Optical and electronic properties of low-density InAs/InP quantum dot-like structures devoted to single-photon emitters at telecom wavelengths

Due to their band-structure and optical properties, InAs/InP quantum dots (QDs) constitute a promising system for single-photon generation at third telecom window of silica fibers and for applications in quantum communication networks. However, obtaining the necessary low in-plane density of emitters remains a challenge. Such structures are also still less explored than their InAs/GaAs counterparts regarding optical properties of confined carriers. Here, we report on the growth via metal-organic vapor phase epitaxy and investigation of low-density InAs/InP QD-like structures, emitting in the range of 1.2-1.7 ${\mu}$m, which includes the S, C, and L bands of the third optical window. We observe multiple photoluminescence (PL) peaks originating from flat QDs with height of small integer numbers of material monolayers. Temperature-dependent PL reveals redistribution of carriers between families of QDs. Via time-resolved PL, we obtain radiative lifetimes nearly independent of emission energy in contrast to previous reports on InAs/InP QDs, which we attribute to strongly height-dependent electron-hole correlations. Additionally, we observe neutral and charged exciton emission from spatially isolated emitters. Using the 8-band k${\cdot}$p model and configuration-interaction method, we successfully reproduce energies of emission lines, the dispersion of exciton lifetimes, carrier activation energies, as well as the biexciton binding energy, which allows for a detailed and comprehensive analysis of the underlying physics.Comment: 13 pages, 9 figure

Multipole plasmons and their disappearance in few-nanometre silver nanoparticles

Electron energy-loss spectroscopy can be used for detailed spatial and spectral characterization of optical excitations in metal nanoparticles. In previous electron energy-loss experiments on silver nanoparticles with radii smaller than 20 nm, only the dipolar surface plasmon resonance was assumed to play a role. Here, applying electron energy-loss spectroscopy to individual silver nanoparticles encapsulated in silicon nitride, we observe besides the usual dipole resonance an additional surface plasmon resonance corresponding to higher angular momenta for nanoparticle radii as small as 4 nm. We study the radius and electron beam impact position dependence of both resonances separately. For particles smaller than 4 nm in radius the higher-order surface plasmon mode disappears, in agreement with generalized non-local optical response theory, while the dipole resonance blueshift exceeds our theoretical predictions. Unlike in optical spectra, multipole surface plasmons are important in electron energy-loss spectra even of ultrasmall metallic nanoparticles

Supplemental data 1 Shape analysis

The diameters of our nanoparticles are determined by using the free online image analysis tool ImageJ [1] which includes a particle analysis package. We use the 2D images taken in STEM mode to measure the surface area A of the nanoparticle, whereafter we determine the mean nanoparticle diameter D using the relation A = π ( D /2) 2 . The particle analysis tool also evaluates the maximum D max and minimum D min diameters of the nanoparticle and the difference between these two diameters, i.e., Δ D = D max -D min provides us a measure for error in the nanoparticle diameter (shown as the error bar in Article Supplementary In order to understand the scattering of the SP resonance energies observed in Articl

Persistent template effect in InAs/GaAs quantum dot bilayers

The dependence of the optical properties of InAs/GaAs quantum dot(QD) bilayers on seed layer growth temperature and second layer InAs coverage is investigated. As the seed layer growth temperature is increased, a low density of large QDs is obtained. This results in a concomitant increase in dot size in the second layer, which extends their emission wavelength, reaching a saturation value of around 1400 nm at room temperature for GaAs-capped bilayers. Capping the second dot layer with InGaAs results in a further extension of the emission wavelength, to 1515 nm at room temperature with a narrow linewidth of 22 meV. Addition of more InAs to high density bilayers does not result in a significant extension of emission wavelength as most additional material migrates to coalesced InAs islands but, in contrast to single layers, a substantial population of regular QDs remains

The Substrate Effect in Electron Energy-Loss Spectroscopy of Localized Surface Plasmons in Gold and Silver Nanoparticles

Electron energy-loss spectroscopy (EELS) has become increasingly popular for detailed characterization of plasmonic nanostructures, owing to the unparalleled spatial resolution of this technique. The typical setup in EELS requires nanoparticles to be supported on thin substrates. However, as in optical measurements, the substrate material can modify the acquired signal. Here, we have investigated how the EELS signal recorded from supported silver and gold spheroidal nanoparticles at different electron beam impact parameter positions is affected by the choice of a dielectric substrate material and thickness. Consistent with previous optical studies, the presence of a dielectric substrate is found to red-shift localized surface plasmons, increase their line widths, and lead to increased prominence of higher order modes. The extent of these modifications heightens with increasing substrate permittivity and thickness. Specific to EELS, the results highlight the importance of the beam impact parameter and substrate-related \u10cerenkov losses and charging. Our experimental results are compared with and corroborated by full-wave electromagnetic simulations based on the boundary element method. The results present a comprehensive study of substrate-induced modifications in EELS and allow identification of optimal substrates relevant for EELS studies of plasmonic structures

A valence force field-Monte Carlo algorithm for quantum dot growth modeling

We present a novel kinetic Monte Carlo version for the atomistic valence force fields algorithm in order to model a self-assembled quantum dot growth process. We show our atomistic model is both computationally favorable and capture more details compared to traditional kinetic Monte Carlo models based on continuum elastic models. We anticipate the model will be useful to experimentalists in understanding better the growth dynamics of quantum dot systems

Optical Property-Composition Correlation in Noble Metal Alloy Nanoparticles Studied with EELS

Noble metals are currently the most common building blocks in plasmonics and thus define the available range of optical properties. Their alloying provides a viable strategy to engineer new materials with a tunable range of optical responses. Despite this attractive prospect, the link between composition and optical properties of many noble metal alloys is still not well understood. Here, electron energy-loss spectroscopy is employed to systematically study AuAg and AuPd nanoparticles of varying compositions. The localized surface plasmons, the bulk plasmons, and the permittivity functions of these two sets of alloys are investigated as functions of their composition. In the case of the more widely studied AuAg alloy system, good agreement is found with previous experimental and theoretical studies. The results on the less scrutinized AuPd system provide highly valuable experimental data that complements other experimental investigations and supports the development of theoretical models

Towards quantitative three-dimensional characterisation of buried InAs quantum dots

InAs quantum dots grown on InP or InGaAsP are used for optical communication applications operating in the 1.3 - 1.55 μm wavelength range. It is generally understood that the optical properties of such dots are highly dependent on their structural and chemical profiles. However, morphological and compositional measurements of quantum dots using transmission electron microscopy can be ambiguous because the recorded signal is usually a projection through the thickness of the specimen. Here, we discuss the application of scanning transmission electron microscopy tomography to the morphological and chemical characterisation of surface and buried quantum dots. We highlight some of the challenges involved and introduce a new specimen preparation method for creating needle-shaped specimens that each contain multiple dots and are suitable for both scanning transmission electron microscopy tomography and atom probe tomography