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We report phonon renormalisation in bilayer graphene as a function of doping. The Raman G peak stiffens and sharpens for both electron and hole doping, as a result of the non-adiabatic Kohn anomaly at the Γ point. The bilayer has two conduction and valence subbands, with splitting dependent on the interlayer coupling. This results in a change of slope in the variation of G peak position with doping, which allows a direct measurement of the interlayer coupling strength.Graphene is the latest carbon allotrope to be discovered [1,2,3,4,5]. Near-ballistic transport at room temperature and high carrier mobilities [2,3,4,5,6,7,8], make it a potential material for nanoelectronics [9,10,11], especially for high frequency applications. It is now possible to produce areas exceeding thousands of square microns by means of micro-mechanical cleavage of graphite. An ongoing effort is being devoted to large scale deposition and growth on different substrates of choice.Unlike single layer graphene (SLG), where electrons disperse linearly as massless Dirac fermions [1,2,3,4,5], bilayer graphene (BLG) has two conduction and valence bands, separated by γ 1 , the interlayer coupling [12,13]. This was measured to be∼0.39eV by angle resolved photoelectron spectroscopy [14]. A gap between valence and conduction bands could be opened and tuned by an external electric field (∼100meV for∼10 13 cm −2 doping) [15,16], making BLG a tunable-gap semiconductor.Graphene can be identified in terms of number and orientation of layers by means of elastic and inelastic light scattering, such as Raman [17] and Rayleigh spectroscopies [18,19]. Raman spectroscopy also allows V TG Laser Spectrometer 50 X Objective SLG BLG Si Si SiO SiO 2 2 Pt A u A u FIG. 1: (color online). Experimental setup. The black dotted box on SiO2 indicates the polymer electrolyte (PEO + LiClO4). The left inset shows an SEM image of the SLG and BLG. Scale bar: 4µm. The right inset, the 2D Raman band monitoring of doping and defects [4,20,21,22,23,24,25]. Indeed, Raman spectroscopy is a fast and non-destructive characterization method for carbons [26]. They show common features in the 800-2000 cm −1 region: the G and D peaks, around 1580 and 1350 cm −1 , respectively. The G peak corresponds to the E 2g phonon at the Brillouin zone center (Γ). The D peak is due to the breathing modes of sp 2 atoms and requires a defect for its activation [27,28,29]. The most prominent feature in SLG is the second order of the D peak: the 2D peak [17]. This lies at ∼ 2700 cm −1 and involves phonons at K+∆q [17,23]. ∆q depends on the excitation energy, due to double-resonance, and the linear dispersion of the phonons around K [17,29,30]. 2D is a single peak in SLG, whereas it splits in four in BLG, reflecting the evolution of the band structure [17]. The 2D peak is always seen, even when no D peak is present, since no defects are required for overtone activation.In SLG, the effects of back and top gating on Gpeak position (Pos(G)) and Full Width at Half Maximum (FWHM(G)) were reported in Refs[20,21,...

We show the evolution of Raman spectra with a number of graphene layers on different substrates, SiO 2 /Si and conducting indium tin oxide (ITO) plate. The G mode peak position and the intensity ratio of G and 2D bands depend on the preparation of sample for the same number of graphene layers. The 2D Raman band has characteristic line shapes in single and bilayer graphene, capturing the differences in their electronic structure. The defects have a significant influence on the G band peak position for the single layer graphene: the frequency shows a blue shift up to 12 cm -1 depending on the intensity of the D Raman band, which is a marker of the defect density. Most surprisingly, Raman spectra of graphene on the conducting ITO plates show a lowering of the G mode frequency by ~ 6 cm -1 and the 2D band frequency by ~ 20 cm -1 . This red-shift of the G and 2D bands is observed for the first time in single layer graphene.

Femtosecond laser pulses and coherent two-phonon Raman scattering were used to excite KTaO3 into a squeezed state, nearly periodic in time, in which the variance of the atomic displacements dips below the standard quantum limit for half of a cycle. This nonclassical state involves a continuum of transverse acoustic modes that leads to oscillations in the refractive index associated with the frequency of a van Hove singularity in the phonon density of states.

Graphene has generated great sensation due to its amazing properties, and extensive research is being pursued on single-as well as bi-and few-layer graphenes. In this Perspective, we highlight some aspects of graphene synthesis, surface, magnetic, and mechanical properties, as well as effects of doping and indicate a few useful directions for future research.