52 research outputs found
Electrodynamic models of 2D materials: can we match thin film and single sheet approaches?
The electromagnetic properties of 2D materials are modeled either as single
sheets with a surface susceptibility or conductivity, or as thin films of
finite thickness with an effective permittivity. Their intrinsic anisotropy,
however, has to be fully described to reliably predict the optical response of
systems based on 2D materials or to unambiguously interpret experimental data.
In the present work, we compare the two approaches within the transfer matrix
formalism and provide analytical relations between them. We strongly emphasize
the consequences of the anisotropy. In particular, we demonstrate the crucial
role of the choice of the thin film's effective thickness compared with the
parameters of the single sheet approach and therefore the computed properties
of the 2D material under study. Indeed, if the isotropic thin film model with
very low thickness is similar to an anisotropic single sheet with no
out-of-plane response, with larger thickness it matches with a single sheet
with isotropic susceptibility, in the reasonable small phase condition. We
illustrate our conclusions on extensively studied experimental quantities such
as transmittance, ellipsometry and optical contrast, and we discuss
similarities and discrepancies reported in the literature when using single
sheet or thin film models
Photonic Structures Optimization Using Highly Data-Efficient Deep Learning: Application To Nanofin And Annular Groove Phase Masks
Metasurfaces offer a flexible framework for the manipulation of light
properties in the realm of thin film optics. Specifically, the polarization of
light can be effectively controlled through the use of thin phase plates. This
study aims to introduce a surrogate optimization framework for these devices.
The framework is applied to develop two kinds of vortex phase masks (VPMs)
tailored for application in astronomical high-contrast imaging. Computational
intelligence techniques are exploited to optimize the geometric features of
these devices. The large design space and computational limitations necessitate
the use of surrogate models like partial least squares Kriging, radial basis
functions, or neural networks. However, we demonstrate the inadequacy of these
methods in modeling the performance of VPMs. To address the shortcomings of
these methods, a data-efficient evolutionary optimization setup using a deep
neural network as a highly accurate and efficient surrogate model is proposed.
The optimization process in this study employs a robust particle swarm
evolutionary optimization scheme, which operates on explicit geometric
parameters of the photonic device. Through this approach, optimal designs are
developed for two design candidates. In the most complex case, evolutionary
optimization enables optimization of the design that would otherwise be
impractical (requiring too much simulations). In both cases, the surrogate
model improves the reliability and efficiency of the procedure, effectively
reducing the required number of simulations by up to 75% compared to
conventional optimization techniques
Direct observation of ideal electromagnetic fluids
Near-zero-index (NZI) media have been theoretically identified as media where electromagnetic radiations behave like ideal electromagnetic fluids. Within NZI media, the electromagnetic power flow obeys equations similar to those of motion for the velocity field in an ideal fluid, so that optical turbulence is intrinsically inhibited. Here, we experimentally observe the electromagnetic power flow distribution of such an ideal electromagnetic fluid propagating within a cutoff waveguide by a semi-analytical reconstruction technique. This technique provides direct proof of the inhibition of electromagnetic vorticity at the NZI frequency, even in the presence of complex obstacles and topological changes in the waveguide. Phase uniformity and spatially-static field distributions, essential characteristics of NZI materials, are also observed. Measurement of the same structure outside the NZI frequency range reveals existence of vortices in the power flow, as expected for conventional optical systems. Therefore, our results provide an important step forward in the development of ideal electromagnetic fluids, and introduce a tool to explore the subwavelength behavior of NZI media including fully vectorial and phase information. © 2022, The Author(s).Y.L. acknowledges partial support from National Natural Science Foundation of China (NSFC) under grant 62022045. I.L. acknowledges support from Ramón y Cajal fellowship RYC2018-024123-I and project RTI2018-093714-301J-I00 sponsored by MCIU/AEI/FEDER/UE and ERC Starting Grant 948504
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