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Flat optical devices thinner than a wavelength promise to replace conventional free-space components for wavefront and polarization control. Transmissive flat lenses are particularly interesting for applications in imaging and on-chip optoelectronic integration. Several designs based on plasmonic metasurfaces, high-contrast transmitarrays and gratings have been recently implemented but have not provided a performance comparable to conventional curved lenses. Here we report polarization-insensitive, micron-thick, high-contrast transmitarray micro-lenses with focal spots as small as 0.57 l. The measured focusing efficiency is up to 82%. A rigorous method for ultrathin lens design, and the trade-off between high efficiency and small spot size (or large numerical aperture) are discussed. The micro-lenses, composed of silicon nano-posts on glass, are fabricated in one lithographic step that could be performed with high-throughput photo or nanoimprint lithography, thus enabling widespread adoption.

Varifocal lenses, conventionally implemented by changing the axial distance between multiple optical elements, have a wide range of applications in imaging and optical beam scanning. The use of conventional bulky refractive elements makes these varifocal lenses large, slow, and limits their tunability. Metasurfaces, a new category of lithographically defined diffractive devices, enable thin and lightweight optical elements with precisely engineered phase profiles. Here we demonstrate tunable metasurface doublets, based on microelectromechanical systems (MEMS), with more than 60 diopters (about 4%) change in the optical power upon a 1-μm movement of one metasurface, and a scanning frequency that can potentially reach a few kHz. They can also be integrated with a third metasurface to make compact microscopes (~1 mm thick) with a large corrected field of view (~500 μm or 40 degrees) and fast axial scanning for 3D imaging. This paves the way towards MEMS-integrated metasurfaces as a platform for tunable and reconfigurable optics.

Optical metasurfaces are two-dimensional arrays of nano-scatterers that modify optical wavefronts at subwavelength spatial resolution. They are poised to revolutionize optics by enabling complex low-cost systems where multiple metasurfaces are lithographically stacked and integrated with electronics. For imaging applications, metasurface stacks can perform sophisticated image corrections and can be directly integrated with image sensors. Here we demonstrate this concept with a miniature flat camera integrating a monolithic metasurface lens doublet corrected for monochromatic aberrations, and an image sensor. The doublet lens, which acts as a fisheye photographic objective, has a small f-number of 0.9, an angle-of-view larger than 60° × 60°, and operates at 850 nm wavelength with 70% focusing efficiency. The camera exhibits nearly diffraction-limited image quality, which indicates the potential of this technology in the development of optical systems for microscopy, photography, and computer vision.

Metasurfaces are nanostructured devices composed of arrays of subwavelength scatterers (or meta-atoms) that manipulate the wavefront, polarization, or intensity of light. Like most other diffractive optical devices, metasurfaces are designed to operate optimally at one wavelength. Here, we present a method for designing multiwavelength metasurfaces using unit cells with multiple meta-atoms, or meta-molecules. A transmissive lens that has the same focal distance at 1550 and 915 nm is demonstrated. The lens has a NA of 0.46 and measured focusing efficiencies of 65% and 22% at 1550 and 915 nm, respectively. With proper scaling, these devices can be used in applications where operation at distinct known wavelengths is required, like various fluorescence microscopy techniques.

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Dielectric metasurfaces are two-dimensional structures composed of nano-scatterers that manipulate phase and polarization of optical waves with subwavelength spatial resolution, enabling ultra-thin components for free-space optics. While high performance devices with various functionalities, including some that are difficult to achieve using conventional optical setups have been shown, most demonstrated components have a fixed functionality. Here we demonstrate highly tunable metasurface devices based on subwavelength thick silicon nano-posts encapsulated in a thin transparent elastic polymer. As proof of concept, we demonstrate a metasurface microlens operating at 915 nm, with focal distance tuning from 600 µm to 1400 µm through radial strain, while maintaining a diffraction limited focus and a focusing efficiency above 50%. The demonstrated tunable metasurface concept is highly versatile for developing ultra-slim, multi-functional and tunable optical devices with widespread applications ranging from consumer electronics to medical devices and optical communications.Metasurfaces are composed of a large number of discrete nano-scatterers (meta-atoms) that locally modify phase and polarization of light with subwavelength spatial resolution. The meta-atoms can be defined lithographically, thus providing a way to mass-produce thin optical elements [1][2][3][4] that could directly replace traditional bulk optical components or provide novel functionalities [4,5]. The two dimensional nature and the subwavelength thickness of metasurfaces make them suitable for tunable and reconfigurable optical elements. Some efforts have recently been focused on developing tunable and reconfigurable metasurfaces using different stimuli for tuning the meta-atoms. Examples include frequency response tuning using substrate deformation [6,7], refractive index tuning via thermo-optic effects [8,9], phase change materials [10,11], and electrically driven carrier accumulation [12,13].Stretchable substrates have been used to demonstrate tunable diffractive and plasmonic metasurface components [14][15][16], but they have exhibited low tunability, poor efficiency, polarization dependent operation, or significant optical aberrations. Here we present mechanically tunable dielectric metasurfaces based on elastic substrates, simultaneously providing a high tuning range, polarization independence, high efficiency, and diffraction limited performance. As a proof of principle, we experimentally demonstrate an aspherical microlens with over 130% focal distance tuning (from 600 µm to 1400 µm) while keeping high efficiency and diffraction limited focusing.Figure 1(a) shows a schematic of a metasurface microlens encapsulated in an elastic substrate with radius r and focal distance f . The phase profile of the lens has the following form, and is drawn in Fig. 1(c) (solid blue curve):where ρ is the distance to the center of the lens. Equation (1) in the paraxial approximation (ρ f ) reduces to By stretching the metasurface microlens with a stretch ratio o...

Diffraction gratings disperse light in a rainbow of colors with the opposite order than refractive prisms, a phenomenon known as negative dispersion. While refractive dispersion can be controlled via material refractive index, diffractive dispersion is fundamentally an interference effect dictated by geometry. Here we show that this fundamental property can be altered using dielectric metasurfaces, and we experimentally demonstrate diffractive gratings and focusing mirrors with positive, zero, and hyper-negative dispersion. These optical elements are implemented using a reflective metasurface composed of dielectric nano-posts that provide simultaneous control over phase and its wavelength derivative. In addition, as a first practical application, we demonstrate a focusing mirror that exhibits a five-fold reduction in chromatic dispersion, and thus an almost three-times increase in operation bandwidth compared with a regular diffractive element. This concept challenges the generally accepted dispersive properties of diffractive optical devices and extends their applications and functionalities.