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arXiv 2202.02636physics.optics
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优化超透镜的消色差性和层状薄膜超透镜的研制

Optimizing achromaticity in metalenses, and development of a layered thin-film metalens

Calvin M. Hooper, Sarah E. Bohndiek, Calum Williams

摘要:

超透镜是设计成利用纳米级谐振结构来重定向入射光以复制常规折射透镜或透镜阵列的行为的微透镜光学装置。这些通常由离散的元原子组成,例如纳米级电介质柱。消色差聚焦-与传统的多元件折射对应物相关联-经常尝试单层超透镜设计,这已被证明难以用有界折射率和总透镜厚度实现。最近的一项研究(F.Presutti和F.Monticone,2020年)将此形式化,将光学延迟线限制应用于超透镜,导致聚焦系统的消色差性的普遍权衡。在这项工作中,我们(1)从理论上探讨了消色差超透镜设计,(2)提出了一个薄膜多层设计作为替代离散元原子的方法大数值孔径(NA)消色差超透镜。它示出,波前调制也可以实现光谱变化的传输幅度,而不是纯粹匹配的相位分布。事实上,即使折射率有界,在给定光谱范围内的完美消色差操作也可能被其他地方的不完美操作所抵消,导致NA受到控制的最小一般光谱特征的限制。这些考虑导致了一个广义的相位匹配优化程序,并模拟了一个薄膜超透镜,使用分层的TiO 2/MgF 2的总厚度小于1 μm(20层),在6个同时波长(350-740 nm,Δλ~65 nm)聚焦。对于中等NA(~0.35),反射光的显著比例(>40%光谱平均值)被聚焦。随着光学涂层工业的成熟,保形薄膜方法从其离散纳米级元原子等效物降低了制造复杂性。

英文摘要:

Metalenses are ultrathin optical devices designed to replicate behavior of conventional refractive lenses, or lens arrays, utilizing nanoscale resonant structures to redirect incident light. These are often comprised of discrete meta-atoms such as nanoscale dielectric pillars. Achromatic focusing - associated with traditional multi-element refractive counterparts - is frequently attempted with single-layer metalens designs, which has proven difficult to achieve with bounded refractive indices and total lens thickness. A recent study (F.Presutti and F.Monticone, 2020) formalized this, applying optical delay-line limitations to metalenses, resulting in a generalized trade-off in achromaticity for focusing systems. In this work, we (1) theoretically explore achromaticity in metalens design, and (2) propose a thin-film multilayer design as an alternative to the discrete meta-atom approach for large numerical aperture (NA) achromatic metalenses. It is shown that wavefront modulation can also be achieved with spectrally-varying transmission magnitudes, rather than purely matching a phase profile. In fact, even with a bounded refractive index, perfect achromatic operation over a given spectral range can be offset by imperfect operation elsewhere, resulting in a NA limited by the smallest general spectral feature controlled. These considerations lead to a generalized phase-matching optimization routine, and a thin-film metalens is simulated, utilizing layered TiO2/MgF2 with total thicknesses under 1 μm (20 layers), focusing across 6 simultaneous wavelengths (350-740 nm, Δλ~65 nm). A significant proportion (>40% spectral average) of the reflected light is focused for moderate NA (~0.35). With the maturity of the optical coating industry, the conformal thin-film approach reduces manufacturing complexity from its discrete nanoscale meta-atom equivalents.

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