tip-enhanced spectroscopy and electron generation

Tip-Enhanced Spectroscopy and Electron Generation

Thomas ElsaesserMax-Born-Institute for Nonlinear Optics and Short-Pulse Spectroscopy,

D-12489 Berlin, Germany, http://www.mbi-berlin.de

• Sub-wavelength optics and nanoplasmonics

• Light scattering from metal nanotips

• Tip-enhanced spectroscopies

• Generation of ultrashort electron pulses

Tutorial Sonderforschungsbereich 658

Coauthors

Markus Raschke David Solli

Catalin Neacsu UC Los Angeles

Markus Breusing

Claus Ropers Dong Ha Kim

Christoph Lienau Wolfgang Knoll

Max-Born-Institute MPI Polymer Research Mainz

Optical spectroscopy allows for generation and observation of elementary excitations with well-defined properties.

Optics on a nanometer = sub-wavelength length scale for

• 'imaging' single nanoobjects

• studying spatio-temporal processes.

Ensemble of nanosystems: size fluctuations result in inhomogeneous broadening.

Motivation: Why optics on a sub-wavelength scale ?

Far-field microscopy:resolution Δx=0.61λ/NA >λ/5

Near-field microscopy:resolution λ/50 - λ/10

Optical Methods with Sub-Wavelength Spatial Resolution

Aperture near-field probes Apertureless scattering

scattered light

illumination

excitation

detection

100 nm aperture r=20 nm Au tip

Spatial resolution 100 nm 10 nmTime resolution 100 fs - cw 10 fs - cwSpectral resolution (cw) 30 µeV several meV

Sample temperatures 10 - 300 K 300 K

Other methods: Illumination of metallic slit and hole masks ('nanoplasmonics'),Stimulated emission depletion microscope (STED)

Optical Methods with Sub-Wavelength Spatial Resolution

Aperture near-field probes

excitation

detection

100 nm aperture

Spatial resolution 100 nmTime resolution 100 fs - cwSpectral resolution (cw) 30 µeV

Sample temperatures 10 - 300 K

Spectroscopy of a semiconductorquantum dot

0 1 2 3 4

X'

0 1 2X' (μm )

-8 -4 0 4 8-3.0

-2.8

-2.6

-2.4

Cross -Correlation

Delay Time (ps)

log(

|ΔR

/R|)

-10 -5 0 5 10Energy (meV)

Near-field PL

T. Guenther et al., Phys. Rev. Lett. 89, 057401 (2002)T. Unold et al., Phys. Rev. Lett. 94, 137404 (2005)

Light Scattering from Metallic Nanotips

a

r=10-20 nm

Scatterer dimension a=2r << λ(Mie scattering)

Electric field enhancement for field vector parallel to tip axis:

- lightning rod effect (field singularity at tip apex)

- surface plasmon excitation

Field enhancement factor 10 – 100

E E

Dielectric function of bulk metals:

Drude model (independent electrons)

Lindhard: interacting electrons, no collisions

)(1

)(1)( 2

02

02

20

2

2

0

2

γωωγω

γωω

γωωω

ωε+

++

−=+

−= ppp ii

∫ +−+

−+= +

ωππωε

hrvrrrr

)()(441),( 3

3

2

2

kEqkE

ffkdqeq kqk


a

r=10-20 nm

Scatterer dimension a=2r << λ Numerically calculated intensity distribution(Mie scattering)

Electric field enhancement for field vector parallel to tip axis:

- lightning rod effect (field singularity at tip apex)

- surface plasmon excitation

Field enhancement factor 10 – 50 L. Novotny et al., Phys. Rev. Lett. 79, 645 (1997),Annu. Rev. Phys. Chem. 57, 303 (2006)

E E


a

r=10-20 nm

Scatterer dimension a=2r << λ Induced surface charge density(Mie scattering)

Surface plasmon excitation:

- Calculate induced surface charge density and electric field from Maxwell‘sequations by multiple multipole method (MMP)

- Standing wave of oscillating surface charge with wavelength smaller thanoptical wavelength

L. Novotny et al., Phys. Rev. Lett. 79, 645 (1997)

E E


a

r=10-20 nm

Au tip Au sphereAnalytical model: consider illuminated sphere of diameter a (linear response):

λlight=810 nm, Au sphere r0=10 nm: ε=-24.9+1.57i , fe=-2.9+11.8i

E

)(00

0000

)( ωα

αα

ω light

par

pp

pp

Eprr

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

=

)(8

2)(1)(4

300

300

ωπεα

ωεωεπεα

epar

pp

fr

r

=

+−

=


a

r=10-20 nm

Au nanoparticle (a=80 nm)Analytical model: consider illuminated sphere of diameter a (linear response):

E

)(00

0000

)( ωα

αα

ω light

par

pp

pp

Eprr

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

=

)(8

2))(Re(,:

2)(1)(4

300

*0

2

300

ωπεα

ωεε

ω

ωεωεπεα

epar

pp

pp

fr

mNeplasmon

r

=

−==

+−

=


a

r=10-20 nm

E

Au nanoparticle (a=80 nm)Analytical model: consider illuminated sphere of diameter a (linear response):

Total electric field:

free space dyadic Green‘s function

Lit.: C. Hafner: The generalized multiple multipole technique for computational electromagnetics,Boston 1990, Artech.

:),,(

)(),,(1),(),(

00

00

2

2

00

ω

ωωωε

ωω

rrG

prrGc

rErE

rrt

rrrtrrrr+=

Plasmon Propagation on NanotipsFor direct illumination of tip apex also sample illuminated.Alternative approach: Illuminate tip shaft and propagate surface plasmon.

C. Ropers et al., Nanoletters 7, 2784 (2007)

Apertureless Near-Field Microscopy

Sample in the vicinity of tip apex changes scattered electric field.Images with a spatial resolution of ~a by scanning tip over sample surface.

Image dipole model

[ ]

))0(032/(1

)1(

1

1

))0(016/(1

)1(

3,

3,00 ,

rz

rz

pp

ppppeff

sample

sample

par

parpareffeffimage EEEp

+−

−

+

−

+−

+

==

=≡+=

πεβα

βα

ε

ε

πεβα

βα

αβ

αααrrrv

2r0

εsample

For constant z and r0: scattered field maps εsample (plus topography)

Experimental Setup

100 nm

W

0.2 mm

Au (r=20 nm)

Detection Scheme

I scatter Et0 Es Ets2

I Es Et0 Etsexp i t 2

Ets

z-modulation:

FT

R. Hillenbrand, F. Keilmann, Phys. Rev. Lett. 85, 3029 (2000)M. Raschke, C. Lienau, Appl. Phys. Lett. 83, 5089 (2003)

Detection Scheme


M. Raschke, C. Lienau, Appl. Phys. Lett. 83, 5089 (2003)

W tip, λ=633 nm

Detection Scheme


M. Raschke, C. Lienau, Appl. Phys. Lett. 83, 5089 (2003)

W tipsλ=633 nm

r 12 nm

r 20 nm

r 5 nm

Chemical composition and structure on the nano-scale.

Raman scattering:

Field enhancement at tip apex enhances Ramanscattered field (plasmon resonance).

In addition possible: enhancement of αij by electronicresonance of sample.

Detection of Stokes and/or anti-Stokes component.

Infrared absorption:

Vibrational resonances contribute to εsample.

Requires (tunable) infrared light.

Nonlinear and time-resolved Raman and infrared spectroscopies possible.

Stokes

Tip-Enhanced Vibrational Spectroscopy

ωLaserω

v=0

v=1

v=2

VibLaserStokes ωωω −=

n

N

n n

ijijij q

qEP

0

63

1)0(, ∑

−

=⎥⎦

⎤⎢⎣

⎡∂

∂+==

αααα

rr

n

N

n na q

qµµµiref

0

63

1

2)0(, ∑

−

=⎥⎦

⎤⎢⎣

⎡∂∂

+=∝rrσ

Single-walled carbon nanotubes (SWN) on glass.

Tip-Enhanced Raman Spectroscopy

StokesωLaserω

Topography 1594 cm-1 vibration

resolution

L. Novotny et al.

Single walled carbon nanotubes (SWN) on glass.

Tip-Enhanced Raman Spectroscopy

StokesωLaserω

Topography 1594 cm-1 vibration

resolution

L. Novotny et al.

Raschke et al.: malachite green (MG)

Polystyrene – b – Polyvinylpyridine film: λ=3,39 µm (CH stretch vibration)

Topography: Microscopy

Tip-Enhanced Infrared Spectroscopy

s-SNOM:

vis 632nm

M. Raschke et al., ChemPhysChem. 6, 2197 (2005)

Tip-Enhanced Infrared Spectroscopy

M. Raschke et al., ChemPhysChem. 6, 2197 (2005)

Other Tip-Enhanced Spectroscopies

Local photoluminescence emission

PL, λ=950 nm Raman 1594 cm-1

A. Hartschuh et al., Nanoletters 5, 2310 (2005)

Variation of spectra along nanotube

Also demonstrated: second harmonic spectroscopy

Ultrashort Electron Pulses from Metallic Nano-TipsPhotoelectrons from metallic nano-tip (tip radius 20 nm) with 7 fs pulses(100 pJ, νrep=80 MHz).

field enhancement ~10

Setup:

scattered 800 nm light

2nd harmonic (400 nm)

electrons

C. Ropers et al, Phys. Rev. Lett. 98, 043907 (2007)


field enhancement ~10

Setup:


2nd harmonic (400 nm)

electrons

With increasing bias: 4-photon electron emission (sub-20 fs) → electron tunneling



electrons

Imaging of nanostructures via change ofelectron yield (local field enhancement)

C. Ropers et al, Phys. Rev. Lett. 98, 043907 (2007)

Conclusions

Apertureless tip-enhanced microscopies allow for optical experimentswith a 10 nm spatial resolution.

Variety of linear and nonlinear spectroscopies to study electronic andvibrational excitations.

Sensitivity up to the single-molecule level.

‚Point-like‘ source of femtosecond electron pulses by multiphoton excitationof metallic tips.

Future developments:

Time-resolved spectroscopies, in particular in the ultrafast time domain.

Electron scattering and diffraction experiments.

tip-enhanced spectroscopy and electron generation

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