2,301 research outputs found
Optimized White Reflectance in Photonic Network Structures
Three-dimensional disordered networks are receiving increasing attention as
versatile architectures for highly scattering materials. However, due to their
complex morphology, little is still known about the interplay between their
structural and optical properties. Here, we describe a simple algorithm that
allows to generate photonic network structures inspired by that of the
Cyphochilus beetle, famous for the bright white reflectance of its thin
cuticular scales. The model allows to vary the degree of structural anisotropy
and filling fraction of the network independently, revealing the key
contribution of these two parameters to the overall scattering efficiency.
Rigorous numerical simulations show that the obtained structures can exceed the
broadband reflectance of the beetle while using less material, providing new
insights for the design of advanced scattering materials.Comment: 10 pages, 3 figures. peer reviewed version, published in final form
at https://doi.org/10.1002/adom.20190004
On the relation between the echo-peak shift and Brownian-oscillator correlation function
We show that for systems that exhibit bimodal dynamics in their system-bath correlation function the shift of the stimulated photon-echo maximum as a function of waiting time reflects fairly well the long time part of the correlation function. For early times this correspondence breaks down due to a fundamentally different behaviour of the echo-peak shift in this time domain and because of the effect of finite pulse duration on the echo-peak shift. The method is used to characterize the solvation dynamics in various dye solutions.
Theory of strong localization effects of light in disordered loss or gain media
We present a systematical theory for the interplay of strong localization
effects and absorption or gain of classical waves in 3-dimensional, disordered
dielectrics. The theory is based on the selfconsistent Cooperon resummation,
implementing the effects of energy conservation and its absorptive or emissive
corrections by an exact, generalized Ward identity. Substantial
renormalizations are found, depending on whether the absorption/gain occurs in
the scatterers or in the background medium. We find a finite, gain-induced
correlation volume which may be significantly smaller than the scale set by the
scattering mean free path, even if there are no truly localized modes. Possible
consequences for coherent feedback in random lasers as well as the possibility
of oscillatory in time behavior induced by sufficiently strong gain are
discussed.Comment: Published versio
Light diffusion and localization in 3D nonlinear disordered media
Using a 3D Finite-Difference Time-Domain parallel code, we report on the
linear and nonlinear propagation of light pulses in a disordered assembly of
scatterers, whose spatial distribution is generated by a Molecular Dynamics
code; refractive index dispersion is also taken into account. We calculate the
static and dynamical diffusion constant of light, while considering a pulsed
excitation. Our results are in quantitative agreement with reported
experiments, also furnishing evidence of a non-exponential decay of the
transmitted pulse in the linear regime and in the presence of localized modes.
By using an high power excitation, we numerically demonstrate the
``modulational instability random laser'': at high peak input powers energy is
transferred to localized states from the input pulse, via third-order
nonlinearity and optical parametric amplification, and this process is signed
by a power-dependent non-exponential time-decay of the transmitted pulse.Comment: 5 pages, 4 figures. Revised version with new figure 4 with localized
state
Structured Optical Materials Controlled by Light
Materials of which the optical response is determined by their structure are
of much interest both for their fundamental properties and applications.
Examples range from simple gratings to photonic crystals. Obtaining control
over the optical properties is of crucial importance in this context, and it is
often attempted by electro-optical effect or by using magnetic fields. In this
paper, we introduce the use of light to switch and tune the optical response of
a structured material, exploiting a physical deformation induced by light
itself. In this new strategy, light drives an elastic reshaping, which leads to
different spectral properties and hence to a change in the optical response.
This is made possible by the use of liquid crystalline networks structured by
Direct Laser Writing. As a proof of concept, a grating structure with
sub-millisecond time-response is demonstrated for optical beam steering
exploiting an optically induced reversible shape-change. Experimental
observations are combined with finite-element modeling to understand the
actuation process dynamics and to obtain information on how to tune the time
and the power response of this technology. This optical beam steerer serves as
an example for achieving full optical control of light in broad range of
structured optical materials
Weak localization of light in superdiffusive random systems
L\'evy flights constitute a broad class of random walks that occur in many
fields of research, from animal foraging in biology, to economy to geophysics.
The recent advent of L\'evy glasses allows to study L\'evy flights in
controlled way using light waves. This raises several questions about the
influence of superdiffusion on optical interference effects like weak and
strong localization. Super diffusive structures have the extraordinary property
that all points are connected via direct jumps, meaning that finite-size
effects become an essential part of the physical problem. Here we report on the
experimental observation of weak localization in L\'evy glasses and compare
results with recently developed optical transport theory in the superdiffusive
regime. Experimental results are in good agreement with theory and allow to
unveil how light propagates inside a finite-size superdiffusive system
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