Transcript Slide 1

Nanophotonics
January 9, 2009
Near-field optics
Resolution in microscopy
Why is there a barrier
in optical microscopy
resolution?
And how can it be broken?
Angular spectrum and diffraction limit
Describe field as superposition of plane waves (Fourier transform):
ˆ k , k ; z 
E
x
y

1
4
2
 E  x, y, z  e
 i k x x  k y y 
dxdy




i k x x  k y y 
ˆ
dk x dk y 
 E  x, y, z    E  k x , k y ; z  e



Field at z=0 (object) propagates in free space as
ˆ k , k ; z  E
ˆ  k , k ;0  e  ik z z
E
x
y
x
y
The propagator H is oscillating for
and exponentially decaying for
kz 
 nk0    kx2  k y2 
2
 kx2  k y2    nk0 
2
 kx2  k y2    nk0 
2
High spatial fluctuations do not propagate: diffraction limit
The diffraction limit in conventional microscopy
Image of a point source in a microscope, collecting part of the
angular spectrum of the source:
Rayleigh criterion: two point sources
distinguishable if spaced by the distance
between the maximum and the first
minimum of the Airy pattern
q
+
d  0.61

NA
NA  n sin q
Numerical Aperture determines resolution
Airy pattern (microscope point spread function)
Breaking the diffraction limit in near-field microscopy
A small aperture in the near field of the source can scatter also the
evanescent field of the source to a detector in the far field.
Image obtained by scanning the aperture
Alternatively, the aperture can be used to
illuminate only a very small spot.
Probing beyond the diffraction limit
Single emitter
Metallic particle
Aperture probe
fibre type
Aperture probe
microlever type
Modified slide from Kobus Kuipers and Niek van Hulst et al.
Transmission of light through a near-field tip
200 nm
Excitation light
Al
NSOM probe
FIB treated probe
Aperture ~20-100 nm
Protein, dendrimer, DNA, etc.
single fluorophores
Fluorescence
Thin polymer film,
self-assembled monolayer,
cell membrane, etc.
Focussed ion beam (FIB) etched NSOM probe

– well defined aperture
– flat endface
– isotropic polarisation
– high brightness up 1 mW
35 nm
aperture
100 nm
glass
100 nm
With excitation Ex , kz, :
aluminum
y
Ex
Ey
Ez
x
500 nm
Veerman, Otter, Kuipers, van Hulst, Appl. Phys. Lett. 74, 3115 (1998)
Shear force feedback: molecular scale
topography
Steps on graphite (HOPG)
Feedback loop:
A0
Df
piezo
3 x 3 mm
w0
Tuning fork
32 kHz
Q ~ 500
Lateral
movement,
A0 ~ 0.1 nm
~ 0.8 nm step
~ 3 mono-atomic steps
DNA on mica
sample
Feedback on phase
Tip -sample < 5 nm
RMS ~ 0.1 nm
1.7 x 1.7 mm
DNA
width 14 nm
height 1.4 nm
Rensen, Ruiter, West, van Hulst, Appl. Phys. Lett. 75 1640 (1999)
Ruiter, Veerman, v/d Werf, van Hulst, Appl. Phys. Lett. 71 28 (1997)
van Hulst, Garcia-Parajo, Moers, Veerman, Ruiter, J. Struct. Biol. 119, 222, (1997)
Perylene orange in PMMA
100 nm
0o
90o
1 mm
Ruiter, Veerman, Garcia-Parajo, van Hulst, J. Phys. Chem. 101 A, 7318 (1997)
Single molecular mapping of the near-field distribution
counts / pixel
DiIC18 molecules
in 10 nm PMMA layer
1.2 x 1.2 mm2;
3 nm/pix; 3 ms/pix
c
b
a
120
45 nm
FWHM
80
40
0
0
400
800
distance (nm)
1200
Veerman, Garcia-Parajo, Kuipers, van Hulst, J. Microscopy 194, 477 (1999)
Data from Kobus Kuipers and Niek van Hulst et al.
Mapping the near field of the probe
NFO for Single Molecule Detection :
Reduced excitation volume,
high resolution,
low background
kcounts/s
50
S/B  20
40
30
FWHM = 75 nm
20
10
0
0.0
0.5
1.0
1.5
2.0
2.5
Single DiD molecule
in
30 nm polystyrene
with
70 nm aperture probe
3.0
lateral scan [mm]
van Hulst, Veerman, Garcia-Parajo, Kuipers. J. Chem. Phys. 112, 7799 (2000)
a
Optical discrimination of
individual molecules
separated by
nm mutual distance
a
90o
emission
b
45 ± 2
nm
b
e
c
c
d
0o
emission
Sample area: 440 x 440 nm2
Aperture diameter: 70 nm
Mutual distance: < 10 nm
0
200
400 nm
van Hulst, Veerman, Garcia-Parajo, Kuipers. J. Chem. Phys. 112, 7799 (2000)
Data from Kobus Kuipers and Niek van Hulst et al.
Time-resolved near-field scanning tunneling microscopy
120 fs pulses
coupled
into the PhCW
Two arms of the interferometer
equal in length gives
temporal overlap on the detector
Data from Kobus Kuipers and Niek van Hulst et al.
A light pulse caught in time and space
40 nm high
ridge waveguide
239.5 x 7.62 mm
Pulse envelope
239.5 x 7.62
mm
Fixed time delay
TE00 pulse, l =1300 nm
duration : 120 fs
Pulse caught in 1 position