Circular polarization of luminescence caused by the current in quantum wells

The degree of circular polarization of photoluminescence from an n-type III–V-based [001] quantum well (QW) is calculated under an electric current flow in the well plane. It is shown that mixing of the states of light and heavy holes leads to circular polarization of photoluminescence during the propagation of light in the plane of the structure. The role of various terms that are linear in the wave vector in the electron energy spectrum is analyzed for the effects of spin orientation and emergence of circular polarization of radiation in the electric field

The manifestation of Coulomb gap in photoluminescence of GaAs/AlGaAs quantum wells with positively charged acceptors

We reportresultsofphotoluminescenceandphotoexcitationstudiescarriedoutonGaAs/AlGaAsquantum well structureswithpositivelychargedacceptors.We explaintheobservedspectralpeculiaritiesbyan appearanceoftheCoulombgap.Anotherevidenceofthe CoulombgapisananomalouslylargeStocksshift which hasbeenobservedinourexperiments

Extended electron states in lateral quantum dot molecules investigated with photoluminescence

InAs quantum dot molecules (QDMs) formed by molecular-beam epitaxy on GaAs (311)B substrates through self-organized anisotropic strain engineering are studied by excitation-power-density- and temperature-dependent macro- and microphotoluminescence (PL). An unusual asymmetric broadening, together with a continuous shift toward higher energies of the PL peak position, most prominent for the p-type modulation-doped QDMs, is observed with increasing excitation power density. The n-type modulation-doped QDMs exhibit a square-shaped PL spectrum, resembling that of modulation-doped quantum wells. In temperature-dependent macro-PL, two distinct minima of the full width at half maximum are observed, indicating thermally activated carrier redistribution within the QDMs through two different channels at lower and higher temperatures. The micro-PL spectra of the p-type modulation-doped QDMs exhibit discrete sets of sharp peaks on top of broad PL bands. The number and intensity of the sharp peaks increase with excitation power density. With increasing temperature, the number and intensity of the sharp peaks decrease while the intensity of the broad PL bands increases, in agreement with the carrier redistribution at lower temperatures. Only broad PL bands are observed for the n-type modulation-doped QDMs with similar behavior. These results are explained by state filling in the presence of extended electron states formed due to lateral electronic coupling of the quantum dots within the QDMs

Polarization of tunneling-assisted recombination of two-dimensional holes

\u3cp\u3eWe have studied the polarization anisotropy of the radiative recombination of a two-dimensional hole gas (2DHG) confined at the interface of a modulation-doped AlxGa1-xAs/GaAs heterostructure. The observed ''cleaved-side'' luminescence from the heavy-hole ground subband is due to indirect optical transitions in real space and is found to be polarized perpendicular to the plane of the 2DHG. This unexpected behavior is ascribed to the admixture of light-hole states to the heavy-hole ground subband. For increasing in-plane wave vector, the degree of linear polarization decreases due to an enhanced penetration of the heavy-hole states into the bulk GaAs layer. These results can be qualitatively explained by our calculations.\u3c/p\u3

Spin-orbit-induced circulating currents in a semiconductor nanostructure

Circulating orbital currents produced by the spin-orbit interaction for a single electron spin in a quantum dot are explicitly evaluated at zero magnetic field, along with their effect on the total magnetic moment (spin and orbital) of the electron spin. The currents are dominated by coherent superpositions of the conduction and valence envelope functions of the electronic state, are smoothly varying within the quantum dot, and are peaked roughly halfway between the dot center and edge. Thus the spatial structure of the spin contribution to the magnetic moment (which is peaked at the dot center) differs greatly from the spatial structure of the orbital contribution. Even when the spin and orbital magnetic moments cancel (for g ¼ 0) the spin can interact strongly with local magnetic fields, e.g., from other spins, which has implications for spin lifetimes and spin manipulation

Growth of InGaAsN/InP structures by chemical beam epitaxy

In III-V semiconductors incorporation of small amounts of nitrogen cause a relatively large bandgap reduction. The problem of InGaAsN is the decrease in luminescence with nitrogen content above 1 - 2%.We have grown a.o. In0.65Ga0.35As0.985N0.015 quantum wells. The as-grown samples show poor photoluminescence at 4K but after a high temperature anneal clear peaks are observed. In comparision to ternary layers of In0.65Ga0.35As, the bandgap has decreased by approximately 70 meV due to 1.4 % of nitrogen. A further increase in the nitrogen content could result in layers with a bandgap of 800 meV at room temperature (l = 1.55 mm)

Design of composite InAsP/InGaAs quantum wells for a 1.55 mu-m polarization independent semiconductor optical amplifier

We investigate a composite InAsP/InGaAs quantum well in which an 8 nm tensile strained InGaAs well is surrounded by two compressively strained InAsP layers which feature a 70:30 conduction band offset ratio. The composite quantum well is found to provide a high TM differential gain. The InAsP layers provide strain compensation while simultaneously shifting the band gap to the relevant 1.55 µm wavelength region and increasing the electron confinement. Composite InAsP/InGaAs quantum wells are a promising candidate for realizing a polarization independent semiconductor optical amplifier at 1.55 µm

Separate electron-hole confinement in composite InAsyP 1-y/Ga xIn1-xAs quantum wells

Composite double qunatum wells made from materials with a type-II band line-up have been grown to realize separate confinement in real space for electrons and holes. We have observed a substantial blue shift of the lowest energy transition in such composite double quantum wells. The photocurrent measurements demonstrate a linear Stark shift due to the separate confinement in real space for electrons and holes. The charge separation is up to 45 Å in the strain balanced InAs0.42P0.58/Ga0.67In0.33As samples. The experimental results agree very well with calculations in the framework of Bir-Pikus strain Hamiltonian

Anisotropy of electron and hole g tensors of quantum dots: An intuitive picture based on spin-correlated orbital currents

Using single spins in semiconductor quantum dots as qubits requires full control over the spin state. As the g tensor provides the coupling in a Hamiltonian between a spin and an external magnetic field, a deeper understanding of the g tensor underlies magnetic-field control of the spin. The g tensor is affected by the presence of spin-correlated orbital currents, of which the spatial structure has been recently clarified. Here we extend that framework to investigate the influence of the shape of quantum dots on the anisotropy of the electron g tensor. We find that the spin-correlated orbital currents form a simple current loop perpendicular to the magnetic moment’s orientation. The current loop is therefore directly sensitive to the shape of the nanostructure: for cylindrical\u3cbr/\u3equantum dots, the electron g-tensor anisotropy is mainly governed by the aspect ratio of the dots. Through a systematic experimental study of the size dependence of the separate electron and hole g tensors of InAs/InP quantum dots, we have validated this picture. Moreover, we find that through size engineering it is possible to independently change the sign of the in-plane and growth direction electron g factors. The hole g tensor is found to be strongly anisotropic and very sensitive to the radius and elongation. The comparable importance of itinerant and localized currents to the hole g tensor complicates the analysis relative to the electron g tensor

Controlling mode degeneracy in a photonic crystal nanocavity with infiltrated liquid crystal

We demonstrate the control of the mode degeneracy when a liquid crystal (LC) is infiltrated into an InGaAsP membrane photonic crystal nanocavity with embedded InAs quantum dots. Mode splitting exists in the anisotropic nematic LC state, and not in the unfilled or isotropic LC state. The degeneracy lifting of the quadrupole mode is attributed to the different interactions of the two orthogonal basis modes of the degenerate mode with the two components of the refractive index of the LC. The interpretation is supported by the quantitative agreement between the experimental results and the three-dimensional finite-difference time-domain computations.The authors acknowledge the support from the BrainBridge project (ZJU-TU/e and Philips Research collaboration)