Multifunctional 2.5D metastructures enabled by adjoint optimization

Abstract

Optical metasurfaces are two-dimensional arrays of meta-atoms that modify different characteristics of light such as phase, amplitude, and polarization. One intriguing feature that distinguishes them from conventional optical components is their multifunctional capability. However, multifunctional metasurfaces with efficiencies approaching those of their single-functional counterparts require more degrees of freedom. Here we show that 2.5D metastructures, which are stacked layers of interacting metasurface layers, provide sufficient degrees of freedom to implement efficient multifunctional devices. The large number of design parameters and their intricate intercoupling make the design of multifunctional 2.5D metastructures a complex task, and unit-cell approaches to metasurface design produce suboptimal devices. We address this issue by designing 2.5D metastructures using the adjoint optimization technique. Instead of designing unit cells individually, our technique considers the structure as a whole, accurately accounting for interpost and inter-layer coupling. As proof of concept, we experimentally demonstrate a double-wavelength metastructure, designed using adjoint optimization, that has significantly higher efficiencies than a similar device designed with a simplified approach conventionally used in metasurface design. The 2.5D metastructure architecture empowered by the optimization-based design technique is a general platform for realizing high-performance multifunctional components and systems.

Revealing Structural Disorder in Hydrogenated Amorphous Silicon for a Low‐Loss Photonic Platform at Visible Frequencies

“…This allows the realization of complex geometric structures with low‐loss a‐Si:H. Furthermore, the PECVD is done at relatively lower temperatures (≈200 °C) than conventional synthesis methods (400–500 °C), [ 47 ] enabling the use of poly(dimethylsiloxane) (PDMS) and polyethylene terephthalate substrates for flexible metasurfaces. Additionally, in multilayered metasurfaces, the bottom nanostructures are usually encapsulated in a polymer such as PDMS or photoresist to mechanically support the top layer, [ 48–50 ] so the high n of low‐loss a‐Si:H is highly desirable to achieve a sufficient refractive index contrast between the nanostructures and the supporting medium. Previously reported multilayered metasurfaces [ 48–50 ] only operate in the infrared region due to the lack of transparent high n materials in the visible regime, but our low‐loss a‐Si:H could be used to expand the working bandwidth of multilayered metasurfaces into the full visible spectrum.…”

Catenary Functions Meet Electromagnetic Waves: Opportunities and Promises

“…Furthermore, as an efficient structural optimization approach, adjoint-simulation-based topology optimization [201][202][203] may further improve the optical performance of catenary optical devices by extending the design space. With the rapid development of micro-/nanofabrication methods, it is also possible to design and fabricate 3D catenary structures with nontrivial performances.…”

Tunable nanophotonics enabled by chalcogenide phase-change materials

“…MSs, the two-dimensional (2D) analog of metamaterials, have attracted significant attention due to their unprecedented ability to control incident electromagnetic fields in the subwavelength regime [7][8][9]. Owing to their judiciously engineered optical scatterers, or the so-called meta-atoms, arranged in a periodic or aperiodic texture, the amplitude, phase, polarization, and frequency of the impinging light can be spatially and spectrally manipulated, making a big step towards the realization of the next-generation flat optics [10][11][12][13]. A myriad of novel phenomena and optical functionalities have thus been demonstrated including beam shaping and steering [14,15], imaging polarimetry [16,17], large-angle holography [18,19], directional lasing [20], analog computing [21,22], quantum emission [23], nonlinear generation [24], structural coloration [25,26], and biosensing [27,28].…”