Study of third-harmonic generation at fluorescent dye solutions interfaces

Abstract

In this project we are going to study third-harmonic generation (THG) at interfaces of fluorescent solutions using femtosecond (fs) laser pulses. The THG is a well known process which occurs at interfaces of isotropic samples at tight focused condition [1]. For example, this process is very important to study the third-order optical properties of materials and for application in nonlinear microscopy because it allows obtaining the tridimensional map of the interfaces of a complex structure. Recently, using fs pulses we observed a strong contribution of self-focusing in the signal of the THG at interfaces of isotropic media [2,3]. Now, using cuvette and liquids, dye solutions, we can explore the use of the third harmonic (TH) generated at interfaces to induce the fluorescence of the proper dye solutions. It is an interesting configuration in which the fluorescence acts as a "sensor" of the TH. Preliminary measurements were made in rhodamine 610 dye solution inside a proper silica cuvette. In this case we have four interfaces: cuvette input, cuvette-solution, solution-cuvette and cuvette output. The fluorescence signal was observed and was easy to be detected. The fluorescence has presented interesting features in term of possible explanation for the shape of the detected signal. It is worth to remember that there is a mechanism of direct excitation of the fluorescence which is the three-photon absorption (3PA) that can induce the bulk fluorescence, on the contrary to THG which happens at interfaces. The presence of two completely distinct processes to induce the fluorescence turns to be very interesting in term of discovering the true excitation mechanism. While the THG is a third-order nonlinear effect, the 3PA is a fifth-order. Several different measurements will be made to prove the one or other mechanism in different situations. Measurements at function of laser power, different dyes, laser polarization and solvents, for instance, will be done. (AU)