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 2016 terahertz science and technology roadmap

The terahertz range possesses significant untapped potential for applications including high-volume wireless communications, noninvasive medical imaging, sensing, and safe security screening However, due to the unique characteristics and constraints of terahertz waves, the vast majority of these applications are entirely dependent upon the availability of beam control techniques Thus, the development of advanced terahertz-range beam control techniques yields a range of useful and unparalleled applications This article provides an overview and tutorial on terahertz beam control The underlying principles of wavefront engineering include array antenna theory and diffraction optics, which are drawn from the neighboring microwave and optical regimes, respectively As both principles are applicable across the electromagnetic spectrum, they are reconciled in this overview This provides a useful foundation for investigations into beam control in the terahertz range, which lies between microwaves and infrared

/pdf/tutorial-terahertz-beamforming-from-concepts-to-realizations-3f8yiao7kl.pdf

Tutorial: Terahertz beamforming, from concepts to realizations

Fuel cells offer the possibility of zero-emissions electricity generation and increased energy security. We review here the current status of solid oxide (SOFC) and polymer electrolyte membrane (PEMFC) fuel cells. Such solid electrolyte systems obviate the need to contain corrosive liquids and are thus preferred by many developers over alkali, phosphoric acid or molten carbonate fuel cells. Dramatic improvements in power densities have been achieved in both SOFC and PEMFC systems through reduction of the electrolyte thickness and architectural control of the composite electrodes. Current efforts are aimed at reducing SOFC costs by lowering operating temperatures to 500–800 °C, and reducing PEMFC system complexity be developing ‘water-free’ membranes which can also be operated at temperatures slightly above 100 °C.

Fuel cell materials and components

Fabrication of sub-micron surface structures on copper, stainless steel and titanium using picosecond laser interference patterning

In this roadmap article, we have focused on the most recent advances in terahertz (THz) imaging with particular attention paid to the optimization and miniaturization of the THz imaging systems. Such systems entail enhanced functionality, reduced power consumption, and increased convenience, thus being geared toward the implementation of THz imaging systems in real operational conditions. The article will touch upon the advanced solid-state-based THz imaging systems, including room temperature THz sensors and arrays, as well as their on-chip integration with diffractive THz optical components. We will cover the current-state of compact room temperature THz emission sources, both optolectronic and electrically driven; particular emphasis is attributed to the beam-forming role in THz imaging, THz holography and spatial filtering, THz nano-imaging, and computational imaging. A number of advanced THz techniques, such as light-field THz imaging, homodyne spectroscopy, and phase sensitive spectrometry, THz modulated continuous wave imaging, room temperature THz frequency combs, and passive THz imaging, as well as the use of artificial intelligence in THz data processing and optics development, will be reviewed. This roadmap presents a structured snapshot of current advances in THz imaging as of 2021 and provides an opinion on contemporary scientific and technological challenges in this field, as well as extrapolations of possible further evolution in THz imaging.

Roadmap of Terahertz Imaging 2021.

Picosecond-Laser 4-Beam-Interference Ablation as a Flexible Tool for Thin Film Microstructuring

Multilevel phase Fresnel lenses (MPFLs) with a high numerical aperture for 0.58 THz frequencies were developed. The components based on a monocrystalline silicon wafer are prepared by patterning by a high-speed industrial-scale laser direct writing (LDW) system. Two consistent series of the terahertz-MPFLs with phase quantization levels varying between 2 and the continuous kinoform shape for the focal lengths of 5 and 10 mm were produced employing inherent flexibility of the LDW fabrication process. The focusing performance was studied at the optimal 0.58 THz frequency using a Gaussian beam profile and scanning 2D intensity distribution with a terahertz detector along the optical axis. The efficiency of the terahertz-MPFL was found to be dependent of the number of subzones. The position and orientation angles of the patterned plane of the silicon wafer were considered to reduce the effect of standing waves formation in the experiment.

Terahertz multilevel phase Fresnel lenses fabricated by laser patterning of silicon.

Laser interference ablation in silicon using femto-, pico-, and nanosecond pulses was investigated. The experimental and computational results provide information about nanoscale thermal diffusion during the ultra-short laser-matter interaction. The temperature modulation depth was introduced as a parameter for quality assessment of laser interference ablation. Based on the experiments and calculations, a new semi-empirical formula which combines the interference period with the laser pulse duration, the thermal modulation depth and the thermal diffusivity of the material was derived. This equation is in excellent agreement with the experimental and modelling results of laser interference ablation. This new formula can be used for selecting the appropriate pulse duration for periodical structuring with the required resolution and quality.

Nanoscale thermal diffusion during the laser interference ablation using femto-, pico-, and nanosecond pulses in silicon

Fibonacci or bifocal terahertz (THz) imaging is demonstrated experimentally employing a silicon diffractive zone plate in continuous wave mode. Images simultaneously recorded in two different planes are exhibited at 0.6 THz frequency with the spatial resolution of wavelength. Multifocus imaging operation of the Fibonacci lens is compared with a performance of the conventional silicon phase zone plate. Spatial profiles and focal depth features are discussed varying the frequency from 0.3 to 0.6 THz. Good agreement between experimental results and simulation data is revealed.

Fibonacci terahertz imaging by silicon diffractive optics.

Interference of several identical laser beams allows for the fabrication of periodical structures over large areas. We present two complementary techniques for the patterning of thin films using the laser beam interference and direct ablation of material. The first one is based on precise control of the laser fluence relative to the ablation threshold of the film. Additional means for pattern management are provided by separate control of the intensity and phase of the interfering laser beams. Interference of four or six laser beams permits fabrication of periodical arrays of circular holes. Flexibility and sensitivity to the process parameters of the method beyond of simple hole arrays are shown for the thin Cr film ablation. Another versatile means for pattern generation in the films is the scanning of the periodic laser beam distribution over a workpiece in a sub-period area. By translation of the sample in small steps between laser shots, the structures with their shape independent of the interfering beam intensity distribution can be fabricated by overlapping the ablated holes. Slit- and cross-shaped periodical structures were fabricated using direct structuring with interfering nanosecond and femtosecond pulses. The thin-film structures fabricated by both techniques can be used as spectrally-selective elements for terahertz and infrared radiation.

https://iopscience.iop.org/article/10.1088/0960-1317/23/9/095034/pdf

Simonas Indrišiūnas

Papers

Picosecond-Laser 4-Beam-Interference Ablation as a Flexible Tool for Thin Film Microstructuring

Terahertz multilevel phase Fresnel lenses fabricated by laser patterning of silicon.

Nanoscale thermal diffusion during the laser interference ablation using femto-, pico-, and nanosecond pulses in silicon

Fibonacci terahertz imaging by silicon diffractive optics.

Two complementary ways of thin-metal-film patterning using laser beam interference and direct ablation