Broadband Multifunctional Efficient Meta-Gratings Based on Dielectric Waveguide Phase Shifters

Molding the wavefront of light is a basic principle of any optical design. In conventional optical components such as lenses and waveplates, the wavefront is controlled via propagation phases in a medium much thicker than the wavelength. Metasurfaces instead typically produce the required phase changes using subwavelength-sized resonators as phase shift elements patterned across a surface. This “flat optics” approach promises miniaturization and improved performance. Here we introduce metasurfaces which use dielectric ridge waveguides (DRWs) as phase shift elements in which the required phase accumulation is achieved via propagation over a subwavelength distance. By engineering the dispersive response of DRWs, we experimentally realize high resolving power meta-gratings with broadband (λ = 1.2–1.7 μm) and efficient routing (splitting and bending) into a single diffraction order, thus overcoming the limits of blazed gratings. In addition, we demonstrate polarization beam splitting capabilities with large suppression ratios

Buried Nanoantenna Arrays: Versatile Antireflection Coating

Reflection is usually a detrimental phenomenon in many applications such as flat-panel-displays, solar cells, photodetectors, infrared sensors, and lenses. Thus far, to control and suppress the reflection from a substrate, numerous techniques including dielectric interference coatings, surface texturing, adiabatic index matching, and scattering from plasmonic nanoparticles have been investigated. A new technique is demonstrated to manage and suppress reflection from lossless and lossy substrates. It provides a wider flexibility in design versus previous methods. Reflection from a surface can be suppressed over a narrowband, wideband, or multiband frequency range. The antireflection can be dependent or independent of the incident wave polarization. Moreover, antireflection at a very wide incidence angle can be attained. The reflection from a substrate is controlled by a buried nanoantenna array, a structure composed of (1) a subwavelength metallic array and (2) a dielectric cover layer referred to as a superstrate. The material properties and thickness of the superstrate and nanoantennas’ geometry and periodicity control the phase and intensity of the wave circulating inside the superstrate cavity. A minimum reflectance of 0.02% is achieved in various experiments in the mid-infrared from a silicon substrate. The design can be integrated in straightforward way in optical devices. The proposed structure is a versatile AR coating to optically impedance matches any substrate to free space in selected any narrow and broadband spectral response across the entire visible and infrared spectrum

Meta-Lens Doublet in the Visible Region

Recently, developments in meta-surfaces have allowed for the possibility of a fundamental shift in lens manufacturingfrom the century-old grinding technology to nanofabricationopening a way toward mass producible high-end meta-lenses. Inspired by early camera lenses and to overcome the aberrations of planar single-layered meta-lenses, we demonstrate a compact meta-lens doublet by patterning two metasurfaces on both sides of a substrate. This meta-lens doublet has a numerical aperture of 0.44, a focal length of 342.5 μm, and a field of view of 50° that enables diffraction-limited monochromatic imaging along the focal plane at a wavelength of 532 nm. The compact design has various imaging applications in microscopy, machine vision, and computer vision

Visualization 1: High-quality-factor planar optical cavities with laterally stopped, slowed, or reversed light

In this finite-difference time-domain simulation (Lumerical Inc.), a dipole emits light into "Structure A" (cf. Table 1 and Fig. 3(a)), then turns off. Much of the emitted light enters a mode which remains localized in the in-plane direction. Originally published in Optics Express on 08 August 2016 (oe-24-16-18399

Supplement 1: Ultracompact metasurface in-line polarimeter

Supplemental document Originally published in Optica on 20 January 2016 (optica-3-1-42

Observation of Nanoscale Refractive Index Contrast via Photoinduced Force Microscopy

Near-field optical microscopy (NSOM) is a scanning probe technique that allows optical imaging of sample surfaces with nanoscale resolution. Generally, all NSOM schemes rely on illuminating the sample surface and collecting the localized scattered light resulting from the interaction of the microscopes nanoscale probe with the sample surface in the illuminated region. Currently, a new set of nanospectroscopic techniques are being developed using Atomic Force Microscopes to detect optical interactions without detecting any light. One of these approaches is photoinduced force microscopy (PiFM), where local optical forces, originated by the illumination of the tip–sample region, are mechanically detected as forced oscillations of the cantilever of an atomic force microscope operating in a multifrequency mode. In this article we show high resolution nanoimaging via PiFM with a contrast only related to the local refractive index of a sample specifically designed to unambiguously decouple morphology from optical response at the nanoscale. Imaging lateral resolution better than 10 nm is obtained, and the optimization of the contrast mechanism is described. Our results represent a step forward in understanding the potential of the PiFM technique, showing the possibility of high resolution imaging of the local polarizability of the sample and subsequently using the mechanism to explore complex spectral behavior at the nanoscale

Holographic Metalens for Switchable Focusing of Surface Plasmons

Surface plasmons polaritons (SPPs) are light-like waves confined to the interface between a metal and a dielectric. Excitation and control of these modes requires components such as couplers and lenses. We present the design of a new lens based on holographic principles. The key feature is the ability to switchably control SPP focusing by changing either the incident wavelength or polarization. Using phase-sensitive near-field imaging of the surface plasmon wavefronts, we have observed their switchable focusing and steering as the wavelength or polarization is changed

Supplement 1: Lasers with distributed loss have a sublinear output power characteristic

Originally published in Optica on 20 January 2015 (optica-2-1-48

Designed Quasi-1D Potential Structures Realized in Compositionally Graded InAs_1–<i>x</i>P_<i>x</i> Nanowires

III–V semiconductor heterostructures are important components of many solid-state optoelectronic devices, but the ability to control and tune the electrical and optical properties of these structures in conventional device geometries is fundamentally limited by the bulk dimensionality and the inability to accommodate lattice-mismatched material combinations. Here we demonstrate how semiconductor nanowires may enable the creation of arbitrarily shaped one-dimensional potential structures for new types of designed device functionality. We describe the controlled growth of stepwise compositionally graded InAs_1–<i>x</i>P_<i>x</i> heterostructures defined along the axes of InAs nanowires, and we show that nanowires with sawtooth-shaped composition profiles behave as near-ideal unipolar diodes with ratchet-like rectification of the electron transport through the nanowires, in excellent agreement with simulations. This new type of designed quasi-1D potential structure represents a significant advance in band gap engineering and may enable fundamental studies of low-dimensional hot-carrier dynamics, in addition to constituting a platform for implementing novel electronic and optoelectronic device concepts

Do bilayer metasurfaces behave as a stack of decoupled single-layer metasurfaces?

Flat optics or metasurfaces have opened new frontiers in wavefront shaping and its applications. Polarization optics is one prominent area which has greatly benefited from the shape-birefringence of metasurfaces. However, flat optics comprising a single layer of meta-atoms can only perform a subset of polarization transformations, constrained by a symmetric Jones matrix. This limitation can be tackled using metasurfaces composed of bilayer meta-atoms but exhausting all possible combinations of geometries to build a bilayer metasurface library is a very daunting task. Consequently, bilayer metasurfaces have been widely treated as a cascade (product) of two decoupled single-layer metasurfaces. Here, we test the validity of this assumption by considering a metasurface made of TiO2 on fused silica substrate at a design wavelength of 532 nm. We explore regions in the design space where the coupling between the top and bottom layers can be neglected, i.e., producing a far-field response which approximates that of two decoupled single-layer metasurfaces. We complement this picture with the near-field analysis to explore the underlying physics in regions where both layers are strongly coupled. Our analysis is general and it allows the designer to efficiently build a multi-layer metasurface, either in transmission or reflection, by only running one full-wave simulation for a single-layer metasurface