In this thesis, we present a systematic study of the photon-assisted tunneling (PAT) through a mesoscopic structure, such as quantum well, quantum wire or quantum dot. The main objective is to fully understand the effect of the time-coherence electron-photon interaction on the spatial-coherence electronic tunneling.; The concept of PAT is originated from Tien and Gordon, who invested the electron tunneling in superconductor junctions by absorbing a number of photons. Theoretically, Wingreen and co-workers present formalism for time-dependent coherent tunneling assuming single electronic level in the mesoscopic structures. We are the first group to investigate coherent transport with the intra-subband transition included. A complete numerical solution is presented for a Hamiltonian describing a two-level atom coherently coupling with both external time-dependant field and reservoirs. From such a solution, we clarified many aspects of electon-photon interaction that were unclear and have yielded new information about the effect of spatial coherence and confinement on the electron-photon interaction.; We also derived the Coulomb blockade model including an external photon field. Based on this model, the PAT in quantum dot system with strong intra-dot electron-electron interaction is investigated. Under such situation, the external light field may modify the intra-dot electron distribution, and indirectly modify the tunneling property.; Most works studying PAT model the photon field as a classical time-dependant function for its computational feasibility. However, the interaction of electrons with the vacuum fluctuation may change the electronic density of states if electron-photon interaction is not in the weak limit. We studied the PAT using a quantum electrodynamic (QED) description of photons. The QED result shows enhance effect of electron-photon interaction.; The non-equilibrium Green function (GF) is originated from Schwinger's time-loop technique, and its perturbation expansion and graph technique is well developed by Keldysh. However, the equation of motion, which is a convenient form for numerical computation, involves undefined singularities, and cannot be applied independently. In this thesis, we developed the equation of motion for non-equilibrium GF, and derive the explicit form of the singularity. The equation of motion for non-equlibrium GF is the main tool for theoretical computation throughout this thesis.
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