Quantum cascade (QC) lasers, invented in 1994 by J. Faist,

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1 REVIEW ARTICLES FOCUS PUBLISHED ONLINE: 8 JUNE 1 DOI: 1.138/NPHOTON Mid-infrared quantum cascade lasers Yu Yao 1, Anthony J. Hoffman * and Claire F. Gmachl 3 Mid-infrared quantum cascade lasers are semiconductor injection lasers whose active core implements a multiple-quantumwell structure. Relying on a designed staircase of intersubband transitions allows free choice of emission wavelength and, in contrast with diode lasers, a low transparency point that is similar to a classical, atomic four-level laser system. In recent years, this design flexibility has expanded the achievable wavelength range of quantum cascade lasers to ~3 5 μm and the terahertz regime, and provided exemplary improvements in overall performance. Quantum cascade lasers are rapidly becoming practical mid-infrared sources for a variety of applications such as trace-chemical sensing, health monitoring and infrared countermeasures. In this Review we focus on the two major areas of recent improvement: power and power efficiency, and spectral performance. Quantum cascade (QC) lasers, invented in 1994 by J. Faist, F. Capasso, and co-workers 1, have progressed rapidly owing to their intrinsic design potential. These semiconductor injection lasers are based on intersubband transitions in multiple-quantumwell structures. The design of alternating wells and barriers, often numbering 5 1, (Fig. 1), allows great flexibility for a creative engineer to explore 9. Having exploited the benefits of mature, established materials InGaAs/AlInAs alloys on InP and later GaAs/ AlGaAs on GaAs thanks to the communications and photonics industries, progress in the development of these new semiconductor lasers was determined first by the imagination of device researchers and later by the pull of application requirements 1. In the years following the first demonstration of the QC laser 1, researchers repeatedly demonstrated the versatility enabled by quantum engineering, including demonstrations of lasers simultaneously emitting at multiple wavelengths 11, lasers with integrated sum-frequency nonlinearities 1 and lasers capable of broadband tuning 13. These new devices with extraordinary emission properties were created by changing the quantum-mechanical structure of the active region. Progress in output power and operating temperature was also made in conventional devices as the active region designs were refined Before, when continuous-wave (CW) operation at room temperature was achieved, QC lasers were limited to low-duty-cycle pulsed operation at room temperature. However, even as device performance improved, most lasers exhibited very low (<1%) conversion efficiencies between electrical power and optical power a metric known as the wall plug efficiency (WPE). Low WPEs limited the use of QC lasers in applications such as portable sensors and infrared countermeasures, where the power consumption of the laser was a major constraint. For applications already employing QC lasers, high WPE was seen as a desirable improvement. Although low threshold and high optical output power were always clear goals (or at least desirable features), only over the past five years has the power efficiency become an essential device parameter. This Review focuses first on power efficiency, as this generally also results in high optical output power and is usually concomitant with a low-threshold current density. High power and power efficiency performance The first QC laser had a WPE of less than.15% in pulsed operation at 1 K, meaning that less than.15% of the input electrical power was converted to optical power from the device 1. Although WPE values improved slightly as better lasers were designed, little work was done with the principal aim of improving the parameter, and the WPE remained of the order of 1% at room temperature for most QC lasers. A low WPE puts heavy demands on system designers; for example, an application requiring 1 W of optical output power from a laser with a WPE of 1% would require 1 W of input power. The system requirements become even more demanding when one considers that any electrical energy not contributing to light output must be dissipated as heat. As the number of proposed applications for QC technology grew, so did the need for improving the WPE. Fig. a shows a plot of the reported WPE over time for both pulsed and CW operation 1, The WPE is a complicated parameter that depends on factors such as the temperature of the device, the quantum-mechanical structure of the energy levels and various device characteristics such as length and waveguide loss 48. The total efficiency of the device can be approximated as the product of four device efficiencies: optical, current, internal and voltage efficiencies. Optical efficiency describes the emission of photons from the laser cavity before they are absorbed by the waveguide; current efficiency describes how far above threshold the laser is operated; internal efficiency describes the properties of the quantum design, such as the radiative and non-radiative recombination rates of the designated optical transition, thermal backfilling of the lower laser level and thermionic emission out of the quantum structure into the continuum states; and voltage efficiency describes the fraction of the voltage drop over the device that contributes to photon generation 48. Designing lasers that maximize these four efficiencies is essential to improving device performance. These four device efficiencies are all functions of the photon energy, which makes the WPE dependent on the wavelength. A reduction in the WPE for longer-wavelength lasers was demonstrated both experimentally and theoretically by considering the effect of wavelength on physical quantities such as oscillator strength, free-carrier absorption and optimal energy separation between the lower laser level and the injector state 48,49. The majority of the effort towards improving the WPE has been concerned with devices operating at wavelengths around 4.6 μm. Devices designed for this wavelength window are driven primarily by security applications such as infrared countermeasures, and benefit from highquality growth of strain-balanced, InP-based heterostructures. These are the devices that are emphasized in this Review. Significant advances in the WPE were achieved by further exploring the design space and re-examining common design strategies. One strategy focused on reducing the voltage defect, the voltage 1 School of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts 138, USA. Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, USA. 3 Department of Electrical Engineering, Princeton University, Princeton, New Jersey 8544, USA. These two authors contributed equally to this work, and significantly more so than the third author. * ajhoffman@nd.edu 43 NATURE PHOTONICS VOL 6 JULY Macmillan Publishers Limited. All rights reserved.

2 NATURE PHOTONICS DOI: 1.138/NPHOTON a PUBLISHED ONLINE: 8 JUNE 1 DOI: 1.138/NPHOTON c Electroplated gold 1 μm SiN insulation Gap in plating to facilitate cleaving Injector 3 Active region Injector b Active core Active region 5 mev 6 nm Figure 1 Concept of a QC laser. a, Photograph of a laser bar with four QC lasers (left, courtesy of Frank Wojciechowski) and scanning electron microscopy image of the front facet of a QC laser (right). b, High-resolution transmission electron microscopy image of a QC laser, showing four periods of active regions and injectors. c, Simplified schematic of the conduction band structure for a basic QC laser, where the laser transition is between sub-bands 3 and. drop in the active region that does not contribute to the generation of light 34,39,43. One particularly successful design achieved this at both the laser turn-on and turn-off voltages 43 ; by ensuring a low voltage defect at these two operating points, the overall electrical efficiency of the device was high and the differential resistance of the device was low. This study also achieved high differential gain by employing a short period length, which allowed denser stacking of the amplifying quantum wells. The high differential gain enabled the fabrication of devices with larger mirror losses and hence increased optical efficiency while minimizing the impact of increased photon emission on the threshold current. This device achieved a record WPE of more than 5% at 4 K in pulsed operation, thus generating more light than heat 43. A second strategy developed in tandem with the above approach also achieved a pulsed WPE of 5% under similar operating conditions by compensating for the effect of interface roughness on the transport of electrons inside the device 44. Theoretical models using interface roughness as the relevant in-plane scattering time showed that broadening due to interface roughness was the limiting factor in resonant tunnelling between the injector and the upper laser level, thus making it the limiting factor in laser performance 5. These results suggested that increasing the coupling strength between the injector and the upper laser level would result in improved gain, which contradicted the previous notion that increased coupling would reduce laser performance due to a broader gain spectrum. The conduction band energy diagram for such a device is shown in Fig. b. Light-current voltage measurements and current-versus-wpe for an ultra-strong coupling laser developed by Liu et al. 44 are shown in Fig. c and Fig. d, respectively. These ultra-strong coupling lasers consistently achieved WPEs in the range of 4 5% below 16 K in pulsed mode, owing to improvements in both the current and internal efficiencies 44. This new understanding of transport in QC lasers is significant to the field because the results are broadly applicable and should lead to improvements in most active-core designs at all temperatures 5. The aforementioned designs achieved record WPEs by operating at low temperatures in low-duty-cycle pulsed mode. However, many applications require lasers that operate at room temperature or in CW mode. Such requirements result in higher active-core temperatures due to an increase in heat sink temperature and/or self-heating 51 53, causing a degradation in laser performance that is most clearly observed as an exponential increase in the threshold current and, somewhat less strongly, as an exponential decrease in the optical slope efficiency of the laser. As a result, both the current efficiency and internal efficiency are negatively impacted. The primary causes for this reduction in laser performance are a reduction in the gain of the laser due to backfilling of the lower laser level, thermionic emission from bound states to the continuum, and increased phonon scattering rates 53. Despite these additional challenges, appropriate active-region designs can be used to mitigate some of the effects of higher operating temperatures. The design considerations for lasers intended to operate at high temperatures or in CW mode are necessarily different from those operating at low temperatures. For example, small voltage defects such as those used in pulsed, low-temperature lasers to achieve record WPEs are not viable design choices because the voltage defect plays an important role in minimizing thermal back-filling of the lower laser level 53,54. Instead, the active core must incorporate a larger voltage defect for devices operating at higher temperatures. Strategies to increase the performance of these devices are multifaceted: reduce the sensitivity of the laser to temperature changes and minimize temperature changes due to self-heating 4,55,56. Reducing the temperature sensitivity of QC laser performance has been achieved using a variety of active-region designs. One particularly effective design incorporates two strategies to achieve high performance. First, shallow wells and barriers are used to increase the separation between the upper laser level and the energy state immediately above it, thus reducing carrier injection into unwanted states. Second, tall barrier inserts are used in the injector region to increase the confinement of the optical transition and minimize carrier loss into the continuum states. The effect of temperature on the threshold of a QC laser can be described by J(T) = J e T/T, where J is a constant and T is the characteristic temperature of the device. High values of T are therefore desirable. This device has a characteristic temperature (T ~ 383 K) that is greater than that of high-performance QC lasers incorporating more conventional designs (T ~ K) 55. The highest-performing QC laser operating at room temperature in CW mode has been demonstrated using this shallow-well architecture; the device operated with a peak WPE of 1% and had a maximum output power of 5.1 W (ref. 46). In addition to reducing the temperature sensitivity of the laser, these highperformance devices also benefit from superb thermal packaging. NATURE PHOTONICS VOL 6 JULY Macmillan Publishers Limited. All rights reserved.

3 REVIEW ARTICLES FOCUS NATURE PHOTONICS DOI: 1.138/NPHOTON a Wall plug efficiency (%) 4 CW 4 Pulsed Year Heat sink temperatue (K) b Energy (mev) 1,8 E = 1 kv cm 1 1,4 19 mev 1, Distance (nm) c Voltage (V) Current density (ka cm ) K 8 K 1 K 16 K K 5 K 3 K P ow er (W ) d Current density (ka cm ) K 8 K 1 K 16 K K 5 K 3 K Current (A) Current (A) Figure WPE. a, Total WPE of selected lasers from the literature at various heat sink temperatures for pulsed and CW operation. Devices were selected from highly cited articles in years The colour of the symbol indicates the temperature of the heat sink. Symbols outlined in black represent devices with one or both facets coated. For non-coated lasers, the output power from a single facet was doubled to obtain the total device efficiency. For research that did not directly report the WPE, the value was estimated using the available data; estimates were required for many of the early devices. The error for the estimated WPE is primarily due to an uncertainty in the voltage applied to the device at peak WPE, which is estimated to be about 3%. In most cases, the estimated WPE is larger than the actual WPE because the turn-on voltage, which is lower than the actual voltage applied to the device, was used for the calculation. b, Conduction-band diagram for an ultrastrong coupling QC laser. c, Pulsed light-current voltage measurements for an ultrastrong coupling laser at various heat sink temperatures. The device was 13.6 μm.9 mm and the total output power was obtained by doubling the output power from a single facet. d, WPE versus current for the device shown in b. Figure b d reproduced with permission from ref. 44, 1 NPG. A complementary strategy that has consistently been employed in high-performance lasers is to minimize the temperature change by effectively removing heat from the active core 35,37,41,4,46. The active core can be subject to significant heating during high-duty-cycle or CW operation, and proper thermal management is essential for ensuring high-performance operation 57. Laser performance when operating in CW mode is greatly improved by fabricating the devices as buriedridge lasers using high-quality processing, employing proper growth conditions and using an InP regrowth technique (owing to the high thermal conductivity of InP),35,36,58. Mounting the device epitaxialside-down on diamond or some other high-thermal-conductivity heat sink further improves heat removal from the active core by minimizing the thermal resistance between the heat-generating active core and the heat-removing heat sink 35. Finally, external cooling mechanisms such as thermoelectric coolers can be used to extract heat from the package and regulate the temperature of the active core. A direct way of minimizing heating is to reduce the amount of power required by the laser. This strategy is particularly useful for applications such as point sensors, where optical powers of the order of a few milliwatts are sufficient, or for applications such as portable network sensors, where the laser is powered by a battery and thus the power budget is limited. In such cases, it is desirable to scale down the input and output powers of these high-performance devices while maintaining a high WPE. Low-power-consumption QC lasers are realized using short, narrow cavities with low-power active regions to ensure that the total power required for lasing is low Because shorter cavities have larger photon emission rates, the mirror loss of the laser must be modified to maintain a low threshold current. One strategy for achieving this is to use high reflectance and partial-high-reflectance coatings on the back and front facets, respectively. Such lasers have been demonstrated in CW mode at room temperature with a threshold power consumption of.83 W. Conventional designs, in contrast, have threshold power consumptions of W (ref. 6). Recently, even lower powers were reached by optimizing the doping density in the active region. These devices were fabricated as short-cavity, distributed feedback (DFB) lasers with antireflection coatings on the front and back facets to suppress Fabry Pérot modes. These devices have threshold power consumptions of around.7 W when operated in CW mode at room temperature. Their emission spectrum is single mode with a side-mode suppression ratio of 3 db, which makes them particularly useful for portable spectroscopic applications NATURE PHOTONICS VOL 6 JULY Macmillan Publishers Limited. All rights reserved.

4 NATURE PHOTONICS DOI: 1.138/NPHOTON a Surface grating Buried grating Highly doped cap layer Upper cladding Active region Bottom cladding Substrate Surface grating II c 1 μm 1 mm Angle ( ) Experiment Simulation Angle ( ) Intensity (normalized) b d Laser waveguide top view Laser ridge Air hole L + P θ 1 mm P +L Laser facet Intensity (log scale) e DFB master oscillator Single-pass power amplifier 1, 1,15 1,1 Wavenumber (cm 1 ) 1,5 Output facet with antireflection coating Figure 3 DFB QC lasers. a, Schematic of DFB QC laser structures. b, Scanning electron microscopy picture of a DFB QC laser array 9 and output laser spectra 88. c, Scanning electron microscopy image of a two-dimensional ring cavity DFB QC laser array (top-left) and the far-field of the laser output beam (bottom-right) 89. d, Schematic of a photonic-crystal DFB QC laser. e, Schematic of a QC laser master oscillator power amplifier with simulated light intensity distribution 9. Figure reproduced with permission from: b (spectra) ref. 88, 9 IEEE; b (inset), ref. 9, 1 SPIE; c (left), ref. 89, 11 AIP; c (right), ref. 77, 1 AIP; e, ref. 9, 11 OSA. High spectral performance For many applications in spectroscopy, particularly trace-gas sensing, single-mode narrow-linewidth tunable mid-infrared lasers are desirable to achieve high selectivity, high sensitivity and multiple-species detection. The most common approaches for achieving tunable single-mode operation are DFB QC lasers 63,64 and external cavity (EC) QC lasers 65. Versatile cavity designs 66 7 have also shown the potential to achieve single-mode lasing spectra. This section focuses on DFB QC lasers and EC QC lasers, both of which have shown rapid progress in terms of laser performance and tuning range. DFB QC lasers are fabricated by integrating a Bragg grating into the laser waveguide. Repeated scattering from a Bragg grating favours a single wavelength the Bragg wavelength which is determined by the grating period. There are two ways to introduce gratings into QC laser waveguides: a surface grating in the highly doped cap layer (Fig. 3a, top) 63 ; or a buried grating close to the active region, sandwiched between the waveguide core and the top cladding layers (Fig. 3a, bottom-left) 64,71,7. Applying a grating to the laser ridge sidewalls has also been attempted but is not as common. A surface grating is easier to fabricate than a buried grating, which requires high-quality upper waveguide cladding regrowth after fabrication. However, surface gratings require larger grating depths to achieve the necessary coupling strength, as the grating is further away from the laser mode centred in the active region. Furthermore, this type of surface grating provides complex coupling; that is, a modulation in both refractive index and loss 16,76. Although this helps to break the degeneracy of the two Bragg resonance modes and thus enforce single-mode operation, the additional loss makes it difficult to achieve CW operation at room temperature. An alternative surface grating technique is to remove the metal from the grooves (Fig. 3a, bottomright), which reduces the loss at the metal semiconductor interface 77. The first room-temperature CW DFB QC laser was demonstrated using a buried grating in Since then, steady improvements have been made towards higher output powers, high-temperature operation and low power consumption 7, Buried gratings were generally regarded as the only way to achieve room-temperature CW DFB QC lasers until recently, when the design principle of surface grating was revisited 8. Taking into account the coupling between the dielectric waveguide mode and the grating surface plasmonic mode allowed researchers to predict the grating coupling coefficient and waveguide loss more accurately than previous models based on coupled-mode theory 63,67. The optimized structure exhibits a shallow grating with a complex-coupling coefficient Researchers have not only demonstrated room-temperature CW operation, but also boosted the output power level to above the watt-level 47,86. However, the weak coupling strength requires a high-quality antireflective coating on at least one facet to maintain single-mode operation at injection currents high above the lasing threshold 85. Another important feature of the DFB QC laser is that its emission wavelength can be continuously tuned by changing the heat sink temperature or injection current 79,85, which in turn heats the laser. Both methods of tuning lead to a change in the effective refractive index of the laser waveguide and thus shift the resonance wavelengths of the Bragg gratings, with a tuning rate of around.1. cm 1 K 1. This capability allows DFB QC lasers to be tuned across the absorption peaks of different gas species, thereby NATURE PHOTONICS VOL 6 JULY Macmillan Publishers Limited. All rights reserved.

5 REVIEW ARTICLES FOCUS NATURE PHOTONICS DOI: 1.138/NPHOTON a d Intensity (a.u.) M GR ,95 CL lens, QCL b Energy (ev), Distance (Å),1 6,15 I 8, E , 1,, Gain (normalized) c Wavelength (μm) Year Intensity (cm/mole e 19 ) DFB BC 88 DFB BC 115 EC BC 113 EC BC stack 94 CO CO H O O 3 CH 4 C H 4 RDFB CB 89 EC DAU 111 DFB TPR11 18 EC CC EC DAU 111 EC BC 114 EC CB 16 EC TPR 97 NH 3 NO NO N O SO EC BC 5 stack 15 EC BC 97 EC BC 1 EC TPR 11 1, 1,5,,5 Wavenumber (cm 1 ) Wavenumber (cm 1 ) Figure 4 EC QC lasers, broadband QC laser design and wavelength tuning ranges achieved in QC lasers. a, Schematic of EC QC laser in a Littrow configuration (M: mirror; GR: grating reflector; CL: collimating lens). b, Conduction-band diagram of the continuum-to-continuum QC laser design 45. c, Top: Wavelength tuning ranges achieved with DFB arrays and EC lasers based on different QC laser designs. Red indicates CW operation; blue indicates pulsed operation. Bottom: HITRAN simulated absorption spectra for several molecules in the mid-infrared wavelength range of 4 1 μm. TPR, BC, CB, CC and DAU are different types of QC laser designs: TPR, two-phonon resonance; BC, bound-to-continuum; CB, continuum-to-bound; CC, continuum-tocontinuum; DAU, dual upper state. EC, DFB and RDFB are tuning methods of laser spectra: EC, external cavity; DFB, linear DFB laser array; RDFB, DFB ring laser array. d, EC tuning laser spectra of the continuum-to-continuum QC laser and laser gain spectra at the same biased electric field 18. The intensity of the highest frequency component (grey) is magnified by a factor of 1, for clarity. Figure b reproduced with permission from ref. 45, 1 AIP. providing high sensitivity in the detection of trace gases. The maximum tuning range of a single DFB QC laser is limited, however, by both the technologically feasible temperature swing and the tuning mismatch between the laser mode and the peak gain. Fully exploiting the gain bandwidth of a QC gain chip requires multiple DFB QC lasers with different grating periods to cover a broader wavelength range 87. By integrating an array of 4 DFB lasers, a wavelength tuning range of cm 1 ( μm) has been demonstrated (Fig. 3b) 88 using a bound-to-continuum active-region design. Using a two-dimensional array of ring cavity second order DFB QC lasers, a wavelength coverage of 18 cm 1 around 8. μm (1, cm 1 ) was achieved with a surface-emitting laser beam at a very small diverging angle of less than 3º (Fig. 3c) 89. The availability of such on-chip widely tunable single-laser sources provides great opportunity for achieving ultracompact mid-infrared spectrometers 9. Photonic-crystal DFB QC lasers 91 and QC laser master oscillator power amplifiers 9 have been employed to meet the requirements of high peak power (tens of watts) and high brightness for particular applications, with promising results. Figure 3d shows the schematic of a photonic-crystal DFB QC laser, in which a photonic crystal lattice is incorporated into the laser waveguide at a tilted angle θ with respect to the laser facets. The four coupling directions (P+, P, L+ and L ) cause the laser beam to propagate in a zigzag pattern. The DFB selects the resonance wavelength and the lateral coupling helps to establish lateral coherence over a wide ridge width (several hundred micrometers, or even up to 1 mm) 93. A nearly diffraction-limited beam has been obtained with a total peak power of 34 W from a 4-μm-wide laser ridge 91. With high fabrication accuracy, this method is a very good solution for highpower brightness, single-mode lasers in pulsed mode operation. However, the wide ridges lead to severe heating in CW operation and are therefore unsuitable for CW room-temperature lasers given the performance of today's laser gain chips. The QC laser master oscillator power amplifier incorporates a DFB QC laser as the master oscillator and a power amplifier at the output 9 (Fig. 3e). The two sections are separately pumped, which allows the DFB laser to be operated with moderate injection current to maintain single-mode emission while ensuring a large injection current in the amplifier section. A high-quality antireflective coating is essential for increasing the self-lasing threshold and allowing higher transmission gain in the amplifier region. Although DFB QC lasers provide compact, reliable single-mode tunable laser emission, EC QC lasers are the most commonly used widely tunable single-mode sources across numerous successful applications 94 1, owing to their rapid progress in tunability, output power 11 and CW operation at room temperature 11,1. Figure 4a shows one typical configuration the Littrow configuration for EC QC lasers. The set-up is carefully aligned to ensure that the collimated laser output beam is incident on the diffraction grating. The first-order diffraction is then coupled back into the laser cavity while the zeroth-order diffraction provides the output laser beam. This system is essentially a coupled cavity system formed by two cavities; that is, the Fabry Pérot cavity formed by the laser s front and back facets, and the grating cavity formed by the back facet and the grating. Detailed analyses have been performed into laser mode selection in such a configuration 13, NATURE PHOTONICS VOL 6 JULY Macmillan Publishers Limited. All rights reserved.

6 NATURE PHOTONICS DOI: 1.138/NPHOTON By rotating the angle of the grating, one can change the grating reflection spectra and thus tune the laser emission wavelength. However, this results in step-wise tuning to discrete wavelengths at which the Fabry Pérot cavity and grating-cavity mode coincide 65. Achieving continuous tuning requires complete mode tracking. A piezo-activated mode-tracking system that provides independent control of the EC length and diffraction grating angle was implemented to achieve mode-hop-free tuning of EC QC lasers 97,13. A spectral resolution of <.1 cm 1 (<3 MHz, limited by the laser linewidth) was obtained in CW operation, which is enough to resolve the narrow absorption spectra of trace gases 97. A major advantage of EC QC lasers is their potential to cover a wavelength range as wide as that allowed by the gain medium using a single QC laser chip. The maximum tuning range of a QC laser achieved so far is more than 43 cm 1 over 39% of the centre frequency in an EC configuration employing a heterogeneous QC laser design 15. The key to a wide tuning range, besides an optimized, well-aligned EC set-up, is a broad gain bandwidth. However, the conventional QC laser gain medium has an intrinsically narrow linewidth because it is based on the intersubband transition. The heterogeneous design covers a broad wavelength range by integrating many sub-stacks at different wavelengths. This technique has been used to achieve a supercontinuum lasing spectrum of 6 8 μm in a single device 13, thanks to its inhomogeneous broadening nature. This is an advantage for applications that require broadband laser spectra. However, for single-mode widely tunable sources, a homogeneously broadened gain medium is preferred because it helps to maintain the single-mode emission at the selected wavelength, even at high pumping currents. Another issue with the heterogeneous cascade design is that it sacrifices the laser gain at individual wavelengths because the confinement factor for each sub-stack becomes smaller as the number of sub-stacks increases, while the total core thickness of the laser is roughly maintained. Several techniques have been developed during the search for a homogeneously broadened QC gain medium with a wide gain spectra and high laser performance, including bound-to-continuum 19, continuum-to-bound/ continuum 45,16 and dual-upper laser state designs 17. Bound-to-continuum QC lasers have multiple transitions from the upper laser state to several lower laser states 19. This design simultaneously provides a short upper laser lifetime and a short depopulation time from the lower laser state a compromise that leads to a laser performance that comparable to the conventional two-phonon resonance design at longer wavelengths (>7 8 μm). For wavelengths shorter than 5 μm, however, the smaller energy gap between the injector states and the continuum above the quantum wells becomes detrimental to laser performance because the upper laser state is close to the band edge of the barriers, which leads to a higher probability of carrier leakage into the continuum states. Another challenge for achieving broad tuning in lasers based on bound-to-continuum design is the potentially unsymmetrical, multi-peaked shape of the gain spectrum, due to the difference in oscillator strengths for individual transitions. In QC laser designs based on continuum-to-bound 16 and continuum-to-continuum 45 (Fig. 4b) active regions, ultra-strong coupling between injector states and the upper/lower laser states provides multiple laser transitions, which contribute to a broadband gain spectrum. The strong coupling facilitates ultrafast carrier transport from the injector to the upper laser states, which compensates for the decrease in gain coefficient as a result of the broadening. Continuum-to-bound 16 and continuum-to-continuum 45 designs are therefore promising techniques for achieving high-performance QC lasers with a broad gain spectrum. The homogeneous broadening mechanism is verified through the broad EC tuning range (35 cm 1, 9% of the gain spectral full width at half maximum, Fig. 4d) and strong gain-clamping effect 18. The dual-upper laser state design is characterized by two strongly coupled upper laser states and a very broad gain spectra (57 cm 1, around 4% of the emission frequency) 19,11. Broad tuning ranges have been demonstrated in both pulsed (31 cm 1 ) and CW operation (48 cm 1 ) 111. Figure 4c presents a summary of recent results for single-mode broad-tuning QC lasers using an EC configuration or DFB arrays based on different QC laser designs 88,89,94,97,11,1,14 16,18, Given the recent progress on broadband gain designs and tuning techniques, we believe there is still much room for expanding the tuning range and improving the laser performance for single-mode tunable mid-infrared light sources based on QC structures. Outlook This Review has highlighted the rapid progress of QC lasers in areas of power efficiency, optical output power and spectral performance, as determined by wide single-mode tunability. Canvassing the field of mid-infrared QC laser research, an obvious speculation concerns the areas of major future progress of the field. Several open questions invite major research investment. Fundamental issues include the development of sub-picosecond, high-pulse-energy pulsed QC lasers, and the development of QC lasers for (or, more specifically, the application of designed intersubband transitions to) materials systems that are very different from conventional group iii v semiconductor alloys. Applied challenges include developing QC lasers whose power specifications are suitable for consumer electronics. Such achievements would highlight the significant potential of QC lasers and open up new fields for research and development. References 1. Faist, J. et al. Quantum cascade laser. Science 64, (1994).. Xie, F. et al. Room temperature CW operation of short wavelength quantum cascade lasers made of strain balanced Ga x In 1 x As/Al y In 1 y As material on InP substrates. IEEE J. Sel. Top. Quant. 17, (11). 3. Williams, B. S. Terahertz quantum-cascade lasers. Nature Photon. 1, (7). 4. Ulrich, J., Kreuter, J., Schrenk, W., Strasser, G. & Unterrainer, K. 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Widely tunable, single-mode, high-power quantum cascade lasers. SPIE Proc. Integrated Photonics: Materials, Devices, and Applications 869, 8695 (11) Mohan, A. et al. Room-temperature continuous-wave operation of an external-cavity quantum cascade laser. Opt. Lett. 3, (7) Maulini, R., Beck, M., Faist, J. & Gini, E. Broadband tuning of external cavity bound-to-continuum quantum-cascade lasers. Appl. Phys. Lett. 84, (4) Lee, B. G. et al. Widely tunable single-mode quantum cascade laser source for mid-infrared spectroscopy. Appl. Phys. Lett. 91, 3111 (7). Acknowledgements The authors acknowledge collaborations with colleagues at Princeton University and associated with the NSF Engineering Research Center MIRTHE. A.J.H. thanks S. Howard for valuable discussions. They also acknowledge partial support by MIRTHE (NSF-ERC) and DTRA. Additional information Correspondence and requests for materials should be addressed to A.J.H. NATURE PHOTONICS VOL 6 JULY Macmillan Publishers Limited. 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