engleski

Accumulation effects in a multi-level atom excited by a train of ultrashort pulses

Femtosecond (fs) mode-locked lasers generate ultrashort pulses by establishing a fixed phase relationship between all the lasing longitudinal modes. Laser output is characterized by a train of identical pulses, separated by a fixed time interval. The corresponding frequency spectrum of a mode-locked laser consists of a discrete, regularly spaced series of sharp lines, known as a frequency comb. The unification of the time- and frequency-domain treatment of the mode-locked lasers and the pulse train they generate allows us simultaneous investigation of the time-resolved atomic dynamic and spectral probing in the frequency domain. This unification is based on coherences accumulation effects in systems characterized by relaxation times longer than the laser repetition period. Detailed theoretical investigation, based on numerically solving the Bloch equations, provide the complete description of the temporal evolution of the multi-level atomic system excited by a train of fs pulses. Also, as direct results of the frequency comb excitation, the accumulation of populations and coherences were obtained. Since the numerical procedures for multi-level atomic systems is time-consuming, iterative analytical solution to optical Bloch equations describing fs pulse-train resonant excitation of four-level atoms is presented. Comparison of the analytical solution with the results obtained by direct numerical integration of the Bloch equations reveals that given analytical solution completely follows time dynamics, thus providing a practical tool for fast and accurate description of atom-laser field interaction. We applied the analytic solution to investigate typical coherences accumulation effects both in time- and frequency-domain. The influence of various pulse-train parameters, such as the number of excitation pulses, pulse peak power, pulse repetition frequency and frequency shift of the comb spectrum on atomic time dynamics and spectral properties are presented.

ultrashort pulses; femtosecon lasers; accumulation effects

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