Page 36 - 00. Complete Version - Progress Report IPEN 2014-2016

36 Lasers Technology | Progress Report

plification) technologies allow the study, in temperatures and pressures that can induce
conventional laboratory, of phenomena that phase transitions; these phase transitions were
only 15 years ago were restricted to national studied in graphite, in which many allotropes
laboratories with annual budgets amounting were crated, including nanodiamonds. The
to billions of dollars. In the Center for Lasers nonthermal ablation was also used to remove
and Applications at IPEN, a hybrid Ti:Sapphire/ necrosed material from burned animals and
Cr:LiSAF TW peak power laser system is under its effects on the tissue regeneration are un-
continuous development. A flashlamp pump- der investigation.
ing cavity for a Cr:LiSAF rod gain medium was
developed and built, aiming to minimize the The ablation of solids by ultrashort pulses is
thermal load on the Cr:LiSAF crystal by the use due to a Coulomb explosion following the ejec-
of absorption filters between the lamps and tion of surface electrons accelerated by the la-
the gain medium, allowing the amplification ser electric field, or by a phase explosion re-
of ultrashort pulses to the terawatt peak pow- sulting from in a high density of free electrons
er region at high repetition rates. The pumping generated by avalanche ionization. The puls-
cavity was initially used in a laser configura- es (have) very brief duration, shorter than the
tion, and generated 60 μs pulses with energy typical phonon period, mainly heats the elec-
up to 2.8 J, with an average power of 30 W at trons and the explosions that remove materi-
15 Hz repetition rate, the highest reported to al occur after the pulse has finished, with min-
date. The utilization of the pumping cavity in imal material heating. The avalanche occurs
a hybrid Ti:Sapphire/Cr:LiSAF CPA configura- when seed electrons, either already present
tion produced 60 fs pulses with 30 mJ of ener- in metals or created by tunneling or multi-
gy at 5 Hz repetition rate, reaching 0.5 TW of photon ionization in other materials, are ac-
peak power, the highest in the southern hemi- celerated by the ultrashort pulse electric field
sphere. Among other applications, these puls- into a quivering motion and generate more
es can be focused to relativistic intensities to free electrons by impact ionization in an ex-
accelerate electrons up to MeV energies. Our ponential growth process that is almost inde-
laboratory also has another amplified laser pendent of the material being irradiated. The
system capable of generating up to 800 μJ, 25 high intensities reached by ultrashort pulses
fs pulses or 300 μJ, 5 fs pulses. Even at lower easily induce the nonlinear phenomena that
peak powers, the pulses very short duration create the initial free electrons, making these
generates nonlinear phenomena, particular- pulses efficient tools to etch any kind of mate-
ly those initiated by multiphotonic and tun- rial. Due to this nonselective mechanism, the
neling processes that generate free electrons. only parameter that must be known to etch a
material with ultrashort pulses is its ablation
Ultrashort pulses were utilized to ablate and threshold fluence (fluency), Fth. This ablation
machine technological materials with preci- threshold derives from the material atoms
sion on the micrometer scales with negligi- bonding energies, electronic density and its
ble heat affected zones, and to study how la- ionizing energies, which depend on the pres-
ser created defects (color centers, vacancies) ence of dopants, impurities or other defects.
affect the ablation dynamics in various ma- As a consequence of the defects presence, the
terial such as metals and dielectrics. As a sec- seed electrons are created more easily and the
ondary effect of the ablation, shockwaves prop- avalanche ones are freed at lower impact en-
agate into the substrate, generating extreme ergies, decreasing the Fth value. These defects

Instituto de Pesquisas Energéticas e Nucleares