Holography, which generates natural three-dimensional images, is one of the most common anti-counterfeiting techniques [1
1. R. L. Van Renesse (ed), Optical document scanning, (Altech House Optoelectronics Library, 1998).
]. In the case of a volume hologram, the surface is ingeniously formed into microscopic periodic structures which diffract incident light in specific directions. A number of diffracted light beams can form an arbitrary three-dimensional image. Generally, these microscopic structures are recognized as being difficult to duplicate, and therefore, holograms have been widely used in the anti-counterfeiting of bank notes, credit cards, etc. However, conventional anti-counterfeiting methods based on the physical appearance of holograms are less than 100% secure [2
2. S. P. McGrew, “Hologram counterfeiting: problems and solutions,” Proc. SPIE, Optical Security and Anticounterfeiting Systems , 1210, 66–76 (1990).
]. Although they provide ease of authentication, adding other security functions without degrading the appearance is quite difficult.
Previously, we have proposed a hierarchical hologram
, which is created by applying nanometric structural changes to the surface structure of a conventional hologram [3
3. N. Tate, W. Nomura, T. Yatsui, M. Naruse, and M. Ohtsu, “Hierarchical Hologram based on Optical Near- and Far-Field Responses,” Opt. Express 16, 607–612 (2008). [CrossRef] [PubMed]
]. The physical scales of the nanometric structural changes and the elemental structures of the hologram are less than 100 nm and larger than 100 nm, respectively. In principle, a structural change occurring at the subwavelength scale does not affect the optical response functions, which are 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 such information retrieval near
retrieval) without having any influence on the optical responses obtained via the conventional far-field light (what we call far
retrieval). By applying this hierarchy, new functions can be added to conventional holograms.
In this paper, we propose embedding a nanophotonic code, which is physically a subwavelength-scale shape-engineered metal nanostructure, in a hierarchical hologram to implement a near-mode function. The basic concept of the nanophotonic code and fabrication of a sample device are described. In particular, since our proposed approach is to embed a nanophotonic code within the patterns of the hologram, which is basically composed of one-dimensional grating structures, it yields clear polarization dependence compared with the case where it is not embedded within the hologram or arrayed structures. There are also other benefits with the proposed approach; a major benefit is that we can fully utilize the existing industrial facilities and fabrication technologies that have been developed so far for conventional holograms, yet adding novel new functionalities to the hologram.
Here we numerically and experimentally demonstrate those features of embedding a nanophotonic code in an embossed hologram for hierarchical information retrieval. Section 2 describes the design and fabrication of the prototype device. Section 3 shows numerical characterizations, and Section 4 gives experimental results. Section 5 concludes the paper.
2. Design and fabrication of the hierarchical hologram: A nanophotonic code embedded in an embossed hologram
Our nanophotonic code is defined by induced optical near-fields, which are generated by irradiating a nanometric structure with light. An optical near-field is a non-propagating light field generated in a space extremely close to the surface of a nanometric structure [4
4. M. Ohtsu, K. Kobayashi, T. Kawazoe, T. Yatsui, and M. Naruse (ed), Principles of Nanophotonics, (Taylor and Francis, Boca Raton, 2008). [CrossRef]
]. Because the light distribution depends on several parameters of the structure and the retrieving setup, various types of coding can be considered. Moreover, several novel features of nanophotonics, such as energy transfer [5
5. M. Ohtsu, T. Kawazoe, T. Yatsui, and M. Naruse, “Single-photon emitter using excitation energy transfer between quantum dots,” IEEE J. Sel. Top. Quantum Electron. 14, 1404–1417 (2008). [CrossRef]
] and hierarchy [6
6. M. Naruse, T. Yatsui, W. Nomura, N. Hirose, and M. Ohtsu, “Hierarchy in optical near-fields and its application to memory retrieval,” Opt. Express 13, 9265–9271 (2005). [CrossRef] [PubMed]
], may be exploited.
As shown in Fig. 1
, 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 × 20 mm, was fabricated by electron-beam lithography on a Si substrate, followed by sputtering a 50-nm-thick Au layer, as schematically shown in the cross-sectional profile in Fig. 1(b)
Fig. 1. (a) Fabrication of a nanometric structure as a nanophotonic code within the embossed structure of Virtuagram®. (b) Schematic diagram of fabricated sample device, and (c) SEM images of various designed patterns serving as nanophotonic codes.
As indicated in the left-hand side of Fig. 1(a)
, we can observe a three-dimensional image of the earth from the device. More specifically, our prototype device was essentially based on the design of Virtuagram
®, developed by Dai Nippon Printing Co., Ltd., Japan, which is a high-definition computer-generated hologram composed of binary-level one-dimensional modulated gratings, as shown in Fig. 1(a)
. Within the device, we slightly modified the shape of the structure so that near-mode information is accessible only via optical near-field interactions. As shown in Figs. 1(a)
, square- or rectangle-shaped structures, whose associated optical near-fields correspond to the near-mode information, were embedded in the original hologram structures. We call such embedded nanostructures nanophotonic codes
. The unit size of the nanophotonic codes ranged from 40 nm to 160 nm.
Note that the original hologram was composed of arrays of one-dimensional grid structures, spanning along the vertical direction in Fig. 1(c)
. To embed the nanophotonic codes, the grid structures were partially modified in order to implement the nanophotonic codes. Nevertheless, the grid structures remained topologically continuously connected along the vertical direction. On the other hand, the nanophotonic codes were always isolated from the original grid structures. Those geometrical characteristics provide interesting polarization dependence, which is discussed in detail in Sec. 3.
3. Numerical evaluations
First, electric fields at the surface of nanometric structures were numerically calculated by a finite-difference time-domain (FDTD) method based on electromagnetic simulation with Poynting for Optics, a product of Fujitsu, Japan.
As shown in Figs. 2
, two types of calculation models were created in order to examine polarization dependencies due to the existence of environmental structures in retrieving the nanophotonic code. The calculated layer is set 10 nm above the surface of structures. The nanophotonic code was represented by a square-shaped Au structure whose side length was 150 nm and whose depth was 100 nm, which is shown near the center in Figs. 2(a)
Fig. 2. (a) Calculation model of embedded nanophotonic code with environmental structures and calculated intensity distribution of electric field produced by (b) x-polarized input light and (c) y-polarized input light.
Fig. 3. (a) Calculation model of isolated nanophotonic code and calculated intensity distribution of electric field produced by (b) x-polarized input light and (c) y-polarized input light.
As shown in Fig. 2
, 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. 3
, on the other hand, the square-shaped structure, whose size was the same as that in Fig. 2(a)
, 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
directions in order 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 μ
m away from the center of the square-shaped structure. The wavelength was set to 785 nm.
show the electric field intensity distribution on the surface of the structure assuming x
-polarized and y
-polarized input light irradiation, respectively. We then investigated how the environmental structures affected the electric fields in the vicinity of the nanophotonic code and the influence of input light polarization. For such purposes, we first evaluated the average electric field intensity in the area of the nanophotonic code, denoted by 〈I
, and that in the area including the surrounding areas, denoted by 〈I
. More specifically, 〈I
represents the average electric field intensity in the 0.6 μ
m 0.6 μ
m area covering the nanophotonic code, as shown by the dotted square in Fig. 4(a)
, where as 〈I
indicates that in the 2.5 μ
m × 2.5 μ
m area marked by the dashed square in Fig. 4(a)
. Figure 4(b)
summarizes the calculated 〈I
, respectively shown by the red and blue bars.
Fig. 4. (a) Schematic diagram explaining definition of average electric field intensity 〈I〉signal and 〈I〉env, and (b) their graphical representations in each calculation model. Evident polarization dependency was exhibited in the case of nanometric code embedded in environmental structures. (c) The ratio of 〈I〉signal with x-polarized input to that with y-polarized input light for the embedded and isolated structures. (d) Numerical recognizability R
num in two types of models with y-polarized input light. The result indicates that the recognizability of the nanophotonic code was greatly enhanced by embedding it in the environmental structure.
We first investigated the polarization dependencies. In the case of the nanophotonic code embedded in environmental periodic structures, evident polarization dependency was observed for both 〈I
. For example, 〈I
-polarized input was about two times larger than 〈I
-polarized input light. On the other hand, the isolated nanophotonic code did not show any polarization dependency. Figure 4(c)
compares the ratio of 〈I
-polarized input light to that with y
-polarized input light for the embedded and isolated structures.
Second, from the viewpoint of facilitating recognition of the nanophotonic code embedded in the hologram, it would be important to obtain a kind of higher recognizability for the signals associated with the nanophotonic codes. In order to evaluate such recognizability, here we define a figure-of-merit R
which yields a higher value with higher contrast with respect to 〈I
(indicated by the term 〈I
) and with higher signal intensity (indicated by 〈I
). Figure 4(d)
shows the calculated R
in the case of y
-polarized light input to the two types of models. The result indicates that the nanophotonic code embedded in environmental structure is superior to that of the isolated code in terms of the recognizability defined by eq. (1
We consider that such a polarization dependency and the recognizability of nanophotonic codes are based on the environmental grid structures that span along the vertical direction. The input light induces oscillating surface charge distributions due to the coupling between the light and electrons in the metal. In the present case, the y-polarized input light induces surface charges along the vertical grids; since the grid structure continuously exists along the y-direction, there is no chance for the charges to be concentrated. However, in the area of the embedded nanophotonic code, we can find structural discontinuity in the grid; this results in higher charge concentrations at the edges of the embedded nanophotonic code.
On the other hand, the x-polarized input light sees structural discontinuity along the horizontal direction due to the vertical grid structures, as well as in the areas of the nanophotonic codes. It turns out that charge concentrations occur not only in the edges of the nanophotonic codes but also at other horizontal edges of the environmental grid structures. In contrast to these nanophotonic codes embedded in holograms, for the isolated square-shaped nanophotonic codes, both x- and y-polarized input light have equal effects on the nanostructures.
These mechanisms indicate that such nanophotonic codes embedded in holograms could also exploit these polarization and structural dependences, not only 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.
In the experimental demonstration, optical responses during near-mode observation were detected using a near-field optical microscope (NOM). A schematic diagram of the detecting setup is shown in Fig. 5(a)
, in which the NOM was operated in an illumination-collection mode with a near-field probe having a tip with a radius of curvature of 5 nm. The fiber probe was connected to a tuning fork. Its position was finely regulated by sensing a shear force with the tuning fork, which was fed back to a piezoelectric actuator. The observation distance between the tip of the probe and the sample device was set at less than 50 nm. The light source used was a laser diode (LD) with an operating wavelength of 785 nm, and scattered light was detected by a photomultiplier tube (PMT). A Glan-Thomson polarizer (extinction ratio 10-6
) selected only linearly polarized light as the radiation source, and a half-wave plate (HWP) rotated the polarization.
Fig. 5. (a) Schematic diagram of the experimental setup for retrieving a nanophotonic code, and (b) observed optical image as basic retrieval results.
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 in the positions marked by the dashed circles in Fig. 5(b)
. 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.
show other retrieved results of nanophotonic codes that were embedded in the hologram and not
embedded in the hologram, respectively, using a linearly polarized radiation source. Figures 6(a)
respectively show observed NOM images of the nanophotonic code embedded in the hologram with a standard polarization (defined as 100-degree polarization) and 60-degree-rotated polarization. Figure 6(c)
summarizes the NOM images obtained with input polarizations from 0-degree to 180-degree rotated polarizations at 20-degree intervals. Also, Figs. 7(a)
, and 7(c)
represent the NOM images of the nanophotonic code which was not embedded in the hologram. As is evident, in the case of the nanophotonic code embedded in the hologram, clear polarization dependence was observed; for example, from the area of the nanophotonic code located in the center, a high-contrast signal intensity distribution was obtained with polarizations around 80 degree.
Fig. 6. Observed NOM images of optical intensity distributions of retrieved nanophotonic code embedded in environmental structures with (a) a standard polarization and (b) 60 deg-rotated polarization, and (c) NOM images observed by irradiating light with various polarizations.
To quantitatively evaluate the polarization dependency of the embedded nanophotonic code, we investigated two kinds of intensity distribution profiles from the NOM images observed. One is a horizontal intensity profile along the dashed line in Fig. 8(a)
, which crosses the area of the nanophotonic code, denoted by I
), 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 μ
m, denoted by 〈I
, which indicates the environmental signal distribution. When a higher intensity is obtained selectively from the area of the nanophotonic code, the difference between I
) and 〈I
can be large. On the other hand, if the intensity distribution is uniform along the vertical direction, the difference between I
) and 〈I
should be small. Thus, the difference of I
) and 〈I
indicates the recognizability of the nanophotonic code. We define an experimental recognizability R
Fig. 7. Observed NOM images of optical intensity distributions of retrieved isolated nanophotonic code with (a) a standard polarization and (b) 60 deg-rotated polarization, and (c) NOM images observed by irradiating light with various polarizations.
shows an example of I
) and 〈I
obtained from the NOM image of the nanophotonic code embedded in the hologram with the standard input light polarization (corresponds to Fig. 6(a)
). Figures 8(c)
as a function of input light polarization based on the NOM results shown in Figs. 6(c)
, respectively. The nanophotonic code embedded in the hologram exhibited much greater polarization dependency, as shown in Fig. 8(c)
, where the maximum R
was obtained at 80-degree input polarization, whereas only slight polarization dependency was observed with the isolated nanophotonic code, as shown in Fig. 8(d)
. Such polarization dependence in retrieving the nanophotonic code agrees well with the results of the simulations in Figs. 2