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Defense & Security

Plasmonic keys for ultra-secure information encryption

Arbitrary 3D polarization orientation is used to excite surface plasmon resonance in gold nanorods, creating highly secure encryption 'keys.'
19 November 2012, SPIE Newsroom. DOI: 10.1117/2.1201211.004538

As the need for information generation and storage increases, so does demand for information security systems. One way to achieve a high level of security is to use optical techniques to encrypt information in the polarization state of a writing beam, requiring a polarization ‘key’ for the visualization of the original information.1

We can use gold nanorods to create these 'plasmonic' keys for encryption. By exciting the surface plasmon resonance in the nanorods, we increase scattering and local field enhancement. These effects can have a variety of applications, such as boosting the efficiency of light-trapping in photovoltaics and increasing photoluminescence quantum yields in metallic nanoparticles.2

The excitation of surface plasmon resonance in gold nanorods is dependent on the polarization orientation of the incidence beam, so only nanorods that are aligned parallel to the polarization orientation of the beam can be excited efficiently. Until recently, we could only generate strongly enhanced longtidudinal polarization by focusing a radially polarized light through a high numerical aperture (NA) objective.3 Here we describe how we achieved arbitrary 3D polarization orientation in the focal region4 using diffraction of a configured single beam (see Figure 1).

We configured the input polarization of the single beam by superposing a radially polarized beam and an azimuthally polarized beam with weighting factors γ and δ, respectively, at the back aperture of a high NA objective: see Figure 1(a). We further modulated the amplitude of the superposed beam using an apodizer with function P(α, ε), where α is the azimuthal angle of the transmission aperture and ε the normalized radius of the obstruction within the apodizer. By configuring the polarization and the amplitude of single beams, we could freely tune the out-of-plane orientation (θ) and the in-plane orientation (β) of the focal polarization (see Figure 1).

Figure 1. Arbitrary 3D focal polarization control. (a) Schematic diagram of superposition of a weighted radially polarized beam (γ) and a weighted azimuthally polarized beam (δ). (b) Configuring the weighting factor and the normalized radius of the concentric obstruction of the apodizer P(α=0, ε)enables an orientation-unlimited flexibility of tuning the ratio of the longitudinal to transverse polarization intensity components from 0 to over 40, corresponding to a tunability of θ from to approximately 0.

This 3D polarization control allowed us to tailor the excitation of the randomly aligned gold nanorods. Figure 2(a)–(c) shows the calculated two-photon (2p) fluorescence rate images of gold nanorods with in-plane, out-of-plane, and arbitrary 3D alignments interacting with the configured 3D focal polarization. The experimental results in Figure 2(d) and (e) are consistent with the calculation. Once the illumination power exceeds the threshold, it can selectively melt correspondingly aligned gold nanorods.5 Figure 2(d) and (e) shows the 2p images of gold nanorods with in-plane (blue) and out-of-plane (red) alignments before and after exposure to the laser illumination, configured with and θ∼0, respectively. The results show that the orientation flexibility enables 3D selective melting of gold nanorods with corresponding alignments. We can control the melting rate by the power of the laser illumination, as well as the configured focal polarization orientation. Figure 3 shows nearly identical melting rates of gold nanorods with in-plane (blue squares) and out-of-plane (red circles) alignments excited by the illumination configured with .

Figure 2. Selective excitation and melting of gold nanorods. The fluorescence rate imaging of gold nanorods with (a) in-plane, (b) out-of-plane and (c) arbitrary 3D alignment excited by the illumination configured at γ=1, δ=0 and P(α=0,ε=0.7). The two-photon fluorescence images of nanorods with in-plane (blue) and out-of-plane (red) alignments before and after high-power illumination under the excitation configured with (d) θ=π/14 and (e) θ∼0, respectively. (f) The two-photon fluorescence contrast reduction of gold nanorods with in-plane (blue squares) and out-of-plane (red circles), respectively, under the excitation configured at =1, δ=0 and P(α=0,ε=0).

Figure 3. Demonstration of ultra-secure polarization information encryption. The red arrows in (a-e) indicate the five configured polarization orientations used for the information encryption at (a) , β=0, (b) , , (c) , , (d) θ∼0, β=0, and (e) , β=0∼2π. The scale bar is 10μm.

We can use the arbitrary 3D selective melting of gold nanorods as a means to encrypt the ‘key’ polarization information, providing the encryption with a far greater level of security than that of in-plane polarization control methods. In Figure 3, we demonstrate the process using five configured polarization orientations (indicated by the directions of red arrows). We can retrieve the five encrypted patterns only by 2p fluorescence imaging through raster scanning with the ‘key’ polarization orientation, which is the same as that used for information encryption. Otherwise, the information is read out as noise.

We could use arbitrary 3D polarization orientation, as described here, to provide greater information storage capacity without increasing the size of the recording medium. In our future work, we may employ this technique to develop polarization microscopy by tailoring the interaction between light and matter in an orientation-unlimited 3D manner. Possibilities for use include single molecule detection, multi-dimensional optical storage data, 3D displays and spintronics.

Min Gu, Xiangping Li
Centre for Micro-Photonics
Swinburne University of Technology
Hawthorn, Australia

Min Gu is an elected fellow of the Australian Academy of Science, the Australian Academy of Technological Sciences and Engineering, and a Laureate fellow of the Australian Research Council.

Xiangping Li received his PhD in optics in 2009 and joined the faculty as a postdoctoral researcher. In 2010, he won Australian Postdoctoral Fellowship, and in 2012 he won the Vice Chancellor's Research Award (Early Career). His research is focused on nanophotonics, nanotechnologies, and optical information technologies.

Tzu-Hsiang Lan, Chung-Hao Tien
Department of Photonics
National Chiao Tung University
Hsinchu, Taiwan

Tsu-Zian Lan received his PhD in 2011. He was a visiting scholar in Min Gu's group recording 3D polarization.

Chung-Hao Tien received his BSc in communication engineering and PhD in electro-optical engineering in 1997 and 2003, respectively. He is now an associate professor, covering computational imaging, free-form optics, and color engineering in display and lighting.

1. P. Refregier, B. Javidi, Optical image encryption based on input plane and Fourier plane random encoding, Opt. Lett. 20, p. 767-769, 1995.
2. P. Zijlstra, J. W. M. Chon, M. Gu, Five-dimensional optical recording mediated by surface plasmons in gold nanorods, Nature 459, p. 410-413, 2009.
3. X. Li, Y. Cao, M. Gu, Superresolution-focal-volume induced 3:0 Tbytes=disk capacity by focusing a radially polarized beam, Opt. Lett. 36, p. 2510-2512, 2011.
4. X. Li, T. H. Lan, C. H. Tien, M. Gu, Three-dimensional orientation-unlimited polarization encryption by a single optically configured vectorial beam, Nature Commun. 3, 2012.
5. S. Link, C. Burda, B. Nikoobakht, M. A. El-Sayed, Laser-induced shape changes of colloidal gold nanorods using femtosecond and nanosecond laser pulses, J. Phys. Chem. B. 104, p. 6152-6163, 2000.