Ultrawide-Bandwidth Slow-Light System Based on THz Plasmonic Graded Metallic Grating Structures

We explore a novel mechanism for slowing down THz waves based on metallic grating structures with graded depths, whose dispersion curves and cutoff frequencies are different at different locations. Since the group velocity of spoof surface plasmons at the cutoff frequency is extremely low, THz waves are actually stopped at different positions for different frequencies. The separation between stopped waves can be tuned by changing the grade of the grating depths. This structure offers the advantage of reducing the speed of the light over an ultrawide spectral band, and the ability to operate at various temperatures, but demands a stringent requirement for the temperature stability.

Figure 1

Dispersion curves calculated for $d=20\mu \mathrm{m}$ and $p=50\mu \mathrm{m}$ with different groove depths ($h=50$ and $110\mu \mathrm{m}$). Inset: Schematic illustration of the graded grating structure.

Figure 2

Reciprocal of the $v_g$ of the SPP modes are calculated according to the dispersion curves in the gray region in Fig. 1.

Figure 3

Results obtained by using 2D FDTD simulations: (a) corresponds to the 2D field distribution at four different frequencies of 0.6, 0.7, 0.8, and 0.9 THz. (b) and (c) represent 1D optical intensity ($|E{|}^2$) distribution $2\mu \mathrm{m}$ above the structured metal surface. The depth of the grating structure in (b) and (c) changes from 50 to $150\mu \mathrm{m}$ linearly [the inset in (b)] and from 50 to $130\mu \mathrm{m}$ linearly [the inset in (c)] from the left-side end to the right-side end, respectively. The dimension of the simulated region is $3600\mu \mathrm{m}\times 800\mu \mathrm{m}$ with a uniform cell of $\Delta x=2\mu \mathrm{m}$ and $\Delta z=2\mu \mathrm{m}$. This simulated region is surrounded by a perfectly matched layer absorber.