No 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.

Optimizing the shape of metasurface unit cells can lead to tremendous performance gains in several critically important areas. This paper presents a method of generating and optimizing freeform shapes to improve efficiency and achieve multiple metasurface functionalities (e.g., different polarization responses). The designs are generated using a three-dimensional surface contour method, which can produce an extensive range of nearly arbitrary shapes using only a few variables. Unlike gradient-based topology optimization, the proposed method is compatible with existing global optimization techniques that have been shown to significantly outperform local optimization algorithms, especially in complex and multimodal design spaces.

Recent transmissive optical metamaterials that leverage a generalized form of Snell's law to induce an anomalous refraction of light have garnered considerable interest in both optical and materials communities. However, most of these designs have primarily centered around parametric studies of planar canonical structures for their low profile and relative ease of manufacturing. In many of these cases, all-dielectric designs are preferred over metallodielectrics due to their low loss characteristics. Moreover, considering modern advances in nanofabrication techniques, these canonical structures represent only a small portion of the design space that is explorable. In this work, we exploit a generalized Multi-Objective Lazy Ant Colony Optimization (MOLACO) algorithm and a modified Pareto locus search mechanism to optimize arbitrary three-dimensional metamaterial unit cells in the optical regime based on the Membrane Projection Lithography technique. Our exploration has revealed unintuitive metallodielectric structures for phase-gradient metasurface applications in the midwave infrared (MWIR) regime that achieve transmission magnitudes comparable to the highest-performance all-dielectric designs found in the literature. As a proof-of-concept, a beam-steering metasurface is synthesized using these unintuitive unit cell geometries and is shown to achieve over 84% diffraction efficiency, which is among the highest performing metallodielectric metasurfaces in the MWIR reported to date.

Near-field characterization of the acoustical environment near rockets has often involved extrapolating far-field measurements. However, because far-field amplitude data reveals only limited information about source characteristics, a vector intensity measurement system and analysis package has been developed to examine source features more directly. This paper describes the development of the measurement and analysis capability and its application to a horizontal firing of a GEM-60 solid propellant rocket motor firing conducted at ATK Space Systems near Promontory, Utah. An analysis of near-field intensity data provides insight both into the spatial extent and principal radiation lobe as a function of frequency. For 50 Hz, the far-field spectral peak frequency in the maximum radiation direction, the dominant source region derived from tracing the near-field intensity vectors spans 17-32 nozzle diameters, with peak radiation at ~68 o . At high frequencies, the radiation results from a more contracted region that occurs farther upstream and is directed at about ~85°. These results point to the potential utility of near-field vector intensity measurements, in part because the near-field environments represented do not agree with historical far-field data-based models.