305 research outputs found
Efficient room-temperature light-emitters based on partly amorphised Ge quantum dots in crystalline Si
Semiconductor light emitters compatible with standard Si integration
technology (SIT) are of particular interest for overcoming limitations in the
operating speed of microelectronic devices 1-3. Light sources based on group-IV
elements would be SIT compatible but suffer from the poor optoelectronic
properties of bulk Si and Ge. Here, we demonstrate that epitaxially grown Ge
quantum dots (QDs) in a fully coherent Si matrix show extraordinary optical
properties if partially amorphised by Ge-ion bombardment (GIB). The GIB-QDs
exhibit a quasi-direct-band gap and show, in contrast to conventional SiGe
nanostructures, almost no thermal quenching of the photoluminescence (PL) up to
room-temperature (RT). Microdisk resonators with embedded GIB-QDs exhibit
threshold-behaviour and super-linear increase of the integrated PL-intensity
(IPL) with increasing excitation power Pexc which indicates light amplification
by stimulated emission in a fully SIT-compatible group-IV nano-system
ANNINE-6plus, a voltage-sensitive dye with good solubility, strong membrane binding and high sensitivity
We present a novel voltage-sensitive hemicyanine dye ANNINE-6plus and describe its synthesis, its properties and its voltage-sensitivity in neurons. The dye ANNINE-6plus is a salt with a double positively charged chromophore and two bromide counterions. It is derived from the zwitterionic dye ANNINE-6. While ANNINE-6 is insoluble in water, ANNINE-6plus exhibits a high solubility of around 1 mM. Nonetheless, it displays a strong binding to lipid membranes. In contrast to ANNINE-6, the novel dye can be used to stain cells from aqueous solution without surfactants or organic solvents. In neuronal membranes, ANNINE-6plus exhibits the same molecular Stark effect as ANNINE-6. As a consequence, a high voltage-sensitivity is achieved with illumination and detection in the red end of the excitation and emission spectra, respectively. ANNINE-6plus will be particularly useful for sensitive optical recording of neuronal excitation when organic solvents and surfactants must be avoided as with intracellular or extracellular staining of brain tissue
Forces on Dust Grains Exposed to Anisotropic Interstellar Radiation Fields
Grains exposed to anisotropic radiation fields are subjected to forces due to
the asymmetric photon-stimulated ejection of particles. These forces act in
addition to the ``radiation pressure'' due to absorption and scattering. Here
we model the forces due to photoelectron emission and the photodesorption of
adatoms. The ``photoelectric'' force depends on the ambient conditions relevant
to grain charging. We find that it is comparable to the radiation pressure when
the grain potential is relatively low and the radiation spectrum is relatively
hard. The calculation of the ``photodesorption'' force is highly uncertain,
since the surface physics and chemsitry of grain materials are poorly
understood at present. For our simple yet plausible model, the photodesorption
force dominates the radiation pressure for grains with size >~0.1 micron
exposed to starlight from OB stars. We find that the anisotropy of the
interstellar radiation field is ~10% in the visible and ultraviolet. We
estimate size-dependent drift speeds for grains in the cold and warm neutral
media and find that micron-sized grains could potentially be moved across a
diffuse cloud during its lifetime.Comment: LaTeX(41 pages, 19 figures), submitted to Ap
Strong-field terahertz-optical mixing in excitons
Driving a double-quantum-well excitonic intersubband resonance with a
terahertz (THz) electric field of frequency \omega_{THz} generated terahertz
optical sidebands \omega=\omega_{THz}+\omega_{NIR} on a weak NIR probe. At high
THz intensities, the intersubband dipole energy which coupled two excitons was
comparable to the THz photon energy. In this strong-field regime the sideband
intensity displayed a non-monotonic dependence on the THz field strength. The
oscillating refractive index which gives rise to the sidebands may be
understood by the formation of Floquet states, which oscillate with the same
periodicity as the driving THz field.Comment: 4 pages, 6 figure
Non-perturbative electron dynamics in crossed fields
Intense AC electric fields on semiconductor structures have been studied in
photon-assisted tunneling experiments with magnetic field applied either
parallel (B_par) or perpendicular (B_per) to the interfaces. We examine here
the electron dynamics in a double quantum well when intense AC electric fields
F, and tilted magnetic fields are applied simultaneously. The problem is
treated non-perturbatively by a time-dependent Hamiltonian in the effective
mass approximation, and using a Floquet-Fourier formalism. For B_par=0, the
quasi-energy spectra show two types of crossings: those related to different
Landau levels, and those associated to dynamic localization (DL), where the
electron is confined to one of the wells, despite the non-negligible tunneling
between wells. B_par couples parallel and in-plane motions producing
anti-crossings in the spectrum. However, since our approach is
non-perturbative, we are able to explore the entire frequency range. For high
frequencies, we reproduce the well known results of perfect DL given by zeroes
of a Bessel function. We find also that the system exhibits DL at the same
values of the field F, even as B_par non-zero, suggesting a hidden dynamical
symmetry in the system which we identify with different parity operations. The
return times for the electron at various values of field exhibit interesting
and complex behavior which is also studied in detail. We find that smaller
frequencies shifts the DL points to lower field F, and more importantly, yields
poorer localization by the field. We analyze the explicit time evolution of the
system, monitoring the elapsed time to return to a given well for each Landau
level, and find non-monotonic behavior for decreasing frequencies.Comment: REVTEX4 + 11 eps figs, submitted to Phys. Rev.
Primer to Voltage Imaging With ANNINE Dyes and Two-Photon Microscopy
ANNINE-6 and ANNINE-6plus are voltage-sensitive dyes that when combined with two-photon microscopy are ideal for recording of neuronal voltages in vivo, in both bulk loaded tissue and the dendrites of single neurons. Here, we describe in detail but for a broad audience the voltage sensing mechanism of fast voltage-sensitive dyes, with a focus on ANNINE dyes, and how voltage imaging can be optimized with one-photon and two-photon excitation. Under optimized imaging conditions the key strengths of ANNINE dyes are their high sensitivity (0.5%/mV), neglectable bleaching and phototoxicity, a linear response to membrane potential, and a temporal resolution which is faster than the optical imaging devices currently used in neurobiology (order of nanoseconds). ANNINE dyes in combination with two-photon microscopy allow depth-resolved voltage imaging in bulk loaded tissue to study average membrane voltage oscillations and sensory responses. Alternatively, if ANNINE-6plus is applied internally, supra and sub threshold voltage changes can be recorded from dendrites of single neurons in awake animals. Interestingly, in our experience ANNINE-6plus labeling is impressively stable in vivo, such that voltage imaging from single Purkinje neuron dendrites can be performed for 2 weeks after a single electroporation of the neuron. Finally, to maximize their potential for neuroscience studies, voltage imaging with ANNINE dyes and two-photon microscopy can be combined with electrophysiological recording, calcium imaging, and/or pharmacology, even in awake animals
A Survey of Methods for Volumetric Scene Reconstruction from Photographs
Scene reconstruction, the task of generating a 3D model of a scene given multiple 2D photographs taken of the scene, is an old and difficult problem in computer vision. Since its introduction, scene reconstruction has found application in many fields, including robotics, virtual reality, and entertainment. Volumetric models are a natural choice for scene reconstruction. Three broad classes of volumetric reconstruction techniques have been developed based on geometric intersections, color consistency, and pair-wise matching. Some of these techniques have spawned a number of variations and undergone considerable refinement. This paper is a survey of techniques for volumetric scene reconstruction
- …