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): Field at z=0 (object) propagates in free space as The propagator H is oscillating for and exponentially decaying for 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 + Airy pattern (microscope point spread function)  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 probe fibre type Aperture probe microlever type Metallic particle Single 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 probe Aperture ~ 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  35 nm aperture – well defined aperture – flat endface – isotropic polarisation – high brightness up 1  W ExEx EyEy EzEz With excitation E x, k z, : Focussed ion beam (FIB) etched NSOM probe Veerman, Otter, Kuipers, van Hulst, Appl. Phys. Lett. 74, 3115 (1998) x y

Shear force feedback: molecular scale topography Feedback on phase Tip -sample < 5 nm RMS ~ 0.1 nm Feedback loop: sample Lateral movement, A 0 ~ 0.1 nm Tuning fork 32 kHz Q ~ 500 ff 00 A0A0 piezo Rensen, Ruiter, West, van Hulst, Appl. Phys. Lett (1999) Ruiter, Veerman, v/d Werf, van Hulst, Appl. Phys. Lett (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 DNA width 14 nm height 1.4 nm DNA on mica

90 o 0o0o 1  m 100 nm Perylene orange in PMMA Ruiter, Veerman, Garcia-Parajo, van Hulst, J. Phys. Chem. 101 A, 7318 (1997)

a b c nm FWHM counts / pixel distance (nm) DiIC 18 molecules in 10 nm PMMA layer 1.2 x 1.2  m 2 ; 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

kcounts/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 with 70 nm aperture probe van Hulst, Veerman, Garcia-Parajo, Kuipers. J. Chem. Phys. 112, 7799 (2000)

a b c d e 90 o emission 45 ± 2 nm 0 o emission a b c nm Sample area: 440 x 440 nm 2 Aperture diameter: 70 nm Mutual 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 x 7.62  m TE 00 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