International audienceSurface ablation of a dielectric material (fused silica) by single femtosecond pulses is studied as a function of pulse duration (7-450 fs) and applied fluence (F (th)< F < 10F (th)). We show that varying the pulse duration gives access to high selectivity (with resolution similar to 10 nm) for axial removal of matter but does not influence the transverse ablation selectivity, which only depends on the normalized applied fluence F/F (th). The ablation efficiency is shown to be inversely dependent on the pulse duration and saturates with respect to the applied fluence earlier at ultra-short pulse durations (a parts per thousand currency sign30 fs). The deduced optimal fluence F (opt) corresponding to the highest ablation efficiency for each pulse width defines two regimes of laser application. Below F (opt), the removed material depth can be accurately adjusted in a large range (similar to 40-200 nm) as a function of the applied fluence and the morphology of the ablated pattern almost reproduces the Gaussian beam distribution. Above F (opt), the material removal depth tends to saturate and the morphology of the ablated pattern evolves to a top-hat distribution. The coupled evolution of depth and morphology is related to the dynamics of formation of dense plasma at the surface of the material, acting as an ultra-fast optical shutter

We describe a dual, second harmonic generation (SHG) and third harmonic generation (THG) microscope, with the aim to obtain large-scale images of the cornea that can simultaneously resolve the micron-thick thin layers. We use an Ytterbium femtosecond laser as the laser source, the longer wavelength of which reduces scattering and allows simultaneous SHG and THG imaging. We measure one-dimensional SHG and THG profiles across the entire thickness of pig cornea, detected in both the forward and backward directions. These profiles allow us to clearly distinguish all the porcine corneal layers (epithelium, stroma, Descemet's membrane and endothelium). From these profiles, longitudinal cross sectional images of the corneal layers are generated, providing large scale topographic information with high-spatial resolution. The ability to obtain both SHG and THG signals in epi-detection on fresh eyes gives promising hopes for in vivo applications.

We investigate the changes in the optical properties of fused silica exposed to intense infrared femtosecond pulses. The laser-induced absorption spectrum reveals the creation of color centers inside the glass matrix, comparable with those observed in ultraviolet-exposed fused silica. The laser-induced absorption is associated with a laser-induced refractive-index change, which can be used for waveguide fabrication. The change in third-order susceptibility in such waveguides is measured by third-harmonic-generation microscopy as a function of the irradiation parameters.

A theoretical and experimental study of the THG signal from a reference interface in confocal microscope allows precise analysis of beam propagation and optimization of the focusing objectives.

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Advanced imaging methods are essential tools for improved outcome of refractive surgery. Second harmonic generation (SHG) and third harmonic generation (THG) microscopy are noninvasive high-resolution imaging methods, which can discriminate the different layers of the cornea, thus having strong impact on the outcome of laser surgery. In this work, we use an Ytterbium femtosecond laser as the laser source, the longer wavelength of which reduces scattering, and allows simultaneous SHG and THG imaging.We present SHG and THG images and profiles of pig corneas that clearly show the anterior surface of the cornea, the entry in the stroma and its end, and the posterior surface of the cornea. These observations allow localizing the epithelium, the stroma and the endothelium. Other experiments give information about the structure and cytology of the corneal layers.

We demonstrate high spatial resolution imaging of a stromal cut in the ex-vivo pig cornea, using second-and third-harmonic generation microscopy. From these images, we see in detail how the cut affects the corneal layers. In the beginning of the cut, the anterior layers, in which the blade is passing through, are disorganized, which could explain the shadows observed on the images. In the stroma, the cut can be imaged by third harmonic microscopy, probably due to the χ 3 contrast. Although the current results were obtained from the healthy ex-vivo cornea, it already allows one to understand the effects of the cut on the tissue characteristics (such as scattering).