ultrafast microscopy mircea vomir · 2013-02-26 · atomic force microscopy (afm) •3d image...
TRANSCRIPT
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
Advantages and disadvantages
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
Advantages and disadvantages
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
Scanning Near Field Optical Microscopy (SNOM)
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
Scanning Near Field Optical Microscopy (SNOM)
• 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)
Advantages and disadvantages
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
air, gases, liquids, dif. Temperatures
512 x 512
AFMAtomic Force Microscope
L 1nmH 0.1 nm
atomic forces attraction/repulsion, magnetic (MFM), friction
air, gases, liquids, dif. Temperatures
512 x 512
Outline
•Microscopy
•Ultrafast magnetism
•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 magnetism
•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
A. Laraoui, M. Albrecht, J.-Y. Bigot. Opt. lett. 32, 936-398 (2007)
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
A. Laraoui, M. Albrecht, J.-Y. Bigot. Opt. lett. 32, 936-398 (2007)
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 !