Folding-assisted plasmonically induced transparency in coupled graphene nanodisks

Abstract

Plasmonically induced transparency (PIT), a classical analogue of electromagnetically induced transparency in quantum systems, could offer an efficient way to engineer light-matter interaction and thus achieve particular manipulation of light. To realize PIT, the crucial point is the design of two strongly coupled modes, with one bright and the other dark. The common practice often takes dipole resonance as the bright mode, while higher order resonance as the dark one. We present here an alternative scheme to realize PIT in coupled graphene nanodisks through mechanical folding, where only fundamental dipole resonances are involved, and the dark mode is derived from two nondegenerate dipole resonances in folded nanodisk. Two parameters (folding angle and rotating angle ) arising from folding can be utilized to tune PIT. They determine two different PIT windows separately. By varying either one of them, single-window PIT can be switched to double-window PIT, and vice versa. Besides the mechanical operations, the proposed PIT can also be actively tuned by electrostatic gating. To understand the mechanism properly, a theoretical model of two coupled harmonic oscillators is presented, from which the predictions show good agreement with simulations.

Broadband perfect optical absorption enabled by quasi-bound states in the continuum in graphene non-concentric rings

“…As well known, the eigenmodes of a plasmonic system are a set of self-standing electromagnetic excitations with different resonance frequency o and damping rate g. Through structural modulation, the coupling between them can occur, which might be employed to make high-Q resonances (quasi-BICs) and BICs. 45 Here, the involved resonances are a dipole and a quadrupole, and their coupling can be simply described by a model of two coupled harmonic oscillators, 19,46 as presented in a matrix form.…”

“…Practically, the evolution of quasi-BICs can be described quantitatively by deriving an analytical formula of the transmission spectrum from eqn (1). By calculating the displacement x D/Q of two oscillators, 19 their susceptibility defined as w = (q D x D + q Q x Q )/E 0 can be readily written down as…”

“…Through structural modulation, the coupling between them can occur, which might be employed to make high-Q resonances (quasi-BICs) and BICs. 45 Here, the involved resonances are a dipole and a quadrupole, and their coupling can be simply described by a model of two coupled harmonic oscillators, 19,46 as presented in a matrix form.where the time dependent e − iωt has been implemented, and the subscripts D and Q refer to the dipole and the quadrupole, respectively. In general, the damping rates γ D and γ Q consist of radiative loss dictated by geometrical parameters, and intrinsic (non-radiative) loss due to material properties.…”

“…Practically, the evolution of quasi-BICs can be described quantitatively by deriving an analytical formula of the transmission spectrum from eqn (1). By calculating the displacement x D/Q of two oscillators, 19 their susceptibility defined as χ = ( q D x D + q Q x Q )/ E 0 can be readily written down aswhere the auxiliary parameter η = q Q m Q −1 /( q D m D −1 ). Then, based on the Kramers–Kronig relations, the transmission of such plasmonic systems is directly given by T = 1 − lm( χ ), 46 which offers a theoretical method to calculate the transmission spectrum of quasi-BICs.…”

“…[5][6][7] In contrast to plasmons in three-dimensional (3D) noble metals, they exhibit significant advantages in field confinement and propagation, [8][9][10] and more remarkably, their resonance frequencies lying in the terahertz (THz) and/or infrared region can be tuned dynamically through electrostatic gating, 11,12 static magnetic field, [13][14][15][16] and mechanical folding. [17][18][19] By placing a metal reflector, sharing an identical idea of metal-insulator-metal (MIM) designs, graphene plasmons can achieve exact 100% OA at plasmonic resonance frequencies, [20][21][22] thus achieving narrowband complete OA. However, because of the discrete feature of these frequencies, it is quite challenging to engineer broadband complete OA in plasmonic systems, where a basic trade-off occurs between absorption efficiency and bandwidth.…”