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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 22 — Nov. 4, 2013
  • pp: 26311–26322
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Realizing vertical light coupling and splitting in nano-plasmonic multilevel circuits

Mohamed H. El Sherif, Osman S. Ahmed, Mohamed H. Bakr, and Mohamed A. Swillam  »View Author Affiliations


Optics Express, Vol. 21, Issue 22, pp. 26311-26322 (2013)
http://dx.doi.org/10.1364/OE.21.026311


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Abstract

We present a novel technique for vertical coupling of light guided by nanoscale plasmonic slot waveguides (PSWs). A triangularly-shaped plasmonic slot waveguide rotator is exploited to attain such coupling with a good efficiency over a wide bandwidth. Using this approach, light propagating in a horizontal direction is efficiently coupled to propagate in the vertical direction and vice versa. We also propose a power divider configuration to evenly split a vertically coupled light wave to two horizontal channels. A detailed parametric study of the triangular rotator is demonstrated with multiple configurations analyzed. This structure is suitable for efficient coupling in multilevel nano circuit environment.

© 2013 Optical Society of America

1. Introduction

The unrelenting need for faster and more efficient processing with miniaturized components drove electronics into the nanoscale. It is now customary to produce scaled down and ultrafast transistors. Yet, unlike transistors where efficiency improves with miniaturization, copper interconnect efficiency degrades, thus causing delays in the electronic devices [1

1. R. Zia, J. A. Schuller, A. Chandran, and M. L. Brongersma, “Plasmonics: the next chip-scale technology,” Mater. Today 9(7-8), 20–27 (2006). [CrossRef]

]. An alternative solution to electronics is using the traditional silicon photonics technology. Silicon photonics are compatible with CMOS electronics and possess 1000 times the data rate present in electronics [2

2. A. Biberman and K. Bergman, “Optical interconnection networks for high-performance computing systems,” Rep. Prog. Phys. 75(4), 046402 (2012). [CrossRef] [PubMed]

]. However, they are limited in size by the diffraction limit, causing a large size mismatch between silicon photonics and electronics [1

1. R. Zia, J. A. Schuller, A. Chandran, and M. L. Brongersma, “Plasmonics: the next chip-scale technology,” Mater. Today 9(7-8), 20–27 (2006). [CrossRef]

, 3

3. D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics 4(2), 83–91 (2010). [CrossRef]

]. This size mismatch hinders using the advantages offered by both technologies concurrently.

Surface plasmon polaritons (SPPs) are paving the way for nanoscale optical technology that mitigates size, delay, and radiation limitations [4

4. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]

]. SPPs promise to bridge the gap between electronics and photonics. Plasmonics are not limited in size by the diffraction limit and they enjoy, at the same time, optical data rates [1

1. R. Zia, J. A. Schuller, A. Chandran, and M. L. Brongersma, “Plasmonics: the next chip-scale technology,” Mater. Today 9(7-8), 20–27 (2006). [CrossRef]

, 3

3. D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics 4(2), 83–91 (2010). [CrossRef]

, 4

4. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]

]. SPPs are realized using metallic/dielectric interfaces, where the metals develop a negative permittivity at optical frequencies. This causes the wave to be guided on the metal/dielectric surface. As a result, the advantages of both electronics and photonics may be combined in the same setting. A number of subwavelength plasmonic devices have been proposed for guiding and localizing electromagnetic energy for a variety of applications. These applications include imaging [4

4. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]

, 5

5. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]

], biosensors [6

6. I. Choi and Y. Choi, “Plasmonic nanosensors: review and prospect,” IEEE J. Sel. Top. Quantum Electron. 18(3), 1110–1121 (2012). [CrossRef]

], multilevel couplers [7

7. M. H. El Sherif, O. S. Ahmed, M. H. Bakr, and M. A. Swillam, “Polarization-controlled excitation of multilevel plasmonic nano-circuits using single silicon nanowire,” Opt. Express 20(11), 12473–12486 (2012). [CrossRef] [PubMed]

], photovoltaic [8

8. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]

], and cancer treatment [9

9. P. K. Jain, I. H. El-Sayed, and M. A. El-Sayed, “Au nanoparticles target cancer,” Nano Today 2(1), 18–29 (2007). [CrossRef]

].

This paper is organized as follows: The theory of our proposed configuration is introduced in Section 2. In Section 3, we present a thorough parametric study of the plasmonic vertical coupler along with detailed numerical results towards the optimization of the proposed configuration. The stair case realization of the vertical coupler and possible fabrication steps are addressed in Section 4. Section 5 presents an application of the vertical coupling to realize ultra-compact vertical power splitters. Our work is concluded in Section 6.

2. Triangular plasmonic slot waveguide (TPSW)

Ultrafast nano-scale light manipulation ushered the introduction of multilevel optical circuits [14

14. M. Raburn, B. Liu, K. Rauscher, Y. Okuno, N. Dagli, and J. E. Bowers, “3-D photonic circuit technology,” IEEE J. Sel. Top. Quantum Electron. 8(4), 935–942 (2002). [CrossRef]

, 15

15. S. N. Garner, S. Lee, V. Chuyanov, A. Chen, A. Yacoubian, W. H. Steier, and L. R. Dalton, “Three-dimensional integrated optics using polymers,” IEEE J. Quantum Electron. 35(8), 1146–1155 (1999). [CrossRef]

]. In this case the device density is increased due to the utilization of 3-dimensional routing mechanisms. This overcomes the limited number of optical devices that can be integrated on one level of the chip due to cross talk [14

14. M. Raburn, B. Liu, K. Rauscher, Y. Okuno, N. Dagli, and J. E. Bowers, “3-D photonic circuit technology,” IEEE J. Sel. Top. Quantum Electron. 8(4), 935–942 (2002). [CrossRef]

, 15

15. S. N. Garner, S. Lee, V. Chuyanov, A. Chen, A. Yacoubian, W. H. Steier, and L. R. Dalton, “Three-dimensional integrated optics using polymers,” IEEE J. Quantum Electron. 35(8), 1146–1155 (1999). [CrossRef]

]. Clustering these optical chips to multiple levels provides highly dense photonic circuit integration with faster processing and more functionalities.

Plasmonic waveguides have been recently utilized in the design of subwavelength light guiding and routing. A Plasmonic slot waveguide (PSW) made of silver [17

17. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, 1998).

] on a silicon dioxide substrate is shown in Fig. 1
Fig. 1 A plasmonic slot waveguide (PSW).
. It has been utilized in on-chip coupling due to its strong confinement of light and low radiation [11

11. L. Liu, Z. Han, and S. He, “Novel surface plasmon waveguide for high integration,” Opt. Express 13(17), 6645–6650 (2005). [CrossRef] [PubMed]

, 18

18. J. A. Dionne, L. A. Sweatlock, H. A. Atwater, and A. Polman, “Plasmonic slot waveguides: towards chip-scale propagation with subwavelength-scale localization,” Phys. Rev. B 73(3), 035407 (2006). [CrossRef]

]. However, propagation in PSWs is limited to relatively short distances due to the losses that occur as a result of absorption in the metal. The propagation loss for the PSW in Fig. 1 is shown in Fig. 2
Fig. 2 The transmission efficiency at different points along the PSW.
. This figure shows an exponential decrease in the transmission with increased propagation distances (d) along the waveguide. PSWs allow for only one polarization mode to propagate while completely blocking the other one. It was recently demonstrated in [7

7. M. H. El Sherif, O. S. Ahmed, M. H. Bakr, and M. A. Swillam, “Polarization-controlled excitation of multilevel plasmonic nano-circuits using single silicon nanowire,” Opt. Express 20(11), 12473–12486 (2012). [CrossRef] [PubMed]

] that using this polarization-dependent nature of PSWs, we can achieve polarization splitting and multilevel coupling capabilities.

The typical parameters of the triangular waveguide are the width w = 400.0 nm and height h = 400.0 nm. Those values are chosen with correspondence to the dimensions of the input and output PSWs. However, a parametric study and thorough investigation is provided in Section 3 to determine the optimal dimensions of this triangular rotator.

3. Numerical investigation of the TPSW

To determine the optimal dimensions of the proposed TPSW that would maximize the coupling efficiency, a parametric study is carried out. For an input light from the vertical PSW, we calculate the transmission and reflection introduced at the proposed coupling structure for different parameter values. The normalized amplitude in each waveguide is calculated by determining the power flux through the respective detector, integrating it over the cross section, and normalizing it with respect to the source power.

3.1 Study of the triangular rotator width (w) effect

There are two approaches for changing the width (w) of the triangular interface. In both configurations, the width (w) is changed while keeping the width and height of the vertical and horizontal waveguides fixed at 400.0 nm respectively. The proposed configurations are shown in Fig. 6
Fig. 6 The TPSW in a vertical-to-horizontal light bending configuration with a monitor placed 400.0 nm from the straight waveguide edge and h = 400.0 nm
where the vertices of the triangular PSW are labeled as 1, 2, and 3. In the first approach, we vary the width (w) of the TPSW by moving the vertical plane containing the vertices 2 and 3 deeper inside the horizontal PSW. The width (w) of the triangular interface is tuned with steps of 100.0 nm to enhance the coupling efficiency. The vertical and horizontal PSWs are 400.0 nm wide with a 50.0 nm slot. The height of the triangular PSW is fixed at h = 400.0 nm. The source is placed 300.0 nm away from the bottom edge of the triangle and the monitor is placed 400.0 nm from the edge of the vertical PSW as shown in Fig. 6. Figure 7
Fig. 7 The transmission efficiency (T) and reflection (R) for the first configuration in Fig. 6. The width (w) of the TPSW is changed by moving the vertical plane containing the points 2 and 3. The width of the vertical waveguide and the height of the horizontal waveguide are kept fixed at 400.0 nm. The height of the triangular PSW is fixed at h = 400.0 nm.
illustrates the transmission efficiency (T) of this configuration. A maximum overall transmission of 70% can be achieved which provides 30% increase over the abrupt coupling. It can be seen that as (w) increases, the transmission efficiency (T) increases over most of the wavelength spectrum until a width of 500.0 nm. The transmission starts to decrease again beyond w = 500.0 nm. It can also be noted from Fig. 7 that the reflections (R) for the different widths (w) are below 15%. We only show the lowest and highest reflection curves in Fig. 7 for clarity purposes.

Another approach for changing the width (w) of the triangular rotator is through altering the width of the triangular PSW using the vertex labeled 1, while keeping the vertical plane containing the vertices 2 and 3 fixed (see Fig. 6). The points 2 and 3 are placed 100.0 nm to the right side of the vertical PSW. The monitor is placed 100.0 nm away from the edge of the TPSW. Maximum transmission is observed for this configuration at a width of w = 500.0 nm over most of the wavelength range as shown in Fig. 8
Fig. 8 The transmission efficiency for the second configuration in Fig. 6. The width (w) of the TPSW is changed using point 1 only while keeping the height h fixed at 400.0 nm. The width of the vertical waveguide and the height of the horizontal waveguide are kept at 400.0 nm. Points 2 and 3 are placed 100.0 nm to the right side of the vertical PSW.
. As (w) increases or decreases from the 500.0 nm value, transmission falls. This demonstrates that the optimal width for the triangle rotator is w = 500.0 nm. It is observed that having the width of the TPSW equal to that of the vertical PSW or 100.0 nm larger provides a simple realization with very good coupling efficiency.

3.2 Study of the triangular height (h) effect

For the case of h = 600.0 nm, as evident from Fig. 9 and Fig. 11, the transmission is larger when increasing (h) and Hw together (≈80%) as compared to the case of a constant Hw = 400.0 nm (≈71%). From Fig. 11, the transmission in the case of h = 500.0 nm reaches a maximum of ≈74%. Thus, having a TPSW height (h) similar to that of the straight PSW or 100.0 nm higher produces the optimal transmission.

3.3 TPSW Normalized calculations

To compare the losses introduced by the TPSW, we normalize the transmission with respect to the transmission of a straight PSW with a similar length. For our calculations, we assume that most of the wave propagates along the middle of the PSW and the TPSW, and divide the transmission by that of a straight PSW with the same length. The normalized results are shown for varying widths (w) of the TPSW in Fig. 13
Fig. 13 The normalized transmission of the TPSW as compared to a straight PSW for different widths (w). The height of the TPSW (h) is fixed at 400.0 nm.
. It can be noticed that the TPSW achieves a transmission of at least 50% of that of the straight PSW over most of the band. Figure 13 also demonstrates that the value w = 500.0 nm achieves the minimum average coupling losses over the entire bandwidth.

4. The stair case TPSW configuration and possible fabrication steps

One possible realization of the proposed TPSW is through a stair case approximation (see Fig. 14
Fig. 14 The light bending stair case structure.
), which simplifies the fabrication process. This realization provides similar coupling efficiency to that of the continuous triangular case. There are 8 rectangular sections, with each section being 50.0 nm high. The width of the lower section is 500.0 nm. The width decreases by steps of 75.0 nm for each upper section. The highest section is 20.0 nm wide. In Fig. 15
Fig. 15 The transmission and reflection of the stair case configuration as compared to the triangular section and rectangular section of the same dimensions.
, the transmission of the stair case assembly is compared to that of the TPSW and a rectangular rotator with the same dimensions. We notice that the transmission of the stair case section is similar to that of the TPSW over most of the spectrum. Having more rectangle sections in the stair case configuration would result in a transmission efficiency closer to that of the TPSW.

The fabrication of the triangular coupler can be done using an isotropic etch on the (111) silicon plane. The silicon can then be covered by 200.0 nm thick metal to assure similar functionality to the pure metal structure. The stair case can be prepared by successive etching of the silicon. Once the stair feature is achieved, the silicon can be covered by metal to ensure plasmonic functionality.

5. Vertical light power splitter/combiner

In this section we introduce a direct application to the proposed triangular rotator. A power splitter that splits a vertically propagating wave equally to a horizontal plane is demonstrated. This configuration uses two symmetric triangle rotators so that the wave is divided equally in the two opposite horizontal waveguides.

The configuration is shown in Fig. 16
Fig. 16 Vertical power splitter/combiner.
. This design causes the vertically propagating wave to split so that different light manipulations can be performed at different sections. Half of the wave is guided to the right (monitor 1) while the other half is guided to the left (monitor 2). Figure 17
Fig. 17 Transmission (T) and reflection (R) for different (h2) of the triangular section power splitter.
displays the transmission (T) for the power splitter at both monitors 1 and 2. It can be seen that when (h2) varies from 0 nm – 400.0 nm the transmission in the ports decreases. It is noted that the reflection (R) is very low for h2 = 0 nm and 160.0 nm (below 8% over most of the band). It reaches its highest value for h2 = 400.0 nm which corresponds to the case of a rectangular junction.

6. Conclusion

We propose a triangular plasmonic waveguide that works as an efficient out-of-plane coupler. Vertical to horizontal light coupling and vice versa is achieved using this triangular rotator. We demonstrated various configurations of the device and how different parameters affect the operation of the structure. We also proposed a power splitter using the TPSW. This splitter divides the power of a vertically propagating wave equally in two horizontal arms. It can be used for multi-level couplers or for light bending and splitting from one circuit plane to another with minimum losses. The fabrication of this device is under investigation.

References and links

1.

R. Zia, J. A. Schuller, A. Chandran, and M. L. Brongersma, “Plasmonics: the next chip-scale technology,” Mater. Today 9(7-8), 20–27 (2006). [CrossRef]

2.

A. Biberman and K. Bergman, “Optical interconnection networks for high-performance computing systems,” Rep. Prog. Phys. 75(4), 046402 (2012). [CrossRef] [PubMed]

3.

D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics 4(2), 83–91 (2010). [CrossRef]

4.

E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]

5.

W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]

6.

I. Choi and Y. Choi, “Plasmonic nanosensors: review and prospect,” IEEE J. Sel. Top. Quantum Electron. 18(3), 1110–1121 (2012). [CrossRef]

7.

M. H. El Sherif, O. S. Ahmed, M. H. Bakr, and M. A. Swillam, “Polarization-controlled excitation of multilevel plasmonic nano-circuits using single silicon nanowire,” Opt. Express 20(11), 12473–12486 (2012). [CrossRef] [PubMed]

8.

H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]

9.

P. K. Jain, I. H. El-Sayed, and M. A. El-Sayed, “Au nanoparticles target cancer,” Nano Today 2(1), 18–29 (2007). [CrossRef]

10.

G. Veronis and S. Fan, “Bends and splitters in metal-dielectric-metal subwavelength plasmonic waveguides,” Appl. Phys. Lett. 87(13), 311102 (2005). [CrossRef]

11.

L. Liu, Z. Han, and S. He, “Novel surface plasmon waveguide for high integration,” Opt. Express 13(17), 6645–6650 (2005). [CrossRef] [PubMed]

12.

W. Cai, W. Shin, S. Fan, and M. L. Brongersma, “Elements for plasmonic nanocircuits with three-dimensional slot waveguides,” Adv. Mater. 22(45), 5120–5124 (2010). [CrossRef] [PubMed]

13.

M. A. Swillam and A. S. Helmy, “Feedback effects in plasmonic slot waveguides examined using a closed-form model,” IEEE Photon. Technol. Lett. 24(6), 497–499 (2012). [CrossRef]

14.

M. Raburn, B. Liu, K. Rauscher, Y. Okuno, N. Dagli, and J. E. Bowers, “3-D photonic circuit technology,” IEEE J. Sel. Top. Quantum Electron. 8(4), 935–942 (2002). [CrossRef]

15.

S. N. Garner, S. Lee, V. Chuyanov, A. Chen, A. Yacoubian, W. H. Steier, and L. R. Dalton, “Three-dimensional integrated optics using polymers,” IEEE J. Quantum Electron. 35(8), 1146–1155 (1999). [CrossRef]

16.

F. D. T. D. Lumerical, Lumerical Soultions, Inc.http://www.lumerical.com

17.

E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, 1998).

18.

J. A. Dionne, L. A. Sweatlock, H. A. Atwater, and A. Polman, “Plasmonic slot waveguides: towards chip-scale propagation with subwavelength-scale localization,” Phys. Rev. B 73(3), 035407 (2006). [CrossRef]

OCIS Codes
(130.2790) Integrated optics : Guided waves
(130.3120) Integrated optics : Integrated optics devices
(230.1360) Optical devices : Beam splitters
(230.4170) Optical devices : Multilayers
(240.6680) Optics at surfaces : Surface plasmons
(250.5403) Optoelectronics : Plasmonics

ToC Category:
Integrated Optics

History
Original Manuscript: August 19, 2013
Revised Manuscript: October 11, 2013
Manuscript Accepted: October 16, 2013
Published: October 25, 2013

Citation
Mohamed H. El Sherif, Osman S. Ahmed, Mohamed H. Bakr, and Mohamed A. Swillam, "Realizing vertical light coupling and splitting in nano-plasmonic multilevel circuits," Opt. Express 21, 26311-26322 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-22-26311


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References

  1. R. Zia, J. A. Schuller, A. Chandran, and M. L. Brongersma, “Plasmonics: the next chip-scale technology,” Mater. Today9(7-8), 20–27 (2006). [CrossRef]
  2. A. Biberman and K. Bergman, “Optical interconnection networks for high-performance computing systems,” Rep. Prog. Phys.75(4), 046402 (2012). [CrossRef] [PubMed]
  3. D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics4(2), 83–91 (2010). [CrossRef]
  4. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science311(5758), 189–193 (2006). [CrossRef] [PubMed]
  5. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature424(6950), 824–830 (2003). [CrossRef] [PubMed]
  6. I. Choi and Y. Choi, “Plasmonic nanosensors: review and prospect,” IEEE J. Sel. Top. Quantum Electron.18(3), 1110–1121 (2012). [CrossRef]
  7. M. H. El Sherif, O. S. Ahmed, M. H. Bakr, and M. A. Swillam, “Polarization-controlled excitation of multilevel plasmonic nano-circuits using single silicon nanowire,” Opt. Express20(11), 12473–12486 (2012). [CrossRef] [PubMed]
  8. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater.9(3), 205–213 (2010). [CrossRef] [PubMed]
  9. P. K. Jain, I. H. El-Sayed, and M. A. El-Sayed, “Au nanoparticles target cancer,” Nano Today2(1), 18–29 (2007). [CrossRef]
  10. G. Veronis and S. Fan, “Bends and splitters in metal-dielectric-metal subwavelength plasmonic waveguides,” Appl. Phys. Lett.87(13), 311102 (2005). [CrossRef]
  11. L. Liu, Z. Han, and S. He, “Novel surface plasmon waveguide for high integration,” Opt. Express13(17), 6645–6650 (2005). [CrossRef] [PubMed]
  12. W. Cai, W. Shin, S. Fan, and M. L. Brongersma, “Elements for plasmonic nanocircuits with three-dimensional slot waveguides,” Adv. Mater.22(45), 5120–5124 (2010). [CrossRef] [PubMed]
  13. M. A. Swillam and A. S. Helmy, “Feedback effects in plasmonic slot waveguides examined using a closed-form model,” IEEE Photon. Technol. Lett.24(6), 497–499 (2012). [CrossRef]
  14. M. Raburn, B. Liu, K. Rauscher, Y. Okuno, N. Dagli, and J. E. Bowers, “3-D photonic circuit technology,” IEEE J. Sel. Top. Quantum Electron.8(4), 935–942 (2002). [CrossRef]
  15. S. N. Garner, S. Lee, V. Chuyanov, A. Chen, A. Yacoubian, W. H. Steier, and L. R. Dalton, “Three-dimensional integrated optics using polymers,” IEEE J. Quantum Electron.35(8), 1146–1155 (1999). [CrossRef]
  16. F. D. T. D. Lumerical, Lumerical Soultions, Inc. http://www.lumerical.com
  17. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, 1998).
  18. J. A. Dionne, L. A. Sweatlock, H. A. Atwater, and A. Polman, “Plasmonic slot waveguides: towards chip-scale propagation with subwavelength-scale localization,” Phys. Rev. B73(3), 035407 (2006). [CrossRef]

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