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)