Fig. 1. Schematic concept and optical characterization of the all-dielectric metalens. (a) The conceptual image illustrates the trapping of a polystyrene microbead with the help of an all-dielectric metalens, which converts an RCP incident Gaussian beam to a focused LCP beam. (b) SEM image of the fabricated metalens that consists of amorphous silicon nanofin array. The red inset shows a region at the edge of the metalens. (c) and (d) Cross sections of the intensity distribution of the focused beam along the optical axis for incident beams with different circular polarizations, drawn on a logarithmic scale. For LCP (RCP) illumination, the metalens acts as a concave (convex) lens, which results in a virtual (real) focal point at z=−530  μm (z=+530  μm). The metalens is located at z=0. (e) and (g) Transverse intensity distributions at the virtual (real) focal point position, drawn on a linear scale. (f) and (h) Red dots: 1D intensity cross sections along the white dashed line shown in panels (e) and (g), respectively. Black dashed line presents Gaussian fits to the measured data points, which provide an FWHM of 0.9 μm in both considered cases.

Fig. 2. Measurement of the metasurface diffraction efficiency and the optical characterization of the vortex metalens. (a) Schematic image of the diffraction efficiency measurement: the metasurface diffraction grating with LCP incident light deflects the RCP beam to the −1st order of diffraction, while the unconverted LCP part remains at the zeroth diffraction order. (b) The cross-section intensity distribution of the 1st, 0th, and −1st orders of diffraction for the incident LCP and RCP beams, further divided into the respective co- and cross-polarization states. (c) and (d) The cross-section intensity distributions of the beams converted by the vortex metalens along the optical axis on a logarithmic scale for better visibility. For LCP to RCP (RCP to LCP) conversion, the metalens acts as a concave (convex) vortex lens, which results in a virtual (real) focal point at z=−545  μm (z=+545  μm). The helical phase factor results in zero intensity on the optical axis in the focal region. (e) and (g) Transverse intensity distributions of the donut-shaped beam profiles at the focal point positions indicated by white arrows in panels (c), (d), drawn on a linear scale. (f) and (h) Red dots: 1D intensity cross sections along the white dashed lines shown in panels (e) and (g), respectively.

Fig. 3. Schematic illustration of the measurement setup. A nonpolarizing beam splitter (BS) is used to insert white light (WL) for sample illumination at the front side of the metasurface (MS, either metalens, vortex metalens, or metasurface diffraction grating) and to measure the incident laser power with a power meter (PM). Three different configurations can be used (blue dashed boxes). (a) Metalens-based optical tweezers system. Laser light with the desired polarization state is focused by the metalens onto the microbeads sample (S) while the focal plane of the metalens is imaged on the camera (CAM) through a microscope objective (MO) and a tube lens (TL). (b) Optical propagation measurement setup. The polarization states were separated by a polarization analyzer consisting of a quarter-wave plate (Q) and a linear polarizer (P). (c) Setup for efficiency measurement with the metasurface diffraction grating. (d) Arrangement of the MS and S in the metalens-based optical tweezers system. H, half-wave plate; CL, collimating lens; F, filter; f1, f2, and f3, lenses.

Fig. 4. Metalens for 2D polarization-sensitive drag and drop manipulation of particles. Polystyrene particles with diameters of (a) 4.5 μm, (b) 3.0 μm, (c) 2.0 μm, and (d) 4.5 μm are dispersed in water and arranged by polarization-sensitive drag and drop using the metalens. (e) Radial stiffness kr versus power in trapping plane P for polystyrene particles with a diameter of 4.5 μm. (f) GLMT simulation for the radial stiffness of optical traps with different particle refractive indices and various particle diameters. Dashed lines indicate the experimental values of the particle size and the relative refractive index.

Fig. 5. Vortex metalens for OAM transfer. (a) A vortex metalens with a numerical aperture of NA=0.35 is used to generate a donut-shaped intensity distribution in the focal spot region. At t=0  s a particle (4.5 μm diameter) is attracted by the lateral gradient force. The OAM is transferred onto the particle, resulting in a clockwise rotational movement. Simultaneously, the particle is slowly pushed out of the trap in the axial direction. (b) Trajectory plot of the particle’s rotational movement. (c) Radial stiffness kr of the vortex trap versus power in trapping plane P.