Using e-beam lithography and reactive ion beam etching of a 120 nm amorphous silicon film, we fabricate a 30-µm spiral metalens that has a very short focal length that is equal to the incident 633 nm wavelength (numerical aperture about 1) and is composed of 16-sectored subwavelength binary gratings with a 220 nm period. Its unique feature is that the metalens generates a left-hand circularly polarized optical vortex with topological charge 2 when illuminated by a left-hand circularly polarized light but generates a cylindrical second-order vector beam when illuminated by linearly polarized light. Both for linearly and circularly polarized light, an inverse energy flow occurs near the strong focus of the metalens. Transverse near-focus intensity distributions measured with a scanning near-field optical microscope are found to be in qualitative agreement with those obtained via the FDTD-aided numerical simulation. Thus, the near-focus reverse energy flow is experimentally confirmed to occur.

使用电子束光刻和120 nm非晶硅膜的反应性离子束刻蚀，我们制造了30 µm螺旋金属，该金属具有非常短的焦距，该焦距等于入射633 nm波长（数值孔径约为1），并且由16个扇形的亚波长二进制光栅组成，周期为220 nm。其独特之处在于，金属元素在被左旋圆偏振光照射时会产生带有拓扑电荷2的左旋圆偏振光学涡流，而在被线偏振光照射时会产生圆柱二阶矢量束。对于线性和圆偏振光，在金属离子的强焦点附近都会发生逆能量流。发现用扫描近场光学显微镜测得的横向近焦点强度分布与通过FDTD辅助的数值模拟获得的横向定焦质量定性一致。因此，通过实验确认了发生了近焦点反向能量流。