9,377 research outputs found
Lorenz-Mie theory for 2D scattering and resonance calculations
This PhD tutorial is concerned with a description of the two-dimensional
generalized Lorenz-Mie theory (2D-GLMT), a well-established numerical method
used to compute the interaction of light with arrays of cylindrical scatterers.
This theory is based on the method of separation of variables and the
application of an addition theorem for cylindrical functions. The purpose of
this tutorial is to assemble the practical tools necessary to implement the
2D-GLMT method for the computation of scattering by passive scatterers or of
resonances in optically active media. The first part contains a derivation of
the vector and scalar Helmholtz equations for 2D geometries, starting from
Maxwell's equations. Optically active media are included in 2D-GLMT using a
recent stationary formulation of the Maxwell-Bloch equations called
steady-state ab initio laser theory (SALT), which introduces new classes of
solutions useful for resonance computations. Following these preliminaries, a
detailed description of 2D-GLMT is presented. The emphasis is placed on the
derivation of beam-shape coefficients for scattering computations, as well as
the computation of resonant modes using a combination of 2D-GLMT and SALT. The
final section contains several numerical examples illustrating the full
potential of 2D-GLMT for scattering and resonance computations. These examples,
drawn from the literature, include the design of integrated polarization
filters and the computation of optical modes of photonic crystal cavities and
random lasers.Comment: This is an author-created, un-copyedited version of an article
published in Journal of Optics. IOP Publishing Ltd is not responsible for any
errors or omissions in this version of the manuscript or any version derived
from i
Impurity-induced modulation of terahertz waves in optically excited GaAs
The effect of the photoinduced absorption of terahertz (THz) radiation in a
semi-insulating GaAs crystal is studied by the pulsed THz transmission
spectroscopy. We found that a broad-band modulation of THz radiation may be
induced by a low-power optical excitation in the spectral range of the impurity
absorption band in GaAs. The measured modulation achieves 80\%. The amplitude
and frequency characteristics of the resulting THz modulator are critically
dependent on the carrier density and relaxation dynamics in the conduction band
of GaAs. In semi-insulating GaAs crystals, the carrier density created by the
impurity excitation is controlled by the rate of their relaxation to the
impurity centers. The relaxation rate and, consequently, the frequency
characteristics of the modulator can be optimized by an appropriate choice of
the impurities and their concentrations. The modulation parameters can be also
controlled by the crystal temperature and by the power and photon energy of the
optical excitation. These experiments pave the way to the low-power fast
optically-controlled THz modulation, imaging, and beam steering.Comment: 5 pages, 3 figure
Controlling the localization and migration of optical excitation
In the nanoscale structure of a wide variety of material systems, a close juxtaposition of optically responsive components can lead to the absorption of light by one species producing fluorescence that is clearly attributable to another. The effect is generally evident in systems comprising two or more light-absorbing components (molecules, chromophores or quantum dots) with well-characterised fluorescence bands at similar, differentiable wavelengths. This enables the fluorescence associated with transferred energy to be discriminated against fluorescence from an initially excited component. The fundamental mechanism at the heart of the phenomenon, molecular (resonance) energy transfer, also operates in systems where the product of optical absorption is optical frequency up-conversion. In contrast to random media, structurally organised materials offer the possibility of pre-configured control over the delocalization of energy, through molecular energy transfer following optical excitation. The Förster mechanism that conveys energy between molecular-scale components is strongly sensitive to specific forms of correlation between the involved components, in terms of position, spectroscopic character, and orientation; one key factor is a spectroscopic gradient. Suitably designed materials offer a broad scope for the widespread exploitation of such features, in applications ranging from chemical and biological sensing to the detection of nanoscale motion or molecular conformations. Recently, attention has turned to the prospect of actively controlling the process of energy migration, for example by changing the relative efficiencies of fluorescence and molecular energy transfer. On application of static electric fields or off-resonant laser light - just two of the possibilities - each represents a means for achieving active control with ultrafast response, in suitably configured systems. As the principles are established and the theory is developed, a range of new possibilities for technical application is emerging. For example, applications can be envisaged for new forms of all-optical switching and transistor action. There is also interest in engaging with the interplay of optical excitation and local nanoscale force, exploiting local responses to changes in dispersion forces, accompanying molecular energy transfer
Optical activity in the Drude helix model
An old classical one-particle helix model for optical activity, first
proposed by Drude, is reconsidered here. The quantum Drude model is very
instructive because the optical activity can be calculated analytically without
further approximations apart from the Rosenfeld long wavelength approximation.
While it was generally believed that this model, when treated correctly, is
optically inactive, we show that it leads to optical activity when the motion
of the particle is quantum mechanically treated. We also find that optical
activity arises even in the classical regime at non-zero energy, while for zero
energy the model is inactive, in accordance with previous results. The model is
compared with other one-electron models and it is shown that its predicted
optical activity is qualitatively different from those of other one-electron
systems. The vanishing of optical activity in the classical zero-energy limit
for the Drude model is due to the localization of the particle at the
equilibrium position, whereas in the analogous model of a particle moving
freely on a helix without a definite equilibrium position, optical activity
does not vanish but the spectrum is rescaled. The model under study leads to
interesting predictions about the optical properties of e. g. helicene
derivatives
Optical properties of cosmic dust analogs: A review
Nanometer- and micrometer-sized solid particles play an important role in the
evolutionary cycle of stars and interstellar matter. The optical properties of
cosmic grains determine the interaction of the radiation field with the solids,
thereby regulating the temperature structure and spectral appearance of dusty
regions. Radiation pressure on dust grains and their collisions with the gas
atoms and molecules can drive powerful winds. The analysis of observed spectral
features, especially in the infrared wavelength range, provides important
information on grain size, composition and structure as well as temperature and
spatial distribution of the material.
The relevant optical data for interstellar, circumstellar, and protoplanetary
grains can be obtained by measurements on cosmic dust analogs in the laboratory
or can be calculated from grain models based on optical constants. Both
approaches have made progress in the last years, triggered by the need to
interpret increasingly detailed high-quality astronomical observations. The
statistical theoretical approach, spectroscopic experiments at variable
temperature and absorption spectroscopy of aerosol particulates play an
important role for the successful application of the data in dust astrophysics.Comment: 18 pages, 6 figures, invited review for Journal of Nanophotonics,
Special Section to honour C.F. Bohre
- …