Widely Tunable Terahertz Semiconductor Laser Sources

Terahertz Quantum Cascade Lasers (THz QCLs) and Terahertz Difference Frequency Generation Quantum Cascade Laser sources (DFG-QCLs) are two types of semiconductor THz radiation sources that are compact and amenable to production in mass quantities. THz QCL can generate over 1W of power under cryogenic temperatures, while THz DFG-QCL can be operated under room temperature over 1mW level output. For either case, widely tunable solution is highly desired for spectroscopy applications. For THz QCLs, operation is still limited to cryogenic temperature and broad tuning is not available. Our experimental study shows that using variable barriers is a viable approach to enhance the design space for THz QCLs. We also propose to tune the spectral output of these devices using an optically projected variable distributed feedback grating. Tuning will be achieved by changing the projected grating period. Preliminary experimental results support the idea but higher pumping light intensity is required for this method to work. For THz DFG-QCLs, very broad tuning in 1-6 THz range has been demonstrated using rotating diffraction grating in an external cavity setup. Similar tuning range can also be achieved in a monolithic configuration. Based on the previous work which demonstrated an electrical monolithic tuner with 580 GHz tuning range, we design and test in this dissertation a linear array of 10 DFG-QCL devices to cover a 2 THz tuning range. An independent gain control scheme is developed to achieve high yield (~100%) of individual device. It is implemented via independent current pumping of two electrically isolated sections. Surface DFB grating and independent current pumping scheme used in our DFG QCLs is found to be useful for mid-IR QCL array sources. We propose a longitudinal integration scheme of multiple grating sections. It enables a single ridge to emit single mode radiation at different wavelengths upon selection. This helps to reduce mid-IR QCL array far field span. We demonstrated single ridge devices that can emit 2 or 3 different wavelengths upon selection.

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.

Broadly tunable lasers continue to have a tremendous impact in many and diverse fields of science and technology. From a renaissance in laser spectroscopy to Bose-Einstein condensation, the one nexus is the tunable laser. Tunable Laser Applications describes the physics and architectures of widely applied tunable laser sources. Fully updated and ex

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.

This book represents a unique collection of the latest developments in the rapidly developing world of semiconductor laser diode technology and applications. An international group of distinguished contributors have covered particular aspects and the book includes optimization of semiconductor laser diode parameters for fascinating applications. This collection of chapters will be of considerable interest to engineers, scientists, technologists and physicists working in research and development in the field of semiconductor laser diode, as well as to young researchers who are at the beginning of their career.

Room-temperature terahertz (THz) sources analogous to diode lasers in the near-infrared/visible or quantum cascade lasers (QCL) in the mid-infrared (mid-IR), i.e., electrically pumped, compact, widely tunable, and suitable for low-cost production, are highly desired for feasible and inexpensive THz systems. This dissertation focuses on demonstrating broadly tunable, room-temperature THz systems based on intra-cavity difference frequency generation (DFG) in mid-IR QCLs with improved spectral capability for versatile applications. Spectral control using an external cavity provides the widest tuning range and is favored for real-world applications. DFG-THz could be spectrally tuned by either tuning one mid-IR pump or by tuning both mid-IR pumps together. I built a Littrow-type, external cavity THz DFG-QCL system that generated spectral tunable THz radiation by fixing one mid-IR pump frequency with an integrated DFB grating on top of the QCL structure and tuning the other mid-IR pump frequency with an external grating, thus demonstrating record broadband narrow linewidth THz frequency tuning from 1.2 to 5.9 THz. A Cherenkov waveguide is used in this system to extract THz radiation through the semi-insulating InP substrate; however, InP has dispersion in 1–6 THz, resulting in steering far field profiles for different THz frequencies. Replacing the InP substrate with high-resistance silicon through an adhesive bonding process solved the beam steering problem of this THz DFG-QCL system. I also built a double-Littrow, external cavity DFG-THz system that tunes both mid-IR pump frequencies using two external diffraction gratings. Such a system allows performing a comprehensive spectroscopic study of the optical nonlinearity and its dependence on the mid-infrared pump frequencies. Our work shows that the terahertz generation efficiency can vary by a factor of two or more, depending on the spectral position of the mid-infrared pumps, even for a fixed THz difference frequency. Using this system, we investigated different active region designs: bound-to-continuum, continuum-to-continuum, three-phonon-resonance, and dual-upper-state active region design. Our studies show THz DFG-QCL based a bound-to-continuum active region with gain centered around 15 μm has an order of magnitude enhancement of mid-IR to THz conversion efficiency, which provides a trend for future improvement of the power performance of THz DFG-QCLs

The book describes the most advanced techniques for generating coherent light in the mid-infrared region of the spectrum. These techniques represent diverse areas of photonics and include heterojunction semiconductor lasers, quantum cascade lasers, tunable crystalline lasers, fiber lasers, Raman lasers, and optical parametric laser sources. Offering authoritative reviews by internationally recognized experts, the book provides a wealth of information on the essential principles and methods of the generation of coherent mid-infrared light and on some of its applications. The instructive nature of the book makes it an excellent text for physicists and practicing engineers who want to use mid-infrared laser sources in spectroscopy, medicine, remote sensing and other fields, and for researchers in various disciplines requiring a broad introduction to the subject.

The objective of this proposal was to develop a new type of THz semiconductor sources that are injection-pumped, efficient, compact, narrowband, tunable, and can operate at room temperature. The proposed device was a mid-infrared InGaAs/AlInAs Quantum Cascade Laser (QCLs) with the waveguide core containing a multi-functional coupled-quantum-well active region that supports both laser action in the mid-infrared and THz generation due to resonant coherent beating of two laser modes. During the course of the work a path was also found to substantially improve the temperature performance and directionality of conventional THz QCLs in which an electronic transition is used for the direct generation of THz radiation. The following was accomplished: first room temperature operation of an elctrically pumped quantum cascade laser based THz source, using intra-cavity nonlinear optics; record operating temperature (180K) of a conventional THz Quantum Cascade Laser, using a copper double metal waveguide for low loss; surface emitting THz QCLs with low beam divergence.

Learn how the rapidly expanding area of mid-infrared and terahertz photonics has been revolutionized in this comprehensive overview. State-of-the-art practical applications are supported by real-life examples and expert guidance. Also featuring fundamental theory enabling you to improve performance of both existing and future devices.

A number of applications including scientific spectroscopy, security screening, and medical imaging have benefitted from the development and utilization of new and emerging terahertz (THz) generation and detection techniques. Exploring recent discoveries and the advancements of biological behaviors through THz spectroscopy and imaging and the devel