Direct Interband Research Articles

Near-UV electroluminescence in unipolar-doped, bipolar-tunneling GaN/AlN heterostructures

Cross-gap light emission is reported in n-type unipolar GaN/AlN double-barrier heterostructure diodes at room temperature. Three different designs were grown on semi-insulating bulk GaN substrates using molecular beam epitaxy (MBE). All samples displayed a single electroluminescent spectral peak at 360 nm with full-width at half-maximum (FWHM) values no greater than 16 nm and an external quantum efficiency (EQE) of ≈0.0074% at 18.8 mA. In contrast to traditional GaN light emitters, p-type doping and p-contacts are completely avoided, and instead, holes are created in the GaN on the emitter side of the tunneling structure by direct interband (that is, Zener) tunneling from the valence band to the conduction band on the collector side. The Zener tunneling is enhanced by the high electric fields (~5 × 106 V cm−1) created by the notably large polarization-induced sheet charge at the interfaces between the AlN and GaN.

Exciton states and direct interband light absorption in the ensemble of toroidal quantum dots

One electron and exciton states in toroidal quantum dot (QD) have been considered. The convenient coordinate system has been defined and the Schrodinger equation has been solved in these coordinates. The electron energy spectrum and wave function dependence on the geometrical parameters of toroidal QD have been obtained. The comparison with the results obtained by numerical methods has been done. Optical absorption has been considered in both cases of “small” toroid when the interaction of an electron and a hole is neglected and cases of relatively “large” toroid when the correlation between the particles has been taken into account. Interband optical transitions have been considered in the ensemble of toroidal QDs. The selection rules for quantum transitions and absorption edge dependence on the geometrical parameters have been obtained.

Optical conductivity of a commensurate graphene-topological insulator heterostructure

The optical conductivity of a heterostructure formed by a commensurate stacking of graphene and a topological insulator (TI) is investigated using the Kubo formalism. Both the intra- and interband AC conductivities are found to be sensitive to the graphene-TI coupling. The direct interband transition in graphene which is the origin of the universal conductance is forbidden due to the topological nature is the coupling. Furthermore, the graphene-TI coupling gives rise to additional broken symmetries, resulting in both the inter- and intraband conductivity to be reduced in the graphene-TI heterostructure. By varying the Fermi energy of the heterostructure, the band that gives the largest contribution changes, which in turn affects the overall electronic transport.

How Light Is Emitted by Plasmonic Metals.

The mechanism by which light is emitted from plasmonic metals such as gold and silver has been contentious, particularly at photon energies below direct interband transitions. Using nanoscale plasmonic cavities, blue-pumped light emission is found to directly track dark-field scattering on individual nanoconstructs. By exploiting slow atomic-scale restructuring of the nanocavity facets to spectrally tune the dominant gap plasmons, this correlation can be measured from 600 to 900 nm in gold, silver, and mixed constructs ranging from spherical to cube nanoparticles-on-mirror. We show that prompt electronic Raman scattering is responsible and confirm that "photoluminescence", which implies phase and energy relaxation, is not the right description. Our model suggests how to maximize light emission from metals.

Investigation of spectral dependences of the absorption coefficient in InGaAs layers

Values of the absorption coefficient of InGaAs structures grown by means of gaseous-phase epitaxy from metaloranic compounds have been studied and calculated. Experimental data have been compared to the theoretical model of the absorption spectrum based on the phenomenon of fundamental absorption and the general theory of direct interband optical transitions. The energy gap width has been graphically calculated from the slope of the experimental absorption characteristic.

Electronic States and Optical Transitions in an Asymmetric Quantum Dot Molecule

In the framework of adiabatic approximation the electronic states and direct interband absorption of light in the asymmetric double quantum dot molecule (QDM) having a shape of Cassini lemniscate revolution are discussed. Analytical expressions for the wave functions and energy spectrum of the electron in the QDM are derived. Nonmonotonic split and step-like behavior of the energy spectrum is revealed due to the possibility of the electron tunneling between quantum dots (QDs) in the molecule. The corresponding selection rules of quantum transitions for the direct interband absorption of light are obtained. The absorption edge characteristics depending on the QDM geometrical sizes are also revealed.

Control of Exciton Valley Coherence in Transition Metal Dichalcogenide Monolayers.

The direct gap interband transitions in transition metal dichalcogenide monolayers are governed by chiral optical selection rules. Determined by laser helicity, optical transitions in either the K^{+} or K^{-} valley in momentum space are induced. Linearly polarized laser excitation prepares a coherent superposition of valley states. Here, we demonstrate the control of the exciton valley coherence in monolayer WSe_{2} by tuning the applied magnetic field perpendicular to the monolayer plane. We show rotation of this coherent superposition of valley states by angles as large as 30° in applied fields up to 9T. This exciton valley coherence control on the ps time scale could be an important step towards complete control of qubits based on the valley degree of freedom.

Experimental investigation and calculation of the spectral dependence of the absorption coefficient of single-layer epitaxial HgCdTe structures

The absorption coefficient of HgCdTe structures grown by the methods of liquid-phase epitaxy (LPE) and metalorganic chemical vapor deposition (MOCVD) is studied and calculated. The experimental data are compared to a theoretical absorption spectrum model, which is based on the fundamental absorption effect and the general theory of direct interband optical transitions, as well as other empirical dependences. The Fermi level shift in the studied HgCdTe structures and the slope of experimental absorption characteristics are calculated.

Ab initiophonon coupling and optical response of hot electrons in plasmonic metals

Ultrafast laser measurements probe the nonequilibrium dynamics of excited electrons in metals with increasing temporal resolution. Electronic structure calculations can provide a detailed microscopic understanding of hot electron dynamics, but a parameter-free description of pump-probe measurements has not yet been possible, despite intensive research, because of the phenomenological treatment of electron-phonon interactions. We present ab initio predictions of the electron-temperature dependent heat capacities and electron-phonon coupling coefficients of plasmonic metals. We find substantial differences from free-electron and semiempirical estimates, especially in noble metals above transient electron temperatures of 2000 K, because of the previously neglected strong dependence of electron-phonon matrix elements on electron energy. We also present first-principles calculations of the electron-temperature dependent dielectric response of hot electrons in plasmonic metals, including direct interband and phonon-assisted intraband transitions, facilitating complete theoretical predictions of the time-resolved optical probe signatures in ultrafast laser experiments.

Cubic Nonlinearity Driven Up-Conversion in High-Field Plasmonic Hot Carrier Systems

Surface plasmon resonances confine electromagnetic fields to the nanoscale, producing high field strengths suitable for exploiting nonlinear optical properties. We examine the prospect of detecting and utilizing one such property in plasmonic metals: the imaginary part of the cubic susceptibility, which corresponds to two plasmons decaying together to produce high energy carriers. Here we present ab initio predictions of the rates and carrier distributions generated by direct interband and phonon-assisted intraband transitions in one and two-plasmon decay. We propose detection of the higher energy carriers generated from two-plasmon decays that are inaccessible in one-plasmon decay as a viable signature of these processes in ultrafast experiments.

Sonochemically assisted colloidal route to CdSe quantum dot assemblies: an alternative way to further fine-tune the size-dependent properties

Continuous-wave heterogeneous sonochemical method was developed to synthesize 3D assemblies composed by QDs of metastable (cubic) modification of CdSe with sphalerite structural type. 3D QD assemblies were deposited in thin film form and as bulk precipitates. Structure, surface morphology, optical absorption as well as charge carriers’ and phonon confinement effects were studied in details. The average nanocrystal radius, calculated by the Scherrer and Williamson–Hall methods, diminishes from 2.7 nm for samples synthesized by conventional colloidal chemical route to 2.0 nm for samples synthesized by sonochemical route. Upon post-deposition thermal annealing this value increases to 12.0 nm. QD size decrease is followed by increase of the uni-directional lattice strain and dislocation density (as defined by Hirsch), as well as by decrease of the lattice constant value. Previous findings were justified by analysis of X-ray diffraction peaks shape, which were found to be dominated by the particle size-distribution, instead of non-uniform strain. The energy of the fundamental direct \(\Gamma_{8}^{v} \to \Gamma_{6}^{c}\) interband electronic transition, computed from optical absorption data within parabolic approximation for the dispersion relation, shifts from 2.67 eV for sonochemically deposited samples and 2.08 eV for chemically deposited ones, to 1.77 eV upon post-deposition thermal treatment (close to the bulk value of 1.74 eV). The type of “interband” transitions does not change upon dimensionality reduce. Enhanced band gap energy blue shift upon average QD size decrease is followed by oscillator strength increase. Such trends were rationalized in terms of 3D confinement effects on charge carriers’ motion. Experimentally observed band gap energies agree very well with the predictions of the effective mass model of Brus. The next direct interband electronic transition, from the spin–orbit split component of the valence band to the conduction band, has been detected and analyzed. The 1LO bands in the Raman spectra of chemically and sonochemically deposited samples appear at 206 and 204 cm−1, as compared to the bulk value of 210 cm−1. The extent of homogeneous broadening of the corresponding band is larger in sonochemically deposited samples, indicating enhanced frequency of phonon–phonon collisions in smaller QDs. The observed frequency shifts are dominated by phonon-dispersion terms, instead of lattice contraction.

(Invited) Plasmonic Hot Carrier Injection in Wide Bandgap Nitride Semiconductors

Despite more than a decade of intensive scientific exploration, new plasmonic phenomena continue to be discovered, including quantum interference of plasmons, observation of quantum coupling of plasmons to single particle excitations, and quantum confinement of plasmons in single-nm scale plasmonic particles. Simultaneously, plasmonic structures find widening applications in integrated nanophotonics, biosensing, photovoltaic devices, single photon transistors and single molecule spectroscopy. Decay of surface plasmons to hot carriers is a new direction that has attracted considerable fundamental and application interest, yet a theoretical understanding of ultrafast plasmon decay processes and the underlying microscopic mechanisms remain incomplete. Recently we analyzed the quantum decay of surface plasmon polaritons and found that the prompt distribution of generated carriers is extremely sensitive to the energy band structure of the plasmonic material. A theoretical understanding of plasmon-driven hot carrier generation and relaxation dynamics at femtosecond timescales is presented here. We report the first ab initio calculations of phonon-assisted optical excitations in metals, which are critical to bridging the frequency range between resistive losses at low frequencies and direct interband transitions at high frequencies. We also present calculations of energy-dependent lifetimes and mean free paths of hot carriers, accounting for electron-electron and electron-phonon scattering, lending insight towards transport of plasmonically-generated carriers at the nanoscale. We will discuss calculations for multiplasmon and nonlinear processes in the ultrafast regime from the mid-IR to visible and in different geometries. Employing a Feynman diagram approach here has been critical to determine the relevant processes. Based on these theoretical predictions, we designed a solid-state system with wide-bandgap semiconductors and studied the photocurrent generated upon plasmon decay across the extended UV and visible regime for different metals and plasmonic structures. Direct comparison with ab-initio calculations allow us to assess the contribution of direct and indirect transitions and identify guidelines for engineering the generation of plasmonic hot-carriers in the UV-VIS regime.

Optical properties of narrow band prolate ellipsoidal quantum layers ensemble

Electronic states and direct interband light absorption in the ensemble of prolate spheroidal quantum layers are considered. The problem of finding the one-electron wave function and energy spectrum have been solved exactly. Absorption edge dependence on the thickness of the layer in the strong size quantization regime has been obtained. The effect of nonparabolicity of the dispersion law of energy levels and optical absorption have been taken into account and calculations are carried out for the cases of both parabolic and Kane’s dispersion laws. Selection rules have been revealed. Absorption coefficient dependence on the frequency of incident light has been obtained, taking into account dispersion of nanolayer thicknesses for the cases of both symmetric and asymmetric distribution functions.

Electronic and optical properties of a double quantum dot molecule with Kane’s dispersion law

In the framework of the adiabatic approximation the electronic states and direct interband absorption of light in the double quantum dot molecule (QDM) are discussed. Cases of both standard parabolic and Kane's dispersion law are considered. Analytical expressions for the wave functions and energy spectrum of the electron in the QDM are treated. The split of energy levels due to the possibility of the electron tunneling between quantum dots (QDs) in the molecule is revealed. The corresponding selection rules of quantum transitions for the direct interband absorption of light are obtained. The absorption edge behavior characteristics depending on the QDs geometrical sizes and the width of the QDs connecting region are revealed as well.

Theory of electronic states and direct light absorption in the biconical (superellipsoidal) quantum dot

The electronic states in the GaAs biconical (superellipsoidal) quantum dot (QD) in a strong and weak size quantization (SQ) regimes are theoretically investigated within the framework of the geometrical adiabatic approximation. An atypical linear term in the effective confining potential forms a family of non-equidistant sublevels in the electron energy spectrum, whereas the quadratic term leads to the appearance of the equidistant energy levels as a consequence of the symmetry of the QD. In the weak size quantization regime the motion of the exciton's center-of-gravity is quantized, which leads to the appearance of the additional Coulomb sub-levels in the energy spectrum of the QD. The direct interband absorption of light in the biconical QD is also considered in both strong and weak SQ regimes. The dependences of the absorption edge on QD geometrical parameters are revealed in the linear and quadratic approximations of the effective potential adiabatic terms. Corresponding selection rules for quantum transitions are obtained as well.

Photonic Integrated Circuits Based on Quantum well Intermixing Techniques

The passive sections of a monolithic device must have a wider bandgap than the active regions to reduce losses due to direct interband absorption. Such bandgap engineering is usually realized by complicated regrown butt-joint or selective-area growth techniques. We, however, have developed a simple, flexible and low-cost alternative technique – quantum well intermixing (QWI) – to increase the bandgap in selected areas of an integrated device post-growth. To verify the QWI process, we have fabricated the following demonstrators: a 40 GHz semiconductor mode-locked laser producing pulses as short as 490 fs; a 10 GHz passively mode-locked extended cavity laser integrated with surface-etched distributed Bragg reflector (DBR) which can be tuned in both wavelength and pulse repetition rate; four 10 GHz 1.55 μm AlGaInAs/InP mode-locked surfaced-etched DBR lasers integrated combiner, a semiconductor optical amplifier and modulator where the four channels can be operated separately or simultaneously; a CWDM source with 12 nm wavelength separation based on an AlGaInAs/InP integrated distributed feedback laser array; and a 1.55μm DFB laser monolithically integrated with power amplifier array. In all these applications, QWI has the advantage of eliminating crystal regrowth and the associated stringent tolerance requirements that are required in traditional integration schemes.

Nonradiative Plasmon Decay and Hot Carrier Dynamics: Effects of Phonons, Surfaces, and Geometry.

The behavior of metals across a broad frequency range from microwave to ultraviolet frequencies is of interest in plasmonics, nanophotonics, and metamaterials. Depending on the frequency, losses of collective excitations in metals can be predominantly classical resistive effects or Landau damping. In this context, we present first-principles calculations that capture all of the significant microscopic mechanisms underlying surface plasmon decay and predict the initial excited carrier distributions so generated. Specifically, we include ab initio predictions of phonon-assisted optical excitations in metals, which are critical to bridging the frequency range between resistive losses at low frequencies and direct interband transitions at high frequencies. In the commonly used plasmonic materials, gold, silver, copper, and aluminum, we find that resistive losses compete with phonon-assisted carrier generation below the interband threshold, but hot carrier generation via direct transitions dominates above threshold. Finally, we predict energy-dependent lifetimes and mean free paths of hot carriers, accounting for electron-electron and electron-phonon scattering, to provide insight toward transport of plasmonically generated carriers at the nanoscale.

Optical constants and band structure of trigonal NiO

Using spectroscopic ellipsometry and transmission measurements, the authors determined the optical constants (absorption coefficient, complex refractive index, and dielectric function) of bulk trigonal NiO from 0.08 to 6.5 eV. By careful discussion of the data, elastic scattering by oxygen bubbles was ruled out and the effects of surface roughness were removed numerically to obtain an accurate dielectric function of NiO. A direct band gap of 0.85 eV was found from transmission and assigned to direct interband transitions from the Ni-O hybrid valence band states to the Ni 4s conduction band at the center of the Brillouin zone. At 4 eV, the authors find the well-known charge transfer gap from the lower to the upper Hubbard band. Several intermediate sharp peaks were also found. The temperature dependence of the NiO charge transfer gap is similar to the E1 gap of Si between 100 and 700 K. At higher temperatures, heating NiO in vacuum leads to sublimation, which has drastic irreversible consequences for the pseudodielectric function of the sample, including a strong Ni nanoparticle plasmon peak at 2 eV.

Formation, Structure and Optical Properties of Nanocrystalline BaSi<sub>2</sub> Films on Si(111) Substrate

BaSi2 thin films were formed on Si (111) substrate by solid-phase epitaxy (SPE) (UHV deposition) using the template technology followed by vacuum annealing at temperatures of 600 °C and 750 °C. After the deposition and annealing barium silicide films were characterized by Auger electron spectroscopy, grazing incidence x-ray diffraction (GIXRD) and atomic-force microscopy (AFM). It was established that the films annealed at T = 600 °C are polycrystalline with the structure of the orthorhombic BaSi2, with grain sizes of 100-200 nm. Higher anneal temperature (T=750 °C) leads to increase of diffraction peak intensity of BaSi2 phase with grain coagulation into 300-400 nm islands. It was confirmed that nanocrystalline BaSi2 films are characterized by a direct fundamental interband transition at 1.3 eV, the second interband transition with an energy of 2.0 eV, own phonon structure with wave number peaks at 112, 119, 146 and 208 cm-1 and a high density of defect states within the band gap, which provide a noticeable subband absorption at energies of 0.8 – 1.1 eV.

Prolate spheroidal quantum dot: Electronic states, direct interband light absorption and electron dipole moment

Excitonic states and direct interband light absorption are considered in prolate spheroidal quantum dot. The problem of finding the one electron wave function and energy spectrum have been solved exactly. Three regimes of size quantization have been considered. Absorption edge dependence on the small semiaxes of the spheroid has been obtained. The probability of the direct interband transitions has been calculated and the selection rules for quantum numbers have been obtained. It has been shown that in prolate spheroidal quantum dot the exact value of the electron ground state with high accuracy coincides with the value of ground state in strongly prolate ellipsoidal quantum dot obtained in the framework of the adiabatic approximation. The dependence of z component of the electron dipole moment has been studied on the small semiaxes of the prolate spheroidal quantum dot.