**3. Nanophotonic hierarchical hologram**

#### **3.1 Concept**

We can see the hierarchy of optical near-fields and far-fields because optical near-field interactions are distinguishable with propagating light. This characteristic feature has led to hierarchical optical system designs, such as *nanophotonic hierarchical holograms* (Tate et al., 2008), where independent functions are associated with both optical near- and far-fields in the same device. Figure 3 shows the basic concept of the hierarchical hologram.

Nanophotonic Hierarchical Holograms:

**3.2 Near-field – Far-field hierarchy** 

retrieval.

Demonstration of Hierarchical Applications Based on Nanophotonics 345

First of all, in actual use of the hierarchical hologram, it is necessary to demonstrate that a nanometric structural change does not affect the optical response in the far-mode retrieval. To verify this, 500 nm-pitch Si diffraction gratings, in which nanometric depressions were embedded in the grid structures as near-mode data, were fabricated by using electron-beam (EB) lithography. A single isolated depression and multiple periodic depressions were embedded in each grating, as shown in Fig. 4(a) and (b), respectively. The size of each depression was less than 50 nm. The fabricated diffraction gratings were illuminated by the light from a He-Ne laser (λ = 633 nm), and the intensity of each diffracted beam was measured. Figure 4(c) shows the diffracted light pattern from the grating with periodic depressions. Large regular diffraction spots due to the grid structure and a number of tiny spots due to the periodic depressions were observed in the diffraction pattern. The diffraction efficiencies of each order of diffraction spots for each grating, calculated from the measured optical intensity of each spot, are shown in Fig. 4(d). The first-order diffraction efficiencies of a grating with no embedded depressions, the grating with the isolated depression, and the grating with the periodic depressions were 9.8 %, 9.7 %, and 10.3 %, respectively, showing a relative difference of less than 10 % between gratings with and without embedded depressions. No large differences were evident in the other diffraction orders either. This result shows that embedding the nanometric fabricated structures did not have a large effect on the function of conventional optical devices, that is, on their far-mode

Fig. 4. SEM images of fabricated gratings with (a) single isolated depression and (b) multiple periodic depressions. (c) Diffraction pattern from the grating with periodic depressions. (d) Compared diffraction efficiencies of each grating, where *n* represents the diffraction order.

Fig. 2. Conceptual image of hierarchy based on nanophotonics, and innovative optical security system utilizing the hierarchical structure. The important point is that, because induced optical near-fields are the result of interactions between nanometric structures, the hierarchical property can be designed by adjusting the size, shape, alignment, and composition of the nanometric structures.

In a nanophotonic hierarchical hologram, the physical scale of the nanometric structural changes is less than 100 nm, whereas the physical scale of the elemental structures of the hologram is larger than 100 nm. In principle, a structural change occurring at the subwavelength scale does not affect the optical response function, which is dominated by propagating light. Therefore, the visual aspect of the hologram is not affected by such a small structural change on the surface. Additional data can thus be written by engineering structural changes in the subwavelength regime so that they are only accessible via optical near-field interactions (we call this *near-mode* retrieval) without having any influence on the optical response obtained via the conventional far-field light (what we call *far-mode* retrieval). By applying this hierarchy, new functions can be added to conventional holograms.

Fig. 3. Basic concept of functional hierarchy of nanophotonic hierarchical hologram. In principle, no interference occurs between each layer, because optical near-field interactions are distinguishable from the conventional hologram view, which consists of diffracted propagating light.

#### **3.2 Near-field – Far-field hierarchy**

344 Holograms – Recording Materials and Applications

Fig. 2. Conceptual image of hierarchy based on nanophotonics, and innovative optical security system utilizing the hierarchical structure. The important point is that, because induced optical near-fields are the result of interactions between nanometric structures, the

In a nanophotonic hierarchical hologram, the physical scale of the nanometric structural changes is less than 100 nm, whereas the physical scale of the elemental structures of the hologram is larger than 100 nm. In principle, a structural change occurring at the subwavelength scale does not affect the optical response function, which is dominated by propagating light. Therefore, the visual aspect of the hologram is not affected by such a small structural change on the surface. Additional data can thus be written by engineering structural changes in the subwavelength regime so that they are only accessible via optical near-field interactions (we call this *near-mode* retrieval) without having any influence on the optical response obtained via the conventional far-field light (what we call *far-mode* retrieval). By

hierarchical property can be designed by adjusting the size, shape, alignment, and

applying this hierarchy, new functions can be added to conventional holograms.

Fig. 3. Basic concept of functional hierarchy of nanophotonic hierarchical hologram. In principle, no interference occurs between each layer, because optical near-field interactions are distinguishable from the conventional hologram view, which consists of diffracted

composition of the nanometric structures.

propagating light.

First of all, in actual use of the hierarchical hologram, it is necessary to demonstrate that a nanometric structural change does not affect the optical response in the far-mode retrieval. To verify this, 500 nm-pitch Si diffraction gratings, in which nanometric depressions were embedded in the grid structures as near-mode data, were fabricated by using electron-beam (EB) lithography. A single isolated depression and multiple periodic depressions were embedded in each grating, as shown in Fig. 4(a) and (b), respectively. The size of each depression was less than 50 nm. The fabricated diffraction gratings were illuminated by the light from a He-Ne laser (λ = 633 nm), and the intensity of each diffracted beam was measured. Figure 4(c) shows the diffracted light pattern from the grating with periodic depressions. Large regular diffraction spots due to the grid structure and a number of tiny spots due to the periodic depressions were observed in the diffraction pattern. The diffraction efficiencies of each order of diffraction spots for each grating, calculated from the measured optical intensity of each spot, are shown in Fig. 4(d). The first-order diffraction efficiencies of a grating with no embedded depressions, the grating with the isolated depression, and the grating with the periodic depressions were 9.8 %, 9.7 %, and 10.3 %, respectively, showing a relative difference of less than 10 % between gratings with and without embedded depressions. No large differences were evident in the other diffraction orders either. This result shows that embedding the nanometric fabricated structures did not have a large effect on the function of conventional optical devices, that is, on their far-mode retrieval.

Fig. 4. SEM images of fabricated gratings with (a) single isolated depression and (b) multiple periodic depressions. (c) Diffraction pattern from the grating with periodic depressions. (d) Compared diffraction efficiencies of each grating, where *n* represents the diffraction order.

Nanophotonic Hierarchical Holograms:

structures of the original hologram.

structure of the nanometric aperture itself.

**3.3.1 Numerical evaluations** 

Demonstration of Hierarchical Applications Based on Nanophotonics 347

Fig. 6. SEM images of embedded nanometric structures (a) outside and (b) inside the grid

Before conducting the experimental demonstration with fabricated samples, the electric fields at the surfaces of nanometric structures were numerically calculated by the finitedifference time-domain (FDTD) method. As shown in Figs. 7, two types of calculation models were created in order to examine polarization dependency in retrieving the nanophotonic code. The embedded nanometric structure was represented by a square aperture whose side length was 150 nm, shown near the centre of the model. On the other hand, in the model shown in Fig. 7(b), the pitch of the periodic one-dimensional wire-grid structure was 150 nm, and the depth was 100 nm, which models the typical structure of an embossed hologram, and an aperture of the same size as that in Fig. 7(a) was embedded. The material of structures was assumed to be Au, and the structures were assumed to be irradiated with polarized plane waves coming from far above the structures. The wavelength was set to 785 nm. Periodic-conditioned computational boundaries were located 1.5 μm away from the center of the square-shaped aperture. By comparing those two cases, we can predict the effect of the existence of grid structures serving as the environmental structures on nanophotonic code retrieval. Also, we chose the square-shaped structure, which is isotropic in both the *x* and *y* directions in order to clearly evaluate the effects of the environmental structures and ignore the polarization dependency originating in the
