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

Biomedical Applications of Terahertz Spectroscopy and Imaging.

The terahertz region lies between the microwave and infrared regions of the electromagnetic spectrum such that it is strongly attenuated by water and very sensitive to water content. Terahertz radiation has very low photon energy and thus it does not pose any ionization hazard for biological tissues. Because of these characteristic properties, there has been an increasing interest in terahertz imaging and spectroscopy for biological applications within the last few years and more and more terahertz spectra are being reported, including spectroscopic studies of cancer. The presence of cancer often causes increased blood supply to affected tissues and a local increase in tissue water content may be observed: this acts as a natural contrast mechanism for terahertz imaging of cancer. Furthermore the structural changes that occur in affected tissues have also been shown to contribute to terahertz image contrast. This paper introduces terahertz technology and provides a short review of recent advances in terahertz imaging and spectroscopy techniques. In particular investigations relating to the potential of terahertz imaging and spectroscopy for cancer diagnosis will be highlighted.

The potential of terahertz imaging for cancer diagnosis: A review of investigations to date

Several classes of non-planar metallic and dielectric waveguides have been proposed in the literature for guidance of terahertz (THz) or T-ray radiation. In this review, we focus on the development of dielectric waveguides, in the THz regime, with reduced loss and dispersion. First, we examine different THz spectroscopy configurations and fundamental equations employed for characterization of THz waveguides. Then we divide THz dielectric waveguides into three classes: solid-core, hollow-core, and porous-core waveguides. The guiding mechanism, fabrication steps, measured loss, and dispersion are presented for the waveguides in each class in chronological order. The goal of this review is to compare and contrast the current solutions for guiding THz radiation.

Terahertz dielectric waveguides

Combined with terahertz (THz) time-domain spectroscopy, THz near-field microscopy based on an atomic force microscope is a technique that, while challenging to implement, is invaluable for probing low-energy light-matter interactions of solid-state and biomolecular nanostructures, which are usually embedded in background media Here, we experimentally demonstrate a broadband THz pulse near-field microscope that provides subsurface nanoimaging of a metallic grating embedded in a dielectric film The THz near-field microscope can obtain broadband nanoimaging of the subsurface grating with a nearly frequency-independent lateral resolution of 90 nm, corresponding to ∼λ/3300, at 1 THz, while the AFM only provides a flat surface topography

Subsurface Nanoimaging by Broadband Terahertz Pulse Near-Field Microscopy

Rectifiers have been used to detect electromagnetic waves with very low photon energies In these rectifying devices, different methods have been utilized, such as adjusting the bandgap and the doping profile, or utilizing the contact potential of the metal-semiconductor junction to produce current flow depending on the direction of the electric field In this paper, it is shown that the asymmetric application of nano-electrodes to a metal-semiconductor-metal (MSM) structure can produce such rectification characteristics, and a terahertz (THz) wave detector based on the nano-MSM structure is proposed Integrated with a receiving antenna, the fabricated device detects THz radiation up to a frequency of 15 THz with responsivity and noise equivalent power of 108 V/W and [Formula: see text] respectively, estimated at 03 THz The unidirectional current flow is attributed to the thermionic emission of hot carriers accelerated by the locally enhanced THz field at the sharp end of the nano-electrode This work not only demonstrates a new type of THz detector but also proposes a method for manipulating ultrafast charge-carrier dynamics through the field enhancement of the nano-electrode, which can be applied to ultrafast photonic and electronic devices

Terahertz rectifier exploiting electric field-induced hot-carrier effect in asymmetric nano-electrode.

Guided-wave propagation of sub-ps terahertz (THz) pulses in a highly birefringent plastic photonic crystal fiber was studied by using a THz time domain spectroscopy technique. The plastic photonic crystal fiber was fabricated by using high density polyethylene tubes and solid filaments. The fabricated THz plastic photonic crystal fibers exhibit an extremely large birefringence of ~2.1×10-2, which is almost one order of magnitude larger than that of previously reported photonic crystal fibers.

Highly birefringent terahertz polarization maintaining plastic photonic crystal fibers

Photoconductive antennas with nano-structured electrodes and which show significantly improved performances have been proposed to satisfy the demand for compact and efficient terahertz (THz) sources. Plasmonic field enhancement was previously considered the dominant mechanism accounting for the improvements in the underlying physics. However, we discovered that the role of plasmonic field enhancement is limited and near-field distribution of bias field should be considered as well. In this paper, we clearly show that the locally enhanced bias field due to the size effect is much more important than the plasmonic enhanced absorption in the nano-structured electrodes for the THz emitters. Consequently, an improved nano-electrode design is presented by tailoring bias field distribution and plasmonic enhancement. Our findings will pave the way for new perspectives in the design and analysis of plasmonic nano-structures for more efficient THz photonic devices.

/pdf/bias-field-tailored-plasmonic-nano-electrode-for-high-power-43akqyd0xg.pdf

Bias field tailored plasmonic nano-electrode for high-power terahertz photonic devices.

We present quantitative coherent measurements of scattering pulses and spectra in terahertz apertureless near-field microscopes. Broadband near-field image contrasts for both amplitude and phase spectra are measured directly from time-domain scattering signals with an unprecedentedly high single-scan signal-to-noise ratio (∼48 dB), with approach curves for both short (<200 nm) and long (up to 82 μm) ranges. By using the line dipole image method, we obtain quantitative broadband THz imaging contrasts with nanoscale resolution.

Kiwon Moon

Papers

Subsurface Nanoimaging by Broadband Terahertz Pulse Near-Field Microscopy

Terahertz rectifier exploiting electric field-induced hot-carrier effect in asymmetric nano-electrode.

Highly birefringent terahertz polarization maintaining plastic photonic crystal fibers

Bias field tailored plasmonic nano-electrode for high-power terahertz photonic devices.

Quantitative coherent scattering spectra in apertureless terahertz pulse near-field microscopes