The amplitude and polarization of cylindrical vector beams (CVBs) are distributed cylindrically and symmetrically, and the tight CVBs focusing plays an important role in optical micromanipulation, optical storage, laser micromachining, super-resolution imaging, particle acceleration, and other fields. At present, various focusing methods have been developed, such as traditional lenses, plasmonic lenses, negative refractive photonic crystal lenses, parabolic mirrors, and meta-lenses. However, there are limitations including diffraction limit, polarization dependence, and complex preparation. Subwavelength grating lens based on -1st order diffraction can achieve tight focusing of radial and azimuthal polarized lights spontaneously, breaking through the diffraction limit and realizing flexible focal field manipulation. Despite these advantages, the energy efficiency of its focal field still deserves further improvement. Therefore, we explore and propose a structural optimization scheme for a blazed subwavelength grating lens that can increase the energy ratio of -1st order diffracted light energy to enhance the focal field energy.
We employ the full vector calculation of electromagnetic field (COMSOL Multiphysics software) based on the finite element method (FEM) to carry out specific research. The blazed structure is located on each grating step with a consistent height, and the overall lens structure is a uniform dielectric GaN. Firstly, an equivalent triangular prism model is built to verify the enhancement effect of the blazed structure on -1st order diffraction. Next, the energy and morphology changes of the focal field before and after modifying the blazed structure are compared, and the influence of the height, number, and location of blazed structures on the focal field is analyzed. Finally, the dynamic manipulation effect of the incident light amplitude distribution and polarization components on the focal field energy and morphology is studied.
We propose a blazed subwavelength grating lens that can improve the diffraction efficiency of -1st order diffracted light and enhance the focal field energy of the negative refractive grating lens. As the preset focal length increases, the height of the blazed structure that satisfies the maximum diffraction efficiency of the lens also rises. The increasing number of blazed structures leads to more balanced energy of the outgoing beams in different regions and higher energy of the focal field. Meanwhile, the ability of the focal field to suppress the secondary focus is stronger, and the focal position is more accurate. By adjusting incident Gaussian radially polarized light, the dynamic control of the focal field energy is realized. Changing the polarization components of CVBs can also achieve lateral focusing modulation and obtain focal fields with diverse morphology. Finally, our study provides ideas for optimizing the focusing performance of subwavelength grating lenses and has potential applications in optical micromanipulation, super-resolution imaging, and other fields.