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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?
i
2
1, ; , , e d d
4x yk x k y
x yk k z x y z x y
E E
Angular spectrum and diffraction limit
Describe field as superposition of plane waves (Fourier transform):
iˆ, , , ; e d dx yk x k y
x y x yx y z k k z k k
E E
Field at z=0 (object) propagates in free space as
iˆ ˆ, ; , ;0 e zk zx y x yk k z k k E E 2 2 2
0z x yk nk k k
The propagator H is oscillating for
and exponentially decaying for
22 20x yk k nk
22 20x yk k nk
High spatial fluctuations do not propagate: diffraction limit
E
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
+
Airy pattern (microscope point spread function)
0.61dNA
sinNA n
Numerical Aperture determines resolution
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.
Aperture probefibre type
Aperture probemicrolever type
Metallic particleSingle emitter
Probing beyond the diffraction limit
Thin polymer film,self-assembled monolayer,
cell membrane, etc.
single fluorophores
NSOM probe
Excitation light
Fluorescence
Protein, dendrimer, DNA, etc.
FIB treated probeAperture ~20-100 nm
200 nm
Al
Transmission of light through a near-field tip
Modified slide from Kobus Kuipers and Niek van Hulst et al.
glass
aluminum
500 nm
100 nm
100 nm
35 nm aperture
– well defined aperture – flat endface– isotropic polarisation– high brightness up 1 W
Ex Ey Ez
With excitation Ex , kz, :
Focussed ion beam (FIB) etched NSOM probeFocussed ion beam (FIB) etched NSOM probe
Veerman, Otter, Kuipers, van Hulst, Appl. Phys. Lett. 74, 3115 (1998)
xy
Shear force feedback: molecular scale topography
Feedback on phase
Tip -sample < 5 nm
RMS ~ 0.1 nm
Feedback loop:
sample
Lateralmovement,A0 ~ 0.1 nm
Tuning fork32 kHzQ ~ 500
f
0
A0
piezo
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)
1.7 x 1.7 m
3 x 3 m
Steps on graphite (HOPG)
~ 0.8 nm step ~ 3 mono-atomic steps
DNAwidth 14 nm
height 1.4 nm
DNA on mica
90o0o 1 m
100 nm
Perylene orange in PMMA
Ruiter, Veerman, Garcia-Parajo, van Hulst, J. Phys. Chem. 101 A, 7318 (1997)
a b c
0 400 800 12000
40
80
120
45 nmFWHM
coun
ts /
pix
el
distance (nm)
DiIC18 moleculesin 10 nm PMMA layer1.2 x 1.2 m2; 3 nm/pix; 3 ms/pix
Single molecular mapping of the near-field distribution
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
0.0 0.5 1.0 1.5 2.0 2.5 3.00
10
20
30
40
50
kcou
nts/
s
lateral scan [m]
FWHM = 75 nm
S/B 20
NFO for Single Molecule Detection : Reduced excitation volume,
high resolution, low background
Single DiD molecule
in 30 nm polystyrene
with70 nm aperture probe
van Hulst, Veerman, Garcia-Parajo, Kuipers. J. Chem. Phys. 112, 7799 (2000)
a b
c d
e
90o
emission
45 ± 2 nm
0o
emission
a
b
c
0 200 400 nm
Sample area: 440 x 440 nm2
Aperture diameter: 70 nmMutual distance: < 10 nm
Optical discrimination of individual molecules separated by nm mutual distance
van Hulst, Veerman, Garcia-Parajo, Kuipers. J. Chem. Phys. 112, 7799 (2000)
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.
Time-resolved near-field scanning tunneling microscopy
40 nm high ridge waveguide
239.5 x 7.62 m
239.5 x 7.62
m
TE00 pulse, l =1300 nm
duration : 120 fs
Pulse envelope
Pulse caught in 1 position
Fixed time delay
Data from Kobus Kuipers and Niek van Hulst et al.
A light pulse caught in time and space