The document discusses light-induced real-time electron dynamics in solids. It begins by introducing an effective Hamiltonian approach that accounts for band structure renormalization, charge fluctuations, and electron-hole interactions. It then discusses linear response simulations using plane waves and norm-conserving pseudopotentials. Challenges with different gauges in the presence of non-local operators are also covered. The document concludes by mentioning work on non-linear response, exciton relaxation mechanisms, and alternatives to the Berry phase for calculating polarization in insulators.

1. Light induced real-time
dynamics for electrons
Claudio Attaccalite
CNRS/CINaM, Aix-Marseille Universite (FR)
“Quantum dissipation by swift nuclei
in condensed matter”
2022, Lausanne

2. Light induced real-time
electron dynamics

3. Light induced real-time
electron dynamics

4. Light induced real-time
electron dynamics

5. Non-linear response
P(ω)=P0+χ
(1)
E+χ
(2)
E
2
+O(E
3
)
First experiments on linear-optics
by P. Franken 1961
Ref: Nonlinear Optics and
Spectroscopy
The Nobel Prize in Physics 1981
Nicolaas Bloembergen

6. We introduce an effective Hamiltonian
that contains...
We start from the DFT
(Kohn-Sham) Hamiltonian:
hk
universal, parameter free
approach
1)

7. We introduce an effective Hamiltonian
that contains...
We start from the DFT
(Kohn-Sham) Hamiltonian:
hk
universal, parameter free
approach
1) 2)Renormalization of the band
structure due to correlation (GW)
hk+Δhk

8. We introduce an effective Hamiltonian
that contains...
We start from the DFT
(Kohn-Sham) Hamiltonian:
hk
universal, parameter free
approach
1) 2)Renormalization of the band
structure due to correlation (GW)
Charge fluctuations
(time-dependent Hartree)
3)
hk+Δhk
hk+Δhk+V H [Δρ]

9. We introduce an effective Hamiltonian
that contains...
We start from the DFT
(Kohn-Sham) Hamiltonian:
hk
universal, parameter free
approach
1) 2)
4)
Renormalization of the band
structure due to correlation (GW)
Electron-hole interaction
Charge fluctuations
(time-dependent Hartree)
3)
hk+Δhk
hk+Δhk+V H [Δρ] hk+Δhk+V H [Δρ]+Σsex [Δ γ]

10. Linear response
● Wave-function in plane-waves plus norm-conserving pseudo
● Propagation time about 50 fs, with a time-step 0.01 fs
● Smearing included as non-Hermitian operator -ig (Weisskopf-
Wigner)or in post-processing as in Octopus
● Laser shape: delta function for LR/ sinus for Non-LR

11. Linear response
● Wave-function in plane-waves plus norm-conserving pseudo
● Propagation time about 50 fs, with a time-step 0.01 fs
● Smearing included as non-Hermitian operator -ig (Weisskopf-
Wigner)or in post-processing as in Octopus
● Laser shape: delta function for LR/ sinus for Non-LR

12. The Gauge problem
H =
p2
2 m
+r E+V (r) Length gauge:
H =
1
2 m
( p−e A)2
+V (r) Velocity gauge:

13. The Gauge problem
H =
p2
2 m
+r E+V (r) Length gauge:
H =
1
2 m
( p−e A)2
+V (r) Velocity gauge:
Quantum Mechanics is gauge invariant,
both gauges must give the same results

14. The Gauge problem
H =
p2
2 m
+r E+V (r) Length gauge:
H =
1
2 m
( p−e A)2
+V (r) Velocity gauge:
Quantum Mechanics is gauge invariant,
both gauges must give the same results
… but in real calculations the each gauge choice
has its advantages and disadvantages

15. The Gauge problem
H =
p2
2m
+r E+V (r)+V nl (r ,r ' )
H =
1
2m
( p−e A)
2
+V (r)+V nl (r ,r ' )
In presence of a non-local
operator
these Hamiltonians
Are not equivalent anymore
≠
W. E. Lamb, et al. Phys. Rev. A 36, 2763 (1987)
M. D. Tokman, Phys. Rev. A 79, 053415 (2009)

16. The Gauge problem
H =
p2
2m
+r E+V (r)+V nl (r ,r ' )
H =
1
2m
( p−e A)
2
+V (r)+V nl (r ,r ' )
≠
moral of the story:
non-local potential should be introduce
in length gauge and then transformed as
W. E. Lamb, et al. Phys. Rev. A 36, 2763 (1987)
M. D. Tokman, Phys. Rev. A 79, 053415 (2009)
H =
1
2m
( p−e A)
2
+V (r)+e
i Ar
V nl (r ,r ' )e
−i Ar '
In presence of a non-local
operator
these Hamiltonians
Are not equivalent anymore

17. The Gauge problem
H =
p2
2 m
+r E+V (r)
Length gauge:
H =
1
2 m
( p−e A)2
+V (r)
Velocity gauge:
● Non-local operators can be easily introduced
● The dipole operator <r> is ill-defined in solids
you need a formulation in term of Berry-phase
● Non-local operators acquires a dependence
from the external field
● The momentum operator <p> is well defined
also in solids
In recent years different wrong papers using velocity gauge
have been published (that I will not cite here)

18. In solids the polarization is written in terms
of wave-function phase
King-Smith and Vanderbilt formula
Phys. Rev. B 47, 1651 (1993)
Pα=
2ie
(2π)
3 ∫BZ
d k∑n=1
nb
〈un k∣
∂
∂ kα
∣unk 〉
Berry's connection !!
1) it is a bulk quantity
2) time derivative gives the current
3) reproduces the polarizabilities at all orders
4) is not an Hermitian operator

19. Effective Schrodinger equation

20. Non-linear response of solids
P(ω)=P0+χ
(1)
E+χ
(2)
E
2
+O(E
3
)
Second Harmonic Generation
Design Synthesis
New molecular crystals with non-linear response comparable to
LiNbO3
J. Phys. Chem. C, 126, 7, 3713–3726(2022)
Tetraphenyl Tetrel
Molecular Crystals

21. Non-linear response of solids
P(ω)=P0+χ
(1)
E+χ
(2)
E
2
+O(E
3
)
Second Harmonic Generation
P(ω)=P0+χ
(1)
E+χ
(2)
E
2
+χ
(3)
E
3
Two-photon absorption
Design Synthesis
New molecular crystals with non-linear response comparable to
LiNbO3
J. Phys. Chem. C, 126, 7, 3713–3726(2022)
Tetraphenyl Tetrel
Molecular Crystals
Phys. Rev. B 98, 165126(2019)

22. Intra-exciton spectroscopy (exp)
Phys. Rev. B 102, 201402(R) (2020)

23. Intra-exciton spectroscopy (exp)
C. Poellmann, et al .Nature Materials 14, 889 (2015)
S. Cha, et al. Nature Communications 7, 10768 (2016)
P. Steinleitner et al. Nano Letters 18, 1402 (2018)
P. Merkl,et al. Nature Materials 18, 691 (2019)

24. Intra-exciton spectroscopy (exp)
C. Poellmann, et al .Nature Materials 14, 889 (2015)
S. Cha, et al. Nature Communications 7, 10768 (2016)
P. Steinleitner et al. Nano Letters 18, 1402 (2018)
P. Merkl,et al. Nature Materials 18, 691 (2019)

25. Intra-exciton spectroscopy (exp)
S. Cha, et al. Nature Communications 7, 10768 (2016)
MoS2

26. Δ P(t)=Ppp (t )−Pp(t)
Δ P(ω)=∫tpp
∞
Δ P(t)eiωt−τ(t−tpp)
Induced polarization from the pump
χ(ω)=
Δ P(ω)
Eprobe (ω)
Pump and probe (theory real-time)

27. Pump and probe (theory real-time)
Δ P(t)=Ppp (t )−Pp(t)
Δ P(ω)=∫tpp
∞
Δ P(t)eiωt−τ(t−tpp)
Induced polarization from the pump
χ(ω)=
Δ P(ω)
Eprobe (ω)

28. Pump and probe (theory real-time)
Δ P(t)=Ppp (t )−Pp(t)
Δ P(ω)=∫tpp
∞
Δ P(t)eiωt−τ(t−tpp)
Induced polarization from the pump
χ(ω)=
Δ P(ω)
Eprobe (ω)

29. Pump and probe (linear response)
+ -
=
|λ⟩=∑c v k
Ac v k
λ
|c v k ⟩
μλ ,λ '
α
=⟨λ'|μα
|λ⟩=∑c v k ∑c ' v' k'
Ac v k
λ ∗
Ac ' v ' k'
λ '
⟨c v k|μα
|c ' v' k' ⟩
χα ,β
λ
(ω)=
2
V
N λ ∑λ '
μλ ' λ
α
μλ λ '
β
Eλ '−Eλ−ω+i η
We can get excited states from the solution
of the Bethe-Salpeter Equation (a Casida like equation)
excited states are in the form:
Intra-exciton dipoles
Linear response from an excited state

30. χλ
α ,β
(ω)=
2
V
N λ ∑λ '
μλ ' λ
α
μλ λ '
β
Eλ '−Eλ−ω+i η
Linear response vs real-time
Pump and probe (linear response)
For m we used
velocity gauge
in order to avoid ill-defined
and complicated
intra-bands dipole matrix
elements
D. Sangalli, M. D’Alessandro, C. Attaccalite
https://arxiv.org/abs/2211.12241 (2022)

31. Pump and probe (group theory)

32. Pump and probe (group theory)
E(2x)
Failure of hydrogen model for
localized excitons!!!

33. We need relaxation (for electron/excitons)

34. Exciton-phonon from first principle

35. Phonon-assisted absorption/emission

36. Exciton relaxation
M. Bernardi et al.
Phys. Rev. Lett. 125, 107401 (2020)
E. Malic
Nano Lett. 20, 4, 2849(2020)
Exciton-lifetime
C
C

37. Simple relaxation term in the
Hamiltonian C
Finite temperature
absorption GaN
~
H=H +i Γ
non-Hermitian term
Γi∝ℑΣii
ph
(ω=ϵi)
Derived from the
electron-phonon self-energy
~
ϵi(T)=ϵi+i η(T)

38. Simone Sanna group
University Giessen
Acknowledgments
Davide Sangalli
CNR -Milan
Marco D’Alessandro
CNR - Rome
Thank you for your attention
●
Non-linear response from real-time simulations including
excitonic effect
●
Intra-exciton transition from first-principle
●
How to address relaxation in an efficient way?
Pierre Lechifflart
Aix-Marseille Univ.

39. The Gauge problem
The guage transformation connect the different guages
A2=A1+∇ f
ϕ2=ϕ1−
1
c
∂ f
∂ t
Non-local operators do not commute
with gauge transformation
vector potential
scalar potential
wave-function phase
f (r ,t) Arbitrary scalar
function
Vnl ,Σxc ,ΔGW
,etc .…
ψ2=ψ1 e
−ie f (r, t)/ℏ c

42. Design materials for non-linear optics
with ab-initio codes
Simone Sanna group
Justus Liebig University
Giessen
Design Synthesis Properties
New molecular crystals with non-linear response comparable to
LiNbO3
J. Phys. Chem. C, 126, 7, 3713–3726(2022)
Tetraphenyl Tetrel
Molecular Crystals
KNbO3
/ LiNbO3
/ LiTaO3
Our prediction:
Tunable c2
in
LiNbx
Ta1-x
O3
without affecting c3
Nonlinear optical response of ferroelectric oxides
Phys. Rev. Mat. 6 (6), 065202 (2022)

43. Real-time Green’s functions
Kadanoff-Baym
equations
From Green’s functions to density matrix
Generalized Kadanoff-Baym Ansatz (GKBA)
Correspond to the equilibrium
self-energy

44. Electronic relaxation
3 – Measurement
process
1 – Photo-excitation
process
2 – relaxation towards
quasi-equilibrium
Sangalli, D., & Marini, A.
EPL (Europhysics Letters), 110(4), 47004.(2015)

45. Local-fields and excitonic effects
in h-BN monolayer
IPA
independent particles
+quasi-particle corrections
+time-dependent Hartree (RPA)
+screend Hartree-Fock (excitonic effects)

46. Local-fields and excitonic effects
in h-BN monolayer
IPA IPA + GW
independent particles
+quasi-particle corrections
+time-dependent Hartree (RPA)
+screend Hartree-Fock (excitonic effects)

47. Local-fields and excitonic effects
in h-BN monolayer
IPA IPA + GW
IPA + GW + TDSHF
independent particles
+quasi-particle corrections
+time-dependent Hartree (RPA)
+screend Hartree-Fock (excitonic effects)

48. Alternatives to the Berry-phase
Unified formalism for calculating
polarization, magnetization, and
more in a periodic insulator
Kuang-Ting Chen and Patrick A. Lee
Phys. Rev. B 84, 205137(2011)
J. Phys. Chem. Lett. , 9, 24, 7045–7051 2018