G. Lampel

Apertureless near-field optical microscopy: A study of the local tip field enhancement using photosensitive azobenzene-containing films

The local optical field enhancement which can occur at the end of a nanometer-size metallic tip has given rise to both increasing interest and numerous theoretical works on near-field optical microscopy. In this article we report direct experimental observation of this effect and present an extensive study of the parameters involved. Our approach consists in making a “snapshot” of the spatial distribution of the optical intensity in the vicinity of the probe end using photosensitive azobenzene-containing films. This distribution is coded by optically induced surface topography which is characterized in situ by atomic force microscopy using the same probe. We perform an extensive analysis of the influence of several experimental parameters. The results are analyzed as a function of the illumination parameters (features of the incident laser beam, exposure time, illumination geometry) as well as the average tip-to-sample distance and tip geometry. The results obtained provide substantial information about t...

Near-field optical patterning on azo-hybrid sol–gel films

We report on the near-field optical patterning of photochromic sol–gel films with subwavelength resolution. The sample containing functionalized azobenzene species is locally illuminated in the visible absorption band of these photochromes through the aperture of a metallized tapered optical fiber. The surface topography imaged by in situ shear-force microscopy reveals that, due to repeated photoisomerization cycles of the azobenzene molecules, photoinduced matter migration occurs under the tip leading to the formation of a surface relief. The shape of this structure is characteristic of the electromagnetic field distribution and strongly depends on the tip-to-sample distance. In near-field illumination conditions, protrusions of lateral dimension as small as 60 nm (≈λ/10) are currently produced. When repeating this process, compact arrays of nanodots are optically inscribed.

Spin-polarized photoemission from AlGaAs/GaAs heterojunction: A convenient highly polarized electron source

We analyze the operation of a spin‐polarized electron source, consisting of a 100 A GaAs cap on top of Al0.3Ga0.7As, excited at 300 or 120 K by a He‐Ne laser. The cap allows easy activation to negative electron affinity while the alloy permits gap matching to the light source, and thus large electron spin polarization (30% at 300 K, 36% at 120 K). We compare yield curves, energy distribution curves, and polarized energy distribution curves obtained on samples with 100 and 1000 A caps and on bulk GaAs. The X conduction minimum position in the alloy is also determined.

Optical detection of electron spin resonance in CdTe

Abstract We report the optical detection of electron spin resonance in p-type CdTe at 1.7 K in optical pumping conditions. The Overhauser shift of the electronic resonance, of the order of 45 G, is related to the sign of the electron g-factor g∗. We measure g ∗ = -1.59±0.02 . Using this g∗ value and the previous results on the Knight shift, we deduce the value of the electron wavefunction on Cd in CdTe, which is consistent with the value in CdS.

Near-field magneto-optics with polarization sensitive STOM

Abstract A scanning tunneling optical microscope (STOM) operating with polarized light has been developed to study thin magnetic films. The magnetic film is deposited on the external face of a prism and illuminated in total reflection conditions with linearly polarized laser light. The evanescent mode close to the magnetic film surface is detected with a tip-ending monomode optical fiber connected at its other end to a light-polarization analyzer mounted at the entrance of a photomultiplier tube. The polarization sensitivity of the whole system, which was found to depend on the tip condition, was characterized on the bare prism with s- and p-polarized excitations. The magneto-optical effect in the evanescent mode is measured through a lock-in amplifier by modulating the magnetic field produced by a coil surrounding the tip. With this set-up we have studied two different systems, both exhibiting perpendicular magnetization. The first one is a dielectric garnet film. The images, obtained on this sample by measuring the magneto-optical effect under very low amplitude of the external magnetic field modulation, show up submicronic details due to magnetic domain wall motion. The second system is a metallic 25 nm Au/1 nm Co/4 nm Au sandwich with a large coercive field (≈ 1 kOe). The magneto-optical effect is here measured by modulating the field with an amplitude larger than the coercive field so that the saturated magnetization is periodically flipped. In this system we have taken advantage of the possibility to excite surface plasmon resonances in noble metal thin films with p-polarized light. Near-field measurements performed with our microscope demonstrate that the intensity of the evanescent mode is strongly enhanced (two orders of magnitude) at resonance. Moreover, the interaction of the light electric field with the gold surface plasmon leads to a related amplification of the magneto-optical effects in the evanescent mode.

Imaging of magnetic domains with scanning tunneling optical microscopy

Near field magneto-optical images of magnetic domains in ferromagnetic soft garnet films have been obtained with a scanning tunneling optical microscope working in total reflection geometry with shear-force control of the tip-to-sample distance. In this geometry a magneto-optical contrast is observed for the first time between domains of opposite magnetization without using modulation techniques. When applying a static or alternating external magnetic field, the magneto-optical images provide the location of domain wall pinning points.

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...

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.

Kinetics of highly spin-polarized electron photoemission from an InGaAlAs strained layer by energy and spin-resolved measurements

High-resolution energy distribution curves and spin polarization versus energy distribution curves from an AlInGaAs layer, capped by a heavily doped thin GaAs quantum well layer has been measured. Polarization P of up to 83% in conjunction with quantum yield Y=0.5% at T=130 K has been obtained. These results are compared to polarization and quantum yield spectra at high excitation power. The narrow-band quantum well is shown to provide large effective negative electron affinity values with no harm to electron polarization. The studies in linear and nonlinear excitation regimes bring insight into the kinetics of photoemission and favor the photoemission model with elastic electron tunneling through the surface barrier.