Formation of field induced absorption in the probe response signal of a four-level ‘V’ type atomic system: a theoretical study

Abstract

A density matrix based analytical model is developed to study the coherent probe field propagation through a
four-level ‘V’ type system in presence of a coherent control field. The four-level system comprises of two ground levels
and two upper levels. The model allows coupling of the probe field from the upper ground level to both of the excited
levels whereas the control field is kept locked from the upper ground level to one of the excited levels. The addition of an
extra ground level to a conventional three-level ‘V’ type system creates extra decay paths to the ground levels from the
upper levels. A set of sixteen density matrix based equations are formed and then solved analytically under rotating wave
approximation to study the probe response under steady state condition. Under Doppler free condition the simulated
probe absorption shows EIT but under thermal averaging the EIT window displays signature of absorption at its middle.
The absorption dip exhibits shift as well as broadening with variation in the control Rabi frequency. This is termed as
field induced absorption.