@article{oudich_su_deng_benalcazar_huang_gerard_lu_zhan_jing_2021, title={Photonic analog of bilayer graphene}, volume={103}, ISSN={["2469-9969"]}, DOI={10.1103/PhysRevB.103.214311}, abstractNote={Drawing inspiration from bilayer graphene, this paper introduces its photonic analog comprising two stacked graphene-like photonic crystals, that are coupled in the near-field through spoof surface plasmons. Beyond the twist degree of freedom that can radically alter the band structure of the bilayer photonic graphene, the photonic dispersion can be also tailored via the interlayer coupling which exhibits an exponential dependence on the distance between the two photonic crystals. We theoretically, numerically, and experimentally characterize the band structures of AA- and AB-stacked bilayer photonic graphene, as well as for twisted bilayer photonic graphene with even and odd sublattice exchange symmetries. Furthermore, we numerically predict the existence of magic angles in bilayer photonic graphene, which are associated with ultra-flat bands resulted from interlayer hybridization. Finally, we demonstrate that the bilayer photonic graphene at a particular twist angle satisfying even sublattice exchange symmetry is a high-order photonic topological insulator. The proposed bilayer photonic graphene could constitute a useful platform for identifying new quantum materials and inspiring next-generation photonic devices with new degrees of freedom and emerging functionality.}, number={21}, journal={PHYSICAL REVIEW B}, author={Oudich, Mourad and Su, Guangxu and Deng, Yuanchen and Benalcazar, Wladimir and Huang, Renwen and Gerard, Nikhil J. R. K. and Lu, Minghui and Zhan, Peng and Jing, Yun}, year={2021}, month={Jun} }
 @article{zhu_gerard_xia_stevenson_cao_fan_spadaccini_jing_assouar_2021, title={Systematic Design and Experimental Demonstration of Transmission-Type Multiplexed Acoustic Metaholograms}, volume={31}, ISSN={["1616-3028"]}, url={http://dx.doi.org/10.1002/adfm.202101947}, DOI={10.1002/adfm.202101947}, abstractNote={Acoustic holograms have promising applications in sound‐field reconstruction, particle manipulation, ultrasonic haptics, and therapy. This study reports on the theoretical, numerical, and experimental investigation of multiplexed acoustic holograms at both audio and ultrasonic frequencies via a rationally designed transmission‐type acoustic metamaterial. The proposed metahologram is composed of two Fabry–Pérot resonant channels per unit cell, which enables the simultaneous modulation of the transmitted amplitude and phase at two desired frequencies. In contrast to conventional acoustic metamaterial‐based holograms, the design strategy proposed here provides a new degree of freedom (frequency) that can actively tailor holograms that are otherwise completely passive and significantly enhances the information encoded in acoustic metamaterials. To demonstrate the multiplexed acoustic metamaterial, the projection of two different high‐quality metaholograms is first shown at 14 and 17 kHz, with the patterns of the letters N and S. Then, two‐channel ultrasound focusing and annular beams generation for the incident ultrasonic frequencies of 35 and 42.5 kHz are demonstrated. These multiplexed acoustic metaholograms offer a technical advance to tackle the rising challenges in the fields of acoustic metamaterials, architectural acoustics, and medical ultrasound.}, number={27}, journal={ADVANCED FUNCTIONAL MATERIALS}, publisher={Wiley}, author={Zhu, Yifan and Gerard, Nikhil J. R. K. and Xia, Xiaoxing and Stevenson, Grant C. and Cao, Liyun and Fan, Shiwang and Spadaccini, Christopher M. and Jing, Yun and Assouar, Badreddine}, year={2021}, month={Jul} }
 @article{gerard_oudich_xu_yao_cui_naify_ikei_rohde_zheng_jing_2021, title={Three-Dimensional Trampolinelike Behavior in an Ultralight Elastic Metamaterial}, volume={16}, ISSN={["2331-7019"]}, url={https://doi.org/10.1103/PhysRevApplied.16.024015}, DOI={10.1103/PhysRevApplied.16.024015}, abstractNote={Elastic metamaterials possess band gaps, or frequency ranges that are forbidden to wave propagation. Existing solutions for impeding three-dimensional (3D) wave propagation largely rest on high-volume fractions of mass inclusions that induce and tailor negative effective density-based local resonances. This study introduces a class of elastic metamaterials that achieve low-frequency band gaps with a volume fraction as low as 3% (mass density as low as $0.034\phantom{\rule{0.1em}{0ex}}\mathrm{g}/{\mathrm{cm}}^{3}$). The working of the proposed design hinges on a 3D trampolinelike mode behavior that gives rise to wide, omnidirectional, and low-frequency band gaps for elastic waves despite very low-mass densities. Such a 3D trampoline effect is derived from a network of overhanging nodal microarchitectures that act as locally resonating elements, which give rise to band gaps at low frequencies. The dynamic effective properties of the metamaterial are numerically examined, which reveal that the band gap associated with the trampoline effect is resulted from a negative effective modulus coupled with a near-zero yet positive effective density. The experimental characterization is then made possible by fabricating the metamaterial via a light-based printing system that is capable of realizing microarchitectures with overhanging microfeatures. This design strategy could be useful to applications where simultaneous light weight and vibration control is desired.}, number={2}, journal={PHYSICAL REVIEW APPLIED}, author={Gerard, Nikhil Jrk and Oudich, Mourad and Xu, Zhenpeng and Yao, Desheng and Cui, Huachen and Naify, Christina J. and Ikei, Alec and Rohde, Charles A. and Zheng, Xiaoyu and Jing, Yun}, year={2021}, month={Aug} }
 @article{xu_hensleigh_gerard_cui_oudich_chen_jing_zheng_2021, title={Vat photopolymerization of fly-like, complex micro-architectures with dissolvable supports}, volume={47}, ISSN={["2214-7810"]}, DOI={10.1016/j.addma.2021.102321}, abstractNote={Recent advances in additive manufacturing of complex geometries enabled the creation of mechanical metamaterials whose exotic properties are based on local control of complex cell geometries. Overhanging and free-hanging features that lack continuous support layers in the previous build volume cannot be directly manufactured, imposing a major design limitation. The resulting metamaterials are limited to single homogenous structural materials, and inherently self-supporting geometries, resulting in constraints of achievable architectures. Realizing arbitrary features is compelling but is inherently limited by process and material support constraints. Here we present a novel light-based additive manufacturing approach capable of printing arbitrary micro-architectures comprising a large array of internally suspended features, large span overhang, and high aspect ratio struts. This method eliminates the need for manual removal of internal supports and enables a suite of multi-functional metamaterials with a range of designed properties, including wide bandgaps for elastic waves at low frequency, switchable wave transmissions, and products requiring no post support removal. We describe the synthesis and rapid printing of a variety of metamaterials comprising an extensive array of suspended features and demonstrate their metamaterial behaviors. The proposed approach removes scale and unit cell limitations and is capable of achieving embedded features across multiple materials.}, journal={ADDITIVE MANUFACTURING}, author={Xu, Zhenpeng and Hensleigh, Ryan and Gerard, Nikhil J. R. K. and Cui, Huachen and Oudich, Mourad and Chen, Wentao and Jing, Yun and Zheng, Xiaoyu}, year={2021}, month={Nov} }
 @article{gerard_jing_2020, title={Loss in acoustic metasurfaces: a blessing in disguise}, volume={10}, ISSN={["2159-6867"]}, url={http://dx.doi.org/10.1557/mrc.2019.148}, DOI={10.1557/mrc.2019.148}, abstractNote={From being an unfavorable consequence to finding itself as the intended imaginary part of a non-Hermitian system, loss has truly emerged as more of a friend than a foe in the context of acoustic metasurfaces. With the promising features of sub-wavelength geometries and the rapid advances in manufacturing techniques that can enable their realization, loss becomes a central topic of discussion. Further, the capability of introducing and tailoring loss allows it to serve as a new degree of freedom in passive wavefront shaping devices. In this review, the authors look back at the recent progress in the field of lossy acoustic metasurfaces. The background behind loss in deep sub-wavelength geometries and the instinctive responses to treat them and exploit them are overviewed, followed by more recent works that embrace and tailor their behavior for unconventional applications. The forthcoming years for acoustic metasurfaces thus hold several promising avenues for exploration, with loss as the protagonist.}, number={1}, journal={MRS COMMUNICATIONS}, author={Gerard, Nikhil J. R. K. and Jing, Yun}, year={2020}, month={Mar}, pages={32–41} }
 @article{deng_oudich_gerard_ji_lu_jing_2020, title={Magic-angle bilayer phononic graphene}, url={https://doi.org/10.1103/PhysRevB.102.180304}, DOI={10.1103/PhysRevB.102.180304}, abstractNote={Thanks to the recent discovery on the magic-angle bilayer graphene, twistronics is quickly becom11 ing a burgeoning field in condensed matter physics. This letter expands the realm of twistronics to acoustics by introducing twisted bilayer phononic graphene, which remarkably also harbors the magic angle, evidenced by the associated ultra-flat bands. Beyond mimicking quantum mechanical behaviors of twisted bilayer graphene, we show that their acoustic counterpart offers a considerably more straightforward and robust way to alter the interlayer hopping strength, enabling us to unlock magic angles (> 3 degrees) inaccessible in classical twisted bilayer graphene. This study, not only establishes the acoustical analog of twisted (magic-angle) bilayer graphene, providing a testbed more easily accessible to probe the interaction and misalignment between stacked 2D materials, but also points out the direction to a new phononic crystal design paradigm that could benefit applications such as enhanced acoustic emission and sensing.}, journal={Physical Review B}, author={Deng, Yuanchen and Oudich, Mourad and Gerard, Nikhil JRK and Ji, Jun and Lu, Minghui and Jing, Yun}, year={2020}, month={Nov} }
 @article{gerard_cui_shen_xie_cummer_zheng_jing_2019, title={Fabrication and experimental demonstration of a hybrid resonant acoustic gradient index metasurface at 40 kHz}, volume={114}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.5095963}, DOI={10.1063/1.5095963}, abstractNote={Over the past few years, acoustic gradient index metasurfaces (GIMs) have been actively studied for the numerous wave control capabilities that they facilitate. Previous research, however, has primarily focused on GIMs that operate in the audible frequency range, due to the difficulties in fabricating such intricate structures at the millimeter and submillimeter scales, for ultrasonic applications. In this work, we design, fabricate, and experimentally demonstrate the working of a hybrid resonant acoustic gradient index metasurface for airborne ultrasound at 40 kHz. The fabrication of such a GIM is made possible by projection microstereolithography, an emerging additive manufacturing technique. Numerical simulations were conducted to verify the metasurface design, and experiments were performed to corroborate these simulations. The stronger dissipation associated with airborne ultrasound is highlighted in this paper. The experimental demonstration of such a metasurface for airborne ultrasound could further its prospects as a candidate for miniaturized acoustic devices.}, number={23}, journal={APPLIED PHYSICS LETTERS}, author={Gerard, Nikhil J. R. K. and Cui, Huachen and Shen, Chen and Xie, Yangbo and Cummer, Steven and Zheng, Xiaoyu and Jing, Yun}, year={2019}, month={Jun} }
 @article{gerard_li_jing_2018, title={Investigation of acoustic metasurfaces with constituent material properties considered}, volume={123}, ISSN={["1089-7550"]}, url={http://dx.doi.org/10.1063/1.5007863}, DOI={10.1063/1.5007863}, abstractNote={This paper examines the transmission behavior of two acoustic metasurfaces and their constituent structural units while including the various material properties that could affect their functionality. The unit cells and the metasurfaces are modeled numerically, and the impact of the structural interaction and thermoviscosity on sound transmission and phase modulation is studied. Each of these effects is viewed individually in order to better understand their influence. Various cases are presented, and the change in the behavior of the metasurfaces is investigated. The deviations from the ideal desired results are examined and highlighted to show that it is important to incorporate these effects to better predict the behavior of acoustic metasurfaces.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Gerard, Nikhil J. R. K. and Li, Yong and Jing, Yun}, year={2018}, month={Mar} }