Diapositiva 1

1
Dynamics of Non-Equilibrium States in
Solids Induced by Ultrashort Coherent Pulses
Claudio Giannetti
INFM and Università Cattolica del Sacro
Cuore Dipartimento di Matematica e Fisica, Via
Musei 41, Brescia.
2
Introduction
High-Intensity femtosecond coherent pulses ?
Investigation of Photoinduced non-equilibrium
states in Solid State Physics
sample
Spectrometer
Photoemission
Aim OPTICAL CONTROL OF ELECTRON INTERACTIONS AND
PHASE TRANSITIONS IN SPECIFIC SYSTEMS
3
Introduction
High-Intensity femtosecond coherent pulses ?
Investigation of Photoinduced non-equilibrium
states in solids
pump
Photoemission
sample
Spectrometer
probe
4
Introduction
High-Intensity femtosecond coherent pulses ?
Investigation of Photoinduced non-equilibrium
states in solids

• Time-resolved non-linear photoemission on METALS.
  
• W.S. Fann et al., Phys. Rev. Lett. 68, 2834
  (1992)
• U. Höfer et al., Science 277, 1480 (1997)
• G. Ferrini et al., Phys. Rev. Lett. 92, 2668021
  (2004)
• Structural and electronic phase transitions in
  SOLIDS and MOLECULAR CRYSTALS.
• A. Cavalleri et al., Phys. Rev. Lett. 87,
  2374011 (2001)
• E. Collet et al., Science 300, 612 (2003)

5
Introduction
OPTICAL CONTROL OF ELECTRON INTERACTIONS AND
PHASE TRANSITIONS IN TWO SPECIFIC SYSTEMS

• Image Potential States on Ag(100)
• By selecting the excitation photon energy it is
  possible to investigate the properties of IPS in
  different regimes.
• Insulator-Metal phase transition of VO2
• By selecting the excitation photon energy it is
  possible to clarify the physical mechanisms
  responsible for the photoinduced phase-transition.

6
IPS on Ag(100)
IMAGE-POTENTIAL STATES (IPS)
IPS 2-dim electron gas in the forbidden gap
of bulk states
Image Potential
Eigenvalues

• Ry Rydberg-like
• Constant
• n1, 2,
• m electron effective mass

P.M. Echenique et al., Surf. Sci. Rep. 52, 219
(2004).
7
IPS on Ag(100)
MEASUREMENTS on IPS

• Relaxation dynamics
• IPS effective mass

Important test for many-body theories (GW)
Electron self-energy
Electron Green function
Screened interaction potential
damping G 1/t ImS Effective mass ?ok
ReSh2k2/2m
Quasi-particle Energy spectrum
8
IPS on Ag(100)
EXPERIMENTAL SET-UP
Energy resolution 10 meV _at_ 2eV
4th 4.2eV
2nd 2.1eV
9
IPS on Ag(100)
NON-LINEAR PHOTOEMISSION on IPS
Ekin h? - En
t h / G
Population of empty states via resonant 2-photon
photoemission
Phys. Rev. B 67, 235407 (2003)
10
IPS on Ag(100)
ANGLE-RESOLVED PHOTOEMISSION on IPS
m/m0.97?0.02 in agreement with calculated
values ? 2-dimensional free electron gas
Phys. Rev. B 67, 235407 (2003)
11
Non-Equilibrium Electron Distribution
NON-LINEAR PHOTOEMISSION on METALS
when h? lt F a non-equilibrium electron population
is excited in the s-p bands of Ag

• investigation of the non-equilibrium electron
  distribution
• ?
• Excitation mechanisms
• Relaxation dynamics
• Photoemission processes

12
Non-Equilibrium Electron Distribution
PHOTON ABSORPTION MECHANISMS PROBLEMS
The intraband transition between s-s states
within the same branch is FORBIDDEN for the
conservation of the momentum.

• Recently the excitation mechanism has been
  attributed to
• Laser quanta absorption in electron collisions
  with phonons.
• A.V. Lugovskoy and I. Bray, Phys. Rev. B 60,
  3279 (1999)
• Photon absorption in electron-ion collisions.
• B. Rethfeld et al., Phys. Rev. B 65, 2143031
  (2002)

THE ENERGY ABSORPTION IS DUE TO A THREE-BODY
PROCESS AND NOT TO A DIPOLE TRANSITION
13
Non-Equilibrium Electron Distribution
NON-LINEAR PHOTOEMISSION on Ag
The excitation of a non-equilibrium electron
population results in a high-energy electron
tail E gt nh?-F
h?3.14eV
Occupied states
n1 IPS
Log Scale 106 sensitivity
h?
2-Photon Photoemission with p-polarized light
Non-equilibrium Distribution
Iabs13 µJ/cm2
14
Non-Equilibrium Electron Distribution
We exclude

• Coherent 3-photon photoemission

• Direct 3-photon photoemission

• ?
• Scattering-mediated transition

The high-energy electron tail is a fingerprint of
the non-equilibrium electron distribution at k?0
submitted to Phys. Rev. B
15
Non-Equilibrium Electron Distribution
NON-EQUILIBRIUM ELECTRON DYNAMICS RESULTS
Time-Resolved Photoemission Spectroscopy
Photemitted charge autocorrelation of different
energy regions
The Relaxation Time of the high-energy region is
tlt150 fs
Fermi-liquid
submitted to Phys. Rev. B
16
Non-Equilibrium Electron Distribution
ENERGY TRANSFER non-equilibrium
electrons ? Equilibrium distribution
Two-temperature model
The heating of the equilibrium distribution can
be neglected
submitted to Phys. Rev. B
17
IPS as a Probe of Non-Equilibrium Distribution
IPS INTERACTING WITH NON-EQUILIBRIUM ELECTRON
DISTRIBUTION

• h? 4.28 eV
• gt En-EF
• RESONANCE
• Iinc 30 µJ/cm2
• 90 d?d
• ?e 21018 cm-3

• h? 3.14 eV
• lt En-EF
• NO DIRECT
• POPULATION
• Iinc 300 µJ/cm2
• 0 d?d
• ?e 1020 cm-3

when h? 3.14 eV a high-density non-equilibrium
electron distribution cohexists with electrons on
IPS
Phys. Rev. Lett 92, 2568021 (2004)
18
IPS as a Probe of Non-Equilibrium Distribution
IMAGE POTENTIAL STATE
Ag(100)
Ekin h?-Ebin Ebin ? 0.5 eV
n1
K0
Shifting with photon energy ?h?0.39eV
Phys. Rev. Lett 92, 2568021 (2004)
19
IPS as a Probe of Non-Equilibrium Distribution
ELECTRIC DIPOLE SELECTION RULES RESULTS
Expected dipole selection rules J0 in
S-pol J?0 in P-pol
Dipole selection rules
Violated in non-resonant case
Respected in resonant case
Phys. Rev. Lett 92, 2568021 (2004)
20
IPS as a Probe of Non-Equilibrium Distribution
IPS EFFECTIVE MASS
s-polarization m/m 0.880.04
p-polarization m/m 0.880.01
2-D electron system interacting with 3-D
electron system
Role of IPS interaction with the non-equilibrium
distribution in W
Phys. Rev. Lett 92, 2568021 (2004)
21
Insulator-Metal Phase Transition in VO2
Insulator-Metal Phase Transition in VO2
Insulator-to-Metal photoinduced phase transition
in VO2
Solid State properties in highly non-equilibrium
regimes
22
Temperature-Driven IMPT in VO2
High-T Rutile phase Conductor
Low-T Monoclinic phase Insulator Egap0.7 eV
Tc340K
3d energy levels
S. Shin et al., Phys. Rev. B 41, 4993 (1990)
23
Origin of the Insulating Band-Gap
Origin of the insulating band-gap
electron-electron correlations in the d
band (Mott-Hubbard insulator) IMPT Dynamics the
electronic structure stabilizes the distorted
Monoclinic phase
minimization of the ground-state lattice
energy (Peierls or band-like insulator) IMPT
Dynamics a phononic mode drives the phase
transition
A comprehensive review M. Imada et al.., Rev.
Mod. Phys. 70, 1039 (1998)
24
Photo-Induced IMPT in VO2
The Insulator-to-Metal phase transition can be
induced by ultrashort coherent pulses. t150
fs h?1.55 eV I10 mJ/cm2 M. Becker et al..,
Appl. Phys. Lett. 65, 1507 (1994)
Questions opened

• It is the same structural and electronic phase
  transition?

• Structural and electronic transitions are
  simultaneous?

• Which is the mechanism driving the highly
• non-equilibrium phase transition?

25
Photo-Induced IMPT in VO2

• It is the same structural and electronic phase
  transition?

Structural YES
Electronic ?
probe h?1.55 eV
structural dynamics t500 fs
electronic dynamics t500 fs
A. Cavalleri et al.., Phys. Rev. Lett. 87,
2374011 (2001)
M. Becker et al.., Appl. Phys. Lett. 65, 1507
(1994)
26
Optical Properties of VO2
_at_ 790 nm ?R/R -20
H. Verleur et al., Phys. Rev. 172, 788 (1968)
DRUDE
Harmonic Oscillator
27
Experimental Set-Up
time-resolved (t150 fs) near-IR (0.5-1 eV)
reflectivity
PUMP PROBE
three-layer sample
28
Film Thickness
Film thickness wide-range CW reflectivity
L1
L2
Best-matching L120 nm L2330 nm
29
Near-IR Reflectivity
0.5-1 eV reflectivity signature of the band-gap
Multi-film calculation
L1
L2
L120 nm L2330 nm
30
Femtosecond Band-Gap Closing
The Insulator-to-Metal phase transition is
induced by 1.57 eV-pulses and probed by 0.54
eV-pulses (under gap)
Signature of Femtosecond band-gap closing
150 fs
31
Photo-Induced IMPT Mechanism

• Which is the mechanism driving the highly
• non-equilibrium phase transition?

• Removal of the d
• electron-electron correlations?
• band-gap collapse and lattice stabilization

p
e-

• Coherent excitation of the phonon responsible of
  the IMPT?
• lattice transition and electronic rearrangment

d
hole - doping
with Ipumpgt10 mJ/cm2 hole-doping 20-100
In this experimental scheme it is not possible to
discriminate!
32
Photo-Induced IMPT Mechanism
Near-IR photoinduction of the phase transition
in the under-gap region the hole-doping is highly
reduced
p
e-
0.7 eV
d
hole - doping
we can discriminate between the two mechanisms
33
Near-IR Photoinduction of the IMPT
ZOOM IMPT completed in 150 fs NO thermal effect
Pump 0.95 eV Probe 1.57 eV-pulses (under gap)
Two dynamics t1200 fs t21000 fs
Metastable metallic phase
34
Near-IR Photoinduction of the IMPT
The Insulator-to-Metal phase transition can be
induced in the under-gap region, through
near-IR pulses (0.5-1 eV)
The pump fluence necessary for the IMPT is about
constant!
35
Near-IR Photoinduction of the IMPT
The pump fluence necessary for the IMPT is about
constant! No role is played by hole doping
Coherent excitation of phonons modes ?
_at_ 2400 nm hole-doping 10 Pin 16 mJ/cm2
_at_ 1300 nm hole-doping 30 Pin 20 mJ/cm2
36
Conclusions
We have demonstrated that selecting a particular
excitation channel

• It is possible to investigate IPS on Ag
  interacting with a photoinduced non equilibrium
  electron distribution

• It is possible to photoinduce the IMPT of VO2 and
  clarify the physical mechanisms responsible for
  the VO2 electronic properties

37
Publications

• G. Ferrini, C. Giannetti, D. Fausti, G.
  Galimberti, M. Peloi, G.P. Banfi and F.
  Parmigiani, Phys. Rev. B 67, 235407 (2003).
• G. Ferrini, C. Giannetti, G. Galimberti, S.
  Pagliara, D. Fausti, F. Banfi and F. Parmigiani,
  Phys. Rev. Lett. 92, 2568021 (2004).
• C. Giannetti, G. Galimberti, S. Pagliara, G.
  Ferrini, F. Banfi, D. Fausti and F. Parmigiani,
  Surf. Sci. 566-568, 502 (2004).
• G. Ferrini, C. Giannetti, S. Pagliara, F. Banfi,
  G. Galimberti and F. Parmigiani,
• in press on J. Electr. Spectrosc. Relat. Phenom.
• F. Banfi, C. Giannetti, G. Ferrini, G.
  Galimberti, S. Pagliara, D. Fausti and F.
  Parmigiani,
• accepted for publication on Phys. Rev. Lett.
• C. Giannetti, S. Pagliara, G. Ferrini, G.
  Galimberti, F. Banfi and F. Parmigiani,
• submitted to Phys. Rev. B.
• E. Pedersoli, F. Banfi, S. Pagliara, G.
  Galimberti, G. Ferrini, C. Giannetti and F.
  Parmigiani,
• in preparation.