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

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Chemical and spatial resolution with a
SNOMintroduction to near field optics
aperture SNOMSNOM tipsapertureless SNOM
applications in solid state phisicssome
examples in biology
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Snell law 1

• Total reflection in a prism
• Classically Snell law
• No light is classically trasmitted in a medium of
  lower refractive index
• when a critical angle is reached

n2
n1
n2
n1
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Snell law 2
n2 n
TM linear polarization in the plane Oxz

• The transmitted polarization is along z

n1 1

• The wave vector along z is immaginary
  exponential decay
• The wave vector along x is higher than w/c
• Notice that high k means small l higher spatial
  resolution

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Angular spectrum
ê

• Decomposition of the field in plane waves at z
  constant

• The field should satisfy the Helmholtz equation

• Fourier component can be written as

• The evolution along z can be deduced by the field
  at z0

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Angular spectrum 2
ê
Where from Helmholtz equation
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
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Angular spectrum 3
ê
Example 1 1D periodic grating
y
We measure the field intensity far away form O
along the z axis
x
z
In y direction there is no modulation so the only
spatial frequency allowed is v0 in x direction
u assumes discrete values n/d n1,2,n. The only
wave vector allowed are etc. Those
values represent the nth diffraction order of the
grating If dltl 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
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Angular spectrum 4
ê
Example 2 propagation through a small squared
aperture
F ? 0 slowly at high spatial frequency sharp
edges. F has a maximum for u,v ? (0, p/a) but,
when altl/2
And the part or all the light that maximizes F
cannot propagate Again, The spatial information
is retained only in the near field
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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

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Illumination
Near field detection
Sample surface
Aperture SNOM

• 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

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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).

Electrodes
Piezo actuator
Impedance detector
Feedback
xyz piezo
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Aperture SNOM

• Operational modes

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Aperture SNOM

• Typical set up

Monochromator
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Aperture SNOM

• SNOM tips

Optical fiber
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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
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Aperture SNOM

• SNOM tips - pulling

Metal coating
Core
Cladding
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Aperture SNOM

• SNOM tips - etching

Metal coating
Core
Light propagation
Cladding
Holes are dug by various methods The best
results are obtained by FIB
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Aperture SNOM

• SNOM tips - polymerization

• Photopolimerization
• 90 wt Pentaerythritol triacrylate (monomer)
• 8 wt methyldiethanolamine (cosynergist)
• 2 wt eosin (dye)
• High sensitivity to the argon laser light (514
  nm)

Metal coating
Core
Light propagation
Cladding
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Aperture SNOM

• SNOM japanese etching

Three different etching steps Solution
NH4FHFH2O X 1 1 X10 angle
20o X2.7 angle 50o The selectivity between core
and cladding comes from different quartz doping
with Ge
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Aperture SNOM

• Application1 blood cell with malaria disease

Study of blood cells infected by malarias
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
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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
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Aperture SNOM

• Application1 blood cell with malaria disease

• Colocalization of host membrane and PF proteins
• Control experiment
• PfHRP1 is bound with antibodies marked either
  with green or red. The perfect overlap excludes
  any instrumental effect
• Colocalization of host protein (green) and MESA
  protein (red)
• good colocalization Mesa and host proteins
  interact oin the cell surface
• 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

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Aperture SNOM

• Application2 single molecule detection and FRET
  mechanism

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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
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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!
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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 qgtqc The signal is
?0 only closo to the sample
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Aperture SNOM

• Application3 optical quantum corral

Teorical optical LDOS in x, y and z direction
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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 14 mixing of the z?x,y) polarization
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Aperture SNOM

• Application4 excitonic wave function of a
  quantum dot

Low temperature operation Illumination collection
mode
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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
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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
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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

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s-SNOM

• We model the tip as a metallic sphere
• Assuming that l gtgta and using a
  quasi-electrostatic theory

Tip polarization far away from the sample in an
external electric field E
Dipole induced on the sample surface
Dipole induced on the sample surface
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s-SNOM
In a first order iterative process the dipole
induced on the tip becomes
The total dipole (tip sample) is that is
having an effective polarizability
In the case of field parallel to the surface the
induced dipole is opposite to the field and the
effective polarizability is
In a metal b ? 1 and aeff is nearly 0
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s-SNOM
It is evident that aeff is increased by the
interaction only for zltlta, In other words when
the tip very close to the surface
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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
Scatteing and absorption cross section for a gold
sphere on gold and silicon substrates for normal
and parallel polarization
If Im 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
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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

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s-SNOM
The dependence of a(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 (ngt3) the near field signal can be
obtained
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s-SNOM

• It works!
• On the left the 1st harmonic signal is collected
  at fixed amplitude while changing the tip-sample
  distance. Even for tip-sample distance gt 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.

l633nm
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s-SNOM

• Lateral resolution and chemical contrast
• 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)

l633nm
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s-SNOM

• True chemical contrast
• 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

800nm
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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

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Near field detection

• Field enhancement on a tip apex
• Antenna effect

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te-SNOM

• Set-up for tip-enhanced SNOM

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te-SNOM

• Raman scattering from a single CNT
• Here the excitation is localized, while the light
  scattered by the nanotube is then collected in
  far field through the optical microscope.
• Confocal microscope
• SNOM raman image taken at the G band wavelenght

With metal tip without
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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
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te-SNOM

• Confocal vs SNOM microscopy
• AND SNOM WINS!!!!!!!!