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Kadantsev E.S.,University of Ottawa | Kadantsev E.S.,NRC Institute for Microstructural Sciences | Hawrylak P.,NRC Institute for Microstructural Sciences
Solid State Communications | Year: 2012

The electronic structure of a single MoS 2 monolayer is investigated with all electron first-principles calculations based on Kohn Sham Density Functional Theory and variational treatment of spin-orbital coupling. The topologies of the valence band maximum and conduction band minimum are explored over the whole Brillouin zone. The single MoS 2 monolayer is confirmed to be a direct band gap semiconductor. The projected density of states (PDOS) of a single monolayer is calculated and compared to that of bulk MoS 2. The effective masses and the orbital character of the band edges at high-symmetry points of the Brillouin zone are determined. The spin-splittings of the conduction band minimum (CBMIN) and valence band maximum (VBMAX) are calculated over the whole Brillouin zone. It is found that the maximum spin-splitting of VBMAX is attained at the K point of the Brillouin zone and is responsible for the experimentally observed splitting between the A 1 and B 1 excitons. © 2012 Elsevier Ltd. All rights reserved.


Kadantsev E.S.,NRC Institute for Microstructural Sciences | Hawrylak P.,NRC Institute for Microstructural Sciences
Applied Physics Letters | Year: 2011

A model for the evolution of conduction and valence bands of IIIA-VA (InAs, GaAs, and InP) semiconductors under (001) biaxial strain is developed. The model is based on the ab initio calculations which take into account finite strain dependent relaxation of the reference levels. The results of ab initio full potential calculations of absolute deformation potentials (ADPs) and (001) biaxial strain-modified band edges are reported. It is shown that in type I heterostructures subjected to (001) compressive biaxial strain, the corrections due to nonzero ADP of the core reference levels reduce the strained band offset for holes. © 2011 American Institute of Physics.


Zielinski M.,NRC Institute for Microstructural Sciences | Korkusinski M.,NRC Institute for Microstructural Sciences | Hawrylak P.,NRC Institute for Microstructural Sciences
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We present atomistic tight-binding theory of electronic structure and optical properties of InAs/GaAs self-assembled quantum dots. The tight-binding model includes zincblende symmetry, faceting, and s p3 d5 s atomic orbitals accounting for interband and intervalley couplings. The equilibrium positions of atoms are calculated using valence force field method and modification of the tight-binding Hamiltonian due to strain is accounted for using Harrison's law. The electronic and optical properties of multiexciton complexes are then determined by diagonalizing the many-body Hamiltonian for interacting electrons and holes using the configuration-interaction approach. The calculations of strain distribution approach 108 atoms while the electron and valence hole single-particle states are calculated by diagonalization of the Hamiltonian matrix with size on the order of 10 7. The dependence of predicted electronic and optical properties on InAs/GaAs valence-band offset and InAs absolute valence-band deformation potentials are described. The reliability of the atomistic calculations is assessed by comparison with results obtained from the effective bond orbital model and empirical pseudopotentials method. © 2010 The American Physical Society.


Lefebvre J.,NRC Institute for Microstructural Sciences | Finnie P.,NRC Institute for Microstructural Sciences
Nano Research | Year: 2011

Polarized light microscopy (PLM) is used to image individual single-walled carbon nanotubes (SWNTs) suspended in air across a slit opening. The imaging contrast relies on the strong optical anisotropy typical of SWNTs. We combine PLM with a tunable light source to enable hyperspectral excitation spectroscopy and nanotube chirality assignment. Comparison with fluorescence microscopy and spectroscopy confirms the assignment made with PLM. This represents a versatile new approach to imaging SWNTs and related structures. © 2011 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.


Korkusinski M.,NRC Institute for Microstructural Sciences | Voznyy O.,NRC Institute for Microstructural Sciences | Hawrylak P.,NRC Institute for Microstructural Sciences
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Theory of electronic and optical properties of exciton and biexciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic s p3 d5 s⊃- tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, and model surface passivation. The optically excited states are expanded in electron-hole configurations and the many-body spectrum is computed in the configuration-interaction approach. Results demonstrate that the low-energy electron spectrum is organized in shells (s,p,...), while the valence hole spectrum is composed of four low-lying, doubly degenerate states separated from the rest by a gap. As a result, the biexciton and exciton spectrum is composed of a manifold of closely lying states, resulting in a fine structure of exciton and biexciton spectra. The quasidegenerate nature of the hole spectrum results in a correlated biexciton state, which makes it slowly convergent with basis size. We carry out a systematic study of the exciton and biexciton emission spectra as a function of the nanocrystal diameter and find that the interplay of repulsion between constituent excitons and correlation effects results in a change of the sign of biexciton binding energy from negative to positive at a critical nanocrystal size. © 2010 The American Physical Society.


Kadantsev E.,NRC Institute for Microstructural Sciences | Hawrylak P.,NRC Institute for Microstructural Sciences
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Theory of exciton fine structure in semiconductor quantum dots and its dependence on quantum-dot anisotropy and external lateral electric field is presented. The effective exciton Hamiltonian including long-range electron-hole exchange interaction is derived within the kp effective-mass approximation. The exchange matrix elements of the Hamiltonian are expressed explicitly in terms of electron and hole envelope functions. The matrix element responsible for the "bright" exciton splitting is identified and analyzed. An excitonic fine structure for a model quantum dot with quasi-two-dimensional anisotropic harmonic oscillator confining potential is analyzed as a function of the shape anisotropy, size, and applied lateral electric field. © 2010 The American Physical Society.


Dupont E.,NRC Institute for Microstructural Sciences | Fathololoumi S.,NRC Institute for Microstructural Sciences | Liu H.C.,NRC Institute for Microstructural Sciences
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

A simplified density-matrix model describing the population and coherence terms of four states in a resonant phonon scattering based terahertz quantum cascade laser is presented. Despite its obvious limitations and with two phenomenological terms-called the pure dephasing time constants in tunneling and intersubband transition-the model agrees reasonably well with experimental data. We demonstrate the importance of a tunneling leakage channel from the upper lasing state to the excited state of the downstream phonon well. In addition, we identify an indirect coupling between nonadjacent injector and extractor states. The analytical expression of the gain spectrum demonstrates the strong broadening effect of the injection and extraction couplings. The gain is decomposed into three terms: a linear gain and two nonlinear components related to stimulated anti-Stokes scattering processes. The nonlinear gain is not negligible at high temperature. Under certain approximations, analytical forms of population and coherence terms are derived. This model is well suited for structures with only a few states involved. This model can simplify the optimization process for new laser designs; it is also convenient for experimentalists to adopt. © 2010 The American Physical Society.


Finnie P.,NRC Institute for Microstructural Sciences | Lefebvre J.,NRC Institute for Microstructural Sciences
ACS Nano | Year: 2012

Individual air-suspended single-walled carbon nanotubes are imaged both spatially and spectrally in photoluminescence. At low excitation power, photoluminescence is bright and stable with high quantum efficiency; however, higher power initially causes a gradual red shift and then more severe changes. Blinking, the loss of quantum efficiency, and the appearance of new deep levels are all seen and can be explained by the introduction of defects. We propose that optical excitation induces molecular deposition onto the nanotube by optically induced van der Waals interactions, leading to physisorption and ultimately chemisorption which severely degrades the luminescence. © Published 2012 by the American Chemical Society.


Voznyy O.,NRC Institute for Microstructural Sciences
Journal of Physical Chemistry C | Year: 2011

We have performed ab initio calculations of electronic properties of the realistic Cd-rich CdSe nanocrystals with covalently bound carboxylic acid (X-type) ligands. Configurations both with and without surface traps can be prepared depending on the amount and geometry of the adsorbed ligands. We find that Cd and Se dangling bonds do not necessarily create surface traps, whereas traps originating from ligands can form near the top of the valence band. Some of the ligands are found to be mobile on the surface and this mobility is accompanied by a spectral diffusion of the associated trap energy levels. This provides the first atomistic example of the processes required to explain the emission wavelength and lifetime variations, and blinking of the nanocrystals. © Published 2011 by the American Chemical Society.


Xu D.-X.,NRC Institute for Microstructural Sciences
Topics in Applied Physics | Year: 2011

This chapter reviews the characteristics of SOI ridge waveguide birefringence, as governed by the waveguide cross-section geometry, the cladding stress level, and cladding thickness. Typical stress levels in dielectric cladding films such as silicon dioxide and silicon nitride are such that the stress-induced birefringence is of comparable magnitude to the waveguide geometrical birefringence. Therefore the total waveguide birefringence can be precisely controlled by counter-balancing these two factors. The application of this technique for achieving polarization independence in a variety of photonic components is described, as well as an example of polarization splitting. Passive and active tuning of the stress-induced birefringence is discussed. The use of birefringence tuning to enhance the efficiency in optical parametric processes and stress-induced Pockels electro-optic effect are also briefly addressed. © Springer-Verlag Berlin Heidelberg 2011.

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