Infrared, Millimeter, and Terahertz Waves

Planar Lens–Based Ultra-Wideband Dielectric Rod Waveguide Antenna for Tunable THz and Sub-THz Photomixer Sources Abstract In this manuscript, the use of dielectric rod waveguide antenna (DRW) with an embedded planar lens is proposed as a highly directional alternative to an electrically large hyper-hemispheric silicon lens for emission at millimeter and sub-millimeter wave frequencies. DRW antennas radiate properly if only the fundamental mode is excited to the structure. Since photomixer-based terahertz sources excite many modes, single-lobe radiation patterns are obtained only for lower frequencies of their potential working band. The use of embedded planar lenses is proposed for rectifying the wavefront phase and suppressing such higher-order modes in DRW, allowing an ultra-wideband operation.

Gain and Bandwidth Improvement of Empty Substrate Integrated Waveguide H-plane Horn Antenna at W-band Abstract A high-gain broadband empty substrate integrated waveguide (ESIW) H-plane horn antenna constructed by four low-cost printed circuit board (PCB) layers is proposed in this paper. The empty structure is fabricated by using high-precision laser engraving technology. A slotted rectangular dielectric load (SRDL) is introduced by simply extending the substrates, resulting a 4-dB gain enhancement and a 31% bandwidth improvement. The proposed antenna gives a measured realized gain at 14.5-dBi level with a relative impedance bandwidth larger than 19.6% (|S11| ≤ − 10 dB). The proposed antenna features the merits of low cost, high gain, and broad bandwidth.

Attenuation of THz Beams: A “How to” Tutorial Abstract Attenuation of ultrashort THz pulses poses a significant technological challenge due to the broadband nature of such light pulses. Several methods exist for this purpose, including crossed wire grid polarizers, high refractive index, high resistivity silicon wafers, and ultrathin metal films. In this review, we discuss the operational principles of these methods, and highlight some of the advantages and potential pitfalls of the methods. We describe the limits of high-frequency operation of wire grid polarizers, relevant for contemporary ultra-broadband THz sources in air photonics. We discuss the effects of multiple reflections and interference in sequences of silicon wafers for attenuation, and finally discuss the potential of using ultrathin metallic films for broadband attenuation.

Full-wave Analysis of Plasma-Loaded Coaxial Cavity with Wedge-Shaped Corrugations on the Insert Abstract High voltages and the existence of scant amount of gas inside gyrotron overmoded cavities leads to the creation of plasma. The presence of magnetic field leads to this plasma being magnetized. In plasma waveguides, space charge modes, cyclotron modes, and EH and HE modes can exist. Inside the gyrotron cavity, magnetized plasma affects the propagation of desired and competing modes. Therefore, for accurate prediction of gyrotron properties, plasma analysis is necessary. For high frequency megawatt class gyrotron, coaxial cavities provide advantages such as reduction in mode competition and reduction of voltage depression. In this paper, full-wave analysis is applied to plasma-filled waveguide with wedge-shaped corrugations on the insert.

THz Properties of Typical Woods Important for European Forestry Abstract Terahertz (THz) properties of seventeen types of wood typically growing within the European area are investigated. The samples include both coniferous and deciduous timbers. The determined properties can be useful for further THz studies on samples consisting completely or partially of wood, for example, art objects. Especially in the field of art conservation, THz spectroscopy is an excellent non-destructive evaluation method, which shows considerable analysis potential.

On the Potential of THz Time-Domain Spectroscopy to Identify Typical Ancient Egyptian Embalming Materials Abstract We investigated the dielectric properties of typical embalming materials used in ancient Egypt in the THz frequency range. In addition, we evaluated the potential to identify these materials based on a principal component analysis of the spectroscopic data. Based on this, we estimated the composition of two unknown ancient bandages of mummified bodies.

Guided-Mode Filters for Terahertz Frequencies Fabricated by 3D Printing Abstract In this article, we present the design and characterization of the first 3D-printed guided-mode filter for terahertz frequencies. The device consist on a diffractive grating fabricated on top of a rectangular dielectric waveguide by 3D printing. Finite difference numerical modeling shows this device acts as a filter at a frequency that can be geometrically engineered and tuned around such frequency by varying the radiation incidence angle. Three devices were designed to operate at 200 GHz, 250 GHz, and 300 GHz; the devices were fabricated and successfully characterized using terahertz time-domain spectroscopy.

Novel Pentagram THz Hollow Core Anti-resonant Fiber Using a 3D Printer Abstract A novel pentagram THz hollow core anti-resonant fiber (HC-ARF) is proposed and fabricated by a 3D printer in this paper. By utilizing the advantage of 3D print technology, a novel structure of one ring triangular air holes is introduced in the cladding and thus a pentagram hollow core is formed, which breaks through the limitation of the material absorption and effectively lowers the propagation loss of THz wave. Numerical results show that the loss as low as 0.02 cm−1 can be obtained for the proposed fiber within the THz frequency range from 0.5 to 2 THz. The fiber samples with different length of 10 cm and 15 cm are fabricated and measured experimentally. Experimental results demonstrate that the minimum loss of 0.025 cm−1 is obtained at 1.94 THz. Furthermore, the proposed fiber also has the advantage of excellent resistance to structural deformation.

3D Printing Metallised Plastics as Terahertz Reflectors Abstract 3D printing of new metallised plastics is investigated as a means to realise rapid creation of reflective optics for the terahertz regime. The suitability of three commercially available candidate materials was tested across a range of systems which span from 0.2 to 10 THz. Simple reflective planes were printed and characterised by spectroscopy and microscopy. Samples were polished which is shown to give a dramatic improvement to the reflectivity for these materials. The results indicate that metallised plastics have potential uses in low frequency rapid prototyping of reflective optics.

W-band Aperture-Type Scanning Near-Field Microscopy Using Tapered Plastic Probe Abstract An aluminum-coated PMMA tapered probe with 50 μm aperture was employed in near-field imaging at 110 GHz. The probe was chosen for near-field imaging experiments according to finite-difference time-domain electromagnetic simulation, analysis of antenna resonance, and impedance matching consideration. Imaging of a printed circuit board with a repeated structure of 450-μm-wide metal strips spaced by 550-μm-wide dielectric demonstrated a spatial resolution of 15 μm (λ/200), which is not just 100 times below the diffraction limit, but is 3 times smaller than the aperture size. Subsurface buried defect in a plastic plate (polytetrafluoroethylene defect in polyester fiber glass, 0.5 mm below the top surface of the plate) was also imaged, with a spatial resolution of 1.5 mm, and positioning error of the defect less than 0.5 mm.