Stafeev S S, Nalimov A G, Kotlyar M V, Gibson D, Song S, O'Faolain L, Kotlyar V V
Opt Express. 2016 Dec 26;24(26):29800-29813. doi: 10.1364/OE.24.029800.
We have investigated a four-sector transmission polarization converter (4-SPC) for a wavelength of 633 nm, that enables 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-SPC, the incident light can be focused into an oblong subwavelength focal spot whose size is smaller than the diffraction limit (with width and breadth, 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). After passing through the 4-SPC, light propagates in free space over a distance of 300 μm before being focused by a Fresnel zone plate (ZP), resulting in focal spot measuring 0.42λ and 0.81λ. The focal spot was measured by a near-field microscope SNOM, and the transverse E-field component of the focal spot was calculated to be 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. To our knowledge, this is the first implementation of polarization conversion and subwavelength focusing of light using a pair of transmission micro-optic elements.
我们研究了一种用于波长633nm的四扇区传输偏振转换器（4-SPC），它能将线偏振入射光束转换为线偏振光和方位角偏振光的混合光。数值计算表明，在4-SPC之后立即放置一个焦距为532nm的菲涅耳波带片，入射光可以聚焦成一个椭圆形的亚波长焦点，其尺寸小于衍射极限（宽度和长度分别测量为半高宽FWHM = 0.28λ和FWHM = 0.45λ，其中λ是入射波长，FWHM代表强度的半高全宽）。光通过4-SPC后，在自由空间中传播300μm的距离，然后由菲涅耳波带片（ZP）聚焦，得到尺寸为0.42λ和0.81λ的焦点。通过近场显微镜SNOM测量焦点，计算出焦点的横向电场分量为0.42λ和0.59λ。这一数值结果通过实验得到了验证，分别得到了尺寸更小和更大的焦点，测量值为0.46λ和0.57λ。据我们所知，这是首次使用一对传输微光学元件实现光的偏振转换和亚波长聚焦。
