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 =