While not traditional optics by any means, metasurfaces (2D periodic subwavelength structures containing one or both metallic and dielectric components) are improving enough in design that they may one day be added as a component in conventional bulk optical systems.
Metasurfaces deflect light in a predesigned way, either in reflection or transmission. With the proper configuration, they could thus be used as focusing as well as deflecting elements. The simplest metasurface designs deflect optimally at only one wavelength and would therefore be most useful in laser systems.
However, achromatic metasurfaces are being developed. For example, in 2015 Federico Capasso at the School of Engineering and Applied Sciences, Harvard University (Cambridge, MA) and his team of researchers from Hewlett-Packard (Palo Alto, CA), University of Wisconsin (Madison, WI), and SIMTech (Singapore) designed an achromatic metasurface based on low-loss dielectric resonators that had a large number of optical modes, leading to dispersive phase compensation.1 The result was a planar achromatic lens, although the design works only at normal incidence and at low efficiency.
Now, researchers at Shenzhen University (Shenzhen, China) and the University of Birmingham (Birmingham, England) have designed a metasurface (as yet not experimentally tested) that has a wide incidence-angle range of 10° to 80° and high efficiency (see figure).2 The surface contains multiple nanogroove gratings, each having a high-efficiency plasmonic resonance for a certain wavelength range-adding up all the ranges results in achromaticity. The researchers also designed an achromatic planar lens for the visible-light spectral region based on the metasurface.
Design for high efficiency
The metasurface design contains three silver nanogroove gratings of different periods, all designed to diffract into their -1 orders. To raise the efficiency as high as possible, the groove heights of the multiple gratings were chosen so that the lights at the gratings' center wavelengths just excite the corresponding localized gap-plasmon modes in the nanogrooves.
The three design wavelengths of 440, 550, and 660 nm resulted in grating geometries of period 400, 500, and 600 nm and heights of 15, 29, and 42 nm, respectively. For all three gratings, the unwanted 0th-order diffraction efficiencies approached zero at the resonant wavelengths, while the first-order diffraction efficiencies reached 78%, 76%, and 60% for 660, 550, and 440 nm, respectively.
The device achieves achromaticity without unwanted diffractions because light at a specific wavelength causes only the gap-plasmon mode in the nanogrooves that resonate at this wavelength to be excited. Because the resonance frequencies of the different nanogroove gratings are separated and not coupled together, the other nanogrooves do not respond to this wavelength. (This behavior is in stark contrast to that of conventional multiple gratings, which under the same conditions produce many diffraction orders.)
By varying the grating period across the device, the design can be modified to create optics with arbitrary phase profiles. As a simple example, the researchers designed a device to produce a quadratic phase profile (a simple lens), and determined that it functioned well over incidence angles ranging from 33° to 60°.
REFERENCES
1. F. Aieta et al., Science, 347, 1342 (2015); doi:10.1126/science.aaa2494.
2. Z.-L. Deng et al., arXiv:1603.04608v1 [physics.optics] (Mar. 15, 2016).
