmission by a planar interface between two homogeneous media [8]. The advent of metamaterials has further increased the SOI research effort [9], mostly in the use of ultra-thin metasurfaces for manipulating the angular momentum of light [10,11] and for vortex generation [12,13].Epsilon near zero (ENZ) media are nowadays attracting an increasing research interest due to the very unconventional way they affect the electromagnetic radiation. The effective wavelength in ENZ media is much larger than the vacuum wavelength and this entails a regime quite opposite to geometrical optics where the field is slowly-varying over relatively large portions of the bulk. Such feature has been exploited for squeezing electromagnetic waves at will [14], for tailoring the antenna radiation pattern [15] and for enhancing nonlinear response of matter [16][17][18][19][20]. In the context of light SOI, it has recently been proposed that a thin epsilon-near-zero slab can enhance the spin Hall effect of transmitted light [21].In this letter we show that a homogeneous, isotropic and ultra-thin (sub-wavelength thick) slab can support vortex generation. We prove that such genuine SOI effect is physically due to the mutual difference between the dynamics of transverse magnetic and transverse electric fields upon refl...

Experimental Realization of an Epsilon-Near-Zero Graded-Index Metalens at Terahertz Frequencies

“…Metamaterials have demonstrated that they allow successful control of the electromagnetic response of media, opening the gate to manipulation of wave propagation [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Within this realm, metamaterials with a relative permittivity near zero [known as epsilon near zero (ENZ)] exhibit fascinating features that have been recently studied in different frequency bands of the electromagnetic spectrum, mainly, microwaves, millimeter waves, and nearinfrared and optical frequencies [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Much of the attention is due to the fact that the waves traveling inside these media show almost "infinite" phase velocity and wavelength, with nearly uniform spatial phase variation, giving rise to unconventional phenomena such as tunneling, squeezing, and supercoupling [16].…”

Pure and achromatic spin-orbit shaping of light from Fresnel reflection off space-variant anisotropic media

“…However, despite all these approaches having a good level of achromatic performances, the fabricated devices so far have been endowed with poor spatial resolution (greater than 1 mm). Intriguingly, it has also been predicted that planar interfaces between two lossless dielectrics [33] or between vacuum and near-zero ϵ materials [34] can also behave as polarization-dependent optical vortex generators, however without experimental realizations to date. In this context, the present approach has the advantage of having no restriction in terms of the geometric phase pattern imparted to an incident field.…”

Electric Control of Spin‐Orbit Coupling in Graphene‐Based Nanostructures with Broken Rotational Symmetry

“…[ 1–3 ] In the paraxial regime (weak localization), SOI is usually negligible [ 4 ] but may be augmented by inhomogeneities, [ 5–8 ] , anisotropy, [ 9,10 ] , and epsilon‐near‐zero materials. [ 11,12 ] Non‐paraxial focusing [ 13,14 ] by small particles [ 15,16 ] or a doped graphene [ 17 ] enhances SOI and results in spin‐dependent directionalities and optical vortices. [ 3 ] In the case of the extreme confinement at the subwavelength scales, SOI is dominant [ 18–21 ] with applications in quantum optics, [ 22,23 ] high‐resolution microscopy, [ 24,25 ] beam shaping with planar metasurfaces, [ 26 ] optical forces [ 27–29 ] , and nanophotonics.…”