CC BY
15LMI-I-7
Investigating energy deposition of ultrashort lasers at the surface of solids at the femtosecond scale
O. Uteza1, T. Genieys1, C. Pasquier1, M. Sentis1, N. Sanner1 1LP3 - CNRS - AMU, LP3, Marseille, France
Ultrashort laser pulses (<<100 fs) offer remarkable capabilities for laser-matter interaction, especially for ablation of transparent dielectric materials [1,2]. However, achievement of those performances requires precise evaluation and understanding of the optical response of the material under laser irradiation. In this context, determination of laser-induced damage or/and ablation thresholds (LIDT/LIAT) of materials and measurement and understanding of energy deposition are mandatory to calibrate material transformation upon laser excitation. This is particularly crucial, for instance in view of laser damage certification of optical components or of development of micromachining processes [2,3].
By convenience or because it corresponds to their actual routine use, those optics or processes are in general set in operation in air. However, focusing ultrashort laser pulses in air implies natural limitations to linear beam propagation because of the development of nonlinearities related to Kerr effect and air ionization prior the target.
Fig. 1. Evolution of reflection (R), transmission (T) and absorption (A, deduced from energy conservation) as a function of incident energy for fused silica measured at 15 fs (a) and evolution of reflectivity vs fluence and pulse duration for nickel (b). Eth,&02 indicates the ablation energy threshold of fused silica (wo,exp = 11 ^m). The corresponding ablation thresholds (expressed in peak fluence) are: Fth,Si02 = 1.8 J/cm2 and Fth,Ni = 0.33 J/cm2 for
pulse duration ranging from 15 to 100 fs.
In this context, we first analyze the spatial, spectral and temporal properties of the ultrashort laser beam to characterize its propagation till the focal plane where the target is located [4]. We further couple this information to the surface ablation of dielectrics and metals of scientific and industrial interest (fused silica sapphire, aluminum, nickel, copper and tungsten), inferring precise LIDT/LIAT evaluation in a pulse duration range (15 - 100 fs) little explored yet. In operating conditions for which the beam propagation is not affected by nonlinearities in air, we further extend our experimental investigations to the determination of the energy coupling at the surface of those
materials using pulse-time-integrated measurements of reflectivity and transmissivity performed for different pulsed durations and at fluences below and above the ablation threshold. Examples of those energy balance measurements are shown below (Figure 1) for metals (nickel) and dielectrics (fused silica). The main results obtained will be detailed and discussed providing valuable insights of laser energy deposition and transformation of solid materials in the ultrashort irradiation regime.
References
[1] K. Sugioka, Progress in ultrafast laser processing and future prospects, Nanophotonics 6 (2), 393-413, (2017).
[2] O. Uteza, N. Sanner, A. Brocas, B. Chimier, N. Varkentina, M. Sentis, P. Lassonde, F. Legare, J-C. Kieffer, Control of material removal of fused silica with single pulses of few optical cycles to sub-picosecond duration, Applied Physics A 105, 131-141, (2011).
[3] M.E. Shaheen, J.E. Gagnon, J.B. Fryer, Femtosecond laser ablation behavior of gold, crystalline silicon, and fused silica: a comparative study, Laser Phys. 24, 106102 (2014).
[4] C. Pasquier, P. Blandin, R. Clady, N. Sanner, M. Sentis, O. Uteza, Y. Li, S. Yan long, Handling beam propagation in air for nearly 10-fs laser damage experiments, Optics Comm. 355, 230-238, (2015).
