37 research outputs found
Absorption enhancing proximity effects in aperiodic nanowire arrays
Aperiodic Nanowire (NW) arrays have higher absorption than equivalent
periodic arrays, making them of interest for photovoltaic applications. An
inevitable property of aperiodic arrays is the clustering of some NWs into
closer proximity than in the equivalent periodic array. We focus on the modes
of such clusters and show that the reduced symmetry associated with cluster
formation allows external coupling into modes which are dark in periodic
arrays, thus increasing absorption. To exploit such modes fully, arrays must
include tightly clustered NWs that are unlikely to arise from fabrication
variations but must be created intentionally.Comment: Accepted by Optics Expres
Modes of Random Lasers
In conventional lasers, the optical cavity that confines the photons also
determines essential characteristics of the lasing modes such as wavelength,
emission pattern, ... In random lasers, which do not have mirrors or a
well-defined cavity, light is confined within the gain medium by means of
multiple scattering. The sharp peaks in the emission spectra of semiconductor
powders, first observed in 1999, has therefore lead to an intense debate about
the nature of the lasing modes in these so-called lasers with resonant
feedback. In this paper, we review numerical and theoretical studies aimed at
clarifying the nature of the lasing modes in disordered scattering systems with
gain. We will discuss in particular the link between random laser modes near
threshold (TLM) and the resonances or quasi-bound (QB) states of the passive
system without gain. For random lasers in the localized regime, QB states and
threshold lasing modes were found to be nearly identical within the scattering
medium. These studies were later extended to the case of more lossy systems
such as random systems in the diffusive regime where differences between
quasi-bound states and lasing modes were measured. Very recently, a theory able
to treat lasers with arbitrarily complex and open cavities such as random
lasers established that the TLM are better described in terms of the so-called
constant-flux states.Comment: Review paper submitted to Advances in Optics and Photonic
Suppression of Anderson localization in disordered metamaterials
We study wave propagation in mixed, 1D disordered stacks of alternating right- and left-handed layers and reveal that the introduction of metamaterials substantially suppresses Anderson localization. At long wavelengths, the localization length in mixed stacks is orders of magnitude larger than for normal structures, proportional to the sixth power of the wavelength, in contrast to the usual quadratic wavelength dependence of normal systems. Suppression of localization is also exemplified in long-wavelength resonances which largely disappear when left-handed materials are introduced
Effects of polarization on the transmission and localization of classical waves in weakly scattering metamaterials
We summarize the results of our comprehensive analytical and numerical
studies of the effects of polarization on the Anderson localization of
classical waves in one-dimensional random stacks. We consider homogeneous
stacks composed entirely of normal materials or metamaterials, and also mixed
stacks composed of alternating layers of a normal material and metamaterial. We
extend the theoretical study developed earlier for the case of normal incidence
[A. A. Asatryan et al, Phys. Rev. B 81, 075124 (2010)] to the case of off-axis
incidence. For the general case where both the refractive indices and layer
thicknesses are random, we obtain the long-wave and short-wave asymptotics of
the localization length over a wide range of incidence angles (including the
Brewster ``anomaly'' angle). At the Brewster angle, we show that the long-wave
localization length is proportional to the square of the wavelength, as for the
case of normal incidence, but with a proportionality coefficient substantially
larger than that for normal incidence. In mixed stacks with only
refractive-index disorder, we demonstrate that p-polarized waves are strongly
localized, while for s-polarization the localization is substantially
suppressed, as in the case of normal incidence. In the case of only thickness
disorder, we study also the transition from localization to delocalization at
the Brewster angle.Comment: 15 pages, 11 figures, accepted for publication in PR
Optimizing Photovoltaic Charge Generation of Nanowire Arrays: A Simple Semi-Analytic Approach
Nanowire arrays exhibit efficient light coupling and strong light trapping,
making them well suited to solar cell applications. The processes that
contribute to their absorption are interrelated and highly dispersive, so the
only current method of optimizing the absorption is by intensive numerical
calculations. We present an efficient alternative which depends solely on the
wavelength-dependent refractive indices of the constituent materials. We choose
each array parameter such that the number of modes propagating away from the
absorber is minimized while the number of resonant modes within the absorber is
maximized. From this we develop a semi-analytic method that quantitatively
identifies the small range of parameters where arrays achieve maximum short
circuit currents. This provides a fast route to optimizing NW array cell
efficiencies by greatly reducing the geometries to study with full device
models. Our approach is general and applies to a variety of materials and to a
large range of array thicknesses.Comment: Accepted by ACS Photonic
Anderson localization in metamaterials and other complex media
We review some recent (mostly ours) results on the Anderson localization of
light and electron waves in complex disordered systems, including: (i)
left-handed metamaterials, (ii) magneto-active optical structures, (iii)
graphene superlattices, and (iv) nonlinear dielectric media. First, we
demonstrate that left-handed metamaterials can significantly suppress
localization of light and lead to an anomalously enhanced transmission. This
suppression is essential at the long-wavelength limit in the case of normal
incidence, at specific angles of oblique incidence (Brewster anomaly), and in
the vicinity of the zero-epsilon or zero-mu frequencies for dispersive
metamaterials. Remarkably, in disordered samples comprised of alternating
normal and left-handed metamaterials, the reciprocal Lyapunov exponent and
reciprocal transmittance increment can differ from each other. Second, we study
magneto-active multilayered structures, which exhibit nonreciprocal
localization of light depending on the direction of propagation and on the
polarization. At resonant frequencies or realizations, such nonreciprocity
results in effectively unidirectional transport of light. Third, we discuss the
analogy between the wave propagation through multilayered samples with
metamaterials and the charge transport in graphene, which enables a simple
physical explanation of unusual conductive properties of disordered graphene
superlatices. We predict disorder-induced resonances of the transmission
coefficient at oblique incidence of the Dirac quasiparticles. Finally, we
demonstrate that an interplay of nonlinearity and disorder in dielectric media
can lead to bistability of individual localized states excited inside the
medium at resonant frequencies. This results in nonreciprocity of the wave
transmission and unidirectional transport of light.Comment: 37 pages, 30 figures, Review pape
Green's functions and relative local density of states in two-dimensional lossy structured systems
The local density of states (LDOS) is a function of spatial position and frequency which governs the radiation properties of sources placed within structured optical systems. We show how the enhancement or suppression of the relative LDOS, comparing two-dimensional systems, may be computed from Green's tensors obeying the two-dimensional Helmholtz equation and electromagnetic boundary conditions, both around and within a coated, lossy, non-magnetic cylinder. We illustrate the spatial and spectral variation of this relative LDOS with numerical results for both principal cases of polarization, with either the magnetic or the electric field of the Green's function source along the cylinder axis
Radiolarian ages for the sedimentary cover of sevan ophiolite (Armenia, lesser Caucasus)
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