**5. State of the art and outlook**

complex and have thousands of rotational and vibrational transitions. Furthermore, broadband features with no separation observed at low pressures happen in measured transmission spectra. Different explosives have, therefore, unique terahertz spectral fingerprints. Williams et al. [52] developed a high-power QCL emitting at around 4.4 THz. These lasers are based on a resonant-phonon depopulation scheme, and use a semi-insulating surface-plasmon waveguide. They managed a maximum power of ∼248 and 138 mW pulsed and CW, respectively. van Neste et al. [53] have used two QCLs operating simultaneously with tunable wavelength windows matching the absorption peaks of analytes in order to improve sensitivities of standoff spectroscopy techniques, leading to a sensitivity of 100 ng/cm2 and a standoff detection distance of 20 m for surface-adsorbed analytes such as explosives and tributyl

Molecular plasmas are tools in plasma-enhanced chemical vapour deposition and in etching systems to deposit or remove thin films. The analysis of the precursor gas fragmentation and the monitoring of plasma reaction products is the key to an improved understanding and control of these chemical-active discharges. This observation can be done by using absorption spectroscopy methods in the mid-infrared spectral region. Recently, a QCL-based absorption spectroscopic system, the quantum cascade laser measurement and control system (Q-MACS), was developed and used to study industrial process plasmas and for environmental studies,

Finally, QCLs are also applied to aerospace and military uses. For instance, the first of a new generation of miniature tunable laser mid-IR spectrometers operating at room temperature for in situ measurement of atmospheric and evolved planetary gases started to develop not too long ago. These devices are based on newly available room-temperature QCL sources in the 3–12μm wavelength region and have immediate applications to Mars, Titan, Venus and Europa missions, being operated on a descending or penetrating probe, lander, rover or aerobot, and would consume only a few watts of power, with a weight less than 1 kg [55]. Furthermore, features of QCL make them good laser sources to carry out non-destructive-imaging engine combustors, where failure mechanisms, engine performance in aircraft and moisture content in jet fuel are examined. A low thermal noise background is required for such purpose, and ceramic ports exist already on such platforms transparent to terahertz radiation. Scale model radar cross-section experiments have also been carried out in the aerospace industry, to replace the bulky and extremely inefficient molecular lasers with QCLs, at the University of Massachusetts Lowell Sub-Millimetre Wave Laboratory [56], to perform scale model radar crosssection measurements for phenomenology and target-recognition database formation.

The infrared spectrum is also used in the aerospace industry for detection purposes, for instance, in the infrared scene generation, which is presently a critical technology for testing of infrared-imaging systems, for example, in infrared-guided missile systems, and QCLs could have an application in this technology to replace large- and slow-response 'resistor banks' with tailored design QCLs, to mimic the thermal background of a given scene [57]. Moreover, coherent transceiver using a terahertz quantum cascade laser (TQCL) as the transmitter and an optically pumped molecular laser as the local oscillator has been used for imaging purposes along with inverse synthetic aperture radar (ISAR), in which the range of the target was limited

and its applicability for monitoring online processes has been proven [54].

phosphate.

14 Quantum Cascade Lasers

QCLs are also currently facing a number of challenges, which will be summarized below.

First, the extensions of the wavelength range into the far-infrared. Rochat et al. [59] grew farinfrared QC structures based on a vertical transition active region emitting at l around 76 μm. This is a challenging issue, so long as population inversion is difficult to attain for such long wavelengths. Energy levels are generally so close that selective injection into a single level is difficult. Furthermore, LO-phonon scattering is replaced by electron-electron scattering as the dominant electron relaxation mechanism, which is more difficult to model accurately. Moreover, wavewide losses are expected to be high, although even very low-temperature QCL operation would be a great accomplishment due to the lack of narrow band and high-power compact sources in the far-infrared wavelength range. In fact, temperature is a key player when it is required to achieve a certain emission wavelength as **Figure 7** shows.

**Figure 7.** Operating temperature plot as a function of the emission wavelength/frequency for QCLs [60].

Another challenge would also be to fibre-optic wavelengths. In fact, the fabrication of large optical waveguides, powers of 14 and 5 W at respective temperatures of 15 and 280 K, is demonstrated at a wavelength of approximately 5.2 μm [61].

On the other side of the IR spectrum, reaching shorter wavelengths is another motivation for a more selection of materials. Work on this respect has also be done on GaN-based devices [61], with Gmachl et al. [62] having measured intersubband optical absorption in narrow (15–30 Å wide), GaN/AlGaN quantum wells grown by MBE on sapphire substrate. Peak absorption wavelengths ranged from 4.2 μm for 30Å wide wells to 1.77 μm for a 15Å wide well. On the other hand, Iizuka et al. [63] reported ultrafast intersubband relaxation (less than 150 fs) at a wavelength of 4.5 μm in Al0.65Ga0.35N/GaN MQWs, with as many as 200 QWs, making this result promising for fabrication ultrafast optical switches.

Doping level of the active region is a key optimization parameter that determines maximum drive current, optical losses and threshold current. Faist et al. [61] presented a systematic change of the active region doping in an InAlAs-InGaAs/InP lasers emitting at 9 μm. On the other hand, the wavelength tunability of each QC-DFB laser is especially limited to cover the entire molecular absorption spectrum of volatile organic compounds and hydrocarbons, which could be addressed by separating the gain medium from the wavelength-selective element [61].

Other challenges involve the low conversion efficiencies (<1%) between electrical and optical power, also known as wall plug efficiency (WPE) [64], and also the unavoidable fact that farinfrared (terahertz) QCLs lack proper performance at room-temperature operation. Belkin et al. [65] have recently reviewed 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 by using intracavity THz difference-frequency generation (DFG) in dual-wavelength mid-IR QCLs. However, Lu et al. [66] recently presented a strong-coupled strain-balanced quantum cascade laser design for efficient THz generation based on intra-cavity difference-frequency generation, demonstrating continuous-wave, single-mode THz emissions with a wide-frequency tuning range of 2.06–4.35 THz and an output power up to 4.2 μW at room temperature from two monolithic three-section sampled grating distributed feedback-distributed Bragg reflector lasers.

Furthermore, Burghoff et al. [67] demonstrated frequency combs based on terahertz QCLs, combining high power of lasers with the broadband capabilities of pulsed sources. By fully exploiting the quantum-mechanically broadened gain spectrum available to these lasers, 5 mW of terahertz power spread across 70 laser lines can be generated. Therefore, the radiation is sufficiently powerful to be detected by Schottky-diode mixers, and will lead to compact terahertz spectrometers.

About gas detection, Harrer et al. [68] combined the operation mode and low-divergence emission from QCLs with two-dimensional array integration with multiple emission and detection frequencies leading to detecting propane concentrations of 0–70 and 0–90% for isobutane at a laser operation wavelength of 6.5 μm using a 10 cm gas cell in double-pass configuration.

Finally, possibly the best way to assess the current state of the art for volume production of QCLs is to start from the market requirement from the end-user point of view. An example is the continuous emission monitoring (CEM) market, which includes engine emission and power-plant stack monitoring, is fiercely competitive, with a range of different gas-sensing technologies, such as Fourier transform spectrometry, chemiluminescence, non-dispersive infrared, and so on, each trying to increase its market share. Among these technologies, QCLbased gas sensor technology is still in its infancy, since although predictions indicate a great improvement in performance, much demonstration and convincing is still required in this conservative market. Moreover, it is difficult to justify a system whose price is above the market, despite better performance, due to the risk of an unproven and new technology. Hence, a QCL device might be priced below the market, which can have negative consequences that can reach the component supplier. Customers are usually reluctant to acquire a new sensor having other well-proven systems already in the market. Therefore, demonstration of the unique advantages of the QCL-based devices is often necessary, including specifications, performance, mean time to failure (MTTF) parameters, traceability and warranty period with a significant impact [69]. Implementation of high-volume production and proper validation processes on QCLs are producing positive outcomes. Along with a noticeable improvement in gas-sensing performance parameters, volume-cost reduction is allowing QCL systems price decrease to meet market demands. By consistently tackling the challenges of the QCL volume manufacturing, carrying out severe testing procedures, implementing quality-control systems and reaching adequate device costing, QCL developers and manufacturers can complete and come into the marketplace.

Finally, continuing to build consumer confidence in QCL commercial products, it should be easier for customers to digest the unavoidable contract 'warranty' and 'product liability' clauses found in supply contracts [69].

## **6. Summary**

with Gmachl et al. [62] having measured intersubband optical absorption in narrow (15–30 Å wide), GaN/AlGaN quantum wells grown by MBE on sapphire substrate. Peak absorption wavelengths ranged from 4.2 μm for 30Å wide wells to 1.77 μm for a 15Å wide well. On the other hand, Iizuka et al. [63] reported ultrafast intersubband relaxation (less than 150 fs) at a wavelength of 4.5 μm in Al0.65Ga0.35N/GaN MQWs, with as many as 200 QWs, making this

Doping level of the active region is a key optimization parameter that determines maximum drive current, optical losses and threshold current. Faist et al. [61] presented a systematic change of the active region doping in an InAlAs-InGaAs/InP lasers emitting at 9 μm. On the other hand, the wavelength tunability of each QC-DFB laser is especially limited to cover the entire molecular absorption spectrum of volatile organic compounds and hydrocarbons, which could be addressed by separating the gain medium from the wavelength-selective element [61].

Other challenges involve the low conversion efficiencies (<1%) between electrical and optical power, also known as wall plug efficiency (WPE) [64], and also the unavoidable fact that farinfrared (terahertz) QCLs lack proper performance at room-temperature operation. Belkin et al. [65] have recently reviewed 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 by using intracavity THz difference-frequency generation (DFG) in dual-wavelength mid-IR QCLs. However, Lu et al. [66] recently presented a strong-coupled strain-balanced quantum cascade laser design for efficient THz generation based on intra-cavity difference-frequency generation, demonstrating continuous-wave, single-mode THz emissions with a wide-frequency tuning range of 2.06–4.35 THz and an output power up to 4.2 μW at room temperature from two monolithic three-section sampled grating distributed feedback-distributed Bragg reflector

Furthermore, Burghoff et al. [67] demonstrated frequency combs based on terahertz QCLs, combining high power of lasers with the broadband capabilities of pulsed sources. By fully exploiting the quantum-mechanically broadened gain spectrum available to these lasers, 5 mW of terahertz power spread across 70 laser lines can be generated. Therefore, the radiation is sufficiently powerful to be detected by Schottky-diode mixers, and will lead to compact

About gas detection, Harrer et al. [68] combined the operation mode and low-divergence emission from QCLs with two-dimensional array integration with multiple emission and detection frequencies leading to detecting propane concentrations of 0–70 and 0–90% for isobutane at a laser operation wavelength of 6.5 μm using a 10 cm gas cell in double-pass

Finally, possibly the best way to assess the current state of the art for volume production of QCLs is to start from the market requirement from the end-user point of view. An example is the continuous emission monitoring (CEM) market, which includes engine emission and power-plant stack monitoring, is fiercely competitive, with a range of different gas-sensing technologies, such as Fourier transform spectrometry, chemiluminescence, non-dispersive

result promising for fabrication ultrafast optical switches.

lasers.

16 Quantum Cascade Lasers

terahertz spectrometers.

configuration.

In this chapter, an overview of quantum cascade lasers (QCLs) is presented. A historical introduction is first introduced. The basic features of QCLs are outlined, as well as a brief description of the issues that this work deals with. The operation and fundamentals of QCLs are also described. An analysis of the operation of these structures is included. Basically, the use of superlattices and tunability due to the layer thickness are the key features, in conjunction with the intersubband tunnelling transition – cascade. The QCLs are usually three-level lasers. Rate equations are also included. Furthermore, introducing a graphical brief timeline (**Figure 8**), including some of the most important milestones achieved in the world of QCL, would be helpful in this section.

An overview over the materials used for these devices is also included, that is, InGaAs and InAlAs layers, InAs/AlSb, Si/SiGe and GaN-based.

QCLs have a wide range of applications: first, trace-gas detection by optical methods in the mid-infrared, the great suitability of the TILDAS techniques and DFB-QCLs in trace-gassensing applications.

**Figure 8.** Achievements of QCLs over time.

Cavity ring-down spectroscopy is another technique used for gas detection and briefly described above. QCLs are also applied in photoacoustic spectroscopy, to study gas concentrations at the parts per billion or even parts per trillion levels. Applications in gas detection using QCLs with other types of spectroscopy and variations, such as lamb-dip spectroscopy, radiometric detection techniques, and so on, are mentioned.

The latest advances in QCL applications are also described. The extension of the wavelength range into the far-infrared but also shorter wavelengths, fibre-optic wavelengths and fabrication of large optical waveguides, doping issues, room-temperature operation problems, frequency combs based on terahertz devices, new insights on gas detection and finally current state of the art for volume production of QCLs are mentioned.

## **Author details**

Raúl Pecharromán-Gallego\*

Address all correspondence to: anubis\_rpg@hotmail.com

The Last Push Consulting, Heverlee, Leuven, Belgium

## **References**


**Figure 8.** Achievements of QCLs over time.

18 Quantum Cascade Lasers

**Author details**

**References**

Raúl Pecharromán-Gallego\*

radiometric detection techniques, and so on, are mentioned.

state of the art for volume production of QCLs are mentioned.

Address all correspondence to: anubis\_rpg@hotmail.com

The Last Push Consulting, Heverlee, Leuven, Belgium

Cavity ring-down spectroscopy is another technique used for gas detection and briefly described above. QCLs are also applied in photoacoustic spectroscopy, to study gas concentrations at the parts per billion or even parts per trillion levels. Applications in gas detection using QCLs with other types of spectroscopy and variations, such as lamb-dip spectroscopy,

The latest advances in QCL applications are also described. The extension of the wavelength range into the far-infrared but also shorter wavelengths, fibre-optic wavelengths and fabrication of large optical waveguides, doping issues, room-temperature operation problems, frequency combs based on terahertz devices, new insights on gas detection and finally current

[1] Esaki L, Tsu R. Superlattice and negative differential conductivity in semiconductors. IBM Journal of Research and Development. 1970;14(1):61–65. DOI: 10.1147/rd.141.0061.

[2] Kazarinov R, Suris R. Possibility of the amplification of electromagnetic waves in a semiconductor with a superlattice. Soviet Physics—Semiconductors. 1971;5(4):707–709.

[3] Faist J, Capasso F, Sivco D L, Sirtori C, Hutchinson A L, Cho A Y. Quantum cascade

laser. Science. 1994;264:553–556. DOI: 10.1126/science.264.5158.553.


sic&dum=true&tb=t&vl(freeText0)=Pecharroman&vid=SUVU01 DOI: Thesis no : T10902.

[28] Sun G, Soref R A, Khurgin J B. Active region design of a terahertz GaN/Al0.15Ga0.85N quantum cascade laser. Superlattices and Microstructures. 2005;37(2):107–113. DOI: 10.1016/j.spmi.2004.09.046.

[17] Walther C, Scalari G, Faist J, Beere H, Ritchie D. Low frequency terahertz quantum cascade laser operating from 1.6 to 1.8THz. Applied Physics Letters. 2006;89(23):

[18] Revin D G, Cockburn J W, Steer M J, Airey R J, Hopkinson M, Krysa A B, Wilson R, Menzel S. InGaAs/AlAsSb/InP quantum cascade lasers operating at wavelengths close

to 3 μm. Applied Physics Letters. 2007;90(2):021108–3. DOI: 10.1063/1.2431035. [19] Green R, Roberts J, Krysa A, Wilson L, Cockburn J, Revin D, Zibik E, Carder D, Airey P. MOVPE grown quantum cascade lasers. Physica E: Low-dimensional Systems and

[20] Liu Z., Wasserman D, Howard S S, Hoffman A J, Gmachl C F, Wang X, Tanbun-Ek T, Cheng L, Choa F-S. Room-temperature continuous-wave quantum cascade lasers grown by MOCVD without lateral regrowth. IEEE Photonics Technology Letters.

[21] Diehl L, Bour D, Corzine S, Zhu J, Höfler G, Lončar M, Troccoli M, Capasso F. Hightemperature continuous wave operation of strain-balanced quantum cascade lasers grown by metal organic vapor-phase epitaxy. Applied Physics Letters. 2006;89(8):

[22] Teissier R, Barate D, Vicet A, Alibert C, Baranov B A, Marcadet X, Renard X, Garcia M, Sirtori C, Revin D, Cockburn J. Room temperature operation of InAs/AlSb. Applied

[23] Ohtani K, Fujita K, Ohno H. Room-temperature InAs/AlSb quantum-cascade laser operating at 8.9 μm. Electronics Letters. 2007;43(9):520–522. DOI: 10.1049/el:20070251.

[24] Lynch S A, Bates R, Paul D J, Norris D J, Cullis A G, Ikonic Z, Kelsall R W, Harrison P, Arnone D D, Pidgeon C R. Intersubband electroluminescence from Si/SiGe cascade emitters at terahertz frequencies. Applied Physics Letters. 2002;81(9):1543–1545. DOI:

[25] De Rossi A, Carras M, Paul D J. Low-loss surface-mode waveguides for terahertz Si– SiGe quantum cascade lasers. IEEE Journal of Quantum Electronics. 2006;42(2):1233–

[26] Paul D J. Si/SiGe heterostructures: from material and physics to devices and circuits. Semiconductor Science and Technology. 2004;19(10):75–108. DOI: 10.1088/0268-1242/

[27] Pecharromán-Gallego R. Investigations of the luminescence of GaN and InGaN/GaN quantum wells [thesis]. UK: University of Strathclyde; 2004. 216 p. Available from: http://suprimo.lib.strath.ac.uk/primo\_library/libweb/action/display.do?tabs=detail-

VOY682303&recIdxs=0&elementId=0&renderMode=poppedOut&displayMode=full &frbrVersion=&dscnt=1&scp.scps=scope%3A%28SU%29%2Cscope%3A%28cla

sTab&ct=display&fn=search&doc=SUVOY682303&indx=1&recIds=SU-

%29&frbg=&tab=local&dstmp=1463141375653&srt=rank&mode=Ba-

Nanostructures. 2004;21(2-4):863–866. DOI: 10.1016/j.physe.2003.11.133.

2006;18(12):1347–1349. DOI: 10.1109/LPT.2006.877006.

Physics Letter. 2004;85(2):167–169. DOI: 10.1063/1.1768306.

231121–3. DOI: 10.1063/1.2404598.

20 Quantum Cascade Lasers

081101–3. DOI: 10.1063/1.2337284.

1238. DOI: 10.1109/JQE.2006.883496.

10.1063/1.1501759.

19/10/R02.


distributed-feedback lasers. Optics Letters. 1998;23(17):1396–1398. DOI: 10.1364/OL. 23.001396.


[49] Hvozdara L, Gianordoli S, Strasser G, Schrenk W, Unterrainer K, Gornik E, Murthy C S S S, Kraft M, Pustogow V, Mizaikoff B, Inberg A, Croitoru N. Spectroscopy in the gas phase with GaAs/AlGaAs quantum-cascade lasers. Applied Optics. 2000;39(36):6926– 6930. DOI: 10.1364/AO.39.006926.

distributed-feedback lasers. Optics Letters. 1998;23(17):1396–1398. DOI: 10.1364/OL.

[40] Williams R M, Kelly J F, Hartman J S, Sharpe S W, Taubman M S, Hall J L, Capasso F, Gmachl C, Sivco D L, Baillargeon J N, Cho A Y. Kilohertz linewidth from frequencystabilized mid-infrared quantum cascade lasers. Optics Letters. 1999;24(24):1844–1866.

[41] Kosterev A A, Curl R F, Tittel F K, Gmachl C, Capasso F, Sivco D, Baillargeon J N, Hutchinson A L, Cho A Y. Effective utilization of quantum-cascade distributedfeedback lasers in absorption spectroscopy. Applied Optics. 2000;39(24):4425–4430.

[42] Kosterev A A, Curl R F, Tittel F K, Gmachl C, Capasso F, Sivco D L, Baillargeon J N, Hutchinson A L, Cho A Y. Absorption spectroscopy with quantum cascade lasers. Laser

[43] Paldus B A, Harb C C, Spence T G, Zare R N, Gmachl C, Capasso F, Sivco D L, BaillargeonJ N, Hutchinson A L, Cho A Y. Cavity ringdown spectroscopy using mid-infrared quantum-cascade lasers. Optics Letters. 2000;25(9):666–668. DOI: 10.1364/OL.

[44] Paldus B A, Spence T G, Zare R N, Oomens J, Harren F J M, Parker D H, Gmachl C, Cappasso F, Sivco D L, Baillargeon J N, Hutchinson A L, Cho A Y. Photoacoustic spectroscopy using quantum-cascade lasers. Optics Letters. 1999;24(3):178–180. DOI:

[45] Nägele M, Hofstetter D, Faist J, Sigrist M W. Mobile laser photoacoustic spectrometer for multicomponent trace-gas monitoring based on CO2- and quantum-cascade lasers as pump sources. In: Conference on Lasers and Electro-Optics Europe; 10–15 September, 2000; Nice, France. IEEE; 2000. p. 12–22. DOI: 10.1109/CLEOE.2000.909779.

[46] Samman A, Rimai L, McBride J R, Carter R O, Weber W H, Gmachl C, Capasso F, Hutchinson A L, Sivco D L, Cho A Y. Potential use of near, mid and far infrared laser diodes in automotive LIDAR applications. In: 52nd Vehicular Technology Conference; 24–28 September, 2000; Boston. USA. IEEE; 2000. p. 2084–2089, vol. 5. DOI: 10.1109/

[47] Gittins C M, Wetjen E T, Gmachl C, Capasso F, Hutchinson A L, Sivco D L, Baillargeon J N, Cho A Y. Quantitative gas sensing by backscatter-absorption measurements of a pseudorandom code modulated λ ∼ 8-μm quantum cascade laser. Optics Letters.

[48] Sonnenfroh D M, Rawlins W T, Allen M G, Gmachl C, Capasso F, Hutchinson A L, Sivco D L, Baillargeon J N, Cho A Y. Application of balanced detection to absorption measurements of trace gases with room-temperature, quasi-CW quantum-cascade lasers.

23.001396.

22 Quantum Cascade Lasers

25.000666.

10.1364/OL.24.000178.

VETECF.2000.883239.

DOI: 10.1364/OL.24.001844.

DOI: 10.1364/AO.39.004425.

Physics. 2001;11(1):39–49. DOI: PMID 12143896.

2000;25(16):1162–1164. DOI: 10.1364/OL.25.001162.

Applied Optics. 2001;40(6):812–820. DOI: 10.1364/AO.40.000812.

