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

Graphene plasmons were predicted to possess ultra-strong field confinement and very low damping at the same time, enabling new classes of devices for deep subwavelength metamaterials, single-photon nonlinearities, extraordinarily strong light-matter interactions and nano-optoelectronic switches. While all of these great prospects require low damping, thus far strong plasmon damping was observed, with both impurity scattering and many-body effects in graphene proposed as possible explanations. With the advent of van der Waals heterostructures, new methods have been developed to integrate graphene with other atomically flat materials. In this letter we exploit near-field microscopy to image propagating plasmons in high quality graphene encapsulated between two films of hexagonal boron nitride (h-BN). We determine dispersion and particularly plasmon damping in real space. We find unprecedented low plasmon damping combined with strong field confinement, and identify the main damping channels as intrinsic thermal phonons in the graphene and dielectric losses in the h-BN. The observation and in-depth understanding of low plasmon damping is the key for the development of graphene nano-photonic and nano-optoelectronic devices.

Highly confined low-loss plasmons in graphene-boron nitride heterostructures

Plasmon polaritons are hybrid excitations of light and mobile electrons that can confine the energy of long-wavelength radiation at the nanoscale. Plasmon polaritons may enable many enigmatic quantum effects, including lasing1, topological protection2,3 and dipole-forbidden absorption4. A necessary condition for realizing such phenomena is a long plasmonic lifetime, which is notoriously difficult to achieve for highly confined modes5. Plasmon polaritons in graphene—hybrids of Dirac quasiparticles and infrared photons—provide a platform for exploring light–matter interaction at the nanoscale6,7. However, plasmonic dissipation in graphene is substantial8 and its fundamental limits remain undetermined. Here we use nanometre-scale infrared imaging to investigate propagating plasmon polaritons in high-mobility encapsulated graphene at cryogenic temperatures. In this regime, the propagation of plasmon polaritons is primarily restricted by the dielectric losses of the encapsulated layers, with a minor contribution from electron–phonon interactions. At liquid-nitrogen temperatures, the intrinsic plasmonic propagation length can exceed 10 micrometres, or 50 plasmonic wavelengths, thus setting a record for highly confined and tunable polariton modes. Our nanoscale imaging results reveal the physics of plasmonic dissipation and will be instrumental in mitigating such losses in heterostructure engineering applications.The fundamental limits to plasmon damping in graphene are determined using nanoscale infrared imaging at cryogenic temperatures, and plasmon polaritons are observed to propagate over 10 micrometres in high-mobility encapsulated graphene.

Fundamental Limits to graphene plasmonics

Self-consistent optical constants of SiO2 and Ta2O5 films have been obtained for their relevance in optical coatings from the near ultraviolet to the near infrared spectral ranges, where they are transparent and have a high refractive index contrast Particular attention has been paid to wavelengths close to and shorter than each material cutoff The far and the extreme ultraviolet ranges are also covered here, where few (SiO2) or almost no optical constant data (Ta2O5) were available for films of these materials This work is aimed at filling the lack of self-consistent sets of optical constants with data in a very broad spectral range, which can be widely applied in multilayer design for the everyday use of these materials in multilayer coatings Oxide films were deposited by reactive electron-beam evaporation onto various sorts of substrates at 573 K Transmittance, reflectance, and ellipsometry measurements were performed for each oxide in spectral intervals jointly covering from the extreme ultraviolet to the near infrared; starting with these measurements along with extrapolations, an iterative and double Kramers-Kronig analysis procedure has been followed to obtain a self-consistent set of optical constants per material With the final data sets, we have satisfactorily reproduced the experimental measurements Global data self-consistency was successfully evaluated through sum rules, and local consistency at each photon energy range was also evaluated through a novel sum-rule method which involves window functions

Self-consistent optical constants of SiO_2 and Ta_2O_5 films

Transparent heat regulating (THR) materials and coatings for energy saving window applications: Impact of materials design, micro-structural, and interface quality on the THR performance

The invention discloses a low-temperature two-step method for preparing a composite anatase type titanium dioxide visible light catalyst, and belongs to the visible light catalyst. The preparation method comprises the following steps of: 1, adding titanium-containing precursor into alcohol, uniformly mixing and then adding a dopant, fully stirring, then adding an inhibitor and de-ionized water respectively, and performing water bath heating to prepare colloid containing titanium and doping element; and 2, cooling the prepared colloid to room temperature, transferring into a high-pressure-resistant container, adding a thermal decomposition type compound into the colloid, then quickly sealing the container, heating to perform reaction, preserving heat, obtaining precipitate after the reaction is finished, filtering the precipitate and washing with the de-ionized water and then drying to prepare the composite anatase type titanium dioxide visible light catalyst. The method has the advantages that: 1, the catalyst is prepared under the condition of 80-100 DEG C, and the energy consumption is favorably reduced during preparation; and 2, subsequent high-temperature treatment is not required in the method, the defect of high probability of losing the doping element through the high-temperature treatment is avoided, and the photo-catalytic performance is favorably improved.

Low-temperature two-step method for preparing composite anatase type titanium dioxide visible light catalyst

Correlations between the oxygen deficiency and the laser damage resistance of different oxide films

Preparation of high laser-induced damage threshold Ta2O5 films

Ta2O5 films are deposited on fused silica substrates by electron beam evaporation method. The optical property, x-ray photoelectron spectroscopy, band gap and nanosecond laser-induced damage threshold (LIDT) of the films before and after annealing are studied. It is found that the existence of an oxygen vacancy results in the decrease of the transmittance, refractive index, both macroscopic band gap and microscopic band gap, and the LIDT of Ta2O5 films. If the oxygen vacancy forms, the macroscopic band gap decreases 2%. However, when the oxygen vacancy forms the microscopic band gap decreases 73% for crystalline Ta2O5 and 77% for amorphous Ta2O5. The serious decrease of microscopic band gap may significantly increase the absorbance of the micro-area in Ta2O5 films when irradiated by laser, thus the damage probability increases. It is consistent with our experimental results that the LIDT of the as-deposited Ta2O5 films is 7.3J/cm2, which increases 26% to 9.2J/cm2 when the oxygen vacancy is eliminated after annealing.

http://iopscience.iop.org/article/10.1088/0256-307X/29/8/084207/pdf

Effect of Oxygen Vacancy on the Band Gap and Nanosecond Laser-Induced Damage Threshold of Ta 2 O 5 Films

The invention relates to a preparation method for a tantalum oxide film with a high laser damage threshold under a high-temperature environment and belongs to the preparation method for an optical film. The preparation method comprises the following steps: plating a tantalum oxide film on a clean substrate according to a double ion beam sputtering method; performing post-processing on a prepared film in laser pre-processing and annealing manner, thereby achieving better functions of repairing film defect and relieving film stress; and preparing a laser film capable of being applied to high-temperature environment. The preparation method provided by the invention has the advantages that: 1) the film prepared according to the double ion beam sputtering method is compact, the characteristic of easiness in moisture absorption of a loosened film prepared according to an electronic beam preparation method is improved, and the stability is better; 2) the laser pre-processing and annealing combined method is adopted, thereby overcoming the limitation caused by traditionally adopting a single method, and being beneficial to greatly increasing the threshold; and 3) the film prepared according to the method can be used under a high-temperature environment with the highest temperature at 350 DEG C, and the problem of the prior art that only the laser film used under room temperature can be prepared is solved.

Jiongtian Liu

Papers

Low-temperature two-step method for preparing composite anatase type titanium dioxide visible light catalyst

Correlations between the oxygen deficiency and the laser damage resistance of different oxide films

Preparation of high laser-induced damage threshold Ta2O5 films

Effect of Oxygen Vacancy on the Band Gap and Nanosecond Laser-Induced Damage Threshold of Ta 2 O 5 Films

Preparation method for tantalum oxide film with high laser damage threshold under high-temperature environment