53,897 research outputs found
Inverse Design of Perfectly Transmitting Eigenchannels in Scattering Media
Light-matter interactions inside turbid medium can be controlled by tailoring
the spatial distribution of energy density throughout the system. Wavefront
shaping allows selective coupling of incident light to different transmission
eigenchannels, producing dramatically different spatial intensity profiles. In
contrast to the density of transmission eigenvalues that is dictated by the
universal bimodal distribution, the spatial structures of the eigenchannels are
not universal and depend on the confinement geometry of the system. Here, we
develop and verify a model for the transmission eigenchannel with the
corresponding eigenvalue close to unity. By projecting the original problem of
two-dimensional diffusion in a homogeneous scattering medium onto a
one-dimensional inhomogeneous diffusion, we obtain an analytical expression
relating the intensity profile to the shape of the confining waveguide.
Inverting this relationship enables the inverse design of the waveguide shape
to achieve the desired energy distribution for the perfectly transmitting
eigenchannel. Our approach also allows to predict the intensity profile of such
channel in a disordered slab with open boundaries, pointing to the possibility
of controllable delivery of light to different depths with local illumination.Comment: 9 pages, 6 figure
Mode Repulsion and Mode Coupling in Random Lasers
We studied experimentally and theoretically the interaction of lasing modes
in random media. In a homogeneously broadened gain medium, cross gain
saturation leads to spatial repulsion of lasing modes. In an inhomogeneously
broadened gain medium, mode repulsion occurs in the spectral domain. Some
lasing modes are coupled through photon hopping or electron absorption and
reemission. Under pulsed pumping, weak coupling of two modes leads to
synchronization of their lasing action. Strong coupling of two lasing modes
results in anti-phased oscillations of their intensities.Comment: 13 pages, 4 figure
Influence of Spatial Correlations on the Lasing Threshold of Random Lasers
The lasing threshold of a random laser is computed numerically from a generic
model. It is shown that spatial correlations of the disorder in the medium
(i.e., dielectric constant) lead to an increase of the decay rates of the
eigenmodes and of the lasing threshold. This is in conflict with predictions
that such correlations should lower the threshold. While all results are
derived for photonic systems, the computed decay rate distributions also apply
to electronic systems
Ti-rich and Cu-poor grain-boundary layers of CaCuTiO detected by x-ray photoelectron spectroscopy
Cleaved and polished surfaces of CaCuTiO ceramics have been
investigated by x-ray photoelectron spectroscopy (XPS) and energy dispersive
x-ray spectroscopy (EDX), respectively. While EDX technique shows the identical
CaCuTiO stoichiometry for the two surfaces, XPS indicates that
the cleaved surface with grain-boundary layers is remarkably Ti-rich and
Cu-poor. The core-level spectrum of Cu 2 unambiguously shows the existence
of monovalent copper only for the cleaved surface. Possible grain-boundary
structure and its formation are discussed.Comment: 8 pages, 3 figure
Control of coherent backscattering by breaking optical reciprocity
Reciprocity is a universal principle that has a profound impact on many areas
of physics. A fundamental phenomenon in condensed-matter physics, optical
physics and acoustics, arising from reciprocity, is the constructive
interference of quantum or classical waves which propagate along time-reversed
paths in disordered media, leading to, for example, weak localization and
metal-insulator transition. Previous studies have shown that such coherent
effects are suppressed when reciprocity is broken. Here we show that by
breaking reciprocity in a controlled manner, we can tune, rather than simply
suppress, these phenomena. In particular, we manipulate coherent backscattering
of light, also known as weak localization. By utilizing a non-reciprocal
magneto-optical effect, we control the interference between time-reversed paths
inside a multimode fiber with strong mode mixing, and realize a continuous
transition from the well-known peak to a dip in the backscattered intensity.
Our results may open new possibilities for coherent control of classical and
quantum waves in complex systemsComment: Comments are welcom
Anisotropic softening of magnetic excitations in lightly electron doped SrIrO
The magnetic excitations in electron doped (SrLa)IrO with
were measured using resonant inelastic X-ray scattering at the Ir
-edge. Although much broadened, well defined dispersive magnetic
excitations were observed. Comparing with the magnetic dispersion from the
parent compound, the evolution of the magnetic excitations upon doping is
highly anisotropic. Along the anti-nodal direction, the dispersion is almost
intact. On the other hand, the magnetic excitations along the nodal direction
show significant softening. These results establish the presence of strong
magnetic correlations in electron doped SrLa)IrO with close
analogies to the hole doped cuprates, further motivating the search for high
temperature superconductivity in this system
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