1,110 research outputs found
Enhancing the photomixing efficiency of optoelectronic devices in the terahertz regime
A method to reduce the transit time of majority of carriers in photomixers
and photo detectors to ps is proposed. Enhanced optical fields associated
with surface plasmon polaritons, coupled with velocity overshoot phenomenon
results in net decrease of transit time of carriers. As an example, model
calculations demonstrating (or 2800 and 31.8 W at 1
and 5 THz respectively) improvement in THz power generation efficiency of a
photomixer based on Low Temperature grown GaAs are presented. Due to minimal
dependence on the carrier recombination time, it is anticipated that the
proposed method paves the way for enhancing the speed and efficiency of
photomixers and detectors covering UV to far infrared communications
wavelengths (300 to 1600 nm).Comment: 5 pages, 4 figure
Negative Refraction and Subwavelength Lensing in a Polaritonic Crystal
We show that a two-dimensional polaritonic crystal, made of metallic rods
that support well defined plasmon oscillations, can act in a narrow frequency
range as a medium in which a negative refraction and subwavelength lensing can
occur. We show that surface modes are excited on the surface of the lens, and
that they facilitate restoration of the evanescent waves, which carry the
subwavelength image information. We demonstrate that this can occur in the
visible frequency range, for a wide range of materials, including silver and
aluminum rods, and carbon nanotubes. This flexibility should allow for an
experimental demonstration of this phenomenon in the visible frequency range.Comment: 14 pages; 4 figure
Complexity of 2D random laser modes at the transition from weak scattering to Anderson localization
The spatial extension and complexity of the eigenfunctions of an open
finite-size two-dimensional (2D) random system are systematically studied for a
random collection of systems ranging from weakly scattering to localized. The
eigenfunctions are obtained by introducing gain in the medium and pumping just
above threshold. All lasing modes are found to correspond to quasimodes of the
passive system, for all regimes of propagation. We demonstrate the existence of
multipeaked quasimodes or necklace states in 2D at the transition from
localized to diffusive, resulting from the coupling of localized states.Comment: Submitted to PR
Surface-mode microcavity
Optical microcavities based on zero-group-velocity surface modes in photonic
crystal slabs are studied. It is shown that high quality factors can be easily
obtained for such microcavities in photonic crystal slabs. With increasing of
the cavity length, the quality factor is gradually enhanced and the resonant
frequency converges to that of the zero-group-velocity surface mode in the
photonic crystal. The number of the resonant modes with high quality factors is
mainly determined by the number of surface modes with zero-group velocity.Comment: 11 pages, 4 figure
Conditions for waveguide decoupling in square-lattice photonic crystals
We study coupling and decoupling of parallel waveguides in two-dimensional
square-lattice photonic crystals. We show that the waveguide coupling is
prohibited at some wavelengths when there is an odd number of rows between the
waveguides. In contrast, decoupling does not take place when there is even
number of rows between the waveguides. Decoupling can be used to avoid cross
talk between adjacent waveguides.Comment: 6 pages, 2 figure
Lasing in metamaterial nanostructures
A self-consistent computational scheme is presented for one dimensional (1D)
and two dimensional (2D) metamaterial systems with gain incorporated into the
nanostructures. The gain is described by a generic four-level system. The loss
compensation and the lasing behavior of the metamaterial system with gain are
studied. A critical pumping rate exists for compensating the losses of the
metamaterial. There exists a wide range of input signals where the composite
system behaves linearly. Nonlinearities arise for stronger signals due to gain
depletion. The retrieved effective parameters are presented for one layer of
gain embedded in two layers of Lorentz dielectric rods and split ring
resonators with two different gain inclusions: (1) gain is embedded in the gaps
only and (2) gain is surrounding the SRR. When the pumping rate increases,
there is a critical pumping rate that the metamaterial system starts lasing.Comment: 18 pages, 19 figures, submitted to Journal of Optics A: Pure and
Applied Optic
Dispersive Cylindrical Cloaks under Non-Monochromatic Illumination
Transformation-based cylindrical cloaks and concentrators are illuminated
with non-monochromatic waves and unusual effects are observed with interesting
potential applications. The transient responses of the devices are studied
numerically with the Finite-Difference Time-Domain method and the results are
verified with analytical formulas. We compute the effective bandwidth of
several cloaking schemes as well as the effect of losses on the performance of
the structures. We also find that narrowband behavior, frequency shift effects,
time delays and spatial disturbances of the incoming waves are dominant due to
the inherently dispersive nature of the devices. These effects are important
and should be taken into account when designing metamaterial-based devices.Comment: 12 pages, 9 figure
Self-consistent calculation of metamaterials with gain
We present a computational scheme allowing for a self-consistent treatment of
a dispersive metallic photonic metamaterial coupled to a gain material
incorporated into the nanostructure. The gain is described by a generic
four-level system. A critical pumping rate exists for compensating the loss of
the metamaterial. Nonlinearities arise due to gain depletion beyond a certain
critical strength of a test field. Transmission, reflection, and absorption
data as well as the retrieved effective parameters are presented for a lattice
of resonant square cylinders embedded in layers of gain material and split ring
resonators with gain material embedded into the gaps.Comment: 5 pages, 6 figure
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
Finite-size effects of a left-handed material slab on the image quality
The characteristics of an imaging system formed by a left-handed material
(LHM) slab of finite length are studied, and the influence of the finite length
of the slab on the image quality is analyzed. Unusual phenomena such as surface
bright spots and negative energy stream at the image side are observed and
explained as the cavity effects of surface plasmons excited by the evanescent
components of the incident field. For a thin LHM slab, the cavity effects are
found rather sensitive to the length of the slab, and the bright spots on the
bottom surface of the slab may stretch to the image plane and degrade the image
quality.Comment: changes in the content and the title, and also the figure
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