Terahertz fibers used for optical-sensing applications are introduced in this chapter, including the dielectric wires, ribbons and pipes. Different analyte conformations of the liquid, solid particle, thin film and vapor gas are successfully integrated with suitable fibers to perform high sensitivities. Based on the optimal sensitivities, analyte recognitions limited in traditional terahertz spectroscopy are experimentally demonstrated by the terahertz fiber sensors. Using the cladding index-dependent waveguide dispersion and high fractional cladding power of terahertz wire fiber, 20 ppm concentration between polyethylene and melamine particles can be distinguished. When the evanescent mode field of a terahertz ribbon fiber is controlled by a diffraction metal grating, subwavelength-confined surface terahertz waves potentially enable the near-field recognition for nano-thin films. Resonance waveguide field surrounding the terahertz pipe fiber is able to identify the macromolecule deposition in subwavelength-scaled thickness, approximately λ/225. For inner core-confined resonance waveguide field inside the terahertz pipe fiber, low physical density of the vaporized molecules around 1.6 nano-mole/mm3 can also be discriminated.
Part of the book: Terahertz Spectroscopy
Terahertz (THz) wave propagation in the layered media is presented based on the waveguide and artificial-material configurations to sense the gas molecules. The single dielectric layer with a cylindrical conformation works as a pipe waveguide in the wave frequency of 0.1–1 THz. For a long-distance propagation over 10 cm of the pipe, resonant modes are characterized from the transmission power dips. The pipe-waveguide resonator works for a THz refractive-index sensor when the resonance frequency is monitored to sense vapor molecules inside the pipe core. Besides of the waveguide configuration, a multilayer microporous polymer structure (MPS) is considered an artificial material to transmit THz waves for sensing gaseous molecules. The MPS is not only transparent to THz waves but also enhances the detection resolution of THz absorption for the vapor molecules. The porous structure provides a large hydrophilic surface area and numerous micropores to adsorb or fill vapors, thereby leading to greatly enhanced wave-analyte interaction with an apparent THz signal change. Different concentrations of toxic methanol adulterated in alcoholic aqueous solutions are thus identified in their vapor phases by using the MPS-based THz sensor.
Part of the book: Gas Sensors