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.

https://iopscience.iop.org/article/10.1088/1361-6463/50/4/043001/pdf

The 2017 terahertz science and technology roadmap

The 2016 terahertz science and technology roadmap

We present SLIM, a new optical method measuring optical pathlength changes of 0.3 nm spatially and 0.03nm temporally. SLIM combines two classic ideas in light imaging: Zernike’s phase contrast microscopyand Gabor’s holography.

Spatial light interference microscopy (SLIM)

Magnetic topological insulators are narrow-gap semiconductor materials that combine non-trivial band topology and magnetic order1. Unlike their nonmagnetic counterparts, magnetic topological insulators may have some of the surfaces gapped, which enables a number of exotic phenomena that have potential applications in spintronics1, such as the quantum anomalous Hall effect2 and chiral Majorana fermions3. So far, magnetic topological insulators have only been created by means of doping nonmagnetic topological insulators with 3d transition-metal elements; however, such an approach leads to strongly inhomogeneous magnetic4 and electronic5 properties of these materials, restricting the observation of important effects to very low temperatures2,3. An intrinsic magnetic topological insulator—a stoichiometric well ordered magnetic compound—could be an ideal solution to these problems, but no such material has been observed so far. Here we predict by ab initio calculations and further confirm using various experimental techniques the realization of an antiferromagnetic topological insulator in the layered van der Waals compound MnBi2Te4. The antiferromagnetic ordering  that MnBi2Te4  shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to a ℤ2 topological classification; ℤ2 = 1 for MnBi2Te4, confirming its topologically nontrivial nature. Our experiments indicate that the symmetry-breaking (0001) surface of MnBi2Te4 exhibits a large bandgap in the topological surface state. We expect this property to eventually enable the observation of a number of fundamental phenomena, among them quantized magnetoelectric coupling6–8 and axion electrodynamics9,10. Other exotic phenomena could become accessible at much higher temperatures than those reached so far, such as the quantum anomalous Hall effect2 and chiral Majorana fermions3. An intrinsic antiferromagnetic topological insulator, MnBi2Te4, is theoretically predicted and then realized experimentally, with implications for the study of exotic quantum phenomena.

Prediction and observation of an antiferromagnetic topological insulator.

At the Institut fur Kernphysik of Mainz University a harmonic double-sided microtron (HDSM) has been built to extend the experimental capabilities for nuclear and particle physics experiments to higher excitation energies. This novel microtron variant accelerates the 0.855 GeV continuous wave (cw) electron beam of the established three-staged race track microtron (RTM) cascade MAMI B up to 1.5 GeV. It consists of two normal conducting linear accelerators (linacs) through which the electrons are guided up to 43 times by a pair of 90°-bending magnets at each end. For beam dynamical reasons the linacs operate at the harmonic frequencies of 4.90 and 2.45 GHz. The extended facility is called MAMI C. The relatively strong vertical defocussing due to the 45°-pole face rotations (Fig. 1) at both the entrance and exit of the segment-shaped bending magnets is compensated for all recirculations by a suitable field decay in the magnets towards higher orbits. As a consequence, the energy gain of the electrons has to decrease with increasing turn number to maintain coherent acceleration. This occurs by an appropriate phase slip of the electron bunches downwards the rf-waves during the acceleration process. In this paper the functional principle and the beam dynamical concept of the double-sided microtron (DSM) as well as the design and development of its main components are described. Finally, the results of first beam measurements taken after starting up in December 2006 are discussed.

The 1.5 GeV harmonic double-sided microtron at Mainz University

The Mainz Microtron MAMI is a cascade of three racetrack microtrons, delivering since 1991 a high-quality 855MeV, 100μA cw-electron beam for nuclear, hadron and radiation physics experiments. An energy upgrade of this facility to 1.5GeV by adding a Harmonic Double-Sided Microtron (HDSM) as a fourth stage is well underway and first beam is expected during the first half of 2006. A detailed description of the multiple recirculation scheme with normal conducting accelerator structures, the basis for the reliable operation of MAMI, is given and the historical development from MAMI A to MAMI B is described. The natural advancement to MAMI C by realizing a polytron of the next higher order, the HDSM, is covered in the last section and a first glimpse into the future of MAMI is given.

/pdf/the-mainz-microtron-mami-past-and-future-uxr0ep1vps.pdf

The Mainz Microtron MAMI --Past and future

In the quest for detailed spectroscopic insight into the electronic structure at solid surfaces in a large momentum range, we have developed an advanced experimental approach. It combines the 3D detection scheme of a time-of-flight momentum microscope with an optimized filling pattern of the BESSY II storage ring. Here, comprehensive data sets covering the full surface Brillouin zone have been used to study faint substrate-film hybridization effects in the electronic structure of graphene on Ir(111), revealed by a pronounced linear dichroism in angular distribution. The method paves the way to 3D electronic bandmapping with unprecedented data recording efficiency.

Multi-MHz time-of-flight electronic bandstructure imaging of graphene on Ir(111)

We present first ideas of a scheme to develop BESSY II into a variable electron pulse length storage ring. The final goal is, to fill BESSY II with short bunches of 1.5 ps length (rms) and long bunches of 15 ps length simultaneously in the presently applied user optics. All insertion devices are operated as usual, i.e. the helical undulators and 7 T-field insertions. Long bunches of 1.5 mA current per bunch, twice the value of the present user optics, are filled in each second bucket. The other buckets can be filled with short bunches of up to 0.8 mA. The lower current value is required to avoid increase in the bunch length and bunch energy spread, predicted by scaling laws. The total current is e.g. limited by the HOM damping capabilities of the sccavities and the machine impedances. This scheme is achievable with recent developments in sc-rf cavity technology driven by requirements of high current cw applications like for the energy recovering linacs (ERLs). These developments seem to make it feasible to install high gradient HOM damped multi-cell cavities in electron storage rings. With an appropriate choice of the cavity resonance frequencies a beating of the effective longitudinal focusing voltage at the stable fixed points can be introduced and leads to different zero current bunch lengths. Basic parameters of this scheme are presented.

Simultaneous long and short electron bunches in the bessy ii storage ring

https://link.aps.org/pdf/10.1103/PhysRevAccelBeams.21.113401

Andreas Jankowiak

Papers

The 1.5 GeV harmonic double-sided microtron at Mainz University

The Mainz Microtron MAMI --Past and future

Multi-MHz time-of-flight electronic bandstructure imaging of graphene on Ir(111)

Simultaneous long and short electron bunches in the bessy ii storage ring

Towards the operation of Cs-K-Sb photocathodes in superconducting rf photoinjectors