Second harmonic generation as an
optical tool in the characterization of
artificial chirality in self-organized
plasmonic nanostructures.
Alessandro Belardini, M. Centini, G. Leahu, A.
Benedetti, M. Bertolotti, E. Fazio, Concita Sibilia
Dipartimento di Scienze di Base e Applicate per l’Ingegneria
Sapienza Università di Roma
Roma, Italy
Joseph W. Haus, Andrew Sarangan
Department of Electrical and Computer Engineering
University of Dayton
Dayton,Ohio,USA
[email protected]
[email protected]
http://www.sbai.uniroma1.it/laboratori/nonlinear_photonics_lab/
Summary
•
Optical nonlinearity (SHG) as a detector of symmetry breaking (chirality)
•
Chirality in plasmonic meta-surfaces
•
Symmetry breaking in self assembled metallic structures
•
Nonlinear response of tilted gold nanowires
Pagina 2
Summary
•
Optical nonlinearity (SHG) as a detector of symmetry breaking (chirality)
•
Chirality in plasmonic meta-surfaces
•
Symmetry breaking in self assembled metallic structures
•
Nonlinear response of tilted gold nanowires
Pagina 3
Chirality - generalities
Chirality (or handedness) is the lack
of mirror symmetry
so
an object is chiral if it cannot be
superimposed on its mirror images
In nature a huge number of
objects are chiral, from shells
to biomolecules such as DNA,
sugar , proteins
Pagina 4
Also light presents chirality…
Linear polarisation apart, there is also the circular
polarisation state of the light:
The circular polarisation is chiral: can be left handed or
right handed
Circular dichroism: a particular
interaction between chiral light with
chiral molecules
Circular polarised light (chiral
light) can be used in the
investigation of chiral materials:
An example is the circular
dichroism, where chiral materials
absorb in different amount the
right handed and left handed
circular polarised light
LH
RH
Chiral material
Chiral material
Pagina 5
SHG as sensor for symmetry breaking


)
Pi   0 el(1,)ij E j  el( 2,ijk
E j Ek  el(3,)ijkl E j Ek El  .....
(2) terms (responsible for SHG) are identically zero in centro-symmetric media
So SHG is a clear sign for symmetry breaking in a structure under investigation
Metals usually present inversion symmetry  SHG from electric dipole term is zero
But still are present surface electric quadrupolar term (and magnetic dipolar term)
Adler PR (1964)
Jhe PR (1965)
Lee PR (1968)
Rudnick & Stern PRB (1971)
Sipe & Stegeman PRB (1980)
Kauranen PRL (2007)





P2  E (  E )  E  H 
In these cases SHG is very sensitive to the symmetry breaking
in the geometrical arrangement of metallic nanoparticles
Pagina 6
6
Second harmonic generation (SHG) as chiral detector:
SHG is sensitive to symmetry breaking
Nonlinear susceptibility expansion
 E    1   2  E   3 EE  ...


PE    0  1   2  E   3 E 2  ... E
Peff (2 )  PD (2 ) 
1

k xM (2 )
(2) eee
(2) eem
PD (2 )   ijk
E j ( ) Ek ( )  ijk
E j ( ) Bk ( )
j ,k
Electric dipole source
M (2 )   
(2) mee
ijk
E j ( ) Ek ( )
Magnetic dipole source
,mee
, eme
j ,k
Nonlinear magnetization
Pagina 7
The optical response: just a matter of symmetry
Second harmonic generation
is very sensitive to symmetry
breaking
Verbiest, Taylor &
Francis (2009)
V. K. Valev , J.J. Baumberg , C. Sibilia , and
T. Verbiest, Adv. Mat. 18, 2517 (2013).
FIRST HARMONIC
SECOND HARMONIC

Left handed and
Right handed circ.
pol. light
S and P pol. light
SHG-CD can be measured in transmission or in reflection
Pagina 8
Summary
•
Optical nonlinearity (SHG) as a detector of symmetry breaking (chirality)
•
Chirality in plasmonic meta-surfaces
•
Symmetry breaking in self assembled metallic structures
•
Nonlinear response of tilted gold nanowires
Pagina 9
3D and 2D Chirality
The most evident
symmtetry breaking is
the lack of myrror
symmetry - chirality -
chirality is very important in nature and scientists try to replicate it artificially at
the nanoscale
In the last years a lot of effort was devoted to the study on chiral plasmonic
metamaterials
Both 3D- and 2D-chiral metamaterials were studied
3D chiral metamaterial
2D chiral metamaterial
Gansel et al., Science 325, 1513-1515 (2009)
Valev, V. K., et al., Adv. Mater., 26: 4074–4081 (2014)
Pagina 10
10
Pseudo Chirality (or Extrinsic Chirality)
E. Plum, PRL 102,113902 (2009)
no-chirality
Pseudo chirality:
A non-chiral element
breaks the symmetry
when is tilted with
respect the incidence
light.
The element thus
assumes a chiral
behavior
L
R
S.N. Volkov, PRA 79, 043819 (2009)
11
Pagina 11
Summary
•
Optical nonlinearity (SHG) as a detector of symmetry breaking (chirality)
•
Chirality in plasmonic meta-surfaces
•
Symmetry breaking in self assembled metallic structures
•
Nonlinear response of tilted gold nanowires
Pagina 12
Test sample fabrication – Sample A
Low cost self assembling procedure
Step 1
Glass
Ar+ ion flux
500 nm
Step 2
T (%)
Gold flux
Step 3
Sample A
Gold 70 nm
160 nm
Wavelength (nm)
13
Reference sample fabrication – Sample B
Low cost self assembling procedure
Glass
Step 1
Gold deposition
500 nm
Step 2
T (%)
Ar+ ion flux
Step 3
Sample B
Gold 20 nm
160 nm
Wavelength (nm)
14
SHG measurements – P pol
The measurements were performed as a function of the incidence angle 
FIRST HARMONIC
l= 800nm
(130fs pulses)
FIRST HARMONIC
l= 400nm
Second harmonic generation - circular
dichroism (SHG-CD)

Right-handed (or left-handed) circular
polarized light
P component
15
SHG measurements on sample B – P pol
f

n
f’
n’ ’
k
k’
2
SHG-CD
1
0
b09 400nm delta H up
b09 400nm delta H down
-1
-2
-40
-20
0
40
20
 (deg)
16
SHG measurements on sample A – P pol
f’
f

n n’ ’
A.Belardini, PRL 107, 257401 (2011) k
k’
2
SHG-CD
1
0
-1
-2
-40
b08 400nm delta H down
b08 400nm delta H up
-20
0
 (deg)
20
40
17
SHG measurements on sample A – P pol
f
f’

n
k
n’ ’
k’
SHG-CD
2
1
0
b08 400nm H delta sx
b08 400nm delta H dx
-1
 (deg)
-2
-40
-20
0
20
40
18
Self assembled straight and curved nanowires
f

n
f’
n’ ’
k
k’
f’
f

n n’ ’
k’
k
f
f’

A. Belardini et al., Phys. Rev. Lett. 107, 257401 (2011).
k
n
n’ ’
k’
Pagina 19
Summary
•
Optical nonlinearity (SHG) as a detector of symmetry breaking (chirality)
•
Chirality in plasmonic meta-surfaces
•
Symmetry breaking in self assembled metallic structures
•
Nonlinear response of tilted gold nanowires
Pagina 20
What appens with self assembled tilted nanowires?
In collaboration with
Joseph W. Haus, Andrew Sarangan
Univ. Oahio, USA
100 K
300 K
f

n
f’
n’ ’
k’
k

n
n’ ’
f
f’
k’
k
Pagina 21
Morphological characterisation - 1
100 K sample
NWs homogeneously
distributed on 2 cm x 2 cm
surface
Pagina 22
Morphological characterisation - 2
100 K sample
60° tilted image
NWs cluster dimension 100nm
NWs
single
fiber
section
20nm
60° tilted image
NWs lenght 400nm
Pagina 23
Morphological characterisation - 3
300 K sample
NWs homogeneously
distributed on 2cm x
2cm surface
NWs length about 380nm
Section about 40nm
Pagina 24
SHG (V) , reflection at -45°
0.8
au300k  l4 p
au300k  l4 p
0.6
0.4
f

0.2
n
f’
n’ ’
k’
k
0
-100
-50
0
50
100
l/4 orientation (deg), 0°=p, -45°=CR, +45°=CL
In collaboration with V. K. Valev, Univ. of Bath, UK
Pagina 25
SHG (V) , reflection at -45°
0.10
au300k  l4 p
au300k  l4 p
0.08
0.06

n
n’ ’
0.04
f
f’
k’
k
0.02
0
-100
-50
0
50
100
l/4 orientation (deg), 0°=p, -45°=CR, +45°=CL
In collaboration with V. K. Valev, Univ. of Bath, UK
Pagina 26
2
au300k SHG-CD  p
au300k SHG-CD  p
SHG-CD pol p
1
0

f
-1
-2
n
n’ ’
f’
k’
k
-50
-30
incidence angle (deg)
Pagina 27
100k
2
shgcd 100k  pol.p
shgcd 100k  pol.p
1
0
-1
-2
20/10/2015
-65
-55
-45
Titolo Presentazione
-35
-25
-15
Pagina 28
2
au300k SHG-CD  p
au300k SHG-CD  p
SHG-CD pol p
1
0
f

n
f’
n’ ’
-1
k’
k
-2
-50
-30
incidence angle (deg)
Pagina 29
Conclusions
•We show how SHG measurements can be sensitive on symmetries of
metallic nanopatterned surfaces
• we show these effects on self –assembled structures produced by
low cost and large area procedure
• SHG signal through the nonlinear circular dichroism is sensitive also
to extrinsic chirality signature
(good candidates as substrate for nonlinear molecular spectroscopy ?)
Acknowledgements
This work was partially supported by the MARINE project of the Italian
Ministry of the Defence
Pagina 30
Chirality: from tradition to innovation
Thank you for your attention !
Pagina 31
Mirror
Mirror
au300k  l2 p
au300k  l2 p
au300k  l2 p
au300k  l2 p 0.25
AU300K
Au flat l/2 p 2
Au flat l/2 s 2
0.20
SHG (V), reflection
SHG (V), reflection at -45°
0.6
incidence angle -45°
0.4
0.2
0.15
0.10
0.05
0
-200
-100
0
100
pump pol (deg) , 0°=p
200
0
-200
-100
0
100
200
input  pol. angle (deg) , 0°=p-pol. , 90°=s-pol.
Pagina 34
Reflectance spectra - 1
Mainly
horizontal
wires
Reflection at -45° with respect Al
1.00
S pol or
P pol
0.75
Mainly
vertical
wires
S pol or
P pol
au nws 300k  v
au nws 300k  v
au nws 300k  v
au nws 300k  pol. v
au nws 300k  h
au nws 300k  h
au nws 300k  h
au nws 300k  pol. h
0.50
0.25
0
0
250
500
750
1000
wavelength (nm)
Pagina 35