Table of contents

Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.

The rhenium-based layered dichalcogenide ReSe₂ crystallizes in a distorted triclinic structure which results in unique, anisotropic electronic and optical properties. This, along with a weak layer-dependence of band gap has made ReSe₂ a subject of intense contemporary research interest. However, there has been no agreement on the exact crystal structure of this material, or knowledge of its thermal properties like the melting point. In this work, we perform single crystal, Laue, and powder diffraction measurements on high-quality ReSe₂ crystals synthesized using a modified Bridgman technique. We confirm the presence of triclinic symmetry ($P\bar{1}$-space group) and support the view that that ReSe₂ has a distorted CdCl₂-type structure (rather than Cd(OH)₂ as initially proposed) and obtain lattice parameter values of a  =  6.5791(8) Å, b  =  6.6897(10) Å, and c  =  6.7013(11) Å. Further, thermal measurements on these crystals show a clear endothermic peak at around 1115 °C pointing to a melting transition, and show no other phase transitions up to 1300 °C.

Recent developments in the fabrication of lithium niobate (LiNbO₃) structures down to the nanoscale opens up novel applications of this versatile material in nonlinear optics. Current nonlinear optical studies in sub-micron waveguides are mainly restricted to the generation of second and third harmonics. In this work, we demonstrate the generation and waveguiding of the sum-frequency generation (SFG) signal in a single LiNbO₃ nanowire with a cross-section of 517 nm  ×  654 nm. Furthermore, we enhance the guided SFG signal 17.9 times by means of modal phase matching. We also display tuning of the phase-matched wavelength by varying the nanowire cross-section and changing the polarization of the incident laser. The results prove that LiNbO₃ nanowires can be successfully used for nonlinear wave-mixing applications and assisting the miniaturization of optical devices.

The realization of photoelectrochemical tandem cells for efficient solar-to-hydrogen energy conversion is currently impeded by the lack of inexpensive, stable, and efficient photocathodes. The family of sulfide chalcopyrites (CuIn_xGa_1−xS₂) has recently demonstrated a remarkable stability and performance even when prepared by solution-based routes that potentially lower the cost of fabrication. However, the photovoltage delivered by the photocathodes is still well-below the attainable values, a classical limitation linked to a large density of surface states in these materials. In the present work, we show that the identity of halide present during the growth of the solution-processed CuIn_0.3Ga_0.7S₂ (CIGS) thin-films governs the overall performance by directing the crystal growth and the passivation of surface states. Replacing chlorine by iodine leads to CIGS photocathodes that deliver photocurrents of 5 mA cm⁻² (at 0 V versus RHE) and a turn-on voltage of 0.5 V versus RHE without charge extracting overlayer nor any sign of deterioration during stability test.

This article presents a kinetic Monte-Carlo study of thermally activated magnetisation dynamics in clusters of statistically distributed magnetic nanoparticles. The structure of clusters is assumed to be of fractal nature, consistently with recent observations of magnetic particle aggregation in cellular environments. The computed magnetisation relaxation decay and frequency-dependent hysteresis loops are seen to significantly depend on the fractal dimension of aggregates, leading to accelerated magnetisation relaxation and reduction in the size of hysteresis loops as the fractal dimension increases from one-dimensional-like to three-dimensional-like clusters. Discussed are implications for applications in nanomedicine, such as magnetic hyperthermia or magnetic particle imaging.

La_0.7Sr_0.3MnO₃ manganite thin films are interesting since they have a fully spin-polarized conduction band at room temperature and this opens the way for applications in electronics. An important issue is their magnetic heterogeneity, which is very difficult to detect. We address here the heterogeneity detection issue in two epitaxial LSMO thin films (57 nm and 90 nm thick) on Si substrate fabricated by reactive molecular beam epitaxy (MBE) deposition. Combining three complementary analytic techniques, we measured structural and magnetic behavior of these films. The high frequency ferromagnetic resonance behavior observed in these two LSMO samples put in evidence a standard dynamic behaviour in the case of the homogeneous material and an uncommon multi-mode behavior in the heterogeneous bi-layered film. The multi-mode behavior can be attributed to the presence of two magnetic sub-layers inside the LSMO film. Indeed, transmission electron microscopy observations and neutron reflectivity measurements are essential to give a microscopic description of the structure and intrinsic magnetic homo/heterogeneity of the composite film.

A series of FeNiN magnetic films with different substrate temperatures were fabricated using radio frequency magnetron oblique sputtering technology. The static and dynamic magnetic properties of samples were studied systematically. The results show that substrate temperature affects the magnetic properties of samples significantly. With the rise of substrate temperature, the coercivity of samples decreases firstly, and then increases when the substrate temperature is higher than 200 °C. The sample fabricated at 200 °C exhibits a well-defined in-plane uniaxial anisotropy and soft magnetic properties. Dynamic magnetic properties of samples were studied by using vector network analyzer and electron spin resonance. All of the results indicate that FeNiN film with good soft magnetic properties can be obtained at a substrate temperature of 200 °C.

Very high magnetic anisotropies have been theoretically predicted for strained Fe–Co(–X) and indeed several experiments on epitaxial thin films seemed to confirm strain induced anisotropy enhancement. This study presents a critical analysis of the different contributions to perpendicular anisotropy: volume, interface and surface anisotropies. Tracing these contributions, thickness series of single layer films as well as multilayers with Au–Cu buffers/interlayers of different lattice parameters have been prepared. The analysis of their magnetic anisotropy reveals a negligible influence of the lattice parameter of the buffer. Electronic effects, originating from both, the Au–Cu interface and the film surface, outrange the elastic effects. Surface anisotropy, however, exceeds the interface anisotropy by more than a factor of three. A comparison with results from density functional theory suggests, that the experimentally observed strong perpendicular surface anisotropy originates from a deviation from an ideal oxide-free surface. Accordingly, tailored Fe–Co–X/oxide interfaces may open a route towards high anisotropy in rare-earth free materials.

We report theoretical and experimental investigation on a single right-angled trapezoid metallic nanoslit for efficient unidirectional generation of surface plasmon polaritons (SPPs) under normal incidence. The propagated SPPs intensity ratio in two directions is sensitive to the taper angle and metal thickness. Significant intensity ratio at the same propagation distances from the respective slit edges in opposite directions is demonstrated. We believe that the proposed compact unidirectional SPPs generator has high potential for applications in nanolithography and photonic integration.

The electrical stability of transparent conductive oxides is an important criterion for evaluating their performance, especially when they are employed at elevated temperatures or in long-term operation. In this work, indium-doped ZnO thin films with various doping concentrations were prepared by RF sputtering. The electrical properties, electrical thermal stability, and time stability of films with differing indium contents were investigated. The results showed that the degradation of the films' conductivity is primarily attributable to the reduction in oxygen vacancies at high temperatures under oxygenated conditions. The aggregation of indium atoms, which cannot replace Zn³⁺ cations at temperatures above 200 °C, can improve the carrier concentration. Further reaction with oxygen degraded the performance of the films due to the formation of insulating oxides. Long-term analysis showed the IZO films to have quite stable electrical properties. Their conductivity remained almost unchanged after two months at room temperature under normal atmospheric conditions.

The coexistence of nonvolatile memory switching and volatile threshold switching in a single device is of importance for suppressing the sneak-path currents in crossbar resistive memory architectures. This study demonstrates that the combination of a thin film of TiO₂ with hafnium nanoparticles in Au/Ti/TiO₂/Hf nanoparticles/Au device configuration enables conversion between memory switching and volatile threshold switching by adjusting the current compliance through the materials stack. The presence of hexagonal closed packed Hf nanoparticles, a synthesis of which has not been reported before, is critical for the device operation that exhibits beneficial features as it is forming free and operates at low voltage and power consumption. Analysis of measured current–voltage (I–V) characteristics reveal a filamentary nature of switching phenomena and present operating similarities with electrochemical metallization cells suggesting that Hf metal atoms and not only oxygen vacancies are responsible for conductive filament formation.

This paper presents a novel design concept, which is verified by analytical and simulation results, of a single-mode small-core terahertz Bragg fibre exhibiting the properties of low loss and low dispersion. Conventionally, a single-TE₀₁-mode Bragg fibre requires a large core and many cladding layer periods to achieve a significant propagation loss discrimination between the desired mode and other unwanted competing modes. The use of a second-order bandgap in this paper completely eliminates this requirement, and enhances propagation loss discrimination using just a small core with a diameter at least 50% smaller than the conventional design and only four cladding layer periods. Furthermore, a generalized half-wavelength condition is proposed, promoting the manipulation of photonic bandgap for Bragg fibre. The TE₀₁ mode has a null point in the electric field close to the boundary interface between the core and the cladding, and this phenomenon has been exploited to minimize the impact of support bridges, which mechanically maintain the air gaps, on the propagation loss of the fibre. Finally, we propose a novel design of a tightly confined single-TE₀₁-mode small-air-core Bragg fibre with propagation loss and group velocity dispersion less than 1.2 dB m⁻¹ and  −0.6 ps/THz/cm, respectively, between frequencies of 0.85 THz and 1.15 THz.

We have systematically investigated a general approach to optimize the optical performances of a 2D array of crossing metal nanoparticle (MNP) thin film. These functionalized metasurface MNPs are designed for use as wavelength-selection filters in high-sensitivity infrared spectroscopic plasmonic sensors. The effects of different structural parameters corresponding to the gap-enhancement and bonded transmittance modes on MNP arrays are studied. Two types of sensor configurations based on gold MNP arrays are thoroughly investigated by using the finite element method. The calculated transmittance spectra of the proposed metasurfaces demonstrate near-infrared transmittance dips with a sensitivity range of 120–700 nm RIU⁻¹ in a dielectric constant (ɛ) ranging from 1.0–3.0. We illustrate that it is possible to increase their sensitivity in the detection of chemical and biological substances. The proposed metasurfaces supporting both core-medium sensitivity and bonded-mode resonances are desirable for label-free sensing applications.

We study the variation of electronic property for zigzag-edge phosphorene nanoribbons (ZPNRs) under a perpendicular electric field (PEF). Using the tight-binding Hamiltonian combined with the surface lattice Green's function (GF) approach, we show that the response of edge states to PEF for a N-ZPNR with even- or odd-N (number of zigzag chains) is qualitatively different. The field opens a gap between two edge bands near the Fermi energy for even-N ribbons, but for odd-N ones where the two edge bands are always nearly degenerated. This difference is originally from that the Stark-effect-induced energies at the upper and lower edges for even- and odd-N ZPNRs are different due to the peculiar lattice structure of phosphorene. In consequence, the electronic densities are more localized at the edges driven by the field for even-N ZPNRs but not for odd-N ones. This even–odd effect is also reflected in conductance, which indicates that the odd-N ZPNRs may be more suitable for the usage of field-effect transistor.

Room temperature random lasing is demonstrated from a GaN epitaxy film with defect pits that result from growth imperfection. The optical coherence feedback is attributed to the formation of closed-loop paths of light through the scattering effect of the defect pits, which can avoid the difficulty of fabricating an artificial cavity. The random lasing action was also investigated through near and far-field patterns that imaged onto the CCD camera. In addition, the angle distribution of the laser beam was illustrated by use of an angle-resolved spectrometer. The lasing threshold, based on the weak scattering diffusive mode of GaN, is about one order of magnitude lower than that strong scattering random laser (RL). Hence, the results in this paper represent a low-cost technique to realize GaN-based laser diodes without the fabrication difficulty of cavity facets that result from the hardness of the sapphire substrate.

To absorb the infrared part of the solar spectrum more efficiently, narrow bandgap hydrogenated nanocrystalline germanium (nc-Ge:H) thin films were fabricated by radio frequency plasma enhanced chemical vapor deposition at a low temperature of 180 °C. While the incubation layer of the nc-Ge:H was reduced to less than 5 nm by using the ultra-high hydrogen dilution, the negative photoconductivity behavior was still observed as the thickness of nc-Ge:H up to 30 nm. Therefore, as the best candidate for solar cells application, the nc-Ge:H (20 nm)/nc-Si:H (10 nm) periodic multilayer structure was prepared and used as the absorption layer of nc-Ge:H nip solar cells. More importantly, the spectral sensitivities extending into the wavelength of 1450 nm were achieved in the nc-Ge:H nip solar cells. In addition, the annealing for the nc-Ge:H nip solar cells was carried out. While the overall short circuit current density of the device is improved after 500 °C annealing, the spectral sensitivities in the infrared region is decreased due to the the coalescence of Ge crystallites.

Heterostructures of atomically thin 2D materials could have improved physical, mechanical and chemical properties as compared to its individual components. Here we report, the effect of heterostructure coatings of hBN and MoS₂ on the corrosion behavior as compared to coatings employing the individual 2D layer compositions. The poor corrosion resistance of MoS₂ (widely used as wear resistant coating) can be improved by incorporating hBN sheets. Depending on the atomic stacking of the 2D sheets, we can further engineer the corrosion resistance properties of these coatings. A detailed spectroscopy and microscopy analysis has been used to characterize the different combinations of layered coatings. Detailed DFT based calculation reveals that the effect on the electrical properties due to atomic stacking is one of the major reasons for the improvement seen in corrosion resistance.

We present a direct method to measure the oxygen stoichiometry in an oxide film with an accuracy of about 2%. It is based on a combination of ¹⁸O annealing and high mass resolution secondary ion mass spectroscopy. Calibration has been done on a LaNiO₃ film whose electrical properties dependence on oxygen stoichiometry are well documented. The method is illustrated with a series of LaNiO₃ films grown on SrTiO₃ substrates prepared with different oxygen stoichiometries. The large influence of the surface state on oxygen exchange is evidenced in films grown on different substrate orientations or coated with a thin layer of LaAlO₃. Oxygen surface exchange and bulk diffusion is then discussed for both LaNiO₃ and SrVO₃ films.

Highly porous ferroelectric ceramics possess remarkably less polarizability than dense ceramics; instead they display high tunability of various physical properties. Particularly, the shape and orientation of pores as well as the total porosity exhibit a great effect on the polarization-switching dynamics. In the present work, finite-element simulations of the electric-field distributions and related statistical distributions of local switching times are analysed and compared with the switching characteristics of porous lead zirconate titanate ceramics, extracted from the experiment by means of the inhomogeneous field mechanism model of polarization switching. Surprisingly, the simulated statistical field-distributions turn out to be virtually independent of the pore-size distribution; however, they are sensitive to the anisometric shape and orientation of the pores. Additionally, they exhibit notable broadening with increasing porosity; an effect confirmed by experimental observations.

Understanding interfacial electronic properties between electron donors and acceptors in hybrid optoelectronic solar cells is crucial in governing the device parameters associated with energy harvesting. To probe the electronic localized states at an electron donor/acceptor interface comprising a representative hybrid solar cell, we investigated the electrical contact properties between Al-doped zinc oxide (AZO) and poly (3-hexylthiophene) (P3HT) using AZO as the source and drain electrodes, pumping carriers from AZO into P3HT. The injection efficiency was evaluated using the transmission line method (TLM) in combination with field effect transistor characterizations. Highly conductive AZO films worked as the source and drain electrodes in the devices for TLM and field effect measurements. A comparable contact resistance difference between AZO/P3HT/AZO and Au/P3HT/Au structures contradicts the fact that a far larger energy barrier exists for electrons and holes between AZO and P3HT compared with between P3HT and Au based on the Schottky–Mott model. It is suggested that band to band tunneling accounts for the contradiction through the initial hop from AZO to P3HT for hole injection. The involvement of the tunneling mechanism in determining the contact resistance implies that there is a high density of electronic traps in the organic side.

Based on first principles calculations, we investigate the geometry, electronic structure, and diffusion mechanism of Na ions in Na₃MnPO₄CO₃ using density functional theory with a Hubbard potential correction. Our results suggest that the structure of Na₃MnPO₄CO₃ can be deintercalated with more than one Na ion, and that the removal of a Na ion can form a bound polaron. We find that our calculations of the intercalation voltages for the redox couples Mn²⁺ /Mn³⁺ and Mn³⁺ /Mn⁴⁺ agree very well with the experimental data. In addition, we demonstrate that Na in Na₃MnPO₄CO₃ can diffuse in three directions with low activation energy barriers, allowing a fast charging rate.

Superconducting strips coated with boron were engineered with a view to subnuclear particle detection. Combining the characteristics of boron as a generator of α-particles (as a consequence of neutron absorption) and the ability of superconducting strips to act as resistive switches, it is shown that fabricated Nb–boron and NbN–boron strips represent a promising basis for implementing neutron detection devices. In particular, the superconducting transition of boron-coated NbN strips generates voltage outputs of the order of a few volts thanks to the relatively higher normal state resitivity of NbN with respect to Nb. This result, combined with the relatively high transition temperature of NbN (of the order of 16 K for the bulk material), is an appealing prospect for future developments. The coated strips are meta-devices since their constituting material does not exist in nature and it is engineered to accomplish a specific task, i.e. generate an output voltage signal upon α-particle irradiation.

A comparative study of optical properties and laser induced heating effects in Tm³⁺, Yb³⁺ doped YNbO₄, GdNbO₄ and LaNbO₄ phosphors is presented in this work. The phosphors were structurally characterized by x-ray diffraction and scanning electron microscopy measurements. The vibrational structures of the phosphors were studied using FTIR measurements. The optical band gaps (E_g), calculated from the Wood and Tauc plot, are found to be 3.78, 4.50 and 3.27 eV for YNbO₄, GdNbO₄ and LaNbO₄, respectively. The luminescence property (downshifting (DS) and upconversion (UC)) was studied in the powder and the pellet forms of the phosphor samples. The DS emission of Tm³⁺ doped ANbO₄ phosphors (λ_ex  =  265 nm) consists of broad blue emission due to (NbO₄)³⁻ group overlapped with sharp peaks due to f–f transition of Tm³⁺ ion with most prominent emission one in the case of YNbO₄ phosphor. The DS emission is comparatively more intense in the pellet form. The NIR excited UC emission spectra of Tm³⁺, Yb³⁺ co-doped ANbO₄ phosphors contain intense blue and NIR emissions due to the Tm³⁺ ion. Contrary to the DS study, the best UC result is found for LaNbO₄ phosphor in pellet form. Further, the laser induced heating effect in UC emission with respect to laser pump power and irradiation time has also been studied in Tm³⁺, and Yb³⁺ co-doped ANbO₄ phosphors. It was found to be more effective in the case of the YNbO₄ host where the heating effect is more prominent in the powder sample. We discuss the mechanisms involved in these observations in detail.