Ultrashort pulse optimization via two-photon absorption

Author(s)

Daniel Luiz da Silva

• Document type: Master's Dissertation
• Press: São Carlos. , gráficos, ilustrações, tabelas.
• Institution: Universidade de São Paulo (USP). Instituto de Física de São Carlos (IFSC/BT)
• Defense date: 2005-03-31

Examining board members

Cleber Renato Mendonca; Ricardo Rego Bordalo Correia; Paulo Barbeitas Miranda

• Advisor: Cleber Renato Mendonca
• Field of knowledge: Physical Sciences and Mathematics - Physics
• Indexed in: Banco de Dados Bibliográficos da USP-DEDALUS; Biblioteca Digital de Teses e Dissertações - USP
• Location: Universidade de São Paulo. Biblioteca do Instituto de Física de São Carlos; IFSC-F/Te1691

Abstract

In this work it is described the implementation of an ultrashort pulse optimization system (15 fs modelocked oscillator) that employs pulse shaping methods via two-photon absorption in organic materials. This technique uses an evolutionary strategy based on a Genetic Algorithm, where the pulse shape is controlled by a deformable mirror, while a feedback signal is monitored. In this way, this system allows both, the two-photon absorption process and pulse optimization. After the accomplishment of the pulse shaping system, we have implemented three distinct optimization methods via two-photon absorption monitoring, being two of them proposed in the present dissertation. These three methods differs from each other by the use of different feedback signals for the optimization process: (i) intensity of the two-photon excited fluorescence; (ii) nonlinear transmittance change in organic compounds due to the two-photon absorption; and (iii) intensity of the thermal lens effect. All optimization methods presented similar and satisfactory results, leading the ultrashort pulse, in the end of the optimization process, close to the Fourier transformed limit. In such cases, the pulse duration were determined through the autocorrelation technique. These results indicates that the new methods proposed here can be used as an alternative for both, pulse optimization and control of two-photon absorption process, specially for nonfluorescent samples. (AU)