New Modeling Of Compact High Efficiency And Widely Tunable Gas Phase Terahertz Lasers

The terahertz region, in the heart of the electromagnetic spectrum, has been the least utilized, in part due to inadequacies of available sources. Optically pumped far-infrared (OPFIR) lasers were one of the most powerful continuous-wave terahertz sources. However, such lasers have long been thought to have intrinsically low efficiency, not tunable in frequency, and large sizes. In this thesis, we introduce a compact, frequency-tunable source of terahertz radiation with high efficiency. We first present both an innovative theoretical model and experimental validation of a Methyl Fluoride OPFIR laser at 0.25 THz that exhibits 10x greater efficiency and 1,000x smaller volume than the best commercial lasers. Unlike previous OPFIR-laser models involving only a few energy levels that failed even qualitatively to match experiments at high pressures, our ab-initio theory matches experiments quantitatively, within experimental uncertainties with no free parameters, by accurately capturing the interplay of millions of degrees of freedom in the laser. Moreover, we demonstrate a widely frequency-tunable compact terahertz radiation with laughing gas (nitrous oxide N2O) pumped by a quantum cascade laser (QCL). In experiments, broad tunability is achieved over 31 lines spanning 0.25-0.80 THz, each with kilohertz linewidths. Our comprehensive theoretical model is able to constrain the key molecular parameters and predict the optimal performance of the laser. The concept of QCL-pumped molecular laser (QPML) is a universal while revolutionary concept characterized by unprecedented frequency tunability over a wide range of rotational transitions using a single molecular gas as the gain medium. An analytical theory for QPML is presented to study the key factors for improving the laser performance. We believe that these developments will revive interest in optically pumped molecular laser as a powerful, tunable, and compact source of terahertz radiation.

In this thesis, we will explore numerical modeling and fabrication of laser sources. First, we demonstrate and distinguish experimentally the existence of special type of Fano resonances at k~~0 in a macroscopic two-dimensional photonic crystal slab. We fabricate a square lattice array of holes in silicon nitride layer and perform an angular resolved spectral analysis of the various Fano resonances. We elucidate their radiation behavior using temporal coupled-mode theory and symmetry considerations. The unique simplicity of this system whereby an ultra-long lifetime delocalized electromagnetic field can exist above the surface and consequently easily interact with added matter, provides exciting new opportunities for the study of light and matter interaction. However, we confirmed that achievable quality factor (Q) is limited by fabrication imperfection. Therefore, in the second part, we present an extensive fabrication optimization process, through which we established improved Q by a factor of three. Lastly, we report a comprehensive theoretical analysis and new experimental data of high-pressure (> 1 Torr) lasing action in optically-pumped far-infrared (OPFIR) lasers. No previous models could satisfactorily capture high-pressure operation because of the growing role of excited vibrational levels. Without these additional excited vibrational levels, molecules are artificially trapped in lower energy vibrational levels. This, in turn, prematurely triggers the so-called vibrational bottleneck and quenches the lasing action at low pressures in the previous models. Even though the high-pressure behavior can be more realistically modeled by including numerous excited vibrational levels, it would dramatically increase the computation time, and more importantly, the rate constants connecting all these levels are unknown. We propose a new model with an expandable pool which embodies 120 excited vibrational levels. Moreover, the knowledge of state-to-state rates among the excited vibrational levels is unnecessary in the proposed new model since the net rate related to the expandable level can be found by equilibrium conditions. Together with a detailed calculation of the pump rate and the wall collision rate, our model qualitatively and quantitatively reproduces experimentally measured high-pressure behavior. The model can be universally used for any OPFIR gas laser system. Thus, our work puts forward a theoretical formalism that could enable the advancement of compact terahertz radiation sources.

The portion of the electromagnetic spectrum between roughly 300 GHz and 10 THz is nicknamed the "THz Gap" because of the enormous difficulty encountered by researchers to devise practical sources covering it. Still, the quantum cascade laser (QCL) has emerged over recent years as the most promising approach to a practical source in the 1-5 THz range. First developed in the higher-frequency mid-IR, where they are now widely available, QCLs were later extended to the THz where a host of greater design challenges awaited. Lasing in QCLs is based on intersubband optical transitions in semiconductor quantum wells, the energy of which can be chosen by design ("bandstructure engineering"). However, simply building a THz optical transition is insufficient; a good design must also produce significant population inversion by the applied cascading electron current, and this requires deep understanding of the transport physics. So far, no THz QCL has operated above the temperature of 200 K, even though the reasons prohibiting high temperature operation are well known. The goal of this Thesis is to put novel ideas for high-temperature operation of THz QCL active regions through rigorous theoretical testing. The central enabling development is a density-matrix-based model of transport and optical properties tailored for use in QCLs, which is general enough that widely varying design concepts can be tested using the same core principles. Importantly, by simulating QCLs more generally, fewer a priori assumptions are required on part of the researcher, allowing for the true physics to emerge on its own. It will be shown that this gives rise to new and useful insights that will help to guide the experimental efforts towards realization of these devices. One specific application is a quantum dot cascade laser (QDCL), a highly ambitious approach in which the electrons cascade through a series of quantum dots rather than wells. Benefits are expected due to the suppression of nonradiative scattering, brought about by the discrete spectrum of electronic states. However, this in turn leads to a highly different physics of transport and effects that are not well understood, even in the case of perfect materials. This work will show that while the benefits are clear, naive scaling of existing QCL designs to the quantum dot limit will not work. An alternative strategy is given based on a revised understanding of the nature of transport, and is put to a test of practicality in which the effects of quantum dot size inhomogeneity are estimated. Another application is to the already existing method of THz difference frequency generation in mid-IR QCLs, which occurs via a difference-frequency susceptibility $\chi^{(2)}$ in the active region itself. For this purpose, the model is extended to enable a coherent and nonperturbative calculation of optical nonlinearities. First, the generality of the method is displayed through the emergence of exotic nonlinear effects, including electromagnetically-induced transparency, in mock quantum-well systems. Then, the modeling concepts are applied to the real devices, where two new and important mechanisms contributing to $\chi^{(2)}$ are identified. Most importantly, it is predicted that the QCL acts as an extremely fast photodetector of itself, giving rise to a current response to the mid-IR beatnote that provides a better path forward to the generation of frequencies below ~2 THz. Finally, the fundamentals of density matrix transport theory for QCLs are revisited to develop a model for conventional THz QCL designs eliminating the usual phenomenological treatment of scattering. The new theory is fully developed from first principles, and in particular sheds light on the effects of scattering-induced electron localization. The versatility of the model is demonstrated by successful simulation of varying active region designs.

Diode Lasers and Photonic Integrated Circuits, Second Edition provides a comprehensive treatment of optical communication technology, its principles and theory, treating students as well as experienced engineers to an in-depth exploration of this field. Diode lasers are still of significant importance in the areas of optical communication, storage, and sensing. Using the the same well received theoretical foundations of the first edition, the Second Edition now introduces timely updates in the technology and in focus of the book. After 15 years of development in the field, this book will offer brand new and updated material on GaN-based and quantum-dot lasers, photonic IC technology, detectors, modulators and SOAs, DVDs and storage, eye diagrams and BER concepts, and DFB lasers. Appendices will also be expanded to include quantum-dot issues and more on the relation between spontaneous emission and gain.

Research and development in the terahertz portion of the electromagnetic spectrum has expanded very rapidly during the past fifteen years due to major advances in sources, detectors and instrumentation. Many scientists and engineers are entering the field and this volume offers a comprehensive and integrated treatment of all aspects of terahertz technology. The three authors, who have been active researchers in this region over a number of years, have designed Terahertz Techniques to be both a general introduction to the subject and a definitive reference resource for all those involved in this exciting research area.

Electromagnetics is too important in too many fields for knowledge to be gathered on the fly. A deep understanding gained through structured presentation of concepts and practical problem solving is the best way to approach this important subject. Fundamentals of Engineering Electromagnetics provides such an understanding, distilling the most important theoretical aspects and applying this knowledge to the formulation and solution of real engineering problems. Comprising chapters drawn from the critically acclaimed Handbook of Engineering Electromagnetics, this book supplies a focused treatment that is ideal for specialists in areas such as medicine, communications, and remote sensing who have a need to understand and apply electromagnetic principles, but who are unfamiliar with the field. Here is what the critics have to say about the original work "...accompanied with practical engineering applications and useful illustrations, as well as a good selection of references ... those chapters that are devoted to areas that I am less familiar with, but currently have a need to address, have certainly been valuable to me. This book will therefore provide a useful resource for many engineers working in applied electromagnetics, particularly those in the early stages of their careers." -Alastair R. Ruddle, The IEE Online "...a tour of practical electromagnetics written by industry experts ... provides an excellent tour of the practical side of electromagnetics ... a useful reference for a wide range of electromagnetics problems ... a very useful and well-written compendium..." -Alfy Riddle, IEEE Microwave Magazine Fundamentals of Engineering Electromagnetics lays the theoretical foundation for solving new and complex engineering problems involving electromagnetics.

This volume contains a series of articles on wave phenomena and fluid dynamics, highlighting recent advances in these two areas of mathematics. The collection is based on lectures presented at the conference Fluids and Waves--Recent Trends in Applied Analysis and features a rich spectrum of mathematical techniques in analysis and applications to engineering, neuroscience, physics, and biology. The mathematical topics discussed range from partial differential equations, dynamical systems and stochastic processes, to areas of classical analysis. This volume is intended as an introduction to major topics of interest and state-of-the-art analytical research in wave motion and fluid flows.

The text has been revised to incorporate new developments in lasers and quantum electronics. Other subjects covered include phase-conjugate optics, long wavelength quaternary semiconductor lasers, the physics of semiconductor lasers, laser arrays and free-electron lasers.