Etienne Goovaerts

Electrical spin injection in a ferromagnet/tunnel barrier/semiconductor heterostructure

We demonstrate experimentally the electrical electron spin injection from a ferromagnetic metal/tunnel barrier contact into a III–V semiconductor light emitting diode (LED). The injected electrons have in-plane spin orientation. By applying a relatively small oblique external magnetic field this spin orientation within the semiconductor can be manipulated to have a nonzero out-of-plane component. By measuring the resulting circular polarization of the emitted light, we observe injected spin polarization in excess of 9% at 80 K in a CoFe/AlOx/(Al,Ga)As/GaAs surface-emitting spin-LED.

Nitric oxide binding properties of neuroglobin: A characterization by EPR and flash photolysis

Neuroglobin is a recently discovered member of the globin superfamily. Combined electron paramagnetic resonance and optical measurements show that, in Escherichia colicell cultures with low O2 concentration overexpressing wild-type mouse recombinant neuroglobin, the heme protein is mainly in a hexacoordinated deoxy ferrous form (F8His-Fe2+-E7His), whereby for a small fraction of the protein the endogenous protein ligand is replaced by NO. Analogous studies for mutated neuroglobin (mutation of E7-His to Leu, Val, or Gln) reveal the predominant presence of the nitrosyl ferrous form. After sonication of the cells wild-type neuroglobin oxidizes rapidly to the hexacoordinated ferric form, whereas NO ligation initially protects the mutants from oxidation. Flash photolysis studies of wild-type neuroglobin and its E7 mutants show high recombination rates (k on) and low dissociation rates (k off) for NO, indicating a high intrinsic affinity for this ligand similar to that of other hemoglobins. Since the rate-limiting step in ligand combination with the deoxy-hexacoordinated wild-type form involves the dissociation of the protein ligand, NO binding is slower than for the related mutants. Structural and kinetic characteristics of neuroglobin and its mutants are analyzed. NO production in rapidly growing E. coli cell cultures is discussed.

Hyper-Rayleigh scattering study of η5-monocyclopentadienyl–metal complexes for second order non-linear optical materials

A series of ionic η5-monocyclopentadienyl–metal compounds possessing p-substituted benzonitrile ligands has been studied by hyper-Rayleigh scattering at the fundamental wavelength of 1.064 µm. Upon systematic variation of the metal ion, the first hyperpolarizability β was found to increase along the sequence Co, Ni, Ru, Fe, with about a three-fold increase from Ru to Fe. This yields very high values for the iron complexes, e.g., β=410×10–30 esu for [Fe(η5-C5H5)(dppe)( p-NCC6H4NO2)]+ [PF6 ]– dissolved in methanol. The high β values are attributed to π back-donation resulting in an extension of the conjugated π-system from the FeII organometallic fragment, acting as a good donor group, via the nitrile to the acceptor group NO2 . Complexes with single phenyl rings as conjugated chains perform better than their biphenyl analogues, which is explained in terms of non-planarity of the coordinated biphenyl ligands in solution. By comparing complexes with electron donor and acceptor substituted ligands it is demonstrated that the organometallic moiety can be used as an extremely effective donor group but not as a good acceptor group.

First hyperpolarizability dispersion of the octupolar molecule crystal violet: multiple resonances and vibrational and solvation effects.

The first hyperpolarizability (β) dispersion curve is measured for the first time for an octupolar nonlinear optical (NLO) molecule (crystal violet, CV) and modeled theoretically, yielding an in-depth understanding of the electronic structure and vibronic and solvation effects on such octupolar conjugated systems. Tunable wavelength hyper-Rayleigh scattering (HRS) measurements were performed on this prototypical octupolar molecule in the broad fundamental wavelength range of 620-1580 nm, showing significant shortcomings of the commonly used β dispersion models. Three well-separated β resonances involving the lowest-energy state and several higher excited states are clearly observed, including a significant contribution from a nominally one-photon forbidden transition. The experimental results for second-harmonic wavelengths above 330 nm are successfully modeled by means of a vibronically coupled essential-state description for octupolar chromophores, developed by Terenziani et al. (J. Phys. Chem. B 2008, 112, 5079), which takes into account polar solvation effects. The relative intensities of the various resonances, including the one below 330 nm, are also quantified by quantum chemical calculations. Furthermore, interesting effects of inhomogeneous broadening due to polar solvation of the two-dimensional chromophore are recognized in both linear and nonlinear spectra, allowing us to quantitatively address the long-standing problem of the band shape of the linear absorption spectrum of CV. This clearly demonstrates that extensive wavelength-dependent HRS measurements, as presented in this work, are essential to the characterization and design of NLO materials and represent a powerful tool to gain valuable information on molecular excitations and environmental effects in general.

Highly sensitive setup for tunable wavelength hyper-Rayleigh scattering with parallel detection and calibration data for various solvents.

A very sensitive experimental setup for accurate wavelength-dependent hyper-Rayleigh scattering (HRS) measurements of the molecular first hyperpolarizability beta in the broad fundamental wavelength range of 600 to 1800 nm is presented. The setup makes use of a stable continuously tunable picosecond optical parametric amplifier with kilohertz repetition rate. To correct for multi-photon fluorescence, a small spectral range around the second harmonic wavelength is detected in parallel using a spectrograph coupled to an intensified charge-coupled device. Reliable calibration against the pure solvent is possible over the full accessible spectral range. An extensive set of wavelength-dependent HRS calibration data for a wide range of solvents is presented, and very accurate measurements of the beta dispersion of the well-known nonlinear optical chromophore Disperse Red 1 are demonstrated.

Electroluminescence from bipolar resonant tunneling diodes

The incorporation of an AlAs/GaAs resonant tunneling structure inside a GaAs p‐n junction leads to strong quantum‐well electroluminescence originating from electron and hole tunneling into the resonant tunneling structure. A large peak‐to‐valley ratio of 10:1 in the electroluminescence intensity is achieved at the electron resonance at 4.2 K, which decreases but persists (1.45:1) at room temperature. Resonant tunneling of holes is also apparent from the electroluminescence at low temperature.

Highly efficient room temperature spin injection in a metal-insulator-semiconductor light-emitting diode

We demonstrate highly efficient spin injection at low and room temperature in an AlGaAs/GaAs semiconductor heterostructure from a CoFe/AlOX tunnel spin injector. We use a double-step oxide deposition for the fabrication of a pinhole-free AlOX tunnel barrier. The measurements of the circular polarization of the electroluminescence in the Oblique Hanle Effect geometry reveal injected spin polarizations of at least 24% at 80 K and 12% at room temperature.

High first hyperpolarizability and perfectly aligned crystal packing for an organometallic compound [Fe(η5-C5H5)((R)–PROPHOS)(p-NCC6H4NO2)][PF6]·CH2Cl2

Abstract The molecular first hyperpolarizability, β, and the X-ray crystal structure of the complex [Fe(η5-C5H5)((R)–PROPHOS)(p-NCC6H4NO2)][PF6] ((R)–PROPHOS=(R)–(+)-bis-(1,2-diphenylphosphino)propane) were determined. A near-resonant enhanced β value as high as 545×10−30 esu was obtained from hyper-Rayleigh scattering measurements. In addition, the compound crystallizes in the space group P1 with a perfect alignment of the dipolar molecules (only a single chromophore per unit cell), thus maximizing the macroscopic nonlinearity for electro-optic or parametric frequency conversion applications. The molecular packing in the crystal is analyzed in order to understand the factors leading to the perfect alignment.

EPR-spectroscopic evidence of a dominant His-FeIII-His coordination in ferric neuroglobin

Abstract The ferric form of the wild-type mouse neuroglobin (Ngb), a newly discovered heme protein which is primarily expressed in the brain of mammals, has been characterized by electron paramagnetic resonance (EPR) spectroscopy. The study reveals the simultaneous presence of two related structural forms in a wide range of pH values. The dominant low-spin form (>90%) with g -tensor principal values 3.15, 2.16 and 1.34 can be attributed to a His–Fe III –His configuration. The high-spin form with g ⊥ =5.97 and g ∥ ∼2, can be ascribed either to a hexacoordinated His–Fe III –H 2 O form or to a pentacoordinated His–Fe III . The high-spin to low-spin ratio is found to decrease with increasing pH values.

Design and characterization of organic and organometallic molecules for second order nonlinear optics

Publisher Summary This chapter focuses on how to design, characterize, and optimize the first hyperpolarizability β of organic and organometallic molecules for nonlinear optics (NLO) applications. The chapter also presents NLO effects and examines the relation between molecular and macroscopic NLO properties. The theoretical methods which have been employed to calculate these properties are discussed. The chapter presents a description of methods for the experimental determination of molecular second-order (SO) NLO parameters with emphasis on the recently widely used hyper-Rayleigh scattering (HRS) technique. The different types of compounds which have been synthesized and characterized for SO NLO are discussed for organic and organometallic molecules, respectively, up to the recent developments in this field. A number of criteria which these compounds have to meet for applications and some materials aspects are also considered in the chapter.