A high numerical aperture terahertz all-silicon metalens with sub-diffraction focus and long depth of focus

Abstract A high numerical aperture (NA = 0.92) square all-silicon metalens is designed for a wavelength of λ = 0.11883 mm with a focal length of 16.8 λ and a side length of 104.4 λ . Based on the waveguide effect, anisotropic rectangular silicon pillars with different dimensions are arranged on the silicon substrate according to the conical phase distribution, and then the rotation angle of each rectangular pillar is adjusted to realize the polarization conversion. Furthermore, a polarization-independent square antireflection silicon pillar array is proposed to be patterned at the other side of the substrate. The metalens can efficiently convert the normally incident x - (or y -) polarized plane wave into the radially (or azimuthally) polarized beam and conduct the sub-diffraction tight-focusing simultaneously, thus the shape of the focus can be flexibly switched. The numerical simulation results show that, under high-NA conditions, our metalens can obtain a tighter focus compared with linearly polarized metalens and a longer depth of focus (DOF) compared with metalens based on the hyperboloidal phase distribution. Under x - (or y -) polarized plane wave incidence, a circular-spot-shaped (or doughnut-shaped) focus with a full width at half maximum of 0.4 λ (or 0.3 λ ) is obtained and the DOF is 26.509 λ (or 18.935 λ ). In addition, under x -polarized beam incidence, the influence of the incident beam intensity distribution on the focusing results is also discussed. The use of a single material makes our metalens more favorable in fabrication and it is expected that our study is of great significance for applying terahertz wave in imaging and communication systems, high-resolution microscopy, optical coherence tomography and manipulation of particles.