We develop a microscopic theory of a strong electromagnetic radiation interaction with bilayer graphene where an energy gap is opened by a static electric field perpendicular to graphene planes. We show that an adiabatic changing on time of the gate potentials (that leads to the resonance of the energy gap with electromagnetic field) may produce full inversion of the electron population between valence and conduction bands. Quantum kinetic equations for density matrix are obtained by the use of a tight-binding approach within second quantized Hamiltonian in an intense laser field and taking into account Coulomb correlations between particles. <p> </p>Excitonic absorption in graphene systems (monolayer and bilayer) with opened energy gap is investigated for different values of the gap and the parameters describing the band structure.
We developed a variational approach to investigate the ground state energy and the extend of the wavefunction of a neutral donor located near a semiconductor surface in the presence of scanning tunneling microscope (STM) metallic tip. We apply the effective mass approximation and use a variational wavefunction that takes into account the influence of all image charges that arise due to the presence of a metallic tip. The behavior of the ground state energy when the tip approaches the semiconductor surface is investigated.