Ultracompact Photonic Coupling Splitters Twisted by PTT Nanowires

NANO LETTERS

Ultracompact Photonic Coupling Splitters Twisted by PTT Nanowires

2008 Vol. 8, No. 9 2839-2843

Xiaobo Xing, Heng Zhu, Yuqing Wang, and Baojun Li* State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, China Received May 21, 2008; Revised Manuscript Received July 17, 2008

ABSTRACT We report a series of ultracompact photonic coupling splitters with multi-input/output ports assembled by twisting flexible polymer nanowires, which were fabricated by one-step drawing method from poly(trimethylene terephthalate) (PTT). Experimental demonstration shows that the properties of the splitters are dependent on the operation wavelength and the input branch of the optical signal launched. For a fixed operation wavelength and the input branch, desirable splitting ratio can be tuned by controlling the input/output branching angle. The excess loss of these splitters is less than 1 dB, and the intrinsic loss is less than 0.4 dB. They are desirable for high density photonic integrated circuits (PICs) and nanonetworks, while the twisting technology will be useful in constructing other wire-based photonic devices.

Miniaturization of photonic devices is being intensively focused because they drive great scientific and technological developments in the fields of optical communications, biophotonics, engineered structures, and photonic integrations. Photonic beam splitters are important devices for integrated photonics,1-3 on-chip optical interconnection,4 and biosensors.5 Micrometer-scale waveguide-based splitters have been fabricated in semiconductor materials such as InP,1 silicon-on-insulator,2 and SiGe/Si3 by multistep photolithography and wet/dry etching techniques. However, it is difficult to realize a large-scale waveguide-based photonic beam splitter on a substrate with a diameter of several inches because the waveguide bending radius is too large to allow many 2 × 2 couplers1 or 1 × 2 Y-branches6,7 to be integrated by cascading method. To avoid the use of cascading method, multimode interference (MMI)-based splitters have been fabricated. But the size of the MMI section is very large, which is usually in the range of hundreds to thousands of micrometers (32.2 µm wide and 1209 µm long,3 32 µm wide and 276.5 µm long,8 and 160 µm wide and 7480 µm long9). To miniaturize splitter size, submicro/nanometer-scale photonic crystal,10 plasmonic waveguides,11 and wire waveguides12 are achieved to combine and split optical signals. But the devices using the aforementioned waveguides made by photolithography are quite complicated with high cost. The ability to inject, guide, and manipulate light on a subwavelength-scale volume makes the nanowire components well-suitable for miniaturized integrated devices and PICs. Substantial efforts have been made on the development of nanowires and the nanowire-based devices assembly.13-18 However, the assembly of the nanowire-based devices is * Corresponding author. E-mail: [email protected]. 10.1021/nl8014507 CCC: $40.75 Published on Web 08/15/2008

 2008 American Chemical Society

restricted by their relatively low flexibility; as a result, only simple 2 × 2 splitters with two input/output ports were assembled.15,18 As a promising polymer, PTT possesses much small crystal modulus of 2.59 GPa with strong flexibility and more than 90% elastic recovery owning to its trimethylene units of organized highly contracted and helically coiled gauche-gauche conformation.19-23 Therefore, PTT nanowires with diameters down to 60 nm and lengths up to 500 mm have been obtained with high mechanical strength and excellent flexibility.24 Here, we report a series of ultracompact photonic coupling splitters with multi-input/ output ports assembled by twisting PTT nanowires. The nanowires we used here in twisting photonic coupling splitters were drawn from the molten PTT by one-step process as shown in Figure 1a. The PTT pellets (molecular weight Mw ) 35 000 g·mol-1) from Shell Chemical Company were heated to melt at 250 °C by a heating plate during the drawing.19,22 First, a silica tip is dipped into the molten PTT and then retracted from the molten PTT along the green arrow with a speed of 0.1-1 m/s, leaving a PTT nanowire (PNW) extending between the molten PTT and the tip. The extended PNW is quickly quenched in air and finally, a naked amorphous PNW is formed. Scanning electron microscope (SEM) and high-magnification transmission electron microscope (TEM) images show that the PNWs have high surface smoothness, length uniformity, and excellent flexibility (see Supporting Information, Figure S2). To investigate optical guiding properties of the PNWs, we fixed the PNWs by two microstage supports and launched lights of different wavelengths into the PNWs by evanescent wave through directional coupling. The output powers from the PNWs were measured by an optical power meter together with an optical

Figure 1. Fabrication process and measured optical loss of the PNWs. (a) Schematic diagram for fabricating PNWs by a direct tip-drawing process from molten PTT material. The molten PTT was kept at 250 °C by a heating plate during the drawing. Green arrow shows the drawing direction. (b) Optical loss of the PNWs versus nanowire diameter at wavelengths of 473, 532, 650, 1310, and 1550 nm.

Figure 2. 2 × 2 photonic coupling splitter. (a) SEM image of the splitter with diameters of 460 and 548 nm for branches A-1 and B-2, respectively. (b,c) Optical microscope images of the guided green (532 nm) and blue (473 nm) lights, respectively. The arrows show the propagation directions of the launched lights. (d) Relative output power versus branching angle. The inset shows excess loss versus branching angle.

spectrum analyzer. The measured coupling efficiency is as high as 94.8% when a silica taper is parallel to the PNW (see Supporting Information, Figure S3). The optical losses of the PNWs were measured by a cutback method, and the results were shown in Figure 1b. The optical loss of the PNWs is smaller than that of SnO2 nanoribbon13 and silica nanowires,14,15 which is good for applications in PICs. In device assembly, first, desired lengths of the PNWs were cut by a micromanipulator under an optical microscope. Second, the cut PNWs were placed on one plane in parallel and fixed by microstage supports. Third, a spiral structure was twisted with a high precision. To get a maximum coupling efficiency and avoid undesirable scattering loss at the coupler junction, each PNW of the structure was pulled 2840

by adjusting the microstages to form a tense coupling junction. Finally, an ultracompact photonic coupling splitter was formed. PTT fiber has a tensional elastic recovery of >90%,23 so it can be frequently bent and twisted. In our previous work,24 we found that the PTT nanowires have high mechanical strength and excellent flexibility that can be repeatedly used to fashion a variety of shapes with very good performance. Figure 2a shows an SEM image of a 2 × 2 photonic coupling splitter with a branching angle of 25°, which was formed by twisting two PNWs with diameters of 460 nm (branch A-1) and 548 nm (branch B-2). The inset of Figure 2a shows that there are three twisted turns in the coupling region (about 14.7 µm long and 1 µm wide). To demonstrate Nano Lett., Vol. 8, No. 9, 2008

Figure 3. 4 × 4 photonic coupling splitter. (a) SEM image of the device with diameters of 450, 450, 510, and 570 nm for branches A to D. (b,c) Optical microscope images of the guided red (650 nm) and blue lights. The arrows show the propagation directions of the launched lights. (d) Relative output powers for the guided red, green, and blue lights. The inset shows excess loss of the device.

its optical coupling and splitting properties, we launched visible lights into different input branches by evanescent coupling (see Supporting Information, Figure S3f). As examples, Figure 2b shows a 532 nm green light was launched into the branch A, coupled through the three-turn coupling region, and then divided into the output branches 1 and 2 with a splitting ratio of about 54:46. The excess loss of the device, defined as -10 log(sum[Poutput]/Pinput), is 0.65 dB, including 0.48 dB input and output coupling loss. The propagation loss is 0.007 dB and the scattering loss is 0.163 dB in the twisted region. Similarly, Figure 2c shows that a 473 nm blue light is divided by the device with a splitting ratio of about 60:40. The measured excess loss is 0.63 dB, which is composed of coupling loss (0.48 dB), propagation loss (0.001 dB), and scattering loss (0.149 dB). It is revealed that the device is very efficient in guiding and splitting lights. To investigate the influence of branching angle on the device performance, input/output branching angles of the splitter was changed from 8° to 90°. Figure 2d shows the relative output power, normalized by the input power, as a function of branching angle. The calculated results show that the splitting ratios for branches 1 and 2 are ranging from 46:54 to 56:44 if a green light is launched into the branch A, and ranging from 55:45 to 62:38 if a blue light is launched into the branch B. In our experiment, we found that, if the branching angles of the 2 × 2 splitter are 10° or 40°, a 3-dB splitter can be achieved in the case of the green light was launched into the branch A. A plot of excess loss versus the branching angle (Figure 2d, inset) shows that the excess loss is in the ranges of 0.555 to 1.124 Nano Lett., Vol. 8, No. 9, 2008

dB and 0.555 to 1.051 dB for green and blue lights, respectively. The splitter with large branching angle operates at a large excess loss, because the bending loss and the scattering loss increase with the increase of the branching angle. Figure 3a shows a 4 × 4 photonic coupling splitter assembled by twisting four PNWs with diameters of 450, 450, 510, and 570 nm for branches A to D. The inset of Figure 3a shows that the coupling section is composed of a 3 × 4 and a 1 × 4 couplers, where the total width of the coupling section is 1.98 µm. The maximum length of the coupling region (Figure 3a, inset) is about 16.1 µm, and that of the 1 × 4 splitter is about 8.5 µm. The size of the 4 × 4 coupling splitter is smaller than that of the reported 4 × 4 air-clad holey fiber coupler (coupling region, 10 µm wide and 19.45 mm long)25 and polymer-clad glass-core fiber splitter (coupling region, 140 µm wide and 20 mm long);26 both were fabricated by fusion and tapering techniques. Figure 3b shows that a 650 nm red light is sent into the branch B and divided into the output branches 1 to 4 with a splitting ratio of about 24:25:32:19. When two blue lights are simultaneously launched into the branches A and B with a power ratio of 1:2 (Figure 3c), the splitting ratio is changed to be 26:23:27:24. We also launched different visible lights into the device to measure its splitting performance. Figure 3d shows the relative output powers from branches 1 to 4 when red, green, and blue lights are coupled into the branches B and (A + B) with a launched power ratio of 1:2, respectively. It shows that the splitting ratios are different for red, green, and blue lights. This is because the coupling 2841

Figure 4. 6 × 6 photonic coupling splitter. (a) SEM image of the device with diameters of 520, 540, 540, 540, 420, and 360 nm for branches A to F. (b,c) Optical microscope images of the launched green light from the input branch C, and blue light from the input branch D are split into six parts, respectively. The arrows show the propagation directions of the launched lights. (d) Relative output powers in the six output branches for the guided red, green, and blue lights. The inset shows excess losses of the device.

Figure 5. 8 × 8 photonic coupling splitter. (a) SEM image of the device with diameters of 400, 400, 400, 400, 400, 750, 750, and 600 nm for branches A to H. (b,c) Optical microscope images of the guided red and blue lights, respectively. The arrows show the propagation directions of the launched lights. (d) Relative output powers in branches 1 to 8 for the guided red, green, and blue lights. The inset shows excess loss of the device.

conditions are changed in the coupling region when the operating wavelengths vary for different input branches. The 2842

inset of Figure 3d shows that excess losses for red, green, and blue lights are 0.706, 0.675, and 0.650 dB for input Nano Lett., Vol. 8, No. 9, 2008

branch B, and 0.726, 0.685, and 0.660 dB for input branches (A + B), which is much smaller than that of the reported devices (e.g., 9.9 dB25 and 10.5 dB26). In addition, the excess loss increases with the increase of wavelength, because the propagation loss and scattering loss of the PNWs increase. Figure 4a further shows a 6 × 6 photonic coupling splitter, which was formed by twisting six PNWs with diameters of 520, 540, 540, 540, 420, and 360 nm for branches A to F, respectively. The inset of Figure 4a shows the magnified twisted section with a coupling length of 11 to 20 µm and a coupling width of 2.92 µm. Figure 4b shows green light is launched into the branch C and divided into six parts, with a splitting ratio of 17:16:20:18:15:14 for the output branches 1 to 6. In this case, the power uniformity is approximately 1.55 dB, which was calculated according to 10 log(Pmax/ Pmin), where Pmax is the maximum output power and Pmin is the minimum output power in the six output branches. When blue light is launched into the branch D (Figure 4c), the device exhibits good power distribution uniformity and its power uniformity is about 0.03 dB. Different visible lights were also launched into the branches C and D. Figure 4d shows the relative output powers in the six output branches. The inset of Figure 4d shows that the excess losses of red, green, and blue lights are 0.685, 0.665, and 0.645 dB for input branch C, and 0.680, 0.658, and 0.641 dB for input branch D. An 8 × 8 photonic coupling splitter (Figure 5) with a longer coupling region was further assembled by twisting eight PNWs with diameters of 400, 400, 400, 400, 400, 750, 750, and 600 nm from branches A to H. The coupling section of the splitter (Figure 5a, inset) is 38 µm long and 2.5 µm wide. The size of the coupling section of the device is much smaller than that of the planar waveguide-based coupling splitters (583 µm long)1 and MMI splitter (MMI region, 32 µm wide and 276.5 µm long).8 Here, we launched visible lights into the device to observe the splitting phenomenon. Figure 5b,c shows that red and blue lights are coupled into the branches E and G, respectively, and divided into branches 1 to 8. The output powers of each branch are plotted in Figure 5d, showing the splitting ratio as function as the operating wavelength and input branch. During the experiment, we also twisted and demonstrated 2 × 4, 3 × 3, and 5 × 5 photonic coupling splitters (see Supporting Information, Figure S4-6). In summary, a series of ultracompact photonic coupling splitters were assembled by twisting PNWs. The splitting performances of the photonic coupling splitters are dependent on wavelength and input branch. The desirable splitting ratio can be obtained by controlling the input/output branching angle. We found that the scattering lights are extremely weak in the splitting area, which is desirable for high performance photonic beam splitters. The results demonstrated that the excess loss of these splitters is less than 1 dB and the intrinsic loss (including transmission and scattering loss) is less than 0.4 dB. Compared with the planar waveguide-based and fiber-based splitters, the size of the devices reported here is compact, and the coupling region is 1 to 3 µm wide and tens of micrometers long. We believe that these ultracompact photonic structures will be useful in future miniaturized Nano Lett., Vol. 8, No. 9, 2008

device blocks, high density PICs, nanonetworks, and multiterminal nanosensors. The twisting method is easier and cheaper than the multistep photolithography device fabrication technology and will be useful in constructing other fiber/ wire-based photonic devices with controllable spiral structures. Acknowledgment. The authors thank Professor Xudong Chen for material supply, Wen He, Weiang Luo, and Like Lu for assistance in samples preparation, Professor Yuezhong Meng for assistance in SEM measurements, and Menglin Guo and Jinchun Shi for assistance in optical coupling. This work was supported by the National Natural Science Foundation of China (Grants 60625404 and 60577001). Supporting Information Available: Transmission spectrum of PTT film; electron micrographs of PTT nanowires and nanowire structures; evanescent coupling with different crossing angles; 2 × 4, 3 × 3, and 5 × 5 photonic coupling splitters. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Raburn, M.; Liu, B.; Abraham, P.; Bowers, J. E. IEEE Photon. Technol. Lett. 2000, 12, 1639. (2) Wang, Z. T.; Fan, Z. C.; Xia, J. S.; Chen, S. W.; Yu, J. Z. Jpn. J. Appl. Phys. 2004, 43, 5085. (3) Li, B. J.; Chua, S.-J.; Fitzgerald, E. A.; Chaudhari, B. S.; Jiang, S.; Cai, Z. Appl. Phys. Lett. 2004, 85, 1119. (4) Robitaille, L.; Callender, C. L.; Noad, J. P. IEEE Photon. Technol. Lett. 1996, 8, 1647. (5) Tazawa, H.; Kanie, T.; Katayama, M. Appl. Phys. Lett. 2007, 91, 113901. (6) Hsu, C. W.; Chen, H. L.; Wang, W. S. IEEE Photon. Technol. Lett. 2003, 15, 1103. (7) Chung, K. K.; Chan, H. P.; Chu, P. L. Opt. Commun. 2006, 267, 367. (8) Hyun, K. S.; Yoo, B. S.; Kim, J. S.; Yun, I. Jpn. J. Appl. Phys. 2001, 40, L443. (9) Seo, S. W.; Cho, S. Y.; Jokerst, N. M. Opt. Lett. 2007, 32, 548. (10) Yu, T. B.; Wang, M. H.; Jiang, X. Q.; Liao, Q. H.; Yang, J. Y. J. Opt. A: Pure Appl. Opt. 2007, 9, 37. (11) Bozhevolnyi, S. I.; Volkov, V. S.; Devaux, E.; Laluet, J.-Y.; Ebbesen, T. W. Nature 2006, 440, 508. (12) Yamada, H.; Chu, T.; Ishida, S.; Arakawa, Y. IEEE J. Sel. Top. Quantum Electron. 2006, 12, 1371. (13) Law, M.; Sirbuly, D. J.; Johnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. Science 2004, 305, 1269. (14) Tong, L.; Gattass, R. R.; Ashcom, J. B.; He, S.; Lou, J.; Shen, M.; Maxwell, I.; Mazur, E. Nature 2003, 426, 816. (15) Tong, L.; Lou, J.; Gattass, R. R.; He, S.; Chen, X.; Liu, L.; Mazur, E. Nano Lett. 2005, 5, 259. (16) Sirbuly, D. J.; Law, M.; Pauzauskie, P.; Yan, H.; Maslov, A. V.; Knutsen, K.; Ning, C.-Z.; Saykally, R. J.; Yang, P. PNAS 2005, 102, 7800. (17) Harfenist, S. A.; Cambron, S. D.; Nelson, E. W.; Berry, S. M.; Isham, A. W.; Crain, M. M.; Walsh, K. M.; Keynton, R. S.; Cohn, R. W. Nano Lett. 2004, 4, 1931. (18) Tong, L.; Hu, L.; Zhang, J.; Qiu, J.; Yang, Q.; Lou, J.; Shen, Y.; He, J.; Ye, Z. Opt. Express 2006, 14, 82. (19) Lyoo, W. S.; Lee, H. S.; Ji, B. C.; Han, S. S.; Koo, K.; Kim, S. S.; Kim, J. H.; Lee, J.-S.; Son, T. W.; Yoon, W. S. J. Appl. Polym. Sci. 2001, 81, 3471. (20) Chuah, H. H. Macromolecules 2001, 34, 6985. (21) Kelsey, D. R.; Kiibler, K. S.; Tutunjian, P. N. Polymer 2005, 46, 8937. (22) Hwo, C.; Forschner, T.; Lowtan, R.; Gwyn, D.; Cristea, B. J. Plastic Film Sheeting 1999, 15, 219. (23) Chuah, H. H.; Chang, B. T. A. Polym. Bull. 2001, 46, 307. (24) Xing, X. B.; Wang, Y. Q.; Li, B. J. Opt. Express 2008, 16, 10815. (25) Kim, Y.; Jeong, Y.; Oh, K.; Kobelke, J.; Schuster, K.; Kirchhof, J. Opt. Lett. 2005, 30, 2697. (26) Kim, D. U.; Bae, S. C.; Kim, J.; Kim, T.-Y.; Park, C.-S.; Oh, K. IEEE Photon. Technol. Lett. 2005, 17, 2355.

NL8014507 2843