Design for quality: reconfigurable flat optics based on active metasurfaces

Abstract

Optical metasurfaces, planar subwavelength nanoantenna arrays with the singular ability to sculpt wavefront in almost arbitrary manners, are poised to become a powerful tool enabling compact and high-performance optics with novel functionalities. A particularly intriguing research direction within this field is active metasurfaces, whose optical response can be dynamically tuned postfabrication, thus allowing a plurality of applications unattainable with traditional bulk optics. Designing reconfigurable optics based on active metasurfaces is, however, presented with a unique challenge, since the optical quality of the devices must be optimized at multiple optical states. In this article, we provide a critical review on the active meta-optics design principles and algorithms that are applied across structural hierarchies ranging from single meta-atoms to full meta-optical devices. The discussed approaches are illustrated by specific examples of reconfigurable metasurfaces based on optical phase-change materials.

Intermediate Phase‐Change States with Improved Cycling Durability of Sb2S3 by Femtosecond Multi‐Pulse Laser Irradiation

“…Reconfigurable photonic systems can provide tunable optical response, allowing metasurface-based nanophotonics to explore new functionalities and applications. [1] Generally, optical response can be tuned via mechanically-actuated geometric that the as-deposited amorphous (α-) state can be transformed to the crystalline (c-) state by thermal annealing, accompanied with huge change in optical properties. [23] Later, phase change (PC) from the α-state to the c-state was realized using a continuouswave laser (CWL), suggesting the application of Sb 2 S 3 for writeonce-read-many-times optical storage.…”

Reversible switching of electromagnetically induced transparency in phase change metasurfaces

“…28,39,40 The structural rearrangement in PCMs can be triggered by optical 41 or electrical 42 pulses, enabling optical (e.g., DVD-RW) 43 and nonvolatile phase-change random access memory devices. 44 Moreover, compared to the other active mediums, such as LCs, ENZ, and VO 2 , the amorphous (AM)to-crystalline (CR) state transition in the most investigated PCMs, Ge 2 Sb 2 Te 5 (GST225), is nonvolatility, 45,46 which makes it ideal for tunable metadevices. 28,[47][48][49][50][51][52][53][54][55][56][57] For example, energy is only required for the state transition process rather than to uphold a particular state, enabling the GST225-based metasurface to be attractive from a green technology standpoint.…”

“…Notably, the refractive index of the GST225 experiences the largest variation of ∼2.8 across the phase change in the NIR region. 46 This may lead to the broadest tuning range of the resonant frequency for tunable NIR metasurfaces. Recently, nanostructured GST225 has been designed to obtain specific switching features, 58,59 and it was shown that the phase change ratio of GST225 could be engineered via femtosecond pulses.…”

Multifunctional Metasurface Design with a Generative Adversarial Network

“…Therefore, it can be time-consuming and laborious to find an appropriate set of meta-atoms for a specific design. Meanwhile, multifunctional metasurfaces such as multi-wavelength metasurfaces, [9,10,13,14] multi-polarization metasurfaces, [15] or reconfigurable metasurfaces based on phase change materials, [16][17][18][19][20][21] have presented another major design challenge due to the difficulty in exploring vast parameter spaces containing meta-atoms that possess sufficient complexity to satisfy restrictive design requirements. Therefore, in addition to exploring meta-atoms with basic shapes such as rings, [9,10] cubes, [14,22] or cylinders, [23] previous works have also adopted evolutionary algorithms to search for meta-atoms and metasurface with freeform patterns [13,15,[24][25][26][27] which provides additional degrees of freedom (DOF).…”