In-situ Measurement of Second Harmonic Generation induced by soot particles exposed to femtosecond laser light
Résumé
Numerous diagnostics exist giving access to soot properties as volume fraction, particle size, morphology, fine structure, elemental and molecular composition. Most of these techniques relies on the particles’ optical and radiative properties, which depend on the particles’ maturity. Nevertheless, there are no optical techniques able to inform us about soot surface and atomic structuration at the surface.
A promising approach to gain access to this informationis the use of Non-Linear Optics (NLO), which describes the optical response of a medium to high intensity/femtosecond laser light. With these lasers, effects such as Second Harmonic Generation and Third Harmonic Generation (SHG and THG) can, in theory, be observed, which should depend on the atomic structuration of the medium with a particular emphasis to its surface
Recently, SHG has been detected on different types of molecules that are placed at the surface of particles in aerosol phase (Qian et al., 2018, Zang et al., 2018). However, the particles needed to be coated by specific molecules with a strong SHG response, which would ultimately alter the particles inner structure and thus be contradictory to an application in internal structure analysis. In another study, third-harmonic generation and scattering have been observed on soot particles generated in flames (Zang et al., 2020). Yet, no information is provided on the structure of the soot particles and to the authors’ knowledge, no measurement of SHG has yet been carried out on BC particles in-situ in aerosol phase.
We developed and optimized an optical setup in order to measure extremely weak SHG signals in a reliable and repeatable way. Particular care was taken to ensure that generated light is effectively due to light-particle interactions and not due to other phenomena induced in the other elements of the experimental setup. The setup is used to carry out SHG measurements in a 1D laminar sooty flame generated by a porous injector burner similar to a McKenna burner, whose particles properties depend on their position in the flame. The results show indeed a SHG signature from the soot particles and might give access to new information on their atomic/molecular structure in the future.