Investigation On Ultrafast Optical Nonlinearities Of Ⅱ-Ⅵ Semiconductors And Porphyrin Materials

Semiconductors including organic and inorganic semiconductors are attractive nonlinear optical materials due to their large optical nonlinearities and fast response time, which are potential for applications in optical limiting, optical switch, infrared detection, and optical storage etc. In this dissertation, we invesitgate nonlinear optical properties and ultrafast dynamics of II-VI inorganic compound semiconductors and organic semiconductors meso-tetraphynelprophyrins by combination of Z-scan technique, time-resolved PO pump-probe, and transient absorption spectrascopy techniques. The corresponding physical mechanism are anylized based on ultrashort pulse propagating equations with group velocity dispersion(GVD) effect, energy-level model, and thermal nonlinear theory. The investigations provide a novel option for exploration of new porphyrins, and enrich research contents and theoretical connotation of semicondutoral materials. The detailed arrangement of this dissertation is as follows:We focus on studying the ultrashort pulse spreading in II-VI semiconductors when the ultrashort pulses propagate in the sample by combination of experimental measurements and theoretical analysis. The time-resolved PO pump-probe experiments reveal that the 200 fs-pulses are obviously spreaded when it propagats in a 3 mm Zn S due to GVD effect. The Z-scan measurements further confirm that the effect of GVD on the nonlinear absorption and refraction could not be neglected, which may lead to a fake nonliear optical parameters of Zn S. Here, we theoretically propose a pair of propagating equations of irradiance and nonlinear phase shift including GVD effect, upon which the experimental results are well explained. Moreover, the spreading of ultrashort pulses in Zn Se, Zn Te, and Cd S are also well explained by this coupled equations, which indicates the validity and generalization of the equations. Furthermore, theoretical calculations based on the equations reveal that nonlinear absorption and larger sample length can also result in ultrashort pulse spreading. However, for a thick bulk sample, GVD still plays a dominant role in pulse spreading. These equations may be a useful guidance for the investigations of interactions between ultrashort pulses and bulk semiconduter or other solid materials.The excited-state absorption dyanmics of a series of tetraphenylporhyrins are synthesized and investigated by Z-scan, picosecond pump-probe, and femtosecond transient absorption(TA) spectrascopy techniques. The effect of N-confused porphyrin, peripheral substituents and central metallization on the nonlinear absorption of porphyrins is analyzed. The obtained results indicate that the inverted pyrrole endowed NCTPP a bathochromic-shift in its UV-visible spectra, and narrowed HOMO-LUMO gap, which are attributed to an accelerated internal conversion from S1 to S0 state as compared to that of TPP. TPP(OMe)4 with the electron-donating OMe group exhibits a switch from saturable absorption to reverse saturable absorption in the ps regime. The ps pump-probe and fs transient absorption spectra of TPP(OMe)4 reveal that its internal conversion rate is largely accelerated with S1 as compared to those of TPP. The femtosecond TA spectra of the metallized derivatives of NCTPP indicate that the introduction of Ag2+, Zn2+, Ni2+, Cd2+, and Mn2+ can accelerate the internal conversion rate. The accelerated internal rate mainly depends on the central metal ion, especially their d orbitals. Porphyrins with unfilled d shell metal ions generally have short excited-state lifetimes, due to the rapid charge transfer to the d shell of metal ion. The structure modifications mentioned above are important for exploration of new porphyrin materials for corresponding applications.Based on the nonlinear absorption properties mentioned above, the nonlinear refractions of a series of TPP derivatives are also studied by the Z-scan techniques in both ps and ns regimes. The effect of N-confused porphyrin, peripheral substituents and central metallization on the nonlinear refraction of these porphyrins are studied and analyzed. The second-order hyperpolarizabilities γ of these porphyrins are calculated by their effective nonlinear absorption coefficients βeff and nonlinear refractive coefficients n2 eff. The results indicate that n2 eff and γ values of TPP(CN)4 and TPP(OMe)4 has been largely enhanced as compared to those of TPP. The nanosecond Z-scan results indicate that the introduction of metal ions Zn2+ and Cu2+, and the OMe and CN substituents into NCTPP can not obviously change its nonliear refraction. In addition, the nonlinear refraction of these porphyrins in the ns regime is attributed to the contribution of both electron cloud distortion and thermal nonlinear refraction. Furthermore, the contributions of these two nonlinear refractions are discussed and analyzed.