Broadband gate-tunable terahertz plasmons in graphene heterostructures

Abstract

Graphene, a unique two-dimensional material of carbon in a honeycomb lattice [1], has brought remarkable breakthroughs across the domains of electronics, mechanics, and thermal transport, driven by the quasiparticle Dirac fermions obeying a linear dispersion [2-3]. Here we demonstrate a counter-pumped all-optical difference frequency process to coherently generate and control THz plasmons in atomic layer graphene with an octave tunability and high efficiency. We leverage the inherent surface asymmetry of graphene for a strong second-order nonlinear polarizability  (2) [4-5], which together with tight plasmon field confinement, enables a robust difference frequency signal at THz frequencies. The counter-pumped resonant process on graphene uniquely achieves both energy and momentum conservation. Consequently we demonstrate a dual-layer graphene heterostructure that achieves the charge-and gate-tunability of the THz plasmons over an octave, from 9.4 THz to 4.7 THz, bounded only by the pump amplifier optical bandwidth. Theoretical modeling supports our single-volt-level gate tuning and optical-bandwidth-bounded 4.7 THz phase-matching measurements, through the random phase approximation with phonon coupling, saturable absorption, and below the Landau damping, to predict and understand the graphene carrier plasmon physics. 2The discovery of graphene spurred dramatic advances ranging from condensed matter physics, materials science to physical electronics, mechanics, and thermal processes. In optics [6][7], the additional chiral symmetry of the Dirac fermion quasiparticles of graphene [8] enables an optical conductivity defined only by the fine structure constant  [9], one that is remarkably charge-density tunable [10][11] and with broadband nonlinearities [12][13][14][15]. The collective oscillations of the two-dimensional correlated quasiparticles in graphene [16] naturally make for a fascinating cross-disciplinary field in graphene plasmonics [17], with applications ranging from tight-field-enhanced modulators, detectors, lasers, polarizers, to biochemical sensors [18][19][20][21][22]. Different from conventional noble metal plasmons, graphene plasmons are dominant in the terahertz and far-infrared frequencies [23]. To excite and detect these plasmons, specialized techniques such as resonant scattering nanoscale antennae near-field microscopy or micro-and nano-scale scattering arrays have been pursued, albeit still using terahertz/far-infrared sources [24][25][26][27][28]. Recently nonlinear optical processes, only with free-space experiments, have proven especially effective in generating graphene plasmons with efficiencies up to 10 -5 [4][5]. However, to date, it is challenging to generate, detect, and control on-chip graphene plasmons all-optically, a key step towards planar integration and next-generation high-density optoelectronics.Concurrently THz generation has recently been revisited by a number of studies for imaging, spectroscopy, and communications [29]. While a wide tunability in THz can provide new g...

Doping-Induced Second-Harmonic Generation in Centrosymmetric Graphene from Quadrupole Response

“…We thus expect the EQ-type responses of graphene to have an even higher efficiency in the infrared frequency range. All these may explain the strong surface plasmon enhanced difference-frequency generation observed by Constant et al [32] and Yao et al [33].…”

“…The electron-hole symmetry operator is , which gives , | ⟩ | ̅ ⟩, and . We then have: 33 ( ) ( ), which leads to:…”

“…Weyl semimetals [28]. Therefore, graphene provides a unique platform for investigating unusual nonlinear optical phenomena, which not only can expand the horizon of nonlinear optics in quantum materials, but also has a large potential in novel device applications [33,34]. Supplemental Table S1.…”

Probing Polaritons in 2D Materials

“…Copyright 2015, Springer Nature. f) Gate tunability of the DFG graphene plasmons with V g increase from –0.2 to 0.2 V. Reproduced with permission . Copyright 2017, Springer Nature.…”

“…Constant et al excited graphene plasmons with a defined wavevector and direction across a large frequency range (up to tens of THz) by controlling the phase matching conditions (Figure e). Yao et al demonstrated THz graphene plasmons are generated and controllable with a counter‐pumped all‐optical DFG (Figure f). In addition, Yao et al showed DFG is also suitable for generating surface plasmons in Bi 2 Se 3 (a type of TIs), which has the same massless Dirac fermions feature as graphene.…”