Nonlinear gain and dispersion in the four-level Λ-type system

Mostafa Sahrai, R. Etemadpour, Mohammad Mahmoudi

Li-Ya Pei

We observe experimentally huge enhanced four-wave mixing based on Raman resonance in an 85Rb atomic vapor system. With the decrease of coupling field power or the increase of experimental temperature, the signal tends to be narrowed down in linewidth, and be basically the same spectrum as the stimulated Raman spectroscopy. It is found that the macroscopic polarization interference effect plays a crucial role in determining the nonlinear spectra. Further more, in the Doppler-broadened Λ-type energy level system, there is a strong relationship among electromagnetically induced transparency, four-wave mixing and stimulated Raman spectroscopy. The sharp transparent window in electromagnetically induced transparency can be interpreted as the suppression of Raman gain on the linear absorption of the probe field. However, the four-wave mixing signal is a new field generated by the atomic vapor system, and it comes from the Raman gain which is affected and modified by the absorption and dispersion of the nonlinear optical medium. It shows that in a Λ-type Doppler-broadened system, in essence, both the electromagnetically induced transparency and enhanced four-wave mixing stem from stimulated Raman scattering based on the third-order nonlinear processes, just the spectra of which are from different ways and objects of detection.

Francesco Plastina, Franco Piperno

Xinlian Chen

With the density matrix equations of motion under the electric dipole and the rotating wave approximations,the mechanism of LWI gain was analyzed.By numerical results,the effects of the Rabi frequencies of the driving fields and nonlinear term on the gain without inversion and the population difference of the system were studied.Theoretical model of an openⅤ-type four-level atomic system was presented.Through calculation,when both the probing and driving fields are resonant,the dispersion equals to zero.A LWI gain was got during the region of Rabi frequencies of the driving fields.The gain was saturated because of nonlinear term.

李晓红, Tao Liu, 汪克林

Ali Raheli, Mostafa Sahrai, Abdolrahman Namdar +1 more

This paper presents a new freedom, which is the result of atomic injection and exit rates from a cavity associated with an open system, thus leading to a remarkable giant Kerr nonlinearity with applications in all-optical switching. The influence of atomic injection and exit rate from the cavity on nonlinear susceptibility of an open four-level Λ-type atomic system with twofold lower levels is investigated. It is found that large Kerr nonlinearity accompanied with probe gain can be obtained by adjusting the atomic injection rates and exit rate from the cavity, which is completely different from those presented in previous closed atomic systems. Phase control of Kerr nonlinearity in such an open atomic system is then studied.