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Journal of Shaoguan University

Population Oscillation of Upper Level Excited by Laser Pulses Pair in Two-Level System

Yun Fang

Published Jan 1, 2001• 0 citations
OpenAlex

Abstract

With time evolution operator,it is proved that the population of upper level is in circumstance of cyclic oscillation by laser pulses pair of Gaussian line shape in a homogeneous broadening two level system.The oscillatory time oxide is the delay time between the laser pulse pairs and the oscillatory frequency equals the one of transition between the lower and upper level.

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Journal of Shaanxi Normal University•Jan 1, 2007•0 citations
With time evolution operator,it is proved that the population of upper level is in circumstance of cyclic oscillation by laser pulses pair of Gaussian line shape in a homogeneous broadening two level system.The oscillatory time oxide is the delay time between the laser pulse pairs and the oscillatory frequency equals the one of transition between the lower and upper level.
A useful expression for the quantum interference (QI) signal was derived for an inhomogeneously broadened two-level system when it was excited by an optically phase-controlled laser-pulse pair. It was shown that the QI signal oscillates as a function of a relative optical phase, with the reduced angular frequency given by the relation ωa = (Γ(2)ω0 + γg(2)Ω)/(γg(2) + Γ(2)), where γg and Γ are standard deviations of the system's absorption and the laser spectra both having a Gaussian line shape, respectively, and ω0 and Ω are the center angular frequency of the system absorption and the carrier angular frequency of the laser, respectively.
Introduction We were able to show in Chapter 3 that a medium in which we can obtain a population inversion (i.e. a situation in which the population density in the excited state is greater than that in the fundamental level) allows for optical gain of an electromagnetic wave having a frequency near to the resonant frequency of the system. By introducing feedback of the amplified signal into the medium, the system can be made to oscillate naturally, resulting in laser oscillations . To obtain this population inversion, we must introduce at least a third (and perhaps even a fourth) energy level into the system. (We saw how a two-level system under the influence of an intense pump beam will saturate with no resulting population inversion.) The aim then of this chapter is to introduce the concepts necessary to extend our two-level system into a working model capable of illustrating the phenomenon of laser oscillation. We will not spend too much time discussing atomic transition lasers as they do not figure readily in our treatment of quantum electronic properties of semiconductors. An exception will be made, however; we brush upon the particular topics of a diode pumped laser in Complement 4.E and a quantum cascade laser in Complement 13.H. Population inversion and optical amplification Population inversion We will show how population inversion can be achieved by carrier transfer from higher lying levels to the upper level of a two-level subsystem of interest.
The temporal evolution of atomic transition in a two-level system under the frequency-chirped pulse field and its dynamic analysis are discussed respectively by means of the density matrix equation and the method of Bloch vector model.The result shows that the population probability oscillates in time domain under the effects of frequency-chirped pulse.The dynamic evolution process of the Bloch vector indicates that the oscillation of the population probability reflects the physical transformation process among the population transfer,dispersion and absorption in the system.
The temporal evolution of atomic transition in a two-state system under the frequency-modulated field and its dynamic evolution are discussed by means of the density matrix equation and the method of Bloch vector model respectively. The results show that the oscillation of population probability appears in time domain under the effects of frequency -modulated pulse.The dynamic evolution of the Bloch vector indicates that the oscillation of population probability reflects the physical converted process among the population transfer,dispersion and absorption in the two-level system.
Full Maxwell-Bloch equations are solved with PC-FDTD method to investigate population evolution of dense V-type three-level medium in few-cycle laser pulses.It is shown that the ratio of transition dipole moments(γ) has strong influence on appearing time and oscillation frequency of population inversion.As γ=1 population inversion with LFC appears later than that without LFC and near dipole-dipole(NDD) interaction delays population inversion.As γ1,appearing with LFC is earlier than that without LFC and NDD interaction accelerates the population inversion.The number of population inversion with unequal transition dipole moments is more than that with equal transition dipole moments.Moreover,evolution of populations in a dense medium is quite different from that in a dilute medium.At input surface,in a dilute medium,population inversion appears with time evolution.With increasing of medium density,populations show quasi-periodicity oscillation and oscillation frequencies of populations with LFC are less than that without LFC.
In the dressed state picture,the transition process and the time evolution dynamic process of a two-level system excited by the sinusoidal phase-modulated shaping pulse are investigated with the Bloch vector sphere schematic diagram.The results show that when a two-level system is interacted with the proper sinusoidal phase shaping pulse,the population is inversed and remained in same state and is irrelevant of the!shaping pulse.During the interaction between field and system,the adiabatic states of particles are variant with the different circumstances of light field phase under the adiabatic condition.