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Chemical and spatial resolution
with a SNOM
introduction to near field optics
aperture SNOM
SNOM tips
apertureless SNOM
applications in solid state phisics
some examples in biology
Snell law 1
Total reflection in a prism
Classically Snell law:
n1 sin  1  n 2 sin  2
sin  c 
n2
2
1
n1
n1
n2
No light is classically trasmitted in a medium
of lower refractive index
when a critical angle is reached:
c
n2
n1
Snell law 2
E I  E    cos  I , 0 , sin  I

E
TM = linear polarization in the plane Oxz
 1  n sin
 E  i n sin 
ET  E 
ET
2
2
2
2
I
n2 = n

 1 , 0 , n sin  
z
 I , 0 , n sin  I
k
x
y
I
• The transmitted polarization is along z
kT  
c

 n sin  I , 0 , i n sin  I  1
2
2

• The wave vector along z is immaginary: exponential decay
• The wave vector along x is higher than /c
• Notice that high k means small l: higher spatial resolution
n1 = 1
ê
Angular spectrum
• Decomposition of the field in plane waves at z constant
E x, y, z  

F u , v , z   e
i  ux  vy  
dudv
• The field should satisfy the Helmholtz equation
E 

2
c
2
E0
• Fourier component can be written as:
F u , v , z   A ( u , v ) e
E  x , y ,0  

 iwz
 B (u , v ) e
A u , v   e
i  ux  vy  
 iwz
w
u  v 
2
2
2
c
dudv
• The evolution along z can be deduced by the field at z=0
2
ê
Angular spectrum 2
E x, y, z  

F u , v , 0   e
i  ux  vy  wz  
dudv
Where from Helmholtz equation:
u v w 
2
2
2

2
c
2
u v 
2
2

2
c
2
w is imaginary!
This expression of the electric field is general:
no approximations have been used until now
u and v are spatial frequencies
w introduces a decaying exponential in the expression of
the Field vs z
ê
Angular spectrum 3
Example 1: 1D periodic grating
y
We measure the field intensity far away
form O along the z axis
x
E x, y, z  

F u , v , 0   e
i  ux  vy  wz  
du
z
In y direction there is no modulation so the only spatial frequency allowed is v=0; in
x direction u assumes discrete values n/d n=1,2,…n.
The only wave vector allowed are
 1 d , 0 ,

 c 2  1 d 2 
 2 d , 0 ,
 
 c 2  2 d 2 

etc.
Those values represent the nth diffraction order of the grating
If d<l w becomes imaginary and the only propagating wave vector is (0,0,0) and the
grating is no longer diffracted.
The spatial information is retained only in the near field
ê
Angular spectrum 4
Example 2:
propagation through a small squared aperture
y
x


F (u , v ,0 ) 
 ua 
 va 
sin 
sin



uv
 2 
 2 
z
a
E u , v , 0   e
  i  ux  vy
F  x , y ,0  
dxdy
1
F  0 slowly at high spatial frequency: sharp edges.
F has a maximum for u,v  (0, p/a) but, when a<l/2
u  v  8p
2
2
2
l  4p
2
2
l 
2
2
c  wiw
2
And the part or all the light that maximizes F cannot propagate
Again, The spatial information is retained only in the near field
Near field detection
How to detect the near filed if it not propagating?
Theorem of reciprocity
[Time reversibility of the Maxwell equation]
If a plane wave is diffracted into an evanescent wave by a subwavelenght
scatterer,
A subwavelenght scatterer should be diffracted into a propagating wave by the
same object
Near field detection
Aperture SNOM
Illumination
Sample
surface
• The light is collected near the sample by a tapered optical fiber with a
subwavelenght aperture
• Low light throughput
• Resolution limited to l/10
Physical mechanism SPATIAL FILTERING
•
•
•
•
True spectroscopic information (including PL, EL, etc)
Dependence only on the tip geometrical properties
No dependance on the tip physical properties
No wavelenght dependence
Aperture SNOM
•
•
•
•
The near field decays
exponentially with distance
The tip should be kept at a
controllede distance from the
sample surface
Feed back mechanism: shear
force (similar to AFM tapping
mode)
Feedback detection: quartz
oscillator (STM current is not
suitable for biological samples;
optical methods are disturbing
the optical response).
Piezo actuator
Impedance
detector
Feedback
xyz piezo
Electrodes
Aperture SNOM
• Operational modes
Illumination
Collection
Illumination
collection
Reflection
collection
Transmission Transmission
collection
illumination
Aperture SNOM
• Typical set up
Optical
fiber
Laser
Pol. control
Topographic
image
feedback
control
xyz
Scanner
Inverted
optical
microscope
nf optical
image
Detector
Monochromator
Aperture SNOM
• SNOM tips
Turner etching
method
Al
coating
Hydrofluoric acid
Chemical etching
Glass
Optical fiber
heating
Heating and pulling
method
Aluminum vapor
pulling
breaking
Aperture SNOM
• SNOM tips
• Calculation of the
distribution of electric field
as a function of the tip
geometry
Source: InAs Qdot
Point like source l/40 below the surface
Aperture SNOM
• SNOM tips - pulling
Metal
coating
Core
Cladding
Light propagation
Aperture SNOM
• SNOM tips - etching
Metal
coating
Core
Light propagation
Cladding
Holes are dug by
various methods:
The best results are
obtained by FIB
Aperture SNOM
• SNOM tips - polymerization
• Photopolimerization
90% wt Pentaerythritol triacrylate
(monomer)
Metal
coating
Core
Cladding
8% wt methyldiethanolamine
(cosynergist)
Light propagation
2% wt eosin (dye)
High sensitivity to the argon laser
light (514 nm)
Aperture SNOM
• SNOM “japanese” etching
Three different etching steps
Solution
NH4F:HF:H2O
X :1 :1
X=10 angle 20o
X=2.7 angle 50o
The selectivity between core and
cladding comes from different
quartz doping with Ge
Aperture SNOM
• Application1: blood cell with malaria disease
Study of blood cells infected by malaria’s plasmodium falciparium.(PF)
Pf expresses several proteins in particular PfHRP1 and MESA that arefixed on the cell
membrane.
Proteins on cell membrane are colored with specific antibody marked with a red and a green
fluorophor
Here PfHRP1 is marked red
Aperture SNOM
• Application1: blood cell with malaria disease
Comparison between SNOM and
confocal microscope images in the
sdame blood cell:
SNOM is sensitive to cell surface
CM images a plane section at the
focal plane
Cellular structure is resolved on the
SNOM image but not in CF image
Aperture SNOM
• Application1: blood cell with malaria
disease
Colocalization of host membrane and PF proteins
a)
Control experiment:
PfHRP1 is bound with antibodies marked
either with green or red. The perfect overlap
excludes any instrumental effect
b)
Colocalization of host protein (green) and
MESA protein (red)
good colocalization Mesa and host proteins
interact oin the cell surface
c)
Colocalization of host protein (green) and
PfHRP1 protein (red)
No interaction at the cell membrane
NB the three ijmages refers to different blood cells
groups
Aperture SNOM
• Application2: single molecule detection and FRET mechanism
Aperture SNOM
• Application2: single molecule detection and FRET mechanism
Green and red spot are due to not hybridized ssDNA (red can also arise from complete FRET
effect)
Yellow spot arise from hybridized dsDNA with competing green and red emission
Aperture SNOM
• Application3: optical quantum corral
The experiment:
Testing the subwavelkenght modulation
induced on the local density of states of the
optical modes by the fabrication of nanometric
opticla corrals
Substrate ITO
Modulators 100nm100nm50nm gold
particles deposited by e-beam lithography
To test the real LDOS the tip should act as a perfetct dipole at a nanometric distance
from the surface.
Real tips always pertirb the LDOS and what is measured is the combined LDOS of the
sample and the tip!
Aperture SNOM
• Application3: optical quantum corral
Light Polarization control
Elliptical mirros that selects only the near
field radiation
(propagating radiation is not allowed in the
“forbidden light region with q>qc
The signal is 0 only closo to the sample
Aperture SNOM
• Application3: optical quantum corral
Teorical optical LDOS in x, y and z direction
Aperture SNOM
• Application3: optical quantum corral
Near field results in
trasmission.
Best results obtained with a
gold coated tip without
apertures
(the tip
At the tip the polarization is
tilted along z
The Snom data are fitted
with a 1:4 mixing of the
zx,y) polarization
Aperture SNOM
• Application4: excitonic wave function of a quantum dot
Low temperature operation
Illumination collection mode
Aperture SNOM
• Application4: excitonic wave function of a quantum dot
Different emission spectra at increasing power (LEFT) and on different dots (Right)
The far field spectra average the different contribution and the structure is lost
Aperture SNOM
• Application4:
excitonic wave
function of a
quantum dot
Excitonic wave function
mapping of different dots
showing that bi-exciton is
more confined
A weak alignment along
(1-10) crystallographic
direction can be noticed
Near field detection
Apertureless SNOM
•
Scattering SNOM
Unlimited resolution
Chemical sensitive
Physical mechanism: TIP-SAMPLE INTERACTION
Strong wavelenght dependence
Strong dependance on the tip physical properties
s-SNOM
We model the tip as a metallic sphere
Assuming that l >>a and using a quasi-electrostatic theory
p  E
p'   p
  4p a ( t  1) ( t  2 )
3
  (  s  1) (  s  1)
Tip polarization far away from Dipole induced
the sample in an external
on the sample
electric field E
surface
E ind 
p'
2p r
3

p
2p r
Dipole induced
on the sample
surface
3
s-SNOM
In a first order iterative process the dipole induced on the tip becomes
p    E  E ind
p 

E 

3
16 p r 

The total dipole (tip + sample) is

 1   

p 
3
1


16
p
(
a

z
)


 E



p   
3
1


16
p
(
a

z
)


 E

that is having an effective polarizability
 eff 
 (1   )
1   16 p ( a  z )
3
In the case of field parallel to the surface the induced dipole is opposite to the
field and the effective polarizability is


eff

 (1   )
1   32 p ( a  z )
3
In a metal   1 and eff is nearly 0
s-SNOM
  4p a  tip
3
 tip  (  t  1) (  t  2 )
4p a (  tip   tip  )
8p a
3
 eff 
4p a  tip 
3
1
16 p ( a  z )
3
 eff 
1

3
1
4  1  3z a  3 z
2
It is evident that eff is increased by the interaction only for z<<a,
In other words when the tip very close to the surface
a z
2
3
a
3

s-SNOM
The measurable quantities are the scattered and the absorbed light that is proportional
to the cross section.
Applying Mie theory of light scattering
C sca 
k
4
6p
 ; C abs  k Im  
2
Scatteing and absorption
cross section for a gold
sphere on gold and silicon
substrates for normal and
parallel polarization
If I’m able to scan a gold sphere close to the sample surface I can observe a contrast
in scattered intensity and, therefore, a can obtain a chemical map of the surface
s-SNOM
• Typical experimental set-up
• The main problem is that the light scattered by the tip that carries the
information on tip-sample interaction is overwhelmed by background light
by several orders of magnitude
s-SNOM
The dependence of (z) is not linear.
Oscillating the tip in a non contact mode
(harmonic) fashion, a non-harmonic
response is obtained.
The non-harmonicity increases with the
oscillation amplitude.
By collecting the nth armonic signal
(n>3) the near field signal can be
obtained
•It works!
s-SNOM
l=633nm
On the left the 1st harmonic signal is collected at fixed amplitude while changing the tipsample distance. Even for tip-sample distance > 200nm ther is a huge signal, arising from
cantilever scattering and independent of tip-sample interaction
On the right the 2nd harmonic is collected, the background is suppressed and the near field
signal is restricted to a 20nm distance from the surface.
s-SNOM
Lateral resolution and chemical contrast
l=633nm
Pattern of Au on silicon obtained by evaporation through
a polystyrene lattice.
The chemical contrast arise from differences in the
dielectric constant value at 633nm.
BUT
Topographic effects are not excluded:
It is true chemical contrast?
(This is a big issue in SNOM and the major source of
SNOM artifacts)
s-SNOM
True chemical contrast
800nm
Silicon surface with a laterally modulated p-n doping
structure.
The topogarphic contrast is just 0.1nm: the surface can
be told to be flat, so the contrast is purely
otpical/chemical
The optical-spatial resolution is about 50 nm l is
10mm
So the resolution approaches l/200
Near field detection
Apertureless SNOM
•
Tip-enhanced SNOM
Unlimited resolution
Physical mechanism: FIELD ENHANCEMENT
Suitable only for particular light-matter interaction process
(e.g. Raman scattering, second harmonic generation, etc
Where the light detected has a different wavelenght from the excitation light.)
Strong analogy to SERS and SPR
Near field detection
•Field enhancement on a tip apex
•Antenna effect
te-SNOM
• Set-up for tip-enhanced SNOM
te-SNOM
•
Raman scattering from a single CNT
With metal tip
without
Here the excitation is localized, while the light scattered by the nanotube
is then collected in far field through the optical microscope.
a) Confocal microscope
b) SNOM raman image taken at the G’ band wavelenght
te-SNOM
• Raman scattering from a single CNT
Localization of radial breathin mode
raman scattering along the nanotube
a and b arc-discharge growth
b and d CVD growth
Structural defects along the structure can
be identified by raman snom experiment
te-SNOM
Confocal vs SNOM microscopy
+
AND SNOM WINS!!!!!!!!
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