Min-Suk Kwon

Characterizations of realized metal-insulator-silicon-insulator-metal waveguides and nanochannel fabrication via insulator removal

We investigate experimentally metal-insulator-silicon-insulator-metal (MISIM) waveguides that are fabricated by using fully standard CMOS technology. They are hybrid plasmonic waveguides, and they have a feature that their insulator is replaceable with functional material. We explain a fabrication process for them and discuss fabrication results based on 8-inch silicon-on-insulator wafers. We measured the propagation characteristics of the MISIM waveguides that were actually fabricated to be connected to Si photonic waveguides through symmetric and asymmetric couplers. When incident light from an optical source has transverse electric (TE) polarization and its wavelength is 1318 or 1554 nm, their propagation losses are between 0.2 and 0.3 dB/μm. Excess losses due to the symmetric couplers are around 0.5 dB, which are smaller than those due to the asymmetric couplers. Additional measurement results indicate that the MISIM waveguide supports a TE-polarized hybrid plasmonic mode. Finally, we explain a process of removing the insulator without affecting the remaining MISIM structure to fabricate ~30-nm-wide nanochannels which may be filled with functional material.

Discussion of the Epsilon-Near-Zero Effect of Graphene in a Horizontal Slot Waveguide

Horizontal slot waveguides based on graphene have been considered an attractive structure for optical waveguide modulators for transverse magnetic (TM) modes. Graphene is embedded in the slot region of a horizontal slot waveguide. If graphene were treated as an isotropic material and its dielectric constant were made close to zero by adjusting its Fermi level, the surface-normal electric field component of the fundamental TM mode of a horizontal slot waveguide might be highly enhanced in graphene. This could cause a large increase in the attenuation coefficient of the mode. This is called the epsilon-near-zero (ENZ) effect. This paper discusses that graphene needs to be treated as an anisotropic material that has an almost real surface-normal dielectric constant component. Then, the ENZ effect does not exist. Approximate analytic expressions and numerical simulation are used for the discussion, and they demonstrate that horizontal slot waveguides are not appropriate for graphene-based modulators for TM modes.

Efficient Coupling Between Photonic and Dielectric-Loaded Surface Plasmon Polariton Waveguides With the Same Core Material

We theoretically investigate how to efficiently couple a photonic waveguide to a dielectric-loaded surface plasmon polariton (DLSPP) waveguide when they are based on a common core material. The DLSPP waveguide is tapered and butt coupled to the photonic waveguide. First, we propose the use of a dielectric with a higher refractive index than the dielectrics of previous DLSPP waveguides. The photonic and DLSPP waveguides are designed to reduce the loss and tapering region length of the coupling, and the tapering region is optimized. We achieve the coupling between the photonic and DLSPP waveguides based on a dielectric of refractive index of 1.57 with a coupling loss of 2.3 dB through a 3-μm-long coupling region. The coupling loss is further reduced by modifying the DLSPP waveguide into a double-DLSPP ( D2LSPP) waveguide. The D2LSPP waveguide has an additional low-index dielectric between its high-index dielectric and metal layer. Designed appropriately, the D2LSPP waveguide can be coupled to the photonic waveguide with a coupling loss of 1.1 dB through a 4-μm-long coupling region. Since the photonic and DLSPP or D2LSPP waveguides investigated in this paper can be simultaneously fabricated, they may constitute an easily realizable hybrid planar lightwave circuit with a relatively low loss.

Investigation and Improvement of 90

We investigate 90° direct bends of metal-insulator-silicon-insulator-metal (MISIM) waveguides, which are hybrid plasmonic waveguides with replaceable insulators. First, we fabricate them using fully standard CMOS technology and characterize them. The experimental excess loss of the two consecutive 90° direct bends is 11, 7.4, and 4.5 dB when the width of the Si line of the MISIM waveguide is about 160, 190, and 220 nm, respectively. Second, we analyze the experimental results using the 3-D finite-difference time-domain method. Through the analysis, we investigate possible loss mechanisms of the 90° direct bend, which have not been studied to our knowledge. It has been found that the Si lines should be narrow to reduce the excess losses of the 90° direct bends. However, the wide Si lines are better for ease of fabrication and for small propagation losses. Finally, we demonstrate a modified low-loss 90° direct bend of the MISIM waveguide with a wide Si line.

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We theoretically and experimentally investigate 90° submicrometer radius bends (SRB) of metal-insulator-silicon-insulator-metal (MISIM) waveguides that are plasmonic waveguides fabricated with standard CMOS technology. We focus on the bends of MISIM waveguides with a wide (e.g., 160-220 nm) silicon line. This study shows that the bend efficiently turns the direction of the MISIM waveguide by 90° if its radius is about 0.7 μm. Moreover, we discuss the fact that the bend may be superior to a SRB of a silicon photonic waveguide when it is used to implement a ring resonator with a high quality factor and small volume.

Direct Bends of Metal–Insulator–Silicon–Insulator–Metal Waveguides

Waveguide-coupled silicon ring or disk resonators have been used for optical signal processing and sensing. Large-scale integration of optical devices demands continuous reduction in their footprints, and ultimately they need to be replaced by silicon-based plasmonic resonators. However, few waveguide-coupled silicon-based plasmonic resonators have been realized until now. Moreover, fluid cannot interact effectively with them since their resonance modes are strongly confined in solid regions. To solve this problem, this paper reports realized plasmofluidic disk resonators (PDRs). The PDR consists of a submicrometer radius silicon disk and metal laterally surrounding the disk with a 30-nm-wide channel in between. The channel is filled with fluid, and the resonance mode of the PDR is strongly confined in the fluid. The PDR coupled to a metal-insulator-silicon-insulator-metal waveguide is implemented by using standard complementary metal oxide semiconductor technology. If the refractive index of the fluid increases by 0.141, the transmission spectrum of the waveguide coupled to the PDR of radius 0.9 μm red-shifts by 30 nm. The PDR can be used as a refractive index sensor requiring a very small amount of analyte. Plus, the PDR filled with liquid crystal may be an ultracompact intensity modulator which is effectively controlled by small driving voltage.

Investigation of 90° submicrometer radius bends of metal-insulator-silicon-insulator-metal waveguides

Symmetric 1 × 2 splitters of metal-insulator-silicon-insulator-metal (MISIM) waveguides, which are hybrid plasmonic waveguides with a 190-nm-wide silicon strip covered by 30-nm-thick SiO2 and sandwiched by copper, are theoretically and experimentally investigated. The splitters consist of a Y-branch, two 90° submicrometer-radius bends (SRBs), and two straight MISIM waveguides connecting the Y-branch and the two SRBs. The Y-branch is an overlap of two oppositely directed SRBs. All the SRBs of the splitter have the same radius of curvature. The splitter has a feature that the length of the in-between MISIM waveguides is adjusted to make its input waveguide split over a short distance into its widely separated output waveguides. Simulation of the splitters indicates that the radius of curvature of the SRB needs to be ≥ 0.8 μm to make small the excess losses of the splitters. When the radius of curvature is 0.8 μm, the splitter which can connect its input waveguide just over 1.6 μm to its output waveguides spaced 6.4 μm apart has an excess loss of 1.4 dB. The splitters are realized by using the standard CMOS technology. The measured excess losses of the fabricated splitters are larger than the calculated excess losses. However, the measured dependence of the excess loss on the radius of curvature is similar to the calculated one. Finally, the splitter feature is theoretically confirmed, and the addition of a tapering region to the Y-branch is discussed to reduce the excess loss of the Y-branch.

Plasmofluidic Disk Resonators.

Plasmofluidic waveguides are based on guiding light which is strongly confined in fluid with the assistance of a surface plasmon polariton. To realize plasmofluidic waveguides, metal-insulator-silicon-insulator-metal (MISIM) waveguides, which are hybrid plasmonic waveguides fabricated using standard complementary metal-oxide-semiconductor technology, are employed. The insulator of the MISIM waveguide is removed to form 30-nm-wide channels, and they are filled with fluid. The plasmofluidic waveguide has a subwavelength-scale mode area since its mode is strongly confined in the fluid. The waveguides are experimentally characterized for different fluids. When the refractive index of the fluid is 1.440, the plasmofluidic waveguide with 190-nm-wide silicon has propagation loss of 0.46 dB/μm; the coupling loss between it and an ordinary silicon photonic waveguide is 1.79 dB. The propagation and coupling losses may be reduced if a few fabrication-induced imperfections are removed. The plasmofluidic waveguide may...

Metal–Insulator–Silicon–Insulator–Metal Waveguide Splitters With Large-Arm Separation

This paper reports a metal stripe waveguide based sensor that functions like a Mach-Zehnder interferometer. It consists of three sections. The first and third sections are input and output metal stripe waveguides that support a long-range surface plasmon polariton (LRSPP). The second section is a sensing region; it comprises a substrate, which is common to the first and third sections, an insulator layer with a refractive index larger than 2 (e.g., TiO2), and an Au layer much thicker than the skin depth of gold. For sensing, it is covered by an aqueous solution with a refractive index of about 1.3. Because of the thick Au layer, separate single-interface surface plasmon polaritons (SPPs) propagate along the top and bottom surfaces of the Au layer. Since the top and bottom SPPs rather than an LRSPP are used in the second section, it is not constrained by the condition of supporting an LRSPP, which is that the substrate should have almost the same refractive index as the solution. The top and bottom SPPs play the roles of sensing and reference arms of an interferometer, respectively. In this paper, the sensor is designed, and its bulk-sensing and surface-sensing characteristics are theoretically analyzed. The design results in the compact sensor whose sensing region is ~ 35μm long; the analysis demonstrates that the sensor has sensitivity higher than or comparable to that of previous plasmonic sensors.

Experimental investigation of plasmofluidic waveguides

We theoretically investigate simple, circle-shaped, and diamond-shaped intersections of metal-insulator-silicon-insulator-metal (MISIM) waveguides. Because of the strong light confinement of the hybrid plasmonic waveguides, the simple intersection does not work efficiently. The low efficiency of the simple intersection is improved in the other intersections, and the diamond-shaped intersection is superior to the circle-shaped one. When the footprint of the diamond-shaped intersection is just 1.96 μm2, its throughput is between -0.68 and -0.78 dB in the wavelength interval between 1.45 and 1.60 μ m, and its crosstalk is smaller than -18 dB in the interval. This compact, efficient intersection may pave the way to on-chip hybrid networks of photonic and plasmonic devices.