IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 8, APRIL 15, 2008
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Silicon-Based 2-D Slab Photonic Crystal TM Polarizer at Telecommunication Wavelength Yonghao Cui, Qi Wu, Ethan Schonbrun, Mark Tinker, Member, IEEE, J.-B. Lee, Senior Member, IEEE, and Won Park Abstract—We report an extremely compact (15.4 m 2 8 m) silicon-based 2-D slab nano photonic crystal (PC) transverse-magnetic (TM) polarizer which blocks propagation of the transverse-electric (TE) polarized light but passes TM polarized light around telecommunication wavelength (1550 nm). The TE polarized light totally vanishes but the TM polarized light propagates with some attenuation in a length of mere 4.9 m and it has a great potential to be integrated in a complex photonic integrated circuits. To our knowledge, this is the first experimental demonstration of a silicon-based PC TM polarizer at 1.55-m wavelength. The plane wave expansion method (PWEM) and 2-D and 3-D finite-difference time-domain (FDTD) simulation were utilized to design a periodic triangular array of air holes in 340-nm-thick silicon with a diameter of 170 nm and pitch distance of 347 nm for the TM polarizer and 371 nm for the input and output waveguide. Such a PC TM polarizer was fabricated in silicon-on-insulator wafer using focused ion beam and reactive ion etching. The device was characterized using tunable lasers in the wavelength range of 1528 nm 1604 nm. Transmitted light intensities of the TE and TM polarized lights were measured which clearly showed the TE polarized light is filtered out around 1.55-m wavelength. Index Terms—Photonic crystal, polarizer, silicon, slab, telecommunication.
I. INTRODUCTION
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HOTONIC crystal (PC) with photonic bandgap [1], [2] has been widely investigated since it has great potential to control and manipulate the flow of lights in various wavelengths. Various 1-D, 2-D, and 3-D PCs have been realized in periodic arrays of high index dielectric material with a low index medium such as air. Of those, 2-D slab PC is considered to be a good candidate for realizing photonic integrated circuits [3]. Development of various components and ways to integrate them together on a single chip are necessary steps to realize photonic integrated circuits. Since typical PC devices are designed to use either transverse-electric (TE) or transverse-magnetic (TM) polarized light, a mixture of the TE and the TM polarization in the input signal
Manuscript received October 8, 2007; revised December 4, 2007. This work was supported by the National Science Foundation (NSF) Nanoscale Interdisciplinary Research Team (NIRT) program under Grant BES-0608934. Y. Cui, M. Tinker, and J.-B. Lee are with the Department of Electrical Engineering, University of Texas at Dallas, Richardson, TX 75083 USA (e-mail:
[email protected]). Q. Wu, E. Schonbrun, and W. Park are with the Department of Electrical and Computer Engineering, University of Colorado, Boulder, CO 80309 USA. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2008.919508
Fig. 1. Photonic band diagrams for the line defect PC TM polarizer in the 0 direction for (a) TE polarization and (b) TM polarization.
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will significantly degrade the PC device performance. Therefore, macroscopic external TE or TM polarizers are typically used to filter out unwanted components of the light before the light signal is coupled into the PC device. Recently, there have been theoretical and simulation investigations on PC-based light polarization devices [4]–[8], however, fabrication and device characterizations were not reported. There was a report on fabrication and characterization of a silicon-based light polarizer in visible and near-infrared wavelengths [9]. In this letter, we report a 2-D PC-based TM polarizer at 1.55- m telecommunication wavelength, which is, to our knowledge, the first experimental demonstration of its kind. II. DESIGN AND SIMULATION The PC TM polarizer was designed to have a line defect in a periodic triangular array of air holes in 340-nm-thick single crystal silicon slab on a buried silicon dioxide in a silicon-on-insulator (SOI) wafer. The plane wave expansion method (PWEM) was utilized to generate photonic band diagrams for the TE and the TM polarization along the direction through which line defect lies (Fig. 1). The refractive indexes used in the design for silicon, silicon dioxide, and air were 3.5, 1.46, 1.0, respectively. Normalized radius (r/a) is in the gap varies set to be 0.245. Normalized frequency from 0.2195 to 0.2238 for the TE polarized light. Since our work was focused on 1.55- m wavelength, the lattice constant in the PC TM polarizer and the diameter of air hole were selected to be 347 and 170 nm, respectively, resulting in the light wavelength in the gap in the range of 1550.7–1580.8 nm. For the TM polarized light, there is no gap in the photonic band and consequently 1.55- m TM light passes through the PC TM polarizer with a mode introduced by the line defect.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 8, APRIL 15, 2008
Fig. 3. SEM images of the fabricated input–output waveguides and TM polarizer: (a) top-view of the PC TM polarizer with input and output waveguide; (b) close-up view of the triangular air-hole PC showing near vertically etched silicon.
For experimental verification of the TM polarization, the device was designed to have an input silicon waveguide and an output silicon waveguide (or a silicon deflection block) which are attached to the both ends of the waveguides. III. FABRICATION Fig. 2. (a) 2-D and 3-D FDTD simulation results of the TE polarized light through the TM polarizer at 1.55-m wavelength at room temperature; (b) 2-D simulation result showing transmitted TM polarized light; (c) 2-D simulation of the blocked TE light through the system consisting of the input PCWG, the TM polarizer, and the output PCWG at 1.55-m wavelength at room temperature; (d) 2-D simulation of the transmitted TM polarized light through the system.
Based on the device dimension found through the PWEM, a series of 2-D and 3-D finite-difference time-domain (FDTD) simulations using commercially available software EMPLab (EM Photonics) were utilized to design the TM polarizer. Fig. 2(a) shows both 2-D and 3-D simulation results of the blocking of the TE polarized light and Fig. 2(b) shows transmission of the TM polarized light for 1.55- m wavelength at room temperature. In order to reduce insertion loss of the device, line defect input and output waveguides were designed to be connected to both ends of the PC TM polarizer and the channel width of the tapered input waveguide was designed to be narrowed down gradually from 3.5 to 0.5 m. The lattice constant was designed as 371 nm and the diameter of the air hole was the same as the PC TM polarizer. Fig. 2(c) and (d) shows 2-D simulation results of the blocking of the TE polarized light and transmission of the TM polarized light, respectively, in a system consisting of the input waveguide, the TM polarizer and the output waveguide for the 1.55- m wavelength at room temperature. For the 1550-nm wavelength, a transmission ratio of the TE light was found to be 45 dB, while that of the TM polarized light is 4.5 dB. The degree of polarization was found to be 0.99982 at 1.55- m wavelength. It should be noted that in this limited silicon thickness design, because lattice constant (347 nm) is close to the thickness of the silicon (340 nm), lights propagating through the PC TM polarizer are TE-like or TM-like rather than pure TE or TM polarized light, even though the input is pure TE or TM mode.
The device was fabricated on a piece of SOI wafer with the top silicon thickness of 340 nm and 2- m buried silicon dioxide. Photoresist S1813 (Shipley Co., Marlborough, MA) was spin coated on a piece of SOI wafer and patterned to create a 20-nm-thick chromium layer by the lift-off process. This micrometer scale pattern defines the area of input waveguide, PC region, and deflection block. In order to pattern nanometer scale chromium etch mask for triangular lattice air-holes in the PC region, focused ion beam (FIB) with the ion-beam voltage of 30 kV and beam-current of 50 pA was used. Then, 340-nm-thick silicon was dry etched through the patterned chromium mask by CF –O (91.25% : 8.75%) plasma using a reactive ion etch (RIE) process. The silicon dioxide layer beneath the silicon layer works as a silicon etch stop. Input–output silicon waveguides, a TM polarizer, and a deflection block were formed simultaneously by this RIE process. The chromium etch mask was then removed by a wet chromium etchant. Fig. 3(a) shows the TM polarizer and the input–output PCWG. Total length of the TM polarizer in this design is merely 4.9 m. The channel width of the input PCWG is narrowed down gradually from 3.5 to 0.5 m. In order to inspect the sidewall profile of the air hole, a trench was formed by FIB [Fig. 3(b)] using a low ion-beam current (1 pA) to minimize physical damage to the sidewall. It is shown that the sidewall etch profile is nearly vertical and clean. In order to couple laser lights into input silicon waveguide, the fabricated SOI sample was cleaved. IV. CHARACTERIZATION The fabricated PC-based TM polarizer was tested at room temperature using 1.55- m TE and TM polarized laser lights. The laser lights were coupled into the cleaved input waveguide edge and infrared camera images were taken. Fig. 4 shows infrared camera images of the top view of the output waveguide cleaved edge with 1.55- m laser light coupled into the input waveguide. Fig. 4(a) shows bright light spots at the cleaved edge which is the result of the transmitted TM polarized light. On the
CUI et al.: SILICON-BASED 2-D SLAB PC TM POLARIZER AT TELECOMMUNICATION WAVELENGTH
Fig. 4. Infrared camera images of the top view of the (a) transmitted TM polarized light and (b) blocked TE polarized light at the edge of the cleaved output waveguide at 1.55-m light at room temperature.
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Fig. 6. Measured light intensities at the cleaved edge of the output waveguide for the TE and TM polarized lights. Scattered points are actual experimental result and red and blue lines are trend line of the results.
Fig. 6 shows analyzed light intensities as a function of wavelengths for the TE and the TM polarized lights. It can be seen that the measured transmitted TM polarized light intensity is much higher than the TE polarized light intensity at a wavelength around 1.55 m. V. CONCLUSION Fig. 5. Infrared camera images of the top view of the (a) transmitted TM polarized light and (b) blocked TE polarized light through the TM polarizer at 1.55-m light at room temperature.
contrary, Fig. 4(b) shows no noticeable light spots at the cleaved edge which is the result of the blocked TE polarized light. The PC-based TM polarizer with the silicon deflection block instead of the output silicon waveguide was also tested. Fig. 5 shows infrared camera images of the top view of the input waveguide, TM polarizer, and the deflection block with 1.55- m laser light coupled into the input waveguide. Fig. 5(a) shows scattered light spots at the deflection block for the transmitted TM polarized light. Fig. 5(b) shows strong reflection at the input of the TM polarizer for the TE polarized light input. In order to fully characterize the PC-based TM polarizer’s light propagation property, two tunable laser sources which cover wavelengths in the range of 1528 1564 nm and 1579 1604 nm were used. A total of 167 infrared camera images each of the TE and TM polarized lights were taken over the 1528 1604 nm wavelength range. The light intensity data was extracted by a specifically programmed Matlab image process code using 167 captured infrared images. We assumed that the scattered light intensity at the cleaved output waveguide edge is proportional to light intensity of the transmitted light through the PC TM polarizer.
A 2-D silicon-based extremely compact PC TM polarizer was designed, fabricated, and characterized around 1.55- m wavelength. Since it is silicon-based, it has high potential to realize an integrated in situ light polarizer in various PC-based optical applications. Moreover, this is another small, but good step toward the development of the photonic integrated circuits. REFERENCES [1] E. Yablonovitch, “Photonic band gap structures,” J. Opt. Soc. Amer. B, vol. 10, no. 2, pp. 283–295, Feb. 1993. [2] J. Joannopoulos, P. Villeneuve, and S. Fan, “Photonic crystals: Putting a new twist on light,” Nature, vol. 386, pp. 143–149, Mar. 13, 1997. [3] M. Notomi, “Ultrasmall photonic integrated circuits using photonic crystal,” NTT Tech. Rev., vol. 4, no. 1, pp. 6–11, Jan. 2006. [4] R. Sinha and Y. Kalra, “Design of a photonic bandgap polarizer,” Opt. Eng. Lett., vol. 45, no. 11, p. 110503, Nov. 2006. [5] X. Shen, K. Han, Y. Shen, H. Li, Y. Wu, and G. Tang, “Dispersionbased all photonic crystals polarization beam splitter,” Phys. Lett. A, vol. 369, no. 5–6, pp. 524–527, Oct. 2007. [6] K. Bayat, S. Chaudhuri, and S. Safavi-Naeini, “Polarization and thickness dependent guiding in the photonic crystal slab waveguide,” Opt. Express, vol. 15, no. 13, pp. 8391–8400, Jun. 2007. [7] R. Sinha and Y. Kalra, “Design of optical waveguide polarizer using photonic band gap,” Opt. Express, vol. 14, no. 22, pp. 10790–10794, Oct. 2006. [8] T. Liu, A. Zakharian, M. Fallahi, J. Moloney, and M. Mansuripur, “Design of a compact photonic-crystal-based polarizing beam splitter,” IEEE Photon. Technol. Lett., vol. 17, no. 7, pp. 1435–1437, Jul. 2005. [9] J. Diener, N. Künzner, E. Gross, and D. Kovalev, “Planar silicon-based light polarizers,” Opt. Lett., vol. 29, no. 2, pp. 195–197, Jan. 2004.
