Abstract
It is commonly admitted that radial polarization leads to the best light beam focalisation in terms of spot size. The work presented here shows that the choice of polarization is in fact more subtle. As an example, it is pointed out that, for numerical apertures of focusing systems below 0.8, circular polarization induces the best light confinements in air, whereas for numerical apertures beyond 0.8, radial polarization seems to be the right solution. Moreover, for numerical apertures larger than 1, we propose an original solution to achieve the ultimate focal spots in linear optical regime. They are estimated to 0.36λ/NA (λ is the wavelength and NA is the numerical aperture), the current technological limit being around 65nm.
Highlights
In previous papers [1,2], Bessel beams have been presented as one powerful solution to generate confined focal spots usable in superresolution imaging [3,4,5,6,7,8,9]
It turns out that the spot size lowest limit is reached in radial polarization when the index of refraction ns of the medium containing the transmitted Bessel beam matches the numerical aperture
The influence of polarization on Bessel beam generation has been analyzed in order to find out the procedure which generates the smallest focal spots, whatever the NA
Summary
In previous papers [1,2], Bessel beams have been presented as one powerful solution to generate confined focal spots usable in superresolution imaging [3,4,5,6,7,8,9]. Since the generation of a light spot requires a non null central intensity, Bessel beams (and more generally, focused beams) can generate a light spot provided that the polarization produces function J0 in one of the three field components at least. We see from Eq (12) that no light confinement can be generated with azimuthally polarized Bessel beams Such beams give the smallest dark spot which can be generated by focusing systems [23,2]. The role of polarization on the light confinement process is double It fully defines whether a light spot can be generated or not. It participates in the extension of the light spot by setting the relative weights of the various Bessel functions involved in the intensity expression. The influence of polarization on the spot size we will be analyzed
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