TOWARDS 3-D NEAR FIELD MICROSCOPY

Transcription

1 TOWARDS 3-D NEAR FIELD MICROSCOPY Gael Moneron, Alexandra Fragola, Florian Formanek, Laurent Williame, Arnaud Dubois, Lionel Aigouy, Yannick de Wilde, Samuel Grésillon and Claude Boccara Laboratoire d Optique Physique, Ecole Supérieure de Physique et Chimie Industrielles, Centre National de la Recherche Scientifique, UPR A0005, 10 rue Vauquelin, F Paris Cedex 5, France

2 Introduction : Near field Optics detector Propagative waves : low spatial frequencies Far field??/2? near field Evanescent waves : high spatial frequencies Sample No wave propagation when k // >(2?/?).

3 Near field Microscopes (SNOM) with a scattering probe nanobead detector detector PZT Translation PZT Translation Concept : scattering particle Application : scattering tip

4 Principle of apertureless near-field optical microscopy High N.A. objective high N.A. objective Tungsten tip fluorescent sample detection of evanescent waves? subwavelength details Application to fluorescence imaging

5 Tip effect in near field optical microscopy Plane wave Metal tip 1 nm Observation plane r I/I0 Electric field Intensity nm nm E 2?? 1 p1? sin(? )?? ( kr) 2, where p 1? 1

6 Field enhancement Scattering Efficiency Field distribution around the tip

7 Electromagnetic field distribution on semicontinuous gold films Collaboration with P. Gadenne (Univ. Versailles) Collaboration with V. Shalaev (Purdue University) Surface field Scattered film EM Image glass Laser Ti : Al 2 O 3 (800 nm) Semicontinuous gold film Localized field enhancement

8 Magnetic domains imaging in Pt/Co/Pt films Trilayer Pt/Co/Pt Collaboration avec C. Chappert (Univ. Orsay) laser Diode (670 nm) Near Field Far Field PE Modulator Al 2 O Magnétic coating 3 Scattered field Magnetic domains (Perpendicular magnetization)

9 Visible and Infrared SNOM

10 Subwavelength holes (200nm) in a chromium film (glass subtrate, visible illumination): Ez AFM antisymmétric lobes Sensitivity to vertical field component Ez S. Ducourtieux, et al. SNOM (F. Formanek et al., J. Appl. Phys. (Juin 2003) ) Reflexion mode visible SNOM (1.5? mx1.5? m) ;? =655nm

11 Subwavelength holes in a chromium film on a glass subtrate (IR illumination?=10,6? m) Topography (3?m x 3?m) Near field microscopy - Résolution ~ 50 nm (?/200) - single dark lobe circular symmetry : polarisation effet - Tiny structures with inversed Minatec Grenoble contrast regarding topography

12 Electrostatic dipole image model E p r h 2h? eff? p sample k 4 C? sca?? 6 eff? (1?? )??? 1? 16 3? h 2??? 3 4? r (? t? 1) (?? 2) (? s? 1)? (?? 1) s t B. Knoll and F. Keilmann, Optics Communications 182, 321 (2000).

13 Expected contrast Glass : n v =2.694+i*0.509 Chromium : n c =13+i*27 Tungsten : n w =10+i*45?=10?m Radius = 30 nm Amplitude = 70 nm R?? verre Glass??? Chromium chrome? 0,68

14 Contrast analysis R= LIopt( centre) 0,6 Good agreement with LIopt( background )? the electrostatic model

15 Near Field IR Thermal Imaging 0.8µm?

16 Fluorescent nanospheres Samples realisation : Othman Bouloussa (Institut Curie, Paris) r = 100 nm? excitation = 488 nm? fluorescence? 550 nm Far-field image SNOM image Resolution Minatec? a few Grenoble 10 nm SNR? 6 to 8

17 Fluorescence SNOM imaging in liquids 1 ) AFM imaging in liquids AFM image on 500 nm spheres in liquid (+paraffin to avoid evaporation)

18 Fluorescence SNOM imaging in liquids AFM image 2 ) Near-field imaging of 500 nm fluorescent spheres in liquid SNOM images tip is in contact tip is 700 nm above the sphere

19 From 2-D to 3-D NEAR FIELD TECHNIQUES ONLY REVEAL VARIOUS CONTRASTS OF 2-D STRUCTURES Let us start with Optical Coherence Tomography

20 Conventional OCT principle Fiber-optic Michelson interferometer Fringe envelope detection Single transverse mode broadband light source Reference Mirror (Z scanning) (X Scanning) Coherence gate = L c /2 (a few?m) Sample

21 White light full field OCT set-up Linnik-type microscope CCD camera (200 Hz) White-light thermal source Microscope objectives PZT Reference mirror Oscillation (50 Hz) < 1µm Sample CCD records 4 images (E 1, E 2, E 3, E 4 ) per modulation period Tomographic image calculated in real-time :? E? E? E? E??? E? E? E? E?

22 Theoretical axial resolution Theoretical axial resolution = = 0.6 µm (in water) 2n L c

23 Sensitivity 200 images accumulated Glass / water R = -24 db (0.004) Noise ~ -92 db (5?10-8 ) (Shot-noise limit)

24 Image acquisition XY (en face) tomographic images without scanning y x z Coherence plane High-resolution motorized translation stage

25 Animal tissues Xenopus laevis tadpole (anesthetized) 0 < depth < 550 µm Animal model Used to demonstrate high-resolution OCT

26 XY 240 µm 0 < depth < 500 µm

27 XZ YZ 240 µm

28 Plant tissues Tobacco leaf (in vivo) Upper epidermis Palisade parenchyma Spongy parenchyma

29 460 µm XY 210 µm XZ

30 From 3-D far field to 3-D near field OCT is a 3-D diffraction limited imaging technique Let us continue with Optical Coherence Tomography and scattering probes

31 Improvement of the 3D resolution Scanning probe microscopy Tunnelling Atomic force Near-field optics Nanometric resolution limited to external surfaces

32 Improvement of the 3D resolution Idea : Replacing the tip by several beads Access to surfaces inside the sample Parallel exploration

33 3D Microscope using a single local probe (r =100 nm) 3D images of polymer network structures (resolution ~ 20 nm) C. Tischer et al., Appl. Phys. Lett. 79, p (2001)

34 Is our instrument able to detect the beads? Rayleigh theory Q scat? m? 1 x 2 3 m? 2 2 x? 2? r m = complex index? Gold beads in water Qscat? 10x 4 radius r = 50 nm? = 600 nm Focused on 1? m scattered Light scattered by a bead collected by the objective ~ 10-4

35 3-D Gold beads Imaging Gold beads (50 nm radius) diluted in a Laponite gel 110 µm 30µm < depth < 50 µm 3D view

36 Application of parallel exploration Measurement of local deformations in soft materials Initially After deformation Mouvement affine

37 Material = Laponite gel? optically transparent

38 Many beads in the field Requirements X (px) Full-field image µm Y (px)

39 Requirements Localization of each bead by over-sampling its diffraction pattern X (px) µm Y (px)

40 Requirements Localization of each bead by over-sampling its diffraction pattern a.u µm X (px) Y (px)

41 Transverse localization accuracy s ~ 20 nm Measurment number

42 Axial localization (Z) x Bead Localization of the coherence peak Resolution ~ 50 nm better when using the phase z

43 Shear strain experiment

44 Shear strain experiment Mouvement affine 1 2 3

45 Shear strain experiment 1,2,3 4,5,6

46 Experimental set-up

47 Lets go back to our project for 3-D Imaging Thermal fluctuations change positions

48 Goal : improvement of the 3D resolution Requirements : freeze the bead movement during the position measurement The beads in water move by a distance equal to their radius (r) during a time : t ~ r 3 (r in µm, t in s) 2r = 50 nm? t ~ 15 µs New setup: Flash illumination (Xe) of 2 µs duration

49 Flash 1 Flash 2 Flash 3... Acquisition of several tomographic images... Narrowing the section (selection and localization of the most brilliant beads)... Transverse localization (over-sampling + interpolation)... Analysis of the positions Reconstruction of the border (2D) repeat at different depths 3D reconstruction

50 CONCLUSION Scattering probes have proved their usefulness in 2-D and their potential use in 3-D imaging Are other approaches using multi-beads possible for 3-D imaging? - Fluorescent probes (e.g. quantum dots) - Photothermal probes Boyer et al. Science 297 (2002) nm gold particles

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