Scanning Near Field Optical Microscopy: Principle, Instrumentation and Applications

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1 Scanning Near Field Optical Microscopy: Principle, Instrumentation and Applications Saulius Marcinkevičius Optics, ICT, KTH 1

2 Outline Optical near field. Principle of scanning near field optical microscope and its operation modes. Probes and distance control. Examples: Imaging through wavelength, refraction and polarization variations, SNOM measurements on AlGaN and UV LEDs, imaging of exciton wave functions, time-resolved SNOM measurements. 2

3 Generation of optical near field Problem in microscopy diffraction limit. To generate strongly-localised radiation with conventional propagating light: A sphere with a diameter << wavelength illuminated by light, Scattered light 1 propagates into the far field, Optical near field cloud around the sphere. Near field thickness ~a<< λ. Alternative a subwavelength aperture. 3

4 Optical near fields Near field and scattered light at a sample sphere 2. Perturbation of the near field by the probe sphere 3. Principle of scanning near-field microscopy

5 The machine, the probe, the feedback Metal-coated thinned fibre probe Instrument schematics Shear force control of the sample-to-probe distance 5

6 Basic modes of operation Illumination/ collection mode Transmission, reflection, luminescence, scattering Apertureless SNOM a) Collection mode b) Illumination mode Transmission, luminescence, scattering, refraction 6

7 Tapered fiber tips: Heating & pulling Different core profiles in etched and pulled probes Heating Pulling Breaking Final shape depends on: pulling strength and symmetry, temperature during breaking. Often sharp tips. 7

8 Tapered fiber tips: Meniscus etching Final shape depends on: acid concentration, buffer solution concentration, temperature, etching time, nature of the material (doping). Fig. 4. Schematic illustrations of meniscus etching of a fiber at (a) the start, (b) tapering and (c) stop Principle: Meniscus between oil and acid forms because of the surface tension difference between the acid and the oil. As the fibre is being etched, the etchant (acid and etching products) become heavier and meniscus smaller. Typical etchant % HF, oil silicon, toluene. Lazarev et al. Review of Scientific Instruments 74, 3679 (2003) 8

9 Tapered fiber tips: Selective etching The tip is designed by choosing appropriate etching rates and time. Etching rates depend on the etchant concentration and fibre doping (refr. index) Fig. 5. (a) Cross-sectional profile of a refractive index of a silica fiber. Here, n 1 and n 2 are the refractive indices of the core and clad, respectively; r 1 and r 2 are the radii of the core and clad, respectively. (b) Top, a geometrical model for the tapering process; Here, τ is the etching time required for making the apex diameter zero. θ is the cone angle of the tapered core. L is the length of the tapered core. Bottom, cross-sectional profiles of the dissolution rates R 1 and R 2 of the core and clad Fig. 8. SEM micrographs of (a) a shoulder-shaped probe and (b) the magnified apex region. D = 25 μm; θ = 20º; d < 10 nm 9

10 Distance control Tuning fork Optical feedback laser excitation laser beam splitter segmented photodiode Morphology through probe feedback (like AFM) cantilever sample 10

11 Example: modes of a VCSEL perpendicular polarizations GaAs/AlGaAs QW VCSEL spectrally-integrated fundamental mode transverse mode at nm transverse mode at nm 11

12 Examples of SNOM imaging refraction, reflection Contrast in near field image due to different refractive indices is induced by different coupling of the near field (coupling depends on polarization, scanning direction, angle of incidence). PPMA resist film 12

13 Examples of SNOM imaging polarization, luminescence Magneto-optic materials with domains (magnetisation up or down), Bi-doped yttrium iron garnet Y 3 Fe 5 O 12 film Domains rotate linear polarisation of the transmitted light (Faraday effect). Polarisation analyser at the output. Simultaneous measurement of refractive index profile (dotted line) and Er distribution (luminescence) in Er-doped fibre. PL Er spreading into cladding. 13

14 Spectrally-resolved scan, deep UV AlGaN QW LED 14

15 AlGaN inhomogeneities measured by SNOM Al 0.42 Ga 0.58 N morphology µm size areas of lower PL peak energy and emission intensity. Clear correlation (0.45): lower energy lower intensity (NR recombination). (Contrary to InGaN!). Clear correlation (-0.44) between FWHM and peak energy. FWHM larger than for 30% Al layer In some places a clear 15 dual peak shape. 10 Intensity, a.u Wavelength, nm 15

16 Dual localization potential 100 nm few µm scale localization: SNOM Localization <100 nm: fitting PL spectra and taking into account homogeneous broadening. 16

17 Direct measurement of exciton wave functions in a quantum dot A SNOM test in ultimate resolution: Measurement of exciton and biexciton wave function distribution in a single quantum dot. AlGaAs/GaAs monolayer fluctuation QDs. 9K. Optical fibre probe! Resolution ~20 nm. SNOM measurement of exciton (X) and biexciton (XX) wave functions (for three different dots) From Matsuda et al. Phys. Rev. Lett. 91, (2003) 17

18 Short pulse propagation in a waveguide Schematics of time- and phase-resolved SNOM set-up. Time-resolved heterodyne interference measurement of pulse propagation in a waveguide (TE 00 mode). a) morphology, b) from lock-in, c) amplitude, d), e) phase. 1.3 μm pulses, 170/1150 nm Si 3 N 4 /SiO 2 From Gersen et al., PRE 68, (2003) 18

19 Short pulse propagation in a waveguide phase and group velocities 262 μm a) scan along the waveguide, measurement amplitude x cos(phase). b) zoomed oscillations, period in waveguide phase velocity Amplitude for 200 fs steps. Group velocity. v ph m/s, v gr m/s 19

20 Short pulse propagation in a PhC waveguide TE00 TE01 Excited different spatial modes. Pulse splitting into different modes. Different v ph and v gr for different modes. Allows experimental determination of PhC band structure. From Gersen et al. PRL 94, (2005). 20