5,236 research outputs found
Optimal control of light propagation through multiple-scattering media in the presence of noise
We study the control of coherent light propagation through
multiple-scattering media in the presence of measurement noise. In our
experiments, we use a two-step optimization procedure to find the optimal
incident wavefront. We conclude that the degree of optimal control of coherent
light propagation through a multiple-scattering medium is only determined by
the number of photoelectrons detected per single speckle spot. The prediction
of our model agrees well with the experimental results. Our results offer
opportunities for imaging applications through scattering media such as
biological tissue in the shot noise limit
Light propagation and emission in complex photonic media
We provide an introduction to complex photonic media, that is, composite
materials with spatial inhomogeneities that are distributed over length scales
comparable to or smaller than the wavelength of light. This blossoming field is
firmly rooted in condensed matter physics, in optics, and in materials science.
Many stimulating analogies exist with other wave phenomena such as sound and
seismology, X-rays, neutrons. The field has a rich history, which has led to
many applications in lighting, novel lasers, light harvesting, microscopy, and
bio optics. We provide a brief overview of complex photonic media with
different classes of spatial order, varying from completely random to
long-periodically ordered structures, quasi crystalline and aperiodic
structures, and arrays of cavities. In addition to shaping optical waves by
suitable photonic nanostructures, the realization is quickly arising that the
spatial shaping of optical wavefronts with spatial light modulators
dramatically increases the number of control parameters. As a result, it is
becoming possible for instance to literally see through completely opaque
complex media. We discuss a unified view of complex photonic media by means of
a photonic interaction strength parameter. This parameter gauges the
interaction of light with any complex photonic medium, and allows to compare
complex media from different classes for similar applications.Comment: 8 pages, 2 figures, Light Localisation and Lasing: Random and
Quasi-Random Photonic Structures, Eds. M. Ghulinyan and L. Pavesi, (Cambridge
Univ. Press, Cambridge, 2015) Ch. 1, p.
Design of a 3D photonic band gap cavity in a diamond-like inverse woodpile photonic crystal
We theoretically investigate the design of cavities in a three-dimensional
(3D) inverse woodpile photonic crystal. This class of cubic diamond-like
crystals has a very broad photonic band gap and consists of two perpendicular
arrays of pores with a rectangular structure. The point defect that acts as a
cavity is centred on the intersection of two intersecting perpendicular pores
with a radius that differs from the ones in the bulk of the crystal. We have
performed supercell bandstructure calculations with up to
unit cells. We find that up to five isolated and dispersionless bands appear
within the 3D photonic band gap. For each isolated band, the electric-field
energy is localized in a volume centred on the point defect, hence the point
defect acts as a 3D photonic band gap cavity. The mode volume of the cavities
resonances is as small as 0.8 (resonance wavelength cubed),
indicating a strong confinement of the light. By varying the radius of the
defect pores we found that only donor-like resonances appear for smaller defect
radius, whereas no acceptor-like resonances appear for greater defect radius.
From a 3D plot of the distribution of the electric-field energy density we
conclude that peaks of energy found in sharp edges situated at the point
defect, similar to how electrons collect at such features. This is different
from what is observed for cavities in non-inverted woodpile structures. Since
inverse woodpile crystals can be fabricated from silicon by CMOS-compatible
means, we project that single cavities and even cavity arrays can be realized,
for wavelength ranges compatible with telecommunication windows in the near
infrared.Comment: 11 figure
Local density of optical states in the band gap of a finite photonic crystal
We study the local density of states (LDOS) in a finite photonic crystal, in
particular in the frequency range of the band gap. We propose a new point of
view on the band gap, which we consider to be the result of vacuum fluctuations
in free space that tunnel in the forbidden range in the crystal. As a result,
we arrive at a model for the LDOS that is in two major items modified compared
to the well-known expression for infinite crystals. Firstly, we modify the
Dirac delta functions to become Lorentzians with a width set by the crystal
size. Secondly, building on characterization of the fields versus frequency and
position we calculated the fields in the band gap. We start from the fields at
the band edges, interpolated in space and position, and incorporating the
exponential damping in the band gap. We compare our proposed model to exact
calculations in one dimension using the transfer matrix method and find very
good agreement. Notably, we find that in finite crystals, the LDOS depends on
frequency, on position, and on crystal size, in stark contrast to the
well-known results for infinite crystals.Comment: 22 pages, 8 figure
Intrinsic fluctuations in random lasers
We present a quantitative experimental and theoretical study of shot-to-shot
intensity fluctuations in the emitted light of a random laser. A model that
clarifies these intrinsic fluctuations is developed. We describe the output
versus input power graphs of the random laser with an effective spontaneous
emission factor (beta factor).Comment: accepted by Phys. Rev. A. submitted; 7 pages, 5 figure
Star formation in the central regions of galaxies
Massive star formation in the central regions of spiral galaxies plays an
important role in the dynamical and secular evolution of their hosts. Here, we
summarise a number of recent investigations of the star formation history and
the physical conditions of the gas in circumnuclear regions, to illustrate not
only the detailed results one can achieve, but also the potential of using
state-of-the-art spectroscopic and analysis techniques in researching the
central regions of galaxies in general. We review how the star formation
history of nuclear rings confirms that they are long-lived and stable
configurations. Gas flows in from the disk, through the bar, and into the ring,
where successive episodes of massive star formation occur. Analysing the ring
in NGC 7742 in particular, we determine the physical conditions of the line
emitting gas using a combination of ionisation and stellar population
modelling, concluding that the origin of the nuclear ring in this non-barred
galaxy lies in a recent minor merger with a small gas-rich galaxy.Comment: Invited contribution, to appear in "Mapping the Galaxy and other
galaxies", Eds. K. Wada and F. Combes, Springer, in pres
The contribution of small farms and commercial large farms to the food security of Trinidad and Tobago
The purpose of this study is to investigate and analyze Trinidad and Tobago’s recent agricultural policy decisions and their effect on the way small farms and commercial large farms coexist to contribute to the country\u27s food security. Food security is defined as when all persons can access sufficient and nutritious food to maintain a healthy and active life, and have enough income to access that food in a safe, socially acceptable way. This research uses qualitative data from farmers, representatives of agricultural organizations, and consumers/vendors at open air markets in Tobago, and secondary data from local government offices and international organizations. Information also comes from the country’s three main daily newspapers. The research suggests that the State has been striving to integrate the commercial farms into domestic, regional and international markets by emphasizing the role of technology and aggressive marketing. The research also suggests that the State is striving to improve the circumstances of small farmers through education, training and technology with regard to farming methods based on best practices; higher output quality standards; inclusion into the commodity value chain process; marketing support; and increased access to credit and financial incentives. The link between increased output at the commercial farms and lower domestic food prices has not been fully explored in this research, but initial reactions from farmers suggest that small farm incomes have been negatively affected
Observation of Intensity Statistics of Light Transmitted Through 3D Random Media
We experimentally observe the spatial intensity statistics of light
transmitted through three-dimensional isotropic scattering media. The intensity
distributions measured through layers consisting of zinc oxide nanoparticles
differ significantly from the usual Rayleigh statistics associated with
speckle, and instead are in agreement with the predictions of mesoscopic
transport theory, taking into account the known material parameters of the
samples. Consistent with the measured spatial intensity fluctuations, the total
transmission fluctuates. The magnitude of the fluctuations in the total
transmission is smaller than expected on the basis of quasi-one-dimensional
(1D) transport theory, which indicates that quasi-1D theories cannot fully
describe these open three-dimensional media.Comment: 4 pages 3 figure
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