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Novel CuB2O4–Cu3B2O6 substrates doped with three Li-based sintering aids, Li2WO4, LiBO2 and Li2CO3, were prepared using the LTCC (low temperature cofired ceramic) technology.

Purpose This paper aims to report on fabrication procedure and presents microstructure and dielectric behaviour of LiZn0.92Cu0.08PO4 ceramic material with Li2CO3 as a sintering aid. Design/methodology/approach Substrates based on LiZn0.92Cu0.08PO4 with Li2CO3 addition were prepared via solid-state synthesis, doping, milling, pressing and sintering. Characterization of the composition, microstructure and dielectric properties was performed using X-ray diffractometry, energy dispersive spectroscopy, scanning electron microscopy, impedance spectroscopy in the 100 Hz to 2 MHz range and time-domain spectroscopy in the 0.1–3 THz range. Findings Doped LiZnPO4 ceramic, which exhibits a low dielectric constant of 5.9 at 1 THz and low sintering temperature of 800 °C, suitable for low temperature co-fired ceramics (LTCC) technology, was successfully prepared. However, further studies are needed to lower dielectric losses by optimising the doping level, synthesis and sintering conditions. Originality/value Search for new low dielectric constant materials applicable in LTCC technology and optimization of processing are essential tasks for developing modern microwave circuits. The dielectric characterization of doped LiZnPO4 ceramic in the terahertz range, which was performed for the first time, is crucial for potential millimetre-wave applications of this substrate material.

Novel optical materials and manufacturing methods are especially important in the fast growing terahertz (THz) market, where new solutions for shaping and redirecting of radiation are widely investigated. An innovative method of manufacturing refractive and diffractive optical elements (DOEs) for THz spectral range using commercially available and cost-effective devices and materials is presented. The 3D-printing method is used to produce molds, which are then filled with paraffin. The optical elements obtained with this method outperform many commercial ones, especially in higher frequencies. Moreover, a different diffractive element design (as kinoforms of higher order) is proposed to match the resolution limitations of 3-D printing.

Coordination polymers with multiple non-centrosymmetric phases have sparked substantial research efforts in the materials community. We report the synthesis and properties of a hitherto unknown cadmium dicyanamide coordination polymer comprising benzyltrimethylammonium cations (BeTriMe+). The room-temperature (RT) crystal structure of [BeTriMe][Cd(N(CN)2)3] (BeTriMeCd) is composed of Cd centers linked together by triple dca-bridges to form one-dimensional chains with BeTriMe+ cations located in void spaces between the chains. The structure is polar, the space group is Cmc21, and the spontaneous polarization in the c-direction is induced by an arrangement of BeTriMe+ dipoles. BeTriMeCd undergoes a second-order phase transition (PT) at T1 = 268 K to a monoclinic polar phase P21. Much more drastic structural changes occur at the first-order PT observed in DSC at T2 = 391 K. Raman data prove that the PT at T2 leads to extensive rearrangement of the Cd-dca coordination sphere and pronounced disorder of both dca and BeTriMe+. On cooling, the HT polymorph transforms at T3 = 266 K to another phase of unknown symmetry. Temperature-resolved second harmonic generation (TR-SHG) studies at 800 nm confirm the structural non-centrosymmetry of all the phases detected. Optical studies indicate that BeTriMeCd exhibits at low temperatures an intense emission under 266 nm excitation. Strong temperature dependence of both one-photon excited luminescence and SHG response allowed for the demonstration of two disparate modes of optical thermometry for a single material. One is the classic ratiometric luminescence thermometry employing linear excitation in the ultraviolet region while the other is single-band SHG thermometry, a thus far unprecedented subtype of nonlinear optical thermometry. Thus, BeTriMeCd is a rare example of a dicyanamide framework exhibiting polar order, SHG activity, photoluminescence properties and linear and nonlinear optical temperature sensing capability.

Rapid development of terahertz technologies causes constant search for new possibilities of making optical elements, needed for proper beam shaping. Large optical elements are often used in THz setups to collect as much power as possible. Unfortunately, traditional refractive optical lenses significantly absorb already weak signal. This is the reason for searching for new solutions in the form of diffractive optical elements (DOEs). For higher terahertz frequencies however, it is challenging to find an appropriate material as well as uncomplicated manufacturing methods. In this article, we present a higher-order kinoform (HOK) made of paraffin, cast from 3D-printed mold, efficiently working for 0.6 THz.

We report density functional theory (DFT) studies of vibrational modes for benzyltrimethylammonium cations (BeTriMe+) as well as THz, IR and Raman studies of [BeTriMe][M(dca)3(H2O)] (dca = N(CN)2−, dicyanamide; M = Mn2+, Co2+, Ni2+) and their anhydrous analogues. These studies show that the anhydrous BeTriMeMn and BeTriMeNi have the same or very similar structures and loss of water molecules leads to significant changes in the metal-dicyanamide frameworks. In particular, the number of dca modes decreases, suggesting increase of crystal symmetry, probablly related with decrease in the number of non-equivalent dca bridges from two to one. Although it is possible that dehydration leads to a replacement of the coordinate Mn-O (Ni-O) bonds by Mn-N (Ni-N) bonds, wherein N atoms come from the C≡N groups of previously non-bridged dca units, reversibility of the dehydration process indicates that such new bonds are either not formed or are very weak. The anhydrous Mn and Ni compounds undergo similar reversible phase transitions to lower symmetry phases. The driving force for these transitions is most likely ordering of dca linkers but this process is accompanied by weak distortion of the metal-dicyanamide frameworks. In the case of BeTriMeCo, the loss of water molecules also leads to significant changes in the cobalt-dicyanamide framework. However, the structure of this analogue is different from the structures of the Mn and Ni counterparts, the number of unique dca linkers is preserved and the dehydration process is irreversible, suggesting more drastic rearrangement of the metal-dicynamide framework.

Two diffractive optical elements are used to create a compact raster THz scanning setup in reflective configuration. The first one focuses the radiation into the small focal spot on the sample, while the second one collects reflected radiation and focuses it on the detector. To assure small size of the setup and large apertures of optical elements, structures work in the off-axis geometry. Thus, the focal spot is formed 100 mm after and 60 mm below the optical axis of the element, which measures 75 mm in diameter. The designed iterative algorithm allows further minimization of these values.

Tape casting was used to prepare green tapes based on pure and doped Zn4B6O13 ceramics that are destined for LTCC high frequency substrates. In order to broaden the sintering temperature range, three additives which were based on Zn2SiO4–Li2TiO3, AlF3–CaB4O7–Li2TiO3 and Zn2SiO4–MgTiO3, were selected using heating microscope studies. Differential thermal analysis was used to establish the optimal firing profiles. The microstructure and composition of the sintered ceramics were examined by scanning electron microscopy, energy dispersive spectroscopy and the X-ray diffraction method. Owing to the low sintering temperatures and good compatibility with commercial Ag thick films, multilayer substrates based on green tapes made of the developed materials can be cofired at 900–920 °C. The dielectric properties were studied in a broad range up to the THz frequencies. The most advantageous properties for LTCC substrates, such as high densification degree, a low dielectric permittivity (6.9 at 1 THz) and a low dissipation factor (0.005 at 1 THz) were exhibited by the Zn4B6O13 ceramic to which 10% Zn2SiO4 and 5% MgTiO3 were added.

The paper describes the results of experimental research on the mobile verification of travellers based on fingerprints. Three-day tests were carried out at the border crossing in Terespol, Poland. The developed system automatically acquires personal and biometric data (fingerprints) from the Polish biometric passport, determines their quality and compares with the live data collected from the traveller. In addition, the system measures the time of individual stages of the process and determines total transaction time. For total number of correctly scanned travellers equal to 128, false acceptance rate equals to 0, while rejection rate is less than 1%. The average transition time of border check was 37 seconds.

Results of a theoretical and experimental investigation of THz detection of randomly-oriented metal particles smaller than 1 mm embedded in pressboard are reported and critically compared with X-ray scanner results. A theoretical description based on Gaussian beam theory and a transfer function approach enabled simulation of signals transmitted around and reflected from real particles and was able to predict signal detectability. Simulated signals and detection limits were compared with experimental transmission and reflection time domain spectroscopy (TDS) raster scanning data. Simulated and measured pulse amplitude and shape were in agreement. Reflection mode was superior to transmission mode in terms of both particle visibility and hit ratio. Imaging of particles was also possible in “reflection in transmission” mode, which can be used to complement classical transmission mode. X-ray images did not detect the smallest particles sufficiently and, therefore, could not be used as reference data for comparison with THz data. The results demonstrate that TDS reflection scanning is able to determine the position of small metal particles and the technique could be possibly used for quality control of pressboard insulation.

The article discusses the reasons for the decrease in interest in the traditional library. The solutions proposed in the literature of the subject and examples of practical solutions used in European libraries are presented.

We investigated six prospective explosive materials in the terahertz range using time-domain spectroscopy. A family of energetic azotetrazolate salts and two caged nitramines were studied. A number of distinct spectral features were observed in the 0.8–3.2 THz frequency range. In transmission configuration in ambient temperature, we determined the absorption coefficient and the refractive index of the materials, which were compressed as pellets. Because the visibility of some absorption peaks was not clear, additionally we performed characterization of these materials in a temperature range from −175°C to 0°C, which resulted in highlighting peaks with low amplitude. Because the considered explosives are insensitive to compression, we also measured them using an attenuated total reflection (ATR) technique, in which sample preparation is easier than with pressed pellets. The absorption peaks measured by ATR agree well with those determined in transmission. This suggests that ATR also can be used for identification of these classes of materials.

[EN] A 3D printer was used to realize custom-made diffractive THz lenses. After testing several materials, phase binary lenses with periodic and aperiodic radial profiles were designed and constructed in polyamide material to work at 0.625 THz. The nonconventional focusing properties of such lenses were assessed by computing and measuring their axial point spread function (PSF). Our results demonstrate that inexpensive 3D printed THz diffractive lenses can be reliably used in focusing and imaging THz systems. Diffractive THz lenses with unprecedented features, such as extended depth of focus or bifocalization, have been demonstrated., Ministerio de Economia y Competitividad (MINECO) (FIS2011-23175); Generalitat Valenciana (PROMETEOII/2014/072); National Center for Research and Development in Poland (LIDER/020/319/L-5/13/NCBR/2014).

Diffractive optical elements such as hyperbolic lens and quasi-spherical lens converting a divergent terahertz beam into the focal line segment perpendicular to the optical axis was designed. Due to the fact that the length of the line segment was longer than the aperture of the designed elements the non-paraxial approach was used. The structures were designed for the narrowband application as kinoform elements. The theoretical approach, computer simulations and experimental results are presented. In the imaging system, the intensity of the radiation in the focal plane was measured by THz point-like detector based on InP heterojunction bipolar transistor.

Date of Conference: 5-9 May 2014 Conference name: SPIE Sensing Technology + Applications, 2014 - Proceedings - Terahertz Physics, Devices, and Systems VIII: Advanced Applications in Industry and Defense We present results on the comparison of different THz technologies for the detection and identification of a variety of explosives from our laboratory tests that were carried out in the framework of NATO SET-193 THz technology for stand-off detection of explosives: from laboratory spectroscopy to detection in the field under the same controlled conditions. Several laser-pumped pulsed broadband THz time-domain spectroscopy (TDS) systems as well as one electronic frequency-modulated continuous wave (FMCW) device recorded THz spectra in transmission and/or reflection. © 2014 SPIE.

We report on the developed system for the detection of the objects hidden under people’s clothes. The system is based on a commercially available THz camera equipped with specially designed software for image processing and fusion. Additionally, a camera for visible light registers the image that is later processed and combined with the THz image by means of the designed fusion algorithm. Such final fused image significantly increases the visibility of the detected elements. A very important aspect of the usefulness of various image processing techniques is the proper evaluation of the image quality. We propose a combination of two image quality assessment methods as a methodology used for the quality estimation of fused images and for comparing image fusion algorithms. Moreover, we designed and constructed a moving, wirelessly-controlled thermal phantom for testing of the functioning of the cameras in the terahertz range. The terahertz camera operating at 0.25 THz and the standard thermal camera were used for tests. We compared the images taken for the phantom and a man and we obtained satisfactory similarities both for naked and dressed targets with hidden items (guns, knives, bombs). The temperature stability of the phantom is sufficient for the evaluation of those cameras.