Session Y36: Graphene: Optical Properties II

Raman spectroscopy of graphite in high magnetic fields

Recently, much attention has been paid to electron-phonon coupling in graphene. In particular, significant re-normalization and broadening of long-wavelength optical phonons are predicted to occur through resonant interaction with Landau-quantized Dirac fermions. We report here on a high-field magneto-Raman spectroscopy study of highly-oriented pyrolytic graphite (HOPG) and natural graphite at temperatures down to 5 K and in magnetic fields up to 45 T. The E$_{2g}$ graphite phonon line exhibits anticrossing-like behavior at approximately 30 T, which we attribute to the magneto-phonon resonance (MPR) of graphite's massless holes at the H-point. Additionally, we observed features related to inter-Landau-level transitions at the K-point of graphite. We also observed weak graphene-like signatures of MPR, indicating the existence of graphene flakes on the graphite surface.   

Raman measurements of graphene in magnetic fields

Electron phonon interactions in graphene are effectively measured using Raman spectroscopy. For example the G-Band of graphene grown on SiC shows characteristic anticrossings when tuning an external magnetic field exactly at the resonances between the G-Band phonon and the electronic Landau Levels. We measure the micro Raman spectra of mechanically exfoliated graphene lithographically prepared as field effect devices. Unlike prior high magnetic field studies, this provides charge tunability and allows simultaneous Raman and transport measurements under variable B-field. Our initial results show a Landau Level dependent splitting of the G-band for magnetic fields B $>$ 10T. We present our latest results of studies of the Raman G and 2D Band and single and bilayer graphene at T=4.2K and fields to 12T.   

Raman Maps of Carbon Nanoscrolls and their Bundles

Recent theoretical simulations and experimental data have shown that carbon nanoscrolls, formed by rolling a single layer of graphene, exhibit different electronic and optical properties from carbon nanotubes or the planar graphene sheet. They hold promise in a variety of applications areas such as energy storage and nanomechanics. Here, we present our investigation on the formation of carbon nanoscrolls and their bundles from large-area graphene originally grown on copper foil by chemical vapor deposition. When graphene is transferred from a copper foil onto a silicon wafer using wet chemistry, both filled and unfilled carbon nanoscrolls are produced upon the rupture of graphene. These nanoscrolls can further form bundles. The results from Raman mapping, optical microscopy, scanning electron microscopy, and atomic force microscopy measurements will be presented and the formation mechanism will be discussed.   

Controlling Inelastic Light Scattering Quantum Pathways in Graphene

Graphene exhibits unique tunable optical properties. Researchers have observed infrared absorptions in graphene interband transitions as well as intraband transitions can be modified substantially through electrostatic gating. At the same time, inelastic Raman scattering from few layer graphene is readily observable and widely used to characterize graphene quality, and to probe graphene electron-phonon interactions. In strongly gated graphene, Raman scattering from graphene can also be varied from electrical doping through direct change of electronic transitions. In this talk, I will describe how the Raman intensity of G-mode and 2D-mode Raman varies with the Fermi energy in doped graphene.   

Ab initio calculation of double-resonant Raman spectra for bilayer graphene

The discovery that the application of an external electric field induces a band gap opening in bilayer graphene attracted a lot of interest on this system, due to important applications in nanoelectronics [1]. Raman spectroscopy is one of the most important experimental techniques for the characterisation of carbon based materials, providing informations on carriers concentration [2], disorder [3], number of layers on multi-layers graphene systems [4], and phonon properties. Most of the theoretical studies on multi-layers graphene are performed using a Tight Binding (TB) model, and full calculation of Raman matrix elements to obtain frequencies, intensities and linewidths of Raman bands has not been performed up to now. The developpement of a fully ab initio theoretical tool to compute Raman spectra is therefore higly desirable and particularly relevant for systems where a simple TB parametrization of the electronic structure and of the electron-phonon interaction is not available. In this talk I will discuss a recently developped methodology to compute fully ab initio double-resonant Raman spectra and I will present results for bilayer graphene.\\[4pt] [1] Ohta et al, Science {\bf 313}, 951 (2006), [2] Malard et al, PRL {\bf 101}, 257410 (2008), [3] Lucchese et al, Carbon {\bf 48}, 1592 (2010), [4] Ferrari et al, PRL {\bf 97}, 187401 (2006)   

Observation of out-of-plane vibrations in few-layer graphene using combination and overtone Raman modes

We have studied three distinct higher-order Raman features, appearing at $\sim $ 1660, 1730 and 1760 cm$^{-1}$, in graphene samples of 1-6 layers thickness and both Bernal and rhombohedral stacking. By detailed analysis of the measured dispersions of these lines using double-resonance theory, we have identified the features, respectively, as the LO+ZA, LO+ZO' combination modes and the 2ZO overtone mode. Here LO, ZA, and ZO, and ZO' denote, respectively, the in-plane longitudinal optical mode, the out-of-plane acoustic, optical and layer-breathing modes. All three of these Raman features are absent in single-layer graphene, which lacks the layer-breathing vibration and exhibits particularly high symmetry. The line shape of LOZO' mode shows a dramatic dependence on the stacking order of the layers and can serve as a means of identifying stacking order in few-layer graphene. In addition, the LOZO' mode allows us to access the properties of the low-energy layer-breathing (ZO') mode in few-layer graphene samples.   

Infrared Kerr and Faraday Measurements in Gated, Multi-Layer SiC Graphene

Magneto-optical Kerr and Faraday measurements are used to probe the Landau level structure of SiC graphene in the mid- and far-infrared regimes (100-1000 meV and 3-10 meV, respectively). Transmittance/reflectance spectroscopy probes the longitudinal conductivity ($\sigma _{xx})$, which is related to the sum of chiral response. In contrast, polarization sensitive techniques provide new insights into the electronic structure by probing the Hall conductivity ($\sigma _{xy})$, which reflects the difference in the chiral response. Samples, which are studied in applied fields (B) up to 7T and temperatures ranging from 10-300K, show robust features arising from two distinct sets of Landau level transitions. One set displays transition energies that are $\sqrt B $ dependent as expected of monolayer graphene. Interestingly, below a critical photon energy ($\sim $100 meV) these features become symmetric with B. The other set is consistent with expectations of bilayer graphene and graphite, showing a linear B dependence and the expected odd symmetry in B. Further investigation of Landau level behavior is accomplished by tuning the Fermi energy in samples with a gate. Work supported by NSF-DMR1006078.   

Landau level-phonon resonances in graphene and their spectroscopic signatures in magneto-optical measurements

The excited states and the optical spectra of a two-dimensional electronic system under a magnetic field are strongly influenced by the electron-phonon interaction when the energy spacing of the Landau levels is resonant with the frequency of an optical phonon. We have performed a theoretical study of these excited states in graphene, and have calculated the optical absorption spectra for a range of magnetic fields. Electron-electron interactions are found to redistribute the spectral weight of the coupled modes and have important consequences for the absorption spectra. Our results are in good agreement with recent magneto-optical transmission experiments on epitaxial graphene on SiC. This work was supported by NSF Grant No. DMR10-1006184 and U.S. DOE Contract No. DE- AC02-05CH11231.   

Probing plasmons in graphene

Plasmon behaviour in graphene is important for the understanding of many body interaction of 2D Dirac fermions. It also provides physical background for potential graphene applications in optoelectronics and ultrahigh speed THz electronics. In this talk, we will describe our study of plasmon behaviour in graphene using far-infrared spectroscopy and compare our experimental results to theoretical predictions.   

Plasmon-Enhanced Photocurrent in a Graphene Nanoconstriction

A plasmonic nanostructure can act like an optical antenna, concentrating light into a deep sub-wavelength volume and enabling manipulation of light-electron interactions at the nanometer scale. Achieving efficient coupling from such antennas to functional electrical devices has been challenging, because the region of field enhancement is so small. We report the use of a use a self- aligned fabrication process to couple a gold break junction acting as a plasmonic antenna with a sub-10-nm graphene constriction. The nonlinear electrical characteristics of the graphene device allow it to serve as a photodetector. We observe a photocurrent that is peaked at the plasmon frequency and strongly modulated by the polarization direction of the incident light. The enhancement of the local optical-frequency electric field induced by the plasmon is a factor of 1.5-10.   

Thermal broadening effects on unstable plasmons in extrinsic graphene with injected carriers

We study theoretically the charge density collective oscillations (plasmons) of an extrinsic ({\em i.e.}, doped) graphene system into which charge carriers (either electrons or holes) are injected. When the injected carriers are sharply peaked so that the distribution function of the injected carriers can be well approximated by $f_{\mbox{\tiny inject}}(\mathbf p) = n\delta(\mathbf p - \mathbf p_0)$, some of the plasmons in the system become unstable, in the sense that the amplitudes of these plasmons grow exponentially in time (at least initially, in the linear-response regime). This effect is analogous to the two-stream instability that is seen in classical plasma systems. As with the classical plasma system, thermal broadening of the injected carriers tends to suppress the instability. We report a theoretical study of the effect of the thermal broadening of the injected carriers on the plasmon instability in graphene, and we delineate the parameters where the thermal effects completely suppress the instability.   

Electromagnetic wave propagation through a graphene-based photonic crystal

A novel type of photonic crystal formed by embedding a periodic array of constituent stacks of alternating graphene and dielectric discs into a background dielectric medium is proposed [1]. The frequency band structure of a 2D photonic crystal with the square lattice of the metamaterial stacks of the alternating graphene and dielectric discs is obtained. The electromagnetic wave transmittance of such photonic crystal is calculated. The graphene-based photonic crystals have the following advantages that distinguish them from the other types of photonic crystals. They can be used as the frequency filters and waveguides for the far infrared region of spectrum at the wide range of the temperatures including the room temperatures. The photonic band structure of the graphene-based photonic crystals can be controlled by changing the thickness of the dielectric layers between the graphene discs and by the doping. The sizes of the graphene-based photonic crystals can be much larger than the sizes of metallic photonic crystals due to the small dissipation of the electromagnetic wave. The advantages of the graphene-based photonic crystal are discussed.\\[4pt] [1] O. L. Berman, V. S. Boyko, R. Ya. Kezerashvili, A. A. Kolesnikov, and Yu. E. Lozovik, Phys. Letts. A 374, 4784 (2010).   

Effects of Layer Stacking on the Combination Raman Modes in Graphene

We have observed new combination modes in the range from 1650 -- 2300 cm$^{-1}$ in single-(SLG), bi-, few-layer and incommensurate bilayer graphene (IBLG) on silicon dioxide substrates. The M band at $\sim $1750 cm$^{-1}$ is suppressed for both SLG and IBLG. A peak at $\sim $1860 cm$^{-1}$ (iTALO$^{-})$ is observed due to a combination of the iTA and LO phonons. The intensity of this peak decreases with increasing number of layers and this peak is absent in bulk graphite. Two previously unidentified modes at $\sim $1880 cm$^{-1}$ (iTALO$^{+})$ and $\sim $2220 cm$^{-1}$ (iTOTA) in SLG are tentatively assigned as combination modes around the K point of the graphene Brillouin zone. The peak frequencies of the iTALO$^{+}$ (iTOTA) modes are observed to increase (decrease) linearly with increasing graphene layers.