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Surface-Plasmon Holography with White-Light Illumination

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Science  08 Apr 2011:
Vol. 332, Issue 6026, pp. 218-220
DOI: 10.1126/science.1201045

Abstract

The recently emerging three-dimensional (3D) displays in the electronic shops imitate depth illusion by overlapping two parallax 2D images through either polarized glasses that viewers are required to wear or lenticular lenses fixed directly on the display. Holography, on the other hand, provides real 3D imaging, although usually limiting colors to monochrome. The so-called rainbow holograms—mounted, for example, on credit cards—are also produced from parallax images that change color with viewing angle. We report on a holographic technique based on surface plasmons that can reconstruct true 3D color images, where the colors are reconstructed by satisfying resonance conditions of surface plasmon polaritons for individual wavelengths. Such real 3D color images can be viewed from any angle, just like the original object.

Noble metal films, such as silver and gold foil, contain free electrons that collectively oscillate and propagate as the surface wave in optical frequency region. The quantum of this surface wave is called surface plasmon polariton (SPP) (1). The electromagnetic field generated by SPP can be enhanced and strongly confined spatially in the near field (with the distance less than the wavelength) from the metal surface as a nonirradiative evanescent field (2, 3). The ability to confine and enhance the optical field to the vicinity of the metal surface or nanometal particle has been applied to immuno-sensor (4), fluorescence sensor (5), solar cell (6), plasmonic laser (7, 8), nanomicroscopy (9, 10), super-lens (11, 12) and photodynamic cancer cell treatment (13).

We report an application of SPP to three-dimensional (3D) color holography with white-light illumination. The first idea to use plasmons for holography was published as a reflection type by Cowan (14), and since then authors have reported holographic reconstruction of transmission type (1517). In those configurations, plasmons have been used for enhancing the diffraction efficiency. In this report, we use color selectivity of SPPs for holographic color reconstruction with white-light illumination.

We recorded the hologram on a photoresist as an interference pattern between a light field coming from the object as scattered light and the unscattered reference beam. Exposure was repeated three times with rotation of the illumination angle for a single color hologram recording. Instead of rotation, one can also use three lasers simultaneously to obtain the hologram in a single exposure. A thin metal film is then coated on the photoresist hologram, which is precoated on a glass plate. For image reconstruction, the SPP is excited by a color component of white light that is incident on the metal film through a prism with an angle satisfying the condition of total internal reflection (Fig. 1A). The SPP associates with a nonradiative evanescent light wave on metal film and then is converted by the grating component of hologram into a radiative light field, which represents the reconstructed wavefront of light that scattered at the object. The reconstruction of the prerecorded object is seen with the eyes through an SPP hologram in color. The reference beam or the zeroth-order diffraction as background beam does not exist in reconstruction for this configuration because the illumination of the hologram is made by total internal reflection.

Fig. 1

Surface plasmon hologram and its color reconstruction with white-light illumination. (A) The SPP hologram is illuminated by white light at a given angle θ in high-index medium. Surface plasmons of a selected color are excited and diffracted by the SPP hologram to reconstruct the wavefront of the object. (B) Dispersion curve of the SPP hologram in reconstruction as a function of the incident angle of white light. The 3D images of red, green, and blue cranes made of paper are obtained at different angles with white-light illumination. This curve was obtained through calculations based on Fresnel’s equations. (C) Reconstruction of a color object through SPP hologram. The hologram is illuminated simultaneously with a white light in three directions at different angles θ and ϕ for each.

Figure 1B shows the dispersion curve of SPP with wavelength λ as a function of the incident angle θ of illumination (excitation). This relationship is given asEmbedded Image (1)where nglass, nm, and n(λ) are the refractive index of the glass substrate, effective index of the medium on the metal surface, and the index of metal (which is a function of λ), respectively (2).

Individual colors are reconstructed by illumination at corresponding incident angles by satisfying the above relationship for each color. Figure 1C shows the optical setup for reconstructing a three-color object in different azimuthal and incident angle sets [(ϕR, θR), (ϕG, θG), and (ϕB, θB)] with white-light illumination.

Reconstruction of an object, an apple with a leaf, is seen in 3D from the thin film plasmon hologram (Fig. 2A) (18). A movie of the object taken with a camera moving around the object is provided (movie S1) in the supporting online material. For plasmon color holography, the adjustment of the white color balance is important for representing the natural color of an object or a scene. We obtained the white color balance by carefully controlling the power of the laser with multiple exposures for red (R), green (G), and blue (B). Figure 2B shows an image of a bar taken by three-time exposure in R, G, and B; each exposure was made by rotating the object (bar) and by taking shots at every 120° of rotation. It includes a white hexagon in the center where all three colors overlap and yellow triangles where red and green bars overlap. For reconstruction of the object, a 100-W Halogen lamp is used.

Fig. 2

Reconstruction of three-dimensional color objects through surface plasmon holograms. (A) Red apple with green leaf in three dimensions (see movie S1). (B) Color bar recorded three times with red, green, and blue by rotating the bar by 120° for each color. In the center, where the three colors overlap, a hexagonal area is reconstructed as white, whereas yellow triangles are reconstructed where red and green overlap.

Figure 3A shows the multilayer system of an SPP hologram. The role of the top-layer dielectric film is to enhance the spectral separation in reconstruction. If this film is absent, the angular separation for the three colors R, G, and B in reconstruction is not large enough (less than 3° between red and blue; more precisely, the angles for the three colors are at 42.8°, 43.7°, and 45.2°, respectively) in a dispersion relationship (Fig. 3B) and makes it difficult to practically reconstruct a color object. If the SiO2 film is coated, the angular separation between red and blue becomes as large as 10° due to the higher index nm of film; more precisely, the angles for the three colors are at 45.6°, 48.6°, and 54.1°, respectively. In Fig. 3B, theoretical curves are plotted from the calculation of a multilayer system based on Fresnel’s equations for noncoated and coated SPP holograms. A rose pendant in the figure is decomposed into three colors in reconstruction by choosing the angle for white-light illumination (Fig. 3B).

Fig. 3

Multilayer system and the color dispersion of an SPP hologram. (A) Hologram configuration. From the top, dielectric layer (25-nm-thick SiO2), metal layer (55-nm-thick silver), and dielectric substrate with 25-nm depth modulation (150-nm-thick photoresist on the glass). (B) The top SiO2 layer works for expanding the color dispersion to incident angle. Without the SiO2 layer on the hologram, the angular separation for color reconstruction is small. Rose pendants of red, green, and blue are separated in the reconstruction, as shown in the insets.

Our results show that plasmon color holography provides a view of an object or a scene seen naturally and vitally with white-light illumination. A typical amplitude modulation in plasmon hologram is ~25 nm (fig. S2), which is much thinner compared with Lippmann-Denisyuk’s color hologram (19) based on Bragg diffraction in volume. The rainbow holograms mounted, for example, on credit cards (20) also reconstruct with white light, where color varies with viewing angle but not with the color distribution in the object. Plasmon holography is advantageous in terms of background-beam–free reconstruction because the illumination light is totally reflected back at the hologram (21). Plasmon holography does not suffer from the ghost produced by the diffraction of ambient light or higher orders of diffraction, because those components are not coupled with SPPs.

Supporting Online Material

www.sciencemag.org/cgi/content/full/332/6026/218/DC1

Materials and Methods

Figs. S1 and S2

References

Movie S1

References and Notes

  1. Materials and methods are available as supporting materials on Science Online.
  2. S. Kawata conceived the research and planned the experiments. M. Ozaki conducted the experiment and calculations. All authors discussed the results and contributed in preparing the manuscript. The authors thank R. Furutani for discussion and P. Verma for his review.
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