Strong Purcell enhancement at telecom wavelengths afforded by spinel FeO nanocrystals with size-tunable plasmonic properties.

Developments in the field of nanoplasmonics have the potential to advance applications from information processing and telecommunications to light-based sensing. Traditionally, nanoscale noble metals such as gold and silver have been used to achieve the targeted enhancements in light-matter interactions that result from the presence of localized surface plasmons (LSPs). However, interest has recently shifted to intrinsically doped semiconductor nanocrystals (NCs) for their ability to display LSP resonances (LSPRs) over a much broader spectral range, including the infrared (IR).

Temperature-Induced Plasmon Excitations for the - Lattice in Perpendicular Magnetic Field.

We have investigated the α-T3 model in the presence of a mass term which opens a gap in the energy dispersive spectrum, as well as under a uniform perpendicular quantizing magnetic field. The gap opening mass term plays the role of Zeeman splitting at low magnetic fields for this pseudospin-1 system, and, as a consequence, we are able to compare physical properties of the the α-T3 model at low and high magnetic fields. Specifically, we explore the magnetoplasmon dispersion relation in these two extreme limits.

Second-harmonic generation in nonlinear plasmonic lattices enhanced by quantum emitter gain medium.

We report on a theoretical study of second-harmonic generation (SHG) in plasmonic nanostructures interacting with two-level quantum emitters (QEs) under incoherent energy pump. We generalize the driven-dissipative Tavis-Cummings model by introducing the anharmonic surface plasmon-polariton (SPP) mode coupled to QEs and examine physical properties of corresponding SPP-QE polariton states. Our calculations of the SHG efficiency for strong QE-SPP coupling demonstrate orders of magnitude enhancement facilitated by the polariton gain.

Magnetoplasmons for the-Tmodel with filled Landau levels.

Using the-Tmodel, we carried out analytical and numerical calculations for the static and dynamic polarization functions in the presence of a perpendicular magnetic field. The model involves a parameterwhich is the ratio of the hopping strength from an atom at the center of a honeycomb lattice to one of the atoms on the hexagon to the hopping strength around its rim. Our results were employed to determine the longitudinal dielectric function and the magnetoplasmon dispersion relation.

Resonance Raman signature of intertube excitons in compositionally-defined carbon nanotube bundles.

Electronic interactions in low-dimensional nanomaterial heterostructures can lead to novel optical responses arising from exciton delocalization over the constituent materials. Similar phenomena have been suggested to arise between closely interacting semiconducting carbon nanotubes of identical structure. Such behavior in carbon nanotubes has potential to generate new exciton physics, impact exciton transport mechanisms in nanotube networks, and place nanotubes as one-dimensional models for such behaviors in systems of higher dimensionality.

Plasmonic giant quantum dots: hybrid nanostructures for truly simultaneous optical imaging, photothermal effect and thermometry.

Hybrid semiconductor-metal nanoscale constructs are of both fundamental and practical interest. Semiconductor nanocrystals are active emitters of photons when stimulated optically, while the interaction of light with nanosized metal objects results in scattering and ohmic damping due to absorption. In a combined structure, the properties of both components can be realized together.

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.

Effect of localized surface-plasmon mode on exciton transport and radiation emission in carbon nanotubes.

We report on a general theoretical approach to study exciton transport and emission in a single-walled carbon nanotube (SWNT) in the presence of a localized surface-plasmon (SP) mode within a metal nanoparticle interacting via near-field coupling. We derive a set of quantum mechanical equations of motion and approximate rate equations that account for the exciton, SP, and the environmental degrees of freedom. The material equations are complemented by an expression for the radiated power that depends on the exciton and SP populations and coherences, allowing for an examination of the angular distribution of the emitted radiation that would be measured in experiment.

Photon dressed electronic states in topological insulators: tunneling and conductance.

We have obtained analytic results for the surface states of three-dimensional topological insulators in the presence of circularly polarized light. This electron-photon interaction results in an energy gap as well as a novel energy dispersion of the dressed electron-photon states, different from both graphene and the standard two-dimensional electron gas (2DEG). Additionally, we made calculations of the ballistic conductance and Klein tunneling in both two- and three-dimensional topological insulators as well as investigating how these phenomena are affected in the presence of circularly polarized light.

Effects of nonlocal plasmons in gapped graphene micro-ribbon array and two-dimensional electron gas on near-field electromagnetic response in the deep subwavelength regime.

A self-consistent theory involving Maxwell's equations and a density-matrix linear-response theory is solved for an electromagnetically coupled doped graphene micro-ribbon array (GMRA) and a quantum well (QW) electron gas sitting at an interface between a half-space of air and another half-space of a doped semiconductor substrate, which supports a surface-plasmon mode in our system. The coupling between a spatially modulated total electromagnetic (EM) field and the electron dynamics in a Dirac-cone of a graphene ribbon, as well as the coupling of the far-field specular and near-field higher-order diffraction modes, are included in the derived electron optical-response function. Full analytical expressions are obtained with nonlocality for the optical-response functions of a two-dimensional electron gas and a graphene layer with an induced bandgap, and are employed in our numerical calculations beyond the long-wavelength limit (Drude model).

Anomalous photon-assisted tunneling in graphene.

We investigated the transmission of Dirac electrons through a potential barrier in the presence of circularly polarized light. An anomalous photon-assisted enhanced transmission is predicted and explained. It is demonstrated that the perfect transmission for nearly head-on collision in infinite graphene is suppressed in gapped dressed states of electrons, which is further accompanied by a shift of peaks as a function of the incident angle away from head-on collision.

Plasma excitations in graphene: their spectral intensity and temperature dependence in magnetic field.

In this paper, we calculated the dielectric function, the loss function, the magnetoplasmon dispersion relation and the temperature-induced transitions for graphene in a uniform perpendicular magnetic field B. The calculations were performed using the Peierls tight-binding model to obtain the energy band structure and the random-phase approximation to determine the collective plasma excitation spectrum. The single-particle and collective excitations have been precisely identified based on the resonant peaks in the loss function.

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.

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.

Generalized Kramers-Heisenberg expressions for stimulated Raman scattering and two-photon absorption.

The frequency-domain pump-probe signal of a material system interacting with two quantum modes of the radiation field is recast in terms of products of scattering amplitudes (T matrix elements) rather than the third-order susceptibility Im chi((3)). The resulting expression offers a more intuitive physical picture for the optical process compared with the semiclassical approach which treats the radiation field as classical. It can be derived and interpreted using closed-time-path-loop diagrams which represent the joint state of the matter and the field for each contribution to the signal.

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

A superoperator non-equilibrium Green's 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.

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.

Nonlinear spectroscopy with entangled photons; manipulating quantum pathways of matter.

Optical signals obtained by the material response to classical laser fields are given by nonlinear response functions which can be expressed by sums over various quantum pathways of matter. We show that some pathways can be selected by using nonclassical fields, through the entanglement of photon and material pathways, which results in a different-power law dependence on the incoming field intensity. Spectrally overlapping stimulated Raman scattering (SRS) and two-photon-absorption (TPA) pathways in a pump probe experiment are separated by controlling the degree of entanglement of pairs of incoming photons.

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.

Multidimensional pump-probe spectroscopy with entangled twin-photon states.

We show that entangled photons may be used in coherent multidimensional nonlinear spectroscopy to provide information on matter by scanning photon wave function parameters (entanglement time and delay of twin photons), rather than frequencies and time delays, as is commonly done with classical pulses. Signals are expressed and interpreted intuitively in terms of products of matter and field correlation functions using a diagrammatic close time path loop formalism which reveals the entangled quantum pathways of the fields and matter. The pump-probe signal measured when the pump and the probe are in a twin entangled state shows two-photon resonant contributions which scale linearly rather than quadratically with the incident beam intensity and reveal frequencies of off-resonant transitions.

Theory of enhanced second-harmonic generation by the quadrupole-dipole hybrid exciton.

We report calculated substantial enhancement of the second-harmonic generation (SHG) in cuprous oxide crystals, resonantly hybridized with an appropriate organic material (DCM2:CA:PS 'solid state solvent'). The quadrupole origin of the inorganic part of the quadrupole-dipole hybrid provides inversion symmetry breaking and the organic part contributes to the oscillator strength of the hybrid. We show that the enhancement of the SHG, compared to the bulk cuprous oxide crystal, is proportional to the ratio of the DCM2 dipole moment and the effective dipole moment of the quadrupole transitions in the cuprous oxide.