ultrafast carrier dynamics in graphene m. breusing, n. severin, s. eilers, j. rabe and t. elsässer...
TRANSCRIPT
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)