Band Mixing Research Articles

Symmetric analysis, categorization, and optical spectrum of ideal pyramid quantum dots

Self-assembled quantum dots possess an intrinsic geometric symmetry. Applying group representation theory, we systematically analyze the symmetric properties of the bound states for ideal pyramid quantum dots, which neglect band mixing and strain effects. We label each bound state by its symmetry group’s corresponding irreducible representation and define a concept called the quantum dots’ symmetry category. A class of quantum dots with the same irreducible representation sequence of bound states are characterized as belonging to a specific symmetry category. This category concept generally describes the symmetric order of Hilbert space or wavefunction space. We clearly identify the connection between the symmetry category and the geometry of quantum dots by the symmetry category graph or map. The symmetry category change or transition corresponds to an accidental degeneracy of the bound states. The symmetry category and category transition are observable from the photocurrent spectroscopy or optical spectrum. For simplicity’s sake, in this paper, we only focus on inter-subband transition spectra, but the methodology can be extended to the inter-band transition cases. We predict that from the spectral measurements, the quantum dots’ geometric information may be inversely extracted.

Control of photoelectron momentum distributions by bichromatic polarization-shaped laser fields

Bichromatic polarization-shaped femtosecond laser pulses are used to control three-dimensional photoelectron momentum distributions (3D-EDs) from resonance enhanced multi-photon ionization of potassium atoms. The light fields consisting of two spectral bands with different ellipticity are produced using an ultrafast polarization pulse shaper equipped with a custom polarizer in the Fourier plane. The tomographically reconstructed 3D-EDs from ionization with counterrotating circularly or orthogonal linearly polarized bichromatic laser pulses show different angular momentum superposition states at four distinct photoelectron energies. The analysis of the measured 3D-EDs reveals that the underlying physical mechanism is based on the interplay of ionization pathway selection via quantum mechanical selection rules for optical transitions and intrapulse frequency mixing of the spectral bands with different ellipticity.

Fano resonance via bonding and antibonding states in nonadiabatically-pumped double-quantum-well systems

In this work, transport properties of the nonadiabatically pumped double-quantum-well (DQW) structure are studied. Different from a single quantum well, band mixing in the DQW generates bonding and antibonding states, whose wave functions have different spatial symmetry. By applying a time-dependent electric potential to the two well regions simultaneously, Floquet sidebands are formed, which constitutes additional quantum tunneling paths. When one of the Floquet sidebands coincides with the bonding or antibonding quasibound states within the DQW structure, sharp Fano resonances are found in the transmission coefficients as well as in the differential shot noise spectra. While such Fano resonances originate from quantum interference, their shapes are strikingly different for transport via the bonding state and via the antibonding state. The Fano resonance via the even-parity bonding state shows a perfect transmission followed by a total reflection and the Fano resonance via the odd-parity antibonding state has a reversed symmetry and shows a total reflection before a perfect transmission.

Systematic study of kinematic and dynamic moments of inertia of superdeformed bands with NpNn scheme

The systematics of kinematic moment of inertia J(1) and dynamic moment of inertia J(2) of superdeformed (SD) bands in A∼130, 150, 190 mass regions have been studied. We have obtained the values of J(1) and J(2) for all the SD bands observed in the A∼130, 150, 190 mass regions by using the experimental intraband E2 transition energies. The result of this work includes the variation of J(1) with the product of valence proton and neutron numbers (NpNn). The phenomenon of band mixing has been observed in the A∼130, 150 mass regions and band crossing has been observed in the A∼190 mass region. The systematics also includes the variation of J(2) with the product of valence proton and neutron numbers (NpNn). Evidence of staggering behaviour has been observed in all three A∼130, 150 and 190 mass regions. We present for the first time the variation of J(1) and J(2) of SD bands in the A∼130, 150, 190 mass regions with NpNn.

Estimation of electron spin polarization from circularly polarized photoluminescence in strained quantum wells

A general method based on the valence band mixing is proposed to establish a correlation between electron spin polarization and the degree of circular polarization of luminescence in a strained quantum well (QW). Key issues that are critical for establishing such a correlation are highlighted by invoking 4 band k.p calculations in a strained QW system. The proposed method takes care of the complex valence band mixing in strained QWs by simultaneously analyzing the heavy hole (e-hh) and light hole (e-lh) related luminescence transitions along the quantization direction. It is shown that the ratio of the degree of circular polarizations measured for the e-hh and e-lh features is a sensitive probe to estimate the values of band mixing. The method is reasonably supported by carefully conducted experiments on a tensile strained GaAsP/AlGaAs QW sample in the temperature range of 10 to 150 K. The proposed method shall be helpful for understanding the spin injection/transfer across the interfaces of QW, especially for the cases where heavy hole-light hole coupling is expected to be strong, and both the features are experimentally observed. It might lead to new directions in the field of semiconductor spin-optoelectronics in QWs.

Analysis of polarized light generation in anisotropic strained quantum dots

A theoretical analysis of the optical characteristics and polarization properties of quantum dot laser which uses interband transitions in shape asymmetric InGaAs/GaAs QDs is presented. We use multi-band effective mass k.p model with the strain effect taken into account to compute the electronic structure of asymmetric QDs. In-plane shape asymmetry in QDs is accounted through the anisotropic parabolic potential. The optical gain spectrum is calculated using the density matrix approach. We have solved coupled set of rate equations numerically using fourth-order Runge Kutta method to analyze the dynamic characteristics of InGaAs/GaAs quantum dot laser. The effect of shape anisotropy, material composition, carrier densities and strain on light emission polarization ratio has been studied. The dependence of gain on polarization direction has been explained in terms of the valence band mixing. The polarization is found to increase almost linearly with anisotropy parameter and decreases with the material composition. Furthermore, it is demonstrated that anisotropy and strain can be used to control the polarization. We found that the output has preferential polarization properties depending on the QD structure and composition.

Reexamining the nuclear structure of Gd154 in the dynamic pairing plus quadrupole model

In a previous study of the collective multiphonon bands in $^{154}\mathrm{Gd}$, using the microscopic dynamic pairing plus quadrupole model, data for eight $K$ bands were analyzed. In the last four decades, its decay scheme is significantly revised and the nuclear theory has undergone a significant change. Special focus is on new weak intensity transitions in several bands and on the reassigned levels in its decay scheme. The present study represents a detailed revised analysis of the collective even parity bands below 2.1 MeV. Also, a discussion is given on the nature of the ${K}^{\ensuremath{\pi}}={0}^{+}$ excited bands, validity of band mixing approach, and of the assumption of shape coexistence of \ensuremath{\beta} band with ground band. Comparison is made with the $X$(5) analytical symmetry and the algebraic interacting boson model predictions. Discussion of the $2n$ transfer reactions is given. The validity of the multiphonon view of the ${K}^{\ensuremath{\pi}}={4}^{+}$ and ${2}_{2}^{+}$ bands is also studied.

Electronic structure of (Ga,Mn)As revisited

The detailed nature of electronic states mediating ferromagnetic coupling in dilute magnetic semiconductors, specifically (Ga,Mn)As, has been an issue of long debate. Two confronting models have been discussed emphasizing host band versus impurity band carriers. Using angle resolved photoemission we show that the electronic structure of the (Ga,Mn)As system is significantly modified from that of GaAs throughout the valence band. Close to the Fermi energy, the presence of Mn induces a strong mixing of the bulk bands of GaAs, which results in the appearance of a highly dispersive band in the gap region of GaAs. For Mn concentrations above 1% the band reaches the Fermi level, and can thus host the delocalized holes needed for ferromagnetic coupling. Overall, our data provide a firm evidence of delocalized carriers belonging to the modified host valence band.

Application of Unmanned Aerial Vehicle Imagery for Algal Bloom Monitoring in River Basin

The purpose of this study is to enhance control capability of river water quality and environment by developing algal bloom monitoring technology using Unmanned Aerial Vehicle. The study acquired remote sensing data using UAV on the midstream area of Nakdong River, one of four major rivers in South Korea. The study also conducted water quality analysis and spectral reflectance measuring in the field to identify emergence aspect of phytoplankton in waters of the river and to induce algal bloom detection index equation through wavelength mixing of sensors. The result showed that the result values of algal bloom detection obtained from wavelength band mixing had strong positive correlation with algae standing crops. Besides, the results of analyzing spatial distribution of algal bloom of the overall investigated water area showed that the area of 0.22 km2 ~ 0.26 km2 had severity equivalent to the level of warning (5,000 ~ 1,000,000 cells/mL) in 2015 criteria for issuing algae warnings. It is expected that when this remote water quality and pollution monitoring technology is applied in the field, it would be able to improve capabilities to deal with the river water quality and pollution at the early stage.

Stability of emergent kinetics in optical lattices with artificial spin-orbit coupling

Artificial spin-orbit coupling in optical lattices can be engineered to tune band structure into extreme regimes where the single-particle band flattens leaving only inter-particle interactions to define many-body states of matter. Lin et al. [Phys. Rev. Lett 112, 110404 (2014)] showed that under such conditions interactions lead to a Wigner crystal of fermionic atoms under approximate conditions: no bandwidth or band mixing. The excitations were shown to possess emergent kinetics with fractionalized charge derived entirely from interactions. In this work we use numerical exact diagonalization to study a more realistic model with non-zero bandwidth and band mixing. We map out the stability phase diagram of the Wigner crystal. We find that emergent properties of the Wigner crystal excitations remain stable for realistic experimental parameters. Our results validate the approximations made by Lin et al. and define parameter regimes where strong interaction effects generate emergent kinetics in optical lattices.

Ultrafast shift and rectification photocurrents in GaAs quantum wells: Excitation intensity dependence and the importance of band mixing

A microscopic approach that is based on the multisubband semiconductor Bloch equations formulated in the basis of a 14-band ${\mathbf k} \cdot {\mathbf p}$ model is employed to compute the temporal dynamics of photocurrents in GaAs quantum wells following the excitation with femtosecond laser pulses. This approach provides a transparent description of the interband, intersubband, and intraband excitations, fully includes all resonant as well as off-resonant excitations, and treats the light-matter interaction non-perturbatively. For linearly polarized excitations the photocurrents contain contributions from shift and rectification currents. We numerically compute and analyze these currents generated by the excitation with femtosecond laser pulses for [110]- and [111]-oriented GaAs quantum wells. It is shown that the often employed perturbative $\chi^{(2)}$-approach breaks down for peak fields larger than about 10~kV/cm and that non-perturbative effects lead to a reduction of the peak values of the shift and rectification currents and to temporal oscillations which originate from Rabi flopping. In particular, we find a complex oscillatory photon energy dependence of the magnitudes of the shift and rectification currents. Our simulations demonstrate that this dependence is the result of mixing between the heavy- and light-hole valence bands. This is a surprising finding since the bandmixing has an even larger influence on the strength of the photocurrents than the absorption coefficient. For [110]-oriented GaAs quantum wells the calculated photon energy dependence is compared to experimental results and a good agreement is obtained which validates our theoretical approach.

Electromodulation spectroscopy of direct optical transitions in Ge1−xSnx layers under hydrostatic pressure and built-in strain

Unstrained Ge1−xSnx layers of various Sn concentration (1.5%, 3%, 6% Sn) and Ge0.97Sn0.03 layers with built-in compressive (ε = −0.5%) and tensile (ε = 0.3%) strain are grown by molecular beam epitaxy and studied by electromodulation spectroscopy (i.e., contactless electroreflectance and photoreflectance (PR)). In order to obtain unstrained GeSn layers and layers with different built-in in-plane strains, virtual InGaAs substrates of different compositions are grown prior to the deposition of GeSn layers. For unstrained Ge1−xSnx layers, the pressure coefficient for the direct band gap transition is determined from PR measurements at various hydrostatic pressures to be 12.2 ± 0.2 meV/kbar, which is very close to the pressure coefficient for the direct band gap transition in Ge (12.9 meV/kbar). This suggests that the hydrostatic deformation potentials typical of Ge can be applied to describe the pressure-induced changes in the electronic band structure of Ge1−xSnx alloys with low Sn concentrations. The same conclusion is derived for the uniaxial deformation potential, which describes the splitting between heavy-hole (HH) and light-hole (LH) bands as well as the strain-related shift of the spin-orbit (SO) split-off band. It is observed that the HH, LH, and SO related transitions shift due to compressive and tensile strain according to the Bir-Pikus theory. The dispersions of HH, LH, and SO bands are calculated for compressive and tensile strained Ge0.97Sn0.03 with the 8-band kp Hamiltonian including strain effects, and the mixing of HH and LH bands is discussed. In addition, the dispersion of the electronic band structure is calculated for unstrained Ge1−xSnx layers (3% and 6% Sn) at high hydrostatic pressure with the 8-band kp Hamiltonian, and the pressure-induced changes in the electronic band structure are discussed.

Renormalization of optical transition strengths in semiconductor nanoparticles due to band mixing

Unique optical properties of semiconductor nanoparticles (SN) make them very promising in the multitude of applications including lasing, light emission and photovoltaics. In many of these applications it is imperative to understand the physics of interaction of electrons in a SN with external electromagnetic fields on the quantitative level. In particular, the strength of electron–photon coupling determines such important SN parameters as the radiative lifetime and absorption cross section. This strength is often assumed to be fully encoded by the so called Kane momentum matrix element. This parameter, however, pertains to a bulk semiconductor material and, as such, is not sensitive to the quantum confinement effects in SNs. In this work we demonstrate that the quantum confinement, via the so called band mixing, can result in a significant suppression of the strength of electron interaction with electromagnetic field. Within the envelope function formalism we show how this suppression can be described by introducing an effective energy-dependent Kane momentum. Then, the effect of band mixing on the efficiencies of various photoinduced processes can be fully captured by the conventional formulae (e.g., spontaneous emission rate), once the conventional Kane momentum is substituted with the renormalized energy-dependent Kane momentum introduced in here. As an example, we evaluate the energy-dependent Kane momentum for spherical PbSe and PbS SNs (i.e., quantum dots) and show that neglecting band mixing in these systems can result in the overestimation of absorption cross sections and emission rates by a factor of ∼2.

Characterization of InN-In0.25Ga0.75N Quantum Well Laser with In0.4Al0.6N Layers for 1300 nm Band

ABSTRACTA wurtzite-strained nitride Quantum Well Laser has been characterized for short distance communication wavelength. InN and In0.25Ga0.75N have been chosen as well material and barrier material respectively with In0.4Al0.6N SCH layers at the end of barrier layers to improve the carrier and photon confinement within the active region. This structure shows less compressive strain (7.33%) with respect to previously proposed structure which makes the structure more suitable for fabrication. To obtain the electronic band structure, self-consistent method with k.p formalism has been performed where valence band mixing effect, strain effect and spontaneous and piezoelectric polarization effect has been included. From the electronic characteristics, the optical properties have been performed with numerical model. From the optical properties, the structure has been found as TE polarized with C1-HH1, C1-LH1, C2-HH1 and C2-LH1 dominating transition elements. From the performance of the numerical model, 4731.98 cm−1 optical gain for TE polarization at 1315.5 nm emission wavelength and 8.017×1027 cm−3s−1eV−1 spontaneous emission rate at 1301.7nm wavelength have been found for 12Å well width, 17Å barrier width and 52Å SCH layer width at 5×1019 cm−3 carrier density. The obtained properties have been shown a good agreement with previously published works.

Electrons, holes, and excitons in GaAs polytype quantum dots

Single and multi-band k⋅p Hamiltonians for GaAs crystal phase quantum dots are used to assess ongoing experimental activity on the role of such factors as quantum confinement, spontaneous polarization, valence band mixing, and exciton Coulomb interaction. Spontaneous polarization is found to be a dominating term. Together with the control of dot thickness [Vainorius et al., Nano Lett. 15, 2652 (2015)], it enables wide exciton wavelength and lifetime tunability. Several new phenomena are predicted for small diameter dots [Loitsch et al., Adv. Mater. 27, 2195 (2015)], including non-heavy hole ground state, strong hole spin admixture, and a type-II to type-I exciton transition, which can be used to improve the absorption strength and reduce the radiative lifetime of GaAs polytypes.

Reduction of collectivity at very high spins inNd134: Expanding the projected-shell-model basis up to 10-quasiparticle states

Background: The recently started physics campaign with the new generation of $\ensuremath{\gamma}$-ray spectrometers, ``GRETINA'' and ``AGATA,'' will possibly produce many high-quality $\ensuremath{\gamma}$ rays from very fast-rotating nuclei. Microscopic models are needed to understand these states.Purpose: It is a theoretical challenge to describe high-spin states in a shell-model framework by the concept of configuration mixing. To meet the current needs, one should overcome the present limitations and vigorously extend the quasiparticle (qp) basis of the projected shell model (PSM).Method: With the help of the recently proposed Pfaffian formulas, we apply the new algorithm and develop a new PSM code that extends the configuration space to include up to 10-qp states. The much-enlarged multi-qp space enables us to investigate the evolutional properties at very high spins in fast-rotating nuclei.Results: We take $^{134}\mathrm{Nd}$ as an example to demonstrate that the known experimental yrast and the several negative-parity side bands in this nucleus could be well described by the calculation. The variations in moment of inertia with spin are reproduced and explained in terms of successive band crossings among the 2-qp, 4-qp, 6-qp, 8-qp, and 10-qp states. Moreover, the electric quadrupole transitions in these bands are studied.Conclusions: A pronounced decrease in the high-spin $B(E2)$ of $^{134}\mathrm{Nd}$ is predicted, which suggests reduction of collectivity at very high spins because of increased level density and complex band mixing. The possibility for a potential application of the present development in the study of highly excited states in warm nuclei is mentioned.

Type-II quantum wells with tensile-strained GaAsSb layers for interband cascade lasers with tailored valence band mixing

Optical properties of modified type II W-shaped quantum wells have been investigated with the aim to be utilized in interband cascade lasers. The results show that introducing a tensely strained GaAsSb layer, instead of a commonly used compressively strained GaInSb, allows employing the active transition involving valence band states with a significant admixture of the light holes. Theoretical predictions of multiband k·p theory have been experimentally verified by using photoluminescence and polarization dependent photoreflectance measurements. These results open a pathway for practical realization of mid-infrared lasing devices with uncommon polarization properties including, for instance, polarization-independent midinfrared light emitters.

Asymmetric Oval-Shaped-Hole Photonic Crystal Waveguide Design by Artificial Intelligence Optimizers

This paper proposes a new kind of photonic crystal waveguide (PCW) called asymmetric oval-shaped-hole PCW (AOPCW). In this PCW, 20 structural parameters are considered, which provides a very flexible PCW to design. The large number of variables makes the design process of this new PCW almost impossible by the current manual try and error methods. Therefore, the AOPCW is optimized automatically by the artificial intelligence optimization technique in three phases. First, the AOPCW is optimized to maximize normalized delay-bandwidth product. Second, it is optimized with respect to two objectives: averaged of group index ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\overline {n_g } $</tex-math> </inline-formula> ) and normalized bandwidth <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$(\Delta \omega /\omega _0)$</tex-math> </inline-formula> . Third, group velocity dispersion is also considered in addition to the two objectives in the second phase, so AOPCW is optimized with respect to three objectives. In all of the three phases, a band mixing avoidance mechanism is also considered and handled. The comparative study of the optimized designs proves that the proposed AOPCW is able to substantially outperform the current PCW structures in the literature. This paper also considers and discusses time-domain simulation issues of the PCW-based optical buffer.

A projection-free method for representing plane-wave DFT results in an atom-centered basis.

Plane wave density functional theory (DFT) is a powerful tool for gaining accurate, atomic level insight into bulk and surface structures. Yet, the delocalized nature of the plane wave basis set hinders the application of many powerful post-computation analysis approaches, many of which rely on localized atom-centered basis sets. Traditionally, this gap has been bridged via projection-based techniques from a plane wave to atom-centered basis. We instead propose an alternative projection-free approach utilizing direct calculation of matrix elements of the converged plane wave DFT Hamiltonian in an atom-centered basis. This projection-free approach yields a number of compelling advantages, including strict orthonormality of the resulting bands without artificial band mixing and access to the Hamiltonian matrix elements, while faithfully preserving the underlying DFT band structure. The resulting atomic orbital representation of the Kohn-Sham wavefunction and Hamiltonian provides a gateway to a wide variety of analysis approaches. We demonstrate the utility of the approach for a diverse set of chemical systems and example analysis approaches.

Optical polarization properties of InAs/InP quantum dot and quantum rod nanowires

The emission polarization of single InAs/InP quantum dot (QD) and quantum rod (QR) nanowires is investigated at room temperature. Whereas the emission of the QRs is mainly polarized parallel to the nanowire axis, the opposite behavior is observed for the QDs. These optical properties can be explained by a combination of dielectric effects related to the nanowire geometry and to the configuration of the valence band in the nanostructure. A theoretical model and finite difference in time domain calculations are presented to describe the impact of the nanowire and the surroundings on the optical properties of the emitter. Using this model, the intrinsic degree of linear polarization of the two types of emitters is extracted. The strong polarization anisotropies indicate a valence band mixing in the QRs but not in the QDs.