89 research outputs found
Time Dependent Theory for Random Lasers
A model to simulate the phenomenon of random lasing is presented. It couples
Maxwell's equations with the rate equations of electronic population in a
disordered system. Finite difference time domain methods are used to obtain the
field pattern and the spectra of localized lasing modes inside the system. A
critical pumping rate exists for the appearance of the lasing
peaks. The number of lasing modes increase with the pumping rate and the length
of the system. There is a lasing mode repulsion. This property leads to a
saturation of the number of modes for a given size system and a relation
between the localization length and average mode length .Comment: 8 pages. Send to PR
Super-reflection of light from a random amplifying medium with disorder in the complex refractive index : Statistics of fluctuations
The probability distribution of the reflection coefficient for light
reflected from a one-dimensional random amplifying medium with {\it
cross-correlated} spatial disorder in the real and the imaginary parts of the
refractive index is derived using the method of invariant imbedding. The
statistics of fluctuations have been obtained for both the correlated telegraph
noise and the Gaussian white-noise models for the disorder. In both cases, an
enhanced backscattering (super-reflection with reflection coefficient greater
than unity) results because of coherent feedback due to Anderson localization
and coherent amplification in the medium. The results show that the effect of
randomness in the imaginary part of the refractive index on localization and
super-reflection is qualitatively different.Comment: RevTex 6 pages, 3 figures in ps file
Study of transmission and reflection from a disordered lasing medium
A numerical study of the statistics of transmission () and reflection
() of quasi-particles from a one-dimensional disordered lasing or amplifying
medium is presented. The amplification is introduced via a uniform imaginary
part in the site energies in the disordered segment of the single-band tight
binding model. It is shown that is a non-self-averaging quantity. The
cross-over length scale above which the amplification suppresses the
transmittance is studied as a function of amplification strength. A new
cross-over length scale is introduced in the regime of strong disorder and weak
amplification. The stationary distribution of the backscattered reflection
coefficient is shown to differ qualitatively from the earlier analytical
results obtained within the random phase approximation.Comment: 5 pages RevTex (twocolumn format), 5 EPS figures, considerably
modifie
Propagation inhibition and wave localization in a 2D random liquid medium
Acoustic propagation and scattering in water containing many parallel
air-filled cylinders is studied. Two situations are considered and compared:
(1) wave propagating through the array of cylinders, imitating a traditional
experimental setup, and (2) wave transmitted from a source located inside the
ensemble. We show that waves can be blocked from propagation by disorders in
the first scenario, but the inhibition does not necessarily imply wave
localization. Furthermore, the results reveal the phenomenon of wave
localization in a range of frequencies.Comment: Typos in Fiures are correcte
Engineering disorder in three-dimensional photonic crystals
We demonstrate the effect of introducing controlled disorder in
self-assembled three-dimensional photonic crystals. Disorders are induced
through controlling the self-assembling process using an electrolyte of
specific concentrations. Structural characterization reveals increase in
disorder with increase in concentrations of the electrolyte. Reflectivity and
transmittance spectra are measured to probe the photonic stop gap at different
levels of disorder. With increase in disorder the stop gap is vanished and that
results in a fully random photonic nanostructure where the diffuse scattered
intensity reaches up to 100%. Our random photonic nanostructure is unique in
which all scatters have the same size and shape. We also observe the resonant
characteristics in the multiple scattering of light.Comment: 13 pages, 3 figure
Localization of Light: Dual Symmetry between Absorption and Amplification
We study the propagation of radiation through a disordered waveguide with a
complex dielectric constant , and show that dual systems, which
differ only in the sign of the imaginary part of , have the same
localization length. Paradoxically, absorption and stimulated emission of
radiation suppress the transmittance of the waveguide in the same way.Comment: Added a reference to the paper by Z.Q. Zhang, Phys.Rev.B. 52, 7960
(1995
Symmetry between absorption and amplification in disordered media
We address the issue of whether amplification, like absorption, suppresses
wave transmission at large gain, as has been claimed in previous studies of
wave propagation in active random media. A closer examination reveals that the
paradoxical symmetry between absorption and amplification is an artifact of
unphysical solutions from the time-independent wave equation. Solutions from
the time-dependent equation demonstrate clearly that when gain is above the
threshold, the amplitude of both the transmitted and the reflected wave
actually increases with time, apparently without bound. The implications of the
current finding is discusse
Manifestation of photonic band structure in small clusters of spherical particles
We study the formation of the photonic band structure in small clusters of
dielectric spheres. The first signs of the band structure, an attribute of an
infinite crystal, can appear for clusters of 5 particles. Density of resonant
states of a cluster of 32 spheres may exhibit a well defined structure similar
to the density of electromagnetic states of the infinite photonic crystal. The
resonant mode structure of finite-size aggregates is shown to be insensitive to
random displacements of particles off the perfect lattice positions as large as
half-radius of the particle. The results were obtained by an efficient
numerical method, which relates the density of resonant states to the the
scattering coefficients of the electromagnetic scattering problem. Generalized
multisphere Mie (GMM) solution was used to obtain scattering matrix elements.
These results are important to miniature photonic crystal design as well as
understanding of light localization in dense random media.Comment: 4 pages, 2 figure
Wave localization at the boundary of disordered photonic lattices
We report on the experimental observation of reduced light energy transport
and disorder-induced localization close to a boundary of a truncated
one-dimensional (1D) disordered photonic lattice. Our observations uncover that
near the boundary a higher level of disorder is required to obtain similar
localization than in the bulk.Comment: 13 pages, 5 figures, to appear in Optics Letter
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