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中国物理B：英文版

SPECTRUM OF A FEW-CYCLE LASER PULSE PROPAGATING IN A TWO-LEVEL ATOM MEDIUM

肖健, 王中阳, 徐至展

Published Jun 12, 2005• 0 citations
Publisher

Abstract

The spectrum evolution of a few-cycle optical pulse in a resonant two-level atom medium is studied theoretically by using the full Maxwell-Bloch equations. On the propagating pulse, significantly much faster oscillation components separated with the main pulse appear due to strong self-phase modulation and pulse reshaping. In this case, ideal self-induced transparency cannot occur for a 2 pi pulse. The spectrum of the 4 pi pulse shows an evident oscillatory feature because of the continuum interference of the separate pulses. For larger pulse areas, continuum generation from near ultraviolet to infrared occurs.

DOI
10.1088/1009-1963/10/10/011

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中国物理B：英文版•Jun 12, 2005•0 citations
The spectrum evolution of a few-cycle optical pulse in a resonant two-level atom medium is studied theoretically by using the full Maxwell-Bloch equations. On the propagating pulse, significantly much faster oscillation components separated with the main pulse appear due to strong self-phase modulation and pulse reshaping. In this case, ideal self-induced transparency cannot occur for a 2 pi pulse. The spectrum of the 4 pi pulse shows an evident oscillatory feature because of the continuum interference of the separate pulses. For larger pulse areas, continuum generation from near ultraviolet to infrared occurs.
The spectrum evolution of a few-cycle optical pulse in a resonant two-level atom medium is studied theoretically by using the full Maxwell-Bloch equations. On the propagating pulse, significantly much faster oscillation components separated with the main pulse appear due to strong self-phase modulation and pulse reshaping. In this case, ideal self-induced transparency cannot occur for a 2π pulse. The spectrum of the 4π pulse shows an evident oscillatory feature because of the continuum interference of the separate pulses. For larger pulse areas, continuum generation from near ultraviolet to infrared occurs.
The phase-dependent feature of few-cycle pulsed laser propagation in a resonant two-level atom medium is demonstrated by solving the full Maxwell-Bloch equations. Even in the perturbative region, the propagating carrier field and the corresponding spectra of the few-cycle pulsed laser are sensitive to the initial phase due to self-phase modulation. For the larger pulse area, the fact that the carrier-wave reshaping comes from the carrier-wave Rabi flopping is also responsible for this sensitivity, and the phase-dependent feature is more evident.
By solving the Maxwell-Bloch equations, we study the area evolution of a few-cycle pulse laser propagating in a resonant two-level-atom medium. We find that in short propagation distance, the pulse envelope, obtained within the slowly varying envelope approximation and the rotating-wave approximation, agrees nicely with the carrier field. In this case, the area theorem can still predict the profile of the area evolution of a few-cycle optical pulse. However, contrary to the long-pulse case, the variation of the few-cycle pulse area is caused by the pulse splitting but not by the pulse broadening or the pulse compression. Furthermore, the negative area occurs when the pulse area decreases. As a result, a pulse with area less than $\ensuremath{\pi}$ is not absorbed rapidly according to the usual Beer's law of absorption but evolves to nonvanishing zero $\ensuremath{\pi}$ pulse.
Full Maxwell-Bloch equations were solved by Predictor-Corrector method and finite-difference time-domain method to investigate spectrum of a sub-cycle laser pulse propagating in a dense Λ-type three-level atomic medium.It is found that both in a dilute medium and a dense medium,the spectrum is broaden due to the self-phase modulation(SPM);however,the width is much larger in the dense medium than that in a dilute medium because of NDD interaction.In the dense medium,the supercontinuum spectrum occurs due to SPM and NDD interaction,and the maximum frequency is 10 ωp.
We have studied the spectral behaviour of few-cycle soliton pulses in a non-resonant two-level atom medium by solving the full Maxwell-Bloch equations. It is demonstrated further that the carrier effects play an important role in the propagation of the few-cycle pulse laser. When the frequency detuning is not very large, both the population distribution and the refractive index of the medium follow the oscillatory carrier field instantaneously; in this case, carrier-wave compression or carrier shock occurs and a supercontinuum broader than that in the resonant medium may be generated. When the frequency detuning is large, the carrier shock is weak and the spectrum is not continuous, only showing an odd harmonic radiation.
We have studied the spectral behaviour of few-cycle soliton pulses in a non-resonant two-level atom medium by solving the full Maxwell-Bloch equations. It is demonstrated further that the carrier effects play an important role in the propagation of the few-cycle pulse laser. When the frequency detuning is not very large, both the population distribution and the refractive index of the medium follow the oscillatory carrier field instantaneously; in this case, carrier-wave compression or carrier shock occurs and a supercontinuum broader than that in the resonant medium may be generated. When the frequency detuning is large, the carrier shock is weak and the spectrum is not continuous, only showing an odd harmonic radiation.