Metalens with convex and concave functionality

Lenses are the key components of telescopes, microscopes, and cameras and are used by the semiconductor industry in optical lithography to manufacture nano-electronic devices and integrated circuits. A conventional lens, which is usually made of glass or other transparent materials, has a curved surface and a fixed focal length. It is either convex (that is, it converges light to a focused spot) or concave (it diverges light). The surface topology of a conventional lens provides the necessary phase accumulation for bending the light in a desired way.

The progress in metamaterials has provided alternative routes for manipulating the propagation of light. Metamaterials are artificial photonic structures with deep-subwavelength building blocks, the so-called meta-atoms or meta-molecules, which play a similar role to the constituent molecules or atoms in natural materials. Benefiting from flexibility in engineering the shape and structure of meta-atoms, researchers have manufactured lenses with various unconventional optical properties, including artificial optical magnetism, invisibility, and negative refractive index.

Recently, scientists proposed a new class of metamaterial surfaces (or metasurfaces) capable of generating abrupt phase discontinuity. This effect occurs for transmitted light with a polarization state orthogonal to that of the incident light. The metasurface can be designed for linear polarizations, such as the ‘V’-shaped plasmonic antennas for the original implementation of the idea.^1,2 In our work, we designed the metasurface for circular polarizations by using simple metallic nanorods with carefully designed orientations.^3,4 In this case, the phase discontinuity only depends on the orientation of the nanorods, and consequently our lens represents a robust way of generating desired phase profiles on the metasurface. More importantly, we can reverse the phase discontinuity by simply flipping the circular polarization of the incident beam.

Based on the polarization-switchable phase discontinuity on a metasurface, we designed a dual-polarity planar ‘metalens’ for visible light: see Figure 1(a).⁴ The device consists of an array of metallic nano-rods fabricated on top of a glass substrate: see Figure 1(b). Each rod, having a dimension of 40nm (width) by 200nm (length) by 40nm (height), is deep subwavelength in size and can thus be considered a dipole antenna. The entire rod pattern works like a lens because each rod controls the local wavefront of the input light beam through its orientation. The rod pattern thus generates a phase deformation of the light wave and can be used for focusing or imaging. Further, when the circular polarization of light is reversed, from a left circular polarization to right circular polarization or vice versa, the phase profile is also reversed. Thus, a concave lens designed for a particular circular polarization would be transformed into a convex lens when illuminated by the opposite circular polarization: see Figure 2. The same device can, therefore, function as a convex or concave lens (and hence can be used to magnify or demagnify objects, and to form real or virtual images) depending on the polarization state of the light passing through it: see Figure 3.

Figure 1. Schematic of a metalens with interchangeable polarity and a scanning electron microscopy (SEM) image of the fabricated plasmonic lens. (a) The focusing property of the same metalens can be interchangeable between a convex lens and a concave lens by controlling the helicity of the incident light. (b) SEM image of the 2D dual-polarity plasmonic lens (top view). RCP: Right circular polarization. LCP: Left circular polarization.

Figure 2. Lens polarity is switched by changing the helicity of the incident light. Optical microscope images at virtual focal plane (left), lens surface (middle), and real focal plane (right) for the incident light with (a) RCP and (b) LCP. The laser beam is incident on the lenses from the left along the z direction, and the lens is at z = 0.

Figure 3. Experimental demonstration of imaging with a metalens. (a) A ‘T’ slit array with a pitch of 20μm along two directions is used as an object. (b) Real and inverted images for RCP incident light. (c) Virtual and upright images for LCP incident light.

Compared to traditional lenses made of glass alone, our metalens is just tens of nanometers thick: it is ideal for imaging on very small length scales and to increase the information-processing capacity of optical circuits. The circular-polarization-encoded information can be demultiplexed and processed separately. This could lead to important applications in quantum optics where photons entangled in polarization states can be more effectively manipulated.

The diameter of metasurface lenses can range from a few tens of microns to 200μm since its size is currently limited by the writing speed of the electron beam lithography system employed. We are looking into innovative nanofabrication techniques for making large-area metalenses for practical applications. The fact that the lens is flat also means that it might easily be integrated into other nanophotonic devices using conventional fabrication techniques.

Metasurfaces consisting of deep-subwavelength plasmonic antennas represent a new powerful way to manipulate light propagation. The polarization-switchable dual-polarity lens we presented is only one example of what is possible with metasurfaces, and more exciting optical devices based on phase-discontinuity metasurfaces are waiting to be explored. In particular, the circular-polarization-based metasurfaces provide reconfigurable optical functionalities not only for controlling free-space light propagation, but also for manipulating surface plasmon polaritons (guided waves at the interface between a metal and a dieletric material).⁵ These are the topics of our current and future work.