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New Frontiers in Quantum Cascade Lasers Physica Scripta INVITED COMMENT Related content - External cavity terahertz quantum cascade New frontiers in quantum cascade lasers: high laser sources based on intra-cavity frequency mixing with 1.2–5.9 THz tuning range performance room temperature terahertz sources Yifan Jiang, Karun Vijayraghavan, Seungyong Jung et al. To cite this article: Mikhail A Belkin and Federico Capasso 2015 Phys. Scr. 90 118002 - Novel InP- and GaSb-based light sources for the near to far infrared Sprengel Stephan, Demmerle Frederic and Amann Markus-Christian - Recent progress in quantum cascade View the article online for updates and enhancements. lasers andapplications Claire Gmachl, Federico Capasso, Deborah L Sivco et al. Recent citations - Optical external efficiency of terahertz quantum cascade laser based on erenkov difference frequency generation A. Hamadou et al - Distributed feedback 2.5-terahertz quantum cascade laser with high-power and single-mode emission Jiawen Luo et al - H. L. Hartnagel and V. P. Sirkeli This content was downloaded from IP address 128.103.224.178 on 26/05/2020 at 19:55 | Royal Swedish Academy of Sciences Physica Scripta Phys. Scr. 90 (2015) 118002 (13pp) doi:10.1088/0031-8949/90/11/118002 Invited Comment New frontiers in quantum cascade lasers: high performance room temperature terahertz sources Mikhail A Belkin1 and Federico Capasso2 1 Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78712, USA 2 Harvard University, School of Engineering and Applied Sciences, 29 Oxford St., Cambridge, MA 02138, USA E-mail: [email protected] and [email protected] Received 19 June 2015, revised 25 August 2015 Accepted for publication 11 September 2015 Published 2 October 2015 Abstract In the last decade quantum cascade lasers (QCLs) have become the most widely used source of mid-infrared radiation, finding large scale applications because of their wide tunability and overall high performance. However far-infrared (terahertz) QCLs have lagged behind in terms of performance and impact due to the inability so far of achieving room temperature operation. Here we review recent research that has led to a new class of QCL light sources that has overcome these limitations leading to room temperature operation in the terahertz spectral range, with nearly 2 mW of optical power and significant tunability, opening up also this region of the spectrum to a wide range of applications. Keywords: quantum cascade lasers, terahertz, difference frequency generation (Some figures may appear in colour only in the online journal) 1. Introduction growth platform for photonic devices at communication wavelengths. MOVPE and MBE are now central to the pro- Last year marked the 20th anniversary of the invention and duction of QCLs. demonstration of the quantum cascade laser (QCL) at Bell The QCL avoids the operating principle of conventional Laboratories [1]. This achievement was the culmination of semiconductor lasers by relying on a radically different pro- 20 years of research on bandstructure engineering of semi- cess for light emission, which is independent of the band gap. conductor heterostructures [2] and in particular nanostructures Instead of using opposite-charge carriers in semiconductors in which quantum effects such as size quantization and (electrons and holes) at the bottom of their respective con- resonant tunneling become important [3]. While the design of duction and valence bands, which recombine to produce light man-made electronic and optical properties is central to band- of frequency ν≈Eg/h, where Eg is the energy band gap and structure engineering, its practical implementation in novel h is Planck’s constant, QCLs use only one type of charge devices was made possible by the epitaxial growth technique carriers (electrons), which undergo quantum jumps between of molecular beam epitaxy (MBE) pioneered by Alfred Y energy levels En and En−1 to create a laser photon of fre- Cho. MBE first enabled the growth of ultrathin semiconductor quency (En −En−1)/h. These energy levels do not exist layers with atomic precision down to a few atomic layers as naturally in the constituent materials of the active region, but well as unprecedented control of the composition of the are artificially created by structuring the active region into material. Later metalorganic vapor phase epitaxy (MOVPE) quantum wells of nanometric thickness. The motion of elec- demonstrated similar capabilities and became the standard trons perpendicular to the layer interfaces is quantized and 0031-8949/15/118002+13$33.001 © 2015 The Royal Swedish Academy of Sciences Printed in the UK Phys. Scr. 90 (2015) 118002 Invited Comment characterized by energy levels whose difference is determined 2. Energy diagram and quantum design by the thickness of the wells and by the height of the energy barriers separating them. The implication of this new This section briefly reviews the design principles of a QCL. approach, based on decoupling light emission from the band The energy diagram of the device at zero bias looks overall gap by utilizing instead optical transitions between quantized like a saw-tooth due to the compositionally graded regions, electronic states in the same energy band (known as inter- which consist of a digitally graded alloy of alternating ultra- subband transitions), are many and far reaching, amounting to thin semiconductor layers. Under high enough bias such that a laser with entirely different operating characteristics from the applied electric field suppresses the saw-tooth energy laser diodes and far superior performance and functionality. barriers, the band diagram is transformed into an energy Initially greeted as a scientific curiosity, due to its com- staircase and electrons are injected efficiently into the excited plex structure and low-temperature operation, the QCL fast state of suitably designed quantum wells, emitting a laser established itself as the leading mid-infrared (mid-IR) source photon at each stage. The energy diagram of the QCL drew of coherent radiation due to the ability to tailor the emission inspiration from the design of the staircase avalanche photo- wavelength across the entire mid-IR spectrum, from 3 to diode (or solid state photomultiplier), which consists of [ ] beyond 20 μm, its unprecedented tunability and high perfor- multiple identical graded gap regions 11 . Under high mance operation at room temperature in the two atmospheric enough applied bias the sawtooth energy diagram turns into a windows with pulsed and continuous-wave (CW) powers up staircase and photogenerated electrons roll down, gaining – to 100 and 10 W respectively. One should recall that before enough energy at the steps to create an electron hole pair by the advent of QCLs the mid-IR spectrum had a very limited impact ionization at the potential step given by band dis- choice of light sources which explains why applications and continuity. The number of stages typically ranges from 20 to – μ commercial development in this sector was very limited. 35 for lasers designed to emit in the 4 8 m range, but Carbon dioxide lasers, while important tools for high power working lasers can have as few as one or as many as over 100 industrial applications such as cutting and welding, have very stages. This cascade effect is responsible for the very high power that QCLs can attain. In QCLs, unlike in a laser diode, limited wavelength range and tunability. Diode lasers in the an electron remains in the conduction band after emitting a mid-IR have had very limited impact due fundamental diffi- laser photon. The electron can therefore easily be recycled by culties in operating at room temperature, very small tunability being injected into an adjacent identical active region, where and lack of robustness in terms of fabrication and reliability it emits another photon, and so forth. on account of their small bandgap. The operating principle of a QCL can be understood from QCL based spectroscopy and its applications to chemical figure 1, which illustrates the band diagram of a QCL sensing, in all its implementations (atmospheric chemistry, designed to emit photons of wavelength λ=7.5 μm. The trace gas analysis for applications such as pollution mon- conduction band slope (units of energy over distance) divided itoring, industrial process control, combustion diagnostics, by the electronic charge is the applied electric field. Each health care (medical diagnostics such as breath analysis, ) fi QCL stage consists of an active region and of an electron surgery have seen an unprecedented growth scienti cally, injector. The former contains three energy levels; the laser technologically and commercially with about thirty compa- photon is emitted in the transition between states 3 and 2, [ ] nies active, large and small 4 . Another sector where QCLs which is controlled primarily by the two wider wells thick- ( – ) are having a major impact is high power 1 10 W CW ness. To achieve a large population inversion between states 3 applications in areas such as infrared countermeasures to and 2 the lifetime of level 3 is designed to be much longer protect aircraft from heat seeking missiles; new emerging than that of state 2. To this end state 1 is positioned applications are material processing for example for 3D approximately an optical phonon energy (∼34 meV in printing where one can use wavelength selective chemistry to InGaAs/AlInAs materials) below level 2, which guarantees mold materials. that electrons in this level resonantly scatter to energy level 1 Recent and earlier reviews of QCLs and their application by emitting an optical phonon, an extremely fast process can be found in [5–9]. characterized by a relaxation time of the order of 0.1–0.2 ps. In 2002 Alessandro Tredicucci and his team at the Uni- Electrons in state 3 relax much more slowly to level 2 owing versity of Pisa demonstrated the first QCL operating in the to the much larger energy difference, a non-resonant phonon far-IR also known as the THz spectral region [10].
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