Flat free-space optical elements based on dielectric metasurfaces

High-performance, ultra-thin optical components that can be mass manufactured at low cost using nanofabrication techniques introduce a new paradigm for optical-system design. These devices can provide capabilities similar to those of conventional components such as lenses, polarizers, prisms, or beam splitters, and may replace them in optical systems. They can also offer new functionalities that are currently difficult to achieve, with a single flat component being optically equivalent to an assembly of conventional components. Metasurfaces—rationally designed 2D arrays of optical scatterers—are promising candidates for implementing flat optical components, and they have attracted significant interest in the past few years.¹

We have recently developed a new dielectric metasurface platform that provides wavefront and polarization control with subwavelength spatial resolution and high transmission efficiency.² The platform is based on silicon nanoposts with elliptical cross section located on a transparent substrate. The nanoposts, which are shorter than the free-space wavelength, are arranged in a hexagonal or square lattice: see Figure 1(a). We vary the diameters and the in-plane orientation of elliptical-nanopost axes across the metasurface, such that optical waves incident at different locations undergo different phase shifts and polarization conversions: see Figure 1(b). We can achieve any desired spatially varying polarization and phase distributions for the transmitted light by properly selecting nanopost diameters and their in-plane rotation angle: see Figure 1(c). The nanoposts operate as weakly coupled scatterers, which allows for independent control of the phase shift and polarization conversion at the location of each nanopost. Therefore, each unit cell of the lattice is considered as a pixel designed to perform any arbitrary polarization and phase conversion: see Figure 1(d). The lattice constants of these metasurfaces are subwavelength (usually close to half a wavelength), which leads to the high-resolution wavefront and polarization shaping.

Figure 1. (a) Schematic drawing of a dielectric metasurface device composed of an array of elliptical silicon nanoposts on a transparent substrate. (b) Spatially varying modification of both polarization and phase of light transmitted through a metasurface by the nanoposts. (c) Top view of the metasurface depicted in (b), showing nanoposts with different diameters (D₁ and D₂) and in-plane rotation angles (θ). (d) A scanning electron microscope image of a fabricated metasurface for simultaneous manipulation of wavefront and polarization. A drawing of the hexagonal lattice is overlaid on the image.

Polarization-insensitive phase masks are a special category of devices that can be realized by using nanoposts with circular cross sections.^3–6Lenses are important examples from this category because of their potential widespread application. We can use lenses as benchmarks for comparing different metasurface platforms. We have experimentally demonstrated efficient metasurface lenses operating at wavelengths ranging from visible (590nm) to mid-IR (4.8μm): see Figure 2. The silicon absorption loss at wavelengths shorter than 550nm becomes appreciable, thus limiting the device transmission, but other materials with lower absorption loss may be used at these wavelengths.⁷

Figure 2. Metasurface lenses for different wavelengths and with different sizes. (a) A 100μm-diameter metasurface lens for 1550nm. (b) A 1cm-diameter birefringent metasurface lens for 590nm. The lens is made of 700 million nanoposts. (c) A 4mm-diameter metasurface lens for 650nm. The inset shows magnification of the letter ‘a’ by the lens. (d) A 5×5array of mid-IR metasurface lenses on a sapphire substrate for operation at 4.8μm. The lenses have a numerical aperture of 0.86 and collimate quantum cascade lasers with 79% efficiency.³

We can also vary the lens size and its numerical aperture (NA) over a wide range. We have demonstrated lenses with diameters ranging from tens of micrometers to 1cm, and larger diameter lenses are also achievable. Our team has also realized metasurface lenses with NA as high as 0.95.⁴ Figure 3 shows the measurement results for five different metasurface lenses with the same diameter of 300μm and different focal lengths. The lenses focus light emitted from the cleaved facet of a fiber to points at different distances (d) from the lens: see Figure 3(a). Focal spot sizes agree well with their corresponding diffraction limited values, indicating very small spherical aberrations: see Figure 3(b). We measured more than 80% focusing efficiency for lenses with NA smaller than 0.55. For higher NAs, there is a trade-off between the focusing efficiency and the NA, which we can ease by using smaller lattice constants. By using a lattice constant of ∼0.4λ (where λ is the wavelength of interest), we recently reported a mid-IR metasurface lens with NA of 0.86 and efficiency of 79%.³ These metasurfaces can also be transferred to flexible substrates and conform to arbitrary shapes to change their optical responses.⁸ Besides lenses, other types of efficient polarization-insensitive phase masks with arbitrary phase profiles can be implemented using this platform. Such phase masks have potential applications in Fourier plane microscopy imaging and free-space optical communications using vortex beams.⁹

Figure 3. Performance characteristics of metasurface lenses.⁴(a) Schematic illustration of high numerical aperture focusing of light emitted from a single-mode fiber using a metasurface lens. (b) Experimental results of the spot size and efficiency for metasurface lenses illuminated as seen in (a). We can see the full width at half-maximum (FWHM) of the focal spot and the focusing and transmission efficiencies for five lenses with different focusing distance (d). The transmission efficiency is defined as the ratio of the transmitted power to the incident power, and the focusing efficiency is the ratio of the power focused by the lens to the incident power. The lenses operate at the wavelength of 1550nm.

In the more general case, the metasurfaces made of elliptical nanoposts can simultaneously shape polarization and phase of light,^{2, 10} thus enabling two different categories of devices. The devices in the first category implement independent phase masks (two phase holograms in the most general case) for two orthogonal polarizations (e.g. vertically and horizontally polarized light): see Figure 4(a). Other examples of devices from this category include birefringent lenses and polarization beam splitters that direct the light at arbitrarily chosen angles. The second category of devices generate any desired vector beams. For example, the device in Figure 4(b) generates converging radially or azimuthally polarized vector beams when illuminated with horizontally or vertically polarized light, respectively. Vector beams have applications in single-molecule localization microscopy¹¹ and laser machining.

Figure 4. Complete control of polarization and phase with dielectric metasurfaces.²(a) A polarization-switchable phase hologram. The metasurface projects two different patterns depending on the polarization of the incident light. (b) Simultaneous generation and focusing of vector beams. The metasurface generates and focuses radially or azimuthally polarized beams depending on the polarization of the incident beam. We show schematics on the left, simulation and measurement results in the middle, and optical and scanning electron micrographs of the metasurfaces on the right. The nanoposts are 715nm tall and the metasurfaces operate at 915nm. a.u.: Arbitrary units.

Our dielectric metasurface devices exhibit impressive performance, which makes them suitable for integration as stand-alone components in various optical setups. However, the greatest advantage of our technology stems from the possibility to lithographically fabricate stacks of multiple components on top of each other, which will eventually impact most technologies that employ free-space optics. For example, one immediate application is in imaging systems where the imaging optics can be fabricated directly on top of image sensors. These are some of the topics of our future work.