Session Q11: Focus Session: Graphene Structure, Stacking, Interactions: Infrared and Terahertz Spectroscopy

Collective modes in gapped bilayer graphene

We calculate the polarizability of gapped bilayer graphene using the four-band continuum model. In the presence of a gap the four-band model and the simplified two-band model return bands that are qualitatively different especially at low energies. We find that because of these differences the static polarizability obtained using the four-band model is qualitatively different from the one obtained using the two-band model. We also find that the differences between the two-band model and the four-band model profoundly affect the dynamical dielectric function, and therefore the properties of the plasmon modes. In addition, we study the effect of trigonal warping and find that it qualitatively modifies the density response function.   

Sum-Frequency Vibrational Spectroscopic Studies of Polymer/Graphene Interface

The interest in alignment of polymer on graphene surfaces has been motivated by a desire to gain a fundamental understanding the interaction between molecule and graphene, and for possible application of graphene for surface chemistry. Graphene-polymer interactions also play an important role in graphene-polymer composites, which exhibit greatly improved electrical conductivity, strength, and thermal stability compared with pure polymer material. Theoretical investigation of graphene-polymer interface has been performed previously using molecular dynamics simulations [1]. However, experimental studies of the interfacial characteristics of graphene-polymer composite has been challenging. Here we investigate the molecular orientation at polymer/graphene interface using phase-sensitive-sum-frequency generation spectroscopy. The sum-fequency spectrum shows clear vibration signatures of CH$_{2}$ groups. In particular, it suggests that CH$_{2}$ groups pointing toward the graphene surface interact with graphene strongly, which leads to a red shift of vibration frequency as large as 15 cm$^{-1}$. In this talk I will discuss the implications of our experimental findings. \\[4pt] [1] C. Lv, Q. Xue, D. Xia, M. Ma, J. Xie, and H. Chen, \textit{J. Phys. Chem. C}, \textbf{2010}, $114$ (14), 6588--6594.   

Tunable Quantum-Enhanced Second-Order Optical Nonlinearity From Bilayer Graphene

Bilayer graphene exhibits a tunable band gap when the inversion symmetry is broken. It therefore stimulates much interest on its physical characterization and practical application for mid-infrared (MIR) optoelectronics. Here we focus on its second order nonlinear optical response to the MIR laser excitation under device condition, following a quantum description of nonlinear optical conductivity. Our theoretical study shows that, for a certain laser-frequency range determined by the band-gap, giant second harmonic generation can be excited due to the intrinsic electronic spectrum of bilayer graphene. Electrically tunable $\chi ^{(2)}$ on the order of $10^5pm/V$ can be achieved, 3 orders of magnitude larger than the widely-used nonlinear crystal AgGaSe2.   

Infrared magneto-spectroscopy of graphene-based systems

The results of infrared magneto-spectroscopy of different graphene-based materials will be presented. These systems involve multi- and mono-layers of epitaxial graphene, decoupled graphene flakes on the surface of graphite as well as bulk graphite. The magneto-optical methods serve us mostly as a tool of Landau level spectroscopy. It is used to study the characteristic response due to massless or massive Dirac-type particles and, e.g., to distinguish materials with graphene layers exhibiting rotational or Bernal stacking. Broadening of inter-Landau level transitions has been traced as a function of magnetic field and energy to evaluate the quality of multi-layer epitaxial graphene in terms of scattering time and carrier mobility. From broadening of overlapping Landau levels we find that the scattering rate increases linearly with energy. Complementary to these experiments, inelastic relaxation processes have been studied in pump-and-probe measurements in THz range. The obtained data indicate that the inelastic processes are significantly slower than the elastic ones, and also that the inelastic relaxation is slowed down when the photon energy is tuned to values below the optical phonon frequency. References: M. Orlita et al., PRL, to be published (2011); S. Winnerl et al., PRL, to be published (2011).   

Far-infrared Kerr rotation spectroscopy of graphite and multilayer graphene

Graphite attracts much attention nowadays as a reference 3D material for graphene. Since the early measurements of the cyclotron effect in graphite over fifty years ago [1], a satisfactory quantitative description of this spectacular phenomenon is missing. The analysis of magneto-optical data was hindered either by a limited set of the used photon energies or by the lack of the optical selectivity between electrons and holes. We overcome this issue by measuring the far-infrared magneto-optical Kerr rotation spectra [2] and achieve a highly accurate unified microscopic description of all spectra in a broad range of magnetic fields (0.5 -- 7 T) by taking rigorously the c-axis band dispersion and the trigonal warping into account. We find that the second- and the forth-order cyclotron harmonics are optically almost as strong as the fundamental cyclotron resonance even at high fields. The same effects are expected to strongly influence the magneto-optical spectra of Bernal stacked multilayer graphene and therefore play a major role in the respective applications. \\[4pt] [1] J. K. Galt, W.A. Yager and H.W. Dail Jr., Phys. Rev. \textbf{103}, 1586 (1956) \newline [2] J. Levallois, M.K. Tran and A. B. Kuzmenko, arXiv:1110.2754v2; submitted.   

``Intrinsic'' terahertz plasmons and magnetoplasmons in single layer graphene on SiC

Plasmons in graphene have lately attracted much attention, to great extent, due to promises for novel technologies. Recently, plasmon absorption in graphene was attained in deliberately patterned structures [1]. We measured the magneto-optical absorption and Faraday rotation response of highly doped single layer graphene, epitaxially grown on Si-terminated SiC substrate. The zero-field spectra show a clear plasmon peak at about 2 THz. In magnetic fields, the plasmon peak splits into two branches, thus showing a characteristic magneto-plasmon behavior which was previously observed in periodic dot structures in GaAs two dimensional electron gases [2]. Hence, in large-scale epitaxial graphene on SiC, light can couple to plasmons in the absence of the intentional patterning of graphene. We suggest that optically-active plasmon absorption in this kind of two-dimensional system arises from laterally confined plasmon modes due to``intrinsic'' imperfections of graphene on Si-face of SiC, such as, grain boundaries which we clearly identify with AFM methods. \newline\noindent [1] L. Ju \emph{et al.} , Nature Nanotechnology \textbf{6}, 630 (2011). \newline\noindent [2] A. J. Allen \emph{et al.} , Phys Rev B \textbf{28}, 4875 (1983).   

Enhanced Optical Dichroism of Graphene Nanoribbons

The optical conductivity of graphene nanoribbons is analytical and exactly derived. It is shown that the absence of translational invariance along the transverse direction allows considerable intra-band absorption in a narrow frequency window that varies with the ribbon width, and lies in the THz band for ribbons 10-100nm wide. In this region the anisotropy in the optical conductivity can be as high as two orders of magnitude, which renders the medium dichroic, and allows near 100\% polarizability with just a single layer of graphene. The interplay between the geometrically induced anisotropy with the anisotropy induced by plasmon absorption is also considered and discussed.   

Quasiparticle properties in graphene

The quasiparticle properties in both single layer and bilayer graphene are presented. We study the electron self-energy as well as the quasiparticle spectral function in graphene, taking into account electron-electron interaction in the leading order dynamically screened Coulomb coupling and electron-impurity interaction associated with quenched disorder. Our calculation of the self-energy provides the basis for calculating all one-electron properties of graphene. We provide analytical and numerical results for quasiparticle renormalization in graphene. Comparison with existing angle-resolved photoemission spectroscopy measurements shows broad qualitative and semiquantitative agreement between theory and experiment, for both the momentum-distribution and energy-distribution curves in the measured spectra. We also present the inelastic quasiparticle scattering rate and the carrier mean free path for energetic hot electrons as a function of carrier energy, density, and temperature, including both electron-electron and electron-phonon interactions. Our results are directly applicable to device structures where ballistic transport is relevant with inelastic scattering dominating over elastic scattering.\\[4pt] S. Das Sarma, S. Adam, E. H. Hwang, and E. Rossi, Rev. Mod. Phys. 83, 407 (2011). \\[0pt] E. H. Hwang, Ben Yu-Kuang Hu, and S. Das Sarma Phys. Rev. B 76, 115434 (2007). \\[0pt] E. H. Hwang and S. Das Sarma Phys. Rev. B 77, 081412 (2008). \\[0pt] Rajdeep Sensarma, E. H. Hwang, and S. Das Sarma, Phys. Rev. B 84, 041408(R) (2011).   

Anisotropy of $\pi$-plasmon Dispersion Relation of AA-stacked Graphite

The dispersion relation of optical $\pi$-plasmons of simple hexagonal intrinsic graphite was calculated within the self-consistent-field approximation. The plasmon frequency $\omega_p$ is determined as functions of the wave vector $\textbf{q}_\parallel$ along the hexagonal plane in the Brillouin zone and its perpendicular component $q_z$. These plasmons are isotropic within the plane in the long wavelength limit. As the in-plane wave vector is increased, the plasmon frequency strongly depends on its magnitude and direction ($\phi$). Our results reveal that interlayer interaction could enhance anisotropy of in-plane $\pi$-plasmons. The group velocity for plasmon propagation along the perpendicular direction may be positive or negative depending on the choice of in-plane wave vector.