Essential Physical Problems On Weak Optical-Elctronic Signals Tansporting Along Low-Dimensional Nano Structures

The need of efficient storage and accurate processing of information urges the modern integrated circuit technology, which is the base of information technology and microelectronics industry, to develop toward higher integration level, faster computing speed, and lower power consumption. However, the micron-sized structures of microelectronic technology is said to have become the bottleneck that restricts the rapid development of information technology. Thus, scientists are anxious to find a revolutionary way to overcome this limitation, which makes the low-dimensional nano structure become the key subject investigated in such fields as modern condensed matter physics, and quantum information. Nowadays, mesoscopic metal rings, graphene, photonic crystal waveguides, nano-metal plasma waveguide, microwave transmission lines, and optical nano-fibers are considered to have potential application in next generation of devices. Because of their controllable quantum effects, the study of the quantum coherent transporting along these low-dimensional nanostructures is one of the active domains in microelectronics, optoelectronics, ultra-high density storage, and quantum computers.In this thesis, we adopt the quantum waveguide theory to investigate the single electron transport along a mesocopic ring driven by time-period magnetic fluxes, Driac fermions transmission along Graphene junction driven by two ac signals, and the single photon transport along a nano quantum optical wavguide. We find that photon assisted tunneling reveals when the mesoscopic ring driven by time-period fluxes and current density waves appear along the mesoscopic ring and the lead connected the ring and the source; The time-depend transmission of the Dirac particles in Graphene nano junction has direct relation on the input angle of the electron; The transmission probabilities of the injected photon can by adjusted by the coupling strength between the photon and the two-level atom. We believe that the findings of our studies will have great effect on the potential experimental schemes of all-optical switches and quantum beam splitters.In chapter3, the photon-assisted tunneling of the elctron transport along the mesoscopic ring driven by time-period fluxes is investigated. We used the quantum waveguide theory combined by the Floquet scattering theorem to investigate the electron transport along an open mesoscopic ring with two leads driven by a time-periodic magnetic flux. Numerical results showed the photon-assisted tunnelings are insensitive of the amplitude of the applied static magnetic flux. The dynamic component of the applied flux induces the photon-assisted tunnelings; i.e., the number of the appeared transmission peaks. The photon-assisted tunneling investigated could be utilized to measure the frequency of the applied oscillating external field.In chapter4, current density wave along the mesoscopic ring is studied. We used the quantum waveguide theory combined with the Floquet scattering theorem to investigate electron transport along an open mesoscopic ring with only one lead driven by a time-periodic magnetic flux. We showed particularly that a current density wave could be excited along the open ring threaded by the time-periodic magnetic flux, and a net current could also be generated in the lead connected to only an electron reservoir. Numerical results showed that the amplitude of the dynamic fluxes can modulate the photon-assisted process between the lead and the ring. When the dynamic amplitude is relatively small, the electrons with different energies are reflected back to the lead and the current in the ring is not influenced. However, with the increase of the amplitude of the dynamic flux, the reflection coefficient could be less than unit.In chapter5, Dirac particles response the ac driven signal in Graphene in considered. By utilizing the massless Dirac equation and Tien-Gordon theory, we have investigated the quasi-particles transporting through a graphene junction driven by two different time-periodic ac signals. Our numerical results show that the time-oscillating behaviors of transmissions depend on the particles’ incident angles. Due to the Klein tunneling, the transmission probabilities for the normal incidence are insensitive to static potentials, phases, and driven frequencies. However, when the electrons are incident with a fixed angle, the probabilities begin to oscillate with time.In chapter6, we investigate the size effect in closed graphene ring. We adopt the massless Dirac equation, combined by the infinite mass boundary condition, to investigate the Aharonov-Bohm effect in a closed graphene ring driven by magnetic flux. The numerical results clearly suggest that the applied magnetic flux breaks the time reversal symmetry and lifts the valley degeneracy in graphene ring. Particularly, the energy level, the energy difference between two valleys, and the absolute amplitude of the oscillating persistent current are all sensitive to the values of the radius and the width.In chapter7, the single photon transport aong the one dimensional qautum optical waveguide is ivestigated. We study the single-photon transport along a one-dimensional optical waveguide containing an asymmetrically-coupled two-level aotm. Differing from the symmetric coupling cases discussed previously, we showed that the transmission probabilities of the incident single photons can vary from0to100%, depending on its asymmetrical couplings with the two-level aotm. This phenomenon is related to the redistribution of energy and momentum of photons after the scattering by the two-level aotm, as the present photon-atom interaction loses the usual spatial-symmetry. When the incident photon and the two-level aotm is on resonance, the amplitude of the excited spectra can still be adjusted by the coupling strength.