Decay, amplification and absorption of initial terahertz pulse in resonant two-level medium based on noninteracting array of zigzag nanotubes and armchair nanoribbons

Abstract

Theoretical investigation and computer simulation of the evolution of terahertz (THz) radiation in a two-level resonance gain medium was carried out on the basis of Maxwell-Bloch equations, using the slowly varying amplitude approximation. The gain medium is based on a system of noninteracting parallel-oriented metallic carbon nanotubes and nanoribbons. Considering the Maxwell-Garnett theory (MGT), the effective dielectric function (EDF) of the medium was obtained. Due to the large nanotube dipole moment (or large value of oscillator strength) not only THz radiation occurs, but also a totality of periodic pulses generation. Results of simulation totally coincide with the theoretical ones. The equation for the phase by analogy with the self-induced transparency (SIT), taking into account the dissipative processes, was obtained. The obtained equation describes a mathematical pendulum, where friction force is proportional to the velocity and directed to the opposite of particle movement. The friction leads to the fact that the pendulum makes a full rotation around the axis or decays, depending on the absorption properties of a gain medium. It was shown that at the certain properties of a gain medium, the equation coincides with the well-known Sine-Gordon equation. It was obtained, that at the initial pumping 6 × 1020 m-3 and at the volume fraction of nanoparticles about 10-7, the stationary value of radiation energy in a resonator is about 1 J/m3.