Site-controlled Quantum Dots Research Articles

Site-controlled InAs quantum dot chains coupled to surface plasmons

Plasmonic hybrid nanostructures are material combinations where the plasmonic metal structure enables optical field confinement, while the other ingredients provide additional functionality, such as emission, absorption, or optical nonlinearity. In particular, epitaxial InAs quantum dots (QDs) embedded in a single-crystal GaAs matrix are highly efficient quantum emitters that can be integrated as plasmonic–semiconductor hybrids to realize various on-chip functions. In this Letter, we demonstrate QD–plasmon coupling in a hybrid structure consisting of site-controlled InAs/GaAs quantum dot chains (QDCs) in the proximity of an Ag film. The optical properties of the QDC–plasmon system are investigated using a cleaved-edge photoluminescence (PL) geometry, which allows us to probe the vertical and horizontal polarizations of the PL emission. We demonstrate plasmonic enhancement of both PL decay rate and vertical polarization of the PL emission with decreasing separation of the QDCs and the Ag film. The ability to couple site-controlled InAs QDCs with surface plasmons is a significant step toward exploitation of high-quality epitaxial quantum dots as gain or loss compensation in subwavelength plasmonic metal structures, such as waveguide networks, quantum plasmonic structures, and metamaterials.

Charge-tunable indium gallium nitride quantum dots

III-Nitride quantum dots have emerged as a new chip-scale system for quantum information science, which combines electrical and optical interfaces on a semiconductor chip that is compatible with non-cryogenic operating temperatures. Yet most work has been limited to optical excitations. To enable single-spin based quantum optical and quantum information research, we demonstrate here quantized charging in optically active, site-controlled III-Nitride quantum dots. Single-electron charging was confirmed by the voltage dependence of the energy, dipole moment, fine structures and polarization properties of the exciton states in the quantum dots. The fundamental energy structures of the quantum dots were identified, including neutral and charged excitons, fine structures of excitons, and A and B excitons. The results lay the ground for coherent control of single charges in III-Nitride QDs, opening a door to III-Nitride based spintronics and spin-qubit quantum information processing.

Observation of resonance fluorescence and the Mollow triplet from a coherently driven site-controlled quantum dot

Resonant excitation of solid state quantum emitters has the potential to deterministically excite a localized exciton while ensuring a maximally coherent emission. In this work, we demonstrate the coherent coupling of an exciton localized in a lithographically positioned, site-controlled semiconductor quantum dot to an external resonant laser field. For strong continuous-wave driving we observe the characteristic Mollow triplet and analyze the Rabi splitting and sideband widths as a function of driving strength and temperature. The sideband widths increase linearly with temperature and the square of the driving strength, which we explain via coupling of the exciton to longitudinal acoustic phonons. We also find an increase of the Rabi splitting with temperature, which indicates a temperature induced delocalization of the excitonic wave function resulting in an increase of the oscillator strength. Finally, we demonstrate coherent control of the exciton excited state population via pulsed resonant excitation and observe a damping of the Rabi oscillations with increasing pulse area, which is consistent with our exciton-photon coupling model. We believe that our work outlines the possibility to implement fully scalable platforms of solid state quantum emitters. The latter is one of the key prerequisites for more advanced, integrated nanophotonic quantum circuits.

Exciton dynamics in a site-controlled quantum dot coupled to a photonic crystal cavity

Exciton and cavity mode (CM) dynamics in site-controlled pyramidal quantum dots (QDs), integrated with linear photonic crystal membrane cavities, are investigated for a range of temperatures and photo-excitation power levels. The absence of spurious multi-excitonic effects, normally observed in similar structures based on self-assembled QDs, permits the observation of effects intrinsic to two-level systems embedded in a solid state matrix and interacting with optical cavity modes. The coupled exciton and CM dynamics follow the same trend, indicating that the CM is fed only by the exciton transition. The Purcell reduction of the QD and CM decay times is reproduced well by a theoretical model that includes exciton linewidth broadening and temperature dependent non-radiative processes, from which we extract a Purcell factor of 17 ± 5. For excitation powers above QD saturation, we show the influence of quantum wire barrier states at short delay time, and demonstrate the absence of multiexcitonic background emission.

Elliptical quantum dots as on-demand single photons sources with deterministic polarization states

In quantum information, control of the single photon's polarization is essential. Here, we demonstrate single photon generation in a pre-programmed and deterministic polarization state, on a chip-scale platform, utilizing site-controlled elliptical quantum dots (QDs) synthesized by a top-down approach. The polarization from the QD emission is found to be linear with a high degree of linear polarization and parallel to the long axis of the ellipse. Single photon emission with orthogonal polarizations is achieved, and the dependence of the degree of linear polarization on the QD geometry is analyzed.

Site-controlled quantum dots coupled to a photonic crystal molecule

Two site-controlled quantum dots (QDs) were integrated in a photonic crystal molecule (PCM) formed by L3 nanocavities. A statistical analysis of the coupled cavity modes demonstrated the formation of bonding and anti-bonding delocalized PCM states. Excitonic transitions belonging to each QD were identified by scanning micro-photoluminescence spectroscopy. Co-polarization of the QDs photoluminescence with the coupled cavity modes provides evidence for the simultaneous coupling of two spatially separated QDs to the same PCM mode.

Spectral diffusion and its influence on the emission linewidths of site-controlled GaN nanowire quantum dots

Fourier transform spectroscopy is used to determine/control the degree of spectral diffusion in high-quality site-controlled GaN nanowire quantum dots. Detailed analysis, including the development of a statistical model and Monte Carlo simulations, provides evidence that the broadening is caused by photoinduced excitation of defects. Furthermore, performing the experiment under weak excitation allows for an estimate of the homogeneous linewidth to be made $(135\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{eV})$. The origins of this linewidth are discussed, and the existence of an alternate dephasing mechanism is inferred. A limit on the emission linewidth at zero excitation is also discussed.

Single pairs of time-bin-entangled photons

Time-bin-entangled photons are ideal for long-distance quantum communication via optical fibers. Here we present a source where, even at high creation rates, each excitation pulse generates, at most, one time-bin-entangled pair. This is important for the accuracy and security of quantum communication. Our site-controlled quantum dot generates single polarization-entangled photon pairs, which are then converted, without loss of entanglement strength, into single time-bin-entangled photon pairs.

Plasmonic Enhancement of Single Photon Emission from a Site-Controlled Quantum Dot

We showed enhanced single photon emission via exciton–plasmon coupling in a metal-encapsulated site-controlled quantum dot (QD) structure. It was observed that the average QD luminescence was enhanced by a factor of 7.33 ± 1.32 and the exciton lifetime was reduced by a factor of 12.8 ± 1.10. The exciton–plasmon coupling was enhanced by matching the exciton energy to the localized surface plasmon resonance. Due to the sensitivity of the plasmonic enhancement to dot-to-dot variations, measurements were performed on the same dot at different stages of the sample processing, enabled by the use of a novel site-controlled InGaN QD structure. This was then repeated over a large number of dots and compared between different metal materials to investigate the nature of the coupling.

Polarization properties and disorder effects in H3 photonic crystal cavities incorporating site-controlled, high-symmetry quantum dot arrays

We report on the effects of optical disorder on breaking the symmetry of the cavity modes of H3 photonic crystal cavities incorporating site-controlled pyramidal quantum dots (QDs) as the internal light source. The high in-plane symmetry of the polarization states of the pyramidal QDs simplifies the analysis of the polarization states of the H3 cavities. It is shown that the optical disorder induced by fabrication imperfections lifts the degeneracy of the two quadrupole cavity modes and tilts the elongation axes of the cavity mode patterns with respect to the ideal, hexagonal symmetry case. These results are useful for designing QD-cavity structures for polarization-entangled photon sources and few-QD lasers.

Optical properties of individual site-controlled Ge quantum dots

We report photoluminescence (PL) experiments on individual SiGe quantum dots (QDs) that were epitaxially grown in a site-controlled fashion on pre-patterned Si(001) substrates. We demonstrate that the PL line-widths of single QDs decrease with excitation power to about 16 meV, a value that is much narrower than any of the previously reported PL signals in the SiGe/Si heterosystem. At low temperatures, the PL-intensity becomes limited by a 25 meV high potential-barrier between the QDs and the surrounding Ge wetting layer (WL). This barrier impedes QD filling from the WL which collects and traps most of the optically excited holes in this type-II heterosystem.

III–V site-controlled quantum dots on Si patterned by nanoimprint lithography

We have successfully grown regular arrays of InAs/GaAs quantum dots on patterned Si substrates. Thanks to the capability of nanoimprint lithography, we were able to obtain uniform patterns extended over some cm2 areas, with periods of 300nm. Ex-situ and in-situ treatments of the surface allowed us to completely remove any residual oxides prior to growth without the use of hydrogen beams, at temperatures compatible with standard III–V molecular beam epitaxy. The growth protocol was optimized in order to obtain a perfect selectivity of InAs/GaAs nanostructures in the holes, without any deposition on the planar areas.

The Role of Groove Periodicity in the Formation of Site-Controlled Quantum Dot Chains.

Structural and optical properties of InAs quantum dot (QD) chains formed in etched GaAs grooves having different periods from 200 to 2000 nm in [010] orientation are reported. The site-controlled QDs were fabricated by molecular beam epitaxy on soft UV-nanoimprint lithography-patterned GaAs(001) surfaces. Increasing the groove periods decreases the overall QD density but increases the QD size and the linear density along the groove direction. The effect of the increased QD size with larger periods is reflected in ensemble photoluminescence measurements as redshift of the QD emission. Furthermore, we demonstrate the photoluminescence emission from single QD chains.

Memristive operation mode of a site-controlled quantum dot floating gate transistor

We have realized a floating gate transistor based on a GaAs/AlGaAs heterostructure with site-controlled InAs quantum dots. By short-circuiting the source contact with the lateral gates and performing closed voltage sweep cycles, we observe a memristive operation mode with pinched hysteresis loops and two clearly distinguishable conductive states. The conductance depends on the quantum dot charge which can be altered in a controllable manner by the voltage value and time interval spent in the charging region. The quantum dot memristor has the potential to realize artificial synapses in a state-of-the-art opto-electronic semiconductor platform by charge localization and Coulomb coupling.

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.

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. Photonics 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.

Probing the excitonic states of site-controlled GaN nanowire quantum dots.

We report the detection of fully confined excited states and the zero-absorption region of individual site-controlled GaN/AlGaN nanowire quantum dots using photoluminescence excitation spectroscopy, which provides evidence of the true zero-dimensional discrete density of states of such quantum dots. Because of the strong quantum confinement in these dots, the p-shell, d-shell, and even higher energy (including some f-shell) states of a single quantum dot are observed, which provides unprecedented insight into the electronic structure. Several emitters are measured and used to build up an average picture of the electronic structure of a single quantum dot via comparison to theoretical simulations.

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.

Integration of multiple site-controlled pyramidal quantum dot systems with photonic-crystal membrane cavities

We describe the fabrication, epitaxial growth and photoluminescence (PL) spectra of systems of site-controlled pyramidal quantum dots (QDs) integrated with photonic crystal (PhC) membrane cavities. The spectra of the individual QDs are characterized by scanning micro-PL spectroscopy, and the cavity modes are identified by polarization-resolved PL measurements and modeling. Weak coupling of systems incorporating up to four identifiable QDs with single modes of the PhC cavities is demonstrated.

Linearly polarized single photon antibunching from a site-controlled InGaN quantum dot

We report on the observation of linearly polarized single photon antibunching in the excitonic emission from a site-controlled InGaN quantum dot. The measured second order coherence function exhibits a significant dip at zero time difference, corresponding to gm2(0)=0.90 under continuous laser excitation. This relatively high value of gm2(0) is well understood by a model as the combination of short exciton life time (320 ps), limited experimental timing resolution and the presence of an uncorrelated broadband background emission from the sample. Our result provides the first rigorous evidence of InGaN quantum dot formation on hexagonal GaN pyramids, and it highlights a great potential in these dots as fast polarized single photon emitters if the background emission can be eliminated.