Both electromagnetically induced absorption (EIA) and transparency (EIT) can be obtained in a
modified quasi-lambda four level system consisting of an optical-radio two-photon coupling field and a
probing field. A physical account of EIA and EIT is given in terms of a transient state picture in this
paper. It can be seen that the optical coupling field in this quasi-lambda four level system has a crucial
effect on the forming of EIA and EIT. An EIA is observed under a resonant optical coupling and it
evolves into an EIT when there is a detuning.
In this paper we present a theoretical study of the nonlinear effect in the quasi-lambda type
four-level system. The system consists of an excited state level and three ground state hyperfine
levels. Probing and coupling field are coupled to between the excited state and two higher ground
states, and the microwave field drives the two lower ground states which are associated with
probing field. By solving the precision solutions of the equations of motion of density matrix, the
absorption properties as a function of Rabi frequencies of the probing field and microwave field
are given. As a result, the switch from double EIT to single EIA is found due to the power
broadening of probing field. However, the splitting frequency of double EIT has some connection
with the Rabi frequency of microwave field.
The effect of the relative phase on the spectral linewidth of electromagnetically induced transparency is studied in a
Λ-type three-level configuration coupled by double coupling fields and the result is presented in this paper. We show that
the relative phase between the double coupling fields has a great degree of influence on the spectral width of
electromagnetically induced transparency window. The linewidth can be controlled by changing the relative phase.
Particularly, as the double coupling fields have opposite phases, the linewidth of EIT window can be extremely narrow
In this paper we study the coherent transient property of a Λ-three-level system (Ωd = 0) and a quasi- Λ -four-level
system (Ωd>0). Optical switching of the probe field can be achieved by applying a pulsed coupling field or rf field. In
Λ -shaped three-level system, when the coupling field was switched on, there is a almost total transparency of the probe
field and the time required for the absorption changing from 90% to 10% of the maximum absorption is 2.9Γ0 (Γ0 is
spontaneous emission lifetime). When the coupling field was switched off, there is an initial increase of the probe field
absorption and then gradually evolves to the maximum of absorption of the two-level absorption, the time required for the
absorption of the system changing from 10% to 90% is 4.2Γ0. In four-level system, where rf driving field is used as
switching field, to achieve the same depth of the optical switching, the time of the optical switching is 2.5Γ0 and 6.1Γ0,
respectively. The results show that with the same depth of the optical switching, the switch-on time of the four-level system
is shorter than that of the three-level system, while the switch-off time of the four-level system is longer. The depth of the
optical switching of the four-level system was much larger than that of the three-level system, where the depth of the
optical switching of the latter is merely 14.8% of that of the former. The speed of optical switching of the two systems can
be increased by the increase of Rabi frequency of coupling field or rf field.
In this paper we present a theoretical study of the effect of a microwave field on an EIT feature. The EIT feature is associated with the well-known three-level Λ type configuration where a pump and probe laser field couples two separate optical transitions. In addition to these two laser fields, there is a microwave field which drives one of the two lower levels of the Λ type three-level system to another hyperfine level. The EIT feature is studied as a function of microwave field frequency and intensity. Our results show that the presence of a microwave field can dramatically modify the EIT feature. When microwave is resonant with the hyperfine transition, the EIT feature can be split into two EIT features. When it is off resonant with the hyperfine transition, it causes a frequency shift of the EIT feature, reminiscent of the well-known light shift effect.