Recent advancements in computational inverse-design approaches — algorithmic techniques for discovering optical structures based on desired functional characteristics — have begun to reshape the landscape of structures available to nanophotonics. Here, we outline a cross-section of key developments in this emerging field of photonic optimization: moving from a recap of foundational results to motivation of applications in nonlinear, topological, near-field and on-chip optics. Starting with a desired optical output it is possible to use computational algorithms to inverse design devices. The approach is reviewed here with an emphasis on nanophotonics.

Inverse design in nanophotonics

Researchers used an inverse design algorithm and experimentally demonstrated an integrated polarization beamsplitter with a footprint of 2.4 × 2.4 μm2.

An integrated-nanophotonics polarization beamsplitter with 2.4 × 2.4 μm2 footprint

Recent advancements in computational inverse design have begun to reshape the landscape of structures and techniques available to nanophotonics. Here, we outline a cross section of key developments at the intersection of these two fields: moving from a recap of foundational results to motivation of emerging applications in nonlinear, topological, near-field and on-chip optics.

Outlook for inverse design in nanophotonics

Recently, there has been an explosion of interest in metalenses for imaging. The interest is primarily based on their subwavelength thicknesses. Diffractive gratings have been used as thin optical elements since the late 19th century. Here, we show that multilevel 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 ∼2λ to ∼λ/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 electromagnetic fields, rather than intensity.

Imaging with flat optics: metalenses or diffractive lenses?

Conventional polarizers operate by rejecting undesired polarization, which limits their transmission efficiency to much less than 50% when illuminated by unpolarized light. We designed, fabricated, and characterized a multilevel metamaterial linear polarizer that rotates light with polarization perpendicular to its principal axis by 90 deg. Light with polarization parallel to its principal axis is transmitted undisturbed. Thereby, such a polarizer is able to output linearly polarized light from unpolarized input with a transmission efficiency that is substantially higher than the theoretical upper limit of 50%. A nonlinear optimization algorithm was used to design the polarizer, while multilevel focused-ion-beam lithography was used to fabricate it in silicon for the vacuum wavelength, λ0=1550  nm. We experimentally confirmed that the fabricated device enhances the transmission of the desired linear polarization by 100% compared to an unpatterned film, corresponding to a transmission efficiency of ∼74% at the design wavelength. Since our method allows for the generalized manipulation of the amplitude, phase, and polarization of light with high transmission efficiency using ultrathin elements, it should enable the efficient generation of complex vector distributions of light.

Ultra-high-efficiency metamaterial polarizer

In this Letter, we report the preliminary demonstration of a new paradigm for photovoltaic power generation that utilizes a broadband diffractive-optical element (BDOE) to efficiently separate sunlight into laterally spaced spectral bands. These bands are then absorbed by single-junction photovoltaic cells, whose band gaps correspond to the incident spectral bands. We designed such BDOEs by utilizing a modified version of the direct-binary-search algorithm. Gray scale lithography was used to fabricate these multilevel optics. They were experimentally characterized with an overall optical efficiency of 70% over a wavelength range of 350-1100 nm, which was in excellent agreement with simulation predictions. Finally, two prototype devices were assembled: one with a pair of copper indium gallium selenide based photovoltaic devices, and another with GaAs and c-Si photovoltaic devices. These devices demonstrated an increase in output peak electrical power of ∼ 42% and ∼ 22%, respectively, under white-light illumination. Because of the optical versatility and manufacturability of the proposed BDOEs, the reported spectrum-splitting approach provides a new approach toward low-cost solar power.

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Howard Lee

Papers

Increased photovoltaic power output via diffractive spectrum separation

Increased Photovoltaic Power Output via Diffractive Spectrum Separation