Interaction Of Terahertz Radiation With Semiconductor Lasers

Terahertz (THz) technology bears great potential in spectroscopy, imaging, material science, security screening and high-speed wireless communication. However, the generation of intensive, directional THz radiation has been difficult and the THz frequency range has long been considered the last final frontier of the electromagnetic spectrum. Recent advancement in optoelectronic terahertz generation techniques and high power electronic sources has helped to bridge the THz gap and has opened up a wealth of new applications for THz technology. However, there is still a major technical limitation in developing THz systems for mass markets, mainly due to the cost of THz hardware components including sources and detectors. In this regard, we investigated the use of semiconductor diode lasers as THz detectors as well as excitation sources for photomixers for THz generation. For THz detection, we investigated the interaction of semiconductor lasers with THz radiation. Intense THz radiation from different sources and at various frequencies was injected into the laser diode. The laser diode was operated in Littman configuration to ensure clean single mode operation in the near infrared. The charge carrier system in the semiconductor was expected to interact with the injected THz radiation and introduce nonlinear frequency mixing. This nonlinear mixing was to induce sidebands in the near infrared optical spectra and was to be analyzed with an optical spectrum analyzer. This may lead to the demonstration of a simple, cost effective and compact room temperature THz spectrometer since the distance between the emission line and the sidebands equals the incident THz frequency. Unfortunatly, due to unprecedented challenges the interaction of THz radiation with diode laser experiment was not successful. Another approach was to demonstrate a compact and cost effective THz source based on monolithic distributed Bragg reflector diode laser emitting two frequencies simulteneously. We successfully demonstrated 300 GHz continuous wave THz radiation, with fiber coupled ion implanted photoconductive antennas used as photomixing devices. The generated THz radiation was tunable via temperature adjustments and current injection. This approach provided a coarse tuning in the range of 286 GHz to 320 GHz. We successfully demonstrated its potential use in non-destructive plant moisture measurements of a leaf induced to drought stresses and for moisture monitoring in drying process of pieces of paper. Due to the fact that the tuning of the developed THz source was coarse, we proposed the use of a new diode which was electrically tunable for fine tuning of the generated THz frequency. The new diode offers optical beat signal adjustments via carrier injection to the DBR section using micro resistor heater integrated on top of the DBR segment. The optical beat tuning via carrier injection was fast and offers tuning which should be free from mode hopping. The injection current to the resistor heater can be adjusted between 0 to 350 mA, an optical beat adjustment of between 100 GHz-300 GHz was realized. This bandwidth was only limited with the overlap of the two modes at higher heater currents of 250 mA to 350 mA. THz radiation emission via photomixing in the range 100 GHz-300 GHz was successfully demonstrated, these results were in good agreement with the optical beat signal measurements. Finally, a simple spectrometer suitable for THz metrology measurements such as thickness determination of Polyethylene sample (PE) was realized and also its application in THz spectroscopy was demonstrated by the determination of the spectroscopic transmission characteristics of a THz filter. In summary, two compact THz sources emitting at 300 GHz were successfully demonstrated and this was a major milestone towards development of compact and cost effective THz system for mass market application.

The book describes distinctive features of the terahertz radiation interaction with various materials, including the Earth’s atmosphere, liquids, dielectrics, superconductors, semiconductors, metals and 2D-structures. It discusses all types of terahertz sources, including lasers, photoconductive antennas, semiconductor diode and transistor generators and amplifiers, and considers terahertz band vacuum devices. The text offers a comparative analysis of these devices and their perspectivity in the terahertz band, and describes the basic types of terahertz detectors. Also investigated here are the reasons for the appearance of the so-called “terahertz gap” and the means of increasing working frequency and output power of the sources of terahertz radiation. The book also considers the electromagnetics and electron-optical systems of vacuum terahertz devices and their manufacturing technology.

Intense Terahertz Excitation of Semiconductors presents the first comprehensive treatment of high-power terahertz applications to semiconductors and low-dimensional semiconductor structures. Terahertz properties of semiconductors are in the center of scientific activities because of the need of high-speed electronics. This research monograph brigdes the gap between microwave physics and photonics. It focuses on a core topic of semiconductor physics providing a full description of the state of the art of the field. _ The reader is introduced to new physical phenomena which occur in the terahertz frequency range at the transition from semi-classical physics with a classical field amplitude to the fully quantized limit with photons. The book covers a wide range of optical, optoelectronic, and nonlinear transport processes, presenting experimental results, clearly visualizing models and basic theories. Background information for future work and exhaustive references of current literature are given. A particularly valuable feature is through the discussion of various technical aspects of the terahertz range like the generation of high-power coherent radiation, optical components, instrumentation, and detection schemes of short intense radiation impulses. The book complements, for the first time in form of a monograph, previous books on infrared physics which dealt with low-power optical and opto-electronic processes. It will be useful not only to scientists but also to advanced students who are interested in terahertz research.

Terahertz (THz) radiation, which is electromagnetic radiation in a frequency int- val from 0.3 to 10 THz (1 mm–30 ?m wavelength), is the next frontier in science and technology. This band occupies a large portion of the electromagnetic sp- trum between the infrared and microwave bands. Basic research, new initiatives, and developments in advanced sensing and imaging technology with regard to the THz band remain unexplored compared to the relatively well-developed science and technology in the microwave and optical frequencies. Historically, THz technologies were used mainly within the astronomy c- munity for studying the background of cosmic far-infrared radiation, and by the laser-fusion community for the diagnostics of plasmas. Since the ?rst demonstration of THz wave time-domain spectroscopy in the late 1980s, there has been a series of signi?cant advances (particularly in recent years) as more intense THz sources and higher sensitivity detectors provide new opportunities for understanding the basic science in the THz frequency range.

Terahertz radiation - also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz - consists of electromagnetic waves within the ITU-designated band of frequencies from 0.3 to 3 terahertz. Wavelengths of radiation in the terahertz band correspondingly range from 1 mm to 0.1 mm. Because terahertz radiation begins at a wavelength of one millimeter and proceeds into shorter wavelengths, it is sometimes known as the submillimeter band, and its radiation as submillimeter waves, especially in astronomy. The book presents information about Terahertz science, Terahertz photodetectors and Terahertz Lasers. A special emphasis is given to room temperature operation of long wavelength photodetectors based on novel quantum dots. Moreover, a complete analysis of systems based on Quantum Cascade structures to detect far infrared wavelengths is provided. Finally, the book presents Terahertz laser principles considering multi-color lasers in this range of wavelengths. It is written as a background for graduate students in the Optics field.

The purpose of this book is two-fold. First, the various different methods of accessing the THz range are discussed, with a view to convince the reader that there have been qualitative and significant improvements over older, more conventional techniques. The text makes it clear that these improvements enable practical "real-world" applications of THz technology, in a manner which would not have been possible before. Second, the demonstrations and feasibility tests described serve as compelling evidence of the utility of such devices. Due to the unique characteristics of THz radiation and its interaction with materials, these devices have substantial advantages over other competing technologies in a number of different areas.

This book covers the latest advances in the techniques employed to manage the THz radiation and its potential uses. It has been subdivided in three sections: THz Detectors, THz Sources, Systems and Applications. These three sections will allow the reader to be introduced in a logical way to the physics problems of sensing and generation of the terahertz radiation, the implementation of these devices into systems including other components and finally the exploitation of the equipment for real applications in some different field. All of the sections and chapters can be individually addressed in order to deepen the understanding of a single topic without the need to read the whole book. The THz Detectors section will address the latest developments in detection devices based on three different physical principles: photodetection, thermal power detection, rectification. The THz Sources section will describe three completely different generation methods, operating in three separate scales: quantum cascade lasers, free electron lasers and non-linear optical generation. The Systems and Applications section will take care of introducing many of the aspects needed to move from a device to an equipment perspective: control of terahertz radiation, its use in imaging or in spectroscopy, potential uses in security, and will address also safety issues. The text book is at a level appropriate to graduate level courses up to researchers in the field who require a reference book covering all aspects of terahertz technology.

Der Terahertz (THz) Frequenzbereich hat in den letzten Jahren für Wissenschaft und Wirtschaft stark an Bedeutung gewonnen. In diesem Bereich des elektromagnetischen Spektrums, der aus Anwendersicht lange Zeit als unzugänglich betrachtet wurde, haben sich inzwischen eine Vielzahl spektroskopischer und bildgebender Verfahren etabliert. Auch Anwendungen aus der Sicherheitstechnik sowie der Telekommunikationsbranche operieren mittlerweile bei Frequenzen im hohen Gigahertz bis unteren THz-Bereich. Die vorliegende Dissertation behandelt zwei Aspekte, die für diese neuartigen Technologien von zentraler Bedeutung sind: die Detektion sowie die Erzeugung von THz-Strahlung. Hierbei kommen optoelektronische Techniken zum Einsatz, basierend auf der Anregung von Halbleitermaterialien mittels Femtosekunden-Lasern. Über den sogenannten THz-Frequenzkamm lassen sich auf diese Weise Dauerstrich-Thz-Quellen mit höchster Messgenauigkeit charakterisieren und räumlich aufgelöste Emissionsprofile der Quellen erstellen. Dies wird anhand zweier Detektionsverfahren, der elektro-optischen sowie der photoleitenden Detektion, untersucht. Die THz-Erzeugung wird über die Anregung kurzer Ströme im Halbleitermaterial realisiert. Hierbei lässt sich die Emissionsrichtung der THz-Felder allein über die Eigenschaften des Anregungslichtes kontrollieren, womit die erste rein optisch gesteuerte THz-Quelle demonstriert werden kann.

The reader will find here a timely update on new THz sources and detection schemes as well as concrete applications to the detection of Explosives and CBRN. Included is a method to identify hidden RDX-based explosives (pure and plastic ones) in the frequency domain study by Fourier Transformation, which has been complemented by the demonstration of improvement of the quality of the images captured commercially available THz passive cameras. The presented examples show large potential for the detection of small hidden objects at long distances (6-10 m). Complementing the results in the short-wavelength range, laser spectroscopy with a mid-infrared, room temperature, continuous wave, DFB laser diode and high performance DFB QCL have been demonstrated to offer excellent enabling sensor technologies for environmental monitoring, medical diagnostics, industrial and security applications. From the new source point of view a number of systems have been presented - From superconductors to semiconductors, e.g. Detection of Terahertz Waves from Superconducting Bi2Sr2CaCu2O8+δ Intrinsic Josephson Junctions. The quest for a compact room temperature THz source and the recent advances in high power mid-IR QCLs lead to the development of a semiconductor THz source based on intracavity difference frequency generation. Furthermore, alternative electrically pumped THz sources based on the high emission efficiency predicted for polaritonic states in the ultra-strong coupling regime led to the demonstration of electroluminescent devices. Finally, antipolaritons in dispersive media were discussed and different aspects of the interaction of THz radiation with biomatter were presented.