Kelsie Niffenegger, Brian Demaske, Vasily Zhakhovsky and Ivan ...

Kelsie
Niffenegger
Auburn
University
and
Materials
Simulation
Laboratory
at
USF
Physics
Department
Faculty
Advisers:
Dr.
Ivan
Oleynik
and
Dr.
Vasily
Zhakhovsky
Graduate
Student
Mentor:
Brian
Demaske

Background
Informa/on
Femtosecond
laser
for
formation
of
nanostructures
Ultrashort
heating
of
electrons
(10 15 
K/s)
Heating
depth
very
small
(~100
nm)
Response
of
metals
to
ultrafast
energy
deposition
Build‐up
of
high
pressure
in
surface
layer
Formation
&
propagation
of
ultrashort
pressure
waves
Strong
tensile
wave
results
in
cavitation
Ablation
&
surface
morphology
Ultrafast
cooling
(10 12 
K/s)

Pump
Probe
Technique
Typical experimental set-up:
Ti-Sapphire laser system
• λ ~ 800 nm
• center ~ 45°
• t pulse ~ 100 fs
• F incident < 1 J/cm 2
Ablation threshold:
F absorbed ≈ 100 - 200 mJ/cm 2
pump pulse
probe pulse
sample
Crater depth at threshold:
d crater ≈ 110 - 130 nm
probe

Experimental
Analysis
a
.
b.
c.
Cross
sections
of
Al
film
after
ultrashort
laser
irradiation
show
that
there
are
bubbles
frozen
underneath
the
material
next
to
the
formed
crater.
Experiments done by S. Ashitkov at Institute for High Temperatures, RAS
Images obtained by Y. Emirov at USF Nanoscience Research and Education Center

What
are
we
trying
to
do
Gain
an
atomic
scale
understanding
of
mechanisms
that
cause
surface
modification
Want
to
show
the
formation
of
a
crater
with
frozen
bubbles
under
the
surface
of
an
irradiated
gold
film
• Past
simulations
of
ablation
were
done
with
1D
heating
–
didn’t
allow
for
generation
of
2D
or
3D
nanostructures,
ours
simulates
a
2D
energy
distribution
on
the
surface
allowing
us
to
make
those
structures
• 2
parts
of
modeling:
• 2
temperature
(ions
&
electrons)
for
short
time
• Then,
1
temperature
(ions)
for
long
time

Two
Temperature
Model
~10 nm
1.Transmission
of
energy
from
laser
pulse
to
conduction
electrons
within
surface
layer
2.Transport
of
heat
by
electrons
through
Au
target
3.Exchange
of
energy
between
electron
and
ion
subsystems
until
thermal
equilibrium
is
established
laser pulse
conduction
electrons
Fit shape to functional form
and use as initial profile for 1-
temperature regime
(MD simulation)

Molecular
Dynamics
(MD)
method
z
periodic
boundary
conditions
reflection y
x
pump pulse
MD
method
is
based
on
tracking
of
the
atom
motions
by
numerical
solving
of
Newton’s
equations.
Periodical
boundary
conditions
are
imposed
on
the
system
along

y,z
–
axes,
with
free
boundary
conditions
on
the
x‐axis.
Laser
heating
is
non‐uniform
in
x‐y
plane,
and
results
in
a
given
profile
T 0 (x,y)
=
T 0 (x)*
cos 8 (












).

3
Main
Processes:
Mel/ng,
JeJng
and
Cavia/on
a. b. c.
Laser
induced
melting
is
shown
in
image
“a”
with
a
small
amount
of
energy
put
into
the
system
Jetting
of
the
material
occurred
when
more
energy
was
added,
as
in
image
“b”
Cavitation
under
a
cupola,
image
“c”,
requires
even
more
energy
to
cause
significant
expansion
of
the
liquid
surface

Abla/on
of
gold
film
by
non‐uniform
laser
beam
Parameters of movie simulation…
5,000
K
with
material
size
160
x
600
x
10
[nm]

Mechanisms
of
crater
forma/on
The
combination
of
these
three
processes
followed
by
recrystallization
(cooling)
results
in
the
final
morphology
which
includes
the
crater
and
frozen
bubbles.
density
velocity

Conclusions
The
major
processes
leading
to
the
formation
of
the
surface
morphology
were
investigated
by
MD
Non‐uniform
energy
distribution
in
laser
beam
induce:
Fast
melting
of
thin
surface
layer
Jetting
of
molten
material
Cavitation
under
a
liquid
shell
Cupola
formation
followed
by
its
breaking
Slow
recrystallization
of
liquid
surface
layer
Qualitative
agreement
with
experiment
found

Acknowledgments
This
research
is
supported
by
NSF
REU
program
in
Computational
Materials
Science
(grant
#
DMR‐0755256)
NSF
grant
#
DMR‐1008676