Tailoring Terahertz Near-Field Enhancement via Two-Dimensional Plasmons

We suggest a novel possibility for electrically tunable terahertz near-field enhancement in flatland electronic materials supporting two-dimensional plasmons, including recently discovered graphene. We employ electric-field effect modulation of electron density in such materials and induce a periodic plasmonic lattice with a defect cavity. We demonstrate that the plasmons resonantly excited in such a periodic plasmonic lattice by an incident terahertz radiation can strongly pump the cavity plasmon modes leading to a deep subwavelength concentration of terahertz energy, beyond $\lambda /1000$, with giant electric-field enhancement factors up to ${10}^4$, which is 2 orders of magnitude higher than achieved previously in metal-based terahertz field concentrators.

Figure 1

Schematic of the split-grating-gate field-effect transistor, where the central gate contact is biased individually. The inset shows an example of the concentration profile in the channel.

Figure 2

(a) Plasmonic cavity resonance frequency as a function of 2D electron density under the defect gate for four cavity modes. The dashed line shows the position of the lattice plasmon resonance. Dots mark the positions of the third and fourth order cavity resonances shown in panel (b). (b) Cavity excitation factor spectrum for the cavity depletion ratio $N_c/N_0=0.12$. Numbers indicate the resonance orders. (c),(d) The mode profiles at the lattice resonance ($\omega =0.336\mathrm{THz}$) and at the fourth order cavity plasmon resonance ($\omega =0.388\mathrm{THz}$), respectively.

Figure 3

(a) Cavity excitation factor spectrum for different depletion ratios, $N_c/N_0$. The dashed line shows the position of the unperturbed lattice plasmon resonance. (b) Dispersions of the third and fourth cavity resonances as functions of the cavity depletion ratio. Solid curves show the dispersion of corresponding unperturbed cavity modes. The dashed line marks the position of the unperturbed lattice plasmon resonance. (c) The power absorbed in a unit cell of the plasmonic lattice with increasing the lateral distance from the cavity (the power absorbed by the cavity is not featured here).

Figure 4

(a) Normalized electric-field profile in the 2D electron channel and (b) maps of the electric-field distribution in the ($x\mathrm{\text{−}}y$) plane for $\tau ={10}^{-11}\mathrm{s}$; (c),(d) the same as in (a) and (b) for $\tau ={10}^{-10}\mathrm{s}$.