Y. Lassailly

Real-time near-field imaging of photoinduced matter motion in thin solid films containing azobenzene derivatives

We present a study of the formation of surface relief gratings in thin solid films containing azobenzene derivatives upon illumination with an interference pattern. This study is based on near-field microscopy techniques that provide real-time imaging of both the photomechanical response of the material and light excitation profile. We demonstrate that the material deformation follows two distinct regimes characterized by different kinetics, a different phase relative to the light intensity pattern, and a specific dependence on light polarization.

Alternating photoinduced mass transport triggered by light polarization in azobenzene containing sol-gel films

Combined shear-force and near-field optical microscopies are used for real-time monitoring of the formation of photoinduced surface relief gratings in photochromic thin films containing azobenzene derivatives. The correlated optical and topographical images provide evidence that the direction of the photoinduced matter migration is defined by the light polarization pattern and that, for a given light intensity pattern, modulating the polarization between two orthogonal states gives rise to alternating mass transport.

Transport and magnetic properties of Fe/GaAs Schottky junctions for spin polarimetry applications

The electrical, magnetic and spin-filtering properties of Pd/Fe/GaAs(001) junctions are investigated. The Pd/Fe thin layers are deposited on GaAs(001) surfaces both clean and passivated by a thin oxide layer. The surface composition, structure and electronic properties of the starting surfaces are studied by means of XPS, LEED, EELS and photoreflectance. The Fe layer magnetic properties are characterized by magneto-optical Kerr effect (MOKE) measurements, while the electronic properties of the junctions are characterized by current-voltage (I-V) and photoreflectance measurements. For both types of substrate surfaces, the magnetization of the Fe layers is found close to that of a bulk Fe slice of equivalent thickness. For the oxide interface, the I-V curve exhibits almost an ideal Schottkylike behavior, since it can be very well interpreted by the thermoionic equation, using the ideality factor of n = 1.02 and surface barrier ϕb = 0.7 eV. For junctions prepared on the reconstructed GaAs(001) surface, the i...

A closer look at the light-induced changes in the mechanical properties of azobenzene-containing polymers by statistical nanoindentation

The mechanical properties of azobenzene-containing polymer films are statistically measured by instrumented nanoindentation experiments in the dark and under illumination in the absorption band of the azobenzene molecules. The material is obtained from a commercial PMMA compound grafted with Disperse Red 1 (DR1) azobenzene derivative. In the dark, DR1 molecules remain in the stable trans isomer state while, under illumination, they undergo photoisomerisation cycling and form a photo-stationary equilibrium between cis and trans isomers. This material is known to exhibit light-induced deformation phenomena related to the photoisomerization cycling of the DR1 units. Statistical loading/unloading tests performed in the tens of μN load range reveal a significant change in the mechanical properties of the film under light excitation. The material hardness and irreversible viscosity are seen to decrease, while the creep coefficient value increases, indicating a significant reinforcement of the viscoplastic response of the film under illumination. Moreover, creep experiments performed at a constant load show striking dissipative effects when light is turned on and also, surprisingly, when light is turned off. These features are supposedly related to the transient changes in the balance between the cis and trans isomer populations.

Optical detection of spin-filter effect for electron spin polarimetry

We have monitored the cathodoluminescence (CL) emitted upon injection of free electrons into a hybrid structure consisting of a thin magnetic Fe layer deposited on a p-GaAs substrate, in which InGaAs quantum wells are embedded. Electrons transmitted through the unbiased metal/semiconductor junction recombine radiatively in the quantum wells. Because of the electron spin-filtering across the Fe/GaAs structure, the CL intensity, collected from the backside, is found to depend on the relative orientation between the injected electronic spin polarization and the Fe layer magnetization. The spin asymmetry of the CL intensity in such junction provides a compact optical method for measuring spin polarization of free electrons beams or of hot electrons in solid-state devices.

Highly polarized photoluminescence from 2‐μm‐thick strained GaAs grown on CaF2

Intense photoluminescence was measured in a strained 2‐μm‐thick GaAs film grown on a (100)‐oriented CaF2 substrate. Circular polarization of 77%±2% was obtained at 77 K under excitation with circularly polarized photons below 1.575 eV. For excitation above 1.610 eV the polarization is limited to 30%±2%. These results are clear indications of strain induced splitting between the Mj=‖±3/2〉 and the Mj=‖± 1/2〉 hole bands. The deduced splitting was 62.5±2.5 meV, corresponding to a stress of the order of −12 kbar. The polarization is maximum for reception energies very close to the excitation. Strained GaAs/CaF2 is thus a good candidate structure for efficient strongly polarized electron sources.

Experimental evidence of nanometer-scale localized recombination due to random In fluctuations in InGaN/GaN quantum wells (Conference Presentation)

In nitride ternary alloys, natural compositional disorder induces strong electronic localization effects. We present a new experimental approach which allows a direct probing at nanometer scale of disorder-induced localization effects in InGaN/GaN quantum wells (QWs). In this experiment, samples are p-type heterostructures incorporating an InGaN/GaN QW nearby the surface. The electrons are locally injected from a scanning tunneling microscope (STM) tip into the conduction band of the thin cladding top GaN layer and captured in the InGaN QW where they radiatively recombine. The injected current is maintained constant by the STM feedback loop and the injection electron energy is controlled by the bias voltage applied to the tip-sample tunnel junction. The luminescence onset voltage coincides with electron injection above the bottom of the conduction band in the bulk GaN (beyond the band bending region). Thereby, scanning the tip allows the high-resolution mapping of the luminescence process in the InGaN QW. Spatial fluctuations of the luminescence peak energy and linewidth are observed on the scale of a few nanometers, which are characteristic of disorder-induced carrier localization. A model based on the so-called localization landscape theory is developed to take into account the effect of alloy disorder into simulations of the structure properties. The localization landscape notably describes an effective confining potential, whose basins and crests define the localization regions of carriers. This theory accounts well for the observed nanometer scale carrier localization and the energy-dependent luminescence linewidth observed for the quantum electron states in the disordered energy band.

Using scanning near-field microscopy to study photo-induced mass motions in azobenzene containing thin films

Scanning near-field optical microscopy (SNOM) is used to study the photo-induced deformation of layered structures containing azobenzene derivatives. This approach is particularly relevant since it allows detecting in real-time, with the same probe the surface topography and the optical field distribution at the nanoscale. The correlation between the local light pattern and the ongoing photo-induced deformation in azobenzene-containing thin films is directly evidenced for different light polarization configurations. This unveils several fundamental photodeformation mechanisms, depending not only on the light field properties, but also on the nature of the material. Controlling the projected electromagnetic field distribution allows inscription of various patterns with a resolution at the diffraction limit, i.e. of a few hundreds of nm. Surface relief patterns with characteristic sizes beyond the diffraction limit can also be produced by using the nearfield probe to locally control the photo-mechanical process. Finally, the photo-mechanical properties of azo-materials are exploited to optically patterned metal/dielectric hybrid structures. Gratings are inscribed this way on thin gold films. The characteristic features (enhancement and localization) of the surface plasmons supported by these noble metal structures are studied by near-field optical microscopy.

Spin Filters as High‐Performance Spin Polarimeters

A spin‐dependent transport experiment in which hot electrons pass through a ferromagnetic metal / semiconductor Schottky diode has been performed. A spin‐polarized free‐electron beam, emitted in vacuum from a GaAs photocathode, is injected into the thin metal layer with an energy between 5 and 1000 eV above to the Fermi level. The transmitted current collected in the semiconductor substrate increases with injection energy because of secondary ‐ electron multiplication. The spin‐dependent part of the transmitted current is first constant up to about 100 eV and then increases by 4 orders of magnitude. As an immediate application, the solid‐state hybrid structure studied here leads to a very efficient and compact device for spin polarization detection.

Photoelectronic processes in semiconductors activated to negative electron affinity

Note: invited communication Reference LOEQ-CONF-1992-001 Record created on 2007-08-31, modified on 2016-08-08