It is shown that, when two femtosecond chirped Gaussian pulses with equal pulse area and same size but opposite sign of the chirp coefficient, which will be simply called as double pulses, overlap and propagate in the three-level Λ-type atomic medium, both coherence between the double pulses and interaction between the double pulses and the atomic medium arise. Adjusting size of the chirp coefficient can change shape, i.e. field distribution, of the composite pulse of the double pulses, at the same time affect the interaction between the composite pulse field and atoms, and thereby control oscillation process and value of the atomic population. By selecting suitable size of the chirp coefficient, we can make the atoms at the lowest level exciting completely to the higher level; moreover the new population distribution is stationary. It is also show that, for the double pulses with any area, efficient and stable population transfer always can be realized by adjusting size of the chirp coefficient. And this conclusion doesn't vary with the pulse width or the medium density varying.
It is shown that, when two femtosecond chirped Gaussian pulses with equal pulse area and same size but opposite sign of the chirp coefficient, which will be simply called as double pulses, overlap and propagate in the three-level Λ-type atomic medium, both coherence between the double pulses and interaction between the double pulses and the atomic medium arise. Adjusting size of the chirp coefficient can change shape, i.e. field distribution, of the composite pulse of the double pulses, at the same time affect the interaction between the composite pulse field and atoms, and thereby control oscillation process and value of the atomic population. By selecting suitable size of the chirp coefficient, we can make the atoms at the lowest level exciting completely to the higher level; moreover the new population distribution is stationary. It is also show that, for the double pulses with any area, efficient and stable population transfer always can be realized by adjusting size of the chirp coefficient. And this conclusion doesn't vary with the pulse width or the medium density varying.
The interference resonant propagation and spectral properties of a superposition of two femtosecond chirped Gaussian pulses with equal pulse area and same size but opposite sign of the chirp coefficient (C) in a three-level Λ-type atomic medium is investigated by using the numerical solution, which is obtained by the finite-difference time-domain (FDTD) method and the iterative predictor–corrector (PC) method for the full Maxwell–Bloch equations. It is found that, for the double pulses with smaller area, (2π, 2π) double pulses, the pulse splitting occurs when the value of the |C| is smaller, and only the variation of pulse shape is present but the pulse splitting no longer occur when the value of the |C| increases to a certain value; New high frequency component doesn’t basically appear and the strength of the spectral component near the central frequency decreases considerably but the strength of blue shift component is not varied obviously with the value of |C| increasing. For the double pulses with larger areas, the case of pulse splitting is similar to that of (2π, 2π) pulses, but the strength of the spectral component with higher frequency increase evidently comparing with the case of (2π, 2π) double pulses. Moreover, the value of the |C| also has an obvious effect on population, different population evolutions can be achieved by adjusting the value of |C|.
We investigate propagation of femtosecond chirped Gaussian laser pulse in a dense three-level Λ-type atomic medium by using the numerical solution of the full Maxwell-Bloch equations without the slowly varying envelope and the rotating-wave approximations, and the solution is obtained by PC-FDTD method. It is shown that, variation of the sign and size of the chirp coefficient has considerable effect on pulse propagation property, and the effect is closely relative to size of the pulse area. When the area of chirped pulse is smaller than 4π, splitting doesn’t occur and the chirped pulse evolves gradually to an approximate normal Gaussian pulse (C=0), and this characteristic doesn’t vary with the chirp coefficient varying. However, variation of the chirp coefficient will changed the amplitude and group velocity of the pulse. For the positive chirp(C>0), amplitude and group velocity of the pulse decrease with chirp coefficient increasing, for the negative chirp(CC increasing. Both the chirped pulses with area equal to larger than 4π will split into sub-pulses of different numbers and shapes, the time and number of the pulse splitting will be determined by the sign and size of the chirp coefficient. But in the two cases, the pulse splitting patterns are much different, and the effects of the coefficient are also different. When the pulse area equals 4π, larger chirp coefficient will lead to increased sub-pulse number, but when the pulse area is larger than 4π, larger chirp coefficient will lead to decreased sub-pulse number. In addition, regardless of pulse area being larger or smaller, changing sign and size of the chirp coefficient always produces obvious effect on the atomic population.
It is shown that, variation of the sign and size of the chirp coefficient (C) of the pulse has considerable effect on spectral properties of the pulse and the effect is closely relative to size of the pulse area. When the pulse with smaller area, 2π pulse, propagates in the medium, pulse splitting doesn't occur and the pulse evolves gradually to an approximate normal Gaussian pulse (C=0); new high frequency component doesn't basically appear; with increasing value of C, oscillation amplitude of blue shift and red shift components increases and blue shift component oscillates more severely; moreover, the strength of the spectral component near the central frequency decreases considerably but the strength of blue shift component increases obviously. When the 4π pulse propagates in the medium, the pulse will split into sub-pulses with different numbers and shapes, new high frequency component can be produced, but the strength of the high frequency component is smaller; similar to the case of 2π pulse, blue shift component oscillates more severely; in addition, the strength of the spectral component with higher frequency decreases evidently with increasing value of C. When the pulse with larger area, 8π pulse, propagates in the medium, the pulse splitting is similar to that in the 4π pulse case, but supercontinuum spectrum with larger strength, higher frequency and wider frequency range than that in the 4π pulse case can be obtained; varying the sign and size of C can not produce new high frequency component, but can change strength of different frequency components in the spectrum, thus can get high frequency components with higher strength
The ultrafast particle population transfer in a ■-type three-level atomic system driven by linearly polarized chirped Gauss femtosecond laser pulse is investigated by using numerical solution of the Schrdinger equation without the rotating wave approximation(RWA).It is shown that the population transfer is mainly determined by the Rabi frequency(intensity) of the driving laser field and chirp rate,and the ratio of two dipole moments and the pulse width also have obvious effect on the population transfer.By choosing suitable values of the above parameters,nearly complete population transfer(99.6%) can be realized.
In the present paper, we examine the propagation of frequency-chirped laser pulses through a medium of Lambda atoms, pulses that are capable of producing adiabatic population transfer between the stable states. We compare the propagation of these pulses to their propagation in a medium of two-level atoms, and show that the distortion of the pulses is greatly reduced in the three-level case. In effect, there exists a transparency phenomenon for frequency-chirped laser pulses propagating in the three-level medium that is somewhat similar to electromagnetically induced transparency. It is concluded that the population transfer induced by chirped pulses does populate the excited state during the process to a slight extent, even if the transfer is perfectly adiabatic. Thus the transparency of the medium is also imperfect, the pulse is eventually distorted and its capability to produce the population transfer is lost, even in the perfectly adiabatic limit.
We report and propose a simple scheme to achieve efficient and fast coherent population transfer by utilizing either a nonlinearly chirped Gaussian-shaped few-cycle laser pulse or an unchirped sinc-shaped few-cycle laser pulse. The proposed scheme is shown to be fairly robust against the variation of the laser parameters such as temporal pulse width, chirp rates, carrier-envelope phases, and Rabi frequencies. We find that compared to the so-called stimulated Raman adiabatic passage technique, our scheme for complete population transfer with few-cycle Gaussian-shaped laser pulses requires less pulse area.