SNOM Challenges and Solutions

Transcription

1 SiO x SiO x Au Au E k SNOM Challenges and Solutions Ralf Vogelgesang, Ph.D. Ralf.Vogelgesang@fkf.mpg.de Nanoscale Science Department (Prof. Kern) Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany

2 Microscopy Some general aspects

3 Nanoscale Microscopy nm scale = only a few atoms, small signals Bulk 1 Surface 10-8 Line Point Must overcome noise and parasitic signals!

4 Nanoscale Microscopy Small signal extraction Selective activation Only in area of interest Whenever possible: Favorable! Blocking/discriminating parasitic signals

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7 The Abbe Diffraction Limit Spatial Resolution: When do you see two points as separate? (The central question in all microscopy) (E. Abbe, Arch. Mikrosk. Anat. 1873, p413) 0λ 0.5λ 1.0λ λ Δx Rayleigh criterion: Δx > λ/2 (Lord Rayleigh, Phil. Mag. 54, 1896, p167)

8 Focussed Beams in Angular Representation Classically Quantum mechanically k x E.H.K. Stelzer et al. Optics Communications 173 (2000) 51

9 Loss of Nearfield Information k = ω / c D + + ( k + π D) ( k + π D) ( ) i( r k + r k ) dk A k exp dk ( ) A k expi r k + D k 2 k 2

10 SNOM Scanning Nearfield Optical Microscopy

11 Nearfield Restauration superlensing material amplitude λ=10.85μm phase λ=11.03μm amplitude λ=9.25μm

12 Nano-Optical Microscopy Techniques Method Confocal (2π) microscope 4π microscope SNOM asnom Scheme Resolution Limit λ/2 4π+STED λ/8! d λ/10 r 250nm < 20nm 60nm 50nm < 10nm

13 Resolution in the Eyes of an Engineer Vary object spatial frequency Fixed PSF Linear system gives convolution image Nonlinear systems improve resolution Resolution Limit

14 SNOM Scanning Nonlinear Optical Microscopy

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16 4π Microscope nm axial

17 Stimulated Emission Depletion (STED) space time

18 Saturated Depletion: Breaks Diffraction Barrier S.W. Hell (2003), Nature Biotechnol. 21, S.W. Hell (2004), Phys. Lett. A S.W. Hell, M. Dyba, S. Jakobs (2004), Curr. Opin. Neurobiol. 14, 599.

19 4π-STED Microscopy Monolayer Monolayer confocal STED-4Pi 53 nm M. Dyba, S. W. Hell Fluorescently tagged microtubuli with an axial resolution of nm M. Dyba, S. Jakobs, S.W. Hell (2003), Nature Biotechnol. 21, 1303.

20 Scanning Near field Optical Microscope SNOM History Envisioned: 1928 & 1956 Synge, Phil. Mag. 6, 356, 1928 J.A. O'Keefe, JOSA 46, incident radiation opaque screen near-field far-field surface

21 SNOM Realization Microwaves: 1972 Ash, Nicholls, Nature 237 Visible: 1984 Pohl, Denk, Lanz, APL 44 Lewis, Isaacson, Harootunian, Murray, Ultramicroscopy 13 tapered optical fiber cut-off diameter aperture Al coating near-field aperture 100 nm far-field surface

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23 SNOM Limits of Scaling Down Energy balance: Reflection 2/3 2 3 Absorption 1/3 1 3 Throughput only 10-6 to (depending on aperture) Skin depth = resolution limit

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26 Reminder: Apertureless SNOM (asnom) nearfield interaction farfield detection opaque and transparent samples wavelength-independent resolution: ~apex radius

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28 Vertical Excitation => {Tip&Sample} E in =E out k in = -k out dipoles? topography optical amplitude 600nm E k 600nm Au disks on glass

29 Horizontal Excitation => Sample => Tip E out E in k in = -k out dipoles topography optical amplitude 680nm E k 680nm Au disks on glass quadrupoles

30 Microscope resolution Spectro-Microscopy microwaves 1mm 100μm 10μm 1μm 100nm 10nm 1nm Vis. IR 100nm 1μm 10μm 100μm 1mm 1cm 10cm 4π Abbe diffraction limit 4π SNOM +STED asnom: Wavelength-independent resolution limit Wavelength