Optics (physics.optics)

Abstract: High-index Mie-resonant dielectric nanostructures provide a new framework to manipulate light at the nanoscale. In particular their local field confinement together with their inherently low losses at frequencies below their band-gap energy allows to efficiently boost and control linear and nonlinear optical processes. Here, we investigate nanoantennas composed of a thin indium-tin oxide layer in the center of a dielectric Gallium Phosphide nanodisk. While the linear response is similar to that of a pure GaP nanodisk, we show that the second and third-harmonic signals of the nanogap antenna are boosted at resonance. Linear and nonlinear finite-difference time-domain simulations show that the high refractive index contrast leads to strong field confinement inside the antenna's ITO layer. Measurement of ITO and GaP nonlinear susceptibilities deliver insight on how to engineer nonlinear nanogap antennas for higher efficiencies for future nanoscale devices.

Abstract: Symmetry breaking plays a crucial role in understanding the fundamental physics underlying numerous physical phenomena, including the electromagnetic response in resonators, giving rise to intriguing effects such as directional light scattering, supercavity lasing, and topologically protected states. In this work, we demonstrate that adding a small fraction of lossy metal (as low as $1\times10^{-6}$ in volume), to a lossless dielectric resonator breaks inversion symmetry thereby lifting its degeneracy, leading to a strong bianisotropic response. In the case of the metasurface composed of such resonators, this effect leads to unidirectional perfect absorption while maintaining nearly perfect reflection from the opposite direction. We have developed more general Onsager-Casimir relations for the polarizabilities of particle arrays, taking into account the contributions of quadrupoles, which shows that bianisotropy is not solely due to dipoles, but also involves high-order multipoles. Our experimental validation demonstrates an extremely thin terahertz-perfect absorber with a wavelength-to-thickness ratio of up to 25,000, where the material thickness is only 2% of the theoretical minimum thickness dictated by the fundamental limit. Our findings have significant implications for a variety of applications, including energy harvesting, thermal management, single-photon detection, and low-power directional emission.

Abstract: Optical imaging deep inside scattering media is an outstanding problem across many disciplines. Numerical modeling can accelerate progress by providing the ground truth, the flexibility to tailor the system and the imaging scheme, and the ease of comparing different methods. Here we realize quantitative modeling of five scattering-based imaging methods through multi-source simulations of the inhomogeneous wave equation in a two-dimensional scattering medium that is roughly 800 wavelengths by 550 wavelengths in size. These large-scale simulations are enabled by a new "augmented partial factorization" numerical approach. We analyze the recently proposed scattering matrix tomography method, reflectance confocal microscopy, optical coherence tomography, optical coherence microscopy, and interferometric synthetic aperture microscopy for imaging embedded nanoparticle targets. Having access to the ground-truth configuration, we can rigorously assess the performance and the limit of these methods while identifying artifacts that are otherwise impossible to detect. Such numerical experiments are cheap, convenient, versatile, and provide an ideal testbed for developing new imaging methods and algorithms.