The Optical Nonlinearity Of Two Photons Absorpation Inducing Free Carrier Absorpation In ZnSe And ZnS

With the invention of the first laser, intensive light source arises at the historicmoment, thus the experiments of optics extends from linear to nonlinear optical optics.It is well known that a wealth of semiconductor materials have extensive and promisingapplications such as optical computing, optical communication and optical storage, it istherefore of great importance to research the nonlinear optical properties of thesemiconductor materials deeply. The main goal in this paper is to study systematicallythe nonlinear optical performances of Zinc group semiconductors in a wide range ofwavelengths by using Z-scan technique.Based on the coupled wave theory and the valence and conduction band theory, thetheory of two-photon absorption induced free carrier absorption is exploredsystematically. In a wide range of wavelengths, the dependences of the two-photonabsorption coefficient and third-order nonlinear refractive index on the incidentwavelengths are further analyzed. Moreover, the two-photon absorption coefficient andthird-order nonlinear refractive index for different band-gap semiconductor materials atsome special wavelengths are compared. Finally, the process of two-photon absorptioninduced free carrier absorption is deduced theoretically in detail, as well as therelationship between the two kinds of absorption mechanisms is illustrated qualitatively.In the aspect of theory, on the basis of beam propagation equation and the carrierrate equation, the field distribution is derived under steady state transient circumstancesby using the slowly varying amplitude approximation. Subsequently, the competitionrelationship between two-photon absorption and free-carrier absorption is analyzed. Asan illustration, we explore systematically the competitive processes of the two kinds ofabsorption mechanisms for ZnS and ZnSe at several of wavelengths. Finally, theabsorbed energy of two-photon absorption or the free-carrier absorption as a function ofthe incident energy is examined by virtue of numerical simulation. The results indicatethe threshold energy of the dominant absorption mechanism.In the aspect of experiment, the optical nonlinearity of ZnSe and ZnS isinvestigated in a range of various energies and wavelengths based on Z-scan technique.The effective third-order nonlinear refractive index and the effective two-photonabsorption are obtained with the aid of numerical fitting. The dependences of valleyvalue of open aperture and the peak-valley value of closed aperture on the utilizedenergy are researched under ns conditions, and here the competition relationshipbetween two-photon absorption and free-carrier absorption is mainly analyzed. Inparticular, the nonlinear refractive indexes of ZnS at532nm and440nm with different level of energy are negative. However, the corresponding refractive coefficients arepositive under small level of energy circumstance at580nm and600nm. Thisphenomenon can be explained by employing the competition relationship betweentwo-photon absorption and free-carrier absorption. Also, we further verify theself-consistency of the proposed theory...