Optical Gas Sensors Using Terahertz Waves in the Layered Media

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

Terahertz Fiber Sensing

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

Investigation of spectral properties and lateral confinement of THz waves on a metal-rod-array-based photonic crystal waveguide

Terahertz (THz) waves laterally confined in a 1 mm-thick microstructured planar waveguide are demonstrated on a free-standing metal rod array (MRA), and one apparent rejection band of a transmission spectrum, resembling the bandgap of a photonic crystal, is found in 0.1–0.6 THz. The visibility of the photonic bandgap in the spectral width and power distinction can be manipulated by changing the MRA geometry parameters, including the rod diameter, the interspace between adjacent rods, and the propagation length based on the interactive MRA-layer number. THz transmission ratio enhanced by a large interactive length is verified in 30 MRA layers due to the longitudinally resonant guidance of transverse-magnetic-polarized waveguide modes along the MRA length, which is critical to the interspace width of adjacent rods and the metal coating of the rod surface. For an MRA with respective rod diameter and interspace dimensions of about 0.16 and 0.26 mm, the highest transmission of the guided resonant THz waves are performed at 0.505–0.512 THz frequency with strong confinement on the metal rod tips and a low scattering loss of 0.003 cm−1

Geometry-dependent modal field properties of metal-rod-array-based terahertz waveguides

One terahertz (THz) waveguide based on the metal rod array (MRA) structure is numerically demonstrated in 0.1–1 THz, including the fundamental and high-order transverse magnetic (TM) modes. The high-order TM-mode THz waves are strictly confined inside the MRA structure and are thus sensitive to the metal rod interspace for their spectral positions, bandwidths, transmittances, and attenuation coefficients. Arranging metal rods with fine-tuning the interspaces across the optic axis is presented as the critical stratagem to optimize the transportation efficiency of THz waves through an MRA structure. The maximum propagation length of MRA-confined THz waves is over 30 mm with the lowest attenuation coefficients of approximately 0.05–0.1 cm−1. The MRA is, therefore, applicable as one deformable artificial structure in THz frequency region because simply one-axial adjustment of the metal-rod interspace enables the modulation purpose without uniform adjustment on the two-dimensional metal rod interspace

Terahertz Plasmonic Sensor Based on Metal–Insulator Composite Woven-Wire Mesh

Terahertz (THz) spectroscopy has been proven as an effective detection means for the label-free and nondestructive sensing of biochemical molecules based on their unique roto-vibrational transitions. However, the conventional THz spectroscopic system is unsuitable for minute material sensing due to its far-field detection scheme, low sample amount, and lack of spectral characteristics, leading to low absorption cross-sections and sensitivity. In this study, a 3D plasmonic structure based on a metal-coated woven-wire mesh (MCWM) was experimentally and numerically demonstrated for sensing trace amounts of analytes combined with THz spectroscopy. Dual sharp spectral features were exhibited in the transmission spectrum, originating from the resonant excitation of THz surface electromagnetic modes via the aperture and periodicity of the MCWM unit cell. According to the finite element simulation, an enhanced and localized surface field was formed at THz resonant frequencies and was concentrated at the metal gaps near the periodic corrugations of the MCWM, resulting in enormous resonant dip shifts caused by the tiny variations in membrane thicknesses and refractive indices. Different types and quantities of analytes, including hydrophilic biopolymer (PAA) membrane, nonuniformly distributed microparticles to mimic macro-biomolecules or cells, and electrolyte salts of PBS, were successfully identified by the MCWM sensor with the best thickness and refractive index sensitivities approaching 8.26 GHz/μm and 547 GHz/RIU, respectively. The demonstrated detection limit of thickness and molecular concentration could respectively achieve nanometer and femtomolar scales in PAA macromolecular detection, surpassing the available metallic mesh devices. The MCWM-based sensing platform presents a rapid, inexpensive, and simple analysis method, potentially paving the way for a new generation of label-free microanalysis sensors

A dual-function tunable terahertz multiband bandpass filter based on PET woven-wire meshes with conductive coating surfaces

A multifunctional terahertz (THz) device with frequency tunability, amplitude modulation and spectral filtering abilities, and substrate flexibility is highly desired for various applications, such as THz communication, sensing, wearable optoelectronics. Therefore, this study proposed a 3D artificial material based on a flexible polyethylene terephthalate (PET) woven-wire mesh with conductive coating surfaces. This material was conceptually demonstrated in experiments as a dual-functional THz device. In addition, the transmission properties of this artificial material were experimentally and numerically explored with reasonable agreement. Then, the underlying mechanism of the spectral characteristics was qualitatively elucidated. The amplitudes and frequencies of the spectral characteristics were successfully validated to be passively manipulated by tailoring the structural parameters of a unit cell and changing the surface conductivities of a device. This process opened the application feasibility of a frequency tunable bandpass filter and a power switch at the THz regime. For the bandpass filter application, the tri-frequency modulation capability was experimentally demonstrated, and a frequency tuning bandwidth of 0.249 THz was achieved by varying the conductivity of the device’s surface-coating layer from 0.44 to 77 MS/m. For the power switch application, the largest modulation depth between the on and off states of a device was successfully demonstrated at approximately 21.3 dB at 0.271 THz. This dual-function THz device has the advantages of compactness, low cost, and easy fabrication, thus providing an alternative strategy to develop tunable, flexible, and versatile THz devices for broad applications