1. Introduction

Many anti-counterfeiting techniques have been proposed in the fields of security and product authenticity verification [1]. Optical techniques, which represent one kind of anti-counterfeiting, have been well established, i.e., confidential information can be hidden in any of the physical attributes of light, such as phase, wavelength, spatial frequency, or polarization [2–4]. For example, holography, which generates natural three-dimensional images, is the most common anti-counterfeiting techniques [5]. The surface of the hologram is ingeniously designed into a complicated structure, and it diffracts incident light in a specific direction. A number of diffracted lights can form an arbitrary three-dimensional image. Generally, the structures are recognized as being difficult to duplicate. Because of the difficulty, a hologram has been widely used in the anti-counterfeiting of bills, credit cards, etc. However, conventional anti-counterfeiting methods based on the physical appearance of holograms are less than 100% secure [6]. Although they provide ease of authentication, adding another securing feature without causing any losses to the appearance is quite difficult.

However, advances in nanophotonics, by utilizing optical near-field interactions, allow optical devices and systems to be designed at densities beyond those conventionally constrained by the diffraction limit of light [7]. Several nanophotonic fabrications and device operations based on optical near-field techniques have been proposed [8, 9]. Because several physical parameters of “propagating” light are not affected by nanometric structures, conventional optical responses in the far-field also are not affected by these structures. This means another functional hierarchy in an optical near-field regime can be added in conventional optical devices and systems without any loss of the primary quality, such as reflectance, absorptance, refractive index, and diffraction efficiency.

In this paper, we describe our application of the nanophotonics techniques to holography: a “hierarchical hologram.” We also describe our demonstration of the concept using commercial optical devices. In Section 2, we discuss the fundamental concept of our hierarchical hologram. Section 3 shows the experimental setup and the results of the demonstration using an embossed hologram. For a quantitative evaluation of our method, we measured the efficiency of a diffraction grating with and without additional nanostructures. Section 4 concludes the paper.

2. Principles of hierarchical hologram

Existing optical devices and systems operate based on several phenomena of “propagating” light. However, the performance is generally limited by the diffraction of light, which is one of the most typical features of propagating light [10]. Therefore, reducing the physical scale beyond over the scale of the optical wavelength is difficult. The critical dimension of the conventional hologram is also bounded by the diffraction limit, because the function of a conventional hologram is based on the diffraction of light.

Nanophotonics has been proposed as an innovative technology that exceeds the diffraction limit [7]. It is defined as utilizing local electromagnetic interactions between nanometric matter and an optical near-field. An optical near-field is free from diffraction, because its optical energy localizes on the nanometric material surface and because the size of the localization is equivalent to the size of the material. Generally, the optical near-field is observed by scanning a fiber probe on the material surface. During the scan, the optical near-field is scattered by an interaction between the fiber probe and the material. The scattered energy is observed via the fiber probe as the local optical response of the nanometric material. Therefore, the resolution of observing the optical near-field depends directly on the size of the fiber probe, and it is independent of the diffraction limit.

Our “nanophotonic hierarchical hologram” is defined as a hologram that has multiple observing layers. It can be created by adding a nanometric structural change (<100nm) to a conventional hologram (>100nm). Figure 1 shows the basic composition of the hierarchical hologram. In principle, the phenomenon occurring at a subwavelength scale does not affect the function induced by propagating light. Therefore, the visual aspect of the hologram is not affected by such a small structural change on the surface. Figure 1 shows the optical response of the hologram, which can be revealed by the propagating light during a “far-mode” observation, and that of a nanometric structural change can be revealed by the optical nano-field during a “near-mode” observation. Because the nanometric structural changes have an insignificant effect on the optical response at the far-mode observation, additional data can be written-in to the nanometric layer without any incident. By applying this hierarchy, new functions can be added to a conventional hologram.

 

Fig. 1. Basic concept of functional hierarchy of hierarchical hologram. In principle, no interference occurs between the two layers, “far-mode” and “near-mode”.

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The number of layers can be increased in the “near-mode” observation to further extend the hierarchical function. An optical near-field interaction between multiple nanometric structures causes characteristic spatial distribution depending on the size, the alignment, etc. Therefore, various optical signal patterns can be observed depending on the size of the fiber probe, and another layer is added in the “near-mode” observation [11, 12].

3. Determining of optical response at near-field and far-field

In actual use of the hierarchical hologram, we needed to demonstrate that nanometric structural change does not affect the optical response during the far-mode observation. At the same time, obvious change must be observed during the near-mode observation.

We used a commercially available embossed hologram in our experiment as a sample of nanometric fabrication. Because an embossed hologram is easily mass produced at a low cost, it is the type used in most security applications, such as credit cards and bank bills [13]. A 40-nm-thick Au layer was coated on the sample surface of the hologram at the nanometric level. Then, 40 nanometric holes were fabricated in a 10 µm×10 µm region using a focused ion beam (FIB) system. Figure 2(a) shows an observed CCD image of the sample hologram. Figure 2(b) and (c) show optical microscope images of the non-fabricated hologram and fabricated hologram, respectively, which are optical responses of the hierarchical hologram during the far-mode observation. No difference was evident in a comparison between Figs. 2(b) and (c). The compared results indicate the independence of the far-mode observation from nanometric fabrication of the hierarchical hologram.

 

Fig. 2. (a) Observed CCD image of sample hologram. Magnified image of (b) non-fabricated hologram and (c) fabricated hologram. The marked region in (a) corresponds with the magnified region at (b) and (c).

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Optical responses during a near-mode observation were detected using a near-field optical microscope (NOM). A schematic diagram of the detecting setup is shown in Fig. 3(a), in which a NOM was operated in an illumination-collection mode with a near-field probe having a tip with a curvature radius of 5 nm (see Figs. 3(b) and (c)). The fiber probe was connected to a tuning fork. Its position was finely regulated by sensing of shear force with the tuning fork and was fed back to a piezoelectric actuator. The light source used was a laser diode (LD) with an operating wavelength of 785 nm, and scattered light was detected by a photomultiplier (PMT).

 

Fig. 3. (a) Schematic diagram of near-mode observation using NOM and composition of sample hologram. (b) SEM images of fiber probe.

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Figure 4(a) shows a scanning electron microscope (SEM) image of three nanometric holes that were fabricated on a hologram. The diameter of each hole is less than 100 nm, and some structural changes were observed on the rim of each hole. Magnified SEM images of each hole and the optical response during the near-mode observation are shown in Figs. 4(b)–(d). Evident optical responses were observed, which were attributed to an optical near-field generated on the rim of each hole.

 

Fig. 4. (a) SEM image of fabricated nanometric holes on hologram. (b)–(d) Magnified images of each hole at (a) (left), and corresponding optical response observed by NOM (right).

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The experimental results in Figs. 2 and 4 show that nanometric fabrications do not affect far-mode observations, and they evidently affect near-mode observations. These results indicate that conventional functions of a hologram at the far-field were not adversely affected by adding another functional layer in the near-field.

We replaced the embedded hologram with a diffraction grating for a quantitative evaluation of the independence between nanometric fabrications and a far-mode observation. After fabricating nanometric holes on the surface of the grating, we measured the diffraction efficiency and compared the efficiency with that of grating with no holes. Figure 5 shows a 40-nm-thick Au layer that was coated on the surface of the grating (600 lines/mm) and 25 nanometric holes (ϕ 100 nm) that were fabricated using a 100 µm pitch with an FIB system. The fabricated region was illuminated by the light from the LD (λ=532 nm), and the diffracted light intensity was measured. Figures 6(a) and (b) show the experimental results. For example, the first-order diffraction intensity of each result was 30.9% and 29.6%, respectively, and the difference was only about 10%. No differences were evident recognized in other orders of diffraction lights, as well. This means that the nanometric fabrications do not have a profound effect on the optical devices.

 

Fig. 5. (a) Composition of fabricated grating, and (b) magnified SEM images of nanometric fabrication.

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Fig. 6. Diffraction efficiencies of (a) non-fabricated grating and (b) fabricated grating.

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4. Summary

In this paper, we described a demonstration of the basic concept of our “hierarchical hologram” and an experiment involving two hierarchical layers using a far-mode and near-mode observation. Our concept can be applied not only to a hologram but also any other media, such as lens and jewelry. Adding extra functions creates value-added media with only a few deficits in the primary functions. However, a trade-off occurs between the conditions of nanometric fabrications (e.g., size and pitch) and deficit of the primary functions. For actual use to several media, the trade-off in each media is under investigation by the authors.