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