Class 8
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):
 k k z x y
;



E
 



4




1
2



E

, ,
 e
 i 

 k k z x y
 e i 
 k k x y



Field at z=0 (object) propagates in free space as
 k k z x y
;
  k k x y

 i k z
, ; 0 e z k z
  
0
2 
 k x
2  k
2 y

The propagator H is oscillating for and exponentially decaying for
 k
2 x
 k
2 y

 k
2 x
 k
2 y

  nk
0
0
   
2
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.

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.


– well defined aperture
– flat endface
– isotropic polarisation
– high brightness up 1 m
W
100 nm
35 nm aperture
With excitation E x
, k z,
:
100 nm y x
E x
E y
E z aluminum glass
500 nm
Veerman, Otter, Kuipers, van Hulst, Appl. Phys. Lett . 74, 3115 (1998)

A
0
Steps on graphite (HOPG)
Feedback loop:
D f piezo w
0
3 x 3 m m
~ 0.8 nm step
~ 3 mono-atomic steps
Lateral
A movement,
0
~ 0.1 nm
Tuning fork
32 kHz
Q ~ 500 sample
Feedback on phase
Tip -sample < 5 nm
RMS ~ 0.1 nm
DNA on mica
1.7 x 1.7 m m
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)

100 nm
Perylene orange in PMMA
0 o 90 o 1 m m
Ruiter, Veerman, Garcia-Parajo, van Hulst , J. Phys. Chem. 101 A , 7318 (1997)

a b c
DiIC
18 molecules in 10 nm PMMA layer
1.2 x 1.2 m m 2 ;
3 nm/pix; 3 ms/pix
120
80
40
0
0
45 nm
FWHM
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

Reduced excitation volume, high resolution, low background
50
40 S/B

20
30
FWHM = 75 nm
20
10
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
lateral scan [ m m]
Single DiD molecule in
30 nm polystyrene with
70 nm aperture probe van Hulst, Veerman, Garcia-Parajo, Kuipers . J. Chem. Phys. 112 , 7799 (2000)

a b e c d a b
90 o emission
45 ± 2 nm c
0 o emission
Sample area: 440 x 440 nm 2
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 m m
Pulse envelope
TE
00 pulse, l =1300 nm duration : 120 fs
239.5 x 7.62 m m
Fixed time delay
Pulse caught in 1 position

Class 1 - Resonances and refractive index
Class 2 - Nanoparticle scattering
Class 3 - Surface plasmon polaritons
Class 4 - Photonic crystals
Class 5 - Local density of optical states
Class 6 – Rare earth ions and quantum dots
Class 7 – Microcavities
Class 8 - Nanophotovoltaics
Class 9 - Metamaterials
Class 10 – Near-field optics

Class schedule
Class 1 - Resonances and refractive index
Class 2 - Nanoparticle scattering
Class 3 - Surface plasmon polaritons
Tour through Ornstein Lab
Homework assistance
Class 4 - Photonic crystals
Class 5 - Local density of optical states
Excursion to AMOLF-Amsterdam
Class 6 – Rare earth ions and quantum dots
Class 7 – Microcavities
Visit to Nanoned conference
Class 8 - Near field optics
Class 9 - Nanophotovoltaics
Excursion to Philips Research- Eindhoven
Class 10 - Metamaterials
Class 11 – Near-field optics
Nanophotonics summary
Closing symposium