Lenses with tunable focal length are crucial to the operation of many optical systems, as in photography, mixed reality, and microscopy. Various technologies exist that support this behavior, but they often entail high power consumption and rely on bulky and expensive optical components. With recent advances in metasurface optics in miniaturizing and augmenting traditional systems, these devices may enable the next generation of varifocal lenses. These devices are flat optical elements comprising arrays of subwavelength-spaced scatterers that can impart spatially varying phase, amplitude, and polarization changes on wavefronts. In recent years, this field has attracted substantial research interest and has produced several demonstrations of focal length adjustable metalenses. These techniques, however, often rely on high control voltages to apply a strain to a flexible substrate or depend on microelectromechanical actuators that require sophisticated fabrication and cannot scale to large area apertures. Here, we discuss our work developing and building a 1 cm aperture Alvarez lens metasurface system with which we demonstrate a focal length tuning range of 6 cm (>200% change) at 1550 nm wavelength. We also design 1 mm Alvarez lens-inspired higher order metasurfaces for full-color imaging when combined with post-capture deconvolution. Using both designs, we demonstrate varifocal zoom imaging.
In recent years, diffractive, discrete scatterer based optics such as metasurfaces have shown considerable promise in the realization of arbitrary optical functions. However, these optical elements are systems large numbers of tunable degrees of freedom that are impractical to tune using forward design methods. In parallel, there has been great progress in using computational inverse design methods to produce high quality nanophotonic elements. We show that this inverse design method is capable of handling the large scale of the three-dimensional electromagnetic scattering problem, and leads to a realistic path towards the computational design and optimization of these discrete scatterer based optics capable of performing arbitrary optical functions in the far field. Then, we present an experimental demonstration of an optical element at 1.55 μm that focuses light into a discrete helical pattern that is designed using an inverse method based on generalized Lorenz-Mie scattering theory. This optical function is realized by specifying a suitable figure of merit that encapsulates the performance of the optical element. The fabrication of these optical elements with such small length scales is done using the Nanoscribe GT two-photon lithography system.
The miniaturization of current image sensors is primarily limited by the volume of optical elements. Using a subwavelength patterned quasi-periodic structure known as a metasurface, we can build planar optical elements based on the principle of diffraction. This platform allows us to mimic complex, asymmetric curvatures with ease and is ideal for the adaptation of freeform optics to the micron scale. The implementation of freeform optics on metasurfaces allows for extreme miniaturization of optical components. In our research we have demonstrated metasurface based optical elements such as lenses, vortex beam generators, and cubic phase plates near visible frequencies. Our fabricated lenses achieved beam spots of less than 1 μm with numerical apertures as high as ~ 0.75. We observed a transmission efficiency of 90% and focusing efficiency of 40% in the visible wavelengths. In addition, we have demonstrated a dynamic metasurface optical system called the Alvarez lens with a tunable focal length range of over 2.5 mm corresponding to a change in optical power of ~1600 diopters with 100 m of total mechanical displacement.
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