scholarly journals Focusing fractional-order cylindrical vector beams

2021 ◽  
Vol 45 (2) ◽  
pp. 172-178
Author(s):  
S.S. Stafeev ◽  
V.D. Zaitsev
Keyword(s):  
Fractional Order ◽  
Energy Flux ◽  
Energy Flow ◽  
Focal Spot ◽  
Numerical Aperture ◽  
Vortex Beam ◽  
Incident Wavelength ◽  
Spot Center ◽  
Vector Beams ◽  
Linearly Polarized

By numerically simulating the sharp focusing of fractional-order vector beams (0≤m≤1, with azimuthal polarization at m=1 and linear polarization at m=0), it is shown that the shape of the intensity distribution in the focal spot changes from elliptical (m=0) to round (m=0.5) and ends up being annular (m=1). Meanwhile, the distribution pattern of the longitudinal component of the Poynting vector (energy flux) in the focal spot changes in a different way: from circular (m=0) to elliptical (m=0.5) and ends up being annular (m=1). The size of the focal spot at full width at half maximum of intensity for a first-order azimuthally polarized optical vortex (m=1) and numerical aperture NA=0.95 is found to be 0.46 of the incident wavelength, whereas the diameter of the on-axis energy flux for linearly polarized light (m=0) is 0.45 of the wavelength. Therefore, the answers to the questions: when the focal spot is round and when elliptical, or when the focal spot is minimal -- when focusing an azimuthally polarized vortex beam or a linearly polarized non-vortex beam, depend on whether we are considering the intensity at the focus or the energy flow. In another run of numerical simulation, we investigate the effect of the deviation of the beam order from m=2 (when an energy backflow is observed at the focal spot center). The reverse energy flow is shown to occur at the focal spot center until the beam order gets equal to m=1.55.

scholarly journals Experimental investigation of the energy backflow in the tight focal spot

2020 ◽  
Vol 44 (6) ◽  
pp. 863-870
Author(s):  
V.V. Kotlyar ◽  
S.S. Stafeev ◽  
A.G. Nalimov ◽  
A.A. Kovalev ◽  
A.P. Porfirev
Keyword(s):  
Circular Polarization ◽  
Energy Flow ◽  
Focal Spot ◽  
Optical Axis ◽  
Numerical Aperture ◽  
Flow Distribution ◽  
Local Maximum ◽  
Flow Tube ◽  
Incident Wavelength ◽  
Direct Energy

Using two identical microobjectives with a numerical aperture NA = 0.95, we experimentally demonstrate that the on-axis intensity near the tight focal spot of an optical vortex with a topological charge 2 is zero for right-handed circular polarization and nonzero for left-handed circular polarization. This serves to confirm that in the latter case there is a reverse energy flow on the optical axis, as testified by a very weak local maximum (the Arago spot) detected at the center of the measured energy flow distribution, caused by diffraction of the direct energy flow by a 300 nm circle (the diameter of a reverse energy flow tube). The comparison of numerical and experimental intensity distributions shows that it is possible to determine the diameter of the reverse energy flow "tube", which is equal to the distance between the adjacent intensity nulls. For NA = 0.95 and a 532 nm incident wavelength, the diameter of the on-axis reverse energy flow "tube" is measured to be 300 nm. It is also experimentally shown that when an optical beam with second-order cylindrical polarization is focused with a lens with NA = 0.95, there is a circularly symmetric energy flow in the focus with a very weak maximum in the center (the Arago spot), whose distribution is determined by diffraction of the direct energy flow by a 300 nm circular region, where the energy flow is reverse. This also confirms that in this case, there is a reverse energy flow on the optical axis.

scholarly journals Vortex energy flow in the tight focus of a non-vortex field with circular polarization

2020 ◽  
Vol 44 (1) ◽  
pp. 5-11
Author(s):  
V.V. Kotlyar ◽  
S.S. Stafeev ◽  
A.G. Nalimov
Keyword(s):  
Angular Momentum ◽  
Orbital Angular Momentum ◽  
Energy Flow ◽  
Poynting Vector ◽  
Focal Spot ◽  
Free Field ◽  
Longitudinal Component ◽  
Circularly Polarized ◽  
Linearly Polarized ◽  
Direct Energy

Using Richards-Wolf formulas, we show that an axisymmetric circularly polarized vortex-free field can be focused into a sharp subwavelength focal spot, around which there is a region where the light energy flow propagates along a spiral. This effect can be explained by the conversion of the spin angular momentum of the circularly polarized field into the orbital angular momentum near the focus, although the on-axis orbital angular momentum remains zero. It is also shown that a linearly polarized optical vortex with topological charge 2 forms near the focal plane an on-axis reverse energy flow (defined by the negative longitudinal component of the Poynting vector) whose amplitude is comparable with the direct energy flow.

scholarly journals Formation of an elongated region of energy backflow using ring apertures

2019 ◽  
Vol 43 (2) ◽  
pp. 193-199
Author(s):  
S.S. Stafeev ◽  
V.V. Kotlyar
Keyword(s):  
Energy Flow ◽  
Poynting Vector ◽  
Numerical Aperture ◽  
Second Order ◽  
Depth Of Focus ◽  
Incident Wavelength ◽  
Entrance Pupil ◽  
High Numerical Aperture ◽  
Vector Beam ◽  
Cylindrical Vector Beam

In this paper, we have investigated the focusing of a second-order cylindrical vector beam by using a high numerical aperture (NA) lens limited by a ring aperture using the Richards-Wolf formulae. It was shown that the range of negative on-axis projections of the Poynting vector could be increased by increasing the depth of focus through the use of a ring aperture. It was shown that when focusing light with a lens with NA = 0.95, the use of a ring aperture limiting the entrance pupil angle to 0.9 of maximum, allows the depth of the region of negative on-axis Poynting vector projections to be four times increased, with the region width remaining almost unchanged and varying from 0.357 to 0.352 of the incident wavelength. Notably, the magnitude of the reverse energy flow was found to be larger than the direct one by a factor of 2.5.

scholarly journals Simulation of laser light focusing with two-layer dielectric microcyl-inders

2021 ◽  
Vol 45 (2) ◽  
pp. 208-213
Author(s):  
A.A. Savelyeva ◽  
E.S. Kozlova
Keyword(s):  
Focal Spot ◽  
Laser Light ◽  
Optical Axis ◽  
Fdtd Method ◽  
Circular Cross Section ◽  
Depth Of Focus ◽  
Incident Wavelength ◽  
The Core ◽  
Linearly Polarized ◽  
Difference Time

Focusing of a linearly polarized laser beam of wavelength 633 nm with two-layer dielectric microcylinders of a circular cross-section and 2-um diameter was simulated using a finite-difference time-domain (FDTD) method, implemented using the FullWAVE software. It was shown that using a cladding whose refractive index (1.8 or 1.9) is higher than that of the core (1.45), it is possible to increase the depth of focus by a factor of 2.57 multiplied by the incident wavelength and shift the focal spot position along the optical axis away from the microcylinder boundary. It was also shown that parameters of the microcylinder could be chosen in such a way that a tighter focal spot was generated, with its full width at half maximum of intensity being 2.27 of the incident wavelength. The intensity at this focus was shown to be 1.4 times higher than that at the focus generated with a homogeneous microcylinder.

scholarly journals A minimal subwavelength focal spot for the energy flux

2021 ◽  
Vol 5 (45) ◽  
pp. 685-691
Author(s):  
S.S. Stafeev ◽  
V.D. Zaicev
Keyword(s):  
Energy Flux ◽  
Topological Charge ◽  
Focal Spot ◽  
Polarized Light ◽  
Spot Size ◽  
Optical Vortices ◽  
Matter Interaction ◽  
Linearly Polarized ◽  
Light Matter Interaction ◽  
Tight Focus

It is shown theoretically and numerically that circularly and linearly polarized incident beams produce at the tight focus identical circularly symmetric distributions of an on-axis energy flux. It is also shown that the on-axis energy fluxes from radially and azimuthally polarized optical vortices with unit topological charge are equal to each other. An optical vortex with azimuthal polarization is found to generate the minimum focal spot measured for the intensity (all other parameters being equal). Slightly larger (by a fraction of a percent) is the spot size calculated for the energy flux for the circularly and linearly polarized light. The spot size in terms of intensity is of importance in light-matter interaction, whereas the spot size in terms of energy flux affects the resolution in optical microscopy.

scholarly journals Formation of Inverse Energy Flux in the Case of Diffraction of Linearly Polarized Radiation by Conventional and Generalized Spiral Phase Plates

Photonics ◽  
2021 ◽  
Vol 8 (7) ◽  
pp. 283
Author(s):  
Andrey Ustinov ◽  
Svetlana Khonina ◽  
Alexey Porfirev
Keyword(s):  
Energy Flux ◽  
Poynting Vector ◽  
Numerical Study ◽  
Longitudinal Component ◽  
Light Fields ◽  
Linearly Polarized ◽  
Linearly Polarized Radiation ◽  
Phase Plates ◽  
Spiral Phase ◽  
Polarized Radiation

Recently, there has been increased interest in the shaping of light fields with an inverse energy flux to guide optically trapped nano- and microparticles towards a radiation source. To generate inverse energy flux, non-uniformly polarized laser beams, especially higher-order cylindrical vector beams, are widely used. Here, we demonstrate the use of conventional and so-called generalized spiral phase plates for the formation of light fields with an inverse energy flux when they are illuminated with linearly polarized radiation. We present an analytical and numerical study of the longitudinal and transverse components of the Poynting vector. The conditions for maximizing the negative value of the real part of the longitudinal component of the Poynting vector are obtained.

scholarly journals Metasurface of Combined Semicircular Rings with Orthogonal Slit Pairs for Generation of Dual Vector Beams

Nanomaterials ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 1718
Author(s):  
Qian Kong ◽  
Manna Gu ◽  
Xiangyu Zeng ◽  
Rui Sun ◽  
Yuqin Zhang ◽  
...  
Keyword(s):  
Time Domain ◽  
Finite Difference Time Domain ◽  
Polarized Light ◽  
Orientation Angle ◽  
Fdtd Simulation ◽  
Dual Vector ◽  
Vector Beams ◽  
Linearly Polarized ◽  
Potential Applications ◽  
Difference Time

Manipulation of multichannel vector beams (VBs) with metasurfaces is an important topic and holds potential applications in information technology. In this paper, we propose a novel metasurface for the generation of dual VBs, which is composed of orthogonal slit pairs arranged on multiple groups of combined semicircular rings (CSRs). A group of CSRs include a right-shifted set and a left-shifted set of semicircular rings, and each set of semicircular rings has two halves of circles with different radii, sharing the same shifted center. Under the illumination of linearly polarized light, the two shifted sets of semicircular rings generate the two VBs at the shifted center positions on the observation plane. The slit units of each set are designed with independent rotation order and initial orientation angle. By adjusting the linear polarization of illumination, both two VBs with their orders and polarization states are independently controlled simultaneously. The principle and design are demonstrated by the finite-difference time domain (FDTD) simulation. The work is of significance for miniatured devices of VB generators and for related applications.

Multi-channeled vortex beam switch with moiré metasurfaces

2021 ◽  
Author(s):  
Cheng Cui ◽  
Zheng Liu ◽  
Bin Hu ◽  
Yurong Jiang ◽  
Juan Liu
Keyword(s):  
Functional Diversity ◽  
Optical Communications ◽  
Topological Charge ◽  
Signal Transmission ◽  
Polarized Light ◽  
Vortex Beam ◽  
Practical Applications ◽  
Linearly Polarized ◽  
Vortex Beams ◽  
Topological Charges

Abstract Tunable metasurface devices are considered to be an important link for metasurfaces to practical applications due to their functional diversity and high adaptability to the application scenarios. Metasurfaces have unique value in the generation of vortex beams because they can realize light wavefronts of any shape. In recent years, several vortex beam generators using metasurfaces have been proposed. However, the topological charge generally lacks tunability, which reduces the scope of their applications. Here, we propose an active tunable multi-channeled vortex beam switch based on a moiré structure composed of two cascaded dielectric metasurfaces. The simulation results show that when linearly polarized light with a wavelength of 810 nm is incident, the topological charge from -6 to +6 can be continuously generated by relatively rotating the two metasurfaces. Meanwhile, different topological charges are deflected to different spatial channels, realizing the function of multi-channeled signal transmission. We also study the efficiency and broadband performance of the structure. The proposed multi-channeled separation method of vortex beams that can actively tune topological charges paves the way for the compactness and functional diversity of devices in the fields of optical communications, biomedicine, and optoelectronics.

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