Gain-Assisted Slow to Superluminal Group Velocity Manipulation in Nanowaveguides

We study the energy propagation in subwavelength waveguides and demonstrate that the mechanism of material gain, previously suggested for loss compensation, is also a powerful tool to manipulate dispersion and propagation characteristics of electromagnetic pulses at the nanoscale. We show theoretically that the group velocity in lossy nanowaveguides can be controlled from slow to superluminal values by the material gain and waveguide geometry and develop an analytical description of the relevant physics. We utilize the developed formalism to show that gain-assisted dispersion management can be used to control the transition between “photonic-funnel” and “photonic-compressor” regimes in tapered nanowaveguides. The phenomenon of strong modulation of group velocity in subwavelength structures can be realized in waveguides with different geometries and is present for both volume and surface modes.

Figure 1

(a) The schematic geometry of a multilayered waveguide; a two-layer cylindrical system with axial symmetry is shown. (b) Dielectric permittivity of Ag; real and imaginary parts are shown in solid and dashed lines, respectively. (c) Real (top) and imaginary (bottom) parts of the dielectric permittivity of the rhodamine-6G model (see text). Solid blue, dashed green, dashed-dotted brown, and dashed-dotted-dotted red curves correspond to gain the values 0%, 33%, 66%, and 100%, respectively.

Figure 2

Group and phase velocities of an SPP on a metal nanorod in Rh6G methanol solution (see text) as functions of gain, (a),(c) frequency, and (b),(d) radius. The radial dependence is given at $\lambda =534\mathrm{nm}$, the wavelength one at $R=35\mathrm{nm}$.

Figure 3

Group and phase velocities of the ${\mathrm{TM}}_{01}$ mode in an anisotropy-based cylindrical subdiffraction waveguide (see text) as functions of gain, (a),(c) frequency, and (b),(d) radius. Radial dependence is given at $\lambda =534\mathrm{nm}$, the wavelength one at $R=35\mathrm{nm}$. Note that the phase velocity is negative.