Due to the natural dispersion effect of the material itself, chromatic aberration is produced when light is incident, so it is important to eliminate chromatic aberration, especially for metalens. We use the phase change material (PCM) Ge2Sb2Te5 (GST-225) in the wavelength range of 4 to 5 μm and design two-layer vortex metalenses, the first one at a single wavelength of 5 μm, to verify the feasibility of a two-layer metasurface for vortex metalens. The second is a two-layer vortex achromatic metalens, where the two layers are modulated by the propagation phase and geometric phase, respectively, and the design of the vortex achromatic metalens is achieved by a combination of compensation phase and adjustment of the crystallization rate of the PCM. The results show that the designed metasurface achieves achromatic effect in the designed wavelength band, which provides an idea for the design of the achromatic vortex metalens.
Metamaterials are widely used as an artificial composite material in optical systems, however, almost all imaging systems suffer from chromatic aberrations due to their inherent dispersion. In this paper, we demonstrate the achromatic bifocal metalens based on phase change materials by tuning the crystalline fraction of Ge2Sb2Te5 in a linear-phasegradient metasurface, which can realize off-axis and co-axial achromatic bifocal metalens in the continuous waveband from 9.5 to12μm. Simulation results show that average focusing efficiency of the off-axis metalens in the working band is about 28.96%, average focusing efficiency of the co-axial metalens is 50.2% with the full width at half-maximum is close to the diffraction limit. This design will play an important role in the field of fluorescence microscopy, which can achieve achromatic objective and eyepiece metalens with different focal lengths over a wide waveband.
Based on double-layer Sb2S3 material, a method is proposed to realize adjustable metalens at 1.31 μm using geometric phase to regulate the incident beam. Different functions are realized by changing different states of Sb2S3. In design 1, the lower layer of phase change material is set as a half-wave plate in the amorphous and crystalline states, and the upper layer can be switched between half-wave plate and full-wave plate when it is in the two states so that the focal length of the metalens can be switched between F1 and F2. The full width at half maximum (FWHM) is close to the diffraction limit, and the focus efficiency can reach 69%. In design 2, the upper layer is always in the amorphous state with the high transmittivity. An optical switch is realized when the lower layer is used as a half-wave plate with the transmittivity close to 0 in the crystalline state. At the same time, a bifocal metalens with the focusing efficiency of 62% can be realized when the lower layer is used as a half-wave plate with the high transmittivity in the amorphous state. By adjusting the material crystallization rate, bifocal metalens with various intensities are realized. Our design has great potential in optical imaging, scanners that combine beam switches and lenses, and dual-function devices.
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