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