Scanning Near Field Optical Microscopy: Principle, Instrumentation and Applications

Saulius Marcinkevičius

Optics, ICT, KTH

1

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

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

Optical near fields

1. 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

1

2

3

The machine, the probe, the feedback

Shear force controlof the sample-to-probedistance

Metal-coatedthinned fibreprobe

Instrument schematics

5

Basic modes of operation

a) Collection modeb) Illumination mode

Transmission, luminescence, scattering,refraction

Illumination/ collectionmodeTransmission, reflection,luminescence, scattering

Apertureless SNOM

6

• Heating• Pulling• Breaking

Final shape depends on: pulling strength and symmetry, temperature during breaking.Often – sharp tips.

Tapered fiber tips: Heating & pulling

Different core profiles in etched and pulledprobes

7

Lazarev et al. Review of Scientific Instruments 74, 3679 (2003)

Tapered fiber tips: Meniscus etching

Final shape depends on:• acid concentration,• buffer solution concentration,• temperature,• etching time,• nature of the material (doping).

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 – 20 -50 % HF, oil – silicon, toluene.

Fig. 4. Schematic illustrations of meniscus etching of a fiber at (a) the start, (b) tapering and (c) stop

8

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, n1and n2 are the refractive indices of the core and clad, respectively; r1 and r2 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 R1 and R2 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

Distance control

sample

segmentedphotodiode

excitationlaser

cantilever

beamsplitter

feedbacklaser

Tuning fork

Optical

Morphology – throughprobe feedback (like AFM)

10

Example: modes of a VCSEL

www.witec.de

GaAs/AlGaAs QW VCSELperpendicular polarizations

spectrally-integrated

fundamental mode

transverse mode at832.0 nm

transverse mode at831.5 nm

11

Examples of SNOM imaging – refraction, reflection

Contrast in near field image due to different refractiveindices is induced by different coupling of the near field(coupling depends on polarization, scanning direction,angle of incidence).

PPMA resist film

12

Magneto-optic materials with domains (magnetisationup or down), Bi-doped yttrium iron garnet Y3Fe5O12 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) inEr-doped fibre. PL – Er spreading into cladding.

Examples of SNOM imaging – polarization, luminescence

13

Spectrally-resolved scan, deep UV AlGaN QW LED

14

AlGaN inhomogeneities measured by SNOM

Al0.42Ga0.58N

• 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 thanfor 30% Al layer.

• In some places – a cleardual peak shape.

morphology

260 270 280 290 300 310-5

0

5

10

15

20

25

30

35

Wavelength, nm

Inte

nsity

, a.u

.

15

Dual localization potential

• 100 nm – few µm scale localization: SNOM• Localization <100 nm: fitting PL spectra and

taking into account homogeneous broadening.

16

Direct measurement of exciton wave functions in a quantum dot

SNOM measurement of exciton (X) and biexciton (XX)wave functions (for three different dots)From Matsuda et al. Phys. Rev. Lett. 91, 177401 (2003)

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.

17

Short pulse propagation in a waveguide

Time-resolved heterodyne interference measurement of pulse propagation in a waveguide (TE00 mode). a) morphology, b) from lock-in, c) amplitude, d), e) phase. 1.3 μm pulses, 170/1150 nm Si3N4/SiO2Schematics of time- and phase-resolved

SNOM set-up. From Gersen et al., PRE 68, 026624 (2003)

18

Short pulse propagation in a waveguide – phase and group velocities

Amplitude for 200 fs steps.Group velocity. a) scan along the waveguide, measurement –

amplitude x cos(phase). b) zoomed oscillations, period in waveguide → phase velocity vph 2.01108 m/s, vgr 1.56108 m/s

262 μm

19

Short pulse propagation in a PhC waveguide

Excited different spatial modes.Pulse splitting into different modes.

From Gersen et al. PRL 94, 123901 (2005).

Different vph and vgr for different modes.Allows experimental determination of PhCband structure.

TE00

TE01

20