Oleksiy Roslyak

Manipulating stimulated coherent anti-Stokes Raman spectroscopy signals by broad-band and narrow-band pulses

A transition-amplitude based representation of heterodyne detected coherent anti-Stokes Raman signals is used to separate them into a parametric component that involves no change in the material and dissipative processes associated with various transitions between states. Qualitatively different contributions from the two processes are predicted for the signal generated by an overlapping narrow (picosecond) and broad-band (femtosecond) pulse.

Influence of exciton dimensionality on spectral diffusion of single-walled carbon nanotubes.

We study temporal evolution of photoluminescence (PL) spectra from individual single-walled carbon nanotubes (SWCNTs) at cryogenic and room temperatures. Sublinear and superlinear correlations between fluctuating PL spectral positions and line widths are observed at cryogenic and room temperatures, respectively. We develop a simple model to explain these two different spectral diffusion behaviors in the framework of quantum-confined Stark effect (QCSE) caused by surface charges trapped in the vicinity of SWCNTs. We show that the wave function properties of excitons, namely, localization at cryogenic temperature and delocalization at room temperature, play a critical role in defining sub- and superlinear correlations. Room temperature PL spectral positions and line widths of SWCNTs coupled to gold dimer nanoantennas on the other hand exhibit sublinear correlations, indicating that excitonic emission mainly originates from nanometer range regions and excitons appear to be localized. Our numerical simulations show that such apparent localization of excitons results from plasmonic confinement of excitation and an enhancement of decay rates in the gap of the dimer nanoantennas.

Graphene nanoribbons in criss-crossed electric and magnetic fields

Graphene nanoribbons (GNRs) in mutually perpendicular electric and magnetic fields are shown to exhibit dramatic changes in their band structure and electron-transport properties. A strong electric field across the ribbon induces multiple chiral Dirac points, closing the semiconducting gap in armchair GNRs. A perpendicular magnetic field induces partially formed Landau levels as well as dispersive surface-bound states. Each of the applied fields on its own preserves the even symmetry Ek=E−k of the sub-band dispersion. When applied together, they reverse the dispersion parity to be odd, which gives Ee,k=−Eh,−k, and mix the electron and hole sub-bands within the energy range corresponding to the change in potential across the ribbon. This leads to oscillations of the ballistic conductance within this energy range. The broken time-reversal symmetry provides dichroism in the absorption of the circularly polarized light. As a consequence, one can observe electrically enhanced Faraday rotation, since the edges of the ribbon provide formation of the substantial density of states.

A unified description of sum frequency generation, parametric down conversion and two-photon fluorescence.

A superoperator non-equilibrium Greens function formalism is presented for computing nonlinear optical processes involving any combination of classical and quantum optical modes. Closed correlation-function expressions based on superoperator time-ordering are derived for the combined effects of causal response and non-causal spontaneous fluctuations. Coherent three wave mixing (sum frequency generation and parametric down conversion) involving one and two quantum optical modes, respectively, are compared with their incoherent counterparts: two-photon-induced fluorescence and two-photon-emitted fluorescence.

Gap-modulated doping effects on indirect exchange interaction between magnetic impurities in graphene

A dilute distribution of magnetic impurities is assumed to be present in doped graphene. We calculate the interaction energy between two magnetic impurities which are coupled via the indirect-exchange or Ruderman-Kittel-Kasuva-Yosida (RKKY) interaction by the doped conduction electrons. The current model is a half-filled AB-lattice structure. Our calculations are based on the retarded lattice Greens function formalism in momentum-energy space which is employed in linear response theory to determine the magnetic susceptibility in coordinate space. Analytic results are obtained for gapped graphene when the magnetic impurities are placed on the A and B sublattice sites of the structure. This interaction, which is important in determining spin ordering, has been found to be significantly different for AA and BB exchange energies in doped graphene due to the existence of an energy gap and is attributed to a consequence of the local fields not being equal on the A and B sublattices. For doped graphene, the osc...

Anisotropy of \pi-Plasmon Dispersion Relation of AA-Stacked Graphite

The dispersion relation of the high energy optical π-plasmons of simple hexagonal intrinsic graphite was calculated within the self-consistent-field approximation. The plasmon frequency ω p is determined as functions of the transferred momentum q || along the hexagonal plane in the Brillouin zone and its perpendicular component q z . These plasmons are isotropic within the plane in the long wavelength limit. As the in-plane transferred momentum is increased, the plasmon frequency strongly depends on its magnitude and direction (φ). With increasing angle, the dispersion relation within the hexagonal plane is gradually changed from quadratic to nearly linear form. There are many significant differences for the π-plasmon dispersion relations between two-dimensional (2D) graphene and three-dimensional (3D) AA-stacked graphite. They include q || - and φ-dependence and π-plasmon bandwidth. This result reveals that interlayer interaction could enhance anisotropy of in-plane π-plasmons. For chosen q || , we also ...

Photon entanglement signatures in difference-frequency-generation

In response to quantum optical fields, pairs of molecules generate coherent nonlinear spectroscopy signals. Homodyne signals are given by sums over terms each being a product of Liouville space pathways of the pair of molecules times the corresponding optical field correlation function. For classical fields all field correlation functions may be factorized and become identical products of field amplitudes. The signal is then given by the absolute square of a susceptibility which in turn is a sum over pathways of a single molecule. The molecular pathways of different molecules in the pair are uncorrelated in this case (each path of a given molecule can be accompanied by any path of the other). However, entangled photons create an entanglement between the molecular pathways.We use the superoperator nonequlibrium Greens functions formalism to demonstrate the signatures of this pathway-entanglement in the difference frequency generation signal. Comparison is made with an analogous incoherent two-photon fluorescence signal.

Second harmonic generation in gapped graphene

The second-order nonlinear optical susceptibility Π(2) for second harmonic generation is calculated for gapped graphene. The linear and second-order nonlinear plasmon excitations are investigated in context of second harmonic generation (SHG). We report a red shift and an order of magnitude enhancement of the SHG resonance with growing gap, or alternatively, reduced electro-chemical potential.

Two-photon-coincidence fluorescence spectra of cavity multipolaritons: novel signatures of multiexciton generation.

In a simulation study, we show that correlated two-dimensional frequency-resolved fluorescence spectra of a quantum dot in a microcavity provide a sensitive probe for the distribution of multiexcitons. Polariton couplings lead to a fine structure of Rabi multiplets that allow us to resolve otherwise overlapping features of the different multiexcitons. These may be used to probe multiexciton generation.