Terahertz Sensors Using Surface Waves in Periodic Metallic Structures
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Terahertz range of frequency has found a fast growing number of applications in material characterization and sensing, imaging and extreme bandwidth communication. Different structures have been proposed for sensing at these frequencies. Surface plasmon waves have successfully been applied to ultra-high precision sensing at optical frequencies, because of their strong field confinement and enhancement. These waves are not as confined in THz due to metal properties over this range of frequencies. However, it has been shown that surface waves on properly designed periodic metallic structure have behavior very similar to plasmonic waves in optical range. These surface wave modes are called surface plasmon-like waves. Here we consider several periodic metallic structures, which support these surface plasmon-like modes, for THz sensing applications. The first one is a two dimensional array of metallic rods which is excited by prism. Many existing plasmonic sensing configurations use prism for plasmonic wave excitation. However, prism is too bulky for integration. Interests in integrated surface plasmonic devices at optical frequencies have been growing recently. As compared with free space configuration, integrated structures have distinct advantages such as small size and multi-channel sensing capabilities. An integrated sensing configuration using plasmonic-like wave is proposed. The new configuration uses a metallic grating that acts as a THz waveguide with a stop-band with a sharp transition edge. Excitation of such metallic grating waveguide through a dielectric waveguide will be described and analyzed. Moreover, it will be shown that the frequency of the transition edge between pass-band and stop-band is highly sensitive to the refractive index of the surrounding medium, and therefore it can be used for dielectric sensing. The excitation requirements of the proposed sensor and its sensitivity will be presented.