179 news & views NANOPHOTONICS Imaging the rainbow Dielectric metalenses made of high-index materials can compensate material dispersion to achieve broadband imaging in the visible. Thomas Zentgraf T raditional optical lenses focus light by altering its wavefront through the addition of a spatially dependent phase to the light field as it propagates through a dispersive medium. With the desire to integrate optical and electronic components for applications such as wearable optics or optical communications, the demand for compact, lightweight, yet high-performance optical systems is steadily increasing. Towards this goal, in the past few years, researchers have reported various approaches to realize ultrathin optical elements based on nanostructured surfaces (metalenses), demonstrating the potential for altering the light propagation in an efficient and convenient way 1,2 . However, these designs suffer from strong chromatic aberration, making them unsuitable for precise imaging of objects with multiple colours. Writing in Nature Nanotechnology, two groups now report metalens designs that allow achromatic imaging in the visible range 3,4 . Both groups use nanofin structures made of high-index dielectrics to add a spatially dependent phase to the light field. As illustrated in Fig. 1a, a common refractive lens made of a regular optical glass focuses white light to different focus spots, corresponding to the different colours, along the propagation axis; thus resulting in a blurred focus spot (the chromatic aberration). The origin of this behaviour is the dispersion of the material’s refractive index with respect to the light wavelength, which is inherent to all materials. In conventional imaging systems, chromatic aberration can be reduced by using two lenses — one with a positive and the other with a negative dispersion property (Fig. 1b). These ‘achromatic’ lenses improve the image quality when using a broadband illumination, as the image will reconstruct for a wide range of colours at the same plane. Unfortunately, as demonstrated in Fig. 1, this adds even more optical components to the imaging system, making it heavier, larger and more expensive — clearly not an option for miniaturized optics. Metalenses can potentially break this cycle of adding more optical components to improve the imaging quality while having the potential of being easily integrated into optical systems. It has been shown, for example, that they can image objects at single wavelengths and control beam shapes 5,6 . Furthermore, they can be used for holographic applications and nonlinear optics. However, to generate high-quality colour images, they need to work over the entire visible wavelength range. Up to now, metalenses have suffered from strong chromatic aberrations in the same way as conventional lenses do, that is, the focal length varies with the wavelength (Fig. 1c), only the sign of the aberrations is opposite due to the diffractive working principle, resulting in a shorter focal length for longer wavelengths. In their reports, Chen et al. and Wang et al. show how to compensate for the chromatic aberration of dielectric metalenses. Chen et al. utilized a meta-atom design consisting of two TiO 2 nanofins spaced close to each other 3 (Fig. 2a). The close proximity of the nanofins results in the formation of optical modes similar to modes found in slot waveguides. The researchers found that such modes can be used to add an additional phase to the wave by adjusting the size of the two fins. As a result, the effective optical mode possesses different fractions in either the high-index fins or the low- index air gap. Based on this added degree of freedom in the design, the researchers were able to independently control the phase and group delay over nearly the entire visible spectral range, and compensate for the chromatic dispersion effect of the lens. Wang et al. followed a slightly different approach in that they used single GaN nanofins as meta-atoms 4 (and inverted versions) that support higher-order resonant modes in each nanofin, in analogy to cavity modes in resonators (Fig. 2b). As these modes depend strongly on the wavelength, the researchers were able to demonstrate that these ‘resonator modes’ can be used to add a wavelength-dependent phase to compensate for the diffractive dispersion effect of the metalens. Uncorrected singlet metalens Corrected singlet metalens Singlet lens Achromatic doublet lens a b c d Blurred Blurred Sharp Sharp Fig. 1 | Focusing of white light with lenses. a, The dispersion of a regular singlet lens made of optical glass leads to different focal distances for different wavelengths. b, Two different lenses made of different materials (like crown and flint glasses) forming a doublet lens can compensate the chromatic aberrations. c, Regular metalenses resemble a singlet lens and show chromatic aberrations. d, Chromatic aberrations can now be compensated solely by design without the need for additional materials or lenses 3,4 . NATURE NANOTECHNOLOGY | VOL 13 | MARCH 2018 | 179–182 | www.nature.com/naturenanotechnology © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.