Ultrafast Carrier Dynamics in Graphene
M. Breusing, N. Severin, S. Eilers, J. Rabe and T. Elsässer
Conclusion
• information about carrier distribution with10fs
time resolution
• Carrier equalibration / formation of Fermi-Dirac
distribution within first 100fs
• Carrier optical phonon scattering with time const.
of about 150fs
• substrate influences observably the carrier
distribution, but not the cooling by phonon scattering
Motivation
• Graphene - building block for future
nanostructured electronic devices (FET, analog
GHz-THz applications)
• Optical application (e.g. saturable absorber)
• carrier relaxation - dominant limit for high
frequency application
• Semi-metal –> tendency towards metals or
semiconductors is still an open issue
• influence of supporting media for monolayer
important
Sample Preparation / Analysis
Graphene on Muscovite (Mica)
600 700 800 900 1000 11000.00
0.02
0.04
0.06
0.08
0.10
Inte
nsity
Wavelength (nm)
0 300 600 900-0.3
0.0
0.3
µ (e
V)
Delay Time (fs)
100020003000
T (K
)
1.4 1.8
0.0
0.7
(a)
250fs150fs
/
(10
-3)
Photon Energy (eV)(b)
800fs
30fs75fs
0 200 400 600 800 1000
1.4
1.6
1.8
Delay Time (fs)
Pho
ton
Ene
rgy
(eV
)
-1.000E-4
4.333E-4
7.000E-4
• Spectrum of laser source offering bandwidth of 0.6eV
• Decrease of sharp spectral features in T/T indicate carrier equilibration
• Spectra for various delays of both sample kinds; in red: best fit assuming Fermi-Dirac distribution
• Extracted carrier Temperature (T) and chem. potential (µ) within the first ps.
• Phonon scattering reduces T within the first 300fs; simultaneously µ rises, but reaches different values
• Different kinds of samples (with / without water-film)
• Spectrally integrated transients and fits of transmission change for sample with water film (blue / green) and without (black / red)
• Inset shows linear dependence on added energy
• Spectral and time resolved transmission change (T/T)
• Shift to lower energies for longer delays clearly visible
)(0 eh ff
abs. after
tD …
Pump-Probe Spectroscopy
• Two delayed ultrashort laserpulses
• Probe detects pump induced sample changes
•Absorption changes () depend on carrier distribution (fe ,fh)
Graphite on Oxidized Silicon
0.10.20.30.40.5
R (%
)
1000
2000
µ=0.0eV
T (K
)
1 2 3
0.1
0.2
0.3
Energy (eV)
µ (e
V)
T=500K
ph
G-band
ph
D‘-band
ph
D-band
• Spectrally resolved R/R, simulated and fitted by Fresnel equations combined with transfer matrix method, assuming Fermi-Dirac distribution
• Temperature (T) drops within first 200fs, chem. potential (µ) rises coevally, but returns to zero within first ps
(1) Sample structure; the well defined oxidized layer induces relevant multiple reflections and thereby Fabry-Perot oscillations in reflected light
(2) spectrally integrated reflection change (R/R) for thick graphite (blue) and graphene (black), corrected for substrate contributions
(3) Sample analysis by Raman spectroscopy – single D‘ peak indicates single layer graphene, absence of, for idealized graphene forbidden, D peak high crystal quality
1.3 1.7
-1
0
150fs300fs900fs
(a)
R/R
(10
-3)
Energy (eV)50fs
0 300 600 900
0.0
0.2
(b)
µ (e
V)
Delay Time (fs)
1000
2000
3000
T (K
)
0.280.33
R
(1) (2) (3)
Properties Graphene
7 fs laser
delay stage
Spectro- graph
sample
M. Breusing et al., Phys. Rev. Lett. 102 (2009)
• 3 layers of graphene (two dimensional carbon lattice)
• Brillouin zone of graphene, showing conical bands centered at K and K‘
• Tips of conduction and valence band cones touch each other at EF=0eV, making graphene a semi-metal
Pump-Probe Set-Up
• Focal spot diameter 8µm
• Lock-in detection
•Time resolution 10fs
• Carrier dynamic simulation for graphene based on Bloch- Boltzmann- Peierls equations
• 3 cases assumed: no varying µ (dash-dotted), istantaneous phonon decay (dashed) and infinite phonon lifetime (solid)