Microscopic electroabsorption line shape analysis for Ga(AsSb)∕GaAs heterostructures

A series of Ga(AsSb)/GaAs/(AlGa) As samples with varying GaAs spacer width are studied by electric-field modulated absorption (EA) and reflectance spectroscopy and modeled using a microscopic theory. The analysis of the Franz-Keldysh oscillations of GaAs capping layer and of the quantum-confined Stark shift of the lowest quantum well (QW) transitions shows the strong inhomogeneity of the built-in electric field indicating that the field modulation due to an external bias voltage differs significantly for the various regions of the structures. The calculations demonstrate that the line shape of the EA spectra of these samples is extremely sensitive to the value of the small conduction band offset between GaAs and Ga(AsSb) as well as to the magnitude of the internal electric field changes caused by the external voltage modulation in the QW region. The EA spectra of the entire series of samples are modeled by the microscopic theory. The good agreement between experiment and theory allows us to extract the strength of the modulation of the built-in electric field in the QW region and to show that the band alignment between GaAs and Ga(AsSb) is of type II with a conduction band offset of approximately 40 meV.</p

Spectroscopic Rationale for Efficient Stimulated-Emission Depletion Microscopy Fluorophores

For decades, the power of far-field fluorescence microscopy has been hampered by the diffraction resolution limit,1,2 d) λ/(2NA)&gt; 200 nm,3,4 where λ denotes the light wavelength and NA the numerical aperture of the lens. Identical fluorophores closer than d cannot be discerned because their signals cannot be separated by the detector. However, in the recent past, it has been demonstrated that the diffraction limit can be fundamentally overcome5 by modulating or switching the ability of the dye to emit fluorescence, causing adjacent objects to be registered sequentially. To this end, stimulated-emission depletion (STED) microscopy5,6 and its deriva-tives modulate the fluorescence capability of dye ensembles using defined spatial patterns of light, whereas the techniques known as (f)PALM,7,8 (d)STORM,9,10 and related concepts11 involve switch-ing individual fluorophores stochastically in space followed by mathematical localization of their coordinates. Hence, all of these concepts harness a molecular mechanism that keeps the dy

Distinct subsets of syt-IV/BDNF vesicles are sorted to axons versus dendrites and recruited to synapses by activity.

BDNF plays a critical role in the regulation of synaptic strength and is essential for long-term potentiation, a phenomenon that underlies learningandmemory.However,whetherBDNFactsinadiffusemanneroristargetedtospecificneuronalsubcompartmentsorsynaptic sites to affect circuit function remains unknown. Here, using photoactivation of BDNF or syt-IV (a regulator of exocytosis present on BDNF-containing vesicles) in transfected rat hippocampal neurons, we discovered that distinct subsets of BDNF vesicles are targeted to axonsversusdendritesandarenotsharedbetweenthesecompartments.Moreover,syt-IV-andBDNF-harboringvesiclesarerecruitedto both presynaptic and postsynaptic sites in response to increased neuronal activity. Finally, using syt-IV knockout mouse neurons, we found that syt-IV is necessary for both presynaptic and postsynaptic scaling of synaptic strength in response to changes in network activity. These findings demonstrate that BDNF-containing vesicles can be targeted to specific sites in neurons and suggest that syt-IV- regulated BDNF secretion is subject to spatial control to regulate synaptic function in a site-specific manner