Understanding of terahertz spectroscopic properties of materials is crucially important for applications of terahertz waves in sensing. Spectroscopic properties of water-rich media, such as biological tissues, gels, and aqueous solutions, are strongly affected by the amount and the dynamics of water in them. Terahertz spectroscopical measurements can clarify the dynamical and/or structural characteristics of water molecules in the hydrogen-bond network in these media. We studied the dynamical properties of water around protein molecules and polymers in aqueous solutions using terahertz spectroscopic measurements. Sensing of liquid using a terahertz waveguide composed of a metal rod array will also be described.
Metallic rod array is successfully demonstrated to be integrated a parallel plate waveguide and used as a slab-waveguide for sensing applications. Both the waveguide properties are characterized from the transmission spectra using different polarization conditions of terahertz electromagnetic fields, which are parallel and perpendicular to the rod axis. When THz field polarization is parallel to the rod axis, there is high-pass filtering feature with structure-period-dependent threshold frequency, corresponding to the plasma frequency of the rod-array composite. For the polarization perpendicular to the rod axis, there is Bragg diffraction frequency as forbidden band with considerably low transmission power. The metal-rod-array integrated parallel-plate waveguide is connected with a microfluidic channel to sense liquids, and the minimum detectable quantity is about 13μmol/mm3. The metal-rod-array slab waveguide is able to sense thinfilm with nanometer level, and the detectable optical path difference is down to λ/2380. The sensor properties of multiple functions for sensing various analyte conformations, microfluidic integration and high sensitivity would be applied in biomedicine and biochemical applications.
A terahertz plasmonic waveguide sensor is experimentally demonstrated to utilize surface waves propagated in a onedimension
metal grating that constructed on a plastic ribbon waveguide. The grating conformation couples evanescent
waves of a subwavelength terahertz waveguide onto the metal surface and highly confines the extended powers within
λ/22-range for the phase-matching condition. The confined terahertz waveguiding waves resemble surface plasmonpolaritons
but transmit with almost zero dispersion when the coupled surface waves interfere along the metal grating.
Based on the dispersion-free guidance, there is Bragg reflection dominated by grating periods and strongly dependent on
the refractive index of surface plasmon-polaritions. We successfully detect different thicknesses of polyethylene layers
covered on the metal grating with thickness resolution of 1μm when the effective waveguide indices are modified in the
vicinity of the metal grating, corresponding to 0.01-index variation. Potentially, terahertz subwavelength ribbon
waveguide based plasmonic sensors could be manipulated to detect molecules with extremely low-density or small
thickness in the metal-dielectric interface for probing pollution particles and any label-free detection.
A dielectric hollow-core tube utilized as a terahertz anti-resonant reflecting hollow-core waveguide (THz-ARRHW)
sensor has been demonstrated to detect the minute variation of both refractive index and thickness in macromolecule
layers, deposited on the tube wall, and to identify liquid vapors from the various core indices. The minimal quantity of
macromolecule layers loaded on the tube wall of a polypropylene tube can be detected at 1.2picomole/mm2 and 0.2%,
corresponding to the variation of 2.9μm-thickness and 0.001-refractive-index. And the sensing performance of a THz-
ARRHW to detect core index variation for identifying volatile liquids is also realized at 0.0001g/cm3- vapor density.
We present the first characterization of a simple subwavelength-diameter plastic wire by using wideband terahertz timedomain
spectroscopy. The propagation characteristics including waveguide dispersion, group velocity, and attenuation
constant of various plastic wires with different diameters and refractive indices are studied. The experimental results
show the subwavelength plastic wire has extremely low waveguide dispersion and low attenuation constant (<0.01cm-1)
at its THz transmission band due to much reduced fractional power delivered inside the lossy core, which is consistent
with the theoretical calculations. With the large evanescent-fields, the subwavelength plastic wire has capability to
integrate with micro-fluid channel for sensitive bio-sensing applications.