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.
We fabricate a PCF coil sensor for water-depth sensing by winding a PCF on a plastic straw. Due to the bending-induced birefringence along the PCF, we can observe clear interference pattern in the output spectrum by placing the PCF coil into a Sagnac fiber loop. As we horizontally immerse the fabricated PCF coil into water, a nonlinear relationship between the water depth and the wavelength shift can be obtained. We have also measured the interference spectrum by vertically immersing the PCF coil into water. We can observe a linear relationship between the water depth and the wavelength shift, and the measured water-depth sensitivity for vertical immersion is -1.17 nm/mm.
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 demonstrate the fabrication of the internally liquid-filled photonic crystal fibers (PCFs) with the liquids filled in
inner air-hole layers of the PCFs. The outer air-hole layers are first blocked by using a selectively blocking technique
with a polished fiber tip. The liquid is then infiltrated into the inner open holes through a vacuum injection method to
obtain the internally liquid-filled PCFs. The measurement results show that the propagation losses of the internally
liquid-filled PCFs can be efficiently reduced with the aid of the outer air-hole layers in the fiber cladding. By varying the
operation temperature, we also demonstrate the thermally tunable optical properties of our fabricated internally liquidfilled
In this paper we employ the full-vector finite-difference frequency-domain (FDFD) method to theoretically investigate
the birefringent characteristics of three kinds of selectively liquid-filled photonic crystal fibers (PCFs). These
birefringent PCFs are fabricated by asymmetrically infiltrating high-index liquids into the air holes of the PCFs. The
birefringence values and the confinement losses of these birefringent selectively liquid-filled PCFs are compared and
discussed. We also demonstrate the fabrication of the birefringent selectively liquid-filled PCFs, and the birefringence
values can be successfully measured by using an optical fiber Sagnac interferometer.
In this paper we demonstrate the fabrication of long-period fiber gratings (LPFGs) based on the photonic crystal fibers
(PCFs) with the UV gel infiltrated in all the air holes. The periodic index modulation of the fabricated LPFGs is induced
by using the UV light through the amplitude masks with periods of ΛG = 400 μm, 500 μm, and 600 μm, respectively. By
measuring the output spectra of these fabricated LPFGs, we can observe clear notches in the transmission spectra. These
notches correspond to the coupling from the core mode to the cladding modes. We also discuss the effect of exposure
time of UV light on the grating properties of the fabricated LPFGs.