In this work, the tight focusing of high-order cylindrical vector beams was investigated. Our analysis relies upon the Richard-Wolf equations, which have been extensively utilized when studying tightly focused laser beams with due regard for the vector properties of light fields. The FDTD method implemented in FullWave software was used to verify the results obtained with the Richards-Wolf integrals. It was shown that in the focus, there are areas with the direction of the Poynting vector opposite to the direction of propagation of the beam and the negative values are comparable in absolute value with positive values. If the order of the beam is equal to two, then the region with negative values is located in the center of the focal spot. In contrast to previous papers, where the inverse energy flow was propagated along a spiral, in this work we investigate a non-vortex inverse flow with a laminar propagation of light.
A large number of scientific papers are currently devoted to the investigation of metasurfaces, based on the subwavelength gratings. Such subwavelength gratings are anisotropic – TE- and TM-polarized waves propagated through them have a different phase. Based on this effect it is possible to create analogues of the classical half-wave plates, which rotate the direction of polarization. In this work we proposed a spiral metalens, which simultaneously converts linearly polarized light into an azimuthally polarized vortex beam and focuses it. This metalens is a combination of a spiral zone plate with a topological charge <i>m</i> = 1 (focal length <i>f</i> = 633 nm) and a sector subwavelength grating (period of 220 nm, relief depth of 120 nm, illumination wavelength of λ = 633 nm). The metalens was fabricated using electron beam lithography and ion etching in 130-nm thick amorphous silicon film. Using FDTD-method it was numerically shown that the metalens illuminated by a plane wave with linear polarization forms a circular focal spot with dimensions smaller than the scalar diffraction limit: FWHMx = 0.435λ and FWHMy = 0.457λ. The focusing by the fabricated metalens was investigated experimentally using scanning near-field optical microscope (Ntegra Spectra, NTMDT).
Using simplified two-dimensional finite element method (FEM) modeling we study such structures where the triangular dielectric prism is used for producing nano-jets. The soda-lime glass (n=1.46), polystyrene (n=1.56), polyester (n=1.59), barium titanate (n=1.8) is considered as material of the prism. The width of the triangular prism base is fixed at 60 um while the height is varied to obtain optimal focal spot parameters such as maximum intensity, full width and depth at half-maximum of intensity. The linear polarized Gaussian beam with a wavelength of 4 um and the waist width of 80 um is taken as the input radiation. All numerical simulations were carried out by COMSOL Multiphysics which used irregular grids with variable steps for FEM. The small step equal to 0.1 um was used in regions which are close to the interface between two media, while the other area is described by grids with steps of 0.2 um. Comparison of simulation results for dielectric triangular prism and simulation results for dielectric circular cylinder shows that prism can produce the narrower focal spot. It can be also noticed that there are resonance modes similar to the whispering gallery modes are formed inside the triangular dielectric prism. The influence of these modes on light focusing is also studied.
In this work, we numerically investigated focusing of a quasi-cylindrical optical vortex with azimuthal polarization and a wavelength of 532 nm. It was shown that the focal spot produced by a beam with four sectors focused with a Fresnel zone plate with a numerical aperture of NA = 0.95 does not differ from the ideally azimuthally polarized optical vortex; the difference in the focal spot diameter does not exceed 0.03λ. The four-sector binary subwavelength grating polarizer was fabricated in a golden film. It was experimentally demonstrated that a linearly polarized 532-nm Gaussian beam reflected at the polarizer was converted to an azimuthally polarized beam. Putting a spiral phase plate (SPP) with the topological charge <i>n</i> = 1 into the azimuthally polarized beam from the micropolarizer was experimentally shown to enable the conversion of the annular intensity pattern into a central intensity peak.
In this work, we fabricated and studied the performance of a 100×100-μm four-sector binary subwavelength grating polarizer in a golden film. It was experimentally demonstrated that a linearly polarized 532-nm Gaussian beam reflected at the polarizer was converted to an azimuthally polarized beam. Putting a spiral phase plate (SPP) with the topological charge <i>n</i> = 1 into the azimuthally polarized beam from the micropolarizer was experimentally shown to enable the conversion of the annular intensity pattern into a central intensity peak.
A binary subwavelength four-zone transmission grating micropolarizer for conversion of a linearly polarized incident laser beam into a azimuthally polarized beam with a phase shift of π at diametrically opposite points of the beam was synthesized and characterized. The proposed micropolarizer consists of four sectors with angles -60°, 60°, -60° and 60° with the y-axis. The micropolarizer has a period 230 nm, width of step 138 nm, and width of groove 92 nm. The micropolarizer was designed for wavelength 633 nm and was manufactured in silicon (refractive index <i>n</i> = 3.87 – 0.016<i>i</i>) spattered on a glass substrate. The size of micropolarizer was equal to 100×100 μm, and the microrelief height was equal to 130 nm. The performance of designed micropolarizer was simulated using FDTD-method. A linearly polarized plane wave of wavelength 633 nm was assumed to illuminate the polarizer at the normal incidence. The mesh of the FDTD method had a λ/30 step. The field distribution at a significant distance from the polarizer was calculated using the Rayleigh-Sommerfeld integral, with the FDTD-aided complex amplitude calculated 100-nm away from the surface taken as an initial field guess. It was shown that the obtained beam focused by Fresnel zone plate with focal length 532 nm produces focal spot with diameters FWHM<sub>x</sub> = 0.42λ and FWHM<sub>y</sub> = 0.81λ. Focal spot formed only by the transverse component of electric field has diameters FWHM<sub>x</sub> = 0.42λ and FWHM<sub>y</sub> = 0.59λ.
We discuss a four-Sector transmission Polarization Converter that enables the conversion of linearly polarized incident light into an azimuthally polarized beam. The resulting azimuthally polarized beam is characterized by a phase shift of π between the diametrically opposite beam points. Using scanning near-field optical microscope we experimentally show that by placing a Fresnel zone plate of focus 532 nm behind the four-sector micropolarizer, light can be focused into a subwavelength focal spot with smaller and larger sizes measuring FWHM = 0.46λ and FWHM = 0.57λ Numerically obtained focal spot of the transverse E-field component, which is measured by our scanning near-field optical microscope, has diameters FWHM<sub>x</sub> = 0.42λ and FWHM<sub>y</sub> = 0.59λ.
We have numerically and experimentally investigated subwavelength grating-polarizer that transform linearly polarized light of wavelength 633 nm into azimuthally polarized beam with a phase shift π at diametrically opposite points of the beam. This beam focused by Fresnel zone plate with focal length 532 nm produces focal spot with diameters equal to 0.42 and 0.81 of wavelength.
Tight focusing of a linearly-polarized asymmetric Bessel beam, which has a crescent profile, was investigated numerically and experimentally. FDTD calculations show that a binary zone plate of numerical aperture NA = 0.995 forms a crescent in the focal plane, which is rotated clockwise around the optical axis, moving away from the focal plane. Using the Debye formulae it was shown that a direction of polarization of incident light has a significant influence on intensity distribution in focal plane. The crescent in the focal plane was also observed experimentally by focusing of the asymmetric Bessel beam using an immersive objective (NA = 1.25).
Binary diffraction optical element was designed for polarization conversion from linear to radial. A grating period was equal to 400 nm, a relief height was equal to 110 nm. Simulation by FDTD method and Rayleight-Zommerfeld integral shown that there are radial polarized light beam in the far field with smooth angle dependence on the beam circle observation position. It was shown experimentally, that a gaussian laser beam with wavelength of 633 nm reflected from the polarization conversion plate contain a radially polarized light.
It was numerically and experimentally shown that square-profile microsteps with width of 0.4 μm, 0.5 μm, 0.6 μm, 0.8 μm and height of 500 nm illuminated by linearly polarized laser light with wavelength λ = 633 nm formed near its’ surface photonic nanojets with intensity that is 6 times higher than the intensity of incident light. Experimentally measured diameters of photonic nanojets were equal to 0.44λ, 0.43λ, 0.39λ and 0.47λ (less than the diffraction limit 0.51λ).
Proc. SPIE. 9031, Saratov Fall Meeting 2013: Optical Technologies in Biophysics and Medicine XV; and Laser Physics and Photonics XV
KEYWORDS: Optical components, Diffraction, Finite-difference time-domain method, Etching, Dielectrics, Near field scanning optical microscopy, Near field diffraction, Micro optics, Near field optics, Photonic nanostructures
The diffraction of a linearly polarized plane wave by a corner dielectric microstep of height equals of two incident
wavelengths was studied using finite-difference time domain method and near-field scanning optical microscopy. It was
shown that the corner step generates an elongated region of enhanced intensity, termed as a curved laser microjet. The
curved laser microjet has a length of about DOF = 9.5λ and the smallest diameter FWHM = (1.94±0.15)λ at distance z =