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Non-invasive, high resolution 3D analysis techniques are very much sought for the characterization of multilayer, multicomponent substrates, as those often encountered in artworks and objects of cultural heritage. The non-linear optical interaction of ultrashort laser pulses with a substrate is the basis of the various modalities of the non-linear optical microscopy (NLOM) techniques, recently introduced for the study of cultural heritage objects. NLOM relies on near-IR, femtosecond laser excitation of transparent or semi-transparent materials to simultaneously induce, with 3D micrometric resolution, and depending on the optical properties of the sample, multiphoton excitation fluorescence (MPEF) and second and third harmonic generation (SHG, THG) signals. MPEF emission is related to the sample chemical composition, SHG identifies the presence of non‐centrosymmetric structures and THG allows imaging interfaces between optically dissimilar materials. For paintings, it has been recently reported that valuable information about composition, layer thickness and state of conservation can be obtained by NLOM [1-3]. Although NLOM is a non-invasive technique, ensuring a correct analytical protocol requires the determination of the laser power thresholds that allow measurements under safe conditions, an aspect especially important when studying sensitive materials such as paintings.
In this work, we present a novel methodology to determine the laser power thresholds for safe analyses by MPFE of painting layers. We also present the results obtained in a set of acrylic paints, extensively used by artists over the past century thanks to their properties and low cost of manufacture. To that purpose, samples were prepared as thin layers over a glass substrate and MPEF signals were induced with two different femtosecond laser sources: a Ti:Sapphire laser with wavelength of 800 nm, repetition rate of 80 MHz, and pulses of 70 femtoseconds; an optical parametric oscillator pumped by a Yb-based laser with repetition rate of 80 MHz and dual output: at 800 nm with pulses of 100 fs and at 1040 nm with pulses of 140 fs. The excitation wavelength affects the determined thresholds and the results obtained show a strong dependence on the light absorption properties and chemical composition of the painting material.
[1] Oujja M., Psilodimitrakopoulos S., Carrasco E., Sanz M., Philippidis A., Selimis A., Pouli P., Filippidis G., Castillejo M. (2017) Phys. Chem. Chem. Phys. 19, 22836-22843.
[2] Liang H., Mari M., Cheung C.S., Kogou S., Johnson P., Filippidis G., (2017) Opt. Express 25, 19640–19653.
[3] Dal Fovo A., Oujja M., Sanz M., Martínez-Hernández A., Cañamares M.V., Castillejo M., Fontana R. (2019) Spectrochim. Acta A 208, 262-270.
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