By using the optical frequency dependence of surface-plasmon polaritons, color images can be reconstructed from holograms illuminated with white light. We report details on the color selectivity of the color holograms. The selectivity is tuned by the thickness of a dielectric film covering a plasmonic metal film. When the dielectric is and the metal is silver, the appropriate thicknesses are 25 and 55 nm, respectively. In terms of spatial color uniformity, holograms made of silver-film corrugations are better than holograms recorded on photographic film on a flat silver surface.
© 2013 Optical Society of America
A quantum of a charge-density wave of free electrons on a metal surface is called a surface-plasmon polariton (SPP), which is excited by coupling with light waves. SPPs can be applied to sensors , solar cells , plasmonic lasers [3,4], near-field optical microscopy , Raman spectroscopy with high spatial resolution , optical nanocircuits , photodynamic cancer cell treatments , and nanolens magnifiers . In some of these applications, the resonant-frequency dependence of SPPs is utilized. Recently, we reconstructed holograms in color with white-light illumination by exploiting the optical frequency dependence of SPPs . Colors were extracted from white light by using this frequency dependence. In this paper, we report on the details of color selectivity and spatial color uniformity. When the color isolation is not sufficiently large, colors become mixed, and the resultant reconstructed image is whitish. When the color selectivity is spatially uneven, color unevenness occurs in a reconstructed image.
2. Color Extraction by SPPs
Figure 1 shows the optical geometry for the excitation of SPPs. Silver film is illuminated by light from the glass side. is the wavenumber of the incident light in the direction of the axis and is given by11,12]. When is equal to , SPPs are excited. Thus, from Eqs. (1) and (2), the excitation condition of SPPs is expressed by
3. Color-Selectivity Tuning
To improve color selectivity, the silver surface must be covered with an layer, as shown in Fig. 2(a). Figure 2(b) is a calculated relationship between the incident angle and the color of the SPPs. The calculations are based on Fresnel’s equations. When the thickness is 0 nm, incident-angle separation between red and blue is less than 3°, which is not sufficiently large to separate colors, and the obtained reconstructed images are whitish. When the thickness is 25 nm, which is the most appropriate thickness for color holograms in the calculations, the incident angle for red, green, and blue broadens to 45.6°, 48.6°, and 54.1°, respectively. If thickness is 50 nm or more, blue SPPs cannot be excited. These color-selectivity responses are caused by the effective value of , which is affected by thickness [13,14].
Figure 3(a) shows a color selectivity identical to that shown in Fig. 2(b), except that the silver-film thickness is thinner (35 nm). Dark bands corresponding to the excitation of SPPs broaden, and hence, the color selectivity gets worse. When the silver thickness is thicker, the bandwidth does not broaden, but the excitation efficiency of SPPs decreases, as shown in Fig. 3(b).
4. Hologram Reconstruction with Spatially Uniform Color
In SPP holography, the hologram to be reconstructed should be located in the near field of the metal surface. In Fig. 4(a), an image hologram  is on a flat silver surface on which SPPs are excited. SPPs propagating on a silver surface are diffracted by the hologram . The diffracted waves focus an image. In this hologram, color separation is broadened by a hologram layer instead of by an film. Figure 4(b) shows a reconstructed image. The center of the image is green, although collimated white light illuminates the hologram to reconstruct a red image. This partial color shift is caused by hologram thickness that is thinner than the surrounding area, as shown in Fig. 4(c). Figure 4(d) shows the relationship between the hologram thickness and the wavelength of the light exciting the SPPs. Red SPPs were excited in the thick hologram area, while green SPPs were excited in the thin area. The uneven thickness of the hologram is mainly caused by the intensity distribution of the exposure light. In this case, the intensity of the center area is larger than that of the surrounding area.
To suppress the color unevenness, a silver-relief SPP hologram [Fig. 5(a)] is needed. Fringes of the hologram consist of corrugations of silver film on which SPPs are excited and diffracted. To create this hologram, we first coated the glass surface with the photoresist and then exposed it to the reference and object light beams. Next, we developed the exposed photoresist and deposited the silver onto the hologram. As a last step, we deposited onto the silver surface for tuning the color selectivity. In this hologram, the spatially uneven photoresist layer recorded is below the silver film, and the layer above it can be uniformly deposited by sputtering. Thus, the color shift is eliminated. Figure 5(b) shows a red bar reconstructed from the silver-relief SPP hologram. A color shift does not occur, and the center of the image, in which exposure intensity is high, is brighter than both ends of the image. Figure 5(c) shows the distribution of image brightness.
The thickness of an film on a silver surface controls color selectivity of SPP holograms reconstructed with white-light illumination. The appropriate thickness is 25 nm and needs to be uniform since its unevenness causes a partial color shift. The thickness of the silver film also affects the color selectivity. The optimal thickness of the silver is 55 nm.
Although gold is well known as a plasmonic material, we use silver because it covers the entire visible range, while gold covers only the range from yellow to red, owing to the resonance frequency of SPPs.
In terms of color uniformity, holograms made of silver-film corrugations are more appropriate than dielectric holograms on flat silver film because the thickness of the hologram layer is uneven, which causes a partial color shift. Although a periodical structure, such as a hologram, also shifts the color of SPPs excited by white light, the depth of our hologram is up to 25 nm, and hence, the color shift is negligible [17,18]. When we suppress an image blur by using the incident angle dependence of the SPP excitation , a silver-relief SPP hologram is preferred because it offers spatially stable excitation conditions.
When the hologram is recorded in our experiments, a thin photoresist film is exposed with interference fringes of the object light and the reference beam, and then it is developed. Therefore, this is a traditional process to record the thin holograms, and it is possible to make fringes of computer-generated holograms in some way. In addition, even though the SPP holograms described here are such thin holograms based on Raman–Nath diffraction, a color image with white-light illumination can be reconstructed.
When the refractive index decreases, the incident angle increases, according to Eq. (3). Thus, the hologram is illuminated at a larger incident angle when a low refractive-index substrate glass is used. Such holograms, with large incident-angle illumination, are called “edge-illuminated holograms”; these have potential for use in slate-type devices and improve the usability of holography [20–22]. An SPP hologram supports slate-type devices based on edge and back illumination, since the opaque metal film prevents stray light from emerging on the observer’s side, while typical back-lit holograms are illuminated through transparent materials.
1. C. Nylander, B. Liedberg, and T. Lind, “Gas detection by means of surface plasmon resonance,” Sens. Actuators 3, 79–88 (1982). [CrossRef]
2. V. E. Ferry, L. A. Sweatlock, D. Pacifici, and H. A. Atwater, “Plasmonic nanostructure design for efficient light coupling into solar cells,” Nano Lett. 8, 4391–4397 (2008). [CrossRef]
3. T. Okamoto, F. H’Dhili, and S. Kawata, “Towards plasmonic band gap laser,” Appl. Phys. Lett. 85, 3968–3970 (2004). [CrossRef]
4. N. I. Zheludev, S. L. Prosvirnin, N. Papasimakis, and V. A. Fedotov, “Lasing spaser,” Nat. Photonics 2, 351–354 (2008). [CrossRef]
5. S. Kawata and Y. Inouye, “Near-field scanning optical microscope with a metallic probe tip,” Opt. Lett. 19, 159–161 (1994). [CrossRef]
6. N. Hayazawa, Y. Inouye, Z. Sekkat, and S. Kawata, “Metallized tip amplification of near-field Raman scattering,” Opt. Commun. 183, 333–336 (2000). [CrossRef]
7. S. A. Maier, M. L. Brongersma, P. G. Kik, S. Meltzer, A. A. G. Requicha, and H. A. Atwater, “Plasmonics—a route to nanoscale optical devices,” Adv. Mater. 13, 1501–1505 (2001). [CrossRef]
8. C. Loo, A. Lowery, N. Halas, J. West, and R. Drezek, “Immunotargeted nanoshells for integrated cancer imaging and therapy,” Nano Lett. 5, 709–711 (2005). [CrossRef]
9. S. Kawata, A. Ono, and P. Verma, “Subwavelength colour imaging with a metallic nanolens,” Nat. Photonics 2, 438–442 (2008). [CrossRef]
10. M. Ozaki, J. Kato, and S. Kawata, “Surface-plasmon holography with white light illumination,” Science 332, 218–220 (2011). [CrossRef]
11. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer-Verlag, 1988).
12. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6, 4370–4379 (1972). [CrossRef]
13. K. Holst and H. Raether, “The influence of thin surface films on the plasma resonance emission,” Opt. Commun. 2, 312–316 (1970). [CrossRef]
14. I. Pockrand, “Surface plasma oscillations at silver surfaces with thin transparent and absorbing coatings,” Surf. Sci. 72, 577–588 (1978). [CrossRef]
15. G. W. Stroke, “White-light reconstruction of holographic images using transmission holograms recorded with conventionally-focused images and ‘in-line’ background,” Phys. Lett. 23, 325–327 (1966). [CrossRef]
16. S. Maruo, O. Nakamura, and S. Kawata, “Evanescent-wave holography by use of surface-plasmon resonance,” Appl. Opt. 36, 2343–2346 (1997). [CrossRef]
17. I. Pockrand, “Reflection of light from periodically corrugated silver films near the plasma frequency,” Phys. Lett. 49A, 259–260 (1974).
18. I. Pockrand and H. Raether, “Surface plasma oscillations in silver films with wavy surface profiles: a quantitative experimental study,” Opt. Commun. 18, 395–399 (1976). [CrossRef]
19. M. Ozaki, J. Kato, and S. Kawata, “Blur suppression in holographic imaging with use of surface plasmons,” Appl. Phys. Lett. 101, 241117 (2012). [CrossRef]
20. L. H. Lin, “Edge-illuminated hologram,” J. Opt. Soc. Am. 60, 714A (1970). [CrossRef]
21. S. A. Benton, S. M. Birner, and A. Shirakura, “Edge-lit rainbow holograms,” Proc. SPIE 1212, 149–157 (1990). [CrossRef]
22. J. Upatnieks, “Edge-illuminated holograms,” Appl. Opt. 31, 1048–1052 (1992). [CrossRef]