Achromatic terahertz quarter-wave retarder in reflection mode

Phase retarders (PRs) are space-variant waveplate elements which are usually utilized to generate radial and azimuthal polarization and find the polarization axis. So far, there have been mainly two types of phase retarders: half-wave phase retarder and quarter-wave phase retarder. Their principles are similar while functions are different. Some researchers may feel confused to make a choice between them. The present work aims to provide an engineering reference for choosing phase retarders with half-wave and quarter-wave retardation in practical applications. We make a comparison of the two types of PRs in both principles and applications to show their properties clearly.

A highly confined spoof surface plasmon (SSP) based sensor to test the dielectric materials in microwave and terahertz (THz) frequency region is presented. The proposed sensor comprised of the main coplanar waveguide (CPW) line and the defected T-shaped SSP line at the top and bottom of the substrate, respectively. The defect in the SSP line provides stop band, and hence localization of the transmission energy at the defect location. The resonance frequency and localization of the transmission energy can primarily be tuned by varying the height of the defect and the periodicity of the SSP strip. The proposed sensor is numerically optimized and tested in the microwave and THz frequency region using the full wave electromagnetic solver (CST-MWS). The designed sensor conforms the planar geometry, which facilitates its ease of integration to the modern radio frequency (RF) and THz frequency systems. The sensor prototype is fabricated and experimentally tested in the microwave frequency region using a number of standard solid and liquid samples. It is found that the test samples with closely spaced dielectric constant can easily be distinguished using the developed SSP sensor owing to its considerably high quality factor (~165) and hence high resolution. Additionally, the fabricated compact sensor prototype has adequate sensitivity (2.84%) and small sensing area, as a result it helps to test small volume of liquid specimen (order of few microliter), which is generally the case with the biological samples due to their low volume availability.

This paper discusses radiation loss of on-chip transmission-line. Since terahertz applications of integrated circuits are emerging, precise modeling in terahertz region is needed. We evaluate conductor loss, dielectric loss and radiation loss of on-chip transmission-line by a full-wave field solver. Simulation results show that design focusing conductor loss may cause large radiation loss in terahertz region. Quantitative evaluation of the impact of radiation loss is demonstrated. Also we show that thick signal wire and differential line is effective to reduce the loss.

The dynamic performance of wide-bandgap 4H-SiC based double drift region (p++ p n n++) IMPATT diode is simulated for the first time at terahertz frequency (0.7 Terahertz) region. The simulation experiment establishes the potential of SiC based IMPATT diode as a high power (2.5×1011 Wm−2) terahertz source. The parasitic series resistance in the device is found to reduce the RF power output by 10.7%. The effects of external radiation on the simulated diode are also studied. It is found that (i) the negative conductance and (ii) the negative resistance of the diode decrease, while, the frequency of operation and the quality factor shift upward under photoillumination. Holes in 4H-SiC based IMPATT are found to dominate the modulation activities. The inequality in the magnitude of electron and hole ionization rates in the semiconductors may be correlated with these findings.

In this paper, we propose a novel structure of anisotropic graphene-based hyperbolic metamaterial (AGHMM) sandwiched as a defect between two one-dimensional photonic crystals (PCs) in the terahertz (THz) region. The proposed structure is numerically simulated and analyzed using the transfer matrix method, effective medium theory and three-dimensional finite-difference time-domain. The defect layer of AGHMM consists of graphene sheets separated by subwavelength dielectric spacers. According to the presented results, a sharp defect mode originated from the optical Tamm state (OTS) takes place at the interfaces between PCs and AGHMM. The platform is then applied as various THz devices with outstanding characteristics compared with their previous counterparts. The performance of all the proposed devices can be tuned by changing the incident angle and graphene chemical potential. Our designed electro-optical modulator exhibits a high extinction ratio of 24.75 dB. The required switching voltage and insertion loss are as low as 9.94 V and 0.05 dB, respectively. Owing to the anisotropy of graphene, a tunable polarizer over wide incident angles is attained. The highest polarization extinction ratios (PERs) for TE- and TM-pass polarizers are 42.4 dB and 76.8 dB, respectively, which are the highest PER ever reported. By introducing more AGHMM defects, multiband tunable filters with quality factors of as high as 75314 are realized. Finally, a narrowband perfect absorber is proposed so that a high absorption of 99.8% with an ultra-narrow FWHM of 0.00055 THz is achieved. Hence, the proposed structure is a promising platform that could have many potential THz applications.

The article presents a rectangular core photonic crystal fiber (RCPCF) structure to detect the major components of blood. We used Topas as bulk fiber material for its extraordinary properties in the terahertz (THz) frequency region and the investigation is accomplished from 0.8 THz to 1.8 THz. The RCPCF sensor provides enriched relative sensitivity (almost 94.38%, 93.72%, 92.94%, 92.14%, and 90.8% for red blood cells (RBCs), hemoglobin (HB), white blood cells (WBCs), plasma, and water respectively at 1.8 THz frequency) with negligible confinement loss (only 2.75 × 10−13 cm−1, 2.87 × 10−13 cm−1, 3.06 × 10−13 cm−1, 3.27 × 10−13 cm−1, and 3.48 × 10−13 cm−1 for RBCs, HB, WBCs, plasma, and water respectively at 1.8 THz frequency). Besides, the proposed sensor offers a large effective area (approximately 3.382 × 105 μm2, 3.436 × 105 μm2, 3.493 × 105 μm2, 3.551 × 105 μm2, and 3.613 × 105 μm2for RBCs, HB, WBCs, plasma, and water respectivelyat 1.8 THz frequency). Also, this RCPCF sensor structure can be fabricated by utilizing the current fabrication techniques.

Three-dimensional (3D) printing technologies enables fast prototyping of complex 3D objects with ever improving printing qualities. To date, 3D printing has been found useful in areas such as manufacturing, industrial design, aerospace, dental and medical industries, and many others. In this article, we review recent advances of 3D printing technologies for terahertz (THz) applications. Different 3D printing technologies and printable materials are first discussed and compared. 3D-printed THz components and devices, which are categorized as waveguides/fibers, antennas, and quasi-optical components, are further demonstrated. It is found that the performances and functionalities of 3D-printed THz devices have been greatly enhanced, while the operating frequencies have been increased from the lower end of THz range to over 1 THz region. With further development of novel materials and printing techniques, it is believed that 3D printing technologies will play an important role in the realization of THz components for efficient control and manipulation of THz waves.

In this paper, the author studies, through numerical simulation, the classical analog of the electromagnetically induced absorption/re∞ection (EIA) in a planar metamaterial structure in the near infrared spectral region. The structure is designed by transforming an electromagnetically induced transparency (EIT) structure into an EIA structure using Babinet's principle. The structure exhibits a coupling between a bright mode (a complementary ring resonator (CRR)) and a dark mode (pair of parallel straight slits) imprinted on a glass substrate. A narrow absorption window, induced in a wide transparent window, is achieved by the structure and the strength of coupling is tuned by the degree of breaking symmetry and relative displacement of the two mode elements. Electromagnetically induced transparency (EIT) is a quantum mechanical phenomenon in which an absorption is transformed into a transparency, and this is associated with strong dispersion that reduces the group velocity (1{3). This phenomena has many potential applications in signal processing, optical flltering and sensing technologies (4{7). The classical analog of the EIT has been achieved and reported by many researchers using a planar metamaterial structures that are generally composed of two metallic elements imprinted on a dielectric substrate. The two elements are metallic resonators that act either as two bright modes or a bright and dark mode. A bright mode element couples strongly with the incident excitation fleld while the dark element does not couple with the incident fleld but couples with the magnetic fleld induced by the bright element. Several EIT metamaterial structures have been employed and reported in the microwave, terahertz and optical frequency regions (8{14). In this paper, we propose a new EIT structure that achieves a more pronounced narrow transparency window sandwiched between two absorption bands compared with a similar structure proposed in (14). Also compared with (14), the structure proposed here is independent of the polarization orientation with respect to the bright mode element. Then we transform this structure into an EIA structure using Babinet's principle (15,16), where a narrow absorption/re∞ection window is sandwiched between two transparency bands. The structures proposed here may be used as obscurant particles that allow the passage of a speciflc window/windows of the electromagnetic spectrum and obscure the neighboring frequencies, for this reason the transmission spectrum will be discussed and addressed for the potential obscurant applications. Although the orientational average response of the structure is what matters, it is of good beneflt to get the behavior in a given orientation. The EIT structure is composed of a metallic ring resonator (RR) which acts as a bright mode and a pair of metallic rods that act as a dark mode. Here we overlap the RR wide antenna resonance and the narrow line width quadrupole rod resonance, which in turn produces the EIT-like efiect. The structure is transformed into an EIA structure by replacing the metallic RR and the pair of rods resonator with complementary ring resonator (CRR) and

In this paper, a multi-band chiral metamaterials (CMMs) based on complementary cross-wire structure is proposed and studied numerically. The finite element method (FEM) method was used to simulate the EM properties of the designed structure. Numerical results indicate that the giant optical activity and multiband negative refractive index can be realized in terahertz region. The proposed CMMs may have some potential applications in novel terahertz device.Particle size of bayerite (d50) was about 24 μm. Al2O3was obtained after calcination at 1050 °C for 1.5 h.

A new achromatic phase retarder based on a metal-multilayer dielectric grating structure is designed using the rigorous coupled wave analysis method and the genetic algorithm. The optimized phase retarder can maintain phase retardation around 90° from 900 nm to 1200 nm, and the maximum deviation is less than 4.5% while the diffraction efficiencies of TE and TM waves are both higher than 95%. Numerical analysis shows the designed phase retarder has a high fabrication tolerance of groove depth, duty cycle and incident angle. This achromatic phase retarder is simple in design and stable in performance, and can be widely used in optical systems.

ABSTRACTIn this article, a compact triple‐band frequency‐tunable (FT) hexagonal‐shaped graphene antenna through a machine learning (ML) approach for terahertz (THz) application is presented. The proposed THz antenna is designed on a polyamide () substrate with a thickness of 10 μm, and graphene is used as an antenna radiator. The size of the substrate is 38 × 46 μm2. The FT is achieved by changing the chemical potential of graphene material. The performance of the proposed THz antenna has been investigated, and the impacts of several conducting materials like gold, aluminum, copper, and graphene and dielectric materials like Rogers RT/duroid 5880, polyamide, quartz, and SiO2 are explored. The proposed THz antenna provides three operating bands. The frequency of operation in Band‐1 is 2.51–5.05 THz, Band‐2 is 5.99–7.43 THz, and Band‐3 is 7.94–9.63 THz. The bandwidth in Band‐1, Band‐2, and Band‐3 are 2.54, 1.44, and 1.69 THz, respectively. The % of impedance bandwidth in Band‐1, Band‐2, and Band‐3 are 67.19%, 24.02%, and 21.28% respectively. The proposed antenna has a maximum peak gain of 5 dBi. The proposed antenna is optimized through various ML algorithms like random forest (RF), extreme gradient boosting (XGB), K‐nearest neighbor (KNN), decision tree (DT), and artificial neural network (ANN). The RF algorithm gives more than 99% accuracy compared to other ML algorithms and accurately predicts the S11 of the proposed antenna. The proposed THz antenna would be suitable for applications related to imaging, medical, sensing, and ultra‐speed short‐distance communication applications in the THz region.

The conductivity of single walled nanotube films is investigatedwith a combination of the Maxwell-Garnett (MG) model and theDrude-Lorentzian (DL) model in the Terahertz region. A theoretical fitfor Jeon's experiment is given and a decrease of the realconductivity with increasing frequency is predicted. Meanwhile, the MGand DL models are also discussed for different samples.

Active broadband terahertz (THz) polarization manipulation devices are challenging to realize, but also of great demand in broadband terahertz systems. Vanadium dioxide (VO2) shows a promising phase transition for active control of THz waves and provides broadband polarization characteristics when integrated within grating-type structures. We creatively combine a VO2-based grating structure with a total internal reflection (TIR) geometry providing a novel interaction mechanism between the electromagnetic waves and the device, to realize a powerful active broadband THz polarization-controlling device. The device is based on a Si-substrate coated with a VO2 layer and a metal grating structure on top, attached to a prism for generating the TIR condition on the Si-VO2-grating interface. The grating is connected to electrodes for electrically switching the VO2 between its insulating and conducting phases. By properly selecting the incident angle of the THz waves, the grating direction, and the incident polarization state, we first achieved a broadband intensity modulator under a fused silica prism with an average modulation depth of 99.75% in the 0.2-1.1 THz region. Additionally, we realized an active ultra-broadband quarter-wave converter under a Si prism that can be switched between a 45° linear rotator and a quarter wave converter in the 0.8-1.5 THz region. This is the first demonstration of an active quarter-wave converter with ultra-broad bandwidth performance. Our work shows a highly flexible and multifunctional polarization-controlling device for broadband THz applications.

Part 1 Fundamentals of terahertz technology for imaging, sensing and communications: Optoelectronic techniques for the generation and detection of terahertz waves Transmission and propagation of terahertz waves in plastic waveguides Fundamental aspects of surface plasmon polaritons at terahertz frequencies Fundamental aspects of terahertz near-field imaging and sensing Room temperature terahertz detectors Terahertz wireless communications. Part 2 Recent progress and novel techniques in terahertz technology: Terahertz bio-sensing techniques Terahertz array imagers: Towards the implementation of terahertz cameras with plasma-wave-based silicon MOSFET detectors Resonant field enhancement of terahertz waves in subwavelength plasmonic structures Fiber-coupled terahertz time-domain spectroscopy systems (THz-TDS) State-of-the-art in terahertz continuous wave photomixer systems Novel techniques in terahertz near-field imaging and sensing Terahertz nano-devices and nano-systems Terahertz integrated devices and systems Terahertz frequency metrology based on frequency comb techniques Semiconductor material development for terahertz applications. Part 3 Applications of terahertz technology: Terahertz applications in tomographic imaging and material spectroscopy: A review Terahertz applications in aerospace industry Terahertz applications in the wood products industry Terahertz applications in the pharmaceutical industry and science Terahertz applications in art conservation Applications of terahertz technology in the semiconductor industry.

Pure rotational spectroscopy of potassium chloride, KCl, up to 930 GHz and isotopically invariant analysis of KCl and NaCl