Non-Degenerate Two-Photon Absorption: Effects of Energy and Polarization on the Two-Photon Absorption Cross-Section

• Grant number: 24/21157-0
• Support Opportunities: Scholarships in Brazil - Doctorate
• Start date: August 01, 2025
• End date: January 31, 2029
• Field of knowledge: Physical Sciences and Mathematics - Physics - Condensed Matter Physics
• Principal Investigator: Cleber Renato Mendonça
• Grantee: Nathan Barbola Marucci
• Host Institution: Instituto de Física de São Carlos (IFSC). Universidade de São Paulo (USP). São Carlos , SP, Brazil
• Associated research grant: 18/11283-7 - Nonlinear photonics: spectroscopy and advanced processing of materials, AP.TEM

Abstract

This doctoral project aims to investigate the effect of non-degeneracy and relative polarization between photons on the two-photon absorption cross-section in fluorescent organic compounds. Two-photon absorption is a nonlinear optical phenomenon with wide applications in fields such as fluorescence microscopy, photodynamic therapy, and microfabrication. Traditionally, the characterization of two-photon absorption is performed using degenerate light beams, which limits the process optimization to structural modifications of the studied compounds. In this project, we will implement an experimental technique based on non-degenerate two-photon-induced fluorescence, using two beams with distinct frequencies and polarizations, to explore new ways to enhance nonlinear absorption efficiency. This technique will allow us to investigate how the cross-section is affected by variations in photon energy and polarization. Additionally, we will employ the Z-scan technique to characterize degenerate two-photon absorption and compare it with non-degenerate conditions. The experimental results will be complemented by quantum chemistry calculations using Density Functional Theory (DFT) to simulate linear absorption and fluorescence spectra, as well as to calculate electric dipole moments. This study is expected to provide new strategies for optimizing two-photon absorption in molecular systems without requiring structural modifications, thereby expanding the potential applications of these materials in photonics-based technologies. (AU)