Preface

Quantum cascade lasers (QCLs) are semiconductor laser structures, the emission range of which varies from mid-infrared range to far-infrared range of the electromagnetic spectrum. These lasers are constructed by a repeated stack of multiple quantum wells and operate on interband transitions of the semiconductor structures. These types of lasers have been com‐ monly used in spectroscopy, remote sensing, optoelectronics, and communications. Due to their potential use in research and industry, advanced growth techniques such as molecular beam epitaxy (MBE), metal-organic vapor phase epitaxy (MOVPE), and metal-organic chemical vapor deposition (MOCVD), among others, have been used to fabricate qualitative quantum-cascade laser structures.

Topics related to the essentials of QCL structures, to the electronic structure of QCLs, to com‐ putational methods, to fabrication techniques, and to techniques in order to achieve low-noise operation with quantum-cascade lasers and their applications are included in the current chapters' collection. This book is divided into two sections. More specifically, in Section 1, the calculations on the band structure of cascade lasers and on the electronic band structure of QCL (e.g., transfer matrix technique, finite element method, and variational method, among others) under the existence of an applied electric field are presented. Moreover, a detailed investigation of electrical and optoelectronic properties of multiple-quantum-well structure is carried out. Furthermore, a theoretical study on the intensity noise characteristics of quantumcascade lasers under the external noncoherent optical injection is carried out. This section ends with an important description on growth methods for QCLs and the possibility of developing GaN-based QCLs among other materials. A few applications of cascade laser structures such as power amplification and THz applications are presented, among others, in Section 2. The THz QCL design and fabrication toward the high-temperature and large-average output pow‐ er operations for the real applications are described. Furthermore, the importance of the power amplification and coherent combination techniques to improve the output power maintaining the single-mode operation is presented.

As an editor of this book, I would like to thank all the authors for their contribution through the up-to-date research of their high-quality work. Lastly, I would like to express my thanks and gratitude to the InTech team for their support during the preparation of this book.

> **Dr. Vasilios N. Stavrou** Hellenic Naval Academy Piraeus, Greece

**Introduction and Theoretical Review**

## **An Overview on Quantum Cascade Lasers: Origins and Development An Overview on Quantum Cascade Lasers: Origins and Development**

Raúl Pecharromán-Gallego Raúl Pecharromán-Gallego

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65003

## **Abstract**

This chapter presents an introductory review on quantum cascade lasers (QCLs). An overview is prefaced, including a brief description of their beginnings and operating basics. Materials used, as well as growth methods, are also described. The possibility of developing GaN-based QCLs is also shown. Summarizing, the applications of these structures cover a broad range, including spectroscopy, free-space communication, as well as applications to near-space radar and chemical/biological detection. Furthermore, a number of state-of-the-art applications are described in different fields, and finally a brief assessment of the possibilities of volume production and the overall state of the art in QCLs research are elaborated.

**Keywords:** quantum cascade lasers, review, history, operation, fundamentals, materials, photoacoustic spectroscopy, sensors, trace-gas detection, plasma species, cavity ring-down spectroscopy

## **1. Introduction**

Quantum cascade lasers (QCLs) are based on a fundamentally different principle to 'classic' semiconductor lasers, that is, they use only one type of charge carriers, electrons, using intersubband transitions, so they can be called unipolar lasers. QCLs were conceived in the early 1970s. First, Esaki and Tsu [1] fabricated the first one-dimensional periodic potential multilayer by periodically varying the composition during epitaxial growth (superlattice). Later, QCLs were proposed by Kazarinov and Suris [2] and finally first demonstrated at Bell Laboratories in 1994 by Faist et al. [3]. Using superlattices leads to both quantum confinement and tunnelling phenomena, the basic processes in QCLs operation.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A conventional laser diode generates light by a single photon that is generated from an electron interband transition; this means that a high-energy electron in the conduction band recombines with a hole in the valence band, being the energy of the photon determined by the band-gap energy of the material system used. However, QCLs do not use bulk semiconductor materials in their optically active region, but a periodic series of thin layers of varying material composition forming a superlattice, which leads to an electric potential that changes across the length of the device (one-dimensional multiple quantum well confinement), splitting the bandpermitted energies into a number of discrete electronic subbands, making electrons cascade down a series of identical energy steps built into the material during crystal growth, and emitting a photon at every step, unlike diode lasers, which emit only one photon over the equivalent cycle. With an appropriate design of the thickness of these layers, population inversion is achieved between discrete conduction band-excited states in the coupled quantum wells by the control of tunnelling, making laser emission possible. Therefore, the position of energy levels is mainly determined by the thickness of the layers, rather than the material, and thus allowing tuning the emission wavelength of QCLs over a wide range in the same material system. Thus, one electron emits a photon during every intersubband transition within the quantum well (QW) in the superlattice, and then can tunnel into the next period of the structure where another electron can be emitted, leading QCLs to outperform diode lasers operating at the same wavelength by a factor even greater than 1000 in terms of power.

Classically, a QCL is made of a periodic repetition of active sections, which consist of tunnelcoupled quantum wells and injector, where a miniband is formed. As **Figure 1** shows [4], from the injector miniband the electrons are injected into the upper laser energy level (4) of the active section, where the laser transition takes place. Afterwards, the lower laser energy level (3) is emptied by longitudinal optical emissions (LO emissions) and the electrons enter the next step by tunnelling.

**Figure 1.** Typical conduction band structure of a QCL [4].

Combining materials in the active region, QCLs could be designed to emit at any wavelength over a wide range of the spectrum [5], as the emission wavelength is determined by quantum confinement. **Figure 2** shows a typical QCL in operation and some commercial examples. These structures are typically grown using either molecular beam epitaxy (MBE) or metal-organic chemical vapour deposition (MOCVD), being the most used growth mechanisms utilized to grow the alternated different semiconductor layers required for heterostructures fabrication on to a substrate. Ever since the first QCL was fabricated using InGaAs/InAlAs grown over InP substrate [1], other materials have been used in order to fabricate QCL structures, such as GaAs/AlGaAs, InGaAs/AlAsSb, InAs/AlSb, Si/SiGe and GaN-based materials, such as AlGaN/ GaN and AlN/GaN.

**Figure 2.** Left: the invisible beam from a high-power quantum cascade laser lights a match in its path. The laser is 2.25 mm long and 17 μm wide, being placed in a cryostat at liquid-nitrogen temperature and emits an optical power in excess of 200 mW from each facet at a wavelength of 8.0 μm. Similar devices emit up to 600 mW at room temperature in pulsed mode. Right: front: mid-IR lasers (4–10 μm) for trace-gas analysis and IR molecular spectroscopy in the front; back: set-up examples.

First commercialized a decade later of their first demonstration [6], the key features of these lasers reside in the fact of their high optical power output and, on the other hand, their tuning range and room-temperature operation. Spectroscopy applications are related to gas detection and analysers (pollutants, components, etc.). Other practical uses include industrial control, plasma chemistry and detection, such as collision avoidance radar or poor visibility-driving condition aids. Finally, the 3 to 5μm atmospheric window would make QCLs perfect candidates for substitution of optical fibre in high-speed and free-space communications.
