In recent years, when reading newspapers and journals or watching TV, one has been able to find feature presentations dealing with the prospects of terahertz (THz) technology and its potential impact on market applications. THz technology aims to fill the THz gap in the electro-magnetic spectrum in order to make the THz frequency regime, which spans the two orders of magnitude from 100 GHz to 10 THz, accessible for applications. From the lower-frequency side, electronics keeps pushing upwards, while photonic approaches gradually improve our technological options at higher frequencies. The popular interest reflects the considerable advances in research in the THz field, and it is mainly advances in the photonic branch, with the highlight being the development of the THz quantum cascade laser, which in recent years have caught the imagination of the public, and of potential users and investors.
This special issue of Semiconductor Science and Technology provides an overview of key scientific developments which currently represent the cutting edge of THz photonic technology. In order to be clear about the implications, we should define exactly what we mean by 'THz photonic technology', or synonymously 'THz photonics'. It is characterized by the way in which THz radiation (or a guided THz wave) is generated, namely by the use of lasers. This may be done in one of two fundamentally different schemes: (i) by laser action in the terahertz frequency range itself (THz lasers), or (ii) by down-conversion processes (photomixing) involving the radiation of lasers which operate in the visible, near-infrared or infrared spectral ranges, either in pulsed or continuous-wave mode.
The field of THz photonics has grown so considerably that it is out of the question to cover all its aspects in a single special issue of a journal. We have elected, instead, to focus our attention on two types of development with a potentially strong impact on the THz field: first, on significant advances of the technology itself, and second, on specific applications considered capable of fostering the transmutation of THz technology as a whole into a market technology. We decided for reasons of conciseness to leave out technologies which require more than table-top equipment (free-electron lasers, THz sources based on electrons accelerated to relativistic speed, etc) as well as fairly mature techniques (such as backward-wave oscillators which, although they are not strictly lasers, also exhibit gain). More difficult was the decision not to consider fascinating ideas for novel sources and detectors which until now have existed only on paper or have just entered the process of fundamental investigation. As we ourselves are working on such a concept (the Bloch-gain laser), we are fully aware of the fact that some of these ideas may have a strong impact on the field of THz photonics in the near future.
After selection of the topics we wanted to cover, we contacted colleagues who are prominent in their respective fields of research and are grateful that most of them responded positively, expressing their willingness to share their knowledge with the readers of this journal. They took care not only to describe their own work but to give ample reference to the status of their respective specialized field of work.
Before summarizing the contributions, we want to address all colleagues in the field who feel that they should have been asked to contribute but were not. To you we want to apologize. We can only hope for your understanding of the constraints of this endeavour.
The collection of invited contributions is grouped into five topics. The first is entitled 'Pulsed THz Systems' and contains four papers dealing with the state of the art in source and detector development of measurement systems employing femtosecond Ti:sapphire lasers. The first paper, by Planken et al, describes the state of the art of the most common types of optoelectronic THz systems, namely those with femtosecond lasers operating at high repetition rate (∼100 MHz). The system described by Planken et al was initially optimized for high-speed pixel-by-pixel THz imaging, which they do not describe here but rather focus on developments in THz microscopy. The second paper, by Kübler et al, presents pioneering work towards ultra-wide-bandwidth THz pulses which exhibit spectral content reaching far into the mid-IR, tremendously widening the covered frequency range, and hence shortening the time resolution, of THz spectroscopy. The third paper, by Löffler et al, deals with the state of the art in THz measurement systems relying on amplified laser pulses. Finally, Krotkus et al focus on low-temperature-grown (LT) GaAs, arguably the most important material for ultrafast optoelectronic switching and present in many THz sources and detectors, and in other emerging materials of similar kind.
This leads directly to the second topic of this collection of papers, 'Continuous-Wave Photomixing Technology', based on THz-wave generation by down-conversion of continuous-wave (cw) laser radiation. This newer branch of THz photonics opens the possibility of obtaining tunable narrow-band THz radiation and of detecting it with high signal-to-noise ratio at room temperature. CW photomixing has received much attention over the last few years mainly because it has the potential to provide the compact and low-cost THz measurement systems needed for market applications beyond the scientific realm, with the sources of light for mixing being semiconductor (or fibre) lasers with or without optical amplifiers. Six papers outline recent developments in this subfield. We should also point towards a seventh paper, by Kawase et al, which is to be found in the section on 'Chemical and Biochemical Recognition', and which discusses an interesting hybrid approach generating tunable quasi-cw THz radiation with the help of nanosecond laser pulses.
Of the six papers mentioned, the first, by Tani et al, summarizes the state of the art which relies on single-point LT-GaAs photoconductive antennae as THz sources and detectors driven by semiconductor lasers operating at wavelengths around 0.8 m. As laser-induced damage to the sources currently limits the achievable output power, researchers have early-on tried to develop travelling-wave mixers with distributed THz-power generation. Michael describes the status quo of this approach. The replacement of lifetime-limited photoconductive antennae with transit-time-limited p–i–n photomixers can be another way towards higher conversion efficiency if the RC frequency roll-off can be controlled. Döhler et al introduce a novel lumped-element device, a quasi-ballistic cascaded p–i–n photomixer, which promises a significantly better conversion efficiency than standard LT-GaAs photomixers at all frequencies. At laser wavelengths in the telecommunication windows, especially at 1.55 m, where InP-based compound semiconductors exhibit an extremely favourable electron mobility, p–i–n mixers have already established themselves as a powerful THz source. The group of Ito et al have set the standards here and describe their achievements in the fourth paper of this subtopic. The challenge remains to develop a similarly effective optoelectronic detector for these operating wavelengths. This, as Brown et al show in their contribution, turns out to be mainly a materials research issue, and as novel ultrafast materials such as those containing ErAs clusters emerge, so do sensitive detectors and photoconductive sources. The section closes with a paper by Hoffmann et al, which is more speculative in its scope but targets a fascinating goal: THz photomixing directly in a dual-colour semiconductor laser itself, and thus the ultimate miniaturization of a THz source based on photomixing.
The third topic is 'THz Laser Technology' and addresses direct laser action at THz frequencies. Hübers et al guide the reader into the topic with a paper presenting the state of the art and the potential of lasers based on germanium and silicon. Tredicucci et al then review their development of the THz quantum cascade laser, the THz radiation source which more than any other currently transforms the field of THz technology. Their paper and the following one by Hu et al, who have introduced major improvements of the laser scheme and the waveguiding technology, present the state of the art of these lasers and discuss their future potential. One of the main challenges will be to raise the operation temperature further, and to bring it as close to room temperature as possible. These improvements will require a more advanced theoretical understanding of how these lasers work. The papers of Hu et al and the following one by Indjin et al address this question and describe the present status of theory.
With this, we leave THz sources and detectors and come to research targeting the application of THz radiation. We have, given the space restraints and the fact that the focus of this journal is on semiconductor technology, decided to address only a single field of strong current interest, 'Chemical and Biochemical Recognition'. Other developing areas, such as THz radar and tomography, aiming at the sensing and diagnostics of surfaces and the inner structure of THz-transparent objects, or semiconductor wafer diagnostics and various other THz measurement modalities, are not covered. Not at all because we might consider them to be less important; quite on the contrary we are certain that they will make a big impact in real-world applications. The field of chemical and biochemical recognition was singled out because in the recent past there was controversial discussion as to what THz spectroscopic signatures to expect, especially from soft and solid chemical or biochemical matter, and the time seems to have come now to review some of the hard data obtained in the mean time.
The topic covers the identification and analysis of chemical and biochemical substances, with a strong motivation stemming from the fact that the knowledge gained by this research opens up broad application areas in such lucrative markets as pharmaceutics, genetics, medical imaging and security screening. It may be interesting to note that until one or two years ago, a buzzword of applications-related research would have been 'biomedical imaging', especially of cancerous tissue or teeth, but for whatever reason none of the researchers contacted by us were interested to represent this subfield here, which seems to indicate that it is not considered to be a hot topic at present.
The first two papers in this section, by Fischer et al and Shen et al, set the stage with an overview of chemical recognition in absorption and reflection spectroscopy, respectively. Kawase et al then demonstrate drug identification with their unique quasi-cw parametric THz system. While the scope of this paper is already security-oriented, Federici et al go further along this line by discussing not only drug detection but also sensing of explosives and weapons. The section closes with a paper by Nagel et al on the detection of DNA-binding states and on the system improvements implemented by this group on the way towards cost-effective sensing.
This brings us to the final theme, 'THz Microscopy, Imaging, and Photonic Crystals'. The three papers in this section deal with three different aspects of THz technology which represent current progress in the use of THz radiation. The first paper, by Cho et al, as well as the one by Planken et al in the section on 'Pulsed THz Systems', discusses developments aiming towards THz microscopy, and reviews the latest results in achieving ultrahigh spatial resolution at THz frequencies. The next paper, by Karpowicz et al, comes back to the issue of THz imaging, which was already addressed by authors of papers in the preceding section, and presents a systematic comparison between two imaging and sensing modalities, time-domain optoelectronic imaging and more conventional GHz all-electronic imaging. This study of high practical interest is followed by the final contribution, by Jian et al, which discusses the development and characterization of photonic crystals for THz frequencies.
We hope that this special issue will provide the readers of this journal with a good overview of the current status of THz photonics. We also hope that we, the Guest Editors and the authors of the papers, will succeed in conveying the fascination of this field of research which comes equally from its interdisciplinarity and from the fact that fundamental and applied research go hand in hand, strongly impacting on each other. For those working in this field it is highly gratifying to help make the last under-used window of the electro-magnetic spectrum accessible for applications.
