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Shaping and focusing light beams with plasmonics

Subwavelength metallic slits can manipulate the shape of light, with potential application in optical interconnections and data storage.
10 March 2011, SPIE Newsroom. DOI: 10.1117/2.1201102.003435

Optical interconnections and data storage offer high-throughput information processing. However, advances in optical interconnection and data storage are hindered by two major bottlenecks: material dispersion and the diffraction limit. The former requires the operation of photonic devices in a dispersionless medium such as a vacuum, and the latter causes structural incompatibility or size-mismatching between electronic devices and their photonic counterparts. Thus, if we could shape light beams or focus them in free space in a given direction or at specific locations, it would open up more possibilities in optical interconnection and data-storage-device design.

Plasmonics, a research field concerned with surface plasmon polaritons (SPPs), has recently become a hot research topic. SPPs are coupled states of electromagnetic waves (light) and collective oscillations of electrons (plasma) at a metallic surface. Light waves can be converted to SPPs or vice versa, and the phases of both can be effectively controlled by dielectric (i.e., insulating) gratings on a metallic surface or slit widths and lengths in the metallic layer.1 We investigated two methods of using this property to shape light beams in interesting ways and to focus light in free space: subwavelength slits surrounded by surface gratings, and multiple subwavelength slits with varying widths and heights.

In the first method p-polarized light (which has a polarization component perpendicular to the surface) passes through the metallic subwavelength slits, exciting SPPs on the exit side that propagate along the metal-dielectric interface. If the SPPs pass through surface gratings, they radiate light in a specific direction in the air determined by the period of the gratings (see Figure 1). By employing gratings that are symmetric, asymmetric, and chirped (i.e., ones in which the period of the grating decreases across it) on the metal surface, on-axis, off-axis directional beaming, and focused light can be achieved, respectively.2,3

Figure 1. Dielectric (nd=1 .72) gratings on the silver (Ag) substrate (εm=-10.1592+0.8294i) are asymmetrically arranged with periods of 305 and 503nm. The height and fill factors of the grating are 100nm and 0.5, respectively. Λ: Grating periods. z: Direction of the normal. x: Direction of the grating periods.

One of the newest applications of this beaming mechanism is ‘bundle beaming.’ When light is incident from the reverse and passes through double subwavelength slits, SPPs are generated that then encounter asymmetric dielectric gratings on the top surface of the metallic layer. We designed the grating period to convert SPPs to radiating light with angles of +20°and −20°, respectively. We found that when the radiating beams from the neighboring slits destructively interfere with each other, they produce a double beam (see Figure 2). When they constructively interfere, they make a strong beam (not shown).4 We can control this interference by changing the incident angle of the plane wave, which enables us to switch dynamically between the single-beam and double-beam states. We can further extend the double beam as a ‘bundle beam’ by increasing the number of slits. These beams are collimated (i.e., nearly parallel).

Figure 2. Intensity distribution (arbitrary units) of off-axis bundle beaming. Arrows indicate the direction of the power flows. z: Direction of the normal. x: Direction of the grating periods.

In the second method, we used Huygens' principle to manipulate output beams that have different phases after passing through multiple subwavelength slits. The slits in this structure have various widths and lengths, and these determine the phases.5,6 The complex wavenumber β depends on the slit width and the permittivities of the metal and dielectric material. If we increase the slit width (keeping the single-mode characteristic of the slit waveguide), the complex wavenumber is reduced, and vice versa. After passing through the subwavelength slit, the SPP mode has an output phase retardation that is mainly determined by Re(βh), where h is the slit length (see Figure 3 for a multislit structure for beam focusing, fabricated by ion-beam machining). The transmitted beam from each slit acts as an independent point light source with the output phase determined by the slit width and length. The light beams focus at the distance of focal length f (see Figure 4 for the intensity distribution). We used a general such rule (called a genetic algorithm) to obtain the optimal structural parameters, such as slit widths and lengths, of the multislit structure. We defined ‘optimal’ as stronger intensity and narrower full-width at half maximum at the focal point. Experimental results and their measurement using holographic microscopy are reported elsewhere.7

Figure 3. Fabricated multiple-slit structure for beam focusing. It consists of five slits with their width wiand length hi arranged along the semicircular profile with radius r carved in a silver slab. λ: Wavelength. Ag: Silver. d: Depth of slab. g: Gap between slits. z: Direction perpendicular to the slits. x: Direction of the incident light.

Figure 4. Intensity distribution of beam focusing when r, w1(=w5), w2(=w4), w3, g1(=g4), and g2(=g3)are 748, 290, 286, 748, 100, and 74nm, respectively. Focal length is obtained as 1.2μm and full-width at half maximum is 310nm. Arrows indicate the direction of the power flows.

In summary, we have described two methods for collimating and focusing beams of light. Converting between the SPP mode and light makes it is possible to control beam shapes using grating structures and multiple slits on a metallic layer. We expect these novel beam-shaping methods will have applications in optical data storage, sensing, tweezing, and interconnection. We are currently developing plasmonic tweezers based on this principle and are also working on dynamic steering methods of bundle beaming.

This work was supported by the National Research Foundation and the Ministry of Education, Science and Technology of Korea through the Creative Research Initiatives Program (Active Plasmonics Application Systems).

Byoungho Lee, Dawoon Choi, Seung-Yeol Lee, Yongjun Lim
School of Electrical Engineering, Seoul National University
Seoul, South Korea

Byoungho Lee received a PhD from the Department of Electrical Engineering and Computer Science, University of California, Berkeley (1993). Since 1994, he has been a faculty member in the School of Electrical Engineering, Seoul National University. He is a Fellow of SPIE and the Optical Society of America. He has served in the SPIE Science, Technology, and Engineering Policy Committee and in 2004 was a co-chair of the SPIE Conference on Holography, Diffractive Optics, and Applications II.

Dawoon Choi received an MS from the School of Electrical Engineering, Seoul National University (2010) and is currently working toward a PhD.

Seung-Yeol Lee received a BS from the School of Electrical Engineering, Seoul National University (2009). He is currently working toward a PhD.

Yongjun Lim received a PhD from the School of Electrical Engineering, Seoul National University (2010). He is currently working as a postdoctoral researcher.

1. W. L. Barnes, A. Dereux, T. W. Ebbesen, Surface plasmon subwavelength optics, Nature 424, pp. 824-830, 2003. doi:10.1038/nature01937
2. S. Kim, H. Kim, Y. Lim, B. Lee, Off-axis directional beaming of optical field diffracted by a single subwavelength metal slit with asymmetric dielectric surface gratings, Appl. Phys. Lett. 90, no. 5, pp. 051113, 2007. doi:10.1063/1.2437730
3. S. Kim, Y. Lim, H. Kim, J. Park, B. Lee, Optical beam focusing by a single subwavelength metal slit surrounded by chirped dielectric surface gratings, Appl. Phys. Lett. 92, no. 1, pp. 013103, 2008. doi:10.1063/1.2828716
4. S. Kim, Y. Lim, J. Park, B. Lee, Bundle beaming from multiple subwavelength slits surrounded by dielectric surface gratings, J. Lightwave Technol. 28, no. 14, pp. 2023-2029, 2010. doi:10.1109/JLT.2010.2051415
5. H. Shi, C. Wang, C. Du, X. Luo, X. Dong, H. Gao, Beam manipulating by metallic nano-slits with variant widths, Opt. Express 13, no. 18, pp. 6815-6820, 2005. doi:10.1364/OPEX.13.006815
6. D. Choi, Y. Lim, S. Roh, I.-M. Lee, J. Jung, B. Lee, Optical beam focusing with a metal slit array arranged along a semicircular surface and its optimization by genetic algorithm, Appl. Opt. 49, no. 7, pp. A30-A35, 2010. doi:10.1364/AO.49.000A30
7. B. Lee, S. Kim, D. Choi, Beam shaping by the use of plasmonics, Proc. SPIE 7848, pp. 784805, 2010. doi:10.1117/12.869095