111 research outputs found
Efficient light coupling from integrated single-mode waveguides to supercollimating photonic crystals on silicon-on-insulator platforms
We propose a practical and efficient solution for the coupling of light from
integrated single-mode waveguides to supercollimating planar photonic crystals
on conventional silicon-on-insulator platforms. The device consists of a rib
waveguide, designed to sustain spatially extended single-modes and matched to a
supercollimating photonic crystal, which has been truncated at its boundary to
improve impedance matching between the two photonic components.
Three-dimensional simulations show transmission efficiencies up to 96 % and
reflections below 0.2 % at wavelengths close to 1.55 microns. This approach
constitutes a significant step toward the integration of supercollimating
structures on photonic chips.Comment: 11 pages, 4 figure
Light Transport and localization in two-dimensional correlated disorder
Structural correlations in disordered media are known to affect significantly the propagation of waves. In this Letter, we theoretically investigate the transport and localization of light in 2D photonic structures with short-range correlated disorder. The problem is tackled semianalytically using the Baus-Colot model for the structure factor of correlated media and a modified independent scattering approximation. We find that short-range correlations make it possible to easily tune the transport mean free path by more than a factor of 2 and the related localization length over several orders of magnitude. This trend is confirmed by numerical finite-difference time-domain calculations. This study therefore shows that disorder engineering can offer fine control over light transport and localization in planar geometries, which may open new opportunities in both fundamental and applied photonics research
First-principles calculation of the temperature dependence of the optical response of bulk GaAs
A novel approach has been developed to calculate the temperature dependence
of the optical response of a semiconductor. The dielectric function is averaged
over several thermally perturbed configurations that are extracted from
molecular dynamic simulations. The calculated temperature dependence of the
imaginary part of the dielectric function of GaAs is presented in the range
from 0 to 700 K. This approach that explicitly takes into account lattice
vibrations describes well the observed thermally-induced energy shifts and
broadening of the dielectric function.Comment: 6 pages, 3 figure
Efficient light coupling into a photonic crystal waveguide with flatband slow mode
We design an efficient coupler to transmit light from a strip waveguide into
the flatband slow mode of a photonic crystal waveguide with ring-shaped holes.
The coupler is a section of a photonic crystal waveguide with a higher group
velocity, obtained by different ring dimensions. We demonstrate coupling
efficiency in excess of 95% over the 8 nm wavelength range where the photonic
crystal waveguide exhibits a quasi constant group velocity vg = c/37. An
analysis based on the small Fabry-P\'erot resonances in the simulated
transmission spectra is introduced and used for studying the effect of the
coupler length and for evaluating the coupling efficiency in different parts of
the coupler. The mode conversion efficiency within the coupler is more than
99.7% over the wavelength range of interest. The parasitic reflectance in the
coupler, which depends on the propagation constant mismatch between the slow
mode and the coupler mode, is lower than 0.6% within this wavelength range.Comment: 11 pages, 7 figures, submitted to Photonics and Nanostructures -
Fundamentals and Application
Composites of resonant dielectric rods: A test of their behavior as metamaterial refractive elements
We report numerical experiments of optical wave propagation in composites of
high refractive index dielectric rods at frequencies where their first electric
and magnetic Mie resonances are excited. The arrays of these particles have
been extensively studied and proposed as non-absorbing and isotropic
metamaterials. We show that negative refraction, observed in ordered particle
arrays, is due to diffraction and that an effective medium theory yields
constitutive parameters that do not reproduce the observations in these
composites, whose transmission also depends on the sample shape. This is
further confirmed by disordering the arrays, a case in which large transmission
losses appear due to extinction by resonant scattering from the particles.
Therefore, these composites although little absorbing have large extinction due
to scattering
Strong magnetic response of submicron Silicon particles in the infrared
High-permittivity dielectric particles with resonant magnetic properties are
being explored as constitutive elements of new metamaterials and devices in the
microwave regime. Magnetic properties of low-loss dielectric nanoparticles in
the visible or infrared are not expected due to intrinsic low refractive index
of optical materials in these regimes. Here we analyze the dipolar electric and
magnetic response of loss-less dielectric spheres made of moderate permittivity
materials. For low material refractive index there are no sharp resonances due
to strong overlapping between different multipole contributions. However, we
find that Silicon particles with refractive index 3.5 and radius approx. 200nm
present a dipolar and strong magnetic resonant response in telecom and
near-infrared frequencies, (i.e. at wavelengths approx. 1.2-2 micrometer).
Moreover, the light scattered by these Si particles can be perfectly described
by dipolar electric and magnetic fields, quadrupolar and higher order
contributions being negligible.Comment: 10 pages, 5 figure
Photon Management in Two-Dimensional Disordered Media
Elaborating reliable and versatile strategies for efficient light coupling
between free space and thin films is of crucial importance for new technologies
in energy efficiency. Nanostructured materials have opened unprecedented
opportunities for light management, notably in thin-film solar cells. Efficient
coherent light trapping has been accomplished through the careful design of
plasmonic nanoparticles and gratings, resonant dielectric particles and
photonic crystals. Alternative approaches have used randomly-textured surfaces
as strong light diffusers to benefit from their broadband and wide-angle
properties. Here, we propose a new strategy for photon management in thin films
that combines both advantages of an efficient trapping due to coherent optical
effects and broadband/wide-angle properties due to disorder. Our approach
consists in the excitation of electromagnetic modes formed by multiple light
scattering and wave interference in two-dimensional random media. We show, by
numerical calculations, that the spectral and angular responses of thin films
containing disordered photonic patterns are intimately related to the in-plane
light transport process and can be tuned through structural correlations. Our
findings, which are applicable to all waves, are particularly suited for
improving the absorption efficiency of thin-film solar cells and can provide a
novel approach for high-extraction efficiency light-emitting diodes
Silicon Mie Resonators for Highly Directional Light Emission from monolayer MoS2
Controlling light emission from quantum emitters has important applications
ranging from solid-state lighting and displays to nanoscale single-photon
sources. Optical antennas have emerged as promising tools to achieve such
control right at the location of the emitter, without the need for bulky,
external optics. Semiconductor nanoantennas are particularly practical for this
purpose because simple geometries, such as wires and spheres, support multiple,
degenerate optical resonances. Here, we start by modifying Mie scattering
theory developed for plane wave illumination to describe scattering of dipole
emission. We then use this theory and experiments to demonstrate several
pathways to achieve control over the directionality, polarization state, and
spectral emission that rely on a coherent coupling of an emitting dipole to
optical resonances of a Si nanowire. A forward-to-backward ratio of 20 was
demonstrated for the electric dipole emission at 680 nm from a monolayer MoS2
by optically coupling it to a Si nanowire
Past Achievements and Future Challenges in 3D Photonic Metamaterials
Photonic metamaterials are man-made structures composed of tailored micro- or
nanostructured metallo-dielectric sub-wavelength building blocks that are
densely packed into an effective material. This deceptively simple, yet
powerful, truly revolutionary concept allows for achieving novel, unusual, and
sometimes even unheard-of optical properties, such as magnetism at optical
frequencies, negative refractive indices, large positive refractive indices,
zero reflection via impedance matching, perfect absorption, giant circular
dichroism, or enhanced nonlinear optical properties. Possible applications of
metamaterials comprise ultrahigh-resolution imaging systems, compact
polarization optics, and cloaking devices. This review describes the
experimental progress recently made fabricating three-dimensional metamaterial
structures and discusses some remaining future challenges
- …