Plasmonic Hologram Based on Bilayer Metallic Nanowire Gratings

Xin Sheng1, Jie Cui1, Jun Zheng2, Zhi-cheng Ye*1 and Han-ping D. Shieh1, 3

1National Engineering Laboratory for TFT-LCD Materials and Technology, Department of Electronic

Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China 2Key Laboratory for Laser Plasmas (Ministry of Education), Department of Physics, Shanghai Jiao Tong

University, Shanghai, 200240, China 3Department of Photonics and the Display Institute, National Chiao Tung University,

Hsinchu 300, Taiwan

Abstract Holographic images were created with high contrast and low

noise by the enhancement of surface plasmon waveguiding effect.

The holograms are sensitive to polarization for compatibility with

further liquid crystal displays and can be reconstructed by white

light.

Author Keywords plasmonic hologram; 3D display; Surface Plasmon Resonance

(SPR); waveguide.

1. Introduction Holography technology has the ability to record the whole

information, which has been applied to many fields such as

holographic interferometry, three-dimensional holographic CAT

scans, credit cards and debit cards. Particularly, 3D holographic

display enjoys great popularity due to the advantage of true 3D

image without glasses. As one of the most successful examples,

3D holographic print from Zebra Imaging can be displayed with a

simple light source without the need of special equipment. 3D

hologram pictures, another example, have been used to present the

artwork for decoration and archival recordings. Unfortunately,

low diffraction efficiency and poor image contrast are still

troubling the holograms especially for the relief ones.

The combination of holography and SPR has been reported to

improve diffraction efficiency of hologram and based on it color

hologram was also demonstrated [1]. Surface Plasmon Resonance

(SPR) is the collective oscillation of electrons on a metal surface.

A light beam incident on the SPR hologram will couple with the

surface plasmon polaritons if they match the light momentum,

thus the diffraction efficiency is increased by the strong resonance

[2]. Normally, a prism or a grating is needed to excite SPR.

However, the prism type is bulky for hologram, and in the grating

coupling, it is difficult for light beam to satisfy the incident angle

precisely.

In fact, for a hologram with metal there are two kinds of surface

plasmon modes: lateral one along the hologram surface and

longitudinal one in the slits of the hologram reliefs. The previous

studies referred to the former, while the latter was neglected. In

our work, a major goal is to take full advantage of longitudinal

surface plasmon mode and to extend the utilization of SPR

holography. In our experiment, we coated the hologram with

aluminum to form a billayer metal film, and this new metal

hologram (Plasmonic Hologram) has a preference for TM

diffraction which shows enhanced efficiency and reduced noise

by improving image contrast. Meanwhile, monochromatic

holographic images were obtained by white light illumination,

which will be discussed in later section.

2. Theoretical and Experimental 2.1 Model of Plasmonic Hologram

As is known to all, the profile of the hologram is random fringe

pattern [3], in other word, an assembly of complex gratings with

line direction mainly perpendicular to the incident plane of the

object and reference beams. Therefore, we can simplify the metal

holographic pattern as metal gratings as shown in figure 1.

To show the waveguide modes in the metal-insulator-metal slits

of metal hologram clearly, equation (1) represents the dispersion

relationship deduced from Bloch theory.

)sin()sin()1

(2

1)cos()cos()cos( 22112211 tktk

ggtktkKT

(1)

Where, K is the Bloch wave number,T is pitch of the grating,

1t and 2t are the width of the dielectric and aluminum in one

period, respectively, 2

0

2

01 zkkk and 22

02 zm kkk are

the wave numbers of the input light in the air and aluminum along

the X-axis, respectively. zk is the wave number of the waveguide

mode along the Z-axis. 021 / kkg m for TM light and

21 / kkg for TE light [4].

Figure 1. Cross-section of metal grating: t1, t2 are the width of

air slit and metal slit, respectively. T denotes the period of

metal grating. For TE light, electric field is perpendicular to

plane of incidence; for TM light, electric field is parallel to

plane of incidence.

46.2 / X. Sheng

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(a)

(b)

Figure 2. Dispersion of the Al-air-Al slits waveguide modes of

the Grating: (a) for TE light; (b) for TM light. The grey zone

is for visible light. The black line is the dielectric line of the

slit. The blue and red dots are the real and imaginary parts of

the wave number kz, respectively.

Figure 1 shows the metal grating, a schematic diagram of

plasmonic hologram model, where T is 320nm, both t1 and t2 are

160nm, the metal and insulator are specified to be aluminum and

air, respectively. According to this model, we can obtain the

solutions of equation (1) shown in figure 2. The intersection point

of kz-r1 and kz-i1 is called first-order cut-off frequency, which

means that only the frequencies above it, the first-order

waveguide modes can pass through the air slit. For TE light in

figure 2(a) the cut-off frequency is ~5×1015 Hz (~378nm) and

there is no real solution below it. For TM light in figure 2(b), a

line marked by kz-r0 is beneath the black dielectric line, which

means that TM zero-order waveguide mode is not cut off.

Accordingly, it is clear that TM is always supported to propagate

in the air slits via surface plasmon waveguide modes; nevertheless

TE is forbidden for its cut-off frequency is larger than the visible

light. In other word, TM light will be diffracted to reconstruct the

object while TE will not. Thus the polarized response of

plasmonic hologram is achieved.

2.2 Experimental

The figure 3 illustrates the experimental setup of hologram

recording. The beam is split into two parts after expansion; one

(the reference beam) is directly illuminating the recording plate

and the other is reflected from the object and then recorded. The

laser we used is a He-Cd gas laser with wavelength of 457.8nm.

The incident angle to the plate is 45° . The intensity after

expansion and the exposure time are 4mW/cm2 and 3minutes,

respectively. The exposed photoresist is developed with NaOH

(0.7%) for 10 seconds. Then, the hologram is rinsed with deioned

water and heated by hot wind for 20s and 30s, respectively.

Subsequently, a 40nm aluminum film is deposited onto the

hologram using e-beam coater (ZSX-500D). Figure 4 shows the

pattern coated with aluminum film. The thickness of aluminum

film is optimized according to that of photoresist. Please note that

the hologram not deposited with aluminum film is called

„ordinary hologram‟ in this paper.

Figure 3. Optical system for recording a transmission

hologram: A plane wave is incident at angle of 45°. The

object to be recorded is close to the recording plate to alleviate

phase difference blur.

Figure 4. The process of the ordinary hologram turning into

plasmonic hologram.

For reconstruction, one can use both the original reference laser

beam and white beam to obtain the image. Figure 5 shows the real

image reconstructed by original reference beam. We can see that

the details on the object (mirror image of the word “CHINA” on a

vertical key) could be represented clearly. As is known to all,

holograms do not reproduce the true colors of the original object.

The image‟s color depends on the color of the laser used to make

the hologram, so the reconstruction image is blue due to 457.8nm

laser beam. In our experiment, white light is also used to

illuminate the plasmonic hologram to achieve the monochromatic

holographic images.

Figure 6 are red/green/blue (R/G/B) real images reconstructed by

parallel white light (unpolarized). The angle of incidence is 75°.

The angles between R/G, G/B are 20°and 21°, respectively.

The viewing distance is 15cm, so that the R/G/B real images can

separate from each other and become more attractive than

traditional rainbow hologram. Because our plasmonic holograms

are image plane hologram, it is easy to obtain the monochromatic

holograms without any slits used in traditional fabrication.

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Figure 5. Reconstructed real image of plasmonic hologram: The incident angle is 80°, and the viewing angle is ~45°.

(a) (b) (c)

Figure 6. Red/green/blue real images observed in different viewing angle: (a) red: 43°; (b) green: 23°; (c) blue: 2°. The

wavelengths of R/G/B are 670, 550, 450nm, respectively.

3. Results and Discussion In the measurement, we compare the plasmonic hologram with

ordinary one. Meanwhile, as reconstruction beam, TM and TE

white light illuminate the hologram above separately [5]. The

results are shown as follows.

Figure 7. TE/TM parallel white light illuminates the

plasmonic hologram and ordinary hologram: (a) Reflection,

(b) Transmission, (c) Sum of transmission and Reflection, (d)

Diffraction efficiency as a function of wavelength.

Figure 7(a)-4(c) are reflection, transmission, the sum of

transmission and reflection as a function of wavelength,

respectively. From figure 7(a), it can be seen that the majority of

TE light is reflected by plasmonic hologram, up to 80%, but the

TM reflection is under 35%. In figure 7(c), compared with

ordinary hologram, there is a great disparity (30%~50%) between

TM and TE light in the sum of transmission and reflection for

plasmonic hologram. Obviously, the plasmonic hologram is

polarization-sensitive.

Figure 7(d) is first-order diffraction efficiency as a function of

wavelength. It is clear in figure 7(d) that the diffraction efficiency

was increased significantly, especially for TM light. At 457.8nm,

for example, diffraction efficiency for TM light jumps from 0.3%

to 4.2%. Moreover, with the increase in wavelength, plasmonic

hologram diffraction efficiency for TM light grows more sharply.

On the contrary, though showing the enhancement, diffraction

efficiency of TE light declines with the growth of wavelength,

which is matching with the cut-off effect of metal-insulator-metal

waveguides in plasmonic hologram. On the whole, for diffraction

efficiency of plasmonic hologram, the ratio of TM to TE light is

ranging from 1.7 to 6.5. These results are in agreement with the

theoretical analysis.

Image contrast is considered to evaluate the image quality. The

image contrast is defined as

minmax

minmax

II

IIV

(2)

In order to measure the image contrast of different wavelength,

we fixed the white light source position and its incident angle at

75 ° . For individual wavelength, the maximum/minimum

intensity was obtained by fine-tuning the height of spectrometer.

It is good that the minimum intensity was localized at the “dark

hole” shown in figure 5(a), because other bark area may be not

within the diffraction zone.

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658 • SID 2014 DIGEST

Table 1. Image contrast of plasmonic hologram

Wavelength (nm)

contrast (TM) contrast (TE)

446.87 61.80% 56.90%

521.73 52.90% 39.90%

551.95 74.50% 48.50%

609.77 58.60% 30.90%

646.56 73.10% 36.20%

From table 1, we can see that the image contrast for TM light is

higher than TE. At 646.56nm, for example, contrast of TM is two

times as much as that of TE. What‟s more, difference between

TM and TE becomes larger with the growth of wavelength.

From what has discussed above, we can see that plasmonic

hologram greatly enhance diffraction efficiency. In addition, TM

light performs better than TE in the improvement of diffraction

efficiency and image contrast. Therefore, ambient light, most of

which is TE mode after being reflected by other objects, will be

restrained to diffract by plasmonic hologram, thus decreasing the

noise and improving the image quality.

4. Conclusions In summary, the plasmonic hologram was fabricated and images

with individual color (R/G/B) were observed. It was demonstrated

that plasmonic hologram significantly enhanced the TM polarized

diffraction efficiency, which shows good agreement with the

results from our model. TM light performed better than TE in

image contrast due to the fact that TM light functions as

reconstruction beam, while TE light is restrained by cut-off

frequency. Meanwhile, our plasmonic hologram can be

compatible with liquid crystals (LC) due to its polarization

character. We believe that this manuscript opens a new door for

high-efficiency and high-contrast holograms by the plasmonic

enhancement. The future work to fabricate reflective full color

plasmonic hologram will be shown in the very near future.

5. Acknowledgments This work was sponsored by 973 Program (2013CB328804) and

supported by National Natural Science Foundation of China

(Grant No. 61007025, 61370047, and 10905039) and Ministry of

Education (Grant No. 20100073120034 and 20090073120076).

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Plasmon Holography with White-Light Illumination,”

Science 332, 218-220 (2011).

[2] Shoji Maruo, Osamu Nakamura, and Satoshi Kawata,

“Evanescent-wave holography by use of surface-plasmon

resonance,” App. Opt. 36, 2343-2346 (1997).

[3] Lee Hyuk, Jin Sang Kyu, “Experimental study of volume

holographic interconnects using random patterns,” Appl.

Phys. Lett. 62, 2191-2193 (1993)

[4] Zhi-Cheng Ye, Jun Zheng, Shu Sun, Lin-Dong Guo, Shieh,

H.-P.D., “Compact Transreflective Color Filters and

Polarizers by Bilayer Metallic Nanowire Gratings on

Flexible Substrates,” Quantum Electronics, 19, 4800205

(2013)

[5] D. E. Smalley, Q. Y. J. Smithwick, V. M. Bover Jr, J.

Barabas and S. Jolly, “Anistropic leaky-mode modulator for

holographic video displays,” Nature 498, 313-317 (2013).

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