Chromatic aberration of an optical lens arises from the variation of focal length with respect to the wavelength of light caused by the material dispersion, which is a common phenomenon in white light imaging. This aberration will lead to the color blurring and severely degrade the image quality.
In refractive lenses, elimination of chromatic aberrations is accomplished by using two or more lenses arranged to achieve the same focal length at the wavelengths of interest. However, this strategy makes lens bulky, heavy and unfavorable for portable and wearable devices.
Metalens, as a flat, thin, and lightweight optical design, provides a stirring new solution for compact imaging devices. Nevertheless, it usually suffers from large diffractive chromatic aberration as well. Till Now, researchers have demonstrated broadband achromatic metalenses by carefully engineering the phase dispersion of each constituent nanostructure.
However, recent works have shown that there exist restriction relations between the lens aperture, thickness, working bandwidth, and efficiency of a broadband achromatic metalens. Generally speaking, it is extremely difficult to design a large scale broadband achromatic metalens with high numerical aperture and high efficiency at the same time.
In this consideration, a possible means to achieve large scale and high efficient achromatic lens is to shrink the bandwidth. In order to avoid reducing the wavelength range, an alternative and efficient method is produce an achromatic lens for several discrete wavelengths instead of a whole spectral range.
In fact, multi-wavelength based image sensors are widely used by adopting a color filter array, i.e., the Bayer pattern in single-sensor color imaging systems. It means a discrete multi-wavelength achromatism for the red, green, and blue (RGB) colors in the visible range is of practical importance.
The research group led by Professor Tao Li from Nanjing University proposed a bandpass-filter-integrated multiwavelength achromatic metalens aiming at RGB wavelengths, which shows remarkably improved efficiency and relatively high quality white-light image. Note that the RGB color filter is important and necessary to block those unwanted light at other wavelengths to make background noises. The results are published in Photonics Research, Vol. 9, Issue 7, 2021 (Hanmeng Li, Xingjian Xiao, Bin Fang, Shenglun Gao, Zhizhang Wang, Chen Chen, Yunwei Zhao, Shining Zhu, Tao Li. Bandpass-filter-integrated multiwavelength achromatic metalens[J]. Photonics Research, 2021, 9(7): 07001384)
The device is composed of two parts, a bandpass-filter and a multiwavelength achromatic metalens. The RGB-color bandpass filter is implemented by six distributed Bragg reflectors (DBRs) and defect dielectric layers sandwiched between them, which is composed of SiNx and SiO2 to form the Fabry–Perot (FP) resonances at the RGB wavelengths.
The dependence of the FP resonances on the parameters of the defect layers provides free control over the spectrum for the bandpass filter. The transmittances of designed bandpass-filter at RGB in simulation are 99.6%, 97.5% and 98.8%, respectively, and the bandwidth at each wavelength is about 10 nm ensuring a good performance in discrete wavelengths selection.
In achromatic metalens design, multiple-shaped nano-pillar structures (square, circle, ring, cross, etc.) are applied following an optimization algorithm (i.e., modified Hooke–Jeeves methods) for achromatism of RGB wavelengths. The diameter of lens is set as 300 μm, with NA equal to 0.2. The experimental focus efficiency of this lens is 50% on average, which is much higher than (~2.5 times) that of a broad achromatic metalens with the same size and NA.
The whole device is schematically shown in Fig.1. It filters the incident white light into RGB color ones, and then focus them into the same spot. This filter successfully blocked those light of other wavelengths without achromatic design, and efficiently decrease the background noises.
As results, this filter integrated metlens show obviously improved resolution and signal-to-noise ratio in the image results as compared with that without filter, revealing its advantage of application in white-light imaging without losing the compactness of flat lens. This work does not only provide a good solution to improve the quality of white-light imaging, but also opens a new avenue towards spectral functional metasurfaces by combing them with multi-layer filter.
It is highly expected in future works to improve the efficiency of the designed RGB achromatic metalens to >90%, which would be fulfilled by further increase the thickness of metalens (i.e., increase the aspect ratio of nano-pillars). Then it will be practically mounted in portable cameras, like cell phone or other compact imaging devices. Besides, there are rich explorations in joint manipulation of wavefront shaping and spectrum tailoring, such as, multi-wavelength laser beam engineering, on-chip spectrum detection and sensors, etc.
Fig.1 Schematics of a bandpass-filter-integrated multiwavelength achromatic metalens.