Page 22 - PR 2014 2016 02 Lasers Technology

36 Lasers Technology | Progress Report

ence of dopants, impurities or other defects. parameter when machining these materials
As a consequence of the defects presence, the to create microstructures such as microfluidic
seed electrons are created more easily and and micro opto-fluidic circuits that are being
the avalanche ones are freed at lower impact developed and built at the Center for Lasers
energies, decreasing the F value. These de- and Applications at IPEN. These parameters
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fects can be created by ultrashort laser pulses were used to investigate the influence of met-
, and in this case the modifications induced by als electron mobility and thermal conductivity
a pulse modify the F value for the following on the ablation dynamics, the glasses ablation
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pulses, until the defects density reaches sat- mechanism dependence on the pulse duration,
uration and F stabilizes at a constant value. and to texturize mechanical tools to improve
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These cumulative phenomena are known as their performance and reduce wear, and to
incubation effects, and the ablation threshold promote selective ablation.
fluency modifications caused by them must
be considered when machining a material. An interferometric system to maintain the
A few years ago, we introduced a simple ex- laser focal point on the surface of a material
perimental technique, denominated D-Scan being processed is under development. This
(Diagonal Scan), to quickly measure the ul- system is based on the ultrashort pulses large
trashort pulses ablation threshold of a solid bandwidth, and will improve the quality of
sample. Recently, we modified this technique the machining.
to allow the ablation threshold measurement
for an arbitrary pulse superposition , and the During the ablation by ultrashort pulses, the
consequently determination of the incuba- explosive material removal creates shock-
tion effects. Knowing the ablation threshold waves that propagate into the substrate un-
for the superposition of multiple pulses is der irradiation. These shockwaves, besides
important when machining samples with promoting a local increase in the pressure,
ultrashort pulses, in which the sample dis- also rise the temperature in a propagating
placement speed and laser repetition rate submicron layer. These conditions can be ex-
play an important role in setting the pulses treme and highly localized, due to the violent
overlap, and determine the morphology and explosion resulting from the ultrashort pulses
quality of the structures etched. Our technique high intensity, and promote modifications in
presents advantages over the traditional one the substrate. We have demonstrated that the
due to its speed and also for reproducing the ablation originated shockwaves in amorphous
machining conditions more closely. Using the graphite can change the hybridization of the
2
3
results obtained for the superposition of many carbon bonds from sp to sp , increasing the
pulses, new methods were developed to etch order of the crystalline array, cumulatively
structures in the surfaces of technological ma- creating different allotropes including nanodi-
terials minimizing the material modifications amonds. Figure 19a shows a High Resolution
on the neighborhood of the etched regions. Electron Diffraction of the diamond-like phase,
and Fig. 19b shows the same analysis of the
Using the D-Scan, we have determined the ab- initial polycrystalline graphite (PG) substrate;
lation threshold of many materials, including Fig. 19c presents a High Resolution Electron
glasses, metals, graphite and ceramics, among Micrography of the laser created structure
others. The ablation threshold dependence with the characteristic 0.205 nm d-spacing
on the pulses superposition is an important of the diamond phase evidenced by the zoom

Instituto de Pesquisas Energéticas e Nucleares