5,966 research outputs found
Self-pulsing dynamics in a cavity soliton laser
The dynamics of a broad-area vertical-cavity surface-emitting laser (VCSEL) with frequency-selective feedback supporting bistable spatial solitons is analyzed experimentally and theoretically. The transient dynamics of a switch-on of a soliton induced by an external optical pulse shows strong self-pulsing at the external-cavity round-trip time with at least ten modes excited. The numerical analysis indicates an even broader bandwidth and a transient sweep of the center frequency. It is argued that mode-locking of spatial solitons is an interesting and viable way to achieve three-dimensional, spatio-temporal self-localization and that the transients observed are preliminary indications of a transient cavity light bullet in the dynamics, though on a non negligible background
Frequency and phase locking of laser cavity solitons
Self-localized states or dissipative solitons have the freedom of translation in systems with a homogeneous background. When compared to cavity solitons in coherently driven nonlinear optical systems, laser cavity solitons have the additional freedom of the optical phase. We explore the consequences of this additional Goldstone mode and analyse experimentally and numerically frequency and phase locking of laser cavity solitons in a vertical-cavity surface-emitting laser with frequency-selective feedback. Due to growth-related variations of the cavity resonance, the translational symmetry is usually broken in real devices. Pinning to different defects means that separate laser cavity solitons have different frequencies and are mutually incoherent. If two solitons are close to each other, however, their interaction leads to synchronization due to phase and frequency locking with strong similarities to the Adler-scenario of coupled oscillators
The Interaction between the ISM and Star Formation in the Dwarf Starburst Galaxy NGC 4214
We present the first interferometric study of the molecular gas in the
metal-poor dwarf starburst galaxy NGC 4214. Our map of the 12CO(1-0) emission,
obtained at the OVRO millimeter array, reveals an unexpected structural wealth.
We detected three regions of molecular emission in the north-west (NW),
south-east (SE) and centre of NGC 4214 which are in very different and distinct
evolutionary stages (total molecular mass: 5.1 x 10^6 M_sun). These differences
are apparent most dramatically when the CO morphologies are compared to optical
ground based and HST imaging: massive star formation has not started yet in the
NW region; the well-known starburst in the centre is the most evolved and star
formation in the SE complex started more recently. We derive a star formation
efficiency of 8% for the SE complex. Using high--resolution VLA observations of
neutral hydrogen HI and our CO data we generated a total gas column density map
for NGC 4214 (HI + H_2). No clear correlation is seen between the peaks of HI,
CO and the sites of ongoing star formation. This emphasizes the irregular
nature of dwarf galaxies. The HI and CO velocities agree well, so do the
H-alpha velocities. In total, we cataloged 14 molecular clumps in NGC 4214. Our
results from a virial mass analysis are compatible with a Galactic CO-to-H_2
conversion factor for NGC 4214 (lower than what is usually found in metal-poor
dwarf galaxies).Comment: accepted for publication in the AJ (February 2001), full ps file at:
ftp://ftp.astro.caltech.edu/users/fw/ngc4214/walter_prep.p
Acquisition of anoikis resistance in human osteosarcoma cells does not alter sensitivity to chemotherapeutic agents
BACKGROUND: Chemotherapy-induced cell death can involve the induction of apoptosis. Thus, aberrant function of the pathways involved might result in chemoresistance. Since cell adhesion to the extracellular matrix acts as a survival factor that homeostatically maintains normal tissue architecture, it was tested whether acquisition of resistance to deadhesion-induced apoptosis (anoikis) in human osteosarcoma would result in resistance to chemotherapy. METHODS: Osteosarcoma cell lines (SAOS-2 and TE-85) obtained from ATCC and were maintained in complete Eagle's MEM medium. Suspension culture was established by placing cells in tissue culture wells coated with poly-HEMA. Cell cytotoxicity was determined using a live/dead cytotoxicity assay. Cell cycle/apoptosis analyses were performed using propidium iodide (PI) staining with subsequent FACS analysis. Apoptosis was also assayed by Annexin-FITC/PI staining. RESULTS: Etoposide, adriamycin, vinblastine, cisplatin and paclitaxel were able to induce apoptosis in human osteosarcoma cells SAOS-2 regardless of their anoikis resistance phenotype or the culture conditions (adhered vs. suspended). Moreover, suspended anoikis resistant TE-85 cells (TE-85ar) retained their sensitivity to chemotherapy as well. CONCLUSION: Acquisition of anoikis resistance in human osteosarcoma cells does not result in a generalized resistance to all apoptotic stimuli, including chemotherapy. Moreover, our results suggest that the pathways regulating anoikis resistance and chemotherapy resistance might involve the action of different mediators
Control of InGaAs facets using metal modulation epitaxy (MME)
Control of faceting during epitaxy is critical for nanoscale devices. This
work identifies the origins of gaps and different facets during regrowth of
InGaAs adjacent to patterned features. Molecular beam epitaxy (MBE) near SiO2
or SiNx led to gaps, roughness, or polycrystalline growth, but metal modulated
epitaxy (MME) produced smooth and gap-free "rising tide" (001) growth filling
up to the mask. The resulting self-aligned FETs were dominated by FET channel
resistance rather than source-drain access resistance. Higher As fluxes led
first to conformal growth, then pronounced {111} facets sloping up away from
the mask.Comment: 18 pages, 7 figure
A Laser System for the Spectroscopy of Highly-Charged Bismuth Ions
We present and characterize a laser system for the spectroscopy on
highly-charged ^209Bi^82+ ions at a wavelength of 243.87 nm. For absolute
frequency stabilization, the laser system is locked to a near-infra-red laser
stabilized to a rubidium transition line using a transfer cavity based locking
scheme. Tuning of the output frequency with high precision is achieved via a
tunable rf offset lock. A sample-and-hold technique gives an extended tuning
range of several THz in the UV. This scheme is universally applicable to the
stabilization of laser systems at wavelengths not directly accessible to atomic
or molecular resonances. We determine the frequency accuracy of the laser
system using Doppler-free absorption spectroscopy of Te_2 vapour at 488 nm.
Scaled to the target wavelength of 244 nm, we achieve a frequency uncertainty
of \sigma_{244nm} = 6.14 MHz (one standard deviation) over six days of
operation.Comment: Contribution to the special issue on "Trapped Ions" in "Applied
Physics B
Development and exploratory cluster-randomised opportunistic trial of a theory-based intervention to enhance physical activity among adolescents
Peer reviewedPostprin
Distinction of disorder, classical and quantum vibrational contributions to atomic mean-square amplitudes in dielectric pentachloronitrobenzene
The solid-state molecular disorder of pentachloronitrobenzene (PCNB) and its
role in causing anomalous dielectric properties are investigated. Normal
coordinate analysis (NCA) of atomic mean-square displacement parameters (ADPs)
is employed to distinguish disorder contributions from classical and
quantum-mechanical vibrational contributions. The analysis relies on
multitemperature (5-295 K) single-crystal neutron-diffraction data. Vibrational
frequencies extracted from the temperature dependence of the ADPs are in good
agreement with THz spectroscopic data. Aspects of the static disorder revealed
by this work, primarily tilting and displacement of the molecules, are compared
with corresponding results from previous, much more in-depth and time-consuming
Monte Carlo simulations; their salient findings are reproduced by this work,
demonstrating that the faster NCA approach provides reliable constraints for
the interpretation of diffuse scattering. The dielectric properties of PCNB can
thus be rationalized by an interpretation of the temperature-dependent ADPs in
terms of thermal motion and molecular disorder. The use of atomic displacement
parameters in the NCA approach is nonetheless hostage to reliable neutron data.
The success of this study demonstrates that state-of-the-art single-crystal
Laue neutron diffraction affords sufficiently fast the accurate data for this
type of study. In general terms, the validation of this work opens up the field
for numerous studies of solid-state molecular disorder in organic materials.Comment: Now published in Physical Review
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