Optical pumping of charged excitons in unintentionally doped InAs quantum dots

As an alternative to commonly used electrical methods, we have investigated the optical pumping of charged exciton complexes addressing impurity related transitions with photons of the appropriate energy. Under these conditions, we demonstrate that the pumping fidelity can be very high while still maintaining a switching behavior between the different excitonic species. This mechanism has been investigated for single quantum dots of different size present in the same sample and compared with the direct injection of spectator electrons from nearby donors.Comment: 4 pages and 3 figures submitted to AP

Espectroscopía óptica confocal aplicada al estudio de cajas cuánticas aisladas de arseniuro de indio (InAS)

RESUMEN La nanotecnología electrónica surge como disciplina científica para dar respuesta a la demanda creciente por parte de la sociedad de dispositivos electrónicos más eficientes y de menor coste. Como su propio nombre indica, esta disciplina involucra los estudios que se llevan a cabo en la actualidad en sistemas electrónicos elementales que, aunque basados en diferentes materiales y técnicas de fabricación, tienen un nexo común en su tamaño nanométrico. En este marco, en el trabajo de tesis presentado se han investigado las propiedades ópticas y electrónicas de cajas cuánticas aisladas de Arseniuro de Indio mediante diferentes técnicas de espectroscopía óptica confocal de alta resolución espacial y espectral. Dicha resolución nos ha permitido profundizar en el estudio de las propiedades individuales de estos objetos nanométricos tanto desde el punto de vista fundamental como aplicado. En primer lugar, se han analizado los espectros de fotoluminiscencia (PL) y el correspondiente espectro de excitación de la PL (PLE) de cajas cuánticas aisladas y sometidas a un campo eléctrico externo. Las cajas cuánticas se hallan en la región activa de un dispositivo de efecto campo que nos permite controlar de manera precisa el número de electrones confinados en cada nanoestructura individual. El estudio de la evolución de la línea de emisión fundamental de cada caja en función del número de electrones en exceso nos ha permitido deducir la magnitud de las distintas interacciones coulombianas existentes entre los diferentes portadores. Algunas de ellas son bien conocidas, como es la de atracción directa entre electrón y hueco que da lugar al excitón, mientras que otras son más sutiles y difíciles de cuantificar hasta hace poco tiempo, como son la de repulsión directa electrón-electrón o su correspondiente interacción de intercambio. Todas ellas han sido investigadas de manera cualitativa y cuantitativa comparando los resultados experimentales con modelos teóricos de reconocida vigencia en este tipo de sistemas. Por otro lado, hemos utilizado un láser sintonizable en el rango de interés para determinar la estructura electrónica asociada a los estados excitados de anillos cuánticos aislados de InAs. Los resultados demuestran que el acoplamiento entre los estados discretos y el continuo determinan en gran medida los procesos de captura y relajación de los portadores hacia el estado fundamental. Este tipo de comportamiento corrobora lo observado por otros autores en nanoestructuras de similar tamaño y composición aunque distinta morfología, si bien ésta es la primera vez que ha sido observado en anillos cuánticos de InAs. En la segunda parte de este estudio, hemos demostrado que es posible determinar los espectros de absorción y reflectividad correspondientes a una sola caja cuántica aislada con una resolución espectral sin precedentes en este tipo de sistemas y con una elevada relación señal-ruido. Para ello, partiendo de un montaje confocal dual de transmisión y reflexión, y aplicando un campo eléctrico externo, hemos explotado el Efecto Stark de Confinamiento Cuántico para sintonizar la energía de la transición interbanda investigada con nuestro láser de prueba mejorando así la eficiencia del experimento respecto a otras aproximaciones basadas en la sintonización del láser. El análisis de los espectros obtenidos de esta manera nos ha proporcionado información valiosa no accesible de manera directa mediante otro tipo de experimentos como son la anchura homogénea de las transiciones interbanda o su fuerza de oscilador, así como indicios de fenómenos de interacción no lineales como el Efecto Stark Óptico. Finalmente, se ha demostrado que este sistema actúa como un interferómetro acoplado a una sola caja cuántica con grados de libertad suficientes para ejercer un control efectivo del grado de interacción, amplitud y fase, entre radiación y materia a estas escalas nanométricas. ____________________________________________________________________________________________________ SUMMARY The electronic and optical properties of InAs single self-assembled quantum dots (SAQDs) have been investigated by using a confocal arrangement adapted to work at 4 K. The layer containing the QDs is embedded in a field effect device with a semitransparent Schottky gate. In this configuration, varying the applied voltage, it is possible to tune the number of electrons confined in a given QD to investigate the exciton radiative recombination as a function of such number of electrons in excess. This way, comparing the experimental results with state of the art theoretical models, we have determined the role played by the interband and intraband coulomb interactions in this system. The electronic structure above the fundamental state is accessible scanning the excitation energy of our laser (PLE spectra). In our nanostructures, we observe that, below the continuum band edges, the expected discrete bound to bound transitions are degenerated with broad bands associated with continuum to bound transitions. As pointed out recently by other authors, we conclude that this result imposes a revision of the concept of an isolated quantum dot embedded in an homogeneous media (macroatom picture). We have demonstrated also the feasibility of a new method, based in the spectral shift of the electronic levels due to the Quantum Confined Stark Effect, to measure the differential absorption spectra of single SAQDs with unmatched spectral resolution and signal to noise ratios. In the linear regime, light and matter interaction can be described by the Lorentz model to extract the oscillator strength and homogeneous linewidth of non-degenerated optical transitions in our QDs. Furthermore, due to the large interlevel spacing in each band, we observe that the two level approximation works finely in this system enabling the study of non linear optical phenomena as the Optical Stark Effect. Finally, the reflectance spectrum of a single electronic level in our QDs has been determined by using a built-in interferometer in our confocal setup. We have demonstrated that the phase relation between probe and reflected fields can be controlled externally enabling the coherent control of the QD polarization in the next future

Study of Growth Parameters for Single InAs QD Formation on GaAs(001) Patterned Substrates by Local Oxidation Lithography

This work studies the selective nucleation of InAs within nanoholes on GaAs(001) substrates patterned by atomic force microscopy local oxidation. The effects of substrate temperature and As4 overpressure during InAs deposition directly on the patterned substrate (without a GaAs buffer layer) are considered. It is found that when InAs is deposited at substrate temperature of 510 °C under low As4 overpressure, a single InAs quantum dot per nanohole is obtained for a broad range of sizes of pattern motifs. The use of these InAs quantum dots as seed nuclei for vertical stacking of optically active single InAs site-controlled quantum dots is investigated.The authors acknowledge financial support by Spanish MINECO through Grants ENE2012-37804-C02-02 and TEC2011-29120-C05-04. Jesús Herranz acknowledges the JAE program for funds.Peer reviewe

Optical transitions and excitonic recombination in InAs/InP self-assembled quantum wires

InAs self-assembled quantum wire structures have been grown on InP substrates and studied by means of photoluminescence and polarized-light absorption measurements. According to our calculations, the observed optical transitions in each sample are consistent with wires of different heights, namely from 6 to 13 monolayers. The nonradiative mechanism limiting the emission intensity at room temperature is related to thermal escape of carriers out of the wires.This work has been supported under project No. TIC99-1035-C02 of the Spanish CICYT.Peer reviewe

Role of re-growth interface preparation process for spectral line-width reduction of single InAs site-controlled quantum dots

We present growth and optical characterization measurements of single InAs site-controlled quantum dots (SCQDs) grown by molecular beam epitaxy on GaAs (001) patterned substrates by atomic force microscopy oxidation lithography. InAs SCQDs directly grown on the patterned surface were used as a seed layer and strain template for the nucleation of optically active single InAs SCQDs. The preservation of the initial geometry of the engraved pattern motifs after the re-growth interface preparation process, the lack of buffer layer growth prior to InAs seed layer deposition and the development of suitable growth conditions provide us an improvement of the SCQDs' active layer optical properties while retaining a high ratio of single occupation (89%). In this work a fivefold reduction of the average optical line-width from 870 μeV to 156 μeV for InAs SCQDs located 15 nm from the re-growth interface is obtained by increasing the temperature of the initial thermal treatment step of the re-growth interface from 490 °C to 530 °C.The authors thank financial support by Spanish MINECO through grants ENE2012-37804-C02-02 and TEC2011-29120-C05-04. Jesús Herranz acknowledges the JAE program for the funds.Peer reviewe

Direct formation of InAs quantum dots grown on InP (001) by solid-source molecular beam epitaxy

We have developed a growth process that leads to the direct formation of self-assembled InAs quantum dots on InP(001) by solid-source molecular beam epitaxy avoiding the previous formation of quantum wires usually obtained by this technique. The process consists of a periodically alternated deposition of In and As correlated with InAs(4×2)(2×4) surface reconstruction changes. Based on the results obtained by in situ characterization techniques, we propose that the quantum dots formation is possible due to the nucleation of In droplets over the InAs(4×2) surface during the In deposition step and their subsequent crystallization under the As step.The authors acknowledge the financial support of the Spanish MICINN (TEC2008-06756-C03-01), Consolider-QOIT (CSD2006-0019), and CAM (S-505/ESP/000200). P.A.G. and A.R. thank the I3P program. B.A. thanks the Ramón y Cajal program.Peer reviewe

High efficient luminescence in type-II GaAsSb-capped InAs quantum dots upon annealing

The photoluminescence efficiency of GaAsSb-capped InAs/GaAs type II quantum dots (QDs) can be greatly enhanced by rapid thermal annealing while preserving long radiative lifetimes which are ∼20 times larger than in standard GaAs-capped InAs/GaAs QDs. Despite the reduced electron-hole wavefunction overlap, the type-II samples are more efficient than the type-I counterparts in terms of luminescence, showing a great potential for device applications. Strain-driven In-Ga intermixing during annealing is found to modify the QD shape and composition, while As-Sb exchange is inhibited, allowing to keep the type-II structure. Sb is only redistributed within the capping layer giving rise to a more homogeneous composition

X-ray nanoimaging of Nd3+ optically active ions embedded in Sr0.5Ba0.5Nb2O6 nanocrystals

[EN] The spatial distribution of Sr0.5Ba0.5Nb2O6 nanocrystals is analyzed in a borate-based glass-ceramic by a synchrotron hard X-ray nanoimaging tool. Based on X-ray excited optical luminescence, we examined 2D projections of the Nd3+ optically active ions in the Sr0.5Ba0.5Nb2O6 nanocrystals, as well as in the glassy phase where they are embedded. Our findings reveal areas of agglomerations and/or clusters of nanocrystals ascribed to the diffusion coefficients of their constituent elements. They are characterized by high Nd3+ concentrations that may act as heterogeneous agents for the nucleation and growth of these nanocrystals. (C) 2017 Optical Society of AmericaMINECO, EU-FEDER and CSIC through the projects MAT2013-46649-C4-4-P, MAT201571070-REDC, MAT2016-75586-C4-2-P, MAT2016-75586-C4-4-P, 201550I021 and 201660I001, respectively. JAS acknowledges the Spanish Program Ramón y Cajal for his fellowship. We also thank the ESRF for the beam time allocated and experimental facilities.Martínez-Criado, G.; Alén, B.; Sans-Tresserras, JÁ.; Lozano-Gorrín, A.; Haro-González, P.; Martin, I.; Lavin, V. (2017). X-ray nanoimaging of Nd3+ optically active ions embedded in Sr0.5Ba0.5Nb2O6 nanocrystals. Optical Materials Express. 7(7):2424-2431. https://doi.org/10.1364/OME.7.002424S2424243177Nagata, K., Yamamoto, Y., Igarashi, H., & Okazaki, K. (1981). Properties of the hot-pressed strontium barium niobate ceramics. Ferroelectrics, 38(1), 853-856. doi:10.1080/00150198108209556Imai, T., Yagi, S., Yamazaki, H., & Ono, M. (1999). Effects of Heat Treatment on Photorefractive Sensitivity of Ce- and Eu-Doped Strontium Barium Niobate. Japanese Journal of Applied Physics, 38(Part 1, No. 4A), 1984-1988. doi:10.1143/jjap.38.1984Volk, T., Isakov, D., Salobutin, V., Ivleva, L., Lykov, P., Ramzaev, V., & Wöhlecke, M. (2004). Effects of Ni doping on properties of strontium–barium–niobate crystals. Solid State Communications, 130(3-4), 223-226. doi:10.1016/j.ssc.2004.01.039Romero, J. J., Andreeta, M. R. B., Andreeta, E. R. M., Bausá, L. E., Hernandes, A. C., & García Solé, J. (2004). Growth and characterization of Nd-doped SBN single crystal fibers. Applied Physics A, 78(7), 1037-1042. doi:10.1007/s00339-003-2151-3Chayapiwut, N., Honma, T., Benino, Y., Fujiwara, T., & Komatsu, T. (2005). Synthesis of Sm3+-doped strontium barium niobate crystals in glass by samarium atom heat processing. Journal of Solid State Chemistry, 178(11), 3507-3513. doi:10.1016/j.jssc.2005.09.002Haro-González, P., Martín, I. R., Martín, L. L., León-Luis, S. F., Pérez-Rodríguez, C., & Lavín, V. (2011). Characterization of Er3+ and Nd3+ doped Strontium Barium Niobate glass ceramic as temperature sensors. Optical Materials, 33(5), 742-745. doi:10.1016/j.optmat.2010.11.026Ivleva, L. I., Volk, T. R., Isakov, D. V., Gladkii, V. V., Polozkov, N. M., & Lykov, P. A. (2002). 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Nanocrystals distribution inside the writing lines in a glass matrix using Argon laser irradiation. Optics Express, 18(2), 582. doi:10.1364/oe.18.000582Haro-González, P., Martín, I. R., Arbelo-Jorge, E., González-Pérez, S., Cáceres, J. M., & Núñez, P. (2008). Laser irradiation in Nd3+ doped strontium barium niobate glass. Journal of Applied Physics, 104(1), 013112. doi:10.1063/1.2952011Kowalska, D., Haro-González, P., Martín, I. R., & Cáceres, J. M. (2010). Analysis of the optical properties of Er3+-doped strontium barium niobate nanocrystals using time-resolved laser spectroscopy. Applied Physics A, 99(4), 771-776. doi:10.1007/s00339-010-5716-yPellicer-Porres, J., Segura, A., Martínez-Criado, G., Rodríguez-Mendoza, U. R., & Lavín, V. (2012). Formation of nanostructures in Eu3+doped glass–ceramics: an XAS study. Journal of Physics: Condensed Matter, 25(2), 025303. doi:10.1088/0953-8984/25/2/025303Martínez-Criado, G., Alén, B., Sans, J. A., Homs, A., Kieffer, I., Tucoulou, R., … Yi, G. (2012). Spatially resolved X-ray excited optical luminescence. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 284, 36-39. doi:10.1016/j.nimb.2011.08.013Martínez-Criado, G., Sans, J. A., Segura-Ruiz, J., Tucoulou, R., Solé, A. V., Homs, A., … Alén, B. (2011). X-ray excited optical luminescence imaging of InGaN nano-LEDs. physica status solidi (c), 9(3-4), 628-630. doi:10.1002/pssc.201100430Villanova, J., Segura-Ruiz, J., Lafford, T., & Martinez-Criado, G. (2012). Synchrotron microanalysis techniques applied to potential photovoltaic materials. Journal of Synchrotron Radiation, 19(4), 521-524. doi:10.1107/s0909049512021383Smith, J., Akbari-Sharbaf, A., Ward, M. J., Murphy, M. W., Fanchini, G., & Kong Sham, T. (2013). Luminescence properties of defects in nanocrystalline ZnO. Journal of Applied Physics, 113(9), 093104. doi:10.1063/1.4794001Armelao, L., Heigl, F., Jürgensen, A., Blyth, R. I. 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III-V semiconductor quantum dots for efficient quantum light sources

Comunicación presentada en el 3rd international Workshop Engineering of quantum emitter properties, celebrado en Linz (Austria) el 17 y 18 de diciembre de 2015.Photonic crystal microcavities (PCMs) with embedded quantum dots (QDs) have been shown as excellent test bed systems for experiments in the field of cavity quantum electrodynamics (c-QED) that may open doors to efficient quantum photonic devices for the generation of single-photons, entangled photon pairs and ultra-low threshold lasing. Based on fundamental excitonic emission and on biexciton-exciton recombination cascade, a single QD embedded in a PCM become efficient emitters of single photons or entangled photon pairs provided that both spectral and spatial matching of the optical cavity mode and the optical emission of the single nanostructure occur. Within this approach, we have explored several systems and growth methods with the aim of fabricating QD which fulfil the requirements for an efficient coupling between a single QD and a PCM. We have fabricated QD by molecular beam epitaxy (MBE) using a) droplet epitaxy and b) selective nucleation at nano-holes fabricated by atomic force microscopy local oxidation (AFMLO) lithography. Results will be presented of QD in GaAs/AlGaAs(111)A, InAs/GaAs(001) and InAs/InP (001). With the aim of obtaining coupled QD-PCM, we have followed two procedures: one is based on the fabrication of a PCM around a buried QD whose position and wavelength emission are previously determined; the other approach consists of locating a single QD by using AFMLO, at the maximum of the electric field of a prefabricated PCM. A MBE re-growth procedure has been developed for completing the PCM membrane thickness.Peer Reviewe