177 research outputs found
Ultrafast spatiotemporal control of photocarriers in doped semiconductors
Control of the spatial and temporal dynamics of photoexcited charge carriers at the surfaces and interfaces of semiconducting materials is pertinent to many of modern technologies such as solar cells, photodetectors and other optoelectronic devices. In inhomogeneous materials such as nanostructured materials, spatial variations in carrier dynamics are inherent via the material design and accordingly allow for improvements in device performance e.g. transport of electrons from donor to acceptor regions in photovoltaic devices. On the other hand, one can also create spatial inhomogeneity in the carrier dynamics in homogeneous systems by using a non-uniform photoexcitation profile. This would have the advantage that the control of the carrier dynamics could be more flexible and not tied down to the fixed material design. In principle, photoexcitation even with a simple Gaussian beam could result in nontrivial inhomogeneous carrier dynamics within the full-width-half-maximum (FWHM) of the optical spot. However, in typical ultrafast spectroscopy measurements performed thus far, one averages out the photocarrier response over the excitation spot and spatial variations therein are inaccessible. In this thesis, we create nontrivial spatiotemporal dynamics of the photoexcited electrons in a homogeneous Zn-doped GaAs using the spatial intensity variation in a simple Gaussian photoexcitation beam. We image these dynamics using time-resolved photoemission electron microscopy (TR-PEEM) - a technique offering both high temporal and spatial resolutions. In particular, we demonstrate the spatial redistribution of the photoexcited electrons in two different regimes: (I) the early time delays where we control the vertical transport of electrons to the sample surface and (II) at long time delays where we manipulate the lateral distribution of the electrons along the sample surface.In the first study, we achieve spatial variations in the screening process of the intrinsic surface field, which influences the vertical drift of photoexcited electrons from the bulk to the sample surface. Combined with the occurrence of Auger recombination in regions of higher intensity, we see a depletion in the electron population at the center, with a gain just away from the center. We show control of these processes and how they affect the electron distribution on the sample surface in nontrivial ways. In the second study, we show that at long time delays the spatially varying screening process leads to the creation of lateral fields along the sample surface. These highly local and spatially varying lateral fields then act upon the photoexcited electrons, eventually pulling them apart into two distinct distributions. Using a simple model that explains the experimental data, we also show the possibility of generating near-arbitrary lateral fields and thus controlling electrons on the sample surface in more general ways. In conclusion, this thesis demonstrates the capability to create and control nontrivial spatiotemporal dynamics of the photoexcited electrons even in a homogeneous semiconductor by exploiting the intensity variation of an ultrafast light pulse. This capability could lead to a promising new handle for use in high-speed optoelectronic devices.Okinawa Institute of Science and Technology Graduate Universit
Improving Signal and Photobleaching Characteristics of Temporal Focusing Microscopy with the Increase in Pulse Repetition Rate
Wide-field temporal focused (WF-TeFo) two-photon microscopy allows for the simultaneous imaging of a large planar area, with a potential order of magnitude enhancement in the speed of volumetric imaging. To date, low repetition rate laser sources with over half a millijoule per pulse have been required in order to provide the high peak power densities for effective two-photon excitation over the large area. However, this configuration suffers from reduced signal intensity due to the low repetition rate, saturation effects due to increased excitation fluences, as well as faster photobleaching of the fluorescence probe. In contrast, with the recent advent of high repetition rate, high pulse energy laser systems could potentially provide the advantages of high repetition rate systems that are seen in traditional two-photon microscopes, while minimizing the negatives of high fluences in WF-TeFo setups to date. Here, we use a 100 microjoule/high repetition rate (50-100 kHz) laser system to investigate the performance of a WF-TeFo two-photon microscope. While using micro-beads as a sample, we demonstrate a proportionate increase in signal intensity with repetition rate, at no added cost in photobleaching. By decreasing pulse intensity, via a corresponding increase in repetition rate to maintain fluorescence signal intensity, we find that the photobleaching rate is reduced by ~98.4%. We then image live C. elegans at a high repetition rate for 25 min. as a proof-of-principle. Lastly, we identify the steady state temperature increase as the limiting process in further increasing the repetition rate, and we estimate that repetition rate in the range between 0.5 and 5 MHz is ideal for live imaging with a simple theoretical model. With new generation low-cost fiber laser systems offering high pulse energy/high repetition rates in what is essentially a turn-key solution, we anticipate increased adoption of this microscopy technique by the neuroscience community
A remark on non-Abelian classical kinetic theory
It is known that non-Abelian classical kinetic theory reproduces the Hard
Thermal/Dense Loop (HTL/HDL) effective action of QCD, obtained after
integrating out the hardest momentum scales from the system, as well as the
first higher dimensional operator beyond the HTL/HDL level. We discuss here its
applicability at still higher orders, by comparing the exact classical
effective action obtained in the static limit, with the 1-loop quantum
effective potential. We remark that while correct types of operators arise, the
classical colour algebra reproduces correctly the prefactor of the 4-point
function only for matter in asymptotically high dimensional colour
representations.Comment: 6 page
Pulling apart photoexcited electrons by photoinducing an in-plane surface electric field
The study and control of spatiotemporal dynamics of photocarriers at the interfaces of materials have led to transformative modern technologies, such as light-harvesting devices and photodetectors. At the heart of these technologies is the ability to separate oppositely charged electrons and holes. Going further, the ability to separate like charges and manipulate their distribution could provide a powerful new paradigm in opto-electronic control, more so when done on ultrafast time scales. However, this requires one to selectively address subpopulations of the photoexcited electrons within the distribution—a challenging task, particularly on ultrafast time scales. By exploiting the spatial intensity variations in an ultrafast light pulse, we generate local surface fields within the optical spot of a doped semiconductor and thereby pull apart the electrons into two separate distributions. Using time-resolved photoemission microscopy, we directly record a movie of this redistribution process lasting a few hundred picoseconds, which we control via the spatial profile and intensity of the photoexciting pulse. Our quantitative model explains the underlying charge transport phenomena, thus providing a roadmap to the more generalized ability to manipulate photocarrier distributions with high spatiotemporal resolution
Molecular gas in NUclei of GAlaxies (NUGA) VII. NGC4569, a large scale bar funnelling gas into the nuclear region
This work is part of the NUGA survey of CO emission in nearby active
galaxies. We present observations of NGC4569, a member of the Virgo Cluster. We
analyse the molecular gas distribution and kinematics in the central region and
we investigate a possible link to the strong starburst present at the nucleus.
70% of the 1.1x10^9 Msolar of molecular gas detected in the inner 20" is found
to be concentrated within the inner 800 pc and is distributed along the large
scale stellar bar seen in near-infrared observations. A hole in the CO
distribution coincides with the nucleus where most of the Halpha emission and
blue light are emitted. The kinematics are modelled in three different ways,
ranging from the purely geometrical to the most physical. This approach allows
us to constrain progressively the physical properties of the galaxy and
eventually to emerge with a reasonable fit to an analytical model of orbits in
a barred potential. Fitting an axisymmetric model shows that the non-circular
motions must be comparable in amplitude to the circular motions (120 km/s).
Fitting a model based on elliptical orbits allows us to identify with
confidence the single inner Lindblad resonance (ILR) of the large scale bar.
Finally, a model based on analytical solutions for the gas particle orbits in a
weakly barred potential constrained by the ILR radius reproduces the
observations well. The mass inflow rate is then estimated and discussed based
on the best fit model solution. The gravitational torques implied by this model
are able to efficiently funnel the gas inside the ILR down to 300 pc, although
another mechanism must take over to fuel the nuclear starburst inside 100 pc.Comment: accepted for publication in A&
Directly visualizing the momentum forbidden dark excitons and their dynamics in atomically thin semiconductors
Resolving the momentum degree of freedom of excitons - electron-hole pairs
bound by the Coulomb attraction in a photoexcited semiconductor, has remained a
largely elusive goal for decades. In atomically thin semiconductors, such a
capability could probe the momentum forbidden dark excitons, which critically
impact proposed opto-electronic technologies, but are not directly accessible
via optical techniques. Here, we probe the momentum-state of excitons in a WSe2
monolayer by photoemitting their constituent electrons, and resolving them in
time, momentum and energy. We obtain a direct visual of the momentum forbidden
dark excitons, and study their properties, including their near-degeneracy with
bright excitons and their formation pathways in the energy-momentum landscape.
These dark excitons dominate the excited state distribution - a surprising
finding that highlights their importance in atomically thin semiconductors.Comment: 34 page
Non-circular Gas Kinematics and Star Formation in the Ringed Galaxy NGC 4736
We analyze the gas kinematics and star formation properties of the nearby
RSab galaxy NGC 4736 using interferometric and single-dish CO(1-0) data and
previously published Halpha and HI data. The CO morphology is dominated by a
central molecular bar and tightly wound spiral arms associated with a bright
ring of star formation. Strong HI emission is also found in the ring, but HI is
absent from the central regions. Comparison of the HI and Halpha distributions
suggests that HI in the ring is primarily dissociated H. Modeling of the CO
kinematics reveals gas motion in elliptical orbits around the central bar, and
we argue that the ring represents both the OLR of the bar and the ILR of a
larger oval distortion. The HI kinematics show evidence for axisymmetric inflow
towards the ring and are inconsistent with streaming in aligned elliptical
orbits, but the highly supersonic (~40 km/s) inflow velocities required,
corresponding to mass inflow rates of ~2 Msol/yr, suggest that more
sophisticated models (e.g., gas orbiting in precessed elliptical orbits) should
be considered. The radial CO and Halpha profiles are poorly correlated in the
vicinity of the nuclear bar, but show a better correlation (in rough agreement
with the Schmidt law) at the ring. Even along the ring, however, the azimuthal
correspondence between CO and Halpha is poor, suggesting that massive stars
form more efficiently at some (perhaps resonant) locations than at others.
These results indicate that the star formation rate per unit gas mass exhibits
strong spatial variations and is not solely a function of the available gas
supply. The localization of star formation to the ring is broadly consistent
with gravitational instability theory, although the instability parameter on average in the ring, only falling below 1 in localized regions.Comment: Revised version accepted by ApJ, with new section on p-v diagrams. 24
pages with 24 figures (emulateapj5). Full resolution and color versions are
available at http://astro.berkeley.edu/~twong/preprint
Errors, biases, and corrections for weighing gauge precipitation measurements from WMO-SPICE
Presentación realizada en: American Meteorological Society Annual Meeting (2017) celebrado en Seattle (Washington) del 22 al 26 de enero de 2017
Errors, Biases, and Corrections for Weighing Gauge Precipitation Measurements from the WMO Solid Precipitation Intercomparison Experiment
Comunicación presentada en: TECO-2016 (Technical Conference on Meteorological and Environmental Instruments and Methods of Observation) celebrada en Madrid, del 27 al 30 de septiembre de 2016.Although precipitation has been measured for many centuries, precipitation measurements are still beset with significant biases and errors. Solid precipitation is particularly difficult to measure accurately, and biases between winter-time precipitation measurements from different measurement networks or different regions can exceed 100%. Using precipitation gauge results from the WMO Solid Precipitation Intercomparison Experiment (WMO-SPICE), errors in precipitation measurement caused by gauge uncertainty, spatial variability in precipitation, hydrometeor type, and wind are quantified. The methods used to calculate gauge catch efficiency and correct known biases are described briefly. Transfer functions describing catch efficiency as a function of air temperature and wind speed are also presented. In addition, the biases and errors associated with the use of a single transfer function to correct gauge undercatch at multiple sites are discussed
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