Subwavelength focusing of laser light of a mixture of linearly and azimuthally polarized beams
S.S. Stafeev, A.G. Nalimov, M.V. Kotlyar, L. O’Faolain
Image Processing Systems Institute оf RAS, – Branch of the FSRC “Crystallography and Photonics” RAS, Samara, Russia,
Samara National Research University, Samara, Russia,
School of Physics and Astronomy of the University of St. Andrews
Full text of article: Russian language.
We investigated a four-zone transmission polarization converter (4-TPC) for a wavelength of 633 nm, enabling the conversion of a linearly polarized incident beam into a mixture of linearly and azimuthally polarized beams. It was numerically shown that by placing a Fresnel zone plate of focal length 532 nm immediately after the 4-TPC, the incident light can be focused into an oblong subwavelength focal spot whose size is smaller than the diffraction limit (with larger and smaller size, respectively, measuring FWHM = 0.28λ and FWHM = 0.45λ, where λ is the incident wavelength and FWHM stands for full-width at half maximum of the intensity). If after passing through a 4-TPC, light propagates in free space over a distance of 300 um before being focused by a Fresnel zone plate, the resulting focal spot was found to measure 0.42λ and 0.81λ (with the focal spot contributed to just by the transverse E-field components measuring 0.42λ and 0.59λ). This numerical result was verified experimentally, giving a focal spot of smaller and larger size, respectively, measuring 0.46λ and 0.57λ.
subwavelength micropolarizer, azimuthal polarization, subwavelength grating, tight focusing, near-field microscopy, FDTD, polarization selective devices.
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