Download Lecture 12 – Nanophotonics in Microscopy

Lecture 12 – Nanophotonics in
Microscopy
EECS 598-002 Winter 2006
Nanophotonics and Nano-scale Fabrication
P.C.Ku
Schedule for the rest of the semester
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Introduction to light-matter interaction (1/26):
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Basic excitations and measurement of ε(r). (1/31)
Structure dependence of ε(r) overview (2/2)
Surface effects (2/7):
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Surface EM wave
Surface polaritons
Size dependence
Case studies (2/9 – 2/16):
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How to determine ε(r)?
The relationship to basic excitations.
Quantum wells, wires, and dots
Nanophotonics in microscopy
Nanophotonics in plasmonics
Dispersion engineering (2/21 – 3/7):
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Material dispersion
Waveguide dispersion (photonic crystals)
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Principles of optical microscopy
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The information contained in the object/image is carried
by the light wave to the detector (e.g. eyes).
Information = F(x,y) Æ F ( k x , k y )
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Optical vs electron microscopy
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Advantages of optical microscopes
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Photon energy is low (~ eV). Electron energy is high (~10-100 keV)
Photon momentum is low (~ 10-27 kg m/s). Electron momentum is
high (~ 10-23 - 10-22 kg m/s)
Especially good to study optical processes (including nonlinear
optical response) in the sample
Ultra-short optical pulse is available to study ultra-fast processes
Can make the fluorescence work Æ useful for biological imaging
Disadvantages of optical microscopes
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Photon does not have charge. Cannot study the Coulomb
interaction in the sample.
Photon does not reveal information regarding the chemical
composition of the sample (unless we go to the x-ray wavelength).
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Resolution limits of optical microscopes
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Small features correspond to large (kx, ky) components.
In traditional optical microscopes, the detector sees the
k
light in the far field
region.
k 2 = ω 2 µ0ε = k x2 + k y2 + k z2
⇒ k x2 + k y2 < ωn / c ⇒ k&,max = 2π n / λ
17.5
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Resolving power
= λ / ( 2n ) ≡ λ / 2
12.5
k-space
real-space
10
eff
7.5
= diffraction limit
5
2.5
−2π n / λ
2π n / λ
k&
-2
-1
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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2
λ /n
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Finite-size lens
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In a real system, the cutoff spatial frequency is often
limited by the size of the lens which is quantitatively
described by a numerical aperture (NA).
NA ≡ n sin θ
k&,max
2π
NA
⇒
= sin θ ⇒ k&,max =
k
λ
θ
Resolving power Æ
λ / ( 2NA ) ≡ λeff / 2
where λeff = λ / NA
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Resolution enhancement
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Shortening of wavelength:
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Decrease the wavelength of light source
Increase the refractive index (e.g. immersion microscope)
Using nonlinear optical effect (e.g. frequency doubling)
Increase the detectable spatial frequency
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Moire grating
Near field optics to detect the evanescent waves
Ref: S. Kawata, chapter 2, figure 1.
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Scanning Near-field Optical Microscope
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Different modes of SNOM
Ref: Prasad, chapter 3, figure 7.
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Detection of near field intensity modulation
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The modulation of the near-field intensity can be detected
at the far-field region:
e.g. Thin slab photon tunneling
d = wavelength/4
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Transmission
Reflection
0.9
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0.5
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0.3
0.2
0.1
0
0
20
40
60
Incident Angle
80
100
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Configuration of SNOM
Ref: Prasad, chapter 3, figure 8.
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Near-field probes
(aperture in an
opaque screen)
(e.g. PSTM)
Green:
evanescent
wave
Ref: S. Kawata, chapter 2, figure 6.
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Advantages of apertureless probes
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No cutoff frequency as existing in a metal-coated fiber
optics probe
Field enhancement due to surface plasmon polaritons
The probe size can be made much smaller
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Controlling the probe-sample distance
(1)
feedback
laser
(2)
excitation
laser
beam
splitter
segmented
photodiode
cantilever
sample
The tip-sample distance changes the damping
of the resonance oscillator.
Ref: Prasad, chapter 3, figure 9.
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Laser trapping for
probe
Ref: S. Kawata, chapter 8.
e.g. 2.5W Nd-YLF traps
a gold particle (~ 10-100 nm)
with a 100X NA=1.35 objective
(oil immersion)
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Examples of SNOM images 1
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Single molecule fluorescence detection
location
Laser detuning
Ref: W. E. Moerner, Science 265 (1994) 46.
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Examples of SNOM images 2
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Single quantum dot
Ref: T. Saiki and Y. Narita, Jpn. J. Appl. Phys., 37 (1998) 1638.
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Examples of SNOM images 3
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Dynamics
using
pump-probe
technique
t = 30 ps
t = 0 ps
Ref: Y. Shen et al., J. Phys. Chem. B 107 (2003) 13551.
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Examples of SNOM images 4
Topography
0
14500
0
28000
0
21000
0
14000
0
80000
0
13000
0
61000
0
18000
0
8000
0
12000
830,25nm
21000
829,53nm
SNOM
0
6000
0
VCSEL
Background
15 microns scan range
Taken from presentation materials of WITEC.
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Examples of SNOM images 5
MFM - Magnetic Force Microscopy
80 microns scan range
20 microns scan range
5 microns scan range
PC hard drive
Taken from presentation materials of WITEC.
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Examples of SNOM images 5
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Semiconductor Surface Study
Topography
SNOM
20% In
(a)
(b)
(c)
24% In
(d)
(e)
(f)
27% In
(g)
(h)
(i)
InGaN films
Jeongyong Kim et al., University of Illinois,
see also APL Vol. 80, 6, 989 (2002).
10 microns scan range
Taken from presentation materials of WITEC.
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Examples of SNOM images 7
Topography
Adhesion
5 microns
scan range
Bodyguard adhesive tape
Taken from presentation materials of WITEC.
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Reading
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S. Kawata, chapter 2.
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