Surface plasmons are of interest for various applications, including optical interconnects and devices, light sources, nanolithography, biosensors, solar cells, and negative-refraction prisms or superlenses. Some of the most important applications are SP-based optical interconnects and devices, which offer the potential of realizing integrated optical nanocircuitry due to the subwavelength confinement and the slow-wave nature of SPs. The fundamental building element of these applications is the plasmonic waveguide. Among the family of various plasmonic waveguides, the metal-insulator-metal waveguide has superior lateral confinement because of the relatively shallow field penetration into the metal claddings (about a skin depth -- usually tens of nanometers). Such subwavelength confinement cannot be achieved by conventional dielectric optical waveguides. However, the loss in the MIM waveguide is substantial due to the strong absorption of metal in the visible or near-infrared spectrum. Therefore, the design, simulation, and measurement of the loss in the MIM waveguide are critically important in the development of SP-based nanocircuitry.
Surface plasmons (SPs) are of interest for various applications, including optical interconnects and devices, light sources, nanolithography, biosensors, solar cells, and negative-refraction prisms or superlenses. Some of the most important applications are SP-based optical interconnects and devices, which offer the potential of realizing integrated optical nanocircuitry due to the subwavelength confinement and the slow-wave nature of SPs. The fundamental building element of these applications is the plasmonic waveguide. Among the family of various plasmonic waveguides, the metal-insulator-metal (MIM) waveguide has superior lateral confinement because of the relatively shallow field penetration into the metal claddings (about a skin depth -- usually tens of nanometers). Such subwavelength confinement cannot be achieved by conventional dielectric optical waveguides. However, the loss in the MIM waveguide is substantial due to the strong absorption of metal in the visible or near-infrared spectrum. Therefore, the design, simulation, and measurement of the loss in the MIM waveguide are critically important in the development of SP-based nanocircuitry.
Owing to the subwavelength sizes of MIM waveguides, the excitation of an MIM plasmonic mode typically requires end-fire coupling with tapered fibers or waveguides. Further, the conventional loss measurements require the usage of a near-field scanning optical microscopy (NSOM) or multiple waveguide samples with various length scales; however, the two aforementioned techniques are both complicated and have issues of sensitivity to uncontrollable environmental factors or variations in coupling strength, respectively. These experimental challenges have been a primary reason for the slow experimental development of the MIM waveguide and device. The research in this thesis focuses on the development of the transverse transmission/reflection (TTR) method, which is a more reliable, accurate, and straightforward method of characterizing the plasmonic modes in the MIM waveguide.
The theory of the TTR method, which incorporates an attenuated total reflection (ATR) configuration, is developed based on the transmission matrix formulation. A methodology for obtaining the propagation constant and attenuation coefficient of a plasmonic mode in an MIM waveguide is illustrated. Using the Metricon Prism Coupler, the TTR method is experimentally applied to planar, single-mode MIM (Au-SiO$_2$-Au) waveguides with various core thicknesses at $lambda=1550$ nm. The experimental results are in very good agreement with the theoretical results. It is also shown experimentally that the TTR method is robust against difficult-to-quantify parameters such as the metal cladding thickness and the air gap thickness between the prism and the waveguide. As a result, the TTR method can be readily applied by using other similar ATR or prism-coupler configurations, without concern for the sensitivity issues caused by the subtle differences between various configurations.
Moreover, the TTR method is also experimentally applied to planar, multimode MIM waveguides. Multimode MIM waveguides, which have larger core sizes, may be of interest for applications in low-loss interconnects or tapered end-couplers. Thanks to the superior angular selectivity of the ATR configuration, the TTR method is capable of detecting the propagation constant and attenuation coefficient of each mode. To the best of the author's knowledge, this is the first time the propagation constant of each mode in a multimode MIM waveguide has been individually measured. Also, to the best of the author's knowledge, this is the first time the attenuation coefficient of each mode in a multimode MIM waveguide has been individually measured.
The TTR method is proved to be a reliable, accurate, and straightforward approach to characterize plasmonic modes in MIM waveguides. Future research will target the extension of the TTR method to 2D MIM waveguides, asymmetric MIM waveguides, and inclusion of scattering loss. Taking full advantage of the TTR method, the development of plasmonic devices can be potentially accelerated.
The theory of the TTR method, which incorporates an attenuated total reflection (ATR) configuration, is developed based on the transmission matrix formulation. A methodology for obtaining the propagation constant and attenuation coefficient of a plasmonic mode in an MIM waveguide is illustrated. Using the Metricon Prism Coupler, the TTR method is experimentally applied to planar, single-mode MIM (Au-SiO$_2$-Au) waveguides with various core thicknesses at $lambda=1550$ nm. The experimental results are in very good agreement with the theoretical results. It is also shown experimentally that the TTR method is robust against difficult-to-quantify parameters such as the metal cladding thickness and the air gap thickness between the prism and the waveguide. As a result, the TTR method can be readily applied by using other similar ATR or prism-coupler configurations, without concern for the sensitivity issues caused by the subtle differences between various configurations.
Moreover, the TTR method is also experimentally applied to planar, multimode MIM waveguides. Multimode MIM waveguides, which have larger core sizes, may be of interest for applications in low-loss interconnects or tapered end-couplers. Thanks to the superior angular selectivity of the ATR configuration, the TTR method is capable of detecting the propagation constant and attenuation coefficient of each mode. To the best of the author's knowledge, this is the first time the propagation constant of each mode in a multimode MIM waveguide has been individually measured. Also, to the best of the author's knowledge, this is the first time the attenuation coefficient of each mode in a multimode MIM waveguide has been individually measured.
The TTR method is proved to be a reliable, accurate, and straightforward approach to characterize plasmonic modes in MIM waveguides. Future research will target the extension of the TTR method to 2D MIM waveguides, asymmetric MIM waveguides, and inclusion of scattering loss. Taking full advantage of the TTR method, the development of plasmonic devices can be potentially accelerated.
| Identifer | oai:union.ndltd.org:GATECH/oai:smartech.gatech.edu:1853/42791 |
| Date | 30 September 2011 |
| Creators | Lin, Chien-I |
| Publisher | Georgia Institute of Technology |
| Source Sets | Georgia Tech Electronic Thesis and Dissertation Archive |
| Detected Language | English |
| Type | Dissertation |
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