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Optics Letters

| RAPID, SHORT PUBLICATIONS ON THE LATEST IN OPTICAL DISCOVERIES

  • Editor: Alan E. Willner
  • Vol. 38, Iss. 16 — Aug. 15, 2013
  • pp: 3005–3008
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Ultracompact and broadband polarization beam splitter utilizing the evanescent coupling between a hybrid plasmonic waveguide and a silicon nanowire

Xiaowei Guan, Hao Wu, Yaocheng Shi, Lech Wosinski, and Daoxin Dai  »View Author Affiliations


Optics Letters, Vol. 38, Issue 16, pp. 3005-3008 (2013)
http://dx.doi.org/10.1364/OL.38.003005


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Abstract

An ultracompact polarization beam splitter (PBS) is proposed based on an asymmetrical directional coupler consisting of a silicon hybrid plasmonic waveguide (HPW) and a silicon nanowire. The widths of the two coupling waveguides are chosen so that the phase-matching condition is satisfied for TE polarization only while the phase mismatch is significant for TM polarization. A sharply bent silicon HPW is connected at the thru port to play the role of polarizer by utilizing its polarization-dependent loss. With the present principle, the designed PBS has a footprint as small as only 1.9μm×3.7μm, which is the shortest PBS reported until now, even when large waveguide dimensions (e.g., the waveguide widths w1,2=300nm and the gap width wgap=200nm) are chosen to simplify the fabrication process. The numerical simulations show that the designed PBS has a very broad band (120nm) with an extinction ratio >12dB and a large fabrication tolerance to allow a waveguide width variation of ±30nm.

© 2013 Optical Society of America

Polarization handling devices play an important role as the basic functional elements for many applications involving polarization [1

1. D. Dai, L. Liu, S. Gao, D. Xu, and S. He, Laser Photon. Rev. 7, 303 (2013). [CrossRef]

], e.g., coherent optical communications for long-haul optical fiber communications [2

2. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, Opt. Express 17, 9421 (2009). [CrossRef]

]. Regarding coherent optical systems used for future network-on-chip for optical interconnects, the polarization diversity components are desired to be ultrasmall. Polarization handling technology also plays an important role in realizing integrated photonic quantum circuits [3

3. L. Sansoni, F. Sciarrino, G. Vallone, P. Mataloni, A. Crespi, R. Ramponi, and R. Osellame, Phys. Rev. Lett. 105, 200503 (2010). [CrossRef]

] and enabling polarization transparent nano-PICs [4

4. T. Barwicz, M. R. Watts, M. A. Popovic, P. T. Rakich, L. Socci, F. X. Kartner, E. P. Ippen, and H. I. Smith, Nat. Photonics 1, 57 (2007). [CrossRef]

], which are very important for the fiber-fed cases.

To realize an asymmetrical coupling system, one can simply choose different widths or heights of the core for the coupling waveguides [18

18. J. Wang, D. Liang, Y. Tang, D. Dai, and J. Bowers, Opt. Lett. 38, 4 (2013). [CrossRef]

], or different types of optical waveguides for the coupling region, e.g., a silicon nanowire (NW) coupling with a silicon nanoslot waveguide [19

19. D. Dai, Z. Wang, and J. Bowers, Opt. Lett. 36, 2590 (2011). [CrossRef]

,20

20. S. Lin, J. Hu, and K. B. Crozier, Appl. Phys. Lett. 98, 151101 (2011). [CrossRef]

] or a silicon hybrid plasmonic waveguide (HPW) [21

21. F. Lou, D. Dai, and L. Wosinski, Opt. Lett. 37, 3372 (2012). [CrossRef]

,22

22. J. Chee, S. Zhu, and G. Q. Lo, Opt. Express 20, 25345 (2012). [CrossRef]

].

The HPW is a kind of novel structure enabling nanoscale optical confinement and strong polarization dependence [23

23. R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, Nat. Photonics 2, 496 (2008). [CrossRef]

,24

24. D. Dai and S. He, Opt. Express 17, 16646 (2009). [CrossRef]

], which makes it feasible to achieve ultracompact PBSs. In [21

21. F. Lou, D. Dai, and L. Wosinski, Opt. Lett. 37, 3372 (2012). [CrossRef]

,22

22. J. Chee, S. Zhu, and G. Q. Lo, Opt. Express 20, 25345 (2012). [CrossRef]

], the PBSs are based on a three-waveguide coupling system consisting of two silicon NWs with a narrow HPW between them. Their asymmetrical coupling system was designed to make TM polarization cross coupled, and the total length of the designed PBSs (including the bent output section) is about 5–10 μm when the gap width is chosen to be as small as 100 nm. In this Letter, we propose a simplified ultrashort PBS using an asymmetrical coupler consisting of a silicon NW and a HPW. More importantly, in our case these two waveguides are designed optimally to make the phase-matching condition satisfied for TE polarization, which is different from the design reported previously in [21

21. F. Lou, D. Dai, and L. Wosinski, Opt. Lett. 37, 3372 (2012). [CrossRef]

,22

22. J. Chee, S. Zhu, and G. Q. Lo, Opt. Express 20, 25345 (2012). [CrossRef]

]. In this case, the two waveguides have widths of 300nm, and the length of the coupling region is only about 2.2 μm even when the gap width is chosen to be as large as 200 nm, which helps to simplify the fabrication. Furthermore, we cascade an ultrasharp bent HPW (R=1.3μm) at the end of the thru port to improve the extinction ratio (ER). With the present principle, the designed PBS has a footprint as small as only 1.9μm×3.7μm, which is the shortest PBS reported to date.

Figure 1 shows the three-dimensional view of the proposed PBS as well as the cross section of the coupling region. The asymmetrical DC used for the PBS consists of a silicon HPW and a silicon NW. The widths (w1, w2) of the silicon HPW and the silicon NW are chosen optimally to make their effective indices equal for TE polarization according to the phase-matching condition. The TE polarized light launched from the input port is then coupled to the adjacent silicon NW efficiently by making the length of the coupling region optimal, while the launched TM polarized light undergoes almost no coupling and outputs from the thru port. A 90° sharply bent section is connected at the end of the straight silicon HPW to make the two waveguides decoupled. This 90° bend section has a small bend radius to work as a polarizer and consequently helps to improve the ER of the PBS.

Fig. 1. Schematic configuration of the proposed PBS. The cross section is in the inset.

As mentioned above, the widths of the silicon HPW and the silicon NW should be chosen to satisfy the phase-matching condition for TE polarization only and to maximize the phase mismatch for TM polarization. As an example, we choose the layer thicknesses as follows: hSi=230nm, hSiO2=50nm, and hm=100nm. Here, the thickness hSiO2 of the low-index layer sandwiched between metal (Ag) and silicon is chosen as a balance between the waveguide loss and the confinement according to [24

24. D. Dai and S. He, Opt. Express 17, 16646 (2009). [CrossRef]

]. The refractive indices of silicon, SiO2, and silver are nSi=3.455, nSiO2=1.445, and nAg=0.1453+11.3587i (at 1550 nm) [23

23. R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, Nat. Photonics 2, 496 (2008). [CrossRef]

]. PMMA is chosen to cover the whole device, and the index is nPMMA=1.481 (at 1550 nm) [25

25. S. Kasarova, N. Sultanova, C. Ivanov, and I. Nikolov, Opt. Mater. 29, 1481 (2007). [CrossRef]

].

Figure 2 shows the calculated effective refractive indices of the fundamental modes (TE0 and TM0) of the silicon HPWs and the silicon NWs. The right side shows electrical field profiles of the major component for the TE0 and TM0 modes in the two types of waveguides. From Fig. 2, one sees that the two types of waveguides have similar effective indices for the TE0 mode, while there is a significant difference between the effective indices of the TM0 mode. The reason is that there is a field enhancement in the low-index region for the TM0 mode due to the plasmonic effect in the silicon HPW (see the right part of Fig. 2). Since a wider silicon HPW has a lower intrinsic loss, we choose a relatively large width, w1=310nm, which also relaxes the requirement for the fabrication. The width of the silicon NW is then chosen as w2=280nm so that the phase-matching condition is satisfied (neff=1.777). On the other hand, for the TM0 mode, the effective indices for the two waveguides are neff1=2.092 and neff2=1.634, which shows a significant enough difference to introduce a huge phase mismatch. The gap width between the two waveguides in the coupling region is chosen to be as large as wgap=200nm.

Fig. 2. Effective indices of the TE0 and TM0 modes in silicon NWs and silicon HPWs. On the right are the electrical field profiles of the major component for TE0 and TM0 modes in the two types of waveguides, when the widths of the HPW and the NW are w1=310nm and w2=280nm, respectively.

For the 90° bend connected at the end of the proposed PBS, we calculate that the polarization-dependent loss as the bend radius R varies by a three-dimensional finite-difference time-domain (3D FDTD) method, as shown in Fig. 3. For the present HPW, there are two loss sources. One is the intrinsic loss due to the metal absorption, which is proportional to the length of the bend section (πR/2). The other is the bend loss (including the transition loss and the pure bend loss), which is inversely proportional to the bend radius. Therefore, there is an optimal bend radius for maximizing the transmission. For the TM0 mode, the present silicon HPW enables submicrometer bending due to the plasmonic effect [26

26. D. Dai, Y. Shi, S. He, L. Wosinski, and L. Thylen, Opt. Express 19, 23671 (2011). [CrossRef]

], and thus the optimal bend radius is about 1.3 μm (as shown in Fig. 3). In contrast, for TE polarization, the bend loss is large and becomes dominant when the radius is smaller than 2 μm, and consequently one sees that the transmission monotonously decreases as the radius decreases. From Fig. 3, it can be seen that the bent silicon HPW is polarization sensitive, so a 90° bend can play a role as a polarizer, which is helpful to improve the ER of the present PBS. In the present design, we choose R=1.3μm to maximize the transmission of TM polarization operation. The corresponding transmissions for TM and TE polarizations are 0.43 and 2.64dB, respectively.

Fig. 3. Transmission of a 90° bent silicon HPW for TM and TE polarizations. Here, w1=310nm and the wavelength is 1550 nm.

Figures 4(a) and 4(b) show the transmissions (Pthru, Pcross) at the cross and thru ports when the input light is with the TE0 and TM0 modes, respectively. The total transmission (Ptotal=Pthru+Pcross) is also shown in these figures. For TE polarization, one sees that the optimal coupling length is 2.4 μm for the highest ER. In contrast, for TM polarization the transmissions are not so sensitive to the coupling length, while there is still an optimal length of 2.0 μm for the highest ER. We choose the coupling length Lc=2.2μm by making a trade-off. One should note that it is possible to reduce the length Lc further by choosing a smaller gap width. For example, one has Lc=1.2μm when choosing wgap=100nm.

Fig. 4. Total transmission and the transmissions at the cross and thru ports as the coupling region length Lc varies. (a) TE and (b) TM. Here, w1=310nm, w2=280nm, and wgap=200nm.

For the present design with wgap=200nm and Lc=2.2μm, the ERs for TE and TM polarizations are 14 and 13 dB, respectively. The insertion losses are as low as 0.025 and 0.66 dB for TE and TM polarizations, respectively.

Figures 5(a) and 5(b) show the simulated light propagation in the designed PBS with R=1.3μm and Lc=2.2μm for TE0(Ex) and TM0(Ey) modes of input, respectively. The simulation tool is a 3D FDTD method, and the grid sizes are chosen as Δx=20nm, Δy=10nm, and Δz=20nm. The input TE-polarized light is coupled to the adjacent silicon NW effectively. However, for TM polarization, the input light is almost not cross coupled and outputs from the thru port.

Fig. 5. Light propagation in the designed PBS with wgap=200nm, Lc=2.2μm, and R=1.3μm. (a) TE and (b) TM. Here, w1=310nm, w2=280nm, and the wavelength is 1550 nm.

Figures 6(a) and 6(b) show the wavelength dependence of the transmissions for the designed PBS when the TE0 and TM0 modes are input, respectively. From the figures, one sees that the transmissions for TM polarization are wavelength-insensitive, since there is a large phase mismatch over a broad band. In contrast, the transmissions for TE polarization are a little sensitive to the wavelength, due to the intrinsic wavelength dependence of an evanescent coupling system. Nevertheless, the present PBS has a relatively high ER (12dB) and a low insertion loss (<1dB) over an 120nm bandwidth.

Fig. 6. Wavelength dependence of the designed PBS. (a) TE and (b) TM.

The fabrication tolerance of the designed PBS is also analyzed, as shown in Figs. 7(a) and 7(b). Here we assume that the two waveguides of the asymmetric coupler have the same width variation Δw due to the fabrication error [5

5. A. Katigbak, J. F. Strother, and J. Lin, Opt. Eng. 48, 080503 (2009). [CrossRef]

]. From Figs. 7(a) and 7(b), it can be seen that the transmission for TM polarization is only slightly sensitive to the width variation Δw and no significant degradation is observed even when the width variation Δw is up to ±50nm. However, for TE polarization, the width variation Δw introduces notable influences on the transmissions. It tolerates a width variation of [5030]nm to achieve 10 dB ER. Such a large tolerance alleviates the fabrication requirement of precision.

Fig. 7. Fabrication tolerance when the designed PBS has a width variation Δw. (a) TE and (b) TM.

In summary, an ultracompact PBS has been proposed by utilizing an asymmetrical coupling system consisting of a silicon HPW and a silicon NW. The waveguide widths have been designed optimally to achieve a phase match for TE polarization only while there is a significant phase mismatch (Δ=0.458) for TM polarization. In this way, the length of the coupling region is as short as 2.2 μm even when the gap width is as large as 200 nm. The PBS size can be reduced further (e.g., to 1.2 μm) by choosing a smaller gap width (e.g., 100 nm). A sharp bend with R=1.3μm is attached at the thru port as a polarizer to enhance the ER. The total size of the designed PBS is only about 1.9μm×3.7μm. The insertion loss of the PBS is only 0.025 and 0.66 dB for TE and TM polarizations, respectively. The presented PBS has a broad bandwidth of >120nm for achieving 12 dB ER. Our numerical simulation has also shown that the PBS tolerates a width variation of [5030]nm for 10 dB ER, which makes the fabrication simple.

This project was partially supported by an 863 project (2011AA010301), the National Natural Science Foundation of China (61077040), Zhejiang provincial grants (Z201121938 and 2011C11024), and the Doctoral Fund of the Ministry of Education of China (20120101110094).

References

1.

D. Dai, L. Liu, S. Gao, D. Xu, and S. He, Laser Photon. Rev. 7, 303 (2013). [CrossRef]

2.

Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, Opt. Express 17, 9421 (2009). [CrossRef]

3.

L. Sansoni, F. Sciarrino, G. Vallone, P. Mataloni, A. Crespi, R. Ramponi, and R. Osellame, Phys. Rev. Lett. 105, 200503 (2010). [CrossRef]

4.

T. Barwicz, M. R. Watts, M. A. Popovic, P. T. Rakich, L. Socci, F. X. Kartner, E. P. Ippen, and H. I. Smith, Nat. Photonics 1, 57 (2007). [CrossRef]

5.

A. Katigbak, J. F. Strother, and J. Lin, Opt. Eng. 48, 080503 (2009). [CrossRef]

6.

B. Yang, S. Shin, and D. Zhang, IEEE Photon. Technol. Lett. 21, 432 (2009). [CrossRef]

7.

I. Kiyat, A. Aydinli, and N. Dagli, IEEE Photon. Technol. Lett. 17, 100 (2005). [CrossRef]

8.

X. Tu, S. S. N. Ang, A. B. Chew, J. Teng, and T. Mei, IEEE Photon. Technol. Lett. 22, 1324 (2010). [CrossRef]

9.

D. Dai and J. Bowers, Opt. Express 19, 18614 (2011). [CrossRef]

10.

T. Yamazaki, H. Aono, J. Yamauchi, and H. Nakano, J. Lightwave Technol. 26, 3528 (2008). [CrossRef]

11.

L. B. Soldano, A. H. Vreede, M. K. Smit, B. H. Verbeek, E. G. Metaal, and F. H. Groen, IEEE Photon. Technol. Lett. 6, 402 (1994). [CrossRef]

12.

T. K. Liang and H. K. Tsang, IEEE Photon. Technol. Lett. 17, 393 (2005). [CrossRef]

13.

Y. Shi, D. Dai, and S. He, IEEE Photon. Technol. Lett. 19, 825 (2007). [CrossRef]

14.

X. Ao, L. Liu, L. Wosinski, and S. He, Appl. Phys. Lett. 89, 171115 (2006). [CrossRef]

15.

J. Xiao, X. Liu, and X. Sun, Jpn. J. Appl. Phys. 47, 3748 (2008). [CrossRef]

16.

J. Feng and Z. Zhou, Opt. Lett. 32, 1662 (2007). [CrossRef]

17.

Z. Wang, Y. Tang, L. Wosinski, and S. He, IEEE Photon. Technol. Lett. 22, 1568 (2010). [CrossRef]

18.

J. Wang, D. Liang, Y. Tang, D. Dai, and J. Bowers, Opt. Lett. 38, 4 (2013). [CrossRef]

19.

D. Dai, Z. Wang, and J. Bowers, Opt. Lett. 36, 2590 (2011). [CrossRef]

20.

S. Lin, J. Hu, and K. B. Crozier, Appl. Phys. Lett. 98, 151101 (2011). [CrossRef]

21.

F. Lou, D. Dai, and L. Wosinski, Opt. Lett. 37, 3372 (2012). [CrossRef]

22.

J. Chee, S. Zhu, and G. Q. Lo, Opt. Express 20, 25345 (2012). [CrossRef]

23.

R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, Nat. Photonics 2, 496 (2008). [CrossRef]

24.

D. Dai and S. He, Opt. Express 17, 16646 (2009). [CrossRef]

25.

S. Kasarova, N. Sultanova, C. Ivanov, and I. Nikolov, Opt. Mater. 29, 1481 (2007). [CrossRef]

26.

D. Dai, Y. Shi, S. He, L. Wosinski, and L. Thylen, Opt. Express 19, 23671 (2011). [CrossRef]

OCIS Codes
(130.2790) Integrated optics : Guided waves
(130.3120) Integrated optics : Integrated optics devices
(230.7370) Optical devices : Waveguides
(130.5440) Integrated optics : Polarization-selective devices

ToC Category:
Integrated Optics

History
Original Manuscript: May 22, 2013
Revised Manuscript: July 15, 2013
Manuscript Accepted: July 15, 2013
Published: August 7, 2013

Citation
Xiaowei Guan, Hao Wu, Yaocheng Shi, Lech Wosinski, and Daoxin Dai, "Ultracompact and broadband polarization beam splitter utilizing the evanescent coupling between a hybrid plasmonic waveguide and a silicon nanowire," Opt. Lett. 38, 3005-3008 (2013)
http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-38-16-3005


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References

  1. D. Dai, L. Liu, S. Gao, D. Xu, and S. He, Laser Photon. Rev. 7, 303 (2013). [CrossRef]
  2. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, Opt. Express 17, 9421 (2009). [CrossRef]
  3. L. Sansoni, F. Sciarrino, G. Vallone, P. Mataloni, A. Crespi, R. Ramponi, and R. Osellame, Phys. Rev. Lett. 105, 200503 (2010). [CrossRef]
  4. T. Barwicz, M. R. Watts, M. A. Popovic, P. T. Rakich, L. Socci, F. X. Kartner, E. P. Ippen, and H. I. Smith, Nat. Photonics 1, 57 (2007). [CrossRef]
  5. A. Katigbak, J. F. Strother, and J. Lin, Opt. Eng. 48, 080503 (2009). [CrossRef]
  6. B. Yang, S. Shin, and D. Zhang, IEEE Photon. Technol. Lett. 21, 432 (2009). [CrossRef]
  7. I. Kiyat, A. Aydinli, and N. Dagli, IEEE Photon. Technol. Lett. 17, 100 (2005). [CrossRef]
  8. X. Tu, S. S. N. Ang, A. B. Chew, J. Teng, and T. Mei, IEEE Photon. Technol. Lett. 22, 1324 (2010). [CrossRef]
  9. D. Dai and J. Bowers, Opt. Express 19, 18614 (2011). [CrossRef]
  10. T. Yamazaki, H. Aono, J. Yamauchi, and H. Nakano, J. Lightwave Technol. 26, 3528 (2008). [CrossRef]
  11. L. B. Soldano, A. H. Vreede, M. K. Smit, B. H. Verbeek, E. G. Metaal, and F. H. Groen, IEEE Photon. Technol. Lett. 6, 402 (1994). [CrossRef]
  12. T. K. Liang and H. K. Tsang, IEEE Photon. Technol. Lett. 17, 393 (2005). [CrossRef]
  13. Y. Shi, D. Dai, and S. He, IEEE Photon. Technol. Lett. 19, 825 (2007). [CrossRef]
  14. X. Ao, L. Liu, L. Wosinski, and S. He, Appl. Phys. Lett. 89, 171115 (2006). [CrossRef]
  15. J. Xiao, X. Liu, and X. Sun, Jpn. J. Appl. Phys. 47, 3748 (2008). [CrossRef]
  16. J. Feng and Z. Zhou, Opt. Lett. 32, 1662 (2007). [CrossRef]
  17. Z. Wang, Y. Tang, L. Wosinski, and S. He, IEEE Photon. Technol. Lett. 22, 1568 (2010). [CrossRef]
  18. J. Wang, D. Liang, Y. Tang, D. Dai, and J. Bowers, Opt. Lett. 38, 4 (2013). [CrossRef]
  19. D. Dai, Z. Wang, and J. Bowers, Opt. Lett. 36, 2590 (2011). [CrossRef]
  20. S. Lin, J. Hu, and K. B. Crozier, Appl. Phys. Lett. 98, 151101 (2011). [CrossRef]
  21. F. Lou, D. Dai, and L. Wosinski, Opt. Lett. 37, 3372 (2012). [CrossRef]
  22. J. Chee, S. Zhu, and G. Q. Lo, Opt. Express 20, 25345 (2012). [CrossRef]
  23. R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, Nat. Photonics 2, 496 (2008). [CrossRef]
  24. D. Dai and S. He, Opt. Express 17, 16646 (2009). [CrossRef]
  25. S. Kasarova, N. Sultanova, C. Ivanov, and I. Nikolov, Opt. Mater. 29, 1481 (2007). [CrossRef]
  26. D. Dai, Y. Shi, S. He, L. Wosinski, and L. Thylen, Opt. Express 19, 23671 (2011). [CrossRef]

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