M. Füle

Regulation of cortical microcircuits by unitary GABA-mediated volume transmission.

GABA (γ-aminobutyric acid) is predominantly released by local interneurons in the cerebral cortex to particular subcellular domains of the target cells. This suggests that compartmentalized, synapse-specific action of GABA is required in cortical networks for phasic inhibition. However, GABA released at the synaptic cleft diffuses to receptors outside the postsynaptic density and thus tonically activates extrasynaptic GABAA and GABAB receptors, which include subtypes of both receptor families especially sensitive to low concentrations of GABA. The synaptic and extrasynaptic action of GABA corroborates the idea that neurons of the brain use synaptic (or wiring) transmission and non-synaptic (or volume) transmission for communication. However, re-uptake mechanisms restrict the spatial extent of extrasynaptic GABA-mediated effects, and it has been proposed that the concerted action of several presynaptic interneurons, the sustained firing of individual cells or an increase in release-site density is required to reach ambient GABA levels sufficient to activate extrasynaptic receptors. Here we show that individual neurogliaform cells release enough GABA for volume transmission within the axonal cloud and, thus, that neurogliaform cells do not require synapses to produce inhibitory responses in the overwhelming majority of nearby neurons. Neurogliaform cells suppress connections between other neurons acting on presynaptic terminals that do not receive synapses at all in the cerebral cortex. They also reach extrasynaptic, δ-subunit-containing GABAA (GABAAδ) receptors responsible for tonic inhibition. We show that GABAAδ receptors are localized to neurogliaform cells preferentially among cortical interneurons. Neurosteroids, which are modulators of GABAAδ receptors, alter unitary GABA-mediated effects between neurogliaform cells. In contrast to the specifically placed synapses formed by other interneurons, the output of neurosteroid-sensitive neurogliaform cells represents the ultimate form of the lack of spatial specificity in GABA-mediated systems, leading to long-lasting network hyperpolarization combined with widespread suppression of communication in the local circuit.

Ultraviolet photoluminescence and its relation to atomic bonding properties of hydrogenated amorphous carbon

Abstract We have observed photoluminescence in hydrogenated amorphous carbon (a-C:H) samples at substantially higher energy than the visible band. The emitted light is in the ultraviolet (UV) region, the spectrum consists of three bands with peak positions of ∼4.46 eV, ∼4.01 eV and ∼3.63 eV, depending slightly on sample properties. The UV photoluminescence (PL) has been observed in samples of different band gaps which show either high or low PL efficiency in the visible region. The excitation spectrum of UV luminescence exhibits high efficiency in the photon energy range of 5.6–6.2 eV and a strong decrease at excitations below this energy range. The experimental fact, that, the peak energies of UV bands exceed the optical gap energy of the studied samples, supports the light emission via radiative recombination of localised geminate electron–hole pairs. Strong localisation is expected for the excitation of π–π* transitions in conjugated double bonded fragments of small sizes. An infrared study of the UV light emitting a-C:H films confirms the presence of conjugated double bonds with aromatic and olefinic local configurations as well, however, the unambiguous relation between UV luminescence and small aromatic structures cannot be established. It is more probable that the olefinic fragments with chain lengths of 2–4 give π electronic levels through which UV light emission takes place. The role of twofold co-ordinated carbon sites cannot be excluded yet.

Multi-band structure of amorphous carbon luminescence

Abstract Structured photoluminescence (PL) spectra of hydrogenated amorphous carbon (a-C:H) layers prepared from benzene are presented. The spectra contain bands with peak positions in the range of 4.34–4.50, 3.93–4.01 and 3.64–3.79 eV in the ultraviolet region, and additional luminescence bands are in the energy regions of 3.19–3.28 and 2.85–2.96 eV, besides the previously mostly measured band, with a peak in the 2.28–2.48 eV photon energy range. Relative efficiency of these bands depends on the deposition conditions. Each of the new bands could be excited above a given photon energy and, therefore, the overall spectral shape as well as the structured feature of a-C:H luminescence varies with excitation energy. It is supposed that the opening of new radiation recombination transitions with increasing excitation energy explains the appearance of new luminescence bands.

Carbon nano-particles prepared by ion-clustering in plasma

The plasma-assisted chemical vapour deposition technique was used to produce carbon nano-particles in a dusty plasma condition, at increased gas pressure and decreased electric field. The negatively biased upper electrode levitates the forming carbon clusters, which grow in a spherical symmetrical way in the relatively large density of the ions of the plasma. The spheres also attach to each other, but their concentration and mobility being low, the aggregation results in a different structure: a necklace-type chain will be formed. The dust particles, leaving the plasma cover the chamber wall, but a substantial amount of them reach the electrically driven substrate; forming an amorphous carbon film. The cluster systems were investigated by transmission electron microscopy, atomic force microscopy, Raman, infrared and photoluminescence spectroscopic methods.

Optical strength in UV region of amorphous carbon

Abstract Optical strength (damage properties) of amorphous carbon films was investigated under the influence of UV laser pulses in the nanosecond (ns) and femtosecond (fs) region on different laser wavelengths. The damaged area was found to depend linearly on the laser fluence in a narrow region around the damage threshold tested by raising the number of pulses and fluence. The optical damage strength was found to be 2–3 times higher for fs pulses on both wavelengths used. This can be explained by the different weights of the ablation and thermal processes in cases where pulse lengths differ strongly. The fs pulse heats the focal spot less effectively, and cools down before the arrival of the next pulse. The ns pulses are better heaters (proven by the lower threshold), they have smaller crater diameters and slower incubation periods.

Electronic structure of pulsed laser deposited carbon thin films monitored by photoluminescence

Abstract The influence of pressure of methane atmosphere on the pulsed laser deposited (PLD) amorphous carbon layers (a-C:H) were studied, on the structure on atomic scale as well as on the electronic levels near the band edges. Raman scattering, measured by two different probing wavelengths, show the absence of graphitic ordering of sp 2 hybridized carbons. The samples prepared at low methane pressure are characterized with spectra, similar to the spectra of diamond-like carbon. Infrared absorption of PLD films is dominated in the CH bond stretching region by the bands due to sp 3 CH 3 and sp 3 CH 2 vibrations, however, a weak, widely distributed band also appears over 3000 cm −1 . Photoluminescence (PL) can be excited in these samples with decreasing wavelength below 400 nm (over 3.1 eV) and a steep increase of PL intensity occurs below 250 nm (over 4.9 eV). Some distinct PL bands can be distinguished in wavelength region of 270–450 nm (4.6–2.7 eV), the appearance and intensity of which shows correlation with the applied methane pressure.

Stimulated oxidation of metals (laser, electric field, etc.): Comparative studies

In this report we demonstrate the importance of metal oxides, e.g. thin films and nanostructures, in modern science and technology. The basic laws of oxide thickness on base of diffusion of specimens versus time in different circumstances (Cabrera-Mott and Wagner laws) under the influence of external fields, e.g. electromagnetic field, static electric and magnetic field, are demonstrated. We give experimental results for various metal oxide layers over a wide range of different metals. Theoretical explanations are provided as well for the most reliable circumstances.

Laser fabricated nanostructures on vanadium foils

In this work we present our results concerning to the nanostructure generation on vanadium surfaces by ultrashort pulsed laser irradiation. The melting free formation of these structures is very important in many fields of science and industry too. We obtain that the nanostructure forming process on vanadium surface is Stransky Krastanov type. The surface covering and the nano‐tower shape are depending on the ambient of the laser matter reaction.

Surface Ripple Structure on V2O5 Single Crystals

In this work we give a phenomenological picture about the properties of the laser interaction with V2O5 single crystals. Our experiments show that the damaged region of the crystal surface is surrounded by a humped surface. The inner part of the examined region is covered by amorphous V2O5 and the outer part of this region has been raised by thermal shock of laser treatment. This heat effect produces the hump on the surface.