In this work, we investigate the scattering behavior of nanorods that are randomly packed at various densities and aspect ratios. We show that the maximum packing density, maximum scattering density, and the percolation threshold are all tightly connected to Onsager excluded-area principle.
KEYWORDS: Beam shaping, Particles, Phase shifts, Near field, Information security, Polarization, Diffraction, Near field optics, Reflection, Optimization (mathematics)
Insensitivity of random systems to the polarization of incident light even for anisotropic and asymmetric particles, larger information capacity and higher level of information transport security as a result of larger degrees of freedom and the absence of spurious diffraction orders observed in periodic structures with large periodicity are among unique features making disordered structures a promising candidate to address challenges in the optical wave manipulations. Most of the metasurfaces are arranged in a periodic grid and the required phase profile for a desired performance is achieved by engineering elements via extracted information from periodic/unit cell simulation definitely not addressing the near field coupling between randomly positioned elements and so not helpful for the design of disordered metasurfaces. In this numerical study, we show how random arrangement of particles affect their phase shift compared to the periodic ones. We propose a new phase-map to design random metasurfaces benefiting from the statistical nature of random media and addressing the near field coupling between resonant elements. This phase-map provides us with the information on the geometry of particles located at random positions for a specific phase shift. Design of random metasurfaces by the proposed random phase-map reveals efficiency improvement compared to those designed based on periodic phase-map. We hope this new phase-map can pave the way towards random optical system outperforming the periodic counterpart in secure optical information processing.
Small semiconductor lasers have attracted a wide interest in academia owing to their potential as highly integrated components in photonic circuits. Particularly, microdisk lasers exploiting whispering gallery modes have been regarded as a good candidate because of ultrasmall modal volume and low threshold property. To exploit large index difference between gain and surrounding medium, microdisk resonators with an underlying post using undercut etching have been proposed and widely investigated in many previous studies. However, it has been challenging in microdisk laser to operate single mode due to the large number of exsiting whispering gallery modes. Here, we propose and demonstrate a novel subwavelength scale microdisk laser. InGaAsP multi-quantum wells microdisk is self-suspended in air by connecting bridges. The behavior of TE-like whispering gallery modes, which belong to most dominant class of mode in thin disk configuration, is both numerically and experimentally investigated. We highlight that bridge does not only provide mechanical stability, but the number of bridges can be an important factor to improve or suppress wave confinement of whispering gallery modes by protecting or breaking spatial symmetry of mode. Moreover, a suitable choice of bridges increases quality factor by up to 79% comparing to the microdisk resonator without bridges. Using this scheme, we numerically and experimentally investigate mode selectivity and further demonstrate single mode microdisk lasers operating at near-infrared telecommunication wavelength.
Mie theory describes how electromagnetic waves scatter at the interface between a homogeneous spherical dielectric particle surrounded by a material of a different optical index. Numerical improvements have allowed studying more complicated geometries with the multipole decomposition of the spherical harmonics. Hence, Mie theory is widely applied in theoretical and applied physics, to enable novel light manipulation, to model Fano resonances, nonlinear optics, or to design dielectric metamaterials. Recently, the anapole state has brought attention to the community as one of the most interesting phenomena. It can be interpreted as a destructive interference in the far field between the fields scattered by the toroidal and electrical dipoles at a given frequency. Such element is therefore transparent to any incoming plane wave. However, things are different if the element is excited in its near field, where it can be excited by an internal source. In this work, we experimentally demonstrate a semiconductor laser based on a single cylindrical resonator suspended in air. An epitaxially grown InGaAsP layer on an InP substrate is patterned by e-beam lithography. We study the shift of the Mie resonance as geometrical parameters are varied, and show how it affects the shift of the lasing frequency. Our investigation of Mie resonances from an active gain medium would is a rich platform to study nontrivial excitation of a complex field and paves the way to designing active devices exploiting Mie theory.
We consider gold plasmonic nanorods in the infrared domain. Such elements are very anisotropic and only polarizable along their longer dimension. Varying the nanorod length from 150 to 500 nm changes the resonant frequency of the element, which allows us to tune the phase-shift provided to an incident plane wave which electric field is parallel to the long axis. On the contrary, the nanorod is transparent to an incoming plane wave with a polarization perpendicular to its main axis. In order to provide a 0 to 2pi phase shift, we chose to work in reflection with metasurfaces made of elements with random positions and orientation. We emphasize that the length of each nanorod is not random, but strongly depends on the position of the element. It is chosen accordingly so that the reflected phase shift follows a parabolic law.
The focusing efficiency strongly depends on the density of nanorods but also of the dimensionality and of the symmetry of the metasurface. Using full wave simulations, we design ordered and random metalens and compare their characteristics. Unfortunately, simulating 2D large area metasurface is numerically challenging. Hence, we extract the transmission matrix parameters for single elements from our FDTD simulation, and model the metasurface as an array of two level atom scatterers
Finally, we present an experimental realization of such random metalens. The latter is made with conventional top-down fabrication techniques and e-beam lithography. We will show that the resulting lens focus light on diffraction limited focal spots for the two polarizations.
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