Agnieszka Gocalinska

Towards quantum-dot arrays of entangled photon emitters

An array of pyramidal site-controlled InGaAs1−δNδ quantum dots is grown on a GaAs substrate to reduce the fine-structure splitting of the intermediate single-exciton energy levels to less than 4 μeV. The quantum dots emit polarization-entangled photons at a maximum fidelity of 0.721 ± 0.043 without external manipulation of the electronic states.

Exciton-Exciton Interaction and Optical Gain in Colloidal CdSe/CdS Dot/Rod Nanocrystals.

Exciton-exciton interaction in dot/rod CdSe/CdS nanocrystals has proved to be very sensitive to the shape of nanocrystals, due to the unique band alignment between CdSe and CdS. Repulsive exciton-exciton interaction is demonstrated, which makes CdSe/CdS dot/rods promising gain media for solution-processable lasers, with projected pump threshold densities below 1 kW cm(-2) for continuous wave lasing.

InP-Based Active and Passive Components for Communication Systems at 2 μm

Progress on advanced active and passive photonic components that are required for high-speed optical communications over hollow-core photonic bandgap fiber at wavelengths around 2 μm is described in this paper. Single-frequency lasers capable of operating at 10 Gb/s and covering a wide spectral range are realized. A comparison is made between waveguide and surface normal photodiodes with the latter showing good sensitivity up to 15 Gb/s. Passive waveguides, 90° optical hybrids, and arrayed waveguide grating with 100-GHz channel spacing are demonstrated on a large spot-size waveguide platform. Finally, a strong electro-optic effect using the quantum confined Stark effect in strain-balanced multiple quantum wells is demonstrated and used in a Mach-Zehnder modulator capable of operating at 10 Gb/s.

InAlAs solar cell on a GaAs substrate employing a graded InxGa1−xAs–InP metamorphic buffer layer

Single junction In0.52Al0.48As solar cells have been grown on a (100) GaAs substrate by employing a 1 μm thick compositionally graded InxGa1−xAs/InP metamorphic buffer layer to accommodate the 3.9% mismatch. Cells processed from the 0.8 μm thick InAlAs layers had photovoltaic conversion efficiency of 5% with an open circuit voltage of 0.72 V, short-circuit current density of 9.3 mA/cm2, and a fill factor of 74.5% under standard air mass 1.5 illumination. The threading dislocation density was estimated to be 3 × 108 cm−2.

Surface organization of homoepitaxial InP films grown by metalorganic vapor-phase epitaxy

We present a systematic study of the morphology of homoepitaxial InP films grown by metalorganic vapor-phase epitaxy which are imaged with ex situ atomic force microscopy. These films show a dramatic range of different surface morphologies as a function of the growth conditions and substrate (growth temperature, V/III ratio, and miscut angle < 0.6deg and orientation toward A or B sites), ranging from stable step flow to previously unreported strong step bunching, over 10 nm in height. These observations suggest a window of growth parameters for optimal quality epitaxial layers. We also present a theoretical model for these growth modes that takes account of deposition, diffusion, and dissociation of molecular precursors, and the diffusion and step incorporation of atoms released by the precursors. The experimental conditions for step flow and step bunching are reproduced by this model, with the step bunching instability caused by the difference in molecular dissociation from above and below step edges, as was discussed previously for GaAs (001).

Selective carrier injection into patterned arrays of pyramidal quantum dots for entangled photon light-emitting diodes

Scalability and foundry compatibility (as apply to conventional silicon-based integrated computer processors, for example) in developing quantum technologies are major challenges facing current research. Here we introduce a quantum photonic technology that has the potential to enable the large-scale fabrication of semiconductor-based, site-controlled, scalable arrays of electrically driven sources of polarization-entangled photons that may be able to encode quantum information. The design of the sources is based on quantum dots grown in micrometre-sized pyramidal recesses along the crystallographic direction (111)B, which theoretically ensures high symmetry of the quantum dots—a requirement for bright entangled-photon emission. A selective electric injection scheme in these non-planar structures allows a high density of light-emitting diodes to be obtained, with some producing entangled photon pairs that also violate Bells inequality. Compatibility with semiconductor fabrication technology, good reproducibility and lithographic position control make these devices attractive candidates for integrated photonic circuits for quantum information processing. Polarization-entangled photons are generated from light-emitting diodes based on site-controlled pyramidal quantum dots. Selective current injection into the vicinity of a quantum dot becomes possible owing to a self-assembled vertical quantum wire.

Conditions for entangled photon emission from (111)B site-controlled pyramidal quantum dots

A study of highly symmetric site-controlled Pyramidal In0.25Ga0.75As quantum dots (QDs) is presented. It is discussed that polarization-entangled photons can be also obtained from Pyramidal QDs of different designs from the one already reported in Juska et al. (Nat. Phot. 7, 527, 2013). Moreover, some of the limitations for a higher density of entangled photon emitters are addressed. Among these issues are (1) a remaining small fine-structure splitting and (2) an effective QD charging under non-resonant excitation conditions, which strongly reduce the number of useful biexciton-exciton recombination events. A possible solution of the charging problem is investigated exploiting a dual-wavelength excitation technique, which allows a gradual QD charge tuning from strongly negative to positive and, eventually, efficient detection of entangled photons from QDs, which would be otherwise ineffective under a single-wavelength (non-resonant) excitation.

Suppression of threading defects formation during Sb-assisted metamorphic buffer growth in InAs/InGaAs/InP structure

A virtual substrate for high quality InAs epitaxial layer has been attained via metalorganic vapor-phase epitaxy growth of Sb-assisted InxGa1−xAs metamorphic buffers, following a convex compositional continuous gradient of the In content from x = 53% to 100%. The use of trimethylantimony (or its decomposition products) as a surfactant has been found to crucially enable the control over the defect formation during the relaxation process. Moreover, an investigation of the wafer offcut-dependence of the defect formation and surface morphology has enabled the achievement of a reliably uniform growth on crystals with offcut towards the [111]B direction.

Indium segregation during III–V quantum wire and quantum dot formation on patterned substrates

We report a model for metalorganic vapor-phase epitaxy on non-planar substrates, specifically V-grooves and pyramidal recesses, which we apply to the growth of InGaAs nanostructures. This model, based on a set of coupled reaction-diffusion equations, one for each facet in the system, accounts for the facet-dependence of all kinetic processes (e.g., precursor decomposition, adatom diffusion, and adatom lifetimes) and has been previously applied to account for the temperature, concentration, and temporal-dependence of AlGaAs nanostructures on GaAs (111)B surfaces with V-grooves and pyramidal recesses. In the present study, the growth of In

Transfer Printing of AlGaInAs/InP Etched Facet Lasers to Si Substrates

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