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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 6 — Mar. 24, 2014
  • pp: 6511–6518
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A systematic approach to enhance off-axis directional electromagnetic wave by two-dimensional structure design

Dongyeal Lim, Dongheok Shin, Hyundo Shin, Kyoungsik Kim, and Jeonghoon Yoo  »View Author Affiliations


Optics Express, Vol. 22, Issue 6, pp. 6511-6518 (2014)
http://dx.doi.org/10.1364/OE.22.006511


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Abstract

In this study, we propose a two-dimensional (2D) dielectric structure tailored by a systematic design approach on the exit side of a metallic aperture to enhance the off-axis electromagnetic (EM) wave. We adopted a phase field method based topology optimization scheme and designed an arbitrary 2D dielectric structure in order to steer outward beaming through an aperture to a specific direction. Beyond previous one-dimensional structure, we proposed an arbitrary 2D dielectric structure through the introduced design process defining not only x- but also y-directional dielectric structural boundaries simultaneously and experimentally confirmed enhanced EM wave transmission to a desired direction.

© 2014 Optical Society of America

1. Introduction

Since the extraordinary transmission of light was measured experimentally through subwavelength hole arrays fabricated in thin metal films [1

1. W. Ebbessen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arays,” Nature 391(6668), 667–669 (1998). [CrossRef]

], intensive researches have been triggered for theoretical and experimental studies on optical field diffraction through a subwavelength aperture. The work by Lezec et al. [2

2. H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, and T. W. Ebbesen, “Beaming light from a subwavelength aperture,” Science 297(5582), 820–822 (2002). [CrossRef] [PubMed]

] showed that an enhanced and directional beaming from subwavelength apertures is allowed via the excitation of surface plasmons on corrugated metallic surfaces. Although EM radiation cannot induce the surface plasmon modes on a smooth metallic surface, the presence of periodic grating structure on the flat surface of a perfect conductor provides the surface bound states which mimic the surface plasmon polaritons of a real metal [3

3. F. J. Garcia-Vidal, L. Martín-Moreno, and J. B. Pendry, “Surfaces swith holes in them: new plasmonic metamaterials,” J. Opt. A, Pure Appl. Opt. 7(2), S97–S101 (2005). [CrossRef]

]. Optical components have been also used for the shaping of anomalous reflected beams [4

4. N. Yu, P. Genevet, M. A. Kats, F. Aieta, J. P. Tetienne, F. Capasso, and Z. Gaburro, “Light propagation with phase discontinuities: generalized laws of reflection and refraction,” Science 334(6054), 333–337 (2011). [CrossRef] [PubMed]

] and it is expansible to metamaterial design for the purpose of reducing the radar cross section for stealth applications [5

5. W. Xu and S. Sokusale, “Microwave diode switchable metamaterial reflector/absorber,” Appl. Phys. Lett. 103(3), 031902 (2013). [CrossRef]

].

The directional beaming effect through a subwavelength metallic aperture combined with dielectric gratings was also demonstrated [6

6. D. Z. Lin, C. K. Chang, Y. C. Chen, D. L. Yang, M. W. Lin, J. T. Yeh, J. M. Liu, C. H. Kuan, C. S. Yeh, and C. K. Lee, “Beaming light from a subwavelength metal slit surrounded by dielectric surface gratings,” Opt. Express 14(8), 3503–3511 (2006). [CrossRef] [PubMed]

] and such dielectric gratings were designed to couple the surface plasmon modes induced by the metallic aperture with surrounding medium along a specific off-axis direction [7

7. S. Kim, H. Kim, Y. Lim, and 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(5), 051113 (2007). [CrossRef]

]. Similar phenomena in the microwave spectrum have been studied for the subwavelength aperture surrounded by using metallic [8

8. H. Caglayan, I. Bulu, and E. Ozbay, “Extraordinary grating-coupled microwave transmission through a subwavelength annular aperture,” Opt. Express 13(5), 1666–1671 (2005). [CrossRef] [PubMed]

] or dielectric [9

9. H. Caglayan, I. Bulu, and E. Ozbay, “Observation of off-axis directional beaming via subwavelength asymmetric metallic gratings,” J. Phys. D Appl. Phys. 42(4), 045105 (2009). [CrossRef]

] grating on the exit side. The emitted beam can be confined to an off-axis specific region when the parameters of the grating structure are appropriately selected.

2. Numerical analysis modeling

Fig. 2 Model for analysis and design.
We compute the scattered field in a simulation region composed of a subwavelength aperture in a metal surface having a perfect electrically conducting (PEC) condition and dielectric gratings on the exit surface. Figure 2 shows FE analysis computation area composed of a PEC metal slit surrounded by dielectric grating design area. The electromagnetic finite element (FE) simulations were carried out for normal incident transverse electric (TE)-polarized Gaussian beam in the X-band (10GHz) frequency. The design area is composed of a rectangular area with 180mm width and the thickness of 4.8mm designated as a dotted rectangular. The measuring region is defined as 30mm square area whose center is located 40mm below and 25mm left from the bottom center of the aperture.

The scattered field is governed by time-harmonic Maxwell’s equation and it is formulated as a Helmholz equation for the TE-polarized wave assuming a nonmagnetic metal condition (μr1.0).
x{1εrxEz}+y{1εryEz}=ω2c2Ez
(1)
where Ez represents the scattered field and εr is the relative permittivity. The analysis of the TE field propagation was carried out by the FE analysis using the RF Module of COMSOLTM, with a monochromatic plane wave as excitation [13

13. COMSOL Multiphysics 3.5a, COMSOL AB, Stockholm.

].

3. Design process

The design objective is defined as the integration of the norm scattered field in the measuring region set on an off-axis area in the simulation region. We evaluate the energy flux at the measuring area based on the Poynting vector value P; therefore, the optimization problem is formulated as follows:
maximizeϕΓ(ϕ,u)=P|atthemeasuringareasubjecttoG(ϕ)=ΩϕdxVreqΩdx=0,0ϕ1
(2)
where Γ is the design objective function. G(ϕ) and Vreq represent the equality constraint on the volume and the required volume fraction, respectively. The analysis is performed for 2D cases and H-field parts need to be changed to E-field because only Ez is measured. Therefore, the state variable u expressed in Eq. (2) becomes Ez.

During the design process, the design variable ϕ is updated and it determines the element density in the design area. The design area is set on the dielectric area as designated in Fig. 2. We adopted a topology optimization approach based on the phase field method [14

14. T. Yamada, K. Izui, S. Nishiwaki, and A. Takezawa, “A topology optimization method based on the level set method incorporating a fictitious interface energy,” Comput. Methods Appl. Mech. Eng. 199(45-48), 2876–2891 (2010). [CrossRef]

] to find single material distribution in the entire design area. The boundary shape of the dielectric part is described by phase field variables which work as design parameters during the design process. The variable is defined as ϕ(x, y) in 2D cases and it is restricted in the interval of [0, 1]. The variable is applied to determine the isotropic dielectric constant as εr = εr-air + ϕ(εr-diel - εr-air) where εr-diel has the relative permittivity of 2.2 of paraffin [15] composing the dielectric gratings.

The double well potential F is assumed to have two global minima at 0 and 1, and is defined taking the equality constraint into account:
F(ϕ)=ap(ϕ)+μΓ(ϕ,Ez)ϕ|t=t1q(ϕ)+λG(ϕ)+γ2G(ϕ)2(t1tt2)
(4)
where p(ϕ)=ϕ2(1ϕ2),q(ϕ)=ϕ3(6ϕ215ϕ2+10) and they are respectively a smooth Dirac delta function and a smooth Heaviside function in the range of 0ϕ1. λ is the Lagrange multiplier and γ is a penalty parameter. μ is a parameter for normalizing the design sensitivity Γ/ϕ. ϕ is assumed to evolve during a virtual time t. The design sensitivity Γ/ϕ at the time t1 is regarded as a constant value. Due to the usage of double well potential functions the sensitivity at outside of the diffuse interface becomes zero so that the approach provides the interface tracking property. The adjoint method [17

17. J. S. Jensen and O. Sigmund, “Topology optimization of photonic crystal structures: a high-bandwidth low-loss T-junction waveguide,” J. Opt. Soc. Am. B 22(6), 1191–1198 (2005). [CrossRef]

] is used to compute the sensitivities necessary for the update scheme.

4. Numerical and experimental results

Optimization has been started from a regular paraffin grating with 9.15mm period and 4.8mm thickness to y-direction. The width and the thickness of the aperture in the PEC are 6mm and 12mm, respectively. The convergence history of the objective function and shape change of the design area at each of iterations are displayed in Fig. 3.
Fig. 3 Convergence history graph of the objective function and the dielectric shape changes during the topology optimization process. The function value is normalized to the initial structure value having a regular grating period.
The shape on the exit side is converging to an arbitrary 2D structure from a regular grating as displayed with the enhancement of the design objective value. The minus sign was originated from minimizing the negative value of the design cost function, i.e., maximizing the field intensity in the measuring region. The transmission boost ratio to the desired direction is expected about three times in comparison with the result by the initial regular grating as can be confirmed in the plot.

Fig. 4 E-field pattern obtained at 10GHz by simulation for (a) the regular gating structure and (b) the proposed 2D structure. Integrated field intensities along an x-directional line (designated as a dotted line) at the measuring area location (designated as a dotted square) are also plotted.
Figures 4 shows electric field distributions at 10 GHz by numerical simulation with the optimized 2D structure as well as the regular grating structure and the structure with the proposed grating shape shows the transmission boost to the desired direction. The transmission intensity plot of Fig. 4(b) estimates that a full-width at half-maximum divergence from the derived structure is about ± 14°. We also confirmed that the proposed dielectric shape gives an enhanced transmission to a desired direction based on intensity plots displayed in the bottom of Figs. 4(a) and 4(b) measured along the designated dotted line parallel to x-axis.

Fig. 5 Parameter study of the proposed structure. (a) 2D dielectric structure proposed from the design process and (b) field intensity evaluations of the measuring area normalized to the intensity value of the regular grating structure by changing t1 and t2. Dotted lines designate values from the derived optimal structure through the design process.
Figure 5(a) represents the 2D dielectric shape derived by the design process. Contrary to ordinary shapes we observed that the exit region on the outlet of the aperture is filled with both dielectric and empty space along the y-axis direction. To investigate the effect of the dielectric portion, we carried out parametric studies for t1 and t2 as designated in the inset of Fig. 5(a). Geometric parameters t1 and t2 are defined on the air portion to evaluate the size dependence of the 2D dielectric structure on the exit side of the metallic aperture. Figure 5(b) shows that the integrated field intensity in the measuring region is dependent on t2 and its value showing the most enhanced performance matches well with the value of the proposed structure. It is important that the derived 2D structure through the proposed approach clearly offers the co-existence of dielectric and air parts exited on the outlet of the metallic aperture and their appropriate sizes at the same time without depending on ordinary parameter study methodologies.

To verify the design result for the off-axis directional beaming, we carried out the experimental measurement in microwave frequency for the structure composed of subwavelength metallic aperture with the dielectric layer having the proposed shape. We made two test samples as illustrated in Fig. 6.
Fig. 6 Photographs of samples of (a) the regular grating structure and (b) the proposed 2D structure.
First sample [Fig. 6(a)] has regular paraffin grating while second sample [Fig. 6(b)] has the optimal grating structure described in Fig. 5(a). We normally launched 10 GHz microwave on the x-y plane waveguide and mapped the TE field distribution in the plane. We used a phase-sensitive, nearfield microwave scanning system inside a planar waveguide and captured the TE field mapping image on the x-y plane [18

18. D. Shin, Y. Urzhumov, Y. Jung, G. Kang, S. Baek, M. Choi, H. Park, K. Kim, and D. R. Smith, “Broadband electromagnetic cloaking with smart metamaterials,” Nat Commun 3, 1213–1220 (2012). [CrossRef] [PubMed]

]. The radiation source was located 22cm away from the metal slit and the experimental setup consisted of a scanning spectrometer to measure the electric field strength and a network analyzer as schematically explained in Fig. 7.
Fig. 7 Experiment set-up for TE field distribution measurement.

Fig. 8 Field pattern comparison experimentally measured for (a) the regular grating structure and (b) the proposed dielectric shape from the design process. Integrated field intensities along an x-directional dotted line at the measuring area location are plotted below each plot.
Figures 8(a) and 8(b) show electric field distributions at 10 GHz from experimental results for the regular grating and the proposed grating structure, respectively. We confirm good agreement with the simulation results in Fig. 4. From both results, our proposed dielectric shape gives an enhanced transmission boost to a desired direction and its intensity plots [bottom of Figs. 8(a) and 8(b)] along the line parallel to x-axis at the measuring location also confirms the off-axis beaming phenomenon. Those plots give similar tendency as numerical simulation based intensity plots displayed in Fig. 4.

5. Conclusion

This study proposes a 2D structure through a simulation based systematic design approach for a dielectric structure attached on the exit side of a metallic aperture to guide an off-axis directional beaming at a microwave frequency. We verified the proposed structure through experiments and proved that the dielectric structure transmits light to a desired direction. The suggested design process for controlling the transmission direction has the potential expansible to structural design of metamaterial at any frequency range such as IR or visible region.

Acknowledgment

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the South Korean government (MEST) (NRF-2011-0017512).

References and links

1.

W. Ebbessen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arays,” Nature 391(6668), 667–669 (1998). [CrossRef]

2.

H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, and T. W. Ebbesen, “Beaming light from a subwavelength aperture,” Science 297(5582), 820–822 (2002). [CrossRef] [PubMed]

3.

F. J. Garcia-Vidal, L. Martín-Moreno, and J. B. Pendry, “Surfaces swith holes in them: new plasmonic metamaterials,” J. Opt. A, Pure Appl. Opt. 7(2), S97–S101 (2005). [CrossRef]

4.

N. Yu, P. Genevet, M. A. Kats, F. Aieta, J. P. Tetienne, F. Capasso, and Z. Gaburro, “Light propagation with phase discontinuities: generalized laws of reflection and refraction,” Science 334(6054), 333–337 (2011). [CrossRef] [PubMed]

5.

W. Xu and S. Sokusale, “Microwave diode switchable metamaterial reflector/absorber,” Appl. Phys. Lett. 103(3), 031902 (2013). [CrossRef]

6.

D. Z. Lin, C. K. Chang, Y. C. Chen, D. L. Yang, M. W. Lin, J. T. Yeh, J. M. Liu, C. H. Kuan, C. S. Yeh, and C. K. Lee, “Beaming light from a subwavelength metal slit surrounded by dielectric surface gratings,” Opt. Express 14(8), 3503–3511 (2006). [CrossRef] [PubMed]

7.

S. Kim, H. Kim, Y. Lim, and 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(5), 051113 (2007). [CrossRef]

8.

H. Caglayan, I. Bulu, and E. Ozbay, “Extraordinary grating-coupled microwave transmission through a subwavelength annular aperture,” Opt. Express 13(5), 1666–1671 (2005). [CrossRef] [PubMed]

9.

H. Caglayan, I. Bulu, and E. Ozbay, “Observation of off-axis directional beaming via subwavelength asymmetric metallic gratings,” J. Phys. D Appl. Phys. 42(4), 045105 (2009). [CrossRef]

10.

M. P. Bendsøe and O. Sigmund, Topology Optimization: Theory, Methods and Applications, (Springer, 2003).

11.

J. Andkær, N. A. Mortensen, and O. Sigmund, “Towards all-dielectric, polarization-independent optical cloaks,” Appl. Phys. Lett. 100(10), 101106 (2012). [CrossRef]

12.

Y. Urzhumov, N. Landy, T. Driscoll, D. Basov, and D. R. Smith, “Thin low-loss dielectric coatings for free-space cloaking,” Opt. Lett. 38(10), 1606–1608 (2013). [CrossRef] [PubMed]

13.

COMSOL Multiphysics 3.5a, COMSOL AB, Stockholm.

14.

T. Yamada, K. Izui, S. Nishiwaki, and A. Takezawa, “A topology optimization method based on the level set method incorporating a fictitious interface energy,” Comput. Methods Appl. Mech. Eng. 199(45-48), 2876–2891 (2010). [CrossRef]

15.

http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_5.html.

16.

A. Takezawa, S. Nishiwaki, and M. Kitamura, “Shape and topology optimization based on the phase field method and sensitivity analysis,” J. Comput. Phys. 229(7), 2697–2718 (2010). [CrossRef]

17.

J. S. Jensen and O. Sigmund, “Topology optimization of photonic crystal structures: a high-bandwidth low-loss T-junction waveguide,” J. Opt. Soc. Am. B 22(6), 1191–1198 (2005). [CrossRef]

18.

D. Shin, Y. Urzhumov, Y. Jung, G. Kang, S. Baek, M. Choi, H. Park, K. Kim, and D. R. Smith, “Broadband electromagnetic cloaking with smart metamaterials,” Nat Commun 3, 1213–1220 (2012). [CrossRef] [PubMed]

OCIS Codes
(220.0220) Optical design and fabrication : Optical design and fabrication
(060.5625) Fiber optics and optical communications : Radio frequency photonics

ToC Category:
Diffraction and Gratings

History
Original Manuscript: January 30, 2014
Revised Manuscript: March 3, 2014
Manuscript Accepted: March 3, 2014
Published: March 12, 2014

Citation
Dongyeal Lim, Dongheok Shin, Hyundo Shin, Kyoungsik Kim, and Jeonghoon Yoo, "A systematic approach to enhance off-axis directional electromagnetic wave by two-dimensional structure design," Opt. Express 22, 6511-6518 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-6-6511


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References

  1. W. Ebbessen, H. J. Lezec, H. F. Ghaemi, T. Thio, P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arays,” Nature 391(6668), 667–669 (1998). [CrossRef]
  2. H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, T. W. Ebbesen, “Beaming light from a subwavelength aperture,” Science 297(5582), 820–822 (2002). [CrossRef] [PubMed]
  3. F. J. Garcia-Vidal, L. Martín-Moreno, J. B. Pendry, “Surfaces swith holes in them: new plasmonic metamaterials,” J. Opt. A, Pure Appl. Opt. 7(2), S97–S101 (2005). [CrossRef]
  4. N. Yu, P. Genevet, M. A. Kats, F. Aieta, J. P. Tetienne, F. Capasso, Z. Gaburro, “Light propagation with phase discontinuities: generalized laws of reflection and refraction,” Science 334(6054), 333–337 (2011). [CrossRef] [PubMed]
  5. W. Xu, S. Sokusale, “Microwave diode switchable metamaterial reflector/absorber,” Appl. Phys. Lett. 103(3), 031902 (2013). [CrossRef]
  6. D. Z. Lin, C. K. Chang, Y. C. Chen, D. L. Yang, M. W. Lin, J. T. Yeh, J. M. Liu, C. H. Kuan, C. S. Yeh, C. K. Lee, “Beaming light from a subwavelength metal slit surrounded by dielectric surface gratings,” Opt. Express 14(8), 3503–3511 (2006). [CrossRef] [PubMed]
  7. 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(5), 051113 (2007). [CrossRef]
  8. H. Caglayan, I. Bulu, E. Ozbay, “Extraordinary grating-coupled microwave transmission through a subwavelength annular aperture,” Opt. Express 13(5), 1666–1671 (2005). [CrossRef] [PubMed]
  9. H. Caglayan, I. Bulu, E. Ozbay, “Observation of off-axis directional beaming via subwavelength asymmetric metallic gratings,” J. Phys. D Appl. Phys. 42(4), 045105 (2009). [CrossRef]
  10. M. P. Bendsøe and O. Sigmund, Topology Optimization: Theory, Methods and Applications, (Springer, 2003).
  11. J. Andkær, N. A. Mortensen, O. Sigmund, “Towards all-dielectric, polarization-independent optical cloaks,” Appl. Phys. Lett. 100(10), 101106 (2012). [CrossRef]
  12. Y. Urzhumov, N. Landy, T. Driscoll, D. Basov, D. R. Smith, “Thin low-loss dielectric coatings for free-space cloaking,” Opt. Lett. 38(10), 1606–1608 (2013). [CrossRef] [PubMed]
  13. COMSOL Multiphysics 3.5a, COMSOL AB, Stockholm.
  14. T. Yamada, K. Izui, S. Nishiwaki, A. Takezawa, “A topology optimization method based on the level set method incorporating a fictitious interface energy,” Comput. Methods Appl. Mech. Eng. 199(45-48), 2876–2891 (2010). [CrossRef]
  15. http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_5.html .
  16. A. Takezawa, S. Nishiwaki, M. Kitamura, “Shape and topology optimization based on the phase field method and sensitivity analysis,” J. Comput. Phys. 229(7), 2697–2718 (2010). [CrossRef]
  17. J. S. Jensen, O. Sigmund, “Topology optimization of photonic crystal structures: a high-bandwidth low-loss T-junction waveguide,” J. Opt. Soc. Am. B 22(6), 1191–1198 (2005). [CrossRef]
  18. D. Shin, Y. Urzhumov, Y. Jung, G. Kang, S. Baek, M. Choi, H. Park, K. Kim, D. R. Smith, “Broadband electromagnetic cloaking with smart metamaterials,” Nat Commun 3, 1213–1220 (2012). [CrossRef] [PubMed]

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