COMMUNICATION
Juan R. Sanchez-Valencia , Johann Toude , * Ana Bo as, * Angel Ba anco , Ru h Lahoz ,
Ge man F. de la Fuen e , Fabian F u os , and Agus in R. Gonzalez-Elipe
Selec i e Dich oic Pa e ning by Nanosecond Lase
T ea men o Ag Nanos ipes
D . J. R. Sanchez-Valencia, D . J. Toude , D . A. Bo as,
D . A. Ba anco, P o . D . A. R. Gonzalez-Elipe
Nano echnology on Su aces Labo a o y
ICMSE: Ma e ials Science Ins i u e o Se ille
CSIC-Uni e si y o Se ille
A d. Ame ico Vespucio 49, 41092, Se illa, Spain
E-mail: [email p o ec ed]; [email p o ec ed]
D . R. Lahoz, P o . G. F. de la Fuen e
Ma e ials Science Ins i u e o A agon
CSIC-Uni e si y o Za agoza
C/Ma ía de Luna 3, 50018, Za agoza, Spain
D . F. F u os
Applied Physics Depa men
E.T.S. Ingenie ía In o má ica
Uni e si y o Se ille
A d. Reina Me cedes s/n, 41012 Se ille, Spain
The s ong abso p ion o ligh
[ 1 ] and he local ampli i ca ion o
he elec omagne ic i eld
[ 2 ] a he plasmon esonance o noble
me al nanos uc u es ha e been he ocus o hund eds o
s udies due o hei p ac ical applica ions o he ab ica ion o
op ical de ices such as i l e s, non-linea op ical componen s,
o Raman enhance s.
[ 3,4 ] The con ol o he plasmon ea u es
such as spec al wid h,
[ 5 ] posi ion, [ 6 ] and shape
[ 7 ] can be accom-
plished by di e en physical deposi ion ou es
[ 8–12 ] p o iding
adequa e g owing condi ions o me al nanopa icles (MNPs).
Pionee wo ks in he 1990s showed he op ical selec i i y o
elonga ed Ag deposi s on SiO
2 wi h applica ions as op ical i l-
e s o windows o con ol sola hea gain and gla e, among
o he s.
[ 13 ] Recen ly, assemblies o pa allel s ipes o MNPs
ha e been ab ica ed on o p e o med su aces p esen ing a 1D
pe iodic oughness
[ 14 , 15 ] o bundled SiO
2 nanocolumns. [ 16 , 17 ] A
signi i can mac oscopic op ical dich oism has been epo ed
o hese sys ems ha can be use ul o he de elopmen o
pola ized ligh emi e s o ma e ials wi h an enhanced IR lumi-
nescence because o he exci a ion o wo dis inc plasmon
esonances in he di ec ions pa allel (longi udinal mode) and
pe pendicula ( ans e se mode) o he s ipes.
[ 18 ] A chi ec u e
con ol o he me al assemblies plays a de e minan ole in he
unc ional p ope ies o he ma e ial. Fo his pu pose, he so -
li hog aphic echniques p o ide means o accu a ely ailo he
nanos uc u e o he ma e ials.
[ 18–21 ] Lase scanning is a so -
li hog aphic echnique widely used o modi y he shape and
s uc u e o me al nanopa icles.
[ 21–24 ] Su ace modi i ca ion
can be easily achie ed by in si u
[ 21 ] o ex si u
[ 22–24 ] pulsed lase
ea men in he case o andom sys ems o MNPs. In con as ,
no hing has been epo ed abou he e ec o a pulsed lase
on he s uc u e and op ical dich oism o au oo ganized me al
nanos uc u es. In his pape we show ha nanosecond (ns)
lase i adia ion can be e ec i ely used o con ol he op ical
dich oism o Ag s ipes suppo ed on SiO
2 nanocolumns (NCs).
This dich oism can be e ec i ely ailo ed along he ull isible
ange. Thus, we p opose he u iliza ion o he AgNPs/SiO
2 NCs
s uc u es o w i ing dich oic pa e ns a he mic oscale wi h
po en ial applica ions o enc yp ion and da a s o age pu poses.
AgNPs/SiO 2 NCs i lms we e g own by a wo-s ep p ocess.
[ 17 ]
Fi s , SiO
2 hin i lms we e deposi ed by glancing angle apo
deposi ion (GLAD) wi h a il ed columna nanos uc u e and
≈ 350 nm hickness (see Figu e S1a in he Suppo ing In o -
ma ion and he Expe imen al Sec ion).
[ 25 , 26 ] These s uc u es
p esen an aniso opic su ace opog aphy known as “bun-
dling”,
[ 17 ] consis ing o he coalescence o he NCs along he
x -di ec ion (Figu e S1b). The sil e nanopa icles we e hen
g own by DC spu e ing a oom empe a u e. The “bundled”
SiO
2 NCs ac as a empla e o he ab ica ion o Ag s ipes
o med by me al NPs.
[ 17 ] Nanosecond lase pos - ea men was
hen pe o med a no mal incidence. The ene gy impinging on
he sample su ace was con olled by a ying he ou pu lase
powe ( P ) be ween 3.5 kW cm
− 2 and 7.0 kW cm
− 2 .
Figu e 1 shows he scanning elec on mic oscopy (SEM)
images o a i lm be o e (Figu e 1a,c,e ) and a e (Figu e 1b,d, )
lase ea men a 7.0 kW cm
− 2 . The su ace o he o iginal i lm
consis s o s ipes o densely packed MNPs ollowing he bun-
dling di ec ion o he nanocolumns ( x -di ec ion) (Figu e 1a ). [ 17 ]
These Ag-s iped nanos uc u es, sepa a ed om each o he
a ound 30 nm in he y -di ec ion, co e 78% ac ion o he
su ace (de i ned as he po ion o he image co esponding
o MNPs) as ob ained by digi al pic u e analysis. Figu e 1c,e
show ha he pa icles p esen a “ca pe -like” p ojec ed shape
wi h a heigh ≈ 15–20 nm. A e lase ea men a 7.0 kW cm
− 2
(Figu e 1b,d, ), he pa allel s ipes a e s ill p esen bu a e
o med by con inuous and ex emely smoo h me al islands
(see also a omic o ce mic oscopy (AFM) image in Figu e 2 ). In
addi ion, hese islands appea o be mo e sepa a ed along he
y -di ec ion. Indeed, while hei cen e - o-cen e dis ance emains
unchanged, hei bo de - o-bo de dis ance has inc eased o
50 nm, wi h a su ace co e age o only 51%. The lase - ea ed
me al s ipes p esen a unca ed ellipsoidal p ojec ed shape in
he y – z plane (Figu e 1d, ). Thei es ima ed heigh o ≈ 30–40 nm
is highe han be o e lase ea men . Fu he mo e, sepa a ed
nanopa icles in he 10–20 nm ange appea on he la e al aces
o he SiO
2 columns (Figu e 1e, and Figu e S2 in he Sup-
po ing In o ma ion) in addi ion o he me al s ipes. Figu e 2
shows he AFM opog aphy pic u es o he as-g own sample
(a) and he lase - ea ed samples a lase powe o 4.0 kW cm
− 2
(b) and 7.0 kW cm
− 2 (c). The su ace o he o iginal sample
COMMUNICATION
esul s can also be in e p e ed
as he compa ison o he size
dis ibu ions o he me al g ains
in he su aces, showing ha
he lase ea men induces a
smoo hing in he me al nano-
pa icles su ace as well as a
sha pening in he size dis ibu-
ion o he ea u es in he su -
ace consis en wi h he enlon-
ga ion o he Ag nano s ipes in
he x -di ec ion. Mo eo e , i is
also wo h men ioning ha he
analysis o he AFM mic oscopy
images by as Fou ie ans-
o m in wo dimensions (2D-
FFT) (see Expe imen al Sec ion
o u he de ails) shows an
inc ease o he aniso opy a e
ea men a high lase powe .
Figu e 3 (le ), which shows he
SEM images o he samples, con-
i ms ha he Ag nanos uc u es
e ol e as a unc ion o he lase
powe (see also Figu e S3, Sup-
po ing In o ma ion). The com-
pa ison be ween Figu e 3a (as-
g own ma e ial) and Figu e 3b
( P = 4.0 kW cm
− 2 ) con i ms he
esul obse ed by AFM ha
con inuous and smoo h me al
s ipes al eady o m a in e me-
dia e powe s. Fu he inc ease
in he powe (Figu e 3c ; P =
7.0 kW cm
− 2 ) p oduces a
dec ease in he me al co e age
ha is likely o be linked o an inc ease o he s ipe heigh .
The consequences o he s uc u al changes p o oked by he
lase ea men on he op ical esponse o he i lms we e s udied
by ansmi ance measu emen s a no mal incidence wi h a
collima ed and linea ly pola ized beam. Spec a in Figu e 3d
we e acqui ed wi h he incoming elec ic i eld o ien ed along
he s ipes ( x -pola iza ion) and hose in Figu e 3e wi h he i eld
pe pendicula o he s ipes ( y -pola iza ion) o AgNPs/SiO 2 NCs
i lms be o e and a e ea men a di e en lase ou pu
powe s. The spec um o he o iginal sample p esen s abso p-
ion bands peaking a 650 nm and 550 nm o x - and y -
pola iza ion, espec i ely. This op ical dich oism mus esul
om he exci a ion o dis inc plasmon esonances along he
s ipes (longi udinal plasmon) and pe pendicula ly o hem
( ans e se plasmon), while he shi be ween he wo bands
indica es ha he elec omagne ic coupling along he s ipes is
s onge han be ween successi e s ipes.
Upon lase ea men a mode a e powe ( P = 4.0 kW cm
− 2 ),
he longi udinal plasmon band (Figu e 3d ) becomes less
in ense, b oadens, and shi s owa ds he in a ed. These ea-
u es a e ypical o he o ma ion o con inuous me al s ipes
in ag eemen wi h Figu e 3b . Inc easing he lase powe has a
smalle e ec on his b oad band, which becomes p og essi ely
(Figu e 2a ) is o med by densely packed nano pa icles, in ag ee-
men wi h he SEM no mal iew mic oscopy image in Figu e 1a .
The analysis o he AFM images shows ha hese Ag NPs ha e
a wide size dis ibu ion anging om 20 o 100 nm. Such a
high dispe sion in he Ag NPs sizes is known o p oduce a wide
plasmon abso p ion band along he isible ange.
[ 1–3 ] The AFM
opog aphy pic u es o he lase ea ed sample a 4.0 kW cm
− 2
does no e eal a subs an ial change in he size dis ibu ion o
he Ag NPs al hough he pa icle edges a e now less de i ned.
This indica es ha he MNPs ha e begun o pe cola e. Such a
pe cola ion can be easily deduced om he de i a i e ep esen-
a ion o he AFM opog aphic images shown in Figu e 2e . This
ep esen a ion enhances he con as a he pa icle edges while
he egions o a cons an slope a e shown wi h he same colo .
Acco dingly, in he o iginal samples he Ag NPs p esen e y
well de i ned bo de s, while in he sample ea ed a 4.0 kW cm
− 2
lase powe he edges a e less de i ned. The mos ou s anding
opog aphic change occu s o he lase ea ed sample a
7.0 kW cm
− 2 (Figu e 2c, ). Fo his ea men he Ag NPs a e
comple ely pe cola ed in e y smoo h and con inuous nano-
s ipes in he x -di ec ion. Figu e 2g ep esen s he s a is ical
analysis o he g ain bo de s o he ea u es in he su ace
ob ained o he h ee modi i ed su aces in Figu e 2d– . The
Figu e 1 . Su ace (a,b) and c oss-sec ion (c,d) SEM images o AgNPs/SiO
2 NCs i lms be o e (le ) and a e
( igh ) lase ea men a 7.0 kW cm
− 2 ; e) and ) co espond o he backsca e ed elec on images o (c) and
(d), espec i ely.
o he sys em was modeled
wi hin he dipole app oxi-
ma ion. De ails a e gi en in
Figu e S4 o he aniso opic
e ec i e medium modeling
[ 27 , 28 ]
sec ion o he Suppo ing In o -
ma ion. Despi e he simplici y o
he model and he c ude app ox-
ima ion o he s ipe mo pholo-
gies (Figu e S4a, Suppo ing
In o ma ion), he simula ions in
Figu e S4b,c ep oduce quali a-
i ely he measu ed e olu ion o
he longi udinal and ans e se
plasmon esonances upon bo h
low and high powe lase ea -
men s. Indeed, he b oadening
and ed-shi o he longi udinal
abso p ion band a e low powe
ea men a e well- ep oduced
by assuming a ansi ion om
densely packed nanopa icles o
con inuous s ipes (Figu e S4b).
Meanwhile, he e e sal o he
ans e se abso p ion band posi-
ion and he loss o in ensi y o
he longi udinal plasmon a e
high-powe ea men s a e ep o-
duced by assuming na owe
s ipes wi h a highe aspec a io
(Figu e S4c).
Lase ea men o me als
a low o mode a e powe s is
gene ally known o a ec hei
nanos uc u e h ough abla ion
mechanisms
[ 29 ] o by hea ing
e ec s, p oducing an enhanced
a omic su ace di usion o
e en olume mel ing.
[ 23 ] In ou
expe imen a signi i can abla-
ion o sil e can be disca ded
because no emo al o sil e was
obse ed, as es ima ed om he
heigh o s ipes and he su ace
co e age. Fo ns-lase annealing
wi h pulse du a ions longe han he me al elec on–phonon
elaxa ion ime,
[ 23 , 30 ] hea accumula es in he me al. Fo con-
inuous me al hin i lms deposi ed on SiO
2 /Si such localized
annealing leads o mel ing and dewe ing o he me al.
[ 31 , 32 ]
Since he condi ions o ou expe imen (i.e., a ound 10 pulses
wi h an a e age powe densi y o ≈ 7 kW cm
− 2 pe pulse o he
ea men a maximum powe eaching a pa icula zone o
he sample su ace) a e compa able o hose used o pa ially
dewe Ag hin i lms om SiO
2 /Si, [ 31 , 32 ] we p opose a wo s ep
mechanism o accoun o he s uc u al e olu ion o he s ipes
upon lase ea men . Fi s ly, he me al nanos uc u es accu-
mula e hea up o hei mel ing empe a u e. Because o hei
small size and he densely packed s a e in he o iginal i lms,
hey mel apidly in o Ag d ople s, which acco ding o li e a u e
less in ense. Unde y -pola iza ion (Figu e 3e ), he ans e se
abso p ion band emains ela i ely na ow a e lase ea men
bu shi s h ough he isible spec um as a unc ion o he lase
powe (Figu e 3 ). T ea men a P = 3.5 kW cm
− 2 esul s in a
ed-shi om 650 o 750 nm, while u he inc easing he lase
powe g adually shi s he band owa ds sho e wa eleng hs (a
blue-shi up o 400 nm). Blue-shi s a e o en a ibu ed in he
li e a u e o a ounding o he nanopa icles o o a weakening
o he elec omagne ic coupling be ween hem.
[ 10 , 11 ]
Also no iceable in he x -pola ized spec a is he enhancemen
o an addi ional abso p ion band a a ound 400 nm (Figu e 3d )
ha is al eady p esen be o e ea men .
To in e p e he e olu ion o plasmon abso p ion bands as
a unc ion o he lase powe , he op ical ex inc ion coe i cien
Figu e 2 . AFM mic oscopy images o he AgNPs/SiO
2 NCs hin i lms be o e he lase ea men (a), a e
ea men a P = 4.0 kW cm
− 2 (b) and a P = 7.0 kW cm
− 2 (c). Panels (d– ) show he co esponding de i a i e
o he images in (a–c) in o de o enhance he bo de g ains be ween he sil e pa icles. g) Compa ison o
he size dis ibu ion o he ea u es in he su ace calcula ed om (d– ). h) Compa ison o he 2D-FFT o he
images in (a–c).
possibili ies o op ical pa e ning and coding. Figu e 4 a
shows a se ies o colo ed samples ob ained by ns-lase ea -
men o AgNPs/SiO
2 NCs deposi ed on used silica sub-
s a es. The pic u es we e acqui ed o samples illumina ed
wi h whi e ligh h ough a linea pola ize (Figu e 4b ). Upon
azimu hal o a ion o he samples, he illumina ion beam
is oggled om x -pola ized o y -pola ized and pola iza ion
modula ed colo s a e hen obse ed. In o he wo ds, hey
display an op ical dich oism ha can be uned by a ying
he lase powe . While x -pola ized exci a ion b ings ou a
monoch oma ic image (g ey-blue ones) ha is li le a ec ed
by he lase powe , a ansi ion om a blue o a yellow colo
do no end o we he silica subs a e.
[ 33 ] Secondly, hese d op-
le s coalesce in he o m o con inuous and l a s ipes o med
along he bundle s uc u e o he SiO
2 wi h NCs ac ing as a
empla e.
[ 34 ] Following he lase sho , he Ag NPs cool down,
ec ys allize, and eeze p oducing an in e connec ed d ople
shape s uc u e as shown by he SEM pic u es on Figu e 1b . As
he lase powe inc eases, he s ipes unde go a pa ial dewe -
ing. In all cases a esidual se o small Ag NPs emain a ached
o he la e al aces o he SiO
2 NCs (see Figu e 1 and Figu es S2
and S3 in he Suppo ing In o ma ion).
Among he possible applica ions o he lase w i ing
echnique de eloped he e, we would like o highligh i s
Figu e 3 . Le ) SEM images o AgNPs/SiO 2 NCs a) be o e lase ea men , b) ea ed a 4.0 kW cm
− 2 , and c) ea ed a 7.0 kW cm
− 2 . Righ ) T ansmi -
ance spec a measu ed be o e and a e i adia ion a di e en lase powe s, as labeled o he incoming elec ic i eld o ien ed along d) he x -di ec ion
and e) he y -di ec ion. The colo code is he same in bo h g aphs. Spec a o he o iginal sample and hose ea ed a 4.0 kW cm
− 2 and 7.0 kW cm
− 2
ha e been highligh ed o acili a e he compa ison wi h he co esponding SEM images (see also Figu e S3, Suppo ing In o ma ion). ) E olu ion o
he spec al posi ion o he ans e se plasmon band as a unc ion o lase ou pu powe .
COMMUNICATION
Expe imen al Sec ion
SiO 2 hin i lms wi h il ed nanocolumns we e g own by GLAD by placing
he subs a es a a glancing angle o 70 ° wi h espec o he e apo a o .
Ag was deposi ed a oom empe a u e on o he SiO
2 NCs by DC
spu e ing a 400 V in an A (2 mba ) a mosphe e. Lase pos - ea men
was pe o med a oom empe a u e wi h a 20 W diode-pumped Nd:YAG
(Powe line E, Ro i n-Baasel Inc.) unpola ized lase emi ing a 1064 nm
wi h an ≈ 100 ns pulse wid h and a 20 kHz epe i ion a e.
[ 35 ] The
samples we e scanned wi h an ≈ 0.5 mm spo a 1 m s
− 1 speed. AgNPs/
SiO
2 NCs nanos uc u e was cha ac e ized by SEM wi h a Hi achi
S5200 mic oscope. Op ical ansmi ance o he i lms was measu ed
wi h a Ca y 100 spec opho ome e a no mal incidence and in he
300–900 nm ange wi h a 1-nm monoch oma o s ep. Noncon ac
AFM cha ac e iza ion was ca ied ou wi h a Ce an es AFM sys em
om NANOTEC and he 2D-FFT and u he analyses o he images
pe o med wi h he WSXM so wa e.
[ 36 ]
Suppo ing In o ma ion
Suppo ing In o ma ion is a ailable om he Wiley Online Lib a y o
om he au ho .
Acknowledgemen s
Au ho s hank he P ojec s FUNCOAT CONSOLIDER-INGENIO
CDS2008 – 0023, MAT2007 – 65764, MAT2010-18447, MAT2010-21228,
CEN2007 – 2014, P09-TEP-5283, Domingo Ma inez Founda ion, and
Juan de la Cie a G an Nº JCI-2009 – 05098 o J.T.
depending on lase powe is seen unde y -pola ized illumina-
ion. Op ical pa e ning wi h la e al esolu ion a he scale o
he lase w i ing is a clea applica ion o his p ocedu e. An
illus a ion a he mac oscale is shown in Figu e 4c whe e he
le e s “CSIC” we e o med by iling a se ies o squa e pla es
subjec ed o lase i adia ion wi h di e en powe s. Beyond
his pola iza ion-dependen colo ing o pa e ns a he
mac o- and mic oscales, he ma e ials and p ocedu e de el-
oped he e migh be used o op ical enc yp ion by w i ing
cha ac e s ha could only be ead wi h a gi en pola iza ion
o he incoming ligh . Un o una ely he le e s “CSIC” can be
dis inguished ega dless o he pola iza ion (Figu e 4c ). This
beha io esul s om he weak dependence on lase powe
o he x -pola ized ansmi ance o he i lms (Figu e 3d ).
Howe e , in iew o da a enc yp ion applica ions, one can
no e in Figu e 3d ,e ha he i lm ansmi ance a 490 nm
is independen (s ongly dependen ) on lase powe when
measu ed unde x -pola iza ion ( y -pola iza ion). Success ul
dich oic enc yp ion migh hus be ob ained by deposi ing he
AgNPs/SiO
2 NCs i lms on o a pho onic s uc u e ac ing as a
490-nm bandpass i l e . Main ad an ages o his p ocedu e
wi h ega d o o he li hog aphic o mo e con en ional lase -
based w i ing p ocedu es ely on i s easy implemen a ion
(i.e., elec on, ion, o esis li hog aphy a e clea ly mo e com-
plica ed me hods) and he ac ha enc yp ion and encoding
e i ciency will no only depend on he size o he w i en pa -
e ns bu also on he need o a gi en pola ized ligh o ead
he in o ma ion.
In his a icle, we ha e demons a ed ha i adia ion o Ag
NPs deposi ed on o bundled SiO
2 nanocolumns wi h a com-
me cial w i ing ns-lase enables a p ecise con ol o hei
op ical dich oism, which is a ibu ed o he o ma ion o long
ange o de ed sil e nanos ipes o med h ough 1D me al pe -
cola ion and pa ial dewe ing along and pe pendicula o he
bundling di ec ion o he SiO
2 NCs, espec i ely. We o esee
ha hese esul s a e a i s s ep owa ds he use o AgNPs/
SiO
2 NCs o op ical nanopa e ning, enc yp ion, and da a
s o age applica ions.
Figu e 4 . a) Pic u es o a ≈ 3.5 × 2 cm
2 glass slide coa ed wi h AgNPs/SiO
2 NCs and subsequen ly ea ed by ns-lase . Pa e ning on he glass slide was
achie ed applying di e en lase po we in each squa e zone. The ela ion be ween he appa en colo o he sample and he pe cen age o he lase
powe is indica ed in he ske ch o he pic u e acquisi ion p ocedu e (b). Unde x -pola iza ion, he pixel colo emains monoch ome, whe eas a ull-colo
ansi ion as a unc ion o he lase powe is seen unde y -pola iza ion. c) Example o dich oic cha ac e enc yp ion on glass slide coa ed wi h AgNPs/
SiO
2 NCs. The size o he pixels is ≈ 0.20 cm, bu i can be dec eased o he la e al esolu ion o he lase ( ≈ 100 μ m).
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