We use inverse design to discover metalens structures that exhibit broadband, achromatic focusing across low, moderate, and high numerical apertures. We show that standard unit-cell approaches cannot achieve high-efficiency high-NA focusing, even at a single frequency, due to the incompleteness of the unit-cell basis, and we provide computational upper bounds on their maximum efficiencies. At low NA, our devices exhibit the highest theoretical efficiencies to date. At high NA-of 0.9 with translation-invariant films and of 0.99 with "freeform" structures-our designs are the first to exhibit achromatic high-NA focusing. arXiv:1905.09213v1 [physics.optics]

Tunable metasurfaces have demonstrated the potential for dramatically enhanced functionality for applications including sensing, ranging and imaging. Liquid crystals (LCs) have fast switching speeds, low cost, and mature technological development, offering a versatile platform for electrical tunability. However, to date, electrically tunable metasurfaces are typically designed at a single operational state using physical intuition, without controlling alternate states and thus leading to limited switching efficiencies (<30%) and small angular deflection (<25°).Here, we use large-scale computational "inverse design" to discover high-performance designs through adjoint-based local-optimization design iterations within a global-optimization search. We study and explain the physics of these devices, which heavily rely on sophisticated resonator design to fully utilize the very small permittivity change incurred by switching the liquid-crystal voltage. The optimal devices show tunable deflection angles ranging from 12°to 144°and switching efficiencies above 80%, exhibiting 6× angular improvements and 6× efficiency improvements compared to the current state-of-the-art.

SUMMARY Despite regulatory concerns over opinion shopping (OS) behavior, there exists little systematic evidence on the prevalence and consequences of OS to avoid a going concern opinion (GCO). Using Lennox's (2000) framework to identify OS, we find that distressed firms successfully engage in OS to avoid a GCO. Moreover, clients engaging in OS exhibit a higher ex post Type II error rate in audit opinions than clients that do not, and the higher Type II error rate is salient for clients switching auditors for OS but not for clients retaining auditors for OS. We continue to find this asymmetric effect of the two types of OS on audit quality measured by restatements. These results indicate that auditor switching for OS not only results in a higher likelihood of audit reporting failures but also impairs other dimensions of audit quality, while auditor retaining for OS has little adverse effect on audit quality.

Nanophotonic devices have enabled microscopic control of light with an unprecedented spatial resolution by employing subwavelength optical elements that can strongly interact with incident waves. However, to date, most nanophotonic devices have been designed based on fixed-shape optical elements, and a large portion of their design potential has remained unexplored. It is only recently that free-form design schemes have been spotlighted in nanophotonics, offering routes to make a break from conventional design constraints and utilize the full design potential. In this review, we systematically overview the nascent yet rapidly growing field of free-form nanophotonic device design. We attempt to define the term “free-form” in the context of photonic device design, and survey different strategies for free-form optimization of nanophotonic devices spanning from classical methods, adjoint-based methods, to contemporary machine-learning-based approaches.

This work presents an accurate finite‐difference time‐domain (FDTD) dispersive modeling of concrete materials with different water/cement ratios in 50 MHz to 1 GHz. A quadratic complex rational function (QCRF) is employed for dispersive modeling of the relative permittivity of concrete materials. To improve the curve fitting of the QCRF model, the Newton iterative method is applied to determine a weighting factor. Numerical examples validate the accuracy of the proposed dispersive FDTD modeling.