ultrafast microscopy mircea vomir · 2013-02-26 · atomic force microscopy (afm) •3d image...

Institute of Physics and Chemistry of Materials Strasbourg

Department of Ultrafast Optics and Nanophotonics (DON)

Ultrafast Microscopy Mircea Vomir

Outline

•Microscopy

•Ultrafast magnetism

•Ultrafast magnetic microscopy• Collaboration (EWHA - IPCMS) between the

• groups of Jeong Weon Wu• groups of Jean-Yves Bigot

The basics of vision : The eye

Limit of the eyes field of vision

P-A. Buffat, EPFL Lausanne

at 25 cm: = 1°, AB (object) = 0.1 mm

Eye “accommodates” between +∞ and 25 cm

eyelens

cornea

retinalfovea

For detecting further details : The Magnifying glass

Starting point of the microscopy :Studying objects invisibles for the eye

F F

f

A

B

B’

p

q

A’

• consists of a convergent lens• the object AB is placed betweeen the lens and F so that : q = • (eye) = 25cm : the minimum distance for a sharp vision• since p f :

The magnification M= q/p /f

Microscope = mikros (small) + skopein (observer)

Antiquity: first etch of convex lenses

XII-XIIIth centuries: magnification

power of convex lenses,

magnifier, glasses

1590 Janssen, first composed microscope

1665 Hooke: first cell imageTita

n™

2010 …

real and virtual image of an object

F F

f

A

B

B’

p

q

A’

F

F

fA

B

B’

p q

A’

imag

e p

lan

e(

real

)

ob

ject

pla

ne

imag

e p

lan

e(

imag

inar

y )

ob

ject

pla

ne

Magnifying more: optical microscope

A

BF

A’

B’

f

F

L

F’

f’

F’

B’’

A’’objective eyepiece

MOb= L / f Mep= / f’M = MOb/ Mep = L / f f’ total magnification

Optical aberrations

correction withcylindrical lenses

correction with a combination of convergent and divergent lenses

Corrected using achromatic and apochromatic lenses

www.olympusmicro.com

Optical microscope

diffraction by an aperture:Smaller the aperture – larger the effect

Airy pattern

• Uses the visible electromagnetic radiation, UV, IR• Glass lenses• Transmission or reflection geometries• Relatively small magnification in classical configurations

• Performance : resolving power (the ability to distinguish fine details)• Limited (not only by optical aberrations)• Origin : diffraction by a finite aperture

Rayleigh criterionPerfect lens:

2 objects

P-A. Buffat

dD=1.22 l/n sin = arcsin(R/L) R/LNA = n sin

particle Wavelength (nm) energie

Photons (visible) 400 – 700 1 – 3 eV

Photons (X rays) 5x10-2 – 1.25 25 keV – 0.1keV

Electrons 10-3 - 3x10-3 1 MeV - 100 keV

Protons or ions ≈ 10-4 ≈ 10 keV

Neutrons ≈ 0,1 ≈ 0.025 eV

Increase resolution decrease λ

Parameters of different types of particles used for microscopy

De Broglie wavelength

For an electron accelerated at 300 kV λe = 2 pm!

eUm

h

vm

h

m

eU

000 2;

2 ll

2

0

0

21

1

2

cm

eUeUm

h

l

Magnetic lenses

• Best solution electromagnets. The field in thecenter of the electromagnet is given by :

Sketch of an electromagnetic lens

BveF

v

B

F

F : forcee : electron chargev : speed B : applied magnetic field

B = 0 N I / L

focal distance approx:

m the electron mass U electrons acceleration potential Bz the axial component of the magnetic field

dzBmU

e

f zz

2

8

1

Lorentz force

• perpendiculaire to B x v

Transmission Electron Microscopy (TEM)

Projection of the back focal plane to the screen - diffraction modeProjection of the intermediate image plane to the screen - image mode

Transmission Electron Tomography core-shell Au/Ag

B)B)A)A) C)C)

Ag / Au

Irregular

“Hexagon”

Ellipse

a)

d)

c)

b)

Bipyramide Au

75°75°

75°75°

compare the morphology of the two constituents

to the surface crystallography

Source: O. Ersen IPCMS

C nanotubes / PtRu nanoparticles

Source: O. Ersen IPCMS

0 1 2 3 4 5 6 7

-20

0

20

40

60

80

100

120

140

160

Interieur du CNT

Nb

of p

art

icle

s

Size (nm)

0 1 2 3 4 5

0

10

20

30

40

50

Nb o

f part

icle

s

Size (nm)

Exterieur

outside

inside

3D resolution better than 1 nm !

Analytical TEM tomography

Average density C N C/N

Source: IPCMS

combining the electron tomography and the energy filtered imaging

Multi-terminal nanotube junctions

Source: F. Banhart IPCMS

connecting carbon nanotubes by electron irradiation

of metal-carbon nanocomposites in the electron microscope

2-terminal 3-terminal 4-terminal

Co

Transmission Electron Microscopy (TEM)

Direct investigation of the structure and composition down to atomic level

• Morphology – surface morphology, structure of small powders…• Crystallography – identification of the crystalline structure, defects, lattice

vibrations• Chemistry – quantitative chemical analysis, determination of the valence

or type of chemical bond• Electronic – excitation and study of surface plasmons• Magnetic – via holography

• time resolution: …

FIG. 4. (Color) Dynamic transmission electron microscope.J. Appl. Phys. 97, 111101 (2005)© 2005 American Institute of Physics

Schematic representation of time-resolved 4D electron tomography.

Not yet with magnetic resolution

Time Resolved Transmission Electron Microscopy

time resolution: 130 fs !

O Kwon, A H Zewail Science 2010;328:1668-1673

Photoemission Electron Microscope (PEEM)

Lawrence Berkeley National Laboratory Advanced Light Source

records electrons emitted from a sample in response to the absorption of ionizing radiationthe image is magnified by a series of magnetic or electrostatic lenses

LEEM - Low Energy Electron Microscopes XPEEM - Xrays PEEM

spatial resolution ~ 10 nm

Magnetic domains in CoPd using TR XPEEM

XMCD amplitude

time resolution of 50 psspatial resolution of 30 nm

T = -300 ps T = 100 ps T = 350 ps

C. Boeglin, O. Ersen, M. Pilard, V. Speisser, F. Kronast, Phys. Rev. B 80 (2009) 035409.

Time Resolved Photoemission Electron Microscope (TRPEEM)

• relatively high resolution (down to 50 nm)• magnetic contrast via XMCD, XMLD• material sensitivity due to resonant absorption• capability to retrieve the spin and orbital angular momentum

• UHV environement• high sensitivity to magnetic fields• time resolution: tens of picoseconds depending on the synchrotron beamline

and of course on the apparatus

Advantages and disadvantages

Lensless imaging of magnetic nanostructures by X-ray spectro-holography

S. Eisebitt, J. Luning, W. F. Schlotter, M. Lorgen, O. Hellwig, W. Eberhardt & J. Stohr, Nature 432, 885–888 (2004)

Lensless imaging of magnetic nanostructures by X-ray spectro-holography

S. Eisebitt, J. Luning, W. F. Schlotter, M. Lorgen, O. Hellwig, W. Eberhardt & J. Stohr, Nature 432, 885–888 (2004)

FFT

Atomic Force Microscopy (AFM)

sample

Piezoelectric motor

Z

-X -Y

Z

-X -Y

tip

feedback

Pie

zoel

ectr

ictu

be

screen

Scan line => AFM image => topology of the sample

sample

Atomic Force Microscopy (AFM)

• 3D image• the samples do not need special treatment • works in all environments (air, liquid, vacuum)• atomic resolution• information on mechanic properties of the surface• the samples do not need to be necessary conductors

• limited image size (50 x 50 µm2)• the tip geometry can induce artifacts• due to the tip geometry, it cannot image very irregular surfaces• long acquisition time : several minutes for one image• time resolution: seconds


Magnetic Force Microscopy (MFM)

N

SS

H

AFM line scan MFM line scan

sample

Software

assisted

lift

xX

Y

Z

AFM line scan MFM line scan

samplesample

Software

assisted

lift

xX

Y

Z

20 to 200nm

Magnetic Force Microscopy (MFM) : A nice example

Nanoscale hysteresis loop of individual Co dots by field-dependent magnetic force microscopy

M. V. Rastei, R. Meckenstock, J. P. Bucher, Appl. Phys. Lett. 87, 222505 (2005)

Magnetic Force Microscopy (MFM)

• 3D image of the magnetic distribution of the sample • image of the magnetic domain structures • works in all environments (air, liquid, vacuum)• resolution of few nanometers• information on magnetization switching

• limited image size (50 x 50 µm2)• the magnetic tip can induce artifacts• relatively longer acquisition time• surface technique ( do not provide information in volume)

•time resolution: seconds


Scanning Tunneling Microscopy (STM)

Piézo-moteur

• tunneling current for distances smaller than 1 nm• tunneling current essentially between the last atom of the tip and an atom from the surface• the tunneling current is an exponential function of the distance (z) between the tip and the surface

J.P. Bucher talk Tuesday

Scanning Tunneling Microscopy (STM)

J.P. Bucher talk on Tuesday

Scanning Near Field Optical Microscopy (SNOM)

support

Piezoelectric motor

Z

-X -Y

Z

-X -Y

sample

tip

Feedback loop

Pie

zo

ele

ctr

ic tu

be

screendetector

or

Evanescent

wave

2 methods for the detection = 2 light pathways


1

2

l

f

screen

D > d

• propagation distance at the interface

1sin2 1

22

1

l

nd

n1 sin1 = n2

Total reflection

Wave vector

1

22

1

2

2z

1122x

sink

sinsink

nn

nn

c

cc

),( yxtrans kkk

1

2

l

f

screen

D < d

z

x

y


• 3D image of the magnetic distribution of the sample• works in all environments (air, liquid, vacuum)• higher resolution than classical optical microscopy (~10 nm)

• limited image size (50 x 50 µm2)• long acquisition time• surface technique ( do not provide information in volume)• problems on maintaining a good polarization in the tip => magnetic contrast ?

•time resolution: can function with femtosecond laser pulses ! (dispersion in the fiber has to be carefully managed)


Confocal Microscopy

www.olympusmicro.com

• 3D image• high magnetic contrast• image of the magnetic domain structures • works in all environments (air, liquid, vacuum)• resolution 200 - 500 nanometers for the visible

• limited image size (100 x 100 µm2)• relatively small spatial resolution compared to other techniques

• time resolution: tenths to hundreds of femtoseconds

Which microscope to choose?

J.P. Eberhart, Analyse structurale et chimique des matériaux, Dunod (1989)

Energy (eV)

Wav

ele

ng

th (

m)

primary radiation

Matter modification

Radiation modification

Secondary radiation

Applications

X RAYS

Electrons vibrations

e reculdefectsexcitation of atomic levels

elastic scatt.inelastic scatt.Absorption

photoelectron

X ray diffractionCompton spectr.Absorption spectr.(XAS,EXAFS)Photoelectron spectr.(XPS,ESCA)

relaxationCharacteristic X raysAuger electrons

X fluorescence(XRF)Auger spectr.(ESCA)

ELECTRONS

thermal vibr. Plasmons excit.brake bondsatoms displ.atomic levels excitation

elastic scatt.inelastic scatt.absorptionenergy loss

X rayssecondary electrons

Electron diffractionX source(MEB)e energy-loss spectr.(EELS)

Relaxation

Characteristic X rays

Auger electrons

microanal. X(EPMA,EDX)X sourceAuger spectroscopy(AES)

NEUTRONSthermal vibr.brake bondsatoms displ.

elastic scatt.inelastic scatt.Absorption

Neutron diffraction

IONS

thermal vibr.brake bondsscatteringions implantationatomic levels excitation

scattering

absorption

secondary ions

Structures

Secondary ions spectroscopy(SIMS)

implantation

Relaxation characteristic rays

microscope resolution interaction / matter environment pixels /usual image

Optic (photonic) ~ 500 nm(< 50 nm hyper focalization)

Electromagnetic radiation / all kinds of samples

air, gases, liquids, dif. Temperatures

3000 x 3000

SEMScanning Electrron Microscope

~ 10 nm electrostatic, magnetic, electronic / binding energy, ionization,atomic lattice

vacuum (gas < 50 mbar)

1000 x 1000

TEMTransmission Electron Microscope

0.2 nm electrostatic, magnetic, electronic / binding energy, ionization,atomic lattice, diffraction

vacuum(no liquids or gases)

4000 x 3000

STMScanning Tunneling Microscope

L 0.2 nmH 0.01 nm

tunneling effect , density of states


512 x 512

AFMAtomic Force Microscope

L 1nmH 0.1 nm

atomic forces attraction/repulsion, magnetic (MFM), friction


512 x 512

Outline

•Microscopy


•Ultrafast magnetic microscopy

Time scales of the magnetization dynamics

Spin-orbit coupling ; coulomb interactions

Collisions spins – conduction electrons

Coupling with the photons: TeraHertz emission

(Gilbert)dt

dMM

M

(Bloch)

-dt

dM

S

t

M

rel

effHM

dt

dM g

effH

M

H

Outline

•Microscopy


•Ultrafast magnetic microscopy

Spatio-temporal scales of the magnetization dynamics

density

speed

Terabit/inch2

TeraHertz

Time Resolved Magneto-Optical Microscope

20

-2

-2 0 2m

A. Laraoui, M. Albrecht, J.-Y. Bigot. Opt. lett. 32, 936-398 (2007)

Time Resolved Magneto-Optical Microscope

.

Pinhole

(20 m)

Dichroic

Beam splitter

y

x

Sample

Focal planeScanning piezo

Probe (400 nm)

Pump (800 nm)

Polarizer

Magnet (± 0.4T)

Objective lens:

N.A = 0.65 (x 40)

5kHz amplified Ti:S

Pulse duration ~120 fs

PM

F

H

l/2

PM

Polarization bridge

(400 nm)

F

Analyzer

R= 5x10- 4 R


Magneto-Optical image of a CoPt3

dot of diameter D = 500 nm

Magnetization dynamics of an individual dot of CoPt3 (D = 1 µm)

Linear dependence of the spin-lattice relaxation as a

function of the laser intensity: Increase of the electronic

specific heat with increasing the electron temperature:

two temperatures model


6

4

2t e

(spin

)-l

108642IP (mJ.cm

-2)

IP= 8 mJ.cm-2

H = ±4 kOe-0.5

0.0

DM

/M

20100Delay (ps)

te(spin)-l = 5.2 ps

1.00.50.0

Delay (ns)

tDiff =630 ps

Magnetization dynamics of an individual dot of CoPt3 (D = 1 µm)

Time resolved magneto-optical imaging -> Magnetization dynamics of a dot of CoPt3

Spatio-temporal dynamics of a single CoPt3 dot

Spatial expansion heat diffusion to the environment

Magnetization dynamics of an individual dot of Py (L = 30 µm)

A. Laraoui, J. Vénuat, V. Halté, M. Albrecht, E. Beaurepaire, J.-Y. Bigot. J. Appl. Phys. 101, 10C105 (2007)

The precession frequency decrease when increasing the laser intensity : decrease of the amplitude

of the effective field via a decrease of the demagnetizing field

eff e l

0

( ) ( )( ) H (T ( ), T ( ), ( ))

( ) ( ) ( )

relax

eff demag anis

d t d tt t t t

dt dt

H t H H t H t

g

M MM M

5.5

5.0

4.5

4.0

Fré

qu

ence

(G

Hz)

8642

IP (mJ.cm-2

)

-1.0

-0.5

0.0

DM

/M

1.00.50.0

Retard (ns)

IP1 = 4 mJ.cm-2

IP2 = 8 mJ.cm-2

5.5

5.0

4.5

4.0

Fré

qu

ence

(G

Hz)

8642

IP (mJ.cm-2

)

-1.0

-0.5

0.0

DM

/M

1.00.50.0

Retard (ns)

IP1 = 4 mJ.cm-2

IP2 = 8 mJ.cm-2

Spin Photonics : manipulating the spins with the laser pulses

What about the time or the smallest dimension ?

Ideally, it should be great to do it as small as few nanometers -> Terabits/inch2

as fast as few femtoseconds -> TeraHertz

CoPt3/Al2O3

Magneto-Optical Pump-Probe Imaging (MOPPI)

Read the written dots with the pump-probe signal for low (≤ 1 mJ.cm-2 ) intensities

of the pump for a fixed delay between the pump and probe

MOPPI image of a magnetic domain D=900 nm

written on a CoPt3/Al2O3 film.

Iread = 1 mJ.cm-2 ; t = 300 fs

Advantages of the MOPPI technique :

- Differential imaging with the modulation of the pump beam : better signal to noise ratio

- Time resolved imaging : read the information at different temporal delaysD dyyxMContrast ),,(t

3 *( , ) ( , ) P P SM r r t dtt tD E E E

-0.2

0.0

DM

/M

1050

Delay (ps)

Reading magnetic domains on a CoPt3 film

Magneto-Optic Imaging for different delays on a CoPt3/Al2O3

Iread = 1 mJ.cm-2 for H = 0

300 fs 2 ps 10 ps

-4 -2 0 2 4

-1

0

1

DM

/M

H (kOe)

Magnetization reversal induced by laser pulses on individual CoPt3 dot

Control the magnetization of a CoPt3 dot using the combination of two

parameters : laser intensity and magnetic field

-0.2

0.0

0.2

DM

/|M|

21

0

210

21

0

210

21

0

210

H = -50 Oe

IP > 6 mJ cm-2

21

0

210 µm

H = 0 Oe

IP = 1 mJ cm-2

H = 0 Oe

IP > 6 mJ cm-2

H = -2 kOe

IP = 3 mJ cm-2

Magnetization reversal induced by laser pulses on CoPt films

-6 -3 0 3 6

-1

0

1

M/M

S

H (kOe)

CoPt3 (15 nm Alloy)/Al2O3 :

Hc = 2.8 kOe(Co0.5nmPt1nm)x8 Multilayer /glass :

Hc = 0.38 kOe

Magnetization reversal induced by laser pulses on CoPt3 film grown on Al2O3

Re-switch towards initial

state for IP = 8 mJ.cm-2 and

H = + 50 Oe.

Local demagnetization for

IP = 8 mJ.cm-2 at

H = 0 Oe

Reversal of M for

IP = 8 mJ.cm-2 at

H = - 50 Oe.

Magnetization reversal induced by laser pulses for H = 0

Influence of the substrate granularity – crystalline structure –

local dipolar magnetic field?

film (Co0.5nmPt1nm)8/glass (multilayer)

Laser intensity : 4 respectively 5 mJ/cm2

MFM study of the spatial laser induced demagnetization

1.8 m0.9 m 1.8 m0.9 m

0.5 m0.5 m 0.5 m0.5 m

CoPt3/Sapphire

CoPt/glass

IP = 4 mJ.cm-2 IP = 8 mJ.cm-2

Domains structure

Davg = 250 nm

Domains structure

Davg = 175 nmSingle domain

Davg = 350 nm

Summary

• Femtosecond confocal Kerr microscopy is a powerful technique

– Magnetization dynamics from femto to nanoseconds with resolutions of 150 fs and 300 nm

– Ultrafast magnetization dynamics on a single ferromagnetic nanostructures

• Spin Photonics

– Writing and reading magnetic domains on ferromagnetic films and dots

– New imaging technique:

– Magneto-Optical Pump-Probe Imaging (MOPPI)

Dream: development of higher resolution techniques• Femtosecond time resolved

– TEM holography using femtosecond electron bunches– SPM (SNOM, MFM,STM using optical switches, scaterring)– XPEEM (using femtoslicing, linear accelerators, tabletop x-ray sources)

Thank you for your attention !

ultrafast microscopy mircea vomir · 2013-02-26 · atomic force microscopy (afm) •3d image...

Documents