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Terahertz vortex beams with different superposition of the orbital angular momentum $l={\pm}1$, ${\pm}2$, ${\pm}3$, and ${\pm}4$ and spin angular momentum ${\sigma}={\pm}1$ were used to study antiferromagnetic (AFM) resonances in TbFe$_3$(BO$_3$)$_4$ and Ni$_3$TeO$_6$ single crystals. In both materials we observed a strong vortex beam dichroism for the AFM resonances that are split in external magnetic field. The magnitude of the vortex dichroism is comparable to that for conventional circular dichroism due to ${\sigma}$. The selection rules at the AFM resonances are governed by the total angular momentum of the vortex beam: $j={\sigma}+l$. In particular, for $l={\pm}2$, ${\pm}3$, and ${\pm}4$ the sign of $l$ is shown to dominate over that for conventional circular polarization ${\sigma}$., Comment: 12 pages, 4 figures; Supplemental Material: 10 pages, 8 figures; Accepted for publication in Physical Review Letters

Optical spectroscopy, x-ray diffraction measurements, density functional theory (DFT), density functional theory + embedded dynamical mean-field theory ($\mathrm{DFT}+eDMFT$), and crystal-field calculations have been used to characterize structural and electronic properties of hexagonal ${M}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ $(M=\mathrm{Fe},\mathrm{Mn})$ polar magnets. Our experimental data are consistent with the room-temperature structure belonging to the space group $P{6}_{3}mc$ for both compounds. The experimental structural and electronic properties at room temperature are well reproduced within $\mathrm{DFT}+eDMFT$ method, thus establishing its predictive power in the paramagnetic phase. With decreasing temperature, both compounds undergo a magnetic phase transition, and we argue that this transition is concurrent with a structural phase transition (symmetry change from $P{6}_{3}mc$ to $P{6}_{3}$) in the Fe compound and an isostructural transition (no symmetry change from $P{6}_{3}mc$) in the Mn compound.

Circularly polarized light with spin angular momentum is one of the most valuable probes of magnetism. We demonstrate that light beams with orbital angular momentum (OAM), or vortex beams, can also couple to magnetism exhibiting dichroisms in a magnetized medium. Resonant optical absorption in a ferrimagnetic crystal depends strongly on both the handedness of the vortex and the direction of the beam propagation with respect to the sample magnetization. This effect exceeds the conventional dichroism for circularly polarized light. Our results demonstrate the high potential of the vortex beams with OAM as a new spectroscopic probe of magnetism in matter.

High-resolution far-infrared reflection and polarized ellipsometry, as well as Raman scattering temperature-dependent measurements are used to study spin-phonon and electron-phonon interactions in rare-earth (RE) iron borates with the R32 structure of a natural mineral huntite, namely, in RFe3(BO3)4 with R ¼Pr, Nd, and Sm. Pronounced peculiarities in the ω(T) dependences at the Neel temperature TN E32 K are observed for all the compounds studied and the origin of these peculiarities is discussed. A coupling between lattice phonons and crystal-field excitations of a RE ion manifests itself by a renormalization of frequencies and intensities of coupled modes. Modeling of the spectra has revealed the value of about 15 cm � 1 for the electron–phonon coupling constant in PrFe3(BO3)4.

Transmittance spectra of $\mathrm{TbFe}{\mathrm{O}}_{3}$ single crystals have been studied in the far-infrared spectral range $15\ensuremath{-}120\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ in the external magnetic fields up to 9 T and at low temperatures down to 1.5 K. The temperature and magnetic-field dependencies of the antiferromagnetic resonance (AFMR) modes have been measured below and above the magnetic ordering temperature of Tb moments ${T}_{N}^{\mathrm{Tb}}=3.3\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Both the quasiferromagnetic and quasiantiferromagnetic modes of AFMR demonstrate hardening at $Tl{T}_{N}^{\mathrm{Tb}}$. The quasiantiferromagnetic mode gains electric-dipole activity along the $c$ axis below ${T}_{N}^{\mathrm{Tb}}$ and thus behaves as a hybrid, i.e., both electric- and magnetic-dipole active, mode. In addition to AFMR modes, below ${T}_{N}^{\mathrm{Tb}}$ we observed the appearance of electromagnon excitation at about $27\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, which is electric-dipole active along the $c$ axis. The electromagnon is optically active only in a narrow temperature range, $2.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}lTl{T}_{N}^{\mathrm{Tb}}$ at $H=0$, and in a narrow range of magnetic fields 2.4--2.7 T applied along the $b$ axis at $T=1.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. We argue that this electromagnon appears in magnetic phases, which are compatible with a spontaneous electric polarization along the $b$ axis.

We report on a new effect caused by the electron-phonon coupling in a stoichiometric rare-earth antiferromagnetic crystal subjected to an external magnetic field, namely, the appearance of a nonzero gap in the spectrum of electronic excitations in an arbitrarily small field. The effect was registered in the low-temperature far-infrared (terahertz) reflection spectra of an easy-axis antiferromagnet PrFe_{3}(BO_{3})_{4} in magnetic fields B_{ext}∥c. Both paramagnetic and magnetically ordered phases (including a spin-flop one) were studied in magnetic fields up to 30 T, and two bifurcation points were observed. We show that the field behavior of the coupled modes can be successfully explained and modeled on the basis of the equation derived in the framework of the theory of coupled electron-phonon modes, with the same field-independent electron-phonon interaction constant |W|=14.8 cm^{-1}.

Rotatable retarders have been developed for applications in spectroscopic, full Mueller Matrix ellipsometry in the far-IR spectral range. Several materials, such as silicon, KRS-5, and a commercial polymer plastic (TOPAS) have been utilized to achieve a fully adjustable retardation between 0° and 90°. Experimental characteristics of the rotatable retarders that utilize three- and four-bounce designs are compared with calculations. We discuss the effect of light focusing on the performance of these rotatable retarders.

Highly relaxed GaN nanodots and submicron ridges have been selectively grown in the NSAG regime using MOVPE on lattice mismatched 6H–SiC and AlN substrates. 2D real space and 3D reciprocal space mapping was performed with a CCD detector using 10.4 keV synchrotron X-ray radiation at the 2-ID-D micro-diffraction beamline at Advanced Photon Source (APS). Calibration procedures have been developed to overcome the unique challenges of analyzing NSAG structures grown on highly mismatched substrates. We studied crystallographic planar bending on the submicron scale and found its correlation with strain relaxation in the NSAG ridges.

The structural properties of InGaN/GaN multiple quantum wells (MQW) were studied using synchrotron based high resolution x-ray diffraction (HRXRD). MQW structures were grown on the top and sidewall facets of triangular and trapezoidal shaped GaN ridges by metalorganic vapour phase epitaxy (MOVPE) in the regime of selective area growth (SAG). Period and strain variations as a function of oxide mask width were determined for both the sidewall and the top facet growth. Oxide mask widths ranged between 2 and 20 µm with openings between adjacent masks of 4 and 6 µm. Analysis of the x-ray diffraction curves revealed a sidewall / vertical growth rate ratio of ∼0.3 through a comparison of the top to sidewall facet MQW periods. Masks orientated along the 〈11-20〉 crystallographic direction showed stronger growth enhancement along with large global strain for MQW growth on the top (0001) plane. Interpreting our results within the framework of vapour phase diffusion revealed that inter-facet migration of group-III species needs to be taken into account. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

One of the central challenges of nanoscience is fabrication of nanoscale structures with well-controlled architectures using planar thin-film technology. Herein, we report that ordered nanocheckerboards in ZnMnGaO4 films were grown epitaxially on single-crystal MgO substrates by utilizing a solid-state method of the phase separation-induced self-assembly. The films consist of two types of chemically distinct and regularly spaced nanorods with mutually coherent interfaces, approximately 4 x 4 x 750 nm3 in size and perfectly aligned along the film growth direction. Surprisingly, a significant in-plane strain, more than 2%, from the substrate is globally maintained over the entire film thickness of about 820 nm. The strain energy from Jahn-Teller distortions and the film-substrate lattice mismatch induce the coherent three-dimensional (3D) self-assembled nanostructure, relieving the volume strain energy while suppressing the formation of dislocations.

Thin films of GaN were grown on template substrates of 4-μm-thick GaN layers on sapphire substrates by low-pressure metal organic vapour-phase epitaxy (LP-MOVPE) in a new-design reactor with the shape T. Wide range of growth temperature from 520 to 1100 °C was explored. At low temperature growth between 550 and 690 °C, dimethylhydrazine (DMHy) was used as source of atomic nitrogen while ammonia (NH 3 ) was used at high temperature growth (1000–1100 °C). At low temperature micro-Raman spectroscopy revealed a significant relaxation of the selection rules for the scattering by the optical phonons in the films grown at lower temperatures. Variation of the intensity ratio for E 2 H and E 1 phonon modes has been attributed to changes in the structural quality of the films grown at different temperatures. At high temperature, the quality of GaN layers were comparable to that of the substrate before growth.

Optical properties and lattice dynamics of hexagonal $2H\ensuremath{-}\mathrm{BaMn}{\mathrm{O}}_{3}$ single crystals are studied experimentally in a wide temperature range by means of rotating analyzer ellipsometry and Raman scattering. The magnitude of the direct electronic band gap is found to be ${E}_{g}=3.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. At room temperature the far-infrared (IR) ellipsometry spectra reveal six IR-active phonons; two of them are polarized along the $c$ axis and four are polarized within the a-b plane. Seven phonon modes are identified in the Raman scattering experiments. Group theoretical mode analysis and complementary density functional theory lattice dynamics calculations are consistent with the $2H\ensuremath{-}\mathrm{BaMn}{\mathrm{O}}_{3}$ structure belonging to the polar $P{6}_{3}mc$ space group at room temperature. All observed vibrational modes are assigned to specific eigenmodes of the lattice. The neutron diffraction measurements reveal a structural phase transition upon cooling below ${T}_{C}=130\ifmmode\pm\else\textpm\fi{}5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is accompanied by a lattice symmetry change from $P{6}_{3}mc$ to $P{6}_{3}cm$. Simultaneously, at temperatures below ${T}_{C}$ several additional IR- and Raman-active modes are detected in experimental spectra. This confirms the occurrence of a structural transition, which is possibly associated with the appearance of electrical polarization along the $c$ axis and a previously known tripling of the primitive cell volume at low temperatures.

A synchrotron microbeam high-angular resolution diffraction setup based on a phase zone plate and a perfect Si(004) analyzer crystal was introduced to generate an x-ray microbeam with a lateral size of 0.24 µm and an angular resolution of 2 arcsec. The microbeam high angular resolution x-ray diffraction was applied to study InGaAlAs-based multiple quantum well (MQW) ridge-waveguide arrays produced by metal–organic vapour-phase epitaxy in a selective area growth regime with a central waveguide width varying from 1.6 to 60 µm. The analysis of the period T and the strain S in MQW ridge structures determined from the high-resolution diffraction data is presented. It was found that the MQW period is uniform across the ridge

An unusual phase of carbon with the fcc crystal structure, the lattice constant of which is very close to that of diamond, was reported before in a number of publications. In all the published works, the existence of fcc carbon could not be unambiguously established, as only insignificant amounts of fcc carbon in mixture with other carbon modifications were obtained so far. The present work provides for the first time clear evidence for the existence of the carbon modification with the fcc crystal structure. Thin films consisting exceptionally of fcc carbon were obtained by various methods, namely by hot-filament CVD, plasma-assisted CVD and treatment of the diamond surface in hydrogen plasmas. Results of electron diffraction, Raman spectroscopy, AES, XPS, EELS and HRTEM of the films consisting of fcc carbon clearly indicate its unique crystal and electron structure. The micro-hardness of the films of fcc carbon was found to be equal to HV0.01 = 250 which is quite different from the hardness of all the known carbon modifications.

We present a new approach to determine the site incorporation of p-type impurities in the doped layers of microelectronic device structures based on X-ray standing wave (XSW) technique. Activation behavior of Zn in InP epitaxial layers grown by metal organic vapor phase epitaxy (MOVPE) on InP(1 0 0) substrates has been studied. The XSW experiments were performed at the A2 beamline of the Cornell high-energy synchrotron source (CHESS). Angular dependences of the Zn–K fluorescence intensity excited by the XSW field inside the InP layer for symmetrical (4 0 0) reflection have been measured along with the Indium L-edge and phosphorus K-edge fluorescence and X-ray reflectivity. Analysis of the XSW data based on the dynamical diffraction theory in layered crystal structures allowed us to determine the fractions of both Zn atoms incorporated into crystal lattice and interstitial Zn. In our example, a 1 lm thickMOCVD-grown InP layer with the nominal concentration of Zn atoms of (2.4 ± 0.2) · 10 18 cm � 3 has the fraction of the substitutional Zn of (65 ± 5)%. This result is in a good agreement with electrical activation of Zn measured with a combination of SIMS and CV profilometry. The accurate knowledge of the interstitial-to-substitutional ratio as a function of growth conditions is required to optimize electrical activation of Zn and to control its diffusion in the device structures. � 2004 Elsevier B.V. All rights reserved.

A new approach to conditioning x-ray microbeams for high angular resolution x-ray diffraction and scattering techniques is introduced. We combined focusing optics (one-bounce imaging capillary) and post-focusing collimating optics (miniature Si(004) channel-cut crystal) to generate an x-ray microbeam with a size of 10 µm and ultimate angular resolution of 14 µrad. The microbeam was used to analyse the strain in sub-micron thick InGaAsP epitaxial layers grown on an InP(100) substrate by the selective area growth technique in narrow openings between the oxide stripes. For the structures for which the diffraction peaks from the substrate and the film overlap, the x-ray standing wave technique was applied for precise measurements of the strain with a Δd/d resolution of better than 10−4.

Properties of selective-area-grown (SAG) ternary and quaternary layers of InGaAs and InGaAsP produced by metal organic vapor phase epitaxy (MOVPE) have been investigated by using a combination of micro-photoluminescence and synchrotron-based micro-beam X-ray fluorescence techniques. Significant variations of the group-III element composition with increase of the oxide mask width and with decrease of the gap between the masks were observed in both ternary and quaternary layers. In contrast, concentration of the group-V elements in quaternary layers does not show any significant change. Effects of the SAG growth on variations of the group-III element composition become more pronounced with increase of the growth pressure.

Infrared-active optical phonons were studied in olivine LiFePO4 oriented single crystals by means of both rotating analyzer and rotating compensator spectroscopic ellipsometry in the spectral range between 50 and 1400 cm−1. The eigenfrequencies, oscillator strengths, and broadenings of the phonon modes were determined from fits of the anisotropic harmonic oscillator model to the data. Optical phonons in a heterosite FePO4 crystal were measured from the delithiated ab-surface of the LiFePO4 crystal and compared with the phonon modes of the latter. Good agreement was found between experimental data and the results of solid-state hybrid density functional theory calculations for the phonon modes in both LiFePO4 and FePO4.

Far-infrared reflection and ellipsometry measurements on multiferroic ${\mathrm{PrFe}}_{3}({\mathrm{BO}}_{3}){}_{4}$ and ${\mathrm{SmFe}}_{3}({\mathrm{BO}}_{3}){}_{4}$ single crystals were used to investigate the interaction between lattice phonons and electronic excitations associated with $4f$ crystal-field transitions. A temperature-dependent interference between two types of excitations was observed in ${\mathrm{PrFe}}_{3}({\mathrm{BO}}_{3}){}_{4}$ in which the frequency of $4f$ crystal-field electronic excitation of ${\mathrm{Pr}}^{3+}$ falls into the TO-LO frequency interval of the optical phonon mode near 50 ${\mathrm{cm}}^{\ensuremath{-}1}$ (1.5 THz). Experimental data were explained on the basis of a theoretical model of coupled electron-phonon modes. The fitting procedure revealed the value 14.8 ${\mathrm{cm}}^{\ensuremath{-}1}$ for the electron-phonon coupling constant. This rather large value points to an essential role played by the electron-phonon interaction in the physics of multiferroics.

Optical properties of hexagonal multiferroic oxides RMnO3 ,w hereR = Ho, Er, Tm, Yb, and Lu, have been studied in the far-infrared spectral range between 100 and 2000 cm −1 and temperatures between 1.5 and 300 K by means of several experimental techniques: Mueller matrix spectroscopic ellipsometry, rotating analyzer ellipsometry, and optical transmission spectroscopy. Spectra of the optical phonons are described in terms of the temperature dependencies of their frequency, damping, and oscillator strength. For all studies, oxide materials’ clear signatures of the spin-phonon interaction have been found below the temperature of the antiferromagnetic phase transition TN due to magnetic ordering of Mn 3+ spins. A decrease of the ionic radius for R 3+ ions between Ho 3+ and Lu 3+ in the corresponding RMnO3 compounds resulted in systematic variation of the frequency for several optical phonons. A magnetic excitation at 190 cm −1 was observed at low temperatures below TN and interpreted as resulting from two-magnon absorption.

Spectroscopic ellipsometry study of Na3Ir3O8 suggests thatelectron-phonon interactions play a prominent role in its electronicstructure.

The high brightness, broad spectral coverage and pulsed characteristics of infrared synchrotron radiation enable time-resolved spectroscopy under throughput-limited optical systems, as can occur with the high-field magnet cryostat systems used to study electron dynamics and cyclotron resonance by far-infrared techniques. A natural extension for magnetospectroscopy is to sense circular dichroism, i.e. the difference in a material’s optical response for left and right circularly polarized light. A key component for spectroscopic circular dichroism is an achromatic 1 4 wave retarder functioning over the spectral range of interest. We report here the development of an in-line retarder using total internal reflection in high-resistivity silicon. We demonstrate its performance by distinguishing electronic excitations of differing handedness for GaAs in a magnetic field. This 1 4 wave retarder is expected to be useful for far-infrared spectroscopy of circular dichroism in many materials.

Electrochemical doping and in situ Raman scattering were used to study charge transfer in K- and Li-doped single wall carbon nanotubes (SWNT) as a function of alkali concentration. An 8 cm ˇ1 downshift was observed for the tangential phonon mode of SWNT doped to stoichiometries of KC24 and Li1:25C6. The shift in both systems is reversible upon de-doping despite an irreversible loss of crystallinity. These results indicate that the tangential mode shifts result from electron transfer from alkali dopants to the SWNT, and that these modes are only weakly aAected by long-range order within the ropes. ” 2001 Elsevier Science B.V. All rights reserved.

We have studied electric-field-induced Raman scattering in ${\mathrm{SrTiO}}_{3}$ thin films using an $\mathrm{indium}\ensuremath{-}\mathrm{tin}\ensuremath{-}\mathrm{oxide}/{\mathrm{SrTiO}}_{3}/{\mathrm{SrRuO}}_{3}$ structure grown by pulsed laser deposition. The soft mode polarized along the field becomes Raman active. Experimental data for electric-field-induced hardening of the soft modes and the tuning of the static dielectric constant are in agreement described by the Lyddane-Sachs-Teller formalism. The markedly different behavior of the soft modes in thin films from that in the bulk is explained by the existence of local polar regions.

Understanding the behaviour of the dielectric constant in ferroelectric thin films remains a challenging problem. These ferroelectric materials have high static dielectric constants, and so are important for their applications in high-storage-density capacitor structures such as dynamic random access memory (DRAM). But the dielectric constant tends to be significantly reduced in thin films, thereby limiting the potential benefit of ferroelectrics for memory devices. Extensive studies have shown that this phenomenon could be caused by a 'dead layer' of very low dielectric constant between the ferroeletric film and the electrode. And, although very few direct measurements are in fact available, it has been recognized that the lattice dynamical properties in the thin films should also play a key role in the reduction of the dielectric constant. Here we report far-infrared ellipsometry and low-frequency dielectric measurements in SrTiO3 thin films, which demonstrate that the Lyddane-Sachs-Teller relation between the optical-phonon eigenfrequencies and the dielectric constant is fully maintained, as is the case in the bulk material. This indicates that the dramatic reduction of the dielectric constant is a consequence of a profound change of the lattice dynamical properties, in particular of the reduced softening of its lowest optical-phonon mode. Our results therefore provide a better understanding of the fundamental limitations of the dielectric constant values in ferroelectric thin films.

Pulsed-laser-deposited SrTiO3 thin films are used as all example to illustrate the effects of strain. oxygen vacancies, and interfacial dead layer on the dielectric and lattice dynamical properties in ferroelectric thin films. We found that strain, both lattice mismatch-induced and due to local defects such as oxygen vacancies, dramatically influences the properties of the thin films. The cubic-to-tetragonal structural phase transition is greatly modified by the strain in the films. Raman scattering shows the symmetry-forbidden optical phonons, indicating the reduction of symmetry. The line shape of the polar phonon shows a Fano asymmetry, which we suggest to be due to the micro polar regions or other local polar structures around the oxygen vacancies. The temperature dependence of the low-frequency complex dielectric constant shows markedly distinct quantum fluctuation behaviors from those of the single crystals. The soft mode is considerably hardened in the thin films compared to the single cry...

We report on in situ studies of the vibrational properties of Si nanoparticles and ultrathin layers grown by dc magnetron sputtering in ultrahigh vacuum on amorphous MgO and Ag buffer layers. The average thickness of the Si layers ranged from monolayer coverage up to 200 A ˚ . Transmission electron microscopy has been used to determine size and shape of the Si nanoparticles. Changes in the phonon spectra of Si nanoparticles during the crystallization process have been studied by interference enhanced Raman scattering technique. Marked size-dependences in the phonon density of states and the relaxation of the k-vector conservation with decrease in size of the Si nanoparticles have been detected. The transition between crystallineand amorphous-like behavior takes place in the particles with an average number of Si atoms equal toO7 ^ 2U £ 10 2 : q 2000

Oxide thin films have been studied for frequency and phase agile electronics. The electric-field tuning of microwave devices employs ferroelectrics, while the Magnetic-field tuning uses ferrites. The critical material parameters for ferroelectric thin films are the tunability of the dielectric constant and the dielectric loss. This paper describes the current understanding of the fundamental mechanisms of these properties and the research efforts to improve them in ferroelectric thin films.

The energy and momentum relaxation of hot electrons in n-type GaAs/AlAs quantum wells is studied. Hot photoluminescence due to the recombination of hot electrons with holes bound on Si acceptors is observed in structures with a high level of doping with silicon. Using the method of magnetic depolarization of hot photoluminescence, the probability of scattering of hot electrons is found to decrease substantially with increasing temperature in the range 4–80 K. This effect is shown to be due to the ionization of donors. It is established that the probability of inelastic scattering by neutral donors is several times greater than the probability of quasielastic electron-electron scattering.

Birefringence measurements for in-plane propagation of light below the absorption edge in GaAs/AlAs multiple quantum wells (MQW{close_quote}s) are reported for different well/barrier widths. A remarkable drop in the low-frequency limit of the birefringence has been observed for MQW structures with small periods and ascribed to the presence of local fields. The temperature dependence of the birefringence is also studied and complementary results in InP quantum dot structures are also presented. The latter exhibit a strong resonant birefringence, which can be explained by the reduced dimensionality in the joint density of states for optical transitions in the dots. {copyright} {ital 1999} {ital The American Physical Society}

Transmission spectroscopy has been used to investigate the in-plane linear birefringence of GaAs/AlAs multiple quantum wells (MQWs) with symmetric wells/barriers from 20/20 to 70/70 A. Varying optical thicknesses of the samples have been used to measure a reference value of the birefringence. While the resonant part in the dispersion of the birefringence has been attributed to the contribution from the confined electronic gaps, a nonresonant background birefringence has been analyzed as a function of the MQW period and explained by the presence of local fields.

We have studied lattice dynamic properties of ${\mathrm{SrTiO}}_{3}$ thin films from 5 to 300 K using metal-oxide bilayer Raman scattering. First-order zone-center optical phonons, symmetry forbidden in single crystals, have been observed in the thin films, indicating strain-induced lowering of symmetry. The asymmetric line shape of the ${\mathrm{TO}}_{2}$ phonon is interpreted as evidence for micropolar regions in the thin films, likely due to oxygen vacancies. The optical phonon lines and the asymmetry persist up to room temperature.

International audience; We report epitaxial growth of GaN layers on z- and x-cut LiNbO3 substrates using MOVPE. GaN layers with the thick- ness of 450 nm were characterized using X-ray diffraction. For both, z- and x-cut orientations of LiNbO3 substrates, the GaN layers have c-axis orientation normal to the substrate plane and the in-plane lattice orientation of GaN layers coin- cides with the primary axes of LiNbO3 substrates. Although GaN layers exhibit almost complete strain relaxation, the re- sidual compressive strain determined with respect to a free- standing GaN is of the order of +0.37% and +0.2% for z- and x-cut substrates, respectively.

We report on Raman scattering in nanostructures with InP quantum dots in an ${\mathrm{In}}_{0.48}{\mathrm{Ga}}_{0.52}\mathrm{P}$ matrix embedded in an ${\mathrm{In}}_{0.48}{\mathrm{Al}}_{0.52}\mathrm{P}$ waveguide. At resonant excitation with the quantum dot excitons, broad Raman peaks corresponding to acoustic and optical vibrations were observed. Their polarization was studied for in-plane propagation of the exciting and scattered light in forward scattering geometry. In comparison with the conventional backscattering configuration, the Raman signals are drastically enhanced due to the increased scattering volume.

We report on spin-flip Raman scattering and on Raman scattering by acoustic phonons in CdS nanocrystals embedded in a glass matrix. We discuss both the spin-flip and the acoustic-phonon Raman spectra on the basis of the interaction of size-quantized electronic excitations with nonconfined acoustic vibrations. The polarization of spin-flip Raman scattering in the Faraday and Voigt configurations is analyzed in comparison with that for bulk CdS. The effective electron $g$ factor has been measured as a function of the quantum dot size.

We report resonant spin-flip Raman scattering measurements in CdTe/Cd${}_{1\ensuremath{-}x}$Mg${}_{x}$Te single quantum wells with $x=0.15, 0.26$, and $0.5$ as well as in Cd${}_{1\ensuremath{-}x}$Mg${}_{x}$Te alloys with $x=0, 0.15$, and $0.5$. Effective $g$ factors of electrons and heavy holes are measured as a function of the quantum well width $(18\ensuremath{-}100$ \AA{}) and the angle of the magnetic field with respect to the growth axis. The electron $g$ factors in quantum wells are between the values for bulk CdTe ${(g}^{e}=\ensuremath{-}1.644\ifmmode\pm\else\textpm\fi{}0.005)$ and Cd${}_{0.85}$Mg${}_{0.15}$Te ${(g}^{e}=\ensuremath{-}0.802\ifmmode\pm\else\textpm\fi{}0.005)$. We show that the change in the energy gap induced by the quantum confinement of the carriers provides the dominant contribution to the electron $g$ factor in quantum wells. A large $g$ factor anisotropy for conduction electrons is observed. It is shown that this anisotropy depends on the splitting between light- and heavy-hole states. The experimentally determined $g$ factors are in good agreement with theoretical predictions.

Symposium W on Current Trends in Optical and X-Ray Metrology of Advanced Materials for Nanoscale Devices III / Spring Meeting of the European-Materials-Research-Society (E-MRS), Strasbourg, FRANCE, MAY 14-18, 2012; International audience; Submicron beam synchrotron-based X-ray diffraction (XRD) techniques have been developed and used to accurately and nondestructively map chemical composition and material quality of selectively grown group nanowires. GaN, AlGaN, and InGaN multi-quantum-well nanowires have been selectively grown on lattice matched and mismatched substrates, and the challenges associated with obtaining and interpreting submicron beam XRD results are addressed and solved. Nanoscale cathodoluminescence is used to examine exciton behavior, and energy-dispersive X-ray spectroscopy is used to verify chemical composition. Scanning transmission electron microscopy is later used to paint a more complete picture. The advantages of submicron beam XRD over other techniques are discussed in the context of this challenging material system.

We report on efficient resonant spin-flip Raman scattering due to localized heavy-hole excitons in single submonolayer InAs insertions in a GaAs matrix. Exciton, heavy-hole and electron g factors are directly measured for samples with different average thicknesses of InAs. The large size uniformity of the InAs islands manifests itself in very narrow heavy- and light-hole exciton photoluminescence peaks which also allow us to measure exciton g factors by magneto-luminescence spectroscopy.

Far-infrared spectra of ferrimagnetic Dy3Fe5O12 single crystals have been studied in the spectral range between 12 and 700 cm(-1) and in a wide temperature range between 5 K and 300 K using transmission spectroscopy and rotating analyzer ellipsometry. In the temperature range below T-C = 16 K for the magnetic ordering of Dy3+ spins, a number of ligand field and Kaplan-Kittel (KK) exchange resonance modes have been found. Temperature dependencies of their frequencies allowed us to estimate the ratio between the Fe-Dy and Dy-Dy exchange constants. We found that variation of the oscillator strength of the KK modes correlates with the magnetic-field-induced changes of magnetic permeability. The low-temperature peak in the static value of the dielectric function at T-C = 16 K may be related to the observed temperature-induced variation of the oscillator strength of the optical phonons.

Far-infrared optical transmission spectra of the antiferromagnetic resonances, or magnons, and crystal-field (CF) transitions have been studied in hexagonal $R$MnO${}_{3}$ ($R$ $=$ Er, Tm, Yb, Lu) single crystals. The magnon and CF frequencies, their oscillator strengths, and effective $g$ factors have been measured using external magnetic fields up to 9 T in the temperature range between 1.5 and 100 K. The magnon frequency increases systematically with a decrease of the $R$ ion radius. The magnetic ordering of $R$ ions ($R$ $=$ Er, Tm, Yb) was observed at low temperatures $Tl3.5$ K and in strong external magnetic fields. The observed effects are analyzed taking into account the main magnetic interactions in the system including exchange of the Mn${}^{3+}$ spins with ${R}^{3+}$ paramagnetic moments.

The dependence of the intensity of resonant Raman scattering by interface (IF) phonons in GaAs/AlAs superlattices on magnetic field, temperature, and pumping power have been investigated. IF phonon spectra, forbidden according to the momentum conservation condition, were observed in a magnetic field. Oscillations of the IF phonon intensity vs. magnetic field, corresponding to the resonance of the exciting light energy with the levels of magneto-excitons which are the intermediate states in the scattering process, have been observed. The IF phonon intensity decreases with increasing temperature. We consider such behaviour to be connected with the temperature-induced delocalization of magneto-excitons bound near static defects which are responsible for the relaxation of the momentum conservation condition. The delocalization energy is observed to increase with magnetic field.

In this letter, we have developed a tandem electroabsorption modulator with an integrated semiconductor optical amplifier that is capable of both nonreturn-to-zero and return-to-zero (RZ) data transmission at 40 Gb/s. The tandem modulator consists of a broad-band data encoder and a narrow-band pulse carver. The pulse carver is able to produce 5-ps pulses with more than 20 dB of extinction. The on-chip semiconductor optical amplifier provides up to 8.5 dB of fiber-to-fiber gain and enables the modulator to be operated with zero insertion loss. Devices have been realized with greater than 40-GHz bandwidth, and 13-dB dynamic extinction for a 2.5-V swing. For optimized designs bandwidths of nearly 60 GHz: have been realized. Using these devices penalty free RZ data transmission over a 100-kin dispersion compensated fiber link has been demonstrated with a received power sensitivity of -29 dBm.

We investigated the carrier responses of [(LaVO${}_{3}$)${}_{6n}$(SrVO${}_{3}$)${}_{n}$] (n = 1, 2, and 4) superlattices using spectroscopic ellipsometry. All the superlattice samples showed an unexpectedly large spectral response in the midinfrared region, which indicated geometrically confined doping near the LaVO${}_{3}$ and SrVO${}_{3}$ interfaces. The intensity of the midinfrared absorption systematically decreased as the superlattice periodicity n increased while the ratio of LaVO${}_{3}$ and SrVO${}_{3}$ layers was kept constant. In addition, we observed that the far-infrared spectral weight of the n = 1 superlattice was reduced by up to 30% with decreasing temperature, while the spectral weights of the n = 2 and n = 4 superlattices were almost temperature independent. This behavior was attributed to the carrier localization effects near the interfaces between LaVO${}_{3}$ and SrVO${}_{3}$.

Neutron diffraction is used to show that small (∼7 MPa, or 70 bar) uniaxial pressure produces significant changes in the populations of magnetic domains in a single crystal of 2% Nd-doped bismuth ferrite. The magnetic easy plane of the domains converted by the pressure is rotated 60° relative to its original position. These results demonstrate extreme sensitivity of the magnetic properties of multiferroic bismuth ferrite to tiny (less than 10(-4)) elastic strain, as well as weakness of the forces pinning the domain walls between the cycloidal magnetic domains in this material.

Far-infrared spectra of magnetodielectric Dy3Fe5O12 garnet were studied using a combination of transmittance, reflectivity, and rotating analyzer ellipsometry. In addition to purely dielectric and magnetic modes, we observed several hybrid modes with a mixed magnetic and electric dipole activity. Using 4 x 4 matrix formalism for materials with mu(omega) 1, we modeled the experimental optical spectra and determined the far-infrared dielectric and magnetic permeability functions. The matching condition mu(omega_h)Se = epsilon(omega_h)Sm for the oscillator strengths Se/m explains the observed vanishing of certain hybrid modes at omega_h in reflectivity.

Forward models for the Mueller Matrix (MM) components of materials with relative magnetic permeability tensor {\mu} \neq 1 are studied. 4x4 matrix formalism is employed to produce general solutions for the complex reflection coefficients and MMs of dielectric-magnetic materials having arbitrary crystal symmetry and arbitrary laboratory orientation. For certain orientations of materials with simultaneously diagonalizable {\epsilon} and {\mu} tensors (with coincident principal axes), analytic solutions to the Berreman equation are available. For the single layer thin film configuration of these materials, analytic formulas for the complex reflection and transmission coefficients are derived for orthorhombic or higher crystal symmetry. The separation of the magnetic and dielectric contributions to the optical properties of a material are demonstrated using measurements of the MM at varying angles of incidence., Comment: presented at ICSE-V, May 2010, submitted to Thin Solid Films presented at the International Conference on Spectroscopic Ellipsometry (ICSE-V, May 2010), submitted to Thin Solid Films