Xijun Fan

Atom localization via interference of dark resonances

A scheme of atom localization based on the interference of resonance of double-dark states is proposed, in which the atom interacts with a classical standing-wave field. It is found that the localization property is significantly improved due to the interaction of double-dark resonances. It is realized that the atom is localized just at the nodes of the standing-wave field with higher precision. Moreover, an improvement by a factor of 2 in the detecting probability of a single atom within the subwavelength domain can be achieved by adjusting the probe-field detuning. This scheme shows more advantages than other schemes of atom localization.

Phase control of gain and dispersion in an open lambda-type inversionless lasing system

The control role of the relative phase between the probe and driving fields on the gain and dispersion in an open Λ-type inversionless lasing system with spontaneously generated coherence (SGC) is investigated. It is shown that the inversionless gain and dispersion are quite sensitive to variation in the relative phase; by adjusting the value of the relative phase, electromagnetically induced transparency (EIT), a high refractive index with zero absorption and a larger inversionless gain can be realized. It is also shown that, in the contributions to the inversionless gain (absorption) and dispersion, the contribution from SGC is always much larger than that from the dynamically induced coherence for any value of the relative phase. Our analysis shows that variation in the SGC effect will cause the spectrum regions and values of the inversionless gain and dispersion to vary evidently. We also found that, under the same conditions, the values of the inversionless gain and dispersion in the open system are evidently larger than those in the corresponding closed system; EIT occurs in the open system but cannot occur in the closed system.

Phase control of gain and dispersion in an open V-type system

The control role of the relative phase between the probe and driving fields on gain, dispersion and populations in an open V-type three-level system with spontaneously generated coherence is studied. The result shows that by adjusting the value of the relative phase, the transformation between lasing with inversion and lasing without inversion (LWI) can be realized and high dispersion (refractive index) without absorption can be obtained. The shape and value range of the dispersion curve are similar to those of the gain curve, and this similarity is closely related to the relative phase. The effects of the atomic exit and injection rates and the incoherent pump rate on the control role of the relative phase are also analysed. It is found easier to get LWI by adjusting the value of the relative phase using the open system rather than the closed system, and using an incoherent pump rather than without using the incoherent pump. Moreover the open system can give a larger LWI gain than the closed system.

Doppler-enhanced gain in an open ladder inversionless lasing system

Abstract An analysis is made of the effect of Doppler broadening on the gain in an open ladder inversionless lasing system. It is shown that for co-propagating probe and driving fields, the gain does not monotonously decrease or increase with increasing Doppler width, and at a suitable Doppler width one can obtain a maximum value much larger than that without Doppler broadening. For counterpropagating probe and driving fields, when the Doppler width is large enough, gain oscillation occurs, and the oscillation amplitude and region increase with increasing Doppler width. This conclusion is very different from that obtained in the corresponding closed system.

Group velocity of probe light pulse in an open lambda system

The group velocity of the probe light pulse (GVPLP) propagating through an open Λ-type atomic system with a spontaneously generated coherence is investigated when the weak probe and strong driving light fields have different frequencies. It is found that adjusting the detuning or Rabi frequency of the probe light field can realize switching of the GVPLP from subluminal to superluminal. Changing the relative phase between the probe and driving light fields or atomic exit and injection rates can lead to GVPLP varying in a wider range, but cannot induce transformation of the property of the GVPLP. The absolute value of the GVPLP always increases with Rabi frequency of the driving light field increasing. For subluminal and superluminal propagation, the system always exhibits the probe absorption, and GVPLP is mainly determined by the slope of the steep dispersion.

Destabilization of the non-lasing solution in open three-level systems

Abstract Lasing without inversion is investigated from a nonlinear dynamics viewpoint for the open resonant lower-ladder and V systems. We find that, for the two systems, destabilization of the non-lasing solution can occur through pitchfork bifurcation, giving rise to continuous-wave lasing emission, and also through Hopf bifurcation, giving rise to self-pulsing lasing emission. This conclusion is very different from that obtained from the corresponding closed systems by Mompart in 1998. The effects of the gain parameter, damping rate, ratio of the atomic injection rates and atomic exit rate on the two types of destabilization are also analysed.

Investigation of transient process and steady output of lasing without inversion

The transient evolution processes and steady outputs of continuous wave lasing without inversion (LWI) and self-pulsing LWI in a resonant open V type three-level system are studied. It was found that the two kinds of LWI have some obvious differences not only from the steady outputs but also from the transient evolution processes. The effects of the unsaturated gain coefficient, cavity loss coefficient, ratio of the atomic injection rates and atomic exit rate on the transient evolution processes and steady outputs are discussed.

Lasing without or with inversion in an open four-level system with a phase-fluctuation driving field

The effect of exit rate and the ratio of atomic injection rate on gain behaviour has been investigated, and the effects of phase fluctuation on absorption, dispersion and population difference in an open four-level system have been analysed by using numerical simulation from the steady linear, analytical solution. The variation of the linewidth, Rabi frequency of the driving field, the exit rate or the ratio of atomic injection rate can change the lasing properties in the open system. The presence of finite linewidth due to driving-field phase fluctuation prevents the open four-level atomic system from obtaining a high refractive index along with zero absorption.

Effect of the ratio of transition dipole moments on few-cycle pulse propagation

Propagation of a few-cycle laser pulses in a dense V-type three-level atomic medium is investigated based on full-wave Maxwell-Bloch equations by taking the near dipole-dipole (NDD) interaction into account. We find that the ratio, γ, of the transition dipole moments has strong influence on the time evolution and split of the pulse: when γ ≤ =1, the NDD interaction delays propagation and split of the pulse, and this phenomenon is more obvious when the value of γ is smaller; when γ = √2, the NDD interaction accelerates propagation and split of the pulse.

Phase-dependent gain without inversion in an inhomogeneous broadened quasi Λ-type four-level system with VIC

The effect of the relative phase (φ) between the probe and driving fields on the gain without inversion (GWI) is studied in a Doppler broadened quasi Λ-type four-level atomic system with vacuum induced coherence (VIC). It is shown that: The probe detuning region in which GWI exists and size of GWI are very sensitive to variation of the relative phase, the Doppler width ( D ) also has dramatically modulation role on the phase-dependent GWI. The GWI maximum value ( Gmax ) varies periodically with the relative phase varying, the period is 2π; but the concrete varying rule is closely related to the value of the Doppler width. In the case of D = 0 (i.e. without Doppler broadening) , when 0 < φ < π, Gmax increases monotonously with φ increasing; when π <φ < 2π, Gmax decreases monotonously with φ increasing; when φ =π, Gmaxhas the largest value. Under the condition of D ≠ 0 (i.e. Doppler broadening presents), in both regions 0 < φ < π and π < φ < 2π, Gmax does not monotonically increase or decrease with φ increasing; the value of φ, which corresponds to the largest value of Gmax, decreases gradually from π with D increasing, when value of D is large enough, the value of φ, which corresponds to the largest value of Gmax, is about π/2. In general speaking, Gmax decreases with D increasing; but Gmax larger than that in the corresponding static atomic system ( D = 0 ) can be gotten by choosing suitable values of φ and D.