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We have performed c.w. and time-resolved photoluminescence measurements on a 30/30 Cd 0.82 Mn 0.18 Te/CdTe superlattice containing two enlarged wells with different widths. Excitation above the alloy band gap allows to study the Bloch transport along the growth direction of the superlattice. Both ambipolar transport and hopping of carriers are observed in the 4100 K temperature range. At low temperatures (

In this work anisotropic porous silicon is investigated as a material for optical sensing. Birefringence and sensitivity of the anisotropic porous silicon membranes are thoroughly studied in the framework of Bruggeman model which is extended to incorporate the influence of environment effects, such as silicon oxidation. The membranes were also characterized optically demonstrating sensitivity as high as 1245 nm/RIU at 1500 nm. This experimental value only agrees with the theory when it takes into consideration the effect of silicon oxidation. Furthermore we demonstrate that oxidized porous silicon membranes have optical parameters with long term stability. Finally, we developed a new model to determine the contribution of the main depolarization sources to the overall depolarization process, and how it influences the measured spectra and the resolution of birefringence measurements.

During the last century, the spatial resolution of the optical microscope reached its theoretical limit which is given by diffraction of light and hence close to its wavelength (Abbe, 1882). As a result, several techniques like electron microscopy, tunnel effect or Scanning Probe Microscopy (SPM), were proposed as alternatives to improve spatial resolution. Despite several optical techniques are beating the diffraction limit (Hell & Stelzer, 1992; Harke et al., 2008) in general optical microscopy resolution is considerably limited at the fast growing field of Nanotechnology. In fact, the optimization of electrooptical devices sometimes requires the study of material properties below the nanometre scale. For that purpose, the resolution of the Transmission Electron Microscope (TEM) is the highest one being able to observe the atomic structure (and composition) of a vast number of compounds. The inconveniences of this powerful technique are related with the preparation of the material under study since the sample thickness must be reduced to 100 nm. Alternatively, the Scanning Electron Microscope (SEM) is often used for structural characterization when atomic resolution is not mandatory. The SEM is a valuable tool for imaging metallic or semiconductor samples, but it results less useful for characterizing dielectric materials. For this reason, Atomic Force Microscopy (AFM) could be considered the best option for studying ferroelectric surfaces. Other than morphology, the measurement of physical magnitudes can be also performed by means of the AFM (Asenjo et al., 2006; Cefali et al., 2003; Kwak et al., 2000), which leads to another important advantage with respect to electron microscopes. In fact, most of the experiments previously performed in different fields of Material Science have been revisited at the nanometer scale by means of advanced SPM techniques. Given the great variety of them, in this chapter we focus our attention in the Near-field Optical Scanning Microscope (NSOM), because of during last years it has been revealed as a powerful technique for studying ferroelectric domains and domain walls in a non-invasive way, (Eng & Gutherodt, 2000; Lamela et al., 2007, 2009; Lifante et al., 2008). The AFM can be considered the simplest but also the most versatile scanning probe instrument, since most of the SPMs are developed on the basis of an AFM, as described in Section 2. The NSOM is one of these rather recent techniques, which allows the microscope user to study optical features and correlate with the topography that is being registered

[EN] This Letter presents practical realization of a two-dimensional low loss acoustic cloak for airborne sound obtained by inverse design. The cloak consists of 120 aluminum cylinders of 15 mm diameter surrounding the cloaked object¿a cylinder of diameter 22.5 cm. The position of each cylinder in the cloak is optimized using the data from two different techniques: genetic algorithm and simulated annealing. The operation frequency of this cloak is 3061 Hz with the bandwidth of about 100 Hz. Being a multi-step approach to the desired cloaking, the inverse design is also valid, in principle, for non-symmetric cylinders and even for three-dimensional objects. VC 2011 American Institute of Physics, Work supported by ONR under Award N000140910554 and the Spanish MCINN under Contract Nos. TEC2010-19751, TEC2008-06756-C03-03, and CSD2008-00066 (CONSOLIDER Program). L. S. thanks the fellowship provided by CSIC with number JAEDoc-08-00351.

4 figuras, 4 páginas.-- PACS number(s): 78.67.Hc, 73.21.La, 78.55.Cr, We investigate the electronic and optical properties of InAs double quantum dots grown on GaAs (001) and laterally aligned along the [110] crystal direction. The emission spectrum has been investigated as a function of a lateral electric field applied along the quantum dot pair mutual axis. The number of confined electrons can be controlled with the external bias, leading to sharp energy shifts which we use to identify the emission from neutral and charged exciton complexes. Quantum tunneling of these electrons is proposed to explain the reversed ordering of the trion emission lines as compared to that of excitons in our system., The authors wish to acknowledge to Spanish MICINN through projects Consolider-Ingenio 2010 QOIT (CSD2006-0019), TEC2008-06756-C03-01/03, and CTQ2008-03344, and to Comunidad Autónoma de Madrid through project Q&CLight (S2009ESP-1503).

The enhancement of the electromagnetic field in the surroundings of nanoparticles via surface plasmon resonance offers promising possibilities for biomedical applications. Here we report on the selective triggering of antibacterial activity using a new type of silver nanoparticles coated with silica, Ag@silica, irradiated at their surface plasmon frequency. The nanoparticles are able to bind readily to the surface of bacterial cells, although this does not affect bacterial growing since the silica shell largely attenuates the intrinsic toxicity of silver. However, upon simultaneous exposure to light corresponding to the absorption band of the nanoparticles, bacterial death is triggered selectively on the irradiated zone. Because of the low power density used in the treatments, we discard thermal effects as the cause of cell killing. Instead, we propose that the switched toxicity is due to the enhanced electromagnetic field in the proximity of the nanoparticles, which either directly (through membrane perturbation) or indirectly (through induced photochemical reactions) is able to cause cell death.

In this work we show two different procedures of fabrication aiming towards the systematic positioning of single InAs quantum dots (QDs) coupled to a GaAs photonic crystal (PC) microcavity. The two approaches are based on the molecular beam epitaxial (MBE) growth of site-controlled QDs (SCQDs) on pre-patterned structures. The PC microcavity (PCM) is introduced previous or after the growth, on each case. We demonstrate the InAs SCQD nucleation on pre-patterned PCMs and a method to perform the QD nucleation respect to an etched ruler that is used to position the PC structure after growth. For both types of structures, we have carried out microphotoluminescence (µPL) spectroscopy experiments at 80 K and 4 K.

We have studied the temperature dependence of the photoluminescence of a single layer of InAs/InP(001) self-assembledquantum wires emitting at 1.5 μm. The non-radiative mechanisms responsible for the quenching of the emission band have been identified. The exciton dynamics has been investigated using time resolved photoluminescence measurements. The results have been explained through the interplay between free excitons and localized states (arising from size fluctuations in the quantum wires)., We want to acknowledge financial support from the Spanish MICINN through Grants: TEC 2005-05781-C03-03, S-0505-TIC-0191, TEC2008-06756-C03, Consolider-Ingenio 2010 QOIT (CSD2006-0019), and CAM S2009ESP-1503. The main author, J.C.-F., thanks also the Spanish MCI for his FPI grant BES-2006-12300.

We have performed cw and time-resolved photoluminescence measurements on a 30/30 Cd 0.82 Mn 0.18 Te/CdTe superlattice containing two enlarged weaks with different widths, namely, 60 and 200 K. The distance between the two enlarged wells is 2400 K and the wider one is located 0.96 film away from the sample surface. Excitation above the alloy band gap allows us to study the Bloch transport along the growth direction of the superlattice. Both ambipolar transport and hopping of carriers are observed. Mobilities of the order of 4×10 4 cm 2 /V s and 3×10 2 cm 2 /V s are estimated for the two processes, respectively, at low temperatures (

In this work we demonstrate the existence of above-barrier quasibound states in ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As/AlAs/GaAs double-barrier quantum-well structures, by using both continuous-wave and time-resolved photoluminescence techniques in addition to reflectivity measurements. A series of clearly resolved resonances is found in the optical spectra. Their energy positions agree fairly well with the energies found for the quasibound states when analyzing the density of states of the heterostructure continuous spectrum. As shown by the analysis of the envelope wave function these resonances are localized in the ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As barrier regions and are found to induce both an increase of the interband absorption and a reduction of the carrier capture efficiency into the wells.

A systematic study of the electron transport and shallow impurity distribution in indium selenide above room temperature or after an annealing process is reported by means of far‐infrared‐absorption and Hall‐effect measurements. Evidences are found for the existence of a large concentration of deep levels (1012–1013 cm−2), related to impurities adsorbed to stacking faults in this material. Above room temperature impurities can migrate from those defect zones and then become shallow in the bulk. The subsequent large increase of 3D electrons can change the dimensionality of the electron transport, which in most cases was 2D. The temperature dependence of the resistivity parallel to the c axis can be explained by the observed increase of the 3D electron concentration, whose motion across the layers is limited by stacking‐fault‐related potential barriers. The observed macroscopic resistivity is thus determined by tunneling through those barriers. Juan.Mtnez.Pastor@uv.es

A photoluminescence-excitation (PLE) study of the exciton binding energy in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well (QW) structures is reported. A line-shape fit of the PLE spectra, based on a correct evaluation of the absorption probability in QW's, is proposed as a powerful tool in order to deduce accurately the exciton binding energies even in samples where the 2s peak is unresolved. The experimental results obtained are in good agreement with recent accurate theories. In particular, we find a strong dependence of the heavy-hole exciton binding energy ${\mathit{E}}_{1}$ on the aluminum concentration of the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As barrier, in agreement with the predicted importance of the dielectric mismatch and conduction-band nonparabolicity in enhancing ${\mathit{E}}_{1}$. Finally, evidence of exciton binding energies larger than the two-dimensional limit in GaAs is found even in relatively thick QW's (50 \AA{}) with AlAs barriers.

We report on continuous-wave and time-resolved measurements of the photoluminescence of good-quality single quantum wells at low temperatures (4-30 K). Luminescence arising from both free and localized excitons is observed and the influence of excitation localization on the photoluminescence decay time is investigated. Resonant pumping at the light-hole exciton transition is found to greatly increase the generation of free heavy-hole excitons. In this case, the free-exciton lifetime increases linearly with temperature (10-30 K) and the dependence of the slope on the well thickness is found to be in good quantitative agreement with the theoretical model of Andreani et al.

Trabajo presentado a la "11th International Conference on Optics of Excitons in Confined Systems" (OECS), celebrada en en Madrid (España) del 7 al 11 de Septiembre de 2009., We have studied the lateral coupling between InAs/GaAs quantum dot pairs embedded in a field-effect structure. Quantum dot pairs and molecules have been identified by the correlated evolution of the Coulomb blockade features of each QD in the pair. This behaviour is largely distorted in the presence of resonant coupling states in the QD molecule. Single QD voltage evolution shows a crossover in the lineshape profile, which is associated to Spectral Diffusion processes due to residual charged environment., The authors gratefully acknowledge financial support by Spanish Ministry of Science and Innovation through grants NANINPHO-QD (TEC-2008-06756-C03-01/03) and Consolider-Ingenio 2010 QOIT(CSD2006-0019) and CAM (S-505/ESP/000200).

Nonradiative recombination processes in thin GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well (QW) and double-barrier quantum-well structures have been investigated by means of continuous-wave and time-resolved photoluminescence (PL) measurements. We find that, due to the temperature dependence of the radiative time constant, the Arrhenius plot of the PL intensity cannot be used for extracting the activation energy of the nonradiative channels. In fact it is the temperature dependence of the PL decay time ${\mathit{T}}_{\mathit{L}}$ which directly gives information on the thermal activation of the loss mechanism. The activation energies obtained from the Arrhenius plot of ${\mathit{T}}_{\mathit{L}}$ demonstrate that, at least in the case of thin wells, the main nonradiative mechanism is the thermal escape of the less-confined species of carriers.

Nonconventional GaAs/${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As quantum-well structures, with one or two monolayers of AlAs inserted between the GaAs layers and the ${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As barriers (double-barrier quantum wells), have been investigated to observe the quasi-three-dimensional behavior of excitons in thin quantum wells. These samples provide higher confinement energies than single quantum wells of the same thickness, in such a way that relatively thick wells produce shallow carrier subbands. An increase of the photoluminescence lifetime of excitons has been found when narrowing the well thickness. The analysis of intensity and shape of the photoluminescence spectra made us confident in interpreting this increase as an effect of the exciton delocalization, and a qualitative agreement with a simple model for the exciton radiative lifetime is obtained.

5 páginas, 5 figuras., We present a fabrication method to produce site-controlled and regularly spaced InAs/GaAs quantum dots for applications in quantum optical information devices. The high selectivity of our epitaxial regrowth procedure can be used to allocate the quantum dots only in positions predefined by ex-situ local oxidation atomic force nanolithography. The quantum dots obtained following this fabrication process present a high optical quality which we have evaluated by microphotoluminescence and photon correlation experiments., The authors gratefully acknowledge financial support by Spanish MEC and CAM through grants NANOSELF2 (TEC-2005-05781-C03-01/03), NANINPHO-QD (TEC-2008- 06756-C03-01/03), NANOCOMIC (S-505/ESP/000200), and Consolider-Ingenio 2010 QOIT (CSD2006-0019).

4 páginas, 3 figuras, 2 tablas.-- MADICA 2006 Conference, Fifth Maghreb-Europe Meeting on Materials and their Applicatons for Devices and Physical, Chemical and Biological Sensors, In this paper we describe the results of temperature dependent photoluminescence intensity and decay time measurements of In(Ga)As/GaAs quantum rings where the depth of barrier is varied from sample to sample. The activation energy found for the reduction of the exciton decay time as a function of the temperature is approximately half the value of the thermionic escape energy of excitons. The temperature dependant behaviour is ascribed to the carriers lost via the excited state to the WL. The time resolved PL study indicates that thermal escape mechanisms is not so affected by reducing the spacer thickness, but it's influenced essentially by the excited state recombination., This work was partially supported by Spanish MCyT Nanoself I and II projects TIC2002-04096-C03 and TEC2005- 05781-C03-03, the SANDiE Network of excellence (Contract No. NMP4-CT-2004-500101) and the AECI Spain-Tunisia bilateral research action no. 2/04/R.

http://link.aip.org/link/?JAPIAU/103/056108/1, We demonstrated the effect of reactive ion beam etching (RIBE) process on the PL properties of CdTe/sapphire metal organic vapor phase epitaxy layers. At optimum conditions, the RIBE attack does not make significant morphological changes but it results in an increase of the concentration of acceptor impurities. This was revealed by an increase of the overall photoluminescence (PL) intensity and, simultaneously, a decrease of the PL decay time, more important on the low energy side of PL spectrum due to the recombination of carriers in acceptor pairs., This work was supported by the Spanish projects TEC-2005-05781-C03-03 and CAM SENSORCDT S-0505/MAT/0209.

A near-field scanning optical microscope (NSOM) is used to perform structural and optical characterization of the surface layer after Zn diffusion in a channel waveguide fabricated on lithium niobate. A theoretical approach has been developed in order to extract refractive index contrast from NSOM optical transmission measurements (illumination configuration). As a result, different solid phases present on the sample surface can be identified, such as ZnO and ZnNb2O6. They appear like submicrometric crystallites aligned along the domain wall direction, whose origin can be ascribed to some strain relaxation mechanism during the annealing process after Zn diffusion. Jose.Canet-Ferrer@uv.es

Continuous wave- and time-resolved micro-photoluminescence spectroscopy has been performed on single InAs self-assembled quantum dots grown on GaAs. The presence of residual impurities (donors and acceptors) in samples with low dot density opens the possibility to switch from trion to neutral exciton states inside quantum dots by selective optical pumping. We propose a microstate model to describe the recombination dynamics of all the excitonic especies (neutral exciton, positive/negative trion and biexciton) under the considered optical pumping conditions when increasing the excitation power. © 2007 Elsevier B.V. All rights reserved.

4 páginas, 5 figuras.-- PACS numbers: 78.67.Lt, 71.30.+h, 71.35. -y.-- Comunicación presentada a la International Conference on the Physics of Semiconductors (ICPS) celebrada en Rio de Jqaneiro (Brasil/2008)., We study the metal-insulator transition in individual self-assembled quantum wires and report optical evidence of metallic liquid condensation at low temperatures. First, we observe that the temperature and power dependence of the single nanowire photoluminescence follow the evolution expected for an electron-hole liquid in one dimension. Second, we find novel spectral features that suggest that in this situation the expanding liquid condensate compresses the exciton gas in real space. Finally, we estimate the critical density and critical temperature of the phase transition diagram at nc~1×105 cm-1 and Tc~35 K, respectively., The authors acknowledge valuable discussions with Dr. Jose M. Llorens, Dr. Richard J. Warburton, and Dr. Khaled Karrai. Financial support by the Spanish MEC and CAM through projects Nos. TEC-2005-05781-C03-01/03, NAN2004-09109-C04-01/03, S-505/ESP/000200 and CSD2006-0019, and by the European Commission through SANDIE Network of Excellence (No. NMP4-CT-2004-500101) is also gratefully acknowledged.

We present a study of the structural and optical properties of InAs quantum dots formed in a low density template of nanoholes fabricated by droplet epitaxy on GaAs (001). The growth conditions used here promote the formation of isolated quantum dots only inside the templated nanoholes. Due to the good optical quality and low density of these nanostructures, their ensemble and individual emission properties could be investigated and related to the particular growth method employed and the quantum dot morphology., The authors gratefully acknowledge the financial support by the Spanish MEC and CAM through project Nos. TEC-2005-05781-C03-01/-03 and NAN2004-09109-C04-01/-03, by the Consolider-Ingenio 2010 CSD2006-0019 and S-505/ESP/000200, and by the European Commission through SANDIE Network of Excellence No. NMP4-CT-2004- 500101 . P.A.G. thanks the I3P program.

We present results on simultaneous oscillation in continuous-wave at 1048 and 1386 nm in Nd3+:LaBGeO5 crystal. LaBGeO5 is a nonlinear positive uniaxial crystal with potential application for intracavity generation of yellow laser radiation at 597 nm. Due to the different cross section in both laser channels, simultaneous oscillation is achieved by axial hole burning in a standing wave (linear cavity). It is concluded that a stable output laser spectrum is obtained for an absorbed pump power of 700 mW. A laser competition with alternating single-channel and dual-channel oscillation at different pump regimes was observed in the experiments.

3D-photonic crystals (opals) based on polystyrene (PS) colloidal nanospheres are being characterized by scanning near-field optical microscopy (SNOM). These crystals offer a huge potential for controlling both the spontaneous emission of embedded light sources and the propagation of light itself.

Summary Scanning near-field optical microscopy is capable of measuring the topography and optical signals at the same time. This fact makes this technique a valuable tool in the study of materials at nanometric scale and, in particular, of ferroelectric materials, as it permits the study of their domains structure without the need of chemical etching and, therefore, not damaging the surface (as will be demonstrated later). We have measured the scanning near-field optical microscopy transmission, as well as the topography, of an RbTiOPO4 single crystalline slab, which exhibits two different of macroscopic ferroelectric domains. A chemical selective etching has been performed to distinguish between them, obtaining areas with a noticeable roughness (C− domain) in comparison with the original flat aspect of the other ones (C+ domain). The effects of the selective chemical etching have been quantified in topographic images obtained by means of our fibre tip probe, and have been compared to topographic images obtained by Atomic Force Microscopy, with a better resolution. The near-field optical transmission images recorded have been obtained under different excitation wavelengths. These images are modulated by the light scattering due to the grains at the rough surface, which depends on the excitation wavelength used. In addition, they show a significant optical contrast due to the variations of the dielectric constant on the proximity of the ferroelectric domain wall.

We have carried out continuous wave and time resolved photoluminescence experiments in self-assembled In(Ga)As quantum dots and quantum rings embedded in field effect structure devices. In both kinds of nanostructures, we find a noticeable increase of the exciton radiative lifetime with the external voltage bias that must be attributed to the field-induced polarizability of the confined electron hole pair. The interplay between the exciton radiative recombination and the electronic carrier tunneling in the presence of a stationary electric field is therefore investigated and compared with a numerical calculation based on the effective mass approximation., The authors gratefully acknowledge financial support by the Spanish MEC and CAM through Projects Nos.TEC-2005-05781-C03-01/03, NAN2004-09109-C04-01/03,CSD2006-0019, and S-505/ESP/000200, and by the European Commission through the SANDIE Network of Excellence (Contract No. NMP4-CT-2004-500101).

Photoluminescence and excitation of photoluminescence spectroscopy have been performed for two kinds of single InAs self-assembled quantum dots grown on GaAs. The presence of unintentional impurities (donors and acceptors) offers the possibility to switch from negative to positively charged excitons by selectively exciting impurity related optical transitions.

In this work Au nanoparticles (AuNPs) are incorporated into poly(methyl methacrylate) (PMMA) waveguides to develop optical couplers that are compatible with planar organic polymer photonics. A method for growing AuNPs (of 10 to 100 nm in size) inside the commercially available Novolak resist is proposed with the intention of tuning the plasmon resonance and the absorption/scattering efficiencies inside the patterned structures. The refractive index of the MNP-Novolak nanocomposite (MNPs: noble metal nanoparticles) is carefully analysed both experimentally and numerically in order to find the appropriate fabrication conditions (filling factor and growth time) to optimize the scattering cross section at a desired wavelength. Then the nanocomposite is patterned inside a PMMA waveguide to exploit its scattering properties to couple and guide a normal incident laser light beam along the polymer. In this way, light coupling is experimentally demonstrated in a broad wavelength range (404-780 nm). Due to the elliptical shape of the MNPs the nanocomposite demonstrates a birefringence, which enhances the coupling to the TE mode up to efficiencies of around 1%.

We present the mapping of the plasmonic properties of gold nanoparticles that are embedded in a TiO2 thin film deposited over two different substrates, glass and silicon. An improved electron energy-loss spectroscopy (EELS) imaging technique was used to extract plasmon maps with nanometre resolution. Several representative cases of randomly dispersed NPs have been examined to carefully evaluate surrounding effects on the optical response of such nanostructured material. Data were compared to analytical calculations and showed good agreement. These results validate previous structural and far-field optical results and provide a clear description of the optical phenomena that take place at a nanometre scale in these materials. They are of primary importance for enlightening the way to the fabrication of thin film materials including metallic nanostructures for photovoltaic applications.

Nanocomposites based on plasmonic nanoparticles and metal-oxide semiconductors are emerging as promising materials for conversion of solar energy into chemical energy. In this work, a Au–ZnO nanocomposite film with notably enhanced photocatalytic activity is successfully prepared by a single-step process. Both ZnO and Au nanoparticles are synthesized in situ during baking of the film spin-coated from a solution of Zn(CH3COO)2 and HAuCl4. Furthermore, it is shown that this precursor solution can be formulated as a nanoink for the generation of micropatterns by microplotter printing, opening the way for the miniaturization of devices with enhanced properties for photocatalysis, optoelectronics, and sensing. The study demonstrates that Au–ZnO films exhibit 4.5-fold enhanced photocatalytic properties for the decomposition of methyl orange upon sunlight exposure in comparison with ZnO films. Au nanoparticles improve significantly the photocatalytic activity of ZnO because they act as photosensitizers, absorbing photons at the localized surface plasmon resonance range (500–600 nm) and transferring electrons to the nearby ZnO semiconductor. XPS analysis of the Au–ZnO nanocomposite supports this explanation, indicating strong interactions between Au and ZnO.

We present results on the optical characterization of InxGa1−xP layers grown by atomic layer molecular beam epitaxy on GaAs (001) substrates at a growth temperature of 420 °C. Our results show that the optical characteristics of these layers, which do not show ordering effects, are strongly dependent on surface stoichiometry during growth. In this way, we can obtain either highly homogeneous alloys with a predictable band-gap energy or layers with optical properties indicative of spatial localization effects, like an anomalous behavior of photoluminescence peak energy with temperature and a large shift between the emission energy and absorption edge.

7 páginas, 5 figuras.-- PACS. 78.67.Hc Quantum dots – 73.21.La Quantum dots – 78.55.Cr III-V semiconductors., We present a study of the primary optical transitions and recombination dynamics in InGaAs self-assembled quantum nanostructures with different shape. Starting from the same quantum dot seeding layer, and depending on the overgrowth conditions, these new nanostructures can be tailored in shape and are characterized by heights lower than 2 nm and base lengths around 100 nm. The geometrical shape strongly influences the electronic and optical properties of these nanostructuctures. We measure for them ground state optical transitions in the range 1.25–1.35 eV and varying energy splitting between their excited states. The temperature dependence of the exciton recombination dynamics is reported focusing on the intermediate temperature regime (before thermal escape begins to be important). In this range, an important increase of the effective photoluminescence decay time is observed and attributed to the state filling and exciton thermalization between excited and ground states. A rate equation model is also developed reproducing quite well the observed exciton dynamics., This work has been supported by the Spanish Nanoself I and II projects TIC2002-04096-C03 and TEC2005-05781-C03-03, Generalitat Valenciana and SANDiE Network of excellence (Contract number NMP4-CT-2004-500101 group TEP-0120).

8 páginas, 8 figuras., In the present work we study the influence of stacking self-assembled InAs quantum wires (QWRs) on the emission wavelength and the excitonic recombination dynamics. The reduction in the InP spacer layer thickness, d(InP), produces both a size filtering effect towards large wire ensembles and an increase in the vertical coupling for electrons and holes along the stack direction. The different vertical coupling for electrons and holes induces a different behaviour in the exciton recombination dynamics, depending on the InP spacer layer thickness: weak electron coupling and negligible hole coupling for d(InP) > 10 nm, intermediate electron coupling and weak hole coupling for 5 nm ≤ d(InP) ≤ 10 nm and strong electron coupling and moderate hole coupling for d(InP) < 5 nm. Such exciton dynamics have been established by comparing the experimental time decay results with a multi-quantum well model accounting for the vertical carrier coupling., The authors gratefully acknowledge the financial support by the Spanish MEC and CAM through projects No TEC-2005- 05781-C03-01 and S-505/ESP/000200 and by the European Commission through SANDIE Network of Excellence (No NMP4-CT-2004-500101, group TEP-0120).

In this work we investigate the exciton recombination dynamics in InAs/ InP semiconductor self-assembled quantum wires, by means of continuous wave and time resolved photoluminescence. The continuous wave photoluminescence results seem to indicate that the temperature quenching of the emission band seems to be more probably due to unipolar thermal escape of electrons towards the InP barrier. On the other hand, the analysis of time resolved photoluminescence reveals that the temperature dependence of the radiative and nonradiative recombination times is mainly determined by the dynamics of excitons localized by disorder shigh energy tail of the PL bandd and strongly localized slow energy tail of the PL bandd in local size fluctuations of the quantum wires.

We present a novel method by wich excitonic interband optical transitions within single InAs self-assembled quantum dots can be directly observed in a transmission experiment. Due to the extremely high resolution of the tecnique, individual peaks associated to single exciton absorption resonances in single quantum dots can be spectrally resolved. Using this technique we investigate the oscillator strength, homogeneous linewidth and fine structure splitting in a collection of such individual resonances.

5 páginas, 5 figuras, 1 tabla.-- Comunicación presentada al E-MRS 2004 Spring Meeting celebrado en Estrasburgo (Francia) Mayo del 2004., For a certain heteroepitaxial system, the optical properties of self-assembled nanostructures basically depend on their size. In this work, we have studied different ways to modify the height of InAs/InP quantum wires (QWrs) in order to change the photoluminescence emission wavelength. One procedure consists of changing the QWr size by varying the amount of InAs deposited. The other two methods explored rely on the control of As/P exchange process, in one case during growth of InAs on InP for QWr formation and in the other case during growth of InP on InAs for QWr capping. The combination of the three approaches provides a fine tuning of QWr emission wavelength between 1.2 and 1.9 µm at room temperature., This work was financed by Spanish MCyT under NANOSELF project (TIC2002-04096) and MAT2003-02655.

Photoluminescence and excitation of the photoluminescence spectroscopy has been performed in single InGaAs self-assembled quantum rings embedded in a field effect structure device. To determine their electronic structure, bias-dependent optical transitions have been analyzed both, for individual quantum rings, and for the averaged ensemble. Our results are compared with a theoretical model, and also with results reported by other authors studying similar nanostructures., The authors would like to thank K. Karrai and R.J. Warburton for helpful discussions. This work was supported by Spanish MEC through NANOSELF Project (No. TIC2002-04096) and European Commission through SANDIE Network of Excellence (No. NMP4-CT-2004-500101).

3 páginas, 5 figuras., High structural and optical quality InxGa1−xP/GaAs quantum wells, with x from 0.51 to 0.45, have been successfully grown by atomic layer molecular beam epitaxy. In that compositional range, an important blue shift of the quantum well luminescence lines is observed, which is explained by an increase of the conduction band gap offset from compressive to tensile strain conditions. The luminescence intensity decreases with temperature above 20–30 K, which is attributed to impurities located at the interfaces and inside the quantum wells. The influence of the In content on the oscillator strength of the optical transitions is also evaluated., One of us (J.M.-P.) thanks the European Community for Human Capital and Mobility financial support, and also the Spanish DGICYT (project No. PB93-0687).

The size and emission wavelength of self-assembled InAs∕InP(001) quantum wires (QWrs) is affected by the P∕As exchange process. In this work, we demonstrate by in situ stress measurements that P∕As exchange at the InAs∕InP interface depends on the surface reconstruction of the InAs starting surface and its immediate evolution when the arsenic cell is closed. Accordingly, the amount of InP grown on InAs by P∕As exchange increases with substrate temperature in a steplike way. These results allow us to engineer the size of the QWr for emission at 1.3 and 1.55 μm at room temperature by selecting the range of substrate temperatures in which the InP cap layer is grown., This work was financed by Spanish MCyT under NANOSELF project (TIC2002-04096) and by NANOMAT project of the EC Growth Program, Contract No. G5RD-CT-2001-00545. One of the authors (D.F.) thanks the Spanish MECyD for funds. The authors also thank the Junta de Andalucía (PAI research group TEP-0120). TEM measurements were carried out in the DME-SCCYT and UCM.

5 páginas, 5 figuras, 2 tablas.-- PACS. 81.16.Dn Self-assembly – 78.67.Lt Quantum wires – 81.07.Vb Quantum wires., In this work we have studied the dependence of the optical properties of self-assembled InAs quantum wires (QWr) grown on InP(001) on the growth temperature of the InP cap layer, as a mean for controlling the InAs QWr size. Our main result is that we can tune the emission wavelength of InAs QWr either at 1.3 μ m m or 1.55 μ m m at room temperature. We suggest that the role of growth temperature is to modify the As/P exchange at the InAs QWr/InP cap layer interface and consequently the amount of InAs involved in the nanostructure. In this way, due to the enhancement of the As/P exchange, the higher the growth temperature of the cap layer, the smaller in height the InAs quantum wires. Accordingly, the emission wavelength is blue shifted with InP cap layer growth temperature as the electron and hole ground state moves towards higher energies. Optical studies related to the dynamics of carrier recombination and light emission quenching with temperature are also included., This work was partially supported by Spanish MCyT under project No. TIC2002-04096-C03 and by Nanomat project of the EC Growth program, contract n◦ G5RD-CT-2001-00545. D. Fuster thanks the Spanish MCyT for funding.

Trabajo presentado en el 7th International Workshop on Nonlinear Optics and Excitation Kinetics in Semiconductors, NOEKS 2003, celebrado en Karlsruhe (Alemania), del 23 al 28 de febrero de 2003, Continuous wave and time-resolved photoluminescence studies as a function of temperature have been performed on disordered InxGa 1-xP/GaAs quantum wells. Simulations of stationary and transient spectra, by using a two-exciton kinetic model, allow the observation of a red shift of the effective mobility edge when temperature increases. From the temperature dependence of the radiative lifetime it can be also deduced that independent localization of electrons and holes seems to be the most likely mechanism for exciton localization in our samples. On the other hand, values ranging from 0.8 to 5.6 ps have been obtained for the radiative lifetime at k∥ = 0., This work has been partially supported by the spanish CICYT under project TIC99-1035.

2 páginas, 2 figuras.-- Trabajo presentado a la XIIth International Conference on Molecular Beam Epitaxy, MBE-XII, San Francisco (EEUU), 2002., It is now well established (1-5) that nanowires formation along [11~0] takes place when an InAs thin layer (about 2.7 monolayers, ML) is grown by MBE on InP [001] substrate. Strain relaxation is the driving force for the unidirectional observed surface roughening due to the asymmetric accumulated stress induced during MBE growth of a lattice mismatch III-V/III-V' heteroepitaxial system. In the case of InAs/InP[001] system, the nanowires covered with an InP cap layer produce photoluminescence (PL) emission at λ≈1.5 μm at room temperature. However it should be desirable to exactly tune the emission wavelength to the optimum for communication devices (λ=1.55 μm) as well as to any other wavelength under design. In this work we have grown by MBE samples consisting of In1-xGaxAs and In1-xAlxAs (Og= 0.1 ML/s and substrate temperature Ts = 490°C. Measurements of surface topography made by atomic force microscopy (AFM) show that the period of the wires change from 17 nm to 45 nm for a change in lattice-mismatch from 3.2% to 2.2%. Results on PL emission wavelength range obtained in similar samples but adding an InP capping layer are shown.

3 páginas, 2 figuras.-- PACS: 73.21.Hb; 78.55.Cr; 78.67.Lt.-- Proceedings of the International Conference on Superlattices, Nano-structures and Nano-devices ICSNN 2002., Multilayer structure containing vertically stacked InAs/InP self-assembled quantum wires (QWR) have been successfully grown by molecular beam epitaxy. We have found that spacer layer thickness fully determines the structural and optical properties ofthe multilayer structure, being observed a strong improvement ofthe homogeneity and uniformity ofthe QWR stacks in the case ofan appropriate layer spacing., This work has been supported by the Spanish CICYT under the project no. TIC99-1035-C02 and European Commission GROWTH program NANOMAT project, Contract no. G5RD-CT-2001- 00545.

6 páginas, 3 figuras., In this work, a study of the relaxation mechanisms in InAs self-assembled quantum dots have been performed by means of photoluminescence (PL), resonant PL (RPL) and PL excitation. The observed phenomenology is different for samples with different InAs coverage. At low InAs coverages, two resonant Raman scattering lines are observed when the excitation energy is chosen inside the emission band. These lines are related to the TO and LO GaAs phonons. Multi-phonon relaxation of carriers (GaAs phonons) is clearly observed in RPL and excitation PL in the lowest InAs coverage sample. Neither such mechanism nor resonant Raman scattering is present in samples with larger InAs coverages, for which the average quantum dot size and areal density remain essentially constant., This work has been partially supported by the spanish Ministerio de Ciencia y Tecnolog ıa under project number TIC99-1035-C02, Ministerio de Asuntos Exteriores (MUTIS program) and the Generalitat Valenciana.

We have investigated the electronic structure of the zig-zag ladder (chain) compound ${\mathrm{SrCuO}}_{2}$ combining polarized optical absorption, reflection, photoreflectance, and pseudo-dielectric-function measurements with the model calculations. These measurements yield an energy gap of 1.42 eV (1.77 eV) at 300 K along (perpendicular to) the Cu-O chains. We have found that the lowest-energy gap, the correlation gap, is temperature independent. The electronic structure of this oxide is calculated using both the local-spin-density approximation with gradient correction method and the tight-binding theory for the correlated electrons. The calculated density of electronic states for noncorrelated and correlated electrons shows quasi-one-dimensional character. The correlation gap values of 1.42 eV (indirect transition) and 1.88 eV (direct transition) have been calculated with the electron hopping parameters $t=0.30\mathrm{eV}$ (along a chain), ${t}_{\mathrm{yz}}=0.12\mathrm{eV}$ (between chains), and the Anderson-Hubbard repulsion on copper sites $U=2.0\mathrm{eV}.$ We concluded that ${\mathrm{SrCuO}}_{2}$ belongs to the correlated-gap insulators.

3 pages, 3 figures.-- PACS: 81.05.Ea; 68.65.+g; 78.55.Cr; 78.66.Fd; 68.35.Bs; 68.55.Jk; 81.15.Hi; 81.15.Ef, We have studied the influence of InP buffer-layer morphology in the formation of InAs nanostructures grown on InP(001) substrates by solid-source molecular-beam epitaxy. Our results demonstrate that when InP buffer layers are grown by atomic-layer molecular-beam epitaxy, InAs quantum dot-like structures are formed, whereas InP buffer layers grown by MBE produce quantum-wire-like structures. The optical properties of these corrugated structures make them potential candidates for their use in light-emitting devices at 1.55 µm., The authors wish to acknowledge the Spanish ‘‘CICYT’’ for financial support under Project No. TIC96- 1020-C02.

6 páginas, 3 figuras., InxGa1—xP/GaAs (x = 0.541 and 0.427) heterostructures, grown by Atomic Layer Molecular Beam Epitaxy (ALMBE) on low temperature substrates, have been characterised by pressure-dependent and time-resolved photoluminescence experiments. The excitonic optical transitions and recombination dynamics are both influenced by the particular band alignments of these systems. The valence band offset has been found to have approximately the same absolute value (ΔEVB ≈ 380 meV), independent of the In content of the alloy in the barrier, whereas the conduction band offset varies appreciably depending on the alloy band gap. The huge valence band offset implies a strong asymmetry in the confinement of carriers, affecting the exciton recombination dynamics in the quantum wells., This work has been supported by the projects TIC96-1020-C02 and HA1997-0129 of the Ministry of Education of Spain.