in which the enhancement is defined by the one point correlation: <|E(r)|>.

attractionlewdsterElectronics - Devices

Oct 18, 2013 (3 years and 10 months ago)

81 views

Copy right
©
Prof. Otto
Copy right ©Prof. Otto
M.Moskovits, L-L.Tay, J.Yang, T. Haslett
SERS and the single molecule
in Optical Properties of Nanostructured Random Media,V.M.Shalaev (Ed.)
Topics in Applied Physics 82,215-227 (2002)
„localization of the EM fields due to particle-particle interactions in an
aggregate results in SERS enhancement of the order of 1011 ; the
remaining enhancement arises from resonance Raman and the so-
called chemical effects.“
„Raman is an incoherent , phase-independent process in which the
enhancement is an incoherent, phase independent process*in which
the enhancement is defined by the one point correlation: <|E(r)|4>.
The very intense fields that exist at hot spots would dominate this
average [V.M.Shalaev, personal communication to M. Moskvits]“
*
This is only correct, when refering to different molecules at different spots.
Raman scattering by a single molecule is a coherent process, seechapter II.
Copy right
©
Prof. Otto
Copy right ©Prof. Otto
2
()Er
4
()Er
106
= 500nm
= 20m
2
()
E
r
4
()
E
r
Theory of giant Raman scattering from semicontinuous metal films,
F. Brouers et al, Phys.Rev B55(1997)13234. Silver at the percolation threshold
107
In this case, |E
(r
)|4
has
nothing to do with
SERS, see below
note, that at most
points |E
(r
)|2<100
Copy right
©
Prof. Otto
Copy right ©Prof. Otto
Near field optical spectroscopy of
individual surface-plasmon modes
in colloid clusters, V.A. Markel et
al., Phys.Rev.B 59 (1999) 10903
topogragraphic image,
height range ca.90nm
near field image, 25nm above surface.
Colour range of a factor of 4
(not 1000 like in foil 2)
silver-colloid
cluster
deposited out
of solution on
pyrex .
Near field images in false-colour, and simultaneously recorded topographic images in thermal colour, obtained by exciting a fibertip scanning 25nm above the surface.
The false colour range corresponds to approximately a factor of 4, with dark blue indicating the low intensity spots and yellow the high intensity regions. The average
optical intensity in the near-field images is approximately 100-fold higher than the signal from a clean pyrexcontrol surface. The height range in the topographic
images is approximately 90nm.
Copy right
©
Prof. Otto
Copy right ©Prof. Otto
Near field optical spectroscopy of individual surface-plasmon modes in colloid clusters. Scanning the Laser wavelength,
V.A. Markel et al., Phys.Rev.B 59(1999)10903
calculated spectra
a)10nmn above surface,
for two points
580nm apart
40nm
b) at a point near A (left), different heights
experimental spectra, 25 nm above
surface, for points A -E
1000cm-1
A given site is a hot spot only at special wavelengths!
20nm
10 nm
Copy right
©
Prof. Otto
Copy right ©Prof. Otto
Near field optical spectroscopy of individual surface-plasmon modes in
colloid clusters. Theory.
V.A. Markel et al., Phys.Rev.B 59(1999)10903
Colour range of field intensity covers approximately a factor of4 (only)
Intensity 100nm above
surface. Vertical range
in the topographical
image is ca. 1100nm
Though directions of
incidence and
polarization remain
constant, hot spots
fluctuate within the
wavelength range of
the Stokes shift
Copy right
©
Prof. Otto
Copy right ©Prof. Otto
Enhanced Raman scattering from self-affine thin films, E.V.Poliakov, V.M.Shalaev, V.A.Markel, R.Botet,
Optics Letters 21(1996) 1628
Copy right
©
Prof. Otto
Copy right ©Prof. Otto
SEM images of the formed silver particle films,
courtesy of Dr. JiawenHu, Xiamen
Copy right
©
Prof. Otto
Copy right ©Prof. Otto
An EM hot spot will be extremely dependent on direction of incidence,
polarisation and wavelength, it „jumps“the more quickly, the higher the
local |Ehot spot
|2
angle of
incidence
changed
angle of
polarisation
changed by
some degrees
wavelength of incident radiation changed by some nm
(this is demonstrated in all previous foils!)
Qualitative picture
Copy right
©
Prof. Otto
Copy right ©Prof. Otto
oscillating dipoles P
i(X
i), electric
vectors of the dipoles E
i(X
j
) valid
in the far field, at frequency ,
ij
P
2(X
2)
matter of any kind
theorem of optical reciprocity :P
1
(X
1
)E
2
(X
1
)= P
2
(X
2
)E
1
(X
2
)
Stokes emission by an adsorbate:How to calculate E
adsorbate(ωStokes, X
detector) p
??
calculate E
detector
(ωStokes
, X
adsorbate
) with fictive unit dipole p
detector
(X
detector; Stokes) !
polarization sensitive
detector p
detector
, idealized at
a point X
detector
|p
detector| = 1
P
adsorbate
(X
adsorbate
,
Stokes
)
p
detector
(X
detector; Stokes)
E
detector (ωStokes, X
ads) P
ads
(X
ads
,
Stokes) = E
ads(ωStokes
, X
detector) p
detector
(X
detector;
Stokes
)
P
1(X
1)
Raman tensor
x E
Laser(X
ads , Laser)
P
ads(X
ads
,
Stokes
)
see M. Arnold, P. Bussemer, K. Hehl, H. Grabhorn, A. Otto, Enhanced Raman scattering from benzene condensed on asilver grating, J. Modern Optics 39(1992) 2329-2346
Copy right
©
Prof. Otto
Copy right ©Prof. Otto
Experimental direction of laser
incidence and Stokes light
( http://www-gdr-cpo.utt.fr
/resumes_CFMCP/rivoal.html
precise backscattering is no
experimental option.
And even if it were possible: One
cannot overcome Stokes
< Laser
Hot spots of Raman excitation and
Raman emission are at different sites!k
L
and p
L
are directions of incident Laser light and
polarization, k
S,p
S are directions of Stokes emission
and polarization, respectively.
2
2
,,(),,()
4
,,()
()()
()
LLStokesL
LS
LS
LL
L
L
kphotspotkphotspot
kphotspot
ErEr
Er
ωω
ω
−−

<<
I do not suscribe to the
statements on the first foil!