Figure 1

Generation of nondiffracting SPPs. Methods used to generate (a) the CGB and (b) the LCGB. Scheme (a) by symmetry launches bidirectional CGBs, when illuminated, while scheme (b) is a one-directional launcher of LCGBs. Each pair of red lines represents grooves ($D\sim 10\mu \mathrm{m}$; $\theta \sim 5°$) that are part of metallic gratings sculpted on an Au film deposited on a glass substrate, as illustrated in the scanning electron micrographs (c),(d). SPPs are excited at normal incidence by a focused Gaussian beam from a laser ($\lambda \sim 0.74\mu \mathrm{m}$) polarized parallel to the $x$ axis. The SPP plane waves of equal amplitude launched by the pair of gratings interfere constructively to form nondiffracting surface waves for a propagation distance $x_{\max }=D/\sin \theta \sim 120\mu \mathrm{m}$. The period of the gratings is designed to match the wavelength of SPPs, so that the latter can be resonantly excited by vertical illumination. Scheme (b) gives a SPP beam more localized in the transverse direction than that in (a). (e) Experimental setup. The SPP field distribution is measured by an apertured NSOM.Reuse & Permissions

Figure 2

CGB generated by two overlapped coupling gratings as shown in Fig. 1c. The near-field intensity distributions (in-plane components) obtained from (a) FDTD simulations and (b) experimentally using a NSOM. (c)–(f) Transverse intensity distributions at specific propagation distances $x$ [green curve, analytical calculations; red curve, simulations by FDTD; blue curve, intensity line scans obtained from the NSOM image in (b)]. Note that it takes a distance of several microns to form the nondiffracting beam, which can be understood by observing that the latter is fully formed only after the surface plasmon polaritons launched by the grooves of the two gratings have sufficiently overlapped Fig. 1a. Similar considerations apply to a localized cosine-Gaussian beam (Fig. 3)Reuse & Permissions

Figure 3

The LCGB generated by two gratings at an angle as shown in Fig. 1d. The near-field intensity distributions (in-plane components) obtained (a) by using FDTD simulations and (b) experimentally with an NSOM. (c)–(f) Transverse intensity distributions at specific propagation distances [green curve, analytical calculations; red curve, simulations by FDTD; blue curve, intensity line scans obtained from the NSOM image (b)].Reuse & Permissions

Figure 4

(a) The FWHM of the main lobe of the LCGB at the propagation distance $x=40\mu \mathrm{m}$ as a function of the half angle $\theta$, which defines the relative orientation of the gratings. The inset shows that the two types of nondiffracting SPPs have comparable width. (b) FWHM of the main lobe of the LCGB versus propagation distance. Experimental data (blue cross) and FDTD simulations (red solid line) are compared for $\theta =5°$. The initial drop in beam width is related to the finite distance required to form the nondiffracting beam.Reuse & Permissions