14 research outputs found
Photon-assistant Fano resonance in coupled multiple quantum dots
Based on calculations of the electronic structure of coupled multiple quantum
dots, we study systemically the transport properties of the system driven by an
ac electric field. We find qualitative difference between transport properties
of double coupled quantum dots (DQDs) and triple quantum dots. For both
symmetrical and asymmetrical configurations of coupled DQDs, the field can
induce the photon-assisted Fano resonances in current-AC frequency curve in
parallel DQDs, and a symmetric resonance in serial DQDs. For serially coupled
triple quantum dots(STQDs), it is found that the -type energy level
has remarkable impact on the transport properties. For an asymmetric (between
left and right dots) configuration, there is a symmetric peak due to resonant
photon induced mixing between left/right dot and middle dot. In the symmetric
configuration, a Fano asymmetric line shape appears with the help of ``trapping
dark state". Here the interesting coherent trapping phenomena, which usual
appear in quantum optics, play an essential role in quantum electronic
transport. We provide a clear physics picture for the Fano resonance and
convenient ways to tune the Fano effects.Comment: 7 pages, 5 figure
Power Amplification and Coherent Combination Techniques for Terahertz Quantum Cascade Lasers
Power amplification and coherent combination are important ways to improve the output power and beam quality of single‐mode terahertz quantum cascade lasers (THz QCLs). Up to date, the tapered waveguide is the most convenient way to amplify the power of THz QCLs. The self‐focusing effect in tapered THz QCLs induces non‐monotonic behaviours of the peak power and far‐field beam divergence, which lead to the existence of optimal structural parameters. The surface and lateral grating techniques have also been employed in tapered THz QCLs to further improve the spectral purity. For coherent combinations, the progress of facet‐emitting phase‐locked arrays of THz QCLs is still limited due to both the lack of the understanding of dynamics of coupled QCLs and the difficulties in designing high‐performance coupled waveguides. We will briefly review the developments of coherent arrays of THz QCLs and present a design of monolithic QCL arrays with common coupled cavity to achieve the optical mutual injection, which may provide a new way for coherent combination of THz QCLs
Optical properties of coupled metal-semiconductor and metal-molecule nanocrystal complexes: the role of multipole effects
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
anti-resonances. 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 strongly-interacting 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.Comment: 9 page
Atomic oxygen adsorption and incipient oxidation of the Pb(111) surface: A density-functional theory study
We study the atomic oxygen adsorption on Pb(111) surface by using
density-functional theory within the generalized gradient approximation and a
supercell approach. The atomic and energetic properties of purely on-surface
and subsurface oxygen structures at the Pb(111) surface are systematically
investigated for a wide range of coverages and adsorption sites. The fcc and
tetra-II sites (see the text for definition) are found to be energetically
preferred for the on-surface and subsurface adsorption, respectively, in the
whole range of coverage considered. The on-surface and subsurface oxygen
binding energies monotonically increase with the coverage, and the latter is
always higher than the former, thus indicating the tendency to the formation of
oxygen islands (clusters) and the higher stability of subsurface adsorption.
The on-surface and subsurface diffusion-path energetics of atomic oxygen, and
the activation barriers for the O penetration from the on-surface to the
subsurface sites are presented at low and high coverages. In particular, it is
shown that the penetration barrier from the on-surface hcp to the subsurface
tetra-I site is as small as 65 meV at low coverage (=0.25). The other
properties of the O/Pb(111) system, including the charge distribution, the
lattice relaxation, the work function, and the electronic density of states,
are also studied and discussed in detail, which consistently show the gradually
stabilizing ionic O-Pb bond with increase of the oxygen coverage.Comment: 31 pages, 16 figure
Quantum blockade and loop current induced by a single lattice defect in graphene nanoribbons
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.Comment: 6 pages, 6 figure
Quasienergy spectra of a charged particle in planar honeycomb lattices
The low energy spectrum of a particle in planar honeycomb lattices is
conical, which leads to the unusual electronic properties of graphene. In this
letter we calculate the quasienergy spectra of a charged particle in honeycomb
lattices driven by a strong AC field, which is of fundamental importance for
its time-dependent dynamics. We find that depending on the amplitude, direction
and frequency of external field, many interesting phenomena may occur,
including band collapse, renormalization of velocity of ``light'', gap opening
etc.. Under suitable conditions, with increasing the magnitude of the AC field,
a series of phase transitions from gapless phases to gapped phases appear
alternatively. At the same time, the Dirac points may disappear or change to a
line. We suggest possible realization of the system in Honeycomb optical
lattices.Comment: 4+ pages, 5 figure
Critical parameters for the one-dimensional systems with long-range correlated disorder
We study the metal-insulator transition in a tight-binding one-dimensional
(1D) model with long-range correlated disorder. In the case of diagonal
disorder with site energy within and having a
power-law spectral density , we investigate the
competition between the disorder and correlation. Using the transfer-matrix
method and finite-size scaling analysis, we find out that there is a finite
range of extended eigenstates for , and the mobility edges are at
. Furthermore, we find the critical exponent of
localization length () to be
. Thus our results indicate that the disorder strength
determines the mobility edges and the degree of correlation
determines the critical exponents.Comment: 6 pages, 6 figure