Investigation of ablation thresholds of optical materials using 1-µm-focusing beam at hard X-ray free electron laser


Koyama, T.; Yumoto, H.; Senba, Y.; Tono, K.; Sato, T.; Togashi, T.; Inubushi, Y.; Katayama, T.; Kim, J.; Matsuyama, S.; Mimura, H.; Yabashi, M.; Yamauchi, K.; Ohashi, H.; Ishikawa, T.

Optics Express 21(13): 15382-8

2013


At the meso-scale we found that the proportion of native forest plays an important role in the reduction

Investigation
of
ablation
thresholds
of
optical
materials
using
1-µm-focusing
beam
at
hard
X-
ray
free
electron
laser
Takahisa
Koyama,
l
'
*
Hirokatsu
Yumoto,
1
Yasunori
Senba,
1
Kensuke
Tono,
1
Takahiro
Sato,
2
Tadashi
Togashi,
1
Yuichi
Inubushi,
2
Tetsuo
Katayama,
1
Jangwoo
Kim,
3
Satoshi
Matsuyama,
3
Hidekazu
Mimura,
4
Makina
Yabashi,
2
Kazuto
Yamauchi,
3
Haruhiko
Ohashi,
1
and
Tetsuya
Ishikawa
2
'Japan
Synchrotron
Radiation
Research
Institute
(JASRI),
1-1-1
Kouto,
Sayo-cho,
Sayo-gun,
Hyogo
679-5198,
Japan
2
RIKEN
SPring-8
center,
1-1-1
Kouto,
Sayo-cho,
Sayo-gun,
Hyogo
679-5148,
Japan
3
Department
of
Precision
Science
&
Technology,
Graduate
School
of
Engineering,
Osaka
University,
2-1
Yamada-
oka,
Suita,
Osaka
565-0871,
Japan
4
Department
of
Precision
Engineering,
Graduate
School
of
Engineering,
The
University
of
Tokyo,
7-3-1,
Hongo,
Bunkyo-kzi,
Tokyo
113-8656,
Japan
[email protected],spring8.or.jp
Abstract:
We
evaluated
the
ablation
thresholds
of
optical
materials
by
using
hard
X-ray
free
electron
laser.
A
1-µm-focused
beam
with
10-keV
of
photon
energy
from
SPring-8
Angstrom
Compact
free
electron
LAser
(SACLA)
was
irradiated
onto
silicon
and
SiO
2
substrates,
as
well
as
the
platinum
and
rhodium
thin
films
on
these
substrates,
which
are
widely
used
for
optical
materials
such
as
X-ray
mirrors.
We
designed
and
installed
a
dedicated
experimental
chamber
for
the
irradiation
experiments.
For
the
silicon
substrate
irradiated
at
a
high
fluence,
we
observed
strong
mechanical
cracking
at
the
surface
and
a
deep
ablation
hole
with
a
straight
side
wall.
We
confirmed
that
the
ablation
thresholds
of
uncoated
silicon
and
SiO
2
substrates
agree
with
the
melting
doses
of
these
materials,
while
those
of
the
substrates
under
the
metal
coating
layer
are
significantly
reduced.
The
ablation
thresholds
obtained
here
are
useful
criteria
in
designing
optics
for
hard
X-ray
free
electron
lasers.
©2013
Optical
Society
of
America
OCIS
codes:
(340.0340)
X-ray
optics;
(140.2600)
Free-electron
lasers
(FELs);
(160.4670)
Optical
materials.
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#187880
-
$15.00
USD
Received
29
Mar
2013;
revised
22
May
2013;
accepted
22
May
2013;
published
20
Jun
2013
(C)
2013
OSA
1
July
2013
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21,
No.
13
I
D01:10.1364/0E.21.015382
I
OPTICS
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15382
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K.
Tono,
T.
Togashi,
Y.
Inubushi,
T.
Sato,
T.
Tanaka,
T.
Kimura,
H.
Yokoyama,
J.
Kim,
Y.
Sano,
Y.
Hachisu,
M.
Yabashi,
H.
Ohashi,
H.
Ohmori,
T.
Ishikawa,
and
K.
Yamauchi,
"Focusing
of
X-ray
free
electron
laser
with
reflective
optics,"
Nat.
Photonics
7(1),
43-47
(2012).
#187880
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I
D01:10.1364/0E.21.015382
I
OPTICS
EXPRESS
15383
20.
K.
Tono,
T.
Kudo,
M.
Yabashi,
T.
Tachibana,
Y.
Feng,
D.
Fritz,
J.
Hastings,
and
T.
Ishikawa,
"Single-shot
beam-position
monitor
for
x-ray
free
electron
laser,"
Rev.
Sci.
Instrum.
82(2),
023108 (2011).
21.
M.
Kato,
T.
Tanaka,
T.
Kurosawa,
N.
Saito,
M.
Richter,
A. A.
Sorokin,
K.
Tiedtke,
T.
Kudo,
K.
Tono,
M.
Yabashi,
and
T.
Ishikawa,
"Pulse
energy
measurement
at
the
hard
x-ray
laser
in
Japan,"
Appl.
Phys.
Lett.
101(2),
023503
(2012).
22.
T.
Kudo,
T.
Hirono,
M.
Nagasono,
and
M.
Yabashi,
"Vacuum-compatible
pulse
selector
for
free-electron
laser,"
Rev.
Sci.
Instrum.
80(9),
093301
(2009).
23.
J.
M.
Liu,
"Simple
technique
for
measurements
of
pulsed
Gaussian-beam
spot
sizes,"
Opt.
Lett.
7(5),
196-198
(1982).
24.
NIST
Chemistry
WebBook,
NIST
Standard
Reference
Database
Number
69.
http://webbook.nist.gov/chemistry/
1.
Introduction
X-ray
free-electron
lasers
(XFELs)
[1],
such
as
the
Linac
Coherent
Light
Source
[2]
and
SPring-8
Angstrom
Compact
free
electron
LAser
(SACLA)
[3],
have
started
to
provide
intense,
coherent,
and
ultrafast
pulses
in
the
hard
X-ray
region,
which
promote
the
development
of
new
approaches
in
various
fields,
such
as
atomic
physics
[4-6]
and
structural
biology
[7,8].
Although
XFEL
light
provides
great
capabilities,
the
intense
beam
could
induce
damage
to
optical
elements,
which
would
lead
to
degradation
of
the
beam
quality.
The
irradiation
tolerance
of
optical
elements
is
evaluated
by
comparing
the
absorption
dose
with
the
melting
threshold.
The
melting
threshold
has
been
considered
as
a
reasonable
guide
in
designing
optical
components
[9-11].
Damage
by
FEL
irradiation
has
been
investigated
in
the
extreme
ultraviolet
(EUV)
and
the
soft
X-ray
regions
[12-16].
David
et
al.
have
reported
on
the
ablation
phenomenon
of
gold
at
a
photon
energy
of
8
keV
[17].
In
this
paper,
we
report
on
our
systematic
study
of
the
damage
thresholds
for
various
optical
materials
by
using
a
hard
X-ray
free
electron
laser
(FEL).
We
used
a
focused
XFEL
beam
at
a
photon
energy
of
10
keV,
which
has
a
sufficient
power
density
to
study
ablation
phenomena.
We
designed
and
installed
a
dedicated
experimental
chamber
for
the
precise
alignment
of
the
position
and
incident
angle
of
the
samples.
We
used
uncoated
silicon
and
Si0
2
substrates,
as
well
as
the
metal
(platinum
and
rhodium)
coating
on
these
substrates,
which
are
widely
used
for
X-ray
optics
as
samples.
We
performed
irradiation
studies
on
a
single
shot
and
for
a
normal
incidence
condition.
2.
Experiment
The
experiments
were
carried
out
at
beamline
3
(BL3)
of
the
SACLA
[3].
During
the
experiments,
SACLA
was
operated
at
a
mean
pulse
energy
of
130
pd,
a
pulse
duration
of
20
fs
[18],
and
a
pulse
repetition
rate
of
10
Hz.
The
X-ray
photon
energy
was
chosen
to
be
10
keV.
The
unwanted
contamination
of
higher-order
harmonics
and
gamma-rays
were
suppressed
using
a
double-mirror
system
in
the
optics
hutch.
The
XFEL
light
was
focused
down
to
a
diameter
of
1
gm
(FWIIM)
using
the
mirror
system
[19]
that
consists
of
two
carbon-coated
elliptical
mirrors
in
a
Kirkpatrick—Baez
configuration.
The
focusing
mirror
system
was
located
115
m
downstream
from
the
exit
of
the
fmal
undulator.
An
irradiation
chamber
was
designed
and
installed
at
the
focal
point
of
the
focusing
mirror
system,
as
shown
in
Fig.
1(a).
The
samples
were
mounted
on
high
precision
stages
at
a
motion
range
of
50
mm
in
the
vertical
and
horizontal
directions
perpendicular
to
the
optical
axis,
as
well
as
at
15
mm
along
the
optical
axis,
as
shown
in
Figs.
1(b)
and
1(c).
A
rotation
stage
was
used
for
adjusting
the
incident
angle,
the
motion
range
of
which
is
from
—10
to
+
100
degrees.
The
stage
specifications
are
summarized
in
Table
1.
The
surface
of
the
samples
is
monitored
by
an
optical
microscope
at
an
angle
of
30
degrees
for
the
normal
incidence
condition,
and
at
an
angle
of
90
degrees
for
the
grazing
incidence
condition,
as
shown
in
Fig.
1(a).
A
knife-edge
scanning
method
was
used
for
measuring
the
profile
of
the
focused
beam.
The
excellent
pointing
stability
of
the
XFEL
light
from
SACLA
made
it
possible
to
accurately
evaluate
the
beam
profile
and
the
irradiation
area.
#187880
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2013;
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22
May
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accepted
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May
2013;
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20
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(C)
2013
OSA
1
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2013
I
Vol.
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I
D01:10.1364/0E.21.015382
I
OPTICS
EXPRESS
15384
(a)
_
T.
en
(c)
/
Z
stage
Sam.
le
-
4
3
.
XFEL
stage
,Y
4
/
X
stage
XFEL
OM
0
stage
Fig.
1.
(a)
Photograph
of
experimental
chamber.
OM:
Optical
microscope
for
observation
of
the
sample
surface.
(b)
Photograph
of
area
around
sample
holder
inside
chamber.
(c)
Schematic
drawing
of
sample
stage
configuration.
The
sample
holder
is
mounted
on
the
XYZ
translation
stages.
These
stages
are
placed
on
the
rotation
stage.
Table
1.
Stage
specifications
Resolution
Motion
range
Horizontal
direction
(X
stage)
0.1
µm
±
25
nun
Vertical
direction
(Z
stage)
0.1
µm
±
25
nun
Optical
axis
direction
(Y
stage)
0.5
µm
±
7.5
nun
Rotation
about
vertical
axis
(0
stage)
0.002
deg.
—10'—+
100
deg.
The
pulse
energy
was
controlled
by
silicon
attenuators
of
various
thicknesses
inserted
in
front
of
the
focusing
mirrors.
The
shot-to-shot
fluctuations
of
the
pulse
energy
were
monitored
by
using
a
scattering-based
beam
intensity
monitor
[20],
which
was
calibrated
by
a
cryogenic
radiometer
[21].
Measured
pulse
energy
accuracy
was
within
the
range
of
±
3.5%
around
the
X-ray
energy
used
in
this
experiment.
Single
shot
irradiations
at
a
normal
incidence
condition
at
a
pulse
energy
ranging
from
0.001
to
100
gJ
were
used
in
this
experiment.
The
sample
was
moved
with
constant
speed
during
the
exposure.
The
number
of
shots
was
controlled
by
using
a
pulse
selector
[22].
The
ablation
thresholds
of
the
samples
were
evaluated
by
measuring
the
diameters
of
the
imprinted
ablation
profiles
using
scanning
probe
microscopy
(SPM)
and
scanning
electron
microscopy
(SEM).
3.
Results
and
discussion
Figure
2
shows
one
of
the
typical
imprints
of
silicon
irradiated
at
high
fluence
without
any
attenuators.
The
optical
microscope
image
for
the
surface
is
shown
in
Fig.
2(a).
The
irradiated
fluence
inside
the
crater
was
57
µJ/µm
2
,
which
was
several
tens
of
orders
of
magnitude
higher
than
the
melting
dose
of
silicon.
Spallation
and
cracks
were
observed
around
an
area
of
40
gm
on
the
surface.
The
cross
sectional
SEM
image
prepared
using
focused
ion
beam
sampling
is
shown
in
Fig.
2(b).
For
the
cross
sectional
SEM
image,
the
crater
depth
and
the
diameter
were
measured
to
be
40
and
4
gm,
respectively.
A
large
volume
of
melted
and/or
evaporated
silicon
was
ejected
from
the
inside,
and
a
straight
side
wall
was
formed.
Therefore,
a
deep
ablation
phenomenon
was
observed.
Although
the
attenuation
length
of
silicon
is
134
gm
for
10
keV
of
photon
energy,
the
40-gm
crater
depth
is
small.
The
reason
for
this
may
be
as
follows.
Ablated
silicon
in
the
region
deeper
than
the
bottom
of
the
crater
cannot
be
ejected
outside,
so
solidification
occurs
and
it
is
probably
in
the
amorphous
state.
It
is
difficult
to
observe
from
the
SEM
images
because
they
are
only
sensitive
to
the
surface
morphology.
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May
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20
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2013
I
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I
D01:10.1364/0E.21.015382
I
OPTICS
EXPRESS
15385
(a)
10
pm
Pt
layer
SiO
2
under
Pt
layer
H
0.023±0.004
µJ/µm
2
0
0
°
0.11±0.03
µJ/µm
2
4
(b)
11111
.
11
11.111101
4
pm
10
pm
77;
Fig.
2.
(a)
Optical
microscope
image
of
irradiated
silicon
viewed
from
surface
at
fluence
of
57
µJ/µm
2
.
(b)
Cross
sectional
SEM
image
of
(a)
prepared
by
focused
ion
beam
sampling.
We
evaluated
the
ablation
thresholds
of
uncoated
silicon
and
SiO
2
substrate
by
varying
the
intensity
using
Liu's
technique
[23].
The
imprint
areas
were
plotted
as
a
function
of
the
fluence,
as
shown
in
Fig.
3(a).
The
obtained
threshold
fluence
Fth
was
0.78
±
0.04
µJ/µm
2
(4.5
±
0.7
µJ/µm
2
)
in
silicon
(SiO
2
),
which
was
converted
to
the
dose
D
for
a
single
atom
[9-
11],
as
follow;
the
dose
D
is
given
by
D
=
F
th
liA
(pN
A
)
,
where
p,
A,
6,
p,
and
N
A
are
the
absorption
coefficient,
the
average
atomic
weight,
the
RMS
beam
size,
the
average
density,
and
the
Avogadro's
constant,
respectively.
Converted
dose
was
0.73
±
0.04
eV/atom
(1.7
±
0.3
eV/atom).
This
value
reasonably
agrees
with
the
calculated
melting
dose
of
0.88
eV/atom
(1.1
eV/atom).
The
melting
dose
was
calculated
from
the
thermodynamic
properties,
which
took
into
consideration
the
temperature
dependent
heat
capacity
and
the
latent
heat
of
melting
[24].
Note
that
we
did
not
include
effects
of
electron
transport
in
this
calculation.
o
Si
A
S10
2
4
5±0.7
µJ
/µm
2
0.78±0
04
µ..141m
2
0.01
0.1
1
Fluence
(µJ
/µm
2
)
10
(a)
12
9
co
co
6
C
,=
E
3
0
(b)
30
N
E
va
20
co
'E
L
10
E
0
10
0.01
0.1
1
Fluence
(0/11m
2
)
Fig.
3.
Imprint
areas
plotted
as
function
of
fluence.
(a)
Uncoated
Si
and
Si0
2
substrates,
(b)
Pt
coating
layer
and
Si0
2
substrate
under
this
layer.
We
used
a
200-nm-thick
platinum
layer
coated
on
silicon
and
SiO
2
substrates
as
the
metal
coating
samples.
The
inserted
adhesive
layer
was
5-nm-thick
chromium.
The
imprint
areas
of
the
platinum
layer
and
the
SiO
2
substrate
under
the
coating
layer
were
plotted
in
Fig.
3(b)
as
a
function
of
the
fluence.
The
ablation
threshold
of
the
platinum
was
evaluated
to
be
0.023
±
0.004
µJ/µm
2
(0.52
±
0.09
eV/atom).
This
value
reasonably
agrees
with
the
calculated
melting
dose
of
0.78
eV/atom.
However,
the
ablation
threshold
of
the
SiO
2
substrate
under
the
coating
layer
was
evaluated
to
be
0.11
±
0.03
µJ/µm
2
(0.04
±
0.01
eV/atom),
while
that
of
#187880
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I
OPTICS
EXPRESS
15386
the
uncoated
SiO
2
substrate
was
4.5
±
0.7
µJ/µm
2
(1.7
±
0.3
eV/atom)
as
obtained
above.
This
value
was
40
times
lower
than
that
of
the
uncoated
substrate.
Figures
4(a)-4(c)
show
the
SEM
images
of
the
imprints
formed
on
the
platinum
layer
coated
SiO
2
substrate
for
a
detailed
observation
of
the
surface
morphology.
Figures
4(d)-4(f)
show
cross
sectional
profiles
of
these
craters.
Shallow
craters
appeared
in
the
substrate
region
for
the
0.3
µJ/µm
2
and
2.6
µJ/µm
2
fluences.
Notably,
these
fluences
are
lower
than
the
threshold
fluence
for
the
uncoated
Si0
2
substrate
(4.5
µJ/µ,m
2
).
Similar
results
were
observed
for
the
silicon
substrate
and
other
coating
materials
such
as
rhodium.
In
the
case
of
silicon
substrate
under
platinum
coating
layer,
the
ablation
threshold
was
evaluated
to
be
0.065
±
0.008µEµ,m
2
(0.060
±
0.007
eV/atom).
This
value
was
10
times
lower
than
that
of
the
uncoated
substrate.
Furthermore,
we
used
a
75-nm-thick
rhodium
layer
coated
on
silicon
and
Si0
2
substrates
as
the
other
metal
coating
samples.
The
inserted
adhesive
layer
was
10-nm-thick
chromium.
We
confirmed
that
the
substrate
behavior
was
the
same
with
platinum
coatings.
0.5
.
(d)
(e)
(f)
Surface
Pt
layer
SiO
2
substrate
-1.0
-
-4
-2
0
2
4
-4
-
2
0
2
4
-4
-2
0
2
Position
(pm)
Position
(µrn)
Position
(pm)
Fig.
4.
(a—c)
Imprint
SEM
images
of
platinum
coated
SiO
2
at
fluences
of
0.3,
2.6,
and
8.6
µJ/µm
2
.
Observed
SEM
images
were
viewed
under
an
angle
of
30°.
The
imprint
diameters
of
the
platinum
layer
were
2.1,
4.0,
and
4.9
pm,
respectively.
(d—f)
Cross
sectional
profiles
of
these
craters
measured
by
SPM.
In
the
case
of
the
crater
irradiated
by
a
fluence
of
8.6
µJ/µm
2
,
the
SPM
probe
cannot
reach
the
bottom
of
the
crater.
The
dashed
line
indicated
the
interface
between
the
Pt
layer
and
the
SiO
2
substrate.
The
measured
threshold
fluences
and
calculated
melting
doses
were
summarized
in
Table
2.
The
measured
threshold
values
for
the
uncoated
substrate
and
metal
coating
layer
agreed
with
the
calculated
melting
dose.
However,
the
thresholds
of
the
substrates
under
the
coating
layer
were
significantly
lower
than
that
for
the
uncoated
substrates;
the
ratio
was
1/10
for
silicon
and
1/40
for
Si02.
As
seen
in
Fig.
3(b),
differential
coefficient
of
the
measured
imprint
area
of
Si02
underneath
coating
as
a
function
of
fluence
is
changed
clearly.
The
inflection
point
is
nearly
threshold
fluence
of
uncoated
substrate.
For
higher
fluence
than
the
threshold,
large
size
craters
were
formed,
as
shown
in
Fig.
4(c),
by
direct
interaction
with
intense
X-rays
to
the
substrate.
Note
that
X-ray
transmissions
through
the
coating
are
as
high
as
95.4%
for
200-nm-
thick
platinum
layer
and
99%
for
75-nm-thick
rhodium
layer
for
10
keV
X-rays.
On
the
other
hand,
for
lower
fluence,
shallow
craters
were
formed
as
shown
in
Fig.
4(a)
and
4(c).
The
damage
of
the
substrate
underneath
coating
could
originate
from
collisions
of
energetic
particles
(electrons,
ions,
and
neutrals)
that
are
generated
with
intense
X-rays
in
the
coating
region.
#187880
-
$15.00
USD
Received
29
Mar
2013;
revised
22
May
2013;
accepted
22
May
2013;
published
20
Jun
2013
(C)
2013
OSA
1
July
2013
I
Vol.
21,
No.
13
I
D01:10.1364/0E.21.015382
I
OPTICS
EXPRESS
15387
Table
2.
Measured
threshold
fluences,
corresponding
doses,
and
calculated
melting
doses.
Measured
threshold
fluence
(µJ/µm
2
)
Corresponding
dose
(eV/atom)
Calculated
melting
dose
(eV/atom)
Uncoated
Si
0.78
±
0.04
0.73
±
0.04
0.88
Si
under
Pt
coating
layer
0.065
±
0.008
0.060
±
0.007
Uncoated
SiO
2
4.5
±
0.7
1.7
±
0.3
1.1
Si0
2
under
Pt
coating
layer
0.11
±
0.03
0.04
±
0.01
Pt
0.023
±
0.004
0.52
±
0.09
0.78
Rh
0.072
±
0.007
0.79
±
0.08
0.90
4.
Summary
We
have
measured
the
ablation
thresholds
of
optical
materials
that
are
widely
use
as
X-ray
mirrors.
A
focusing
hard
X-ray
FEL
beam
at
a
beam
size
of
1µm
was
used.
We
found
that
the
measured
ablation
thresholds
of
uncoated
silicon
and
a
SiO
2
substrate,
as
well
as
a
metal
(platinum
and
rhodium)
thin
film
are
comparable
to
the
melting
dose,
while
the
substrates
under
the
metal
coating
layer
showed
that
they
are
easily
damaged.
These
results
should
be
useful
criteria
for
designing
X-ray
optics.
Acknowledgments
The
authors
would
like
to
sincerely
thank
Takanori
Miura
for
his
support
in
measuring
the
samples,
and
Hikaru
Kishimoto
and
the
SACLA
engineering
team
for
their
help
during
the
beam
time.
This
work
was
performed
at
the
BL3
of
SACLA
with
the
approval
of
the
Japan
Synchrotron
Radiation
Research
Institute
(JASRI)
(Proposal
No.
2012A8056).
This
research
was
partially
supported
by
a
Grant-in-Aid
for
Scientific
Research
(5)
(23226004)
from
the
Ministry
of
Education,
Sports,
Culture,
Science
and
Technology,
Japan
(MEXT).
#187880
-
$15.00
USD
Received
29
Mar
2013;
revised
22
May
2013;
accepted
22
May
2013;
published
20
Jun
2013
(C)
2013
OSA
1
July
2013
I
Vol.
21,
No.
13
I
D01:10.1364/0E.21.015382
I
OPTICS
EXPRESS
15388