We investigate theoretically the effects of interaction between an optical dipole (semiconductor quantum dot or molecule) and metal nanoparticles. The calculated absorption spectra of hybrid structures demonstrate strong effects of interference coming from the exciton-plasmon coupling. In particular, the absorption spectra acquire characteristic asymmetric lineshapes and strong antiresonances. We present here an exact solution of the problem beyond the dipole approximation and find that the multipole treatment of the interaction is crucial for the understanding of stronglyinteracting exciton-plasmon nano-systems. Interestingly, the visibility of the exciton resonance becomes greatly enhanced for small inter-particle distances due to the interference phenomenon, multipole effects, and electromagnetic enhancement. We find that the destructive interference is particularly strong. Using our exact theory, we show that the interference effects can be observed experimentally even in the exciting systems at room temperature.

We investigate theoretically the electronic transport properties in narrow graphene ribbons with an adatom-induced defect. It is found that the lowest conductance step of a metallic graphene nanoribbon may develop a dip even down to zero at certain values of the Fermi energy due to the defect. Accompanying the occurrence of the conductance dip, a loop current develops around the defect. We show how the properties of the conductance dip depend on the parameters of the defect, such as the relative position and severity of the defect as well as the width and edges of the graphene ribbons. In particular, for metallic armchair-edges graphene nanoribbons, whether the conductance dip appears or not, they can be controlled by choosing the position of the single defect.

We design a plasmonic fiber waveguide (PFW) composed of coaxial cylindrical metal-dielectric multilayers in nanoscale, and constitute the corresponding dynamical equations describing the modes of propagation in the PFW with the Kerr nonlinearity in the dielectric layers. The physics is connected to the discrete matrix nonlinear Schrödinger equations, from which the highly confined ring-like solitons in scale of subwavelength are found both for the visible light and the near-infrared light in the self-defocusing condition. Moreover, the confinement could be further improved when increasing the intensity of the input light due to the cylindrical symmetry of the PFW, which means both the width and the radius of the ring are reduced. PACS numbers: 73.20.Mf, 78.67.Pt, 42.65.Tg How to control the propagation of the light is a most important subject in optics. Using the technology of the optical fiber waveguides (OFWs) to pilot the light is a big advance towards all-optical signal processing. When consider ever-accelerated miniaturization of optical devices, however, the conventional OFWs seem to be difficult to fulfill the requirement because of the diffraction limitation for the optical components of dielectric photonic materials. Recently, it is shown that the limitation may be overcome in the rapid developing field of plasmonics[1-3] based on properties of the surface plasmon polariton (SPP), a confined mode localized at the interface of metal and dielectric material [4,5]. Subsequently wide attentions have been paid to miscellaneous nanostructures with metamaterials involved in pursuit of subwavelength confinement of the light [6]. Among these progresses, several researchers have predicted the subwavelength control of the light in various lattices through the formation of solitons when the nonlinearity is considered. It has been found that the planar nonlinear metal-dielectric multilayers (MDM), a stack of alternating metal-dielectric nanolayers, can effectively manipulate the propagation of light [7][8][9] and put a subwavelength confinement on it [10][11][12][13][14][15]. For instance, Zhang's group theoretically found the subwavelength discrete soliton in the nanoscaled periodic structures consisting of MDM [14]. Ye et al. predicted the stable fundamental and vortical plasmonic lattice solitons of subwavelength extent in arrays of metallic nanowires embedded in a nonlinear medium [15].Then could we construct a kind of waveguide in analogy to the optical fiber waveguide to control the light's propagation in two-dimensional (2D) transverse space based on the plasmonics rather than the optics? In this Letter we design a plasmonic fiber waveguide (PFW) by rolling the MDM to a cylindrical shape to guide the light. The nonlinearity in the dielectric layers are employed to realize the subwavelength confinement in radial direction when propagating along the axial direction. Based on the advanced nano-technologies [16][17][18] in addition to the sophisticated fiber fabrications, this kind of PFW is supposed to be a e...

We present the theory of excitonic high-order sideband generation ͑HSG͒ in semiconductors by an intense terahertz ͑THz͒ field. When the Coulomb interaction is neglected, we give an analytical solution to the HSG with the help of Floquet theory. Besides, the HSG is also studied by the quantum trajectory theory ͑saddle-point method͒ by which the HSG is explained by the interference of different quantum trajectories of excitons when accelerating in the external THz field. Both the exact analytic solution and the saddle-point method obtain consistent results: the spectrum of sidebands has an extended plateau where all the sidebands have almost the same intensity, which is similar to the high-order harmonic generation in atomic system. Moreover the HSG provides more flexibility in studying the quantum trajectory theory. When Coulomb interaction is considered, we find considerable Coulomb enhancement of HSG, which is absent in atomic system. The mechanism is discussed based on numerical calculations.

We investigate the midinfrared generation from difference frequency in self-assembled quantum dots near metal nanoparticles with two-color interband excitations. The generated signal strength is enhanced by several orders of magnitude due to the plasmon-exciton resonance in the nanosuperstructures. The signal enhancement is found to be the result of competition between local electric field enhancement and excitonic lifetime shortening. Therefore, there exists an optimal interparticle distance for the difference-frequency generation. This nanomolecule is proposed to be an excellent candidate for the midinfrared quantum dot laser.

Abstract. We theoretically study the excitonic optical absorption in semiconductor superlattices irradiated by an intense terahertz (THz) laser polarized along the growth direction. We calculate the linear excitonic absorption spectra using the finite-difference time-domain method in real space. Our numerical results and qualitative analysis show that when tuning the frequency or intensity of the THz laser, the dynamical localization (DL) and the ac Stark effect (ACSE) are either jointly or individually responsible for the exciton peak shift. Moreover, with the onset of miniband collapse, the excitonic dimensionality crossover and the DL will counteract each other in shifting the exciton peak. The direct experimental verification of the DL through semiconductor optical spectra is still a challenge. Regarding this fact, we propose a method to single out the DL effect from the spectral shift dominated by the ACSE.

The theory of excitonic high-order sideband generation (HSG) in a semiconductor quantum well irradiated by two orthogonal terahertz (THz) fields (one frequency is an integral multiple of the other) is presented. The exact analytical solution to the sideband spectrum is given with the help of the generalized Bessel functions. As a special case, the HSG when the frequencies of these two THz fields are the same is derived and its dependence on the ellipticity of the THz field is discussed. The theory could explain the experiments, especially concerning the sensitive dependence of HSG signals on the ellipticity of the THz field: the signals are strong when the THz field has a linear polarization and totally vanish in case of a circular polarization. More interestingly, it was found that the strongest signal is not produced in the case of linear polarization for some sidebands. The theory is supported by numerical calculations.

An intrinsic topological metal state is found in the T-graphene, a monolayer with both the time-reversal symmetry and the four-fold symmetry. The state distinguishes itself by the nontrivial electric polarization from the ordinary metals and features with two local edge states in the corresponding nanoribbons. The topological metal state is confirmed as a transition state bridging the ordinary metal state and the topological insulator state when the relative neighboring hoppings change in the lattice. The topological nature is further verified by checking the robustness of transport property against randomly-introduced strong disorders. The fact that the multiple topological states indexed by different parameters coexist in such a practical system shows a broad prospect in versatile topological transport devices.