Giant Kerr nonlinearities about twelve orders of magnitudes greater than in glass were measured under negligible absorption conditions within two different closed transitions of the cesium D2line characterized by electromagnetically induced transparency or absorption.
Spontaneous transfer of coherence has proven to be a good interpretative scheme for the explantation of the spectra of a probe laser probing a closed degenerate two-level systems. With its aid it was possible to give an explanation for the emergence of electromagnetically induced absorption and also to make predictions on the associated coupling laser spectra. Here we extend our work on the role of the coupling laser in electromagnetically induced absorption presenting new measurements of the probe and coupling laser absorption and dispersion spectra - taken in the D2 line of caesium - which reassert the importance of spontaneous transfer of coherence in the generation of electromagnetically induced absorption spectra. All measurements were performed with linearly polarised coupling and probe laser of orthogonal polarisation, acting on a perpendicularly propagating caesium atomic beam to minimise the Doppler broadening of the lines. For sake of completeness we compared the electromagnetically induced absorption spectra with electromagnetically induced transparency spectra obtained in another two-level system within the same line. The measured probe spectra were used to calculate the refractive index of caesium in the presence of electromagnetically induced transparency and absorption. On that basis we could calculate the effect of Kerr nonlinearities and measure nonlinear Kerr coefficients of the order of n2 ≈ 10-5 cm2/mW with absorption coefficients of the order of &agr; ≈ 0.1 cm-1.
A theoretical investigation is carried out into the effect of applied coherent fields on electromagnetically induced transparency(EIT) and Kerr nonlinearity in the four-level Y-type energy atom system.It is found that the Absorption curve of the atom system changes from one EIT window to a gain interval which has two transparency points when the intensity of coupling field and pumping field changes from equality to inequality.The position of transparency points,the magnitude of gain value together with the width of gain interval are all closely connected with the changed Rabi frequency of applied fields.It is also found that Kerr nonlinearity is enhanced but accompanied by linear Absorption when applied fields are all weak and their relative intensity reduces.When the intensity of applied fields is enlarged,Kerr nonlinearity is enhanced significantly and its peak moves to the position of transparency point with the enlargement of their relative intensity.That is to say,giant Kerr nonlinearity accompanied by vanishing linear Absorption is obtained.Finally,we find that the effect of the intensity variation of the pumping field on the enhancement of Kerr nonlinearity is more remarkable than that of the coupling field.