Control of Femtosecond Laser Ablation of Thin Films from a Dielectric Surface by Nonlinear Interaction with the Substrate

Controlling the interaction of an ultrafast laser pulse with a thin film remains a difficult task, especially when aiming to confine material modifications to subwavelength scales. We introduce a method to achieve reproducible submicron ablation of thin films from a dielectric surface, in a back-irradiation geometry. First, the pulse of 45-fs duration and 800-nm central wavelength nonlinearly interacts with the dielectric and undergoes strong but reproducible modifications of its intensity profile. Then, the pulse ablates a thin polymer film [four bilayers of poly(allylamine hydrochloride) and poly(sodium 4-styrene-sulfonate), 8 nm thick] from the back surface. We measure the hole with atomic force microscopy and study the influence of laser energy and focal plane position. The radius of the resulting hole is determined by a threshold intensity for ablation. Therefore, we also demonstrate how measuring the radius as a function of focal plane position provides a new approach to profiling a tightly focused laser beam under nonlinear propagation conditions. We compare the beam profile with that predicted by a widely used propagation model and show that the latter can semiquantitatively be applied to estimate the size of achievable holes.

Figure 1

Beam radius measured by the knife-edge technique in both orthogonal directions, when focused with the 0.25-NA microscope objective. Inset: Zoom close to the focal plane position. The dashed curves show the Gaussian radius for different waists.

Figure 2

AFM images of desorbed polymer film holes obtained with 200 nJ and (a) the focus $23.2\mu \mathrm{m}$ before the rear surface and (c) the focus $8.7\mu \mathrm{m}$ before the rear surface. The color bar indicates the height in nanometers. (b),(d) Corresponding cross sections. The dashed red lines indicate the depth of the film.

Figure 3

Experimental radii (symbols) of ablated polymer film holes from the rear surface of a fused silica substrate as a function of focal plane position. Positive (negative) values of the horizontal axis correspond to the focal plane situated inside (outside) the substrate bulk. The solid lines show the calculated radius $R_{\mathrm{th}}$ for which the intensity is equal to the ablation threshold intensity ($I_{\mathrm{th}}=1.6\times 10^{13}\mathrm{W}{\mathrm{cm}}^{-2}$) deduced from Gaussian propagation (55 nJ) and the model of wave propagation discussed in the text (150, 200, and 500 nJ).

Figure 4

AFM image of desorbed polymer holes, obtained by positioning the focal plane inside the dielectric bulk, $23.2\mu \mathrm{m}$ before the rear surface. For each row, the laser energy is indicated.

Figure 5

When the laser focus is positioned in the fused silica bulk, $8.7\mu \mathrm{m}$ before the rear surface, the radius (half width at half maximum) of the ablated polymer film is almost independent of the incident energy (squares). Such is not the case with a Gaussian beam (cross symbols).

Figure 6

(a) AFM image of desorbed polymer film holes obtained with 500 nJ and the focus $49.4\mu \mathrm{m}$ before the rear surface. The color bar indicates the height in nanometers. (b) Corresponding cross section. The dashed red lines indicate the depth of the film.