Because metalenses are able to control the properties of an incoming light beam using just an engineered flat surface, they are useful for making a variety of planar optical components, such as polarizers and tunable phase modulators. Their potential could also be extended to other applications and devices, such as full-colour imaging and displays, if it wasn’t for the limitation of strong chromatic aberration at visible wavelengths.
A long-standing goal has thus been the realization of achromatic metalenses that work in the visible light region with broad bandwidth and high efficiency. Din Ping Tsai and colleagues have now demonstrated such a device (Nat. Nanotech. https://doi.org/10.1038/s41565-017-0052-4; 2018).
The metalens (pictured) is composed of integrated-resonant unit elements (IRUEs) made from a combination of solid and inverse GaN-based nanostructures that are arranged in a subwavelength periodic hexagonal lattice on a double-polished sapphire substrate. The lattice constant is 120 nm and the GaN-based structure height is fixed at 800 nm. The phase behaviour of the achromatic metalens originates from two separate factors associated with the design: one contributing factor for the phase is dispersionless and thus independent of the incident wavelength, the other is wavelength-dependent and can be controlled by the IRUEs. The idea is that the phase difference between the maximum and the minimum wavelengths within the working bandwidth can be compensated by the integrated resonances. To do so, the team arranged and rotated the GaN-based IRUEs carefully to exactly provide the required phase compensation necessary to make the device achromatic.
Experimentally, the focal length of the metalenses remained unchanged for different numerical apertures in the wavelength region 400–600 nm, showing complete elimination of chromatic aberration at about 49% bandwidth of the central working wavelength of 530 nm. With a numerical aperture of 0.106, the efficiency, defined as the ratio of the optical power of the focused circularly polarized beam to the optical power of the incident beam with opposite helicity, can be as high as 67%, while the average efficiency is about 40% over the whole working bandwidth. The team points out that the efficiency spectra vary with working wavelength, which may be due to the fluctuations in the polarization conversion efficiency spectra of IRUEs and imperfections in the fabricated samples. They further demonstrated full-colour imaging using their achromatic metalens and compared its performance with that of a chromatic lens — a clearer colourful image was obtained with the achromatic metalens.
“In principle, this approach is not limited to the wave-band of the electromagnetic spectrum. Moreover, because the two phase components are independent of each other, any achromatic metasurface device can be made using such design principles. For example, to obtain achromatic beam deflection, one can use the geometric phase method to identify a linear phase distribution, and then introduce the integrated-resonances to compensate the phase dispersion between different incident wavelengths,” said Tsai when asked about the advantages of their design. He also added that the visible achromatic metalens could be used in many areas, ranging from industrial and academic research, such as in lithography, optical microscopy and virtual reality, to devices used every day, such as in smartphones and cameras.
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Won, R. Achromatic metalens for full-colour imaging. Nature Photon 12, 130 (2018). https://doi.org/10.1038/s41566-018-0130-7
Advanced Materials (2019)