Nanophotonic code

As shown in Fig. 2,15, we created a sample device to experimentally demonstrate the retrieval of a nanophotonic code within an embossed hologram. The entire device structure, whose size was 15 mm x 20 mm, was fabricated by electron-beam lithography on an Si substrate, followed by sputtering a 50 nm-thick Au layer, as schematically shown in the cross-sectional profile in Fig. 2.15b. 

---

Fig. 2.15 (a) Fabrication of a nanometric structure as a nanophotonic code within the embossed stmcture of Virtuagram , and SEM images of various designed patterns serving as nanophotonic codes. 

Fig. 2.16 (a) Calculation model of embedded nanophotonic code with environmental stnictuies and calculated intensity distribution of electric field produced by (b) Ac-polarized input light and (c) y-polarized input light, (d) Ctilculation model of isolated ntmophotonic code and calculated intensity distribution of electric field produced by (e) Ac-polarized input light and (f) y-polarized... 

As shown in Fig. 2.16a, the square-shaped structure was embedded in a periodic one-dimensional wire-grid structure, whose pitch was 150 nm, which models the typical structure of an embossed hologram. As shown in Fig.2.16d, on the other hand, the square-shaped structure, whose size was the same as that in Fig. 2.16a, was not provided with any grid structure. By comparing those two cases, we can evaluate the effect of the environmental structures around the nanophotonic code. Also, we chose the square-shaped structure that is isotropic in both the x and y directions to clearly evaluate the effects of environmental structures and ignore the polarization dependency originating in the structure of the nanophotonic code itself. Periodic-conditioned computational boundaries were located 1.5 im away from the center of the square-shaped structure. The wavelength was set to 785 nm. 

Second, from the viewpoint of facilitating recognition of the nanophotonic code embedded in the hologram, it is important to obtain a kind of higher visibility for the signals associated with the nanophotonic codes. To evaluate such visibility, here... 

These mechanisms indicate that such nanophotonic codes embedded in holograms could also exploit these polarization and structural dependences, not only for retrieving near-mode information via optical near-field interactions. For instance, we could facilitate near-mode information retrieval using suitable input light polarization and environmental structures. 

Fig. 2.18 (a) Schematic diagram of the experimental setup for retrieving a nanophotonic code, and (b) basic retrieval results... 

Figure 2.18b summarizes the experimental results obtained in retrieving nanophotonic codes which were not embedded in the hologram. In this demonstration, different shapes of nanophotonic codes were formed at the positions marked by the dashed circles in Fig. 2.5b. For the first step of our demonstration, the device was irradiated with randomly polarized light by removing the polarizer from the experimental setup. Clear near-field optical distributions that depended on the structures of the nanophotonic codes were obtained.

Figures 2.19 and 2.20 show other retrieved results of nanophotonic codes that were embedded in the hologram and not embedded in the hologram, respectively.

Fig. 2.19 Observed NOM images of optical intensity distributions of retrieved nanophotonic code embedded in environmental structures with (a) standard polarization tind (b) polarization rotated by 60°, and (c) NOM images observed by radiating light with various polarizations...

Figure 2.21b shows Kxp as a function of input light polarization based on the NOM results shown in Figs. 2.19c and 2.20c. The nanophotonic code embedded in the hologram exhibited much greater polarization dependency, as shown as in a hologram in Fig. 2.21b, where the maximum Fexp was obtained at 80° input polarization, whereas only slight polarization dependency was observed with the isolated nanophotonic code, as shown as w/o a hologram in Fig. 2.21b. Such polarization dependence in retrieving the nanophotonic code agrees well with the results of the simulations in Fig. 2.16.

Fig. 2.21 (a) Schematic diagram explaining definition of I x) and /(x ))env> and (l>) calculated experimental visibility Fexp of embedded nanophotonic code and that of isolated nanophotonic code, showing actualization of optical response as evident visibility and its polarization... 

Fig. 5. Schematic diagram of the sample device for demonstration of nanophotonic hierarchical hologram with a nanophotonic code embedded within the embossed structure of Virtuagram .

Fig. 10. Schematic diagram of the experimental setup for retrieving a nanophotonic code.

Figures 11(a) and (b) show retrieved results of nanophotonic codes that were outside and inside the environmental grid structures of the hologram, respectively, using a linearly polarized radiation source rotated by 0 degree to 180 degree at 20-degree intervals. As is evident in Fig. 11(a), although small and noisy intensity distributions were obtained, clear polarization dependence was observed in Fig. 11(b) for example, from the area of the nanophotonic code located in the center, a high-contrast signal intensity distribution was obtained with polarizations aroimd 80 degree.

To quantitatively evaluate the characteristics of the embedded nanophotonic code, we investigated two kinds of intensity distribution profiles from the observed NOM images. One is a horizontal intensity profile along the dashed line in Fig. 12(a), which crosses the area of the nanophotonic code, denoted by I(x), where x represents the horizontal position. The other was also an intensity distribution as a function of horizontal position x however, at every position x, we evaluated the average intensity along the vertical direction within a range of 2.5 (im, denoted by which indicates the environmental signal... 

---