JW2A.92.pdf CLEO:2016 © OSA 2016 Nano-focusing in an Air-slot Plasmonic Waveguide With a Tapered Grating Coupler Chuan Zhong, David McCloskey, Jing Jing Wang, Brian D. Jennings, Nicolás Abadía, Ertugrul Karademir, Jian-Yao Zheng, and John F. Donegan Advanced Materials and Bioengineering Research Centre (AMBER) and School of Physics, CRANN, Trinity College Dublin, Dublin 2, Ireland Author e-mail address: zhongc@tcd.ie Abstract: We propose an air-slot plasmonic waveguide for nano-focusing. A tapered grating is used as a dipole radiation source for surface plasmon excitation and coupling. Simulations show an effective coupling and field enhancement in our design. OCIS codes: (240.6680) Surface plasmons; (310.1860) Deposition and fabrication; (350.4238) Nanophotonics and photonic crystals. 1. Introduction Nano-focusing is used for confine down light to the nanoscale, as a result, the cross section of the propagating optical modes can be dramatically reduced. Efficiently concentrating optical energy to nanoscale dimension has been performed over the past few decades [1]. This concept has been widely investigated in the design of photonic integrated circuits (PIC). Compared to the integrated electric devices, integrated photonic devices have larger bandwidth, higher immunity to the electromagnetic interference and feasible compatibility with current complementary metal-oxide semiconductor (CMOS) fabrication technologies [2], these advantages are essential for improving the performance of nanoscale integrated photonic devices. Various structures have been demonstrated for nano-focusing [3,4] in recent years. In this work, we propose an air-slot plasmonic waveguide for nano-focusing. A tapered grating is used as a dipole radiation source for surface plasmon excitation and coupling. Our simulation shows that an effective coupling can be achieved through the tapered grating coupler and with high field enhancement and confinement can be achieved through this air-slot plasmonic waveguide. Far-field measurements show the surface plasmon waveguiding and focusing have been achieved in our air-slot plasmonic waveguide. 2. Experimental methods and results The tapered grating is formed by a group of metal slits with varying length as shown in Fig. 1(a). The period of the grating is 500 nm and the duty cycle is 1/2. The wavelength of the input laser is 633 nm, the period of the grating in our design matches the momentum compensation condition for exciting the surface plasmon. The air-slot waveguide consists of two parallel metal strips with a certain separation which form a tapered air-slot as shown in Fig. 1(a). The separation of the air-slot in the input port is 400 nm which is the same dimension as the output of the Fig. 1 Fig. 2 Fig.1 (a) Schematic of the air-slot plasmonic waveguide. (b) SEM image. (c) Field enhancement and confinement in the air-slot waveguide. (d) Propagation length in the air-slot waveguide. Fig. 2 (a) Simulation of the electric field in the tapered grating. (b)-(e) Simulations of the electric field in the air-slot waveguide. 978-1-943580-11-8/16/$31.00 ©2016 Optical Society of America Authorized licensed use limited to: Cardiff University. Downloaded on June 30,2022 at 06:08:35 UTC from IEEE Xplore. Restrictions apply. JW2A.92.pdf CLEO:2016 © OSA 2016 tapered grating, this design not only functions as a dipole radiation source to excite the surface plasmon in the airslot, but also operates with a high coupling efficiency. Fig. 1(b) shows the top-view scanning-electron-microscope (SEM) image of the air-slot plasmonic waveguide sample, which was patterned by an electron beam lithography (EBL) system, then deposited with 50 nm Au with 2 nm Ti adhesion layer through an electron beam evaporation system. After that the sample was put into acetone for 10 hours in order to move the photoresist. Finally the sample went through a lift-off procedure by gentle sonification for 45 seconds under 80 kHz oscillation. The length of the air-slot waveguide shown in Fig. 1(b) is 10 μm. The angle of the taper in the air-slot is 40° which is the optimized angle in order to decrease the back reflection as found in our simulations. Fig. 1(c) shows the electric field enhancement along the air-slot. The enhancement is varying with angle of the taper, a strongest enhancement occurs around 40°. Fig. 1(d) shows the intensity of plasmon along the air-slot. The electric-field in the air-slot occurs with an exponential decay along the direction of propagation due to the large damping in the Au. The propagation length of plasmon is 15 μm in our air-slot waveguide at 633 nm. The simulations shown in Fig. 2 were conducted numerically using a commercially COMSOL software package. For all calculations, we use the wavelength 633 nm. The complex permittivity of Au is ε Au=0.196+3.258i which is from Palik optical constant. The electric field along the waveguide with different perspectives are shown in Fig 2. Fig. 2 (a) shows the light coupling in the tapered grating. As can be seen, there is a strong electric field on the tip of the grating which can be treated as a dipole radiation source for exciting the surface plasmon in the waveguide. Fig. 2(b) and (c) show the cross section of electric field in the input plane and output plane of the air-slot waveguide, respectively. In the input plane, the electric field mainly confined along the lateral side of the metal strip which can be assigned as surface plasmon modes. In our design, a tapered air-slot can confine the light gradually into a spot less than 100 nm as shown in Fig. 2(c). The size of the confined light spot in our calculation can be as small as 50 nm*50 nm. Fig. 2(d) and (e) show the electric field along the waveguide. A strong enhancement is achieved in the taper and the plasmon can be confined strongly in the air-slot. Fig. 3 Light propagating and focusing through far-field measurement. (a)-(c): length of the waveguide is 5 μm, 10 μm and 25 μm respectively. (d)-(f): the corresponding position of the waveguides during the far-field measurement. The scale bar is 2 μm for all the figures. Fig 3 shows the light propagating and focusing in the waveguides with different lengths from a far-field measurement. The background in Fig. 3(a)-(c) has been removed by a subtraction algorithm using MATLAB in order to observe the weak surface wave. The theoretical propagation length of the surface plasmon is 15 μm at 633 nm, the focusing spot shows up in the air-slot instead of on the tip as shown in Fig. 3(c). Future work will focus on optimizing the grating in order to improve the coupling efficiency and acquiring images with high resolution through a scanning near field optical microscopy (SNOM) system. This air-slot plasmonic nano-focusing waveguide has a strong potential to be used in applications such as heated magnetic recording (HAMR). 3. Conclusions An air-slot plasmonic waveguide has been proposed which can be used for nano-focusing of light. A novel tapered grating coupler performances as a dipole radiation source which can be used to excite the surface plasmon in the airslot waveguide. Meanwhile the tapered grating will induce an effective coupling to the air-slot plasmonic waveguide. Our simulation provides a good reference for the fabrication and characterization of these waveguides. References [1] S. Hayashi, and T. Okamoto, "Plasmonics: visit the past to know the future," Journal of Physics D: Applied Physics 45, 433001 (2012). [2] T. L. Koch, and U. Koren, "Photonic integrated circuits," AT&T Technical Journal 71, 63-74 (1992). [3] M. Schnell, P. Alonso-Gonzalez, L. Arzubiaga, F. Casanova, L. E. Hueso, A. Chuvilin, and R. Hillenbrand, "Nanofocusing of mid-infrared energy with tapered transmission lines," Nat Photonics 5, 283-287 (2011). [4] H. Choo, M. K. Kim, M. Staffaroni, T. J. Seok, J. Bokor, S. Cabrini, P. J. Schuck, M. C. Wu, and E. Yablonovitch, "Nanofocusing in a metalinsulator-metal gap plasmon waveguide with a three-dimensional linear taper," Nat Photonics 6, 837-843 (2012). Authorized licensed use limited to: Cardiff University. Downloaded on June 30,2022 at 06:08:35 UTC from IEEE Xplore. Restrictions apply.