Unconventional plasmon-phonon coupling in graphene

Abstract

We predict the existence of coupled plasmon-phonon excitations in graphene by using the selfconsistent linear response formalism. The unique electron-phonon interaction in graphene leads to unconventional mixing of plasmon and optical phonon polarizations. We find that longitudinal plasmons couple exclusively to transverse optical phonons, whereas graphene's transverse plasmons couple only to longitudinal optical phonons. This coupling can serve as a magnifier for exploring the electron-phonon interaction in graphene, and it offers novel electronical control over phonon frequencies.PACS numbers: 73.20. Mf,73.22.Lp,63.22.Rc,78.67.Wj The interaction of electrons and crystal lattice vibrations (phonons) has fundamental implications on properties of materials and leads to diverse many-body phenomena such as superconductivity and charge-density waves. The electron-phonon interaction takes an unusual form in graphene, a recently discovered two-dimensional (2D) material [1] (see, e.g., [2] for a review), and its implications are far from being explored. These include the breakdown of the Born-Oppenheimer approximation [3], anomaly of the optical phonon [4], and nonadiabatic Khon anomaly [5]. However, the interaction of collective electron excitations (plasmons) and optical phonons has not yet been presented for graphene. Plasmons in graphene are of fundamental scientific interest [6][7][8][9][10][11][12], but they also hold potential for technological applications (e.g., in the context of plasmon lasers [9] and metamaterials [12]). Besides the ordinary longitudinal plasmons (transverse magnetic modes) [6][7][8][9]12], graphene also supports unusual transverse plasmons (transverse electric modes) [8]. The hybridization of plasmon and phonon modes is a striking manifestation of the breakdown of the Born-Oppenheimer approximation, because it occurs when phonons and electrons are on comparable energy scales. Plasmon-phonon coupling has been studied in bulk semiconductors [13,14], systems with reduced dimensionality (see e.g. [15][16][17][18]), and in the context of graphene, plasmons were shown to couple to surface optical phonons of the substrate (e.g., SiC, which is a polar material) [11,19,20]. However, to the best of our knowledge, the phenomenon of plasmon-phonon coupling was not yet presented for an isolated 2D material. Here we predict the coupling of plasmons with intrinsic optical phonons in graphene by using the selfconsistent linear response formalism. We find that, in contrast to all other known systems in nature, longitudi- * Electronic address: mjablan@phy.hr † Electronic address: soljacic@mit.edu ‡ Electronic address: hbuljan@phy.hr nal plasmons (LP) couple only to transverse optical (TO) phonons [21], while transverse plasmons (TP) couple only to longitudinal optical (LO) phonons. The LP-TO coupling is stronger for larger concentration of carriers, in contrast to the TP-LO coupling (which is fairly weak). The former could be measured via current experimental techniques. Thus, plasmon-phonon resonance cou...

Spatial control of defect creation in graphene at the nanoscale

“…Recent work has shown that flexural phonon modes have an essential role in thermal conductance in suspended graphene 36 . Graphene also has unusual and strong coupling between plasmons and phonons 37,38 . Thus, an inelastic collision may trigger a flexural phonon mode either directly or through plasmon coupling that causes a temporary increase in the out-of-plane motion of surrounding atoms, and if a subsequent elastic collision were to occur with one of these atoms then sputtering may result.…”

Photocurrent in graphene harnessed by tunable intrinsic plasmons

“…However, the excitation of intrinsic graphene plasmons allows the tuning of strong absorption into the mid-infrared [22][23][24] . Intrinsic graphene plasmons also produce high confinement of electromagnetic energy in sub-wavelength dimensions in combination with a longer lifetime of the excitation as compared with plasmons in metals 25 . Moreover, unlike surface plasmons in metals [18][19][20]26 , plasmons in graphene can be effectively tuned with a backgate voltage, potentially enabling optoelectronic switching devices.…”

“…Plasmons are excited when the standing plasmon resonance of the graphene array, tuned by gate modulation of the chemical potential, coincides with a selected incident light frequency. These plasmons decay into hot electron-hole pairs via scattering from the edges of the nanostructures or via inelastic scattering with optical phonons 25,31 . Longer-lived hybrid plasmon-phonon modes that also have narrower spectral widths owing to the uncertainty principle can be induced through coupling with the underlying SiO 2 substrate 31,32 .…”

Novel Midinfrared Plasmonic Properties of Bilayer Graphene

“…It has many desirable properties such as gate-tunability, extreme light confinement, long plasmon lifetime, and plasmonic resonances in the terahertz to mid-infrared (IR) regime [8][9][10][11][12][13]. Spatially resolved propagating plasmons has been observed with scanning near-field optical microscope [14,15] In this paper, we discuss why Bernal AB-stacked bilayer graphene is important and interesting in its own right as a plasmonic material.…”