Periodic Dielectric Metasurfaces with High‐Efficiency, Multiwavelength Functionalities

Sell, D. D.; Yang, Jianji; Doshay, Sage; Fan, Jonathan A.

doi:10.1002/adom.201700645

Cited by 125 publications

(113 citation statements)

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“…To use standard high-dimensional optimization algorithms, one needs to provide an efficient computation of both the objective (cost) function and its gradient. There is a well-known technique called an adjoint method [55] that can be used to efficiently compute the gradient for any number of parameters with a cost comparable to evaluating the objective function at most twice, which is commonly used in topology optimization [63,39,38,49,50,51,66]. In the case of the two objectives presented in Sec.…”

Section: Appendixmentioning

confidence: 99%

“…In this paper, we present and validate a fast method for optimization-based "inverse design" of large (hundreds of wavelengths λ) aperiodic metasurfaces for wavefront shaping [37,65,34,64,27], incorporating both scattered amplitude and phase for multiple incident λ and angles. Previous methods either optimized the full Maxwell equations [38,49,50,51,66] (which is infeasible for large surfaces), were restricted to weakly coupled scatterers [41], or started with a desired scattered phase and tried to design a corresponding metasurface unit cell [2,4,32,3,31,33,5,56,22,39] (but if attainable unit cells fail to exactly match the desired λ-dependent phase there was no systematic way to choose the best compromise). In contrast, our approach starts with a family of manufacturable unit cells and directly optimizes an aperiodic composition for the desired field pattern by a fast approximate model, automatically finding the best compromise for the given constraints.…”

Section: Introductionmentioning

confidence: 99%

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Inverse design of large-area metasurfaces

et al. 2018

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We present a computational framework for efficient optimization-based "inverse design" of large-area "metasurfaces" (subwavelength-patterned surfaces) for applications such as multiwavelength/multi-angle optimizations, and demultiplexers. To optimize surfaces that can be thousands of wavelengths in diameter, with thousands (or millions) of parameters, the key is a fast approximate solver for the scattered field. We employ a "locally periodic" approximation in which the scattering problem is approximated by a composition of periodic scattering problems from each unit cell of the surface, and validate it against brute-force Maxwell solutions. This is an extension of ideas in previous metasurface designs, but with greatly increased flexibility, e.g. to automatically balance tradeoffs between multiple frequencies or to optimize a photonic device given only partial information about the desired field. Our approach even extends beyond the metasurface regime to non-subwavelength structures where additional diffracted orders must be included (but the period is not large enough to apply scalar diffraction theory).

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Section: Appendixmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Inverse design of large-area metasurfaces

et al. 2018

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“…Among the most successful of these concepts is the adjoint variables method, which uses gradient descent to iteratively adjust the dielectric composition of the devices and improve device functionality [3][4][5][6][7][8]. This design method has enabled the realization of high performance, robust [9] devices with nonintuitive layouts, including new classes of on-chip photonic devices with ultrasmall footprints [10], non-linear photonic switches [11], and diffractive optical components that can deflect [12][13][14][15][16] and focus [17,18] electromagnetic waves with high efficiencies. While adjoint optimization has great potential, it is a local optimizer and depends strongly on the initial distribution of dielectric material in the devices [19].…”

Section: Introductionmentioning

confidence: 99%

Simulator-based training of generative neural networks for the inverse design of metasurfaces

2019

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AbstractMetasurfaces are subwavelength-structured artificial media that can shape and localize electromagnetic waves in unique ways. The inverse design of these devices is a non-convex optimization problem in a high dimensional space, making global optimization a major challenge. We present a new type of population-based global optimization algorithm for metasurfaces that is enabled by the training of a generative neural network. The loss function used for backpropagation depends on the generated pattern layouts, their efficiencies, and efficiency gradients, which are calculated by the adjoint variables method using forward and adjoint electromagnetic simulations. We observe that the distribution of devices generated by the network continuously shifts towards high performance design space regions over the course of optimization. Upon training completion, the best generated devices have efficiencies comparable to or exceeding the best devices designed using standard topology optimization. Our proposed global optimization algorithm can generally apply to other gradient-based optimization problems in optics, mechanics, and electronics.

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“…[236] Arbitrary manipulation of OAM superpositions in four channels was achieved by controlling the helicity of the incident light. [239] Polarization-controlled optical metadevices have been developed based on harmonic-response metasurfaces as well, such as a polarization splitting and focusing metamirror that can simultaneously split orthogonal light polarizations and focus them into different focal spots, as shown in Figure 9h. [237] In addition to multichannel optical vortices generators, multiple optical vortices multiplexers and demultiplexers based on a highly integrated off-axis technique have been presented.…”

Section: Harmonic-response Metasurfacesmentioning

confidence: 99%

Phase Manipulation of Electromagnetic Waves with Metasurfaces and Its Applications in Nanophotonics

Chen

Zhang

et al. 2018

Advanced Optical Materials

120

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elements. Thus, realizing phase manipulation of EM waves at the nanoscale has become a key pursuit for the development of modern optics and nanophotonics.Metamaterials are 3D artificial nanostructures composed of periodic subwavelength unit cells that resonantly couple to the incident EM waves, exhibiting effective electric and magnetic responses not found in nature. [1][2][3] However, these promising potential applications are hindered in their applications due to the challenges of fabricating the required complex 3D nanostructures and the inherent metallic losses and strong dispersion of plasmonic elements at optical frequencies. Planar metamaterials, or so-called metasurfaces, can be fabricated using existing technologies, such as the lithography method and have attracted increasing attention due to their feasibility, low loss, and ease of fabrication. [4,5] The most prominent advantage of metasurfaces is that they can generate spatial phase discontinuities over the full 2π range with an optically thin interface; moreover, the resolution is less than one wavelength. Thus, wavefronts can be shaped with a distance of much less than the wavelength. With the increasing development of metasurfaces, the aforementioned limitations can be solved using various ultrathin optical devices, which have properties superior to their conventional counterparts. [6][7][8][9][10][11][12] Here, we concentrate on the new capabilities of metasurfaces in recent years in manipulating the phase and propagation behaviors of EM waves. In Section 2, we briefly introduce the underlying mechanisms of three types of phase discontinuities. In Section 3, we review the basic applications of phase modulation using metasurfaces. In Section 4, we review more complex and advanced information photonics that have emerged from metasurfaces. In the last section, we provide concluding remarks and an outlook on future development directions. Three Basic Types of Phase Discontinuities Generated by Metasurfaces Resonance PhaseThe pioneering approach to achieve phase discontinuities was to use the dispersion of various metallic nanoantennas, as shown in the left panel of Figure 1a. The optical energy is coupled to surface EM waves propagating back and forth along the antenna surface. Due to the localized surface plasmon resonance, these waves are accompanied by oscillating free electrons Relative to conventional phase-modulation optical elements, metasurfaces (i.e., 2D versions of metamaterials) have shown novel optical phenomena and promising functionalities with more compact platforms and more straightforward fabrication processes. With the ability to generate a spatial phase variation, optical wavefronts can be manipulated into arbitrary shapes at will, enabling new phenomena and integrated ultrathin optical devices to be explored. This review is focused on recent developments regarding phase manipulation of electromagnetic waves with metasurfaces. Starting from their underlying physics for realizing full 2π phase manipulation, an overview of the applica...

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Periodic Dielectric Metasurfaces with High‐Efficiency, Multiwavelength Functionalities

Abstract: Metasurfaces are thin-film optical devices for tailoring the phase fronts of light. The extension of metasurfaces to multiple wavelengths has remained a major challenge, and existing design techniques do not yield devices with high efficiency. We report a

Cited by 125 publications

References 32 publications

Inverse design of large-area metasurfaces

Inverse design of large-area metasurfaces

Simulator-based training of generative neural networks for the inverse design of metasurfaces

Phase Manipulation of Electromagnetic Waves with Metasurfaces and Its Applications in Nanophotonics

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