Applications In Terahertz Regime Research Articles

Design and analysis of low loss solid-core hexagonal photonic crystal fiber for applications in terahertz regime

An index-guiding novel solid-core photonic crystal fiber (SC-PCF) formed by a hexagonal lattice of circular-shaped air holes arranged in silicon background is realized. By varying the radius ‘r’ of the air holes from 0.1a to 0.5a (where ‘a’ is defined as the lattice constant), the characteristic electromagnetic modes of the low loss Terahertz (THz) fiber were solved through eigenmode analysis using finite element method (FEM). The effective mode area and the nonlinearity of the proposed PCF are calculated for different radii of the air holes and it is found that the effective mode area decreases when the radius of the air holes is increased. On the other hand, the nonlinearity increases for an increase in the air holes radii. At 1 THz, the confinement loss of the proposed fiber is in the order of 10−23 dB/m and transmittance efficiency above 96% has been attained. As 5-G technology emanates, THz wave propagation becomes essential and the designed hexagonal lattice SC-PCF will be useful for the advancement of communication systems, sensing devices and several medical applications.

Active controllable dual broadband terahertz absorber based on hybrid metamaterials with vanadium dioxide.

In this paper, we present an active controllable terahertz absorber with dual broadband characteristics, comprised by two diagonal identical patterns of vanadium dioxide in the top layer of the classical three-layer structure of metamaterial perfect absorbers. Simulation results show that two bandwidths of 80% absorption are 0.88 THz and 0.77 THz from 0.56 to 1.44 THz and 2.88 to 3.65 THz, respectively. By using thermal control to change the conductivity of the vanadium dioxide, absorptance can be continuously adjusted from 20% to 90%. The impedance matching theory is introduced to analyze and elucidate the physical mechanism of the perfect absorption. Field analyses are further investigated to get more insight into the physical origin of the dual broadband absorption. In addition, incident polarization insensitivity and wide-angle absorption are also demonstrated. The proposed absorber promises diverse applications in terahertz regime, such as imaging, modulating, sensing and cloaking.

Bandgap engineering of PbTe ultra-thin layers by surface passivations

We calculate the electronic structures of the PbTe (1 1 1) ultra-thin films by performing the first-principles calculations. The PbTe (1 1 1) ultra-thin films possess direct or indirect band gaps depending sensitively on surface passivations with hydrogen or halogen atoms, and the band gaps depend sensitively on the passivation elements. The bandgaps of PbTe (1 1 1) ultra-thin films with hydrogen passivations can be tuned from 15 meV to 65 meV by applying external strains, making PbTe ultra-thin films promising candidates for optoelectronic device applications in terahertz regime.

Terahertz biosensing metamaterial absorber for virus detection based on spoof surface plasmon polaritons

As a promising candidate for rapid and ultrasensitive sensing of biomolecules, terahertz (THz) time-domain spectroscopy is widely used in label-free, noncontact, noninvasive, and nondestructive detection. In this article, planar Jerusalem cross metamaterial absorber for different virus detection based on spoof surface plasmon polaritons (SSPPs) in THz band is investigated. Strongly confined SSPPs modes are extracted from the absorption spectra of biosensing metamaterial absorber associated with local field enhancement. Meanwhile, equivalent complex refractive index (N) of different virus subtypes and protein concentrations is numerically investigated for virus detection. Besides, simulated detections for different virus subtypes (different N) are probed with the proposed biosensing metamaterial absorber. Especially for the simulated detections of three representative Avian Influenza viruses (H5N2, H1N1, H9N2), the proposed THz biosensing metamaterial absorber chip performed ultrasensitive sensitivity and high resolution by extracting the shifted resonance frequencies (ΔF) and the changed values at maximum absorptions (ΔA). All these findings show great significance on a rapid real-time procedure for diseases diagnosis especially for the contagious and time-sensitive target virus. The proposed ultrasensitive and selective THz metamaterial absorber opens up a new way for planar biosensing chip to be developed into practical applications in THz regime.

Highly birefringent and low effective material loss microstructure fiber for THz wave guidance

A novel porous core photonic crystal fiber (PC-PCF) is proposed in this article for efficient transmission of terahertz (THz) wave. The propagation characteristics of the proposed design are investigated by finite element method based COMSOL v 4.2 software. The proposed PC-PCF offers ultrahigh birefringence of order of 10−2 and low effective material loss (EML) of 0.07 cm−1 at an operating frequency of 1 THz. Moreover, the proposed structure exhibits very low and nearly zero flattened dispersion of 1.1 ± 0.02 ps/THz/cm in the frequency ranging between 0.8 and 1.2 THz. In addition, very low bending loss of 5.45 × 10−9 dB/m is observed when bending radius is 1 cm. Other important guiding characteristics such as confinement loss, effective area etc. of the proposed fiber is discussed rigorously. This structure can be a promising candidate for different applications in THz regime.

Efficient Terahertz Plasmonic Absorbers with V-Grooves Using Highly Doped Silicon Substrate and Simple Wet-Etching Techniques

We experimentally demonstrate that at terahertz frequencies perfect plasmonic absorbers made from a 3D V-groove array in a highly doped silicon wafer can be easily realized using simple wet-etching process. The surface plasmon modes can be excited by the V-groove array and get decayed when they propagate along the silicon surface and enter the grooves, inducing a broadband near-zero dip in the reflection spectra. The reflection spectrum of the fabricated absorber is characterized using a terahertz time-domain spectroscopy system, and the experimental results are in good agreement with numerical simulations. The high performance including high absorptivity and large bandwidth together with the easy fabrication processes presented in this paper make this plasmonic absorber promising for a wide range of practical applications in terahertz regime.

A single mode porous-core square lattice photonic crystal fiber for THz wave propagation

Interests on low-loss terahertz (THz) waveguides are increasing due to their remarkable applications in various fields. Since most the materials are highly absorbent to THz waves therefore it is an ongoing challenge to obtain a low-loss waveguide. This paper presents a novel porous-core square lattice photonic crystal fiber (PCF) for efficient transmission of THz waves. The guiding properties of the proposed fiber are characterized by using finite element method (FEM) with circular perfectly matched layer (PML) boundary conditions. It is demonstrated that the designed PCF shows very low effective material loss (EML) of 0.076 cm-1 at 1.0 THz that indicates about 62 % reduction of bulk absorption loss of the background material. In addition to this, the proposed fiber exhibits low confinement loss of 8.96 × 10-3 dB/cm and low flattened dispersion of 0.96 ± 0.086 ps/THz/cm for the optimal design parameters. Other important propagation characteristics such as single mode propagation, power fraction, and bending loss are also investigated thoroughly. A porous-core PCF is an efficient mechanism for the transmission of THz waves. The proposed low-loss and low-dispersion PCF can find numerous applications in THz regime.

Polarization Maintaining Low-Loss Slotted Core Kagome Lattice THz Fiber

A polarization maintaining ultra-low effective material loss based on slotted core kagome lattice fiber is proposed for terahertz (THz) wave propagation. Numerical study demonstrates that by using rectangular slotted air holes in the core of the kagome lattice exhibits simultaneously an ultra-high birefringence of $8.22\times 10^{-2}$ , an ultra-low effective material loss of 0.05 cm $^{-1}$ , and a very low confinement loss of $4.13\times 10^{-5}$ cm $^{-1}$ at the frequency of 1 THz. Further investigation shows that about half of the total mode power confines into the air slots at 50% core porosity. The proposed fiber can be used for polarization maintaining applications in THz regime.

Prospects of photo-sensitive indium phosphide based top-mounted and flip-chip IMPATT oscillators for application in terahertz regime

The prospects of using double-drift-region (DDR) flat and single-low-high-low (SLHL) type InP based impact-ionisation-avalanche-transit-time (IMPATT) diodes as terahertz sources are studied and compared for the first time. Extensive simulation indicates that the devices are capable of generating RF power of 146.8–241.8 mW at around the 0.5 THz frequency. The effects of parasitic series resistance on the THz performance of the devices are further simulated. It is interesting to note that the presence of a charge bump in flatly doped DDR structures reduces the value of parasitic series resistance by 29.5%. The effects of photo-illumination on the top-mounted (TM) and flip-chip (FC) IMPATT devices are also investigated using a modified double iterative simulation technique. Under photo-illumination the negative conductance and the negative resistance of the devices (both flat and SLHL) decrease along with an upward shift in operating frequency. The upward shift in operating frequency is found to be more (at least 22.0 GHz) when the performance of the InP based IMPATT is controlled by the hole dominated saturation current rather than by the electron dominated saturation current. The overall effects of photo-irradiation on the SLHL diodes are found to be much more prominent than for their flat-profile counterparts. These studies reveal the potential of InP IMPATTs as optically controlled high-speed terahertz switching tools.