Enhanced optical conductance in graphene superlattice due to anisotropic band dispersion

Abstract

The optical response of a Kronig-Penney type graphene superlattice is investigated. When an external field is applied along the periodicity of the superlattice, the total optical response of the graphene superlattice is enhanced due to the formation of anisotropic Dirac fermions. Such anisotropy tunes up the total optical spectra while maintaining the same critical electric field regardless of the degree of anisotropy. The optical conductance of anisotropic Dirac fermions exhibits two contrasting behaviours: (i) inversely proportional to the anisotropy and (ii) directly proportional to the anisotropy, depending on the direction of the external field. Interestingly, the anisotropy-induced optical conductance enhancement also occurs in gapped graphene with band structure anisotropy. This suggests that the enhanced electron-photon couplings in the presence of anisotropy is a general feature of the relativistic nature of the Dirac fermions in both massless and massive form. It is also revealed that the strong optical nonlinearity is a consequence of the relativistic nature of the Dirac fermions and the Dirac cone isotropy is not required. © 2012 IOP Publishing Ltd. AbstractThe optical response of a Kronig-Penney type graphene superlattice is investigated. When an external field is applied along the periodicity of the superlattice, the total optical response of the graphene superlattice is enhanced due to the formation of anisotropic Dirac fermions. Such anisotropy tunes up the total optical spectra while maintaining the same critical electric field regardless of the degree of anisotropy. The optical conductance of anisotropic Dirac fermions exhibits two contrasting behaviours: (i) inversely proportional to the anisotropy and (ii) directly proportional to the anisotropy, depending on the direction of the external field. Interestingly, the anisotropy-induced optical conductance enhancement also occurs in gapped graphene with band structure anisotropy. This suggests that the enhanced electron-photon couplings in the presence of anisotropy is a general feature of the relativistic nature of the Dirac fermions in both massless and massive form. It is also revealed that the strong optical nonlinearity is a consequence of the relativistic nature of the Dirac fermions and the Dirac cone isotropy is not required.

Nonlinear optical response of the α−T3 model due to the nontrivial topology of the band dispersion

“…Three-photon valence-to-conduction band process found in similar quantum frameworks [90,91,99]) whereas for α = 1 the single-photon flat-to-conduction band process is dominant. The dominant contribution from the flat band is attributed to its high density of states compared to that of the valence band.…”

Nonlinear optical response in Kronig–Penney type graphene superlattice in terahertz regime

“…Theoretically, electron beam supercollimation in graphene superlattice was investigated; 23 the new massless Dirac fermions in graphene under external periodic potentials have been generated; 24 the optical conductance in graphene superlattice was enhanced due to anisotropic band dispersion. 25 Experimentally, patterns with periodicity as small as 5 nm have been imprinted on graphene through electronbeam induced deposition of adatoms; 26 the triangular patterns with about 10 nm lattice period have been observed for graphene on metal surfaces; 27,28 the graphene superlattice was made using periodically patterned gates. 24 The K-point electron in graphene is usually isotropic in every direction with v F = 10 6 ms −1 .…”

“…Such a quasiparticle nature is similar to a massless Dirac fermions travelling in anisotropic spacetime. 24,30,31 Ang and Zhang investigated the optical response of third-order in terahertz frequency regime, 25 where the optical nonlinearity is perfectly protected from band structure anisotropy.…”

Temperature-dependent of Nonlinear Optical Conductance of Graphene-based Systems in High-intensity Terahertz Field

“…The optical properties of graphene have been studied both experimentally [9][10][11][12][13][14][15][16][17] and theoretically [18][19][20][21][22][23][24][25][26][27][28][29][30]. In the terahertz to the far-infrared (FIR) spectral regime, optical conductance of graphene-based systems attracts many researchers' interests due to the ongoing search on viable terahertz detectors and emitters.…”

“…This may make graphene as a simple and natural frequency multiplier and open up exciting opportunities for using graphene in terahertz electronics. Subsequently, a series of theoretical works were carried out to seek for the strong nonlinear terahertz responses of graphene-based systems, such as single layer graphene (SLG) [23][24][25], bilayer graphene (BLG) [26], graphene p-n junction [27], graphene nanoribbons (GNRs) [28], semihydrogenated graphene (SHG) [29] and Kronig-Penney type (KP) graphene superlattice [30]. As studied by Hendry et al in 2010 [15], they first measured the coherent nonlinear optical response of single-and few-layer graphene using four-wave mixing.…”