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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: - PowerPoint PPT Presentation
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Chemical and spatial resolution with a SNOM
introduction to near field optics aperture SNOMSNOM tipsapertureless SNOM applications in solid state phisicssome examples in biology
Snell law 1Total reflection in a prismClassically Snell law:
No light is classically trasmitted in a medium of lower refractive index when a critical angle is reached:
n1
2211 sinsin nn
n2
n1
n22
1
c2
1sin nn
c
Snell law 2E
k
yx
z
n2 = n
n1 = 1
III E sin,0,cosE
IIT nnE sin,0,sin1 22E
IIT nniE sin,0,1sin 22 E
1sin,0,sin 22 IIT ninc k
• The transmitted polarization is along z
• The wave vector along z is immaginary: exponential decay• The wave vector along x is higher than /c• Notice that high k means small : higher spatial resolution
TM = linear polarization in the plane Oxz
Angular spectrum• Decomposition of the field in plane waves at z constant
dudvezvuFzyx vyuxi ,,,,E
ê
02
2
EEc
• The field should satisfy the Helmholtz equation
• Fourier component can be written as:
iwziwz evuBevuAzvuF ),(),(,,
dudvevuAyx vyuxi ,0,,E
• The evolution along z can be deduced by the field at z=0
2222 cvuw
Angular spectrum 2ê
dudvevuFzyx wzvyuxi 0,,,,E
Where from Helmholtz equation:
2
2222
cwvu
2
222
cvu
w is
imaginary!
This expression of the electric field is general: no approximations have been used until nowu and v are spatial frequenciesw introduces a decaying exponential in the
expression of the Field vs z
Angular spectrum 3ê
duevuFzyx wzvyuxi 0,,,,E
Example 1: 1D periodic grating
y
xz
We measure the field intensity far away form O along the z axis
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 etc.
Those values represent the nth diffraction order of the grating
If d< 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
22 1,0,1 dcd
22 2,0,2 dcd
Angular spectrum 4ê
dxdyevuEyxF vyuxi
a
0,,0,,
Example 2: propagation through a small squared aperture
y
xz
2sin
2sin
1)0,,(
vaua
uvvuF
F 0 slowly at high spatial frequency: sharp edges.F has a maximum for u,v (0, /a) but, when a<
wiwcvu 22222222 48
And the part or all the light that maximizes F cannot propagateAgain, The spatial information is retained only in the near field
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
• The light is collected near the sample by a tapered optical fiber with a subwavelenght aperture
• Low light throughput• Resolution limited to /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
Illumination
Sample surface
Near field detection
• 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).
xyz piezo
Feedback
Impedance detector
ElectrodesPiezo actuator
Aperture SNOM
• Operational modes
Aperture SNOM
Illumination CollectionIlluminationcollection
Reflectioncollection
Transmissionillumination
Transmission collection
• Typical set up
Aperture SNOM
LaserPol. controlfeedbackcontrol
xyzScanner
Detector
Topographic image
nf optical imageInverted
optical microscope
Optical fiber
Monochromator
• SNOM tips
Aperture SNOM
pulling
breaking
heatingHeating and pulling
method
Turner etching method
Hydrofluoric acidChemical etching
Aluminum vapor
Glass
Al coating
Optical fiber
• SNOM tips• Calculation of the
distribution of electric field as a function of the tip geometry
Aperture SNOM
Source: InAs QdotPoint like source /40 below the surface
• SNOM tips - pulling
Aperture SNOM
CoreCore Light propagation
CladdingCladding
Metal Metal coatingcoating
• SNOM tips - etching
Aperture SNOM
CoreCore Light propagation
CladdingCladding
Metal Metal coatingcoating
Holes are dug by various methods:The best results are obtained by FIB
• SNOM tips - polymerization
Aperture SNOM
Core Light propagation
Cladding
Metal coating
• Photopolimerization
90% wt Pentaerythritol triacrylate (monomer)
8% wt methyldiethanolamine (cosynergist)
2% wt eosin (dye)
High sensitivity to the argon laser light (514 nm)
• SNOM “japanese” etching
Aperture SNOM
Three different etching stepsSolution NH4F:HF:H2O
X : 1 : 1X=10 angle 20o
X=2.7 angle 50o
The selectivity between core and cladding comes from different quartz doping with Ge
• Application1: blood cell with malaria disease
Aperture SNOM
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 fluorophorHere PfHRP1 is marked red
• Application1: blood cell with malaria disease
Aperture SNOM
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
• Application1: blood cell with malaria disease
Aperture SNOM
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
• Application2: single molecule detection and FRET mechanism
Aperture SNOM
• Application2: single molecule detection and FRET mechanism
Aperture SNOM
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
• Application3: optical quantum corral
Aperture SNOM
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 ITOModulators 100nm100nm50nm 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!
• Application3: optical quantum corral
Aperture SNOM
Light Polarization control
Elliptical mirros that selects only the near field radiation (propagating radiation is not allowed in the “forbidden light region with >c
The signal is 0 only closo to the sample
• Application3: optical quantum corral
Aperture SNOM
Teorical optical LDOS in x, y and z direction
• Application3: optical quantum corral
Aperture SNOM
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 zx,y) polarization
• Application4: excitonic wave function of a quantum dot
Aperture SNOM
Low temperature operationIllumination collection mode
• Application4: excitonic wave function of a quantum dot
Aperture SNOM
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
• Application4: excitonic wave function of a quantum dot
Aperture SNOM
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
• Scattering SNOM
Unlimited resolutionChemical sensitive
Physical mechanism: TIP-SAMPLE INTERACTIONStrong wavelenght dependenceStrong dependance on the tip physical properties
Apertureless SNOM
s-SNOMWe model the tip as a metallic sphereAssuming that >>a and using a quasi-electrostatic
theory
)2()1(4 3
tta
Ep
Tip polarization far away from the sample in an external electric field E
)1()1(
'
ss
pp
Dipole induced on the sample surface
33 22
'
r
p
r
pEind
Dipole induced on the sample surface
s-SNOM
316 r
pEEEp ind
Eza
p
3)(161
In a first order iterative process the dipole induced on the tip becomes
3)(161
)1(
zaeff
The total dipole (tip + sample) is that is having an effective polarizability
Eza
p
3)(161
1
In the case of field parallel to the surface the induced dipole is opposite to the field and the effective polarizability is
3)(321
)1(
zaeff
In a metal 1 and eff is
nearly 0
s-SNOM
3
3
3
)(16
41
)(4
za
a
a
tip
tiptipeff
)2()1(
4 3
tttip
tipa
3322
3
33141
1
8
azazaz
aeff
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
s-SNOM
Im;6
24
kCk
C abssca
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 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-SNOMThe 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
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 > 200nm ther is a huge signal, arising from cantilever scattering and independent of tip-sample interactionOn 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.
=633nm
s-SNOMLateral 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.BUTTopographic effects are not excluded:It is true chemical contrast?(This is a big issue in SNOM and the major source of SNOM artifacts)
=633nm
s-SNOMTrue 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 is 10mSo the resolution approaches /200
800nm
Near field detection
• Tip-enhanced SNOM
Unlimited resolutionPhysical mechanism: FIELD ENHANCEMENT
Suitable only for particular light-matter interaction process (e.g. Raman scattering, second harmonic generation, etcWhere the light detected has a different wavelenght from the
excitation light.)
Strong analogy to SERS and SPR
Apertureless SNOM
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
Here the excitation is localized, while the light scattered by the nanotube is then collected in far field through the optical microscope.
a) Confocal microscopeb) SNOM raman image taken at the G’ band wavelenght
With metal tipwithout
te-SNOM
• Raman scattering from a single CNT
Localization of radial breathin mode raman scattering along the nanotubea and b arc-discharge growth b and d CVD growth
Structural defects along the structure can be identified by raman snom experiment
te-SNOMConfocal vs SNOM microscopy
AND SNOM WINS!!!!!!!!AND SNOM WINS!!!!!!!!
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