Shekhar Priyadarshi

Structure Property Relationship in Femtosecond 1560nm induced Nonlinearity

In this thesis three apparently disjointed pieces of work are presented, namely, (a)determination of optical nonlinearity in homologous series of alcohols, (b) study of thermal properties of alcohols using thermal lens spectroscopy and (c) a theoretical analysis of a Fourier 4F-Geometry pulse shaper. All the work, however, is centered on a1560nm femtosecond laser. Study of various nonlinear dynamics and control at 1560 nm is gaining prime attention in recent years due to its importance in telecommunication applications. Study and control of third order nonlinear optical (NLO) phenomena permit all optical switching and signal processing. In this thesis molecular NLO has been exploited for determining the lifetime of excited states. The popular technique of z-scan was used for measuring the nonlinear parameters. Non-radiative de-excitation of optically pumped molecules induces thermal agitation in bulk samples, which can be measured by Thermal Lens Spectroscopy (TLS).Determining the thermal excitation and relaxation of hydrocarbons also has great importance in petroleum industry. However, thermal lensing induces sluggish switching behavior and reduced quantum efficiency. Therefore, the determination of thermal relaxation dynamics is important and we used thermal lens pump-probe experiment to determine this. Nonlinear optical and thermal properties of alcohols with increasing carbon chain were investigated at 1560nm wavelength (c-band) to understand structure property relationship. We obtain absorption saturation in various alcohols at 1560nm. Such a characterization of the thermal properties enabled us to measure the nonlinear index of refraction by eliminating most of the thermal effect. Comparative study of degree of thermal excitation and thermal relaxation dynamics was also performed in various alcohols. Further control and manipulation of the parameters reported in these experiments here would require more fineness on the optical interaction with molecules. This can be most effectively achieved, independent of the character of the molecules, if we can have complete control over the femtosecond laser pulses at 1560nm. Pulse shaping at such a short timescale is difficult to achieve through direct time domain approaches. We, therefore, present an alternative Fourier approach of analysis for Femtosecond pulse shaping at 1560nm.