A Metalens with Near-Unity Numerical Aperture (1705.00895v2)

Abstract: The numerical aperture (NA) of a lens determines its ability to focus light and its resolving capability. Having a large NA is a very desirable quality for applications requiring small light-matter interaction volumes or large angular collections. Traditionally, a large NA lens based on light refraction requires precision bulk optics that ends up being expensive and is thus also a specialty item. In contrast, metasurfaces allow the lens designer to circumvent those issues producing high NA lenses in an ultra-flat fashion. However, so far, these have been limited to numerical apertures on the same order of traditional optical components, with experimentally reported values of NA <0.9. Here we demonstrate, both numerically and experimentally, a new approach that results in a diffraction limited flat lens with a near-unity numerical aperture (NA>0.99) and sub-wavelength thickness (~{\lambda}/3), operating with unpolarized light at 715 nm. To demonstrate its imaging capability, the designed lens is applied in a confocal configuration to map color centers in sub-diffractive diamond nanocrystals. This work, based on diffractive elements able to efficiently bend light at angles as large as 82{\deg}, represents a step beyond traditional optical elements and existing flat optics, circumventing the efficiency drop associated to the standard, phase mapping approach.

• The paper introduces a metalens design that achieves a near-unity numerical aperture (>0.99) with a sub-wavelength thickness of approximately λ/3.
• It employs diffractive energy redistribution through asymmetric dimer nanoantennas, resulting in high diffraction efficiencies (over 88% for s-polarized light).
• Numerical simulations and experimental validations confirm this breakthrough design, paving the way for advanced photonic devices and high-resolution imaging applications.

Near-Unity Numerical Aperture Metalens

The paper "A Metalens with Near-Unity Numerical Aperture" introduces a novel approach to the design of metalenses, achieving an unprecedented numerical aperture (NA) greater than 0.99. This design utilizes diffractive elements and nanoantenna inclusions to efficiently direct and focus light, paving the way for advancements in flat optics and their application in high-resolution imaging and photonic devices.

Key Contributions

The paper presents an ultra-flat lens with a sub-wavelength thickness approximately λ/3, operating at a wavelength of 715 nm and achieving a NA of over 0.99. This achievement surpasses the traditional limits of flat and bulk optical components, which have historically been constrained to NAs less than 0.9 in free-space configurations. The metalens demonstrates that it is possible to attain such high focusing capabilities without the inherent disadvantages of bulk optics, namely complexity and expense.

Conceptual Innovation

The paper introduces a mechanism of diffracted energy redistribution facilitated by nanoantenna inclusions with tailored scattering patterns. Contrasting with traditional phase-mapping techniques that struggle with efficiency at large angles, this approach efficiently bends light at angles up to 82°, allowing for more effective focusing and light collection. The lattice structure is engineered in a way that prioritizes energy concentration into a singular diffraction order, thereby maximizing the efficiency of light bending.

Numerical and Experimental Validation

The authors validate their approach through both numerical simulations and experimental results, demonstrating that the novel metalens concept is viable. The unit cell of the metalens, composed of asymmetric dimer nanoantennas, achieves a diffraction efficiency of over 88% for s-polarized light and above 82% for p-polarized light. The experimental characterization aligns closely with the simulations, with the fabricated sample exhibiting a diffraction efficiency into the desired order of approximately 75%.

Implications and Applications

The metalens' capability to focus light with a NA of 0.99 opens new possibilities in fields that require high-resolution imaging and tight light confinement. For instance, such lenses could significantly enhance photolithography processes or be employed in quantum optics experiments where efficient photon collection is critical. Furthermore, the application of the metalens in confocal microscopy configurations points towards its potential in microscopy, offering resolution improvements over conventional objectives.

Future Directions

Given the success of the experimental results, the paper hints at further improvements in nanoantenna design and fabrication processes that could yield even higher efficiencies, mitigating current discrepancies between simulated and measured performance. The exploration of alternative nanoantenna geometries and materials might result in even greater control over light interaction and efficiency enhancements.

In summary, the paper demonstrates a significant advancement in the design of metalenses, surpassing traditional limitations with a near-unity numerical aperture. This work has the potential to influence future developments in optical and photonic devices, suggesting a paradigm shift towards more compact and efficient high-performance optics.