Page 37 - 00. Complete Version - Progress Report IPEN 2014-2016

Lasers Technology | Progress Report 37

can be created by ultrashort laser pulses (,) parameters were used to investigate the influ-
and in this case the modifications induced by ence of metals electron mobility and thermal
a pulse modify the Fth value for the following conductivity on the ablation dynamics, the
pulses, until the defects density reaches sat- glasses ablation mechanism dependence on
uration and Fth stabilizes at a constant value. the pulse duration, and to texturize mechani-
These cumulative phenomena are known as cal tools to improve their performance and re-
incubation effects, and the ablation threshold duce wear, and to promote selective ablation.
fluency modifications caused by them must
be considered when machining a material. A An interferometric system to maintain the la-
few years ago(,) we introduced a simple ex- ser focal point on the surface of a material be-
perimental technique, denominated D-Scan ing processed is under development. This sys-
(Diagonal Scan), to quickly measure the ultra- tem is based on the ultrashort pulses large
short pulses ablation threshold of a solid sam- bandwidth, and will improve the quality of
ple. Recently(,) we modified this technique to the machining.
allow the ablation threshold measurement
for an arbitrary pulse superposition (,) and During the ablation by ultrashort pulses, the
the consequently determination of the incu- explosive material removal creates shockwaves
bation effects. Knowing the ablation thresh- that propagate into the substrate under irra-
old for the superposition of multiple pulses is diation. These shockwaves, besides promoting
important when machining samples with ul- a local increase in the pressure, also rise the
trashort pulses, in which the sample displace- temperature in a propagating submicron lay-
ment speed and laser repetition rate play an er. These conditions can be extreme and highly
important role in setting the pulses overlap, localized(,) due to the violent explosion result-
and determine the morphology and quality ing from the ultrashort pulses high intensity,
of the structures etched. Our technique pres- and promote modifications in the substrate.
ents advantages over the traditional one due We have demonstrated that the ablation origi-
to its speed and for also (for) reproducing the nated shockwaves in amorphous graphite can
machining conditions more closely. Using the change the hybridization of the carbon bonds
results obtained for the superposition of many from sp2 to sp3, increasing the order of the
pulses, new methods were developed to etch crystalline array, cumulatively creating dif-
structures in the surfaces of technological ma- ferent allotropes including nanodiamonds.
terials minimizing the material modifications Figure 1a shows a High Resolution Electron
on the neighborhood of the etched regions. Diffraction of the diamond-like phase, and
Fig. 1b shows the same analysis of the initial
Using the D-Scan, we have determined the ab- polycrystalline graphite (PG) substrate; Fig. 1c
lation threshold of many materials, including presents a High Resolution Electron Microg-
glasses, metals, graphite and ceramics, among raphy of the laser created structure with the
others. The ablation threshold dependence on characteristic 0.205 nm d-spacing of the dia-
the pulses superposition is an important pa- mond phase evidenced by the zoom of a small
rameter when machining these materials to area shown in the bottom inset. Fig. 1d exhib-
create microstructures such as microfluidic and its the corresponding electron diffraction peaks
microoptofluidic (micro opto-fluidic) circuits as a function of the inverse of the d-spacing
that are being developed and built at the Cen- for the polycrystalline graphite (PG, gray spec-
ter for Lasers and Applications at IPEN. These trum) and diamond-like phase (blue spectrum).