Small Fine-structure Splitting Research Articles

Fine structure splitting cancellation in highly asymmetric InAs/InP droplet epitaxy quantum dots

We find the fine structure splitting (FSS) of a single exciton, which splits its degenerate ground-state manifold into singlets, nearly vanishes in highly asymmetric quantum dots (QDs) due to the cancellation of splitting effects with markedly different origin. The dots simulated are those that emerge on top of etch pits through the droplet epitaxy growth process; these etch pit dots break square (C4v) spatial symmetry, which has been previously associated with small FSS. Configuration interaction calculations predict a vanishing FSS at a specific finite etch pit displacement from the center of the dot, for a structure far from square symmetry. We thus predict that highly asymmetric QDs may still display negligible FSS, providing avenues for high-fidelity generation of indistinguishable, polarization entangled photon pairs on demand. Published by the American Physical Society 2024

Light Hole Excitons in Strain-Coupled Bilayer Quantum Dots with Small Fine-Structure Splitting

In this work, we measure polarization-resolved photoluminescence spectra from excitonic complexes in tens of single InAs/GaAs quantum dots (QDs) at the telecom O-band with strain-coupled bilayer structure. QDs often show fine-structure splitting (FSS) ~100 μeV in uniform anisotropy and valence-band mixing of heavy holes (HH) and light holes (LH); the biaxial strain also induces LH excitons with small FSS (especially XX, <5 μeV, 70% of QDs); delocalized LH reduces the Coulomb interaction between holes Vhh and enhances population on LH excitons XX, XX11, X11+ and XX21+.

Single-photon generation from self-assembled GaAs/InAlAs(111)A quantum dots with ultrasmall fine-structure splitting

We present a novel semiconductor single-photon source based on tensile-strained (111)-oriented GaAs/InAlAs quantum dots (QDs) exhibiting ultrasmall exciton fine-structure splitting (FSS) of ≤ 8 µeV. Using low-temperature micro-photoluminescence spectroscopy, we identify the biexciton-exciton radiative cascade from individual QDs, which, combined with small FSS, indicates these self-assembled GaAs(111) QDs are excellent candidates for polarization-entangled photon-pair generation.

Highly uniform and symmetric epitaxial InAs quantum dots embedded inside Indium droplet etched nanoholes

III−V semiconductor quantum dots (QDs) obtained by local droplet etching technology provide a material platform for generation of non-classic light. However, using this technique to fabricate single emitters for a broad spectral range remains a significant challenge. Herein, we successfully extend the QD emission wavelength to 850–880 nm via highly uniform and symmetric InAs QDs located inside indium-droplet-etching nanoholes. The evolution of InGaAs nanostructures by high temperature indium droplet epitaxy on GaAs substrate is revealed. By carefully designing the appropriate growth conditions, symmetric QDs with the a small fine structure splitting of only ∼4.4 ± 0.8 μeV are demonstrated. Averaging over the emission energies of 32 QDs, an ensemble broadening of 12 meV is observed. Individual QDs are shown to emit nonclassically with clear evidence of photon antibunching. These highly uniform and symmetric nanostructures represent a very promising novel strategy for quantum information applications.

Universal Growth Scheme for Quantum Dots with Low Fine-Structure Splitting at Various Emission Wavelengths

Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fibre optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure-splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into 1D optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realise QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs droplet-epitaxy of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at ~900nm, and metal-organic vapour phase epitaxy on InP emitting at 1550 nm, with mean FSS 4x smaller than for Stranski-Krastanow QDs.

Telecom wavelength emitting single quantum dots coupled to InP-based photonic crystal microcavities

Here we report on the fabrication and optical characterization of InP-based L3 photonic crystal (PhC) microcavities embedded with a medium density InAs/InP quantum dots (QDs) emitting at telecom wavelengths. The QDs are grown by solid source molecular beam epitaxy using a ripening technique. Micro-photoluminescence (μ-PL) measurements of PhC samples reveal sharp cavity modes with quality factors exceeding 8500. QDs emit highly linear-polarized light at telecom wavelengths with resolution-limited spectral linewidth below 50 μeV. Enhanced PL intensity of QDs in PhC is observed in comparison to the PL intensity of QDs in bulk semiconductors. The combination of excitation power-dependent and polarization-resolved μ-PL measurements reveal the existence of an exciton-biexciton system with a small fine-structure splitting.

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.

Highly uniform and strain-free GaAs quantum dots fabricated by filling of self-assembled nanoholes

We demonstrate the self-assembled creation of a novel type of strain-free semiconductor quantum dot (QD) by local droplet etching (LDE) with Al to form nanoholes in AlGaAs or AlAs surfaces and subsequent filling with GaAs. Since the holes are filled with a precisely defined filling level, we achieve ultrauniform LDE QD ensembles with extremely narrow photoluminescence (PL) linewidth of less than 10 meV. The PL peaks agree with a slightly anisotropic parabolic potential. Small QDs reveal indications for transitions between electron and hole states with different quantization numbers. For large QDs, a very small fine-structure splitting is observed.

Highly Reduced Fine-Structure Splitting inInAs/InPQuantum Dots Offering an Efficient On-Demand Entangled 1.55-μmPhoton Emitter

To generate entangled photon pairs via quantum dots (QDs), the exciton fine-structure splitting (FSS) must be comparable to the exciton homogeneous linewidth. Yet in the (In,Ga)As/GaAs QD, the intrinsic FSS is about a few tens microeV. To achieve photon entanglement, it is necessary to cherry-pick a sample with extremely small FSS from a large number of samples or to apply a strong in-plane magnetic field. Using theoretical modeling of the fundamental causes of FSS in QDs, we predict that the intrinsic FSS of InAs/InP QDs is an order of magnitude smaller than that of InAs/GaAs dots, and, better yet, their excitonic gap matches the 1.55 microm fiber optic wavelength and, therefore, offers efficient on-demand entangled photon emitters for long distance quantum communication.

Hyperfine structure in the 6snh (l = 5) Rydberg series of barium

The hyperfine structure in the odd-parity 6snh (9 ≤ n ≤ 40) Rydberg series of barium has been investigated, using a single cw ring dye laser and a beam of neutral atoms in the metastable 5d2 1G4 state. Owing to small fine-structure splitting, this hyperfine structure is dominated by the Fermi contact interaction of the 6s electron already at n = 12. The hyperfine-structure analysis results in a determination of the complete fine structure of the 6snh series as well as in values for the singlet–triplet mixing in the 6snh J =5 states. The 6snh series turn out to be perturbed by levels of the 5d4f configuration; J = 4 and J = 5 levels of this configuration have been observed. The Rydberg series perturbations are analyzed, using multichannel quantum-defect theory in the shifted R-matrix formalism. Far from the 5d4f perturbers, the 6snh configuration is almost purely jj coupled.