phd candidate: alessandra virga supervisor: prof. tullio ... · alessandra virga supervisor: prof....
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PhD Candidate: Alessandra Virga Supervisor: Prof. Tullio Scopigno
Ph.D. School Vito Volterra in Physics XXXI cycle
Outline of the Project
Achievement I: the role of e-ph coupling as relaxation channel for an hot Fermi-Dirac distribution in out-of-equilibrium graphene
Achievement II: Spectro-microscopy of monolayer graphene by Coherent Vibrational Spectroscopy
Goal III: Study of electronic distribution in Semiconductor Nanowires by means of transient Absorption
Achievements
Ongoing Work
Prof. A. Polimeni Group: Optical Spectroscopy of Nanostructured Materials
Prof. F. Mauri Dott. L. Benfatto
A. Ferrari
Outline of the Project
Achievement I: the role of e-ph coupling as relaxation channel for an hot Fermi-Dirac distribution in out-of-equilibrium graphene
Achievement II: Spectro-microscopy of monolayer graphene by Coherent Vibrational Spectroscopy
Goal III: Study of electronic distribution in Semiconductor Nanowires by means of transient Absorption
Achievements
Ongoing Work
Prof. A. Polimeni Group: Optical Spectroscopy of Nanostructured Materials
Prof. F. Mauri Dott. L. Benfatto
A. Ferrari
We can recognize 4 fundamental steps:
Creation of electron-hole pairs;
Electron-Electron scattering;
Radiative relaxation: Hot-Luminescence emission;
Electron-phonon coupling.
ħω
Light-matter interaction in Graphene
e--h+
pairs
𝐓 = 𝟎 𝒇𝒔
Light-matter interaction in Graphene We can recognize 4 fundamental steps:
Creation of electron-hole pairs;
Electron-Electron scattering;
Radiative relaxation: Hot-Luminescence emission;
Electron-phonon coupling.
𝐓~𝟏𝟎 𝒇𝒔
+k
-k
ħΩ
We can recognize 4 fundamental steps:
Creation of electron-hole pairs;
Electron-Electron scattering;
Radiative relaxation: Hot-Luminescence emission;
Electron-phonon coupling.
𝐓 > 𝟏𝟎𝟎 𝒇𝒔
Light-matter interaction in Graphene
Raman spectrum in Graphene
Pristine
Defected
Raman Scattering 𝜔𝐿 = 𝜔𝑆𝑐 ± Ω𝑞
𝒌𝐿 = 𝒌𝑆𝑐 ± 𝒒
Spontaneous Raman Scattering
Ferrari et al., Nat Nano 8, 235–246 (2013)
Raman spectrum of Graphene
Impulsive Spontaneous Raman
?
Fs-Pulsed Laser High peak power Broadband Spectrum Out-of-equilibrium x No Raman Spectroscopy
CW Laser Low power Monochromatic Beam x No Out-of equilibrium Raman spectroscopy
Ps-Pulsed Laser High peak power Broadband Spectrum Out-of-equilibrium Raman spectroscopy Convolution effects
Impulsive Raman in Graphene
Hot-Lumiscence in Graphene
Planck’s Law
𝐹 𝜔, 𝑇𝑒𝑙 = 𝜏𝑒𝑚 𝜔𝜔3
2𝜋2𝑐2 𝑒𝑥𝑝ℏ𝜔
𝑘𝑇𝑒𝑙− 1
−1
Evidence of out-of-equilibrium condition
Unpublished Research
Impulsive Raman in Graphene
v
v
v
v
G band 2D band
Unpublished Research Unpublished
Research
Impulsive Raman in Graphene
v
v
v
v
Possible effects at high T:
• Anharmonicity: Pos(G) decreases with Te
• Electron-phonon coupling:
Pos(G) increases with Te but FWHM(G) decreases
Montagnac et al., Carbon, 5 4 ( 2 0 1 3 ) 6 8 –7 5 Yan et al., PRB 80, 121403R 2009
?
Unpublished Research
ω2𝐷 independent on 𝑇𝑒𝑙
Blueshift of 𝜔𝐺
x Increasing of Raman Linewidths
ħΩ ħΩ
Increasing the electronic temperature 𝑇𝑒𝑙
Electron-phonon coupling
𝑞, 𝜔𝑚
𝑠′, 𝑘 − 𝑞, 𝜀𝑛 − 𝜔𝑚
𝑞, 𝜔𝑚
𝑠, 𝑘, 𝜀𝑛
Electron-phonon coupling
ps-pulsed Laser
ℏ𝜔
𝐸𝐹
1585 𝑐𝑚−1 0.20 𝑒𝑉
𝐸𝐹 ℏ𝜔
1585 𝑐𝑚−1 0.20 𝑒𝑉
Smearing out of the Dirac cone
CW Laser
Unpublished Research
Theoretical model of 𝚪𝑮
ps-pulsed Laser
1585 𝑐𝑚−1 0.20 𝑒𝑉
𝛾𝑒~𝛼𝑒𝑘𝐵𝑇𝑒𝑙 = 0.71𝑇𝑒𝑙
Unpublished Research
Outline of the Project
Achievement I: the role of e-ph coupling as relaxation channel for an hot Fermi-Dirac distribution in out-of-equilibrium graphene
Achievement II: Spectro-microscopy of monolayer graphene by Coherent Vibrational Spectroscopy
Goal III: Study of electronic distribution in Semiconductor Nanowires by means of transient Absorption
Achievements
Ongoing Work
Prof. A. Polimeni Group: Optical Spectroscopy of Nanostructured Materials
Prof. F. Mauri Dott. L. Benfatto
A. Ferrari
Coherent Vibrational Spectroscopy
Pump + Stokes
CARS
Filter Sample
Objective in focus
Condensor
Pump: l=780nm, P=120 mW, Dn=15cm
-1
Stokes: l=840-1100nm, P < 10mW
G-band of Graphene
wP wS
W
was
W was = 2wp – ws
In-Resonance: wp – ws = W W
wS wP
wS wP was wP
W
𝐼𝐶𝐴𝑅𝑆 ∝ 𝜒(3) 2𝐼𝑝
2𝐼𝑠𝐿2𝑠𝑖𝑛𝑐2Δ𝑘𝐿
2
Coherent Anti-Stokes Raman Scattering
Input Spectra Output Spectra
Coherent Vibrational Spectroscopy In-Resonance: wp – ws = W
wS wP was wP
W
wS wP
W
was wP
𝐼𝐶𝐴𝑅𝑆 ∝ 𝜒 3 2= 𝝌 𝟑 𝑹 + 𝝌 𝟑 𝑵𝑹 2
=
= ℜ𝑒 𝜒 3 𝑅 2+ ℑ𝑚 𝜒 3 𝑅 2
+ 2ℜ𝑒 𝜒 3 𝑅 𝜒 3 𝑁𝑅
NRB
• When 𝜒 3 𝑁𝑅 ≫ 𝜒 3 𝑅, NRB will overwhelm CARS
signal.
• Distortion of the lineshape: peak position is redshifted.
CARS
W [𝑐𝑚−1]
𝜒
(3)
2 [
arb
. unit
s]
W [𝑐𝑚−1]
𝜒
(3)
2 [
arb
. unit
s]
W [𝑐𝑚−1]
𝜒
(3)
2 [
arb. unit
s]
W [𝑐𝑚−1]
𝜒
(3)
2 [
arb. u
nit
s]
NRB Dispersive Lineshape
Simmetric peak
𝜒 3 𝑅/𝜒 3 𝑁𝑅 =0.25 𝜒 3 𝑅/𝜒 3 𝑁𝑅 =0.60 𝜒 3 𝑅/𝜒 3 𝑁𝑅 = 3 𝜒 3 𝑅/𝜒 3 𝑁𝑅 = ∞
Outline of the Project
Achievement I: the role of e-ph coupling as relaxation channel for an hot Fermi-Dirac distribution in out-of-equilibrium graphene
Achievement II: Spectro-microscopy of monolayer graphene by Coherent Vibrational Spectroscopy
Goal III: Study of electronic distribution in Semiconductor Nanowires by means of transient Absorption
Achievements
Ongoing Work
Prof. A. Polimeni Group: Optical Spectroscopy of Nanostructured Materials
Prof. F. Mauri Dott. L. Benfatto
A. Ferrari
Transient Absorption in ZB InP Nanowires
𝐼 𝜆𝑝𝑟 , 𝜏 =𝑇𝑂𝑁(𝜆𝑝𝑟 , 𝜏)
𝑇𝑂𝐹𝐹(𝜆𝑝𝑟 , 𝜏)
𝐸𝑔 ≈ 1.42 𝑒𝑉 = 880 𝑛𝑚
𝜔𝑝𝑢𝑚𝑝 ≈ 2.3 𝑒𝑉 = 540 𝑛𝑚
𝜔𝑝𝑟𝑜𝑏𝑒 ≈ 1.7 − 3 𝑒𝑉 = 400 − 720 𝑛𝑚
v9
c7E
k
~ 1.5 𝑒𝑉
ħ𝜔𝑃
Transient Absorption in ZB InP Nanowires
𝐸𝑔 ≈ 1.5 𝑒𝑉 = 830 𝑛𝑚
𝜔𝑝𝑢𝑚𝑝 ≈ 2.3 𝑒𝑉 = 540 𝑛𝑚
𝜔𝑝𝑟𝑜𝑏𝑒 ≈ 1.7 − 3 𝑒𝑉 = 400 − 720 𝑛𝑚
Preliminary Results
Thanks for the attention