90 research outputs found
Light mediated emergence of surface patterns in azopolymers at low temperatures
Polymer thin films doped with azobenzene molecules do have the ability to
organize themselves in spontaneous surface relief gratings (SRG) under
irradiation with a single polarized beam. To shed some light in this still
unexplained phenomenon, we use a new method that permits us to access
experimentally the very first steps of the pattern formation process.
Decreasing the temperature, we slow down the formation and organization of
patterns, due to the large increase of the viscosity and relaxation time of the
azopolymer. As a result decreasing the temperature allows us to access and
study much shorter time scales,in the physical mechanisms underlying the
pattern formation, than previously reported. We find that the patterns organize
themselves in sub-structures which size increase with the temperature,
following the diffusion coefficient evolution of the material. That result
suggests that the pattern formation and organization is mainly governed by
diffusive processes, in agreement with some theories of the SRG formation.
Decreasing further the temperature we observe the emergence of small voids
located at the junction of the sub-structures.Comment: 6 figures, 13 pages
Birefringence properties and surface relief grating formation on methlacrylate polymers with photochromic side chains
International audienc
Subwavelength atom localization via amplitude and phase control of the absorption spectrum
We propose a scheme for subwavelength localization of an atom conditioned
upon the absorption of a weak probe field at a particular frequency.
Manipulating atom-field interaction on a certain transition by applying drive
fields on nearby coupled transitions leads to interesting effects in the
absorption spectrum of the weak probe field. We exploit this fact and employ a
four-level system with three driving fields and a weak probe field, where one
of the drive fields is a standing-wave field of a cavity. We show that the
position of an atom along this standing wave is determined when probe field
absorption is measured. We find that absorption of the weak probe field at a
certain frequency leads to subwavelength localization of the atom in either of
the two half-wavelength regions of the cavity field by appropriate choice of
the system parameters. We term this result as sub-half-wavelength localization
to contrast it with the usual atom localization result of four peaks spread
over one wavelength of the standing wave. We observe two localization peaks in
either of the two half-wavelength regions along the cavity axis.Comment: Accepted for publication to Physical Review
Surface relief grating formation on nano-objects
We exploit the photoinduced migration effect in azopolymer thin films to induce surface relief patterning of nano-objects. Manipulation and precise control of the molecular order is achieved at the nanoscale. Interaction between a laser beam from an argon laser and the azopolymer nano-objects induces structures on the surface. The self-patterning process is observed to depend on the laser beam polarization
Impact of sidewall etching on the dynamic performance of GaN-on-Si E-mode transistors
Abstract The aim of this paper is to investigate the role of the etching of the sidewalls of p-GaN on the dynamic performance of normally-off GaN HEMTs with p-type gate. We analyze two wafers having identical epitaxy but with different recipes for the sidewall etching, referred to as "Etch A" (non-optimized) and "Etch B" (optimized). We demonstrate the following relevant results: (i) the devices with non-optimized etching (Etch A), when submitted to positive gate bias, show a negative threshold voltage shift and a decrease in Ron, which are ascribed to hole injection under the gate and/or in the access regions; (ii) transient characterization indicates the existence of two trap states, with activation energies of 0.84 eV (CN defects) and 0.30 eV. The latter (with time-constants in the ms range) is indicative of the hole de-trapping process, possibly related to trap states in the AlGaN barrier or at the passivation/AlGaN interface; (iii) by optimizing the p-GaN sidewall etching (for the same epitaxy) it is possible to completely eliminate the threshold voltage shift. This indicates that hole injection mostly takes place along the sidewalls
Mouvement photomoléculaire sur des nano-objets en azopolymère: Vers de nouvelles applications en photonique
Date du colloque : 08/2010</p
Time-dependent efficiency measurements of polymer solar cells with dye additives: unexpected initial increase of efficiency
We report the effects of the addition of two azo-dye additives on the time-dependent efficiency of polymer solar cells. Although the maximum efficiencies of devices containing different amounts of dye do not vary greatly over the selected concentration range, the time dependence results reveal a surprising initial increase in efficiency in some samples. We observe this effect to be correlated with a leakage current, although a specific mechanism is not yet identified. We also present the measured lifetimes of these solar cells, and find that variations in dye concentrations produce a small effect at most. Characterization of the bulk heterojunction layer (active layer) morphology using atomic-force microscope (AFM) imaging reveals reordering patterns which suggest that the primary effects of the dyes arise via structural, not absorptive, characteristics
Photomolecular motions on azopolymer nano-objects: Toward new applications for photonics
International audienc
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