Varifocal optics have a variety of applications in imaging systems. Metasurfaces offer control of the phase, transmission, and polarization of light using subwavelength engineered structures. However, conventional metasurface designs lack dynamic wavefront shaping which limits their application. In this work, we design and fabricate 3D doublet metalenses with a tunable focal length. The phase control of light is obtained through the mutual rotation of the singlet structures. Inspired by Moiré lenses, the proposed structure consists of two all-dielectric metasurfaces. The singlets have reverse-phase profiles resulting in the cancellation of the phase shift in the nominal position. In this design, we show that the mutual rotation of the elements produces different wavefronts with quadratic radial dependence. Thus, an input plane wave is converted to spherical wavefronts whose focal length depends on the rotation. We use a combination of a nanopillar and a phase plate as the unit cell structure working at a wavelength of 1500 nm. Our design holds promise for a range of applications such as zoom lenses, microscopy, and augmented reality.
Metasurfaces offer a unique platform to precisely control optical wavefronts and enable the realization of flat lenses, or metalenses, which have the potential to substantially reduce the size and complexity of imaging systems and to realize new imaging modalities. However, it is a major challenge to create achromatic metalenses that produce a single focal length over a broad wavelength range because of the difficulty in simultaneously engineering phase profiles at distinct wavelengths on a single metasurface. For practical applications, there is a further challenge to create broadband achromatic metalenses that work in the transmission mode for incident light waves with any arbitrary polarization state. We developed a design methodology and created libraries of meta-units—building blocks of metasurfaces—with complex cross-sectional geometries to provide diverse phase dispersions (phase as a function of wavelength), which is crucial for creating broadband achromatic metalenses. We elucidated the fundamental limitations of achromatic metalens performance by deriving mathematical equations that govern the tradeoffs between phase dispersion and achievable lens parameters, including the lens diameter, numerical aperture (NA), and bandwidth of achromatic operation. We experimentally demonstrated several dielectric achromatic metalenses reaching the fundamental limitations. These metalenses work in the transmission mode with polarization-independent focusing efficiencies up to more »
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- Light: Science & Applications
- Nature Publishing Group
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- National Science Foundation
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