5,110 research outputs found
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
Spatial homogeneity of optically switched semiconductor photonic crystals and of bulk semiconductors
This paper discusses free carrier generation by pulsed laser fields as a
mechanism to switch the optical properties of semiconductor photonic crystals
and bulk semiconductors on an ultrafast time scale. Requirements are set for
the switching magnitude, the time-scale, the induced absorption as well as the
spatial homogeneity, in particular for silicon at lambda= 1550 nm. Using a
nonlinear absorption model, we calculate carrier depth profiles and define a
homogeneity length l_hom. Homogeneity length contours are visualized in a plane
spanned by the linear and two-photon absorption coefficients. Such a
generalized homogeneity plot allows us to find optimum switching conditions at
pump frequencies near v/c= 5000 cm^{-1} (lambda= 2000 nm). We discuss the
effect of scattering in photonic crystals on the homogeneity. We experimentally
demonstrate a 10% refractive index switch in bulk silicon within 230 fs with a
lateral homogeneity of more than 30 micrometers. Our results are relevant for
switching of modulators in absence of photonic crystals
Strongly nonexponential time-resolved fluorescence of quantum-dot ensembles in three-dimensional photonic crystals
We observe experimentally that ensembles of quantum dots in three-dimensional (3D) photonic crystals reveal strongly nonexponential time-resolved emission. These complex emission decay curves are analyzed with a continuous distribution of decay rates. The log-normal distribution describes the decays well for all studied lattice parameters. The distribution width is identified with variations of the radiative emission rates of quantum dots with various positions and dipole orientations in the unit cell. We find a striking sixfold change of the width of the distribution by varying the lattice parameter. This interpretation qualitatively agrees with the calculations of the 3D projected local density of states. We therefore conclude that fluorescence decay of ensembles of quantum dots is highly nonexponential to an extent that is controlled by photonic crystals
Randomized lasso links microbial taxa with aquatic functional groups inferred from flow cytometry
High-nucleic-acid (HNA) and low-nucleic-acid (LNA) bacteria are two operational groups identified by flow cytometry (FCM) in aquatic systems. A number of reports have shown that HNA cell density correlates strongly with heterotrophic production, while LNA cell density does not. However, which taxa are specifically associated with these groups, and by extension, productivity has remained elusive. Here, we addressed this knowledge gap by using a machine learning-based variable selection approach that integrated FCM and 16S rRNA gene sequencing data collected from 14 freshwater lakes spanning a broad range in physicochemical conditions. There was a strong association between bacterial heterotrophic production and HNA absolute cell abundances (R-2 = 0.65), but not with the more abundant LNA cells. This solidifies findings, mainly from marine systems, that HNA and LNA bacteria could be considered separate functional groups, the former contributing a disproportionately large share of carbon cycling. Taxa selected by the models could predict HNA and LNA absolute cell abundances at all taxonomic levels. Selected operational taxonomic units (OTUs) ranged from low to high relative abundance and were mostly lake system specific (89.5% to 99.2%). A subset of selected OTUs was associated with both LNA and HNA groups (12.5% to 33.3%), suggesting either phenotypic plasticity or within-OTU genetic and physiological heterogeneity. These findings may lead to the identification of system-specific putative ecological indicators for heterotrophic productivity. Generally, our approach allows for the association of OTUs with specific functional groups in diverse ecosystems in order to improve our understanding of (microbial) biodiversity-ecosystem functioning relationships.
IMPORTANCE A major goal in microbial ecology is to understand how microbial community structure influences ecosystem functioning. Various methods to directly associate bacterial taxa to functional groups in the environment are being developed. In this study, we applied machine learning methods to relate taxonomic data obtained from marker gene surveys to functional groups identified by flow cytometry. This allowed us to identify the taxa that are associated with heterotrophic productivity in freshwater lakes and indicated that the key contributors were highly system specific, regularly rare members of the community, and that some could possibly switch between being low and high contributors. Our approach provides a promising framework to identify taxa that contribute to ecosystem functioning and can be further developed to explore microbial contributions beyond heterotrophic production
Detection of a methanol megamaser in a major-merger galaxy
We have detected emission from both the 4_{-1}-3_{0} E (36.2~GHz) class I and
7_{-2}-8_{-1} E (37.7~GHz) class II methanol transitions towards the centre of
the closest ultra-luminous infrared galaxy Arp 220. The emission in both the
methanol transitions show narrow spectral features and have luminosities
approximately 8 orders of magnitude stronger than that observed from typical
class I methanol masers observed in Galactic star formation regions. The
emission is also orders of magnitude stronger than the expected intensity of
thermal emission from these transitions and based on these findings we suggest
that the emission from the two transitions are masers. These observations
provides the first detection of a methanol megamaser in the 36.2 and 37.7 GHz
transitions and represents only the second detection of a methanol megamaser,
following the recent report of an 84 GHz methanol megamaser in NGC1068. We find
the methanol megamasers are significantly offset from the nuclear region and
arise towards regions where there is Ha emission, suggesting that it is
associated with starburst activity. The high degree of correlation between the
spatial distribution of the 36.2 GHz methanol and X-ray plume emission suggests
that the production of strong extragalactic class I methanol masers is related
to galactic outflow driven shocks and perhaps cosmic rays. In contrast to OH
and H2O megamasers which originate close to the nucleus, methanol megamasers
provide a new probe of feedback (e.g. outflows) processes on larger-scales and
of star formation beyond the circumnuclear starburst regions of active
galaxies.Comment: Accepted for publication in ApJ
Signature of a three-dimensional photonic band gap observed on silicon inverse woodpile photonic crystals
We have studied the reflectivity of CMOS-compatible three-dimensional silicon
inverse woodpile photonic crystals at near-infrared frequencies.
Polarization-resolved reflectivity spectra were obtained from two orthogonal
crystal surfaces corresponding to 1.88 pi sr solid angle. The spectra reveal
broad peaks with high reflectivity up to 67 % that are independent of the
spatial position on the crystals. The spectrally overlapping reflectivity peaks
for all directions and polarizations form the signature of a broad photonic
band gap with a relative bandwidth up to 16 %. This signature is supported with
stopgaps in plane wave bandstructure calculations and with the frequency region
of the expected band gap.Comment: 9 pages, 5 figure
Programmable two-photon quantum interference in channels in opaque scattering media
We investigate two-photon quantum interference in an opaque scattering medium
that intrinsically supports transmission channels. By adaptive spatial
phase-modulation of the incident wavefronts, the photons are directed at
targeted speckle spots or output channels. From experimentally available
coupled channels, we select two channels and enhance their transmission, to
realize the equivalent of a fully programmable beam splitter. By
sending pairs of single photons from a parametric down-conversion source
through the opaque scattering medium, we observe two-photon quantum
interference. The programmed beam splitter need not fulfill energy conservation
over the two selected output channels and hence could be non-unitary.
Consequently, we have the freedom to tune the quantum interference from
bunching (Hong-Ou-Mandel-like) to antibunching. Our results establish opaque
scattering media as a platform for high-dimensional quantum interference that
is notably relevant for boson sampling and physical-key-based authentication
Classical antennae, quantum emitters, and densities of optical states
We provide a pedagogical introduction to the concept of the local density of
optical states (LDOS), illustrating its application to both the classical and
quantum theory of radiation. We show that the LDOS governs the efficiency of a
macroscopic classical antenna, determining how the antenna's emission depends
on its environment. The LDOS is shown to similarly modify the spontaneous
emission rate of a quantum emitter, such as an excited atom, molecule, ion, or
quantum dot that is embedded in a nanostructured optical environment. The
difference between the number density of optical states, the local density of
optical states, and the partial local density of optical states is elaborated
and examples are provided for each density of states to illustrate where these
are required. We illustrate the universal effect of the LDOS on emission by
comparing systems with emission wavelengths that differ by more than 5 orders
of magnitude, and systems whose decay rates differ by more than 5 orders of
magnitude. To conclude we discuss and resolve an apparent difference between
the classical and quantum expressions for the spontaneous emission rate that
often seems to be overlooked, and discuss the experimental determination of the
LDOS.Comment: 80 pages, 19 figure
Ultrafast optical switching of three-dimensional Si inverse opal photonic band gap crystals
We present ultrafast optical switching experiments on 3D photonic band gap
crystals. Switching the Si inverse opal is achieved by optically exciting free
carriers by a two-photon process. We probe reflectivity in the frequency range
of second order Bragg diffraction where the photonic band gap is predicted. We
find good experimental switching conditions for free-carrier plasma frequencies
between 0.3 and 0.7 times the optical frequency: we thus observe a large
frequency shift of up to D omega/omega= 1.5% of all spectral features including
the peak that corresponds to the photonic band gap. We deduce a corresponding
large refractive index change of Dn'_Si/n'_Si= 2.0% and an induced absorption
length that is longer than the sample thickness. We observe a fast decay time
of 21 ps, which implies that switching could potentially be repeated at GHz
rates. Such a high switching rate is relevant to future switching and
modulation applications
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