Supplemental Material
Intense terahertz field effects on photoexcited carrier
dynamics in gated graphene
H. A. Hafez1, P. L. Lévesque2, I. Al-Naib3,4, M. M. Dignam3, X. Chai1,
S. Choubak5, P. Desjardins5, R. Martel2 and T. Ozaki1*
1
INRS-EMT, Advanced Laser Light Source, INRS, Varennes,
Québec J3X 1S2, Canada
2
Département de Chimie, Université de Montréal, Montréal,
Québec H3C 3J7, Canada
3
Department of Physics, Engineering Physics and Astronomy,
Queen’s University, Kingston, Ontario K7L 3N6, Canada
4
Biomedical Engineering Department, College of Engineering,
University of Dammam, Dammam 31441, Saudi Arabia
5
Département de Génie Physique, Polytechnique Montréal, Montréal,
Québec H3C 3A7, Canada
(*)
Author to whom correspondence should be addressed. Electronic mail: ozaki@emt.inrs.ca.
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1. Sample Preparation and Characterization
1.1. Graphene Growth
Graphene films were grown on 25μm thick Cu foils (Alfa Aesar, item no. 13382) at 1000°C in a
1.5 inch fused quartz tube at low pressure [1]. The system consists of a manifold capable of UHV
conditions where the gases are injected into the chamber. The base pressure of the system was
below 510-6 Torr. Prior to insertion in the tube furnace, the Cu foils were chemically cleaned in 1
M acetic acid (Sigma Aldrich, Reagent Plus > 99%) at 60 ºC followed by acetone and then 2propanol (without drying) for 10 minutes in each step. The Cu substrates were then blown-dried
with nitrogen. The Cu foils were heated to 1000°C and annealed at this temperature for 30 minutes
under the flow of H2 (purified Praxair, UHP, grade 5) at 50 mTorr. In order to grow graphene, CH4
(purified Praxair, UHP, grade 3.7) was injected into the chamber and the total pressure reached 500
mTorr. After a 10 min growth time, the chamber was cooled down to room temperature under the
flow of H2. H2 and CH4 UHP grade gases are further purified from their O2 content down to ppb
level with DEOXOTM and SAES Pure Gas Inc. MC1-950FV, respectively.
1.2. Graphene Transfer
The Cu foil was then dissolved and the graphene was transferred, using a PMMA handle, onto a zcut quartz substrate. The transfer method is adapted from the procedure in Ref.2. Once the graphene
film was grown, a poly methyl methacrylate (PMMA, M.W. 15000 GPC, Acros organics, 4% in
chlorobenzene) solution was spin coated on one side of the graphene/ copper foils at 4000 r.p.m
for 30 s and dried in air. Next the graphene layer on the other side of the copper foil was etched
away by Reactive Ion Etching (RIE) technique using 10 sccm of O2 at 125 mTorr for 1 min at a
100W power. The copper was etched away using 0.1 M ammonium persulfate (Reagent, ACS 98%,
Acros Organics) at room temperature. The PMMA/graphene was then rinsed in a DI water bath.
The floating film is then fished with the desired substrate. After drying, the PMMA layer was
removed in an acetone bath and rinsed with Isopropanol alcohol.
1.3. Contact
The contact electrodes Cr/Au (5 nm/100 nm) were deposited through a custom made polyimide
shadow mask aligned with the graphene layer with a typical EBeam evaporator (EDWARDS
AUTO-306).
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1.4. Gating
The
ionic
liquid,
1-ethyl-3-methylimidazolium
bis
(trifluoromethylsulphonyl)
imide
([EMIM][TFSI]) 99%, was purchased from IoLiTec Ionic Liquids Technologies GmbH. The ionic
liquid was dried at 100°C for about 12h while pumped with a turbo pumping station until a vacuum
of 1×10-6 Torr was reached and then transferred in the vacuum sealed container to an inertatmosphere glove box. Poly (styrene-b-ethylene oxide-b-styrene) (PS-PEO-PS) triblock copolymer
was purchased from Polymer Source and used as received. The quoted molecular weights of the
block copolymermoieties were 9.5-48.0-9.5 kg.mol-1 for the PS-PEO-PS blocks, respectively. The
crosslinked ionic liquid/triblock copolymer gel (ion gel) was produced as follows. In glove box,
0.55 g (360 µL) of [EMIM] [TFSI] was dissolved with 21mg of PS-PEO-PS in 2 ml of dry
dichloromethane. The solution was stirred overnight at room temperature. The ion gel was spincoated on the graphene sample within the glovebox at a rate of 4,000 r.p.m and was allowed to dry
for at least 2 hours. The sample was then exposed to ambient condition for measurements. The
graphene samples were examined under field emission prior to any treatments.
1.5. Raman Spectra of Graphene
FIG. S.1: Raman spectrum of the graphene sample.
The graphene quality is assessed, using Raman Spectroscopy (Renishaw Invia) with a laser
wavelength of 514 nm. A 50X objective (N.A= 0.55) focuses a 0.16 mW laser beam to a probe spot
about 1 µm in diameter. Prior to analysis the graphene films of the same growth batch were
transferred onto SiO2 (100nm thick)/Si substrate. The characteristic graphene G and 2D peaks [3]
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are observed, as shown in Fig. S.1. The very weak D band at 1300 cm-1 is indicative of a low defect
density. The 2D band FWHM is 33 cm-1 denotes that the graphene mostly consists of a single layer,
which was further confirmed by scanning electron microscope (SEM) imaging.
1.6. SEM Analysis
Fig. S.2 shows a representative scanning electron microscope (SEM) image (JEOL 7600) of the
grown graphene on copper before transfer to the quartz substrate. In addition to the single layer,
only sparse multilayer graphene islands can be observed.
FIG. S.2: SEM image of the graphene sample
2. Experimental Setup and Measurements
Our experimental setup for the optical-pump/intense-THz-probe (OPITP) spectroscopy is
described in detail in [4]. Intense single-cycle THz pulses have been generated by optical
rectification of 800 nm, 40 fs, Ti:sapphire laser pulses in a LiNbO3 crystal using the pulse-fronttilting technique [5-7]. The incoming laser beam from the Ti:sapphire laser has been first split into
two using a beam splitter. The transmitted beam enabled the THz generation while the reflected
beam has been further split to provide the optical pump of the graphene sample and a detection
probe that enables coherent electro-optic sampling detection. The THz beam after generation has
been first guided and focused onto the sample using a pair of off-axis parabolic mirrors and the
diverging transmitted beam has been guided and refocused by another pair of off-axis parabolic
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mirrors to enable electro-optic sampling detection using 1-mm-thick (110) ZnTe crystal. The THz
field strength calculation is based on measuring the THz field-induced modulation [7] in the ZnTe
birefringence, probed coherently by the optical probe beam. A pair of wire-grid polarizers has been
used before the sample to control the THz field strength interacting with the sample. We also used
five high-resistivity Si wafers before detection to prevent any nonlinear THz effects in the ZnTe
detection crystal. The graphene sample has been illuminated from the backside of the quartz
substrate. Thus, the THz field interacting with the graphene film is the one transmitted through the
quartz substrate, which is ~12% less in field-strength than the incident beam at the air/substrate
incidence interface. Similarly, the optical pump fluence interacting with the graphene film is the
one transmitted though the quartz substrate. In our experiment, in contrast to the collinear pumpprobe scheme followed in [4], the optical pump beam was incident noncollinearly at an angle of
~30° with respect to the incident THz probe beam that was incident normally to the graphene
sample.
For the accuracy of the THz field strength estimations, it is worth noting here that field strength
calculations based on electro-optic sampling always give lower field values compared with other
estimation schemes based on energy measurements [7-10]. Thus, preferring to be conservative in
our estimations of the field strength, we used the electro-optic sampling based calculations.
In our experiments, the Fermi level (or the electrochemical doping concentration) of the graphene
has been controlled by adjusting the gate voltage VG. The sample has further been pumped optically
at a fixed pump fluence of ~29 μJ/cm2. The photo-induced dynamics has then been probed by the
intense THz probe beam at the peak field of its pulses. The nonlinear THz response of the sample
has been extracted through THz differential transmission measurements [11] at various THz peak
electric field levels ranging from ~1.5 to 70 kV/cm.
The photoexcitation by the optical pump beam results in electron-hole pair generation in graphene
with a total density of ~3.5×1012cm-2 (i.e., density sum for electrons and holes) calculated through
formulas reported in [11]. Moreover, the optical pump results also in an increase in the free carrier
temperature. The photo-induced increase in free carrier density itself is expected to increase the
photoconductivity of graphene, while the rise in temperature leads to increase in the carrier
scattering rate that suppresses the photoconductivity. These two effects always exist as long as the
pump photon energy is higher than twice the Fermi-level energy of graphene, but the dominance
of one over the other depends on the initial Fermi level (or doping concentration) before pump. It
has been found in our study as well as in previous studies [12-14] using low THz probe field that
the first effect dominates when the Fermi level is initially low (less than 115 meV in our case),
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providing negative THz differential transmission indicating semiconductor-like behavior. On the
other hand, the latter effect dominates for higher Fermi levels, providing positive THz differential
transmission corresponding to metal-like behavior. Thus, the crossover from the semiconductorlike to the metal-like behavior has been revealed by THz-probe differential transmission induced
by the optical pump. Observing such a crossover optically requires very sensitive differential
transmission/reflection measurements that can only be enabled by employing pump-probe
techniques. In other words, it is not possible to explore this behavior with the conventional THzTDS techniques. In our work, we focused on the effect of increasing the THz probe field on the
photo-induced carrier dynamics at initially low Fermi level near the CNP. The intense THz field
results in extra heating effects in addition to the heating effect induced by the optical pump, and
led to domination of the scattering rate over the increase in the carrier density induced by the optical
pump.
3. Lattice and Electron Temperatures
As stated in the letter, the lattice and electron temperatures were taken to be equal and are
determined by distributing the absorbed optical pump and THz probe energy between the electron
and phonon populations, taking both to be in quasi-equilibrium. Given that energy from both the
optical and THz pulses initially goes entirely into the electronic subsystem, it is not clear how much
of that energy should be transferred to the phonon population. For simplicity, we have taken those
two temperatures to be equal. We have examined other ways of distributing this energy, but none
of them leads to the observed decreases in the scattering times. If more energy remains in the
electronic subsystem, then the electron-phonon scattering time decrease is even less. It is hard to
imagine a mechanism whereby the phonon temperature could be higher than the electron
temperature. However, even if all of the absorbed energy were transferred to the phonons, the large
heat capacity of the phonons means that increase in the phonon temperature (and thereby the
phonon population) is still not enough to sufficiently decrease the electron-phonon scattering time.
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Wrinkles
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