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Manipulating properties of matter at the nanoscale is the essence of nanotechnology, which has enabled the realization of quantum dots, nanotubes, metamaterials, and two-dimensional materials with tailored electronic and optical properties. Two-dimensional semiconductors have revealed promising perspectives in nanotechnology. However, the tunability of their physical properties is challenging for semiconductors studied until now. Here we show the ability of morphological manipulation strategies, such as nanotexturing or, at the limit, important surface roughness, to enhance light absorption and the luminescent response of atomically thin indium selenide nanosheets. Besides, quantum-size confinement effects make this two-dimensional semiconductor to exhibit one of the largest band gap tunability ranges observed in a two-dimensional semiconductor: from infrared, in bulk material, to visible wavelengths, at the single layer. These results are relevant for the design of new optoelectronic devices, including heterostructures of two-dimensional materials with optimized band gap functionalities and in-plane heterojunctions with minimal junction defect density.

© 2015 Optical Society of America. The development of a thresholdless laser operating above room temperature (RT) is key for the future replacement of electronics with photonic integrated circuits, enabling an increase of several orders of magnitude in computing speeds. Recently, thresholdless lasing characteristics at low temperature (4 K) have been demonstrated. However, for practical applications, RT laser emission becomes necessary. Here we report experimental evidence that is compatible with a laser based on InAsSb quantum dots embedded in a photonic-crystal microcavity that exhibits an ultralow-power threshold (860 nW) and high efficiency (ß = 0.85), thus operating in the near-thresholdless regime at RT in the 1.3 µm spectral window. The results open up a wide range of opportunities for RT applications of ultralow threshold lasers, such as integrated photonic circuitry or high sensitivity biosensors.

The Purcell effect dependence on the excitation power is studied in photonic crystal microcavity lasers embedding InAs/InP quantum wires. In the case of non-lasing modes, the Purcell effect has low dependence on the optical pumping, attributable to an exciton dynamics combining free and localized excitons. In the case of lasing modes, the influence of the stimulated emission makes ambiguous the determination of the Purcell factor. We have found that this ambiguity can be avoided by measuring the dependence of the decay time on the excitation power. These results provide insights in the determination of the Purcell factor in microcavity lasers. © 2013 AIP Publishing LLC.

We have investigated the temperature dependent recombination dynamics in two bimodally distributed InAs self assembled quantum dots samples. A rate equations model has been implemented to investigate the thermally activated carrier escape mechanism which changes from exciton-like to uncorrelated electron and hole pairs as the quantum dot size varies. For the smaller dots, we find a hot exciton thermal escape process. We evaluated the thermal transfer process between quantum dots by the quantum dot density and carrier escape properties of both samples. © 2012 American Institute of Physics.

The spontaneous emission rate and Purcell factor of selfassembled quantum wires embedded in photonic crystal micro-cavities aremeasured at 80 K by using micro- photoluminescence, under transient and steady state excitation conditions. The Purcell factors fall in the range 1.1 - 2 despite the theoretical prediction of ?15.5 for the figure of merit. We explain this difference by introducing a polarization dependence on the cavity orientation, parallel or perpendicular with respect to the wire axis, plus spectral and spatial detuning factors for the emitters and the cavitymodes, taking in account the finite size of the quantum wires. © 2012 Optical Society of America.

[EN] The invention relates to a three-dimensional acoustic lens having rotating symmetry for focusing, collimating or scattering sound beams. The lenses consist of concentric rings of a material having acoustic properties different from those of the medium in which they are inserted, thus allowing them to scatter sound., [ES] Se describe una lente acústica tridimensional con simetría de revolución para focalizar, colimar o dispersar haces sonoros. Las lentes están fonnadas por anillos concéntricos de un material con propiedades acústicas diferentes del las del medio en que se hallan inmersos y por tanto actuando como dispersores del sonido.

Se describe una lente acústica tridimensional con simetría de revolución para focalizar, colimar o dispersar haces sonoros. Las lentes están formadas por anillos concéntricos de un material con propiedades acústicas diferentes del las del medio en que se hallan inmersos y por tanto actuando como dispersores del sonido.

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.

We model the time-resolved and time-integrated photoluminescence of a single InAs/GaAs quantum dot (QD) using a random population description. We reproduce the joint power dependence of the single QD exciton complexes(neutral exciton, neutral biexciton and charged trions). We use the model to investigate the selective optical pumping phenomenon, a predominance of the negative trion observed when the optical excitation is resonant to a nonintentional impurity level. Our experiments and simulations determine that the negative charge confined in the QD after exciting resonance to the impurity level escapes in 10 ns.

Las nanopartículas semiconductoras (puntos cuánticos) son nanomateriales emisores de luz por luminiscencia a partir de una excitación UV, con prometedoras aplicaciones futuras en nano-biomedicina y nano-optoelectrónica. El comportamiento óptico básico de estos nanomateriales luminiscentes depende principalmente de su composición química, forma y tamaño. En este trabajo se realiza un análisis espectral y colorimétrico preliminar de dos variedades químicas de puntos cuánticos (CdSe y CdTe) en función diferentes variaciones en sus procesos de síntesis. Este análisis comprende el paso de los espectros de absorción/emisión relativa a valores colorimétricos convencionales usando espacios de representación del color recomendados por la Comisión Internacional de Iluminación y Color (CIE) con el fin de evaluar el rango de colores perceptibles, y, correlacionar la evolución de color con las variables implicadas en el proceso de sus síntesis.

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 emission properties of lateral and vertical QD pairs grown on GaAs nanoholes are investigated. Vertical QD pairs with different size asymmetry have been fabricated controlling the bottom QD size independently of the areal density. The emission of individual pairs is dominated by spectral diffusion effects and charge instabilities induced by the local charge environment. Lateral QD pairs have been fabricated on GaAs nanoholes and studied as a function of an electric field applied in the growth plane.

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.

We have fabricated an array of closely spaced quantum dashes starting from a planar array of self-assembled semiconductor quantum wires. The array is embedded in a metallic nanogap which we investigate by micro-photoluminescence as a function of a lateral electric field. We demonstrate that the net electric charge and emission energy of individual quantum dashes can be modified externally with the performance limited by the size inhomogeneity of the self-assembling process.

We have investigated the selective optical pumping of charged excitonic species in a sample containing quantum dots of different sizes and low areal density by photoluminescence and excitation of the photoluminescence microspectroscopy. We study the selective optical excitation of negatively charged excitons as an alternative to commonly used electrical methods. We demonstrate that under resonant excitation in impurity related bands, the selective pumping efficiency can be as high as 85% in small quantum dots having one electron shell and emitting at around 930 nm, and around 65% in big quantum dots having four electron shells and emitting at 1160 nm. © IOP Publishing Ltd.

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.

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.

In the presence of a stationary electric field applied in the growth direction tunneling of electrons out of the quantum dots can take place. This mechanism competes with the quantum confined Stark effect (QCSE) that produces an increase of the exciton lifetime by increasing the electric field, mainly due to a net decrease of the electron–hole wavefunction overlap. The electric field range where QCSE dominates over tunneling will be mainly determined by the size of the nanostructure along the vertical direction (height), as demonstrated in this work.

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.

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 paper investigates the electronic structure of self-assembled InAs quantum wires (QWrs), grown under different conditions by molecular beam epitaxy on InP, by means of photoluminescence measurements under pressure. In samples with regularly distributed QWrs, room pressure photoluminescence spectra consist of a broad band centred at about 0.85 eV, which can be easily de-convoluted in a few Gaussian peaks. In samples with isolated QWrs, photoluminescence spectra exhibit up to four clearly resolved bands. Applying hydrostatic pressure, the whole emission band monotonously shifts towards higher photon energies with pressure coefficients ranging from 72 to 98 meV/GPa. In contrast to InAs quantum dots on GaAs, quantum wires photoluminescence is observed up to 10 GPa, indicating that InAs QWrs are metastable well above pressure at which bulk InAs undergoes a phase transition to the rock-salt phase (7 GPa).

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.

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.

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.

We present a systematic study of closely In(Ga)As/InAs quantum rings (QRs) grown by molecular beam epitaxy (MBE). Photoluminescence (PL) experiments show a strong filtering effect in the ring being stacked and simultaneous linewidth narrowing for the appropriate layer thickness (thinner thickness). If the spacer thickness is further reduced, a strong coupling between the nanostructures is produced and the signal shifts to low energy.

Exciton recombination dynamics in vertical stacks of InGaAs quantum rings have been studied by means of continuous wave and time resolved photoluminescence under low excitation density conditions. We have paid special attention to the effect of the carrier coupling on the exciton radiative lifetime: weak (14 nm spacer sample), intermediate (4.5 nm spacer sample), where the size filtering effects (towards small rings) compensate partially that arising from carrier coupling (towards lower energies), and strong electron and hole coupling (1.5 nm spacer sample) between layers. Experimental decay times in the latter two cases have been compared to the times simulated with a multi-quantum well based model, which accounts for the observed change of carrier coupling regime. The most important effect is observed when the hole wave function overlap along the growth direction becomes important (1.5 nm spacer sample). This situation makes important the lateral tunneling of excitons between rings, given their large lateral size, which is characterized by times around 5 ns at the emission peak energy (rings with the most probable size of the distribution).

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.

In this work, an imaging tool to investigate optical inhomogeneities with site and chemical sensitivities has been integrated in a hard x-ray microprobe. Freestanding GaN and epitaxially grown GaN:Mn on -Al2O3 are used to exploit the unprecedented scanning x-ray excited luminescence technique. Optical images of the radiative recombination channels are reported for several impurities and defect centers in sapphire and GaN compounds. Within the experimental accuracy, a visible nonuniformity characterizes the Mn centers in good correlation with former x-ray fluorescence map. Expanding the microprobe versatility, x-ray absorption spectroscopy in both photon collection modes (x-ray excited luminescence and x-ray fluorescence) is finally presented from a freestanding GaN layer.

Isolated InAs/InP self-assembled quantum wires have been grown using in situ accumulated stress measurements to adjust the optimal InAs thickness. Atomic force microscopy imaging shows highly asymmetric nanostructures with average length exceeding more than ten times their width. High resolution optical investigation of as-grown samples reveals strong photoluminescence from individual quantum wires at 1.5 µm. Additional sharp features are related to monolayer fluctuations of the two-dimensional InAs layer present during the early stages of the quantum wire self-assembling process.

We present results concerning the carrier transfer between In(Ga)As quantum rings in a stacked multilayer structure, which is characterised by a bimodal size distribution. This transfer of carriers explains the observed temperature behaviour of diode lasers based on that kind of stacked layer structures. The inter-ring carrier transfer can be possible by phonon assisted tunnelling from the ground state of the smallring family towards the big-ring family of the bimodal size distribution. This process is thermally activated in the range 40–80 K.

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.

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.

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.

The temperature dependence of the effective mobility edge in semiconductor quantum wells containing disorder is reported for the first time. It is deduced by reproducing the experimental continuous wave and time resolved luminescence spectra by means of a two-class exciton kinetic model, which considers the co-existence of free and localized excitons and introduces a mobility edge defined by a Fermi function. The mobility edge varies faster than the PL peak energy when the temperature increases, passing from the high energy side of the PL band at low temperatures (recombination of localized excitons dominates) to the low energy side above 125 K (recombination of quasi-free excitons dominates).

We have performed photoluminescence experiments in samples containing self-assembled quantum dots with different spacer layer thicknesses. A strong filtering effect produced by the GaAs spacer layer on the dots size being stacked is observed for spacers thinner than 10 nm. This effect produces a blue shift of the emission band from stacked dots and a simultaneous line width narrowing. At the same time, given the existence of a broad dot size distribution in the first layer, bigger dots can evolve towards InAs cylinder-like structures, whose emission occurs at appreciably lower energies as compared to the emission band associated to dot stacks (with some GaAs separation).

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.

The effects of an external electric field on the photoluminescence of small self-assembled InAs/GaAs quantum dots (3–4 nm high) have been studied at different temperatures, laser power, and laser wavelength excitation. The shallow nature of the only confined electron level is revealed by the strong photocurrent signal measured at low temperatures. In this QD system, recombination from that electron level with ground and first excited hole states can be observed in high power excited PL and low power excited PL under direct applied bias, when the hole population is high enough under these conditions.

GaAs/InxGa1−xP quantum wells, with x=0.541 and 0.427, have been investigated by continuous wave and time resolved photoluminescence. Spatial localization of excitons due to alloy compositional defects seem to be responsible of the observed phenomenology: non-exponential decay behavior at long times and slow decrease of the effective recombination time by increasing temperature. A two-class exciton kinetic model, based on a mobility edge varying with the temperature explain the experimental results.

The effects of external electric field on the photoluminescence (PL) of self assembled InAs/GaAs quantum dots (QDs) have been studied at different temperatures with different energies and power densities of excitation. Photocurrent measurements on the same sample were carried out for additional information on the transitions . The test structure consists of a single layer of InAs QDs with the InAs wetting layer buried in intrinsic GaAs, sandwiched between an n++GaAs at the back and a semitransparent Cr Schottky contact on the top GaAs capping layer. The power of the excitation source for PL measurements was varied in the range of 200-4000 W/cm2, approximately. The results of comparatively low electric fields are to be reported.

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.

Resonant photoluminescence experiments have been performed on self-assembled quantum dots. The emission bands measured in the investigated samples can be deconvoluted in several Gaussian components, which could be related to different size families of dots. The experimental results reveal the importance of GaAs phonons for the carrier relaxation of excess energy, specially for the sample with low dot density. With increasing temperature, electrons from smaller dots can be activated to reach the wetting layer electron states, from which they can be trapped at larger dots (below 100 K) or escape to the GaAs barriers (above 100 K).