Imaging with flat optics: metalenses or diffractive lenses? (1901.05042v2)

Abstract: Recently, there has been an explosion of interest in metalenses for imaging. The interest is primarily based on their sub-wavelength thicknesses. Diffractive lenses have been used as thin lenses since the late 19th century. Here, we show that multi-level diffractive lenses (MDLs), when designed properly can exceed the performance of metalenses. Furthermore, MDLs can be designed and fabricated with larger constituent features, making them accessible to low-cost, large area volume manufacturing, which is generally challenging for metalenses. The support substrate will dominate overall thickness for all flat optics. Therefore the advantage of a slight decrease in thickness (from ~2{\lambda} to ~{\lambda}/2) afforded by metalenses may not be useful. We further elaborate on the differences between these approaches and clarify that metalenses have unique advantages when manipulating the polarization states of light.

• The paper demonstrates that MDLs achieve superior focusing efficiency, with over 87% narrowband efficiency at high numerical apertures.
• The study utilizes detailed optical analysis, including Zernike polynomial-based aberration evaluations, to validate MDLs' performance across spectra.
• The paper highlights MDLs’ practical advantages through a cost-effective, low-resolution fabrication process compared to the complex production of metalenses.

Imaging with Flat Optics: Metalenses or Diffractive Lenses?

The comparative paper "Imaging with Flat Optics: Metalenses or Diffractive Lenses?" by Banerji et al. investigates the performance and fabrication implications of multi-level diffractive lenses (MDLs) in relation to metalenses within the domain of flat optics. The authors challenge the current perceived advantages of metalenses, mainly their sub-wavelength thickness, by providing a comprehensive analysis and argument favoring MDLs both in optical performance and manufacturability.

The paper delineates the evolution of flat optics from traditional refractive lenses to diffractive and meta-optics. Metalenses, which owe their recent popularity to their capacity to manipulate light through resonant sub-wavelength structures, are discussed alongside diffractive optics that achieve focusing by arranging zones that impart phase shifts necessary for constructive interference at the focal point. While metalenses capitalize on subwavelength antenna arrays for achieving phase manipulation, MDLs use blazed or multi-level diffractive designs to optimize path length delays and phase distribution.

Key numerical results from the paper illustrate that MDLs outperform metalenses in focusing efficiency across both narrowband and broadband spectral regimes, particularly at high numerical apertures (NAs). For instance, MDLs maintain efficiencies exceeding 87% at high NAs for narrowband operations, a performance not paralleled by metalenses. Moreover, MDLs demonstrate broad spectral achromatic capabilities, achieving efficiency above 70% over a wide wavelength range, thus highlighting their advantage in broadband applications.

The fabrication process contributes substantially to the practical advantages of MDLs. MDLs do not necessitate the high-resolution lithography required for metalenses and can be constructed from low-cost, low-index materials like S1813 photopolymer. This less complex fabrication process, along with the compatibility of MDLs with standardized manufacturing techniques akin to those used in the holography industry, renders them more feasible for large-scale and economically viable production compared to the intricate and costly approach required for fabricating metalenses.

The paper also engages with the discussion on aberration correction in MDLs, using Zernike polynomials to quantify wavefront aberrations. The results assert that MDLs exhibit minimal aberrations over varying operational wavelengths, which reinforces their applicability in high-performance imaging tasks. The versatility and adaptability of MDLs in terms of materials, design, and broad-spectrum performance suggest that they hold promise for diverse imaging and optical applications beyond what is realizable with metalenses.

The authors acknowledge that while MDLs have demonstrated superior efficiency for generic imaging applications, meta-optics, including metalenses, retain unique capabilities in manipulating polarization states, exemplified in polarimetric imaging. The extreme form birefringence and the associated fine-tuned control over light polarization achievable with metasurfaces still represent an area where metalenses and equivalent technologies might prove indispensable.

In conclusion, the analysis presented in this paper positions MDLs as a credible alternative to metalenses in many practical scenarios, arguing that the latter's advantages might be overstated in typical imaging applications. As the field of flat optics continues to evolve, the demonstrated efficacy and manufacturability of MDLs could inspire further developments in ultra-thin lens technology, providing innovative solutions to a range of optical challenges while also potentially influencing the direction of future research and applications in the photonics industry.