High and uniform coherence creation in Doppler-broadened double Λ-like atomic system by a train of femtosecond optical pulses

Abstract

We present a detailed analysis of coherence creation in a four level double Ʌ-like atomic system using a train of ultra-short few-cycle Gaussian pulses. The effect of the Doppler broadening has been analyzed. It is possible to create high coherence across all the velocity groups in the atomic ensemble using pulses with low repetition frequency. The accumulation of coherence at different radial locations within the atomic beam cross-section as a function of the number of pulses in the train is also reported. We find that pulse train with lower repetition frequencies are able to generate high and nearly uniform coherence in a substantial part of the atomic system. I. INTRODUCTIONSince the first experimental demonstration of coherent population trapping (CPT) in Ʌ-type system [1], numerous investigations have been made into coherence-induced phenomena like electromagnetically induced transparency (EIT), absorption (EIA), etc. [2][3][4][5][6][7]. In fact, recently a theoretical investigation shows that CPT can occur even in buckminsterfullerene C60 [8]. Arguably, one of the primary reasons behind such an interest is the possibility of modifying the optical properties of the media through quantum interference. Extremely narrow absorption and emission spectral features [9] are associated with CPT and have been identified for use in the implementation of atomic frequency standards [10][11]. Again, the phenomenon of CPT is used to reduce the group velocity of light significantly in certain media [12]. The coherence accumulated in a system can give rise to effects like enhancement of refractive index with zero absorption [13][14][15], light amplification without population inversion [16], high-harmonic generation [17], etc. Kocharovskaya and Khanin predicted the possibility of using a train of pulses to create coherence in 1986 [18] and later demonstrated experimentally by making use of a comb of optical pulses produced from a mode-locked diode laser [19]. With the recent progress in generation of well-characterized short optical pulses it has become possible to accumulate the coherence in a robust and controllable fashion [20][21]. More recently authors have made theoretical investigations into the generation of coherence using a series of short pulses [22][23][24]. In particular, Aumiler [20] investigated EIT of a train of femtosecond pulses interacting with the four level atomic systems in the conditions when the pulse repetition period is shorter than the characteristic atomic relaxation times. He considered a special case where the pulse repetition frequency is a subharmonic of the hyperfine splitting of the ground state. Aumiler's works illustrate how the judicious choice of the frequency comb parameters can provide means to effectively control the degree of coherence between the ground-state hyperfine levels for selective velocity groups of atoms. In this work we present a detailed analysis of coherence creation between the two lower states of a four level double Ʌ-like atomic system using a train of ultra-short f...