2,119 research outputs found
A Silicon-Based Monolithic Optical Frequency Comb Source
Recently developed techniques for generating precisely equidistant optical
frequencies over broad wavelength ranges are revolutionizing precision physical
measurement [1-3]. These frequency "combs" are produced primarily using
relatively large, ultrafast laser systems. However, recent research has shown
that broad-bandwidth combs can be produced using highly-nonlinear interactions
in microresonator optical parametric oscillators [4-11]. Such devices not only
offer the potential for developing extremely compact optical atomic clocks but
are also promising for astronomical spectroscopy [12-14], ultrashort pulse
shaping [15], and ultrahigh-speed communications systems. Here we demonstrate
the generation of broad-bandwidth optical frequency combs from a
CMOS-compatible integrated microresonator [16,17], which is a fully-monolithic
and sealed chip-scale device making it insensitive to the surrounding
environment. We characterize the comb quality using a novel self-referencing
method and verify that the comb line frequencies are equidistant over a
bandwidth that is nearly an order of magnitude larger than previous
measurements. In addition, we investigate the ultrafast temporal properties of
the comb and demonstrate its potential to serve as a chip-scale source of
ultrafast (sub-ps) pulses
Second-Harmonic Generation in Silicon Nitride Ring Resonators
The emerging field of silicon photonics seeks to unify the high bandwidth of
optical communications with CMOS microelectronic circuits. Many components have
been demonstrated for on-chip optical communications, including those that
utilize the nonlinear optical properties of silicon[1, 2], silicon dioxide[3,
4] and silicon nitride[5, 6]. Processes such as second harmonic generation,
which are enabled by the second-order susceptibility, have not been developed
since the bulk vanishes in these centrosymmetric CMOS materials.
Generating the lowest-order nonlinearity would open the window to a new array
of CMOS-compatible optical devices capable of nonlinear functionalities not
achievable with the? response such as electro-optic modulation, sum
frequency up-conversion, and difference frequency generation. Here we
demonstrate second harmonic (SH) generation in CMOS compatible integrated
silicon nitride (Si3N4) waveguides. The response is induced in the
centrosymmetric material by using the nanoscale structure to break the bulk
symmetry. We use a high quality factor Q ring resonator cavity to enhance the
efficiency of the nonlinear optical process and detect SH output with milliwatt
input powers.Comment: 4 pages, 3 figure
Characterizing Mode Anharmonicity and Huang–Rhys Factors Using Models of Femtosecond Coherence Spectra
Femtosecond laser pulses readily produce coherent quantum beats in transient–absorption spectra. These oscillatory signals often arise from molecular vibrations and therefore may contain information about the excited-state potential energy surface near the Franck–Condon region. Here, by fitting the measured spectra of two laser dyes to microscopic models of femtosecond coherence spectra (FCS) arising from molecular vibrations, we classify coherent quantum-beat signals as fundamentals or overtones and quantify their Huang–Rhys factors and anharmonicity values. We discuss the extracted Huang–Rhys factors in the context of quantum-chemical computations. This work solidifies the use of FCS for analysis of coherent quantum beats arising from molecular vibrations, which will aid studies of molecular aggregates and photosynthetic proteins
Complexity, Development, and Evolution in Morphogenetic Collective Systems
Many living and non-living complex systems can be modeled and understood as
collective systems made of heterogeneous components that self-organize and
generate nontrivial morphological structures and behaviors. This chapter
presents a brief overview of our recent effort that investigated various
aspects of such morphogenetic collective systems. We first propose a
theoretical classification scheme that distinguishes four complexity levels of
morphogenetic collective systems based on the nature of their components and
interactions. We conducted a series of computational experiments using a
self-propelled particle swarm model to investigate the effects of (1)
heterogeneity of components, (2) differentiation/re-differentiation of
components, and (3) local information sharing among components, on the
self-organization of a collective system. Results showed that (a) heterogeneity
of components had a strong impact on the system's structure and behavior, (b)
dynamic differentiation/re-differentiation of components and local information
sharing helped the system maintain spatially adjacent, coherent organization,
(c) dynamic differentiation/re-differentiation contributed to the development
of more diverse structures and behaviors, and (d) stochastic re-differentiation
of components naturally realized a self-repair capability of self-organizing
morphologies. We also explored evolutionary methods to design novel
self-organizing patterns, using interactive evolutionary computation and
spontaneous evolution within an artificial ecosystem. These self-organizing
patterns were found to be remarkably robust against dimensional changes from 2D
to 3D, although evolution worked efficiently only in 2D settings.Comment: 13 pages, 8 figures, 1 table; submitted to "Evolution, Development,
and Complexity: Multiscale Models in Complex Adaptive Systems" (Springer
Proceedings in Complexity Series
Radial Star Formation Histories in 32 Nearby Galaxies
The spatially resolved star formation histories are studied for 32 normal
star-forming galaxies drawn from the the Spitzer Extended Disk Galaxy
Exploration Science survey. At surface brightness sensitivities fainter than 28
mag arcsec, the new optical photometry is deep enough to complement
archival ultraviolet and infrared imaging and to explore the properties of the
emission well beyond the traditional optical extents of these nearby galaxies.
Fits to the spectral energy distributions using a delayed star formation
history model indicate a subtle but interesting average radial trend for the
spiral galaxies: the inner stellar systems decrease in age with increasing
radius, consistent with inside-out disk formation, but the trend reverses in
the outermost regions with the stellar age nearly as old as the innermost
stars. These results suggest an old stellar outer disk population formed
through radial migration and/or the cumulative history of minor mergers and
accretions of satellite dwarf galaxies. The subset of S0 galaxies studied here
show the opposite trend compared to what is inferred for spirals:
characteristic stellar ages that are increasingly older with radius for the
inner portions of the galaxies, and increasingly younger stellar ages for the
outer portions. This result suggests that either S0 galaxies are not well
modeled by a delayed- model, and/or that S0 galaxies have a more
complicated formation history than spiral galaxies.Comment: Accepted for publication in the Astronomical Journal. arXiv admin
note: text overlap with arXiv:1511.0328
Octave-spanning frequency comb generation in a silicon nitride chip
We demonstrate a frequency comb spanning an octave via the parametric process
of cascaded four-wave mixing in a monolithic, high-Q silicon nitride microring
resonator. The comb is generated from a single-frequency pump laser at 1562 nm
and spans 128 THz with a spacing of 226 GHz, which can be tuned slightly with
the pump power. In addition, we investigate the RF-noise characteristics of the
parametric comb and find that the comb can operate in a low-noise state with a
30-dB reduction in noise as the pump frequency is tuned into the cavity
resonance
Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosol
Thermodynamic models predict that sulfate aerosol (S(VI)  ≡ 
H2SO4(aq) + HSO4−+ SO42−) should take up
available ammonia (NH3) quantitatively as ammonium (NH4+)
until the ammonium sulfate stoichiometry (NH4)2SO4 is close
to being reached. This uptake of ammonia has important implications for
aerosol mass, hygroscopicity, and acidity. When ammonia is in excess, the
ammonium–sulfate aerosol ratio R =  [NH4+] ∕ [S(VI)] should approach
2, with excess ammonia remaining in the gas phase. When ammonia is in
deficit, it should be fully taken up by the aerosol as ammonium and no
significant ammonia should remain in the gas phase. Here we report that
sulfate aerosol in the eastern US in summer has a low ammonium–sulfate ratio
despite excess ammonia, and we show that this is at odds with thermodynamic
models. The ammonium–sulfate ratio averages only 1.04 ± 0.21 mol mol−1 in
the Southeast, even though ammonia is in large excess, as shown
by the ammonium–sulfate ratio in wet deposition and by the presence of
gas-phase ammonia. It further appears that the ammonium–sulfate aerosol
ratio is insensitive to the supply of ammonia, remaining low even as the wet
deposition ratio exceeds 6 mol mol−1. While the ammonium–sulfate ratio
in wet deposition has increased by 5.8 % yr−1 from 2003 to 2013 in the
Southeast, consistent with SO2 emission controls, the
ammonium–sulfate aerosol ratio decreased by 1.4–3.0 % yr−1.
Thus, the aerosol is becoming more acidic even as SO2 emissions decrease
and ammonia emissions stay constant; this is incompatible with
simple sulfate–ammonium thermodynamics. A tentative explanation is that
sulfate particles are increasingly coated by organic material, retarding the
uptake of ammonia. Indeed, the ratio of organic aerosol (OA) to sulfate in
the Southeast increased from 1.1 to 2.4 g g−1 over the 2003–2013 period
as sulfate decreased. We implement a simple kinetic mass transfer limitation
for ammonia uptake to sulfate aerosols in the GEOS-Chem chemical transport
model and find that we can reproduce both the observed ammonium–sulfate
aerosol ratios and the concurrent presence of gas-phase ammonia. If sulfate
aerosol becomes more acidic as OA ∕ sulfate ratios increase, then controlling
SO2 emissions to decrease sulfate aerosol will not have the co-benefit
of suppressing acid-catalyzed secondary organic aerosol (SOA) formation
In vivo efficacy of the boron-pleuromutilin AN11251 against Wolbachia of the rodent filarial nematode Litomosoides sigmodontis
The elimination of filarial diseases such as onchocerciasis and lymphatic filariasis is hampered by the lack of a macrofilaricidal – adult worm killing – drug. In the present study, we tested the in vivo efficacy of AN11251, a boron-pleuromutilin that targets endosymbiotic Wolbachia bacteria from filarial nematodes and compared its efficacy to doxycycline and rifampicin. Doxycycline and rifampicin that were previously shown to deplete Wolbachia endosymbionts leading to a permanent sterilization of the female adult filariae and adult worm death in human clinical studies. Twice-daily oral treatment of Litomosoides sigmodontis-infected mice with 200 mg/kg AN11251 for 10 days achieved a Wolbachia depletion > 99.9% in the adult worms, exceeding the Wolbachia reduction by 10-day treatments with bioequivalent human doses of doxycycline and a similar reduction as high-dose rifampicin (35 mg/kg). Wolbachia reductions of > 99% were also accomplished by 14 days of oral AN11251 at a lower twice-daily dose (50 mg/kg) or once-per-day 200 mg/kg AN11251 treatments. The combinations tested of AN11251 with doxycycline had no clear beneficial impact on Wolbachia depletion, achieving a > 97% Wolbachia reduction with 7 days of treatment. These results indicate that AN11251 is superior to doxycycline and comparable to high-dose rifampicin in the L. sigmodontis mouse model, allowing treatment regimens as short as 10-14 days. Therefore, AN11251 is represents a promising pre-clinical candidate that was identified in the L. sigmodontis model, and could be further evaluated and developed as potential clinical candidate for human lymphatic filariasis and onchocerciasis. 
AUTHOR SUMMARY
Onchocerciasis and lymphatic filariasis are human filarial tropical diseases, which can cause blindness and severe dermatitis (onchocerciasis) or lymphedema and hydrocele (lymphatic filariasis). Current strategies to eliminate these diseases include the mass drug administration (MDA) of drugs that target the progeny of the filariae, the microfilariae, and temporarily inhibit filarial embryogenesis and, therefore, the transmission of the disease. However, MDA has several limitations that delay the goal of elimination including the lack of a drug with a short term regimen and a potent macrofilaricidal effect. As an alternative approach, the antibiotic doxycycline has been proven to be effective in depleting Wolbachia endosymbionts from adult filariae, which then leads to permanent sterilization and death of the adult worms. Due to contraindications for doxycycline and prolonged treatment regimen of at least 4 weeks, there is an urgent need for new anti-filarial drugs with an improved safety profile and shorter regimens. The current study demonstrates that the boron-pleuromutilin derivative AN11251 provides an excellent in vivo anti-Wolbachia depletion in the Litomosoides sigmodontis filarial mouse model that is superior to doxycycline and comparable to rifampicin, allowing for regimens as short as 10-14 days. Combination with doxycycline for 7 days had no significant beneficial effect on efficacy, achieving Wolbachia reductions of more than 97%. Therefore, AN11251 shows potent anti-Wolbachia activity in the L. sigmodontis mouse model and may also present an alternative pre-clinical candidate for filariasis treatment
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