Spectral signatures of high-symmetry quantum dots and effects of symmetry breaking

Abstract

High symmetry epitaxial quantum dots (QDs) with three or more symmetry planes provide a very promising route for the generation of entangled photons for quantum information applications. The great challenge to fabricate nanoscopic high symmetry QDs is further complicated by the lack of structural characterization techniques able to resolve small symmetry breaking. In this work, we present an approach for identifying and analyzing the signatures of symmetry breaking in the optical spectra of QDs. Exciton complexes in InGaAs/AlGaAs QDs grown along the [111]B crystalline axis in inverted tetrahedral pyramids are studied by polarization resolved photoluminescence spectroscopy combined with lattice temperature dependence, excitation power dependence and temporal photon correlation measurements. By combining such a systematic experimental approach with a simple theoretical approach based on a point-group symmetry analysis of the polarized emission patterns of each exciton complex, we demonstrate that it is possible to achieve a strict and coherent identification of all the observable spectral patterns of numerous exciton complexes and a quantitative determination of the fine structure splittings of their quantum states. This analysis is found to be particularly powerful for selecting QDs with the highest degree of symmetry (C 3v and D h 3 ) for potential applications of these QDs as polarization entangled photon sources. We exhibit the optical spectra when evolving towards asymmetrical QDs, and show the higher sensitivity of certain exciton complexes to symmetry breaking.

Single- and Twin-Photons Emitted from Fiber-Coupled Quantum Dots in a Distributed Bragg Reflector Cavity

“…There is an additional X line, X Z , from lh h 1 polarized in z -axis coexisting with the traditional X lines (power-dependent spectra in Figure 2 c and that of QD3 and QD1 in Figure 3 ), similar to a strain-tuned GaAs QD [ 18 ]; the energy offset between X Z and the traditional X lines reflects e-h separation, Stark shift and donor field intensity: 0.18 meV in QD3 ( Figure 3 a) in a lower field and 1.68 meV in QD2 ( Figure 2 ) and 1.89 meV in QD1 ( Figure 3 b) in a higher field, consistent with monolithic increase of FSS from ~4 μeV in QD3 to ~6–12 μeV in QD2 and 35 μeV in QD1, due to lh h 1 more confined with more anisotropy in the donor field. In contrast, lh h 2 is more delocalized: in QD3 in a lower field with lh h 2 coupled to wetting layer, a fast h 2 –h 1 decay is expected and XXī 1 (2e 1 1h 1 1h 2 ) shows considerable intensity in a broad linewidth from a fast h 2 –h 1 decay of its transition target X 01 (1e 1 1h 2 ); QD1 in a much stronger field with lh h 2 decoupled from wetting layer and confined in QD shows higher excitons related to h 2 such as X 1 ī + , Xī 1 + (1e 1 1h 2 1h 2 ), X 0 ī, XXī 1 , and XX 1 ī [ 19 ]—located around XX [ 11 ] with D 3h symmetric spectral features and slow h 2 –h 1 decay, unlike C 2v featured X and XX with large FSS ~35 μeV; the slow h 2 –h 1 decay is likely from their spatial distribution as the model in Figure 3 b inset indicates: the stress at QD base with large strain distribution [ 20 ] forms lh h 2 strongly confined there in the donor field and leads to lh h 1 being more confined (from h 2 repulsion) for larger V hh and slower h 2 –h 1 decay; the more confined h 1 contains more anisotropy for larger FSS in XX and X; in QD2 in a high field with lh h 2 gradually decoupled from wetting layer by epoxy thermal stress during cryogen circles (see spectra under 2nd and N -th cryogen circles, Figure 2 a inset), lh h 1 gets more confined to show increased e 1 –h 1 overlap for shaper X Z , FSS raising from 6 to 12 μeV (i.e., e 1 –h 1 overlap increased), X 2+ , XX + and XXX blue-shift slightly (i.e., an increased h 1 –h 2 Coulomb interaction). In QD1 in a stronger field with lh h 1 more confined, a shape X Z , a large FSS and an increased h 1 -h 1 Coulomb interaction V hh to enlarge negative XX binding energy E B (XX) = 2V eh − V ee − V hh [ 21 …”

Symmetric Excitons in an (001)-Based InAs/GaAs Quantum Dot Near Si Dopant for Photon-Pair Entanglement

“…Figure 2 presents the QDs with D3h symmetric exciton emissions in a spectral feature as pyramidal (111) QDs [16,17] from valence band mixing and lh h2 for e1-h2 transitions XX2ī + , X1ī + and XX1ī and e1-h1 ones 21 + , Xī1 + and XXī1 (indexes mean the hole numbers in h1 (first) and h2 (second) with a bar classifying the transitions) [16,33], located around XX as a mirror. XX2ī + singlet built a cascade with X + as the bunching peak in their cross-correlation reflects, weaker than before [33] due to a fast electron capture in X + to populate XX; X1ī + doublet (split by Δhh ± (Δ I eh + Δ II eh): 150 μeV in QD1 or 50 μeV in QD2) and XX1ī doublet (split by Δhh−Δ 0 eh [16] of near zero); a broad XXT -(doublet split by ~Δ 0 eh/2 [33]) also appeared. In QD1, the three peaks in XXī1 were well depicted by D3h transition diagram (σpolarized) while the three peaks in 21 + with the branch appearing to reflect C3v [16], the lack of a mirror symmetry; in QD2, both 21 + and XXī1 showed one peak from D3h (z-polarized) [16].…”

“…XX2ī + singlet built a cascade with X + as the bunching peak in their cross-correlation reflects, weaker than before [33] due to a fast electron capture in X + to populate XX; X1ī + doublet (split by Δhh ± (Δ I eh + Δ II eh): 150 μeV in QD1 or 50 μeV in QD2) and XX1ī doublet (split by Δhh−Δ 0 eh [16] of near zero); a broad XXT -(doublet split by ~Δ 0 eh/2 [33]) also appeared. In QD1, the three peaks in XXī1 were well depicted by D3h transition diagram (σpolarized) while the three peaks in 21 + with the branch appearing to reflect C3v [16], the lack of a mirror symmetry; in QD2, both 21 + and XXī1 showed one peak from D3h (z-polarized) [16]. In QD24, the higher field showed a relatively lower emission intensity and made these peaks a little far from XX that has a negative binding energy.…”

“…For strain-driven (001)-based InAs QDs with a flexible wavelength (λ), 870~1600 nm [5][6][7][8], structures (e.g., strain-coupled QD bilayer [5], dot-in-barrier [6], or dot-in-well [7]) and microcavity integration, it shows FSS > 10 μeV [9][10][11][12] typically and spin scattering time Tss ~1.9 ns [2]. The FSS can be reduced by growing C3v QD on (111) surface [13][14][15][16][17] or a post-grown electric field (different Stark shifts for X1 and X2) [18,19] or stress [10,11,20,21] tuning. Due to a deviation of X1 and X2 from the main crystal axis [110] and [1][2][3][4][5][6][7][8][9][10], the erasure of FSS requires a biaxial stress (in-plane) tuning or a combination of stress (in-plane) and electric field (vertical) tuning.…”

“…These parameters are well controlled in the sacrificed-QD-layer growth method we developed to obtain QDs with isotropic geometry. In this work, individual Si dopant intentionally added above a QD proves a new method to build a local electric field for electron-hole (eh) separation in the [001] axis to show D3h symmetric excitons as pyramidal (111) QDs [16,17] and reduce FSS to zero (in an as-grown sample with no fine tuning, the lowest FSS obtained is 3.9 μeV with the lowest-energy X state (LX) anticlockwise rotate from [1][2][3][4][5][6][7][8][9][10], i.e., zero FSS will be crossed in a proper field). QDs show small lh-hh mixing degree: β ≈ 0 with no doping or in a vertical electric field, and β ≈ 0.2 (close to strain-free GaAs QDs, implying no strain contribution) in an in-plane field.…”