Single photon nonlinear absorption spectroscopy of cascade energy level systems

Theoretically discussed the third order nonlinear optical properties and absorption cross-sections in two and three level cascade systems by considering single photon resonant absorption at 671 nm of CW laser. The results indicate that while two level system producing Single Saturable Absorption (SSA), it is possible to obtain Double Saturable Absorption (DSA) and Reverse Saturable Absorption (RSA) with long range and modulation depth by tuning the spectroscopic parameters like absorption cross-section and lifetime of participating levels in three level system. Analytical expressions have been given for nonlinear coefficients of the electromagnetic wave equation by considering the propagation in two and three level cascade media. The gradient refractive index formed in single and double beam interaction with the material was discussed. The origin of large phase distortion in the z-scan closed aperture and formation of nonlinear grating in the degenerative four wave mixing are discussed. To emphasize the theoretical results, the discussion has been extended to explain the third order nonlinear experimental results of porphyrin molecules in the degenerative four wave mixing and z-scan techniques.