In this article we review a selection of recent results on long-wave quantum cascade lasers both for high power and for single-mode emission. Both MBE-grown and MOCVD-grown devices are examined and compared. Currently, LWIR QC lasers exhibit output powers in the Watt-level range and up to double-digit conversion efficiencies in the best cases.
© 2013 Optical Society of America
The long-wave infrared (LWIR, λ = 8-12μm) region of the spectrum is particularly interesting due to the rich variety of molecular species having their “fingerprint” absorption features in this range, and due to the reduced water absorption that allows long-range atmospheric transmission of IR light for a series of applications, including wireless communications, IR beacons, ranging and targeting, standoff detection, among others. Quantum cascade lasers (QCL) [1,2] are one of the few light sources capable of serving this important spectral range. Due to their unique electrical operation and miniature size, QCLs can be included into compact, portable, ruggedized systems that are potentially inexpensive and field-deployable with the ease of use associated to any semiconductor-based platform.
In this paper we will review a selection of recent results on high power and single-mode QC lasers in the LWIR spectral range, including devices based on lattice-matched and strain-compensated material systems. Current laser performances in the LWIR region [3–8] are starting to be comparable to performances in the mid-wave IR (MWIR, λ = 3-5μm) , traditionally more established since high power MWIR devices have seen a strong development due to the need for IRCM solutions. LWIR performances today are delivering Watt-level output powers, with double-digit electro-optical conversion efficiencies in the best cases [5,6]. The biggest challenges for high performance LWIR devices are not easily overcome by simply extending to the LWIR range solutions suitable for QCLs in the MWIR range, nevertheless some significant progress has been shown by using strain-compensated device design and fabrication approaches that leverage the knowledge gained from the development of high efficiency MWIR QC lasers. Overall, the approach to the development of high power, high efficiency LWIR QCLs can be compared to the MWIR development seen in previous years both experimentally [9, 10], and theoretically [11, 12], although affected by peculiar challenges associated with LWIR operation, such as the increased free-electron optical losses at longer wavelengths, the lower intersubband gain, the decreased optical confinement, the thicker semiconductor active layers leading to worse thermal performances and more challenging fabrication, among others. It is worth noting though, that the very first demonstration of room temperature (RT) continuous operation (CW) of a QC laser was based on a 9.1μm InP-matched design by Beck et al. in 2001 , while high power RT CW operation at MWIR wavelengths was demonstrated only later thanks to the refinement and development of highly strained InGaAs/InAlAs materials growth techniques, thus enabling designs that prevent high-energy electrons from leaking from the upper laser level to higher energy levels in the active region. This was one of the key elements to increase the QC lasers’ characteristic temperatures T0 and T1, thus allowing high CW power emission at high temperature.
2. State-of-the-art LWIR QCLs: lattice matched and strain compensated designs
Quantum cascade lasers are today fully deployable devices working at room temperature and above, capable of being both operated and stored in challenging environmental conditions. Overall, the level of maturity of this technology is approaching the one of other semiconductor devices with a much longer history. The leveraging of near-IR laser manufacturing know-how and material development, allowed QC lasers to become available for real-world applications less than 10 years after the very first cryogenic temperature demonstration of lasing in a QCL grown by Molecular-Beam Epitaxy (MBE) in 1994 [1,14]. Among the key steps of this development were the demonstration of RT CW operation of QC lasers in 2001  and subsequently, the room temperature CW operation of a QC laser grown and fabricated using MOCVD technology in 2005 [15,16], which is the platform of choice for industrial III-V semiconductor manufacturing. Today, QC lasers are grown and fabricated using both MBE and MOCVD technologies. Although over the years MBE growth, and in particular gas-source MBE , has preserved a leading edge in the best published performances especially in the MWIR [17–21], for many practical application purposes the performances attainable with MOCVD growth are good enough and allow a more flexible fabrication setup especially in industrial environments.
As mentioned above, the very first RT CW QC laser was also a LWIR one, exhibiting about 10mW at 300K. From that moment on, the race to high power and high efficiency at room temperature shifted towards the shorter wavelengths (3.5-5μm) due to the rising need for IRCM solutions requiring powers in the Watts range and high efficiencies (>10%). Today, MWIR CW powers of several Watts and CW conversion efficiencies in the double-digit (>20%) range have been demonstrated  both in the lab and in devices deployed for field tests [17–21]. The development of high power LWIR lasers has been lagging behind both because of the smaller funding and market need, and because of the bigger technological challenges. Today, partly because of the promise of long distance signal transmission through the atmosphere, partly because of the deployment of power-dependent detection techniques such as photoacoustic spectroscopy, and partly because of the need for smaller and more integrated packages, the push for high power and high efficiency in the LWIR range of the spectrum has been significantly increased.
Currently, Watt-level outputs at wavelengths in the λ = 6-10μm range have been demonstrated  and efficiencies in the double-digit range have been measured . Beyond λ = 10μm, broader emitter devices (14μm) have been shown to output Watt-level peak power in pulsed and CW mode, with conversion efficiencies of about 10% and 4.8%, respectively [8, 21]. The disparity between LWIR performances and the performances of shorter-wavelength lasers is due to several factors, in particular to the increased free-electron absorption at longer wavelengths, and to the fact that the longer emission wavelength leads to wider waveguide designs for optimal mode confinement. This latter feature of LWIR lasers leads to more challenging CW operating characteristics since narrower waveguides would have an optimized thermal dissipation due to the better surface-to-volume ratio of the active material where heat is generated. Overall, a good compromise between optical confinement and CW operation is harder to achieve at LWIR rather than at MWIR wavelengths. A selection of our results is reviewed in the subsequent sections of this paper.
3. Strained material design and current leakage in LWIR QC lasers
The 9μm non-resonant extraction (NRE) active region design presented here, which was first reported in , is based on a strain-balanced In0.58Ga0.42As/Al0.64In0.36As composition (0.36% strain in quantum wells and −1.10% in barriers). A conduction band diagram of two gain stages of the new design is shown in Fig. 1. The radiative transition happens between levels 4 and 3. Energy spacing E54 was designed to be approximately 60meV. The energy spacing between the upper laser level and top of the direct barriers, EC4, was increased up to 430meV respect to standard designs. The calculated laser transition matrix element, and the upper and lower laser lifetimes for this design are 2.44nm, 1.22ps and 0.25ps, respectively. Carrier lifetimes were calculated taking into account only interaction with longitudinal optical phonons and assuming T = 298K. The targeted sheet carrier density for active region doping was 1.5·1011 cm−2.
The optical waveguide was designed to achieve low free-carrier optical losses by keeping the doping level low (2·1016cm−3) in the 3µm cladding layers adjacent to the 45-stage active region design described above. The rest of the waveguide structure consists of 4µm (top) and 2µm (bottom) low doped (5·1016cm−3) InP layers and a highly doped (8·1018cm−3) 1µm plasmon layer, which helps to decouple the optical mode from the lossy metal top contact. This waveguide design resulted in calculated free-carrier waveguide losses of αfc = 1.1cm−1 and optical mode overlap factor with the active region, Γ, of 52%. Loss contribution from free carriers in the active region was ignored in these calculations.
The 45-stage quantum cascade laser active region, along with the waveguide and contact layers was grown by molecular beam epitaxy on a low doped (2·1017 cm−3) InP substrate. The wafer was then processed into a buried heterostructure geometry and cleaved into individual laser chips and into round mesas for electroluminescence measurements. The laser chips were mounted epi-side down on AlN/SiC composite submounts  for pulsed and CW characterization. Pulsed testing was performed with 500 ns pulses and 0.5% duty cycle and CW operation was controlled with a thermoelectric cooler.
4. Limits of LWIR efficiency
Long-wave IR (LWIR) devices designs have traditionally been employing lattice-matched material systems due to the lower photon energy of the long-wave transitions. Typically, for a λ = 9μm emission, i.e. a photon transition energy of ΔE32 = 137 meV, and assuming an electron relaxation energy of ΔE20 = 150meV ≈4x ΔELO to avoid phonon back-filling, we can estimate the energy difference from the upper laser level to the conduction band offset ΔE3C to be: ΔE3C = ΔEoffset - ΔE32 - ΔE20 = 233 meV, where ΔE20 is the energy difference between the lower laser level and the ground state of the quantum cascade active region, ΔE32 is the energy difference between the upper and lower laser level, and ΔEoffset is the conduction band offset, which for the InGaAs/InAlAs material system lattice matched to InP is considered to be ΔEoffset = 520 meV. ΔE3C = 233 meV is considered to be enough to ensure confinement of electrons against transitioning to the conduction band continuum, comparable to the energy level structure in MWIR QCLs, but not quite fully exploiting the advantage of the smaller transition energy to improve electron confinement. On the other hand, the enhanced energy levels spacing obtained with strain-compensated materials, yielded improved results for LWIR emission. It has been pointed out that this may be associated to a lower electronic temperature characteristic of high-CB offset materials [23, 24]. In addition, there may be a contribution due to the interface quality of strain compensated material vs. lattice matched ones that could play a role in the laser performance.
The main reason for using the lattice matched composition is that the spontaneous emission linewidth of the laser transition is expected to increase for larger band offsets, hence in higher strain materials, thus reducing the optical gain. However, it was shown recently by MBE growth that highly strained QCL designs can have spontaneous emission linewidth similar to that of designs based on significantly lower strain composition . Employment of large band offset promises, therefore, a way of improving LWIR QCL performance.
5. Lattice matched material growth
MOCVD growth of layers composing the QC lasers material presented here, both for the active region and the waveguide, was used in the case of lattice-matched (LM) devices only. Strain-compensated (SC) devices were grown by MBE and subsequently fabricated using MOCVD techniques for buried-heterostructures similar to the ones employed for LM structures. Only the MOCVD growth will be discussed here, being the only technology common to both SC and LM processes.
The MOCVD system is designed to include a showerhead gas injection arrangement, located in close proximity with the wafers, thus minimizing the “dead” volume and allowing for a very short transient gas switching time between layers. The system is also equipped with Epison ultrasonic gas concentration monitoring systems which are crucial in controlling the output gas flow of the metal organic sources. This in situ feedback loop automatically modifies the flow rate, and consequently the growth rate and composition, to compensate against fluctuations due to environmental variations. This makes MOCVD particularly suitable for the growth of very thick layers typically included in QCL structures, requiring very long growth times. Repeated optimization of the growth conditions, such as substrate temperature, interface switching mechanism, growth rate, V/III ratio, among others, has been carried out in order to produce very sharp multi-quantum well interfaces. Although not yet completely explained, interface roughness definitely plays a role in the definition of QC lasers performances. X-ray measurements are routinely carried out on the InGaAs/InAlAs multilayers resulting from MOCVD growth. A close match between simulated experimental curves is typically achieved, indicating a good control over the growth parameters. Polaron C-V testing is used to monitor the doping in the structure. High resolution SEM and Nomarski Microscope techniques have been used to ensure the grown wafer surface quality .
Particular care was dedicated to doping calibration in the LWIR. As shown in Fig. 2 the doping can affect both the efficiency and the total power. If we compare lasers grown with the same epi-layer structure but with different doping, we realize that there is an optimal trade-off in the doping density below which the output power is limited by the amount of carriers that can be injected (as highlighted by the comparison of the green vs. red curve in Fig. 2) while as the doping is increased beyond the optimal level, the threshold currents become higher and the slope efficiency starts to drop (red curve vs. grey curve in Fig. 2), with consequently worse CW performances at room temperature.
6. QCL fabrication
The lasers were fabricated in buried-heterostructure waveguides for high power room temperature operation. The laser stripe width generally varies from 4μm to about 10μm, while their cavity length is between 3 and 5mm. After completion of the growth, the ridge waveguides were defined by chemical etching and insulating Fe:InP was re-grown on the sides of the laser waveguide. Polaron C-V and Hall testing have been used to make sure the Fe:InP is a good electrical insulator. The purpose of the lateral re-growth is twofold: it allows optical confinement of the laser mode in the lateral direction, and it contributes to optimizing the heat dissipation, both by improving the lateral transmission of the heat generated in the active region, and by planarizing the top surface of the device, thus allowing epi-down mounting of the lasers. The top and bottom contact metal depositions by electron beam evaporation conclude the device fabrication. The devices were diced and soldered to lattice-matched submounts and subsequently to copper heat spreaders for optimal thermal dissipation. Device temperature was monitored by a temperature sensor installed in close proximity with the device itself.
In the case of DFB lasers, the MOCVD growth was interrupted just after the active core was completed, including an InP buffer and an InGaAs sacrificial grating layers. Electron-beam lithography was performed in order to trace a grating pattern directly on top of the InGaAs sacrificial layer, which was subsequently etched in order to obtain the necessary periodic modulation of the waveguide effective refractive index, necessary for single mode operation. The cladding and top layers were grown on top of the patterned core material, with particular care in order to obtain a flat surface at the end of the re-growth on top of the grating layer. Subsequent device fabrication proceeded as for the case of Fabry-Perot buried heterostructure devices. HR coating, when needed, was performed by electron-beam evaporation of metallic coatings on previously isolated laser facets by dielectric deposition. This method has the advantage to be a total reflector for the back facet radiation at all wavelengths, but the downside is that the yield for epi-down mounting of the devices can be lowered by causing the device to short due to the contact between the soldering material and the back facet metallization. DFB lasers were coated with dielectric AR low-reflectivity (R<5%) coatings on the front facet.
7. LWIR QCLs: current results
We divide the LWIR in two ranges: the one below 10μm and the one above 10μm, due to the additional complexities of the very-long wave IR (λ>10μm), such as non-radiative intersubband processes enhanced by phonon resonances, increased free-electron absorption, and fabrication limitations, among others.
Lattice matched results for QC lasers at λ<10μm
Devices emitting at center wavelength of λ = 7.8μm and processed into FP buried heterostructure waveguides are shown in Fig. 3. The total output power can reach Pout>0.6W at T = 15C heat sink temperature with a conversion efficiency of η = 3% from devices with a 9μm wide, 5mm long waveguide. Increasing the device width to 11μm, brings the total power up to Pout = 0.8W and the efficiency to η = 3.4%. The characteristic temperature in CW operation is T0 = 145K. A smaller sensitivity of the threshold current to temperature, represented by an increased characteristic temperature T0 of devices emitting at longer wavelengths, can be partly justified by the improved injection efficiency due to the smaller fraction of hot-electrons that are leaked to higher energy states in longer-wavelength lasers. Similar effects have been observed in strain-compensated devices discussed in the following sections.
High power devices at λ = 8.9μm have been fabricated from lattice matched material grown by MOCVD and characterized, the results are shown in Fig. 4. The maximum power output is about 1W and the maximum efficiency is about 4%. The devices were HR coated on the back facet and left uncoated on the front facet, then Indium-soldered to a CS-type mount in an epilayer-down configuration. Optimization of the mounting technique may provide additional performance improvements in the CW operation regime.
The device emission spectrum is represented in Fig. 5 at currents close to roll-over conditions.
In a slightly different waveguide geometry, but still using lattice matched designs and MOCVD growth, we have obtained up to 1.4W of LWIR power output in pulsed mode from QC lasers emitting at λ = 9μm. In Fig. 6 we are showing the voltage-current and power-current characteristics of a LWIR device under pulsed operating conditions at room temperature. The device is 6mm long and 12μm wide, with HR coatings on the back facet and is bonded to a CS-type mount as in the previous cases. The front facet was left uncoated. The laser was operated with 200ns pulses at 2% duty cycle while the average output power was measured with a thermopile detector and divided by the duty cycle in order to estimate the peak power during the pulse.
LWIR results at λ = 10μm and beyond
In this range the optical confinement, electron absorption, and intersubband non-radiative transition processes affect even more the QCL operation performances. The free (doping) electron absorption increases with wavelength according to: αel~λn, where the power dependence is in the range n = 2-3; while the intersubband absorption becomes stronger as the transition energies decrease due to the smaller momentum exchange required for transitions between low-energy states in the injection/relaxation regions of the active materials, as compared to higher energy transitions. As for the optical confinement along the vertical direction, if we consider the wavelength in the material with an approximate effective refractive index of n = 3.2, we can estimate λ/n = 2.5μm and 3.3μm for a λ1 = 8.0μm and a λ2 = 10.5μm free-space laser wavelength, respectively. Due to the device geometry and the limitations in fabrication and design, typical active guiding core thicknesses lie in the DAR = 1.5μm-2.5μm range. Therefore, DAR ≈λ1/n, but DAR< λ2/n, and we can expect that while λ1 = 8.0μm lasers will be characterized by a higher confinement factor, typically in the 70% range, λ2 = 10.5μm lasers will not be able to reach comparable overlap factors, typically staying in the 60% range. In addition, in very-long λ QCLs, the optimal waveguide width for thermal dissipation (10μm or lower) is comparable to the optical wavelength, hence limiting the overlap of the optical mode with the active region for lasers designs optimized for CW operation.
In Fig. 7, we show the emission characteristics of a 5mm long, 11μm wide device designed for 10.5μm wavelength. The maximum pulsed optical power is about 750mW but the high current required for the device operation does not allow for efficient CW performances.
The CW performances of these λ = 10.5μm devices are shown in Fig. 8. A maximum power output of 150mW was obtained at 15C from a device mounted epi-side down on a CS-type mount.
Strain compensated devices grown by MBE for high power emission
Electroluminescence (EL) data measured from MBE-grown devices at room temperature for a round mesa in the vicinity of threshold voltage and roll over voltage are shown in Fig. 9(a). EL peak is centered at 9.1µm at threshold and 8.7µm at roll over. Figure 9(b) summarizes EL full width at half maximum (FWHM) dependence on bias. EL is relatively wide at threshold since several radiative transitions contribute to gain. It quickly narrows down at higher bias as a single transition becomes dominant, reaching approximately 14meV at roll over. The narrow EL spectrum at roll over confirms our previous results published in  demonstrating that EL FWHM comparable to that of lattice matched material can be achieved for highly strained compositions.
Figure 10 shows a comparison between pulsed and CW total optical power from both facets vs. current (LI) and voltage vs. current (IV) characteristics at 293 K for an uncoated 3 mm by 10 µm laser. Threshold current density, slope efficiency, maximum WPE and maximum optical power from both facets in pulsed and CW modes at 293K were measured to be 2.1 and 2.5 kA/cm2, 2.8 and 2.1 W/A, 16 and 10%, and 4.4 and 2.0 W, respectively. Both optical power and efficiency in pulsed/CW mode are the highest values reported at this wavelength and exceed the best previously reported result by over factor of two . The inset of Fig. 10 shows that the pulsed laser spectrum taken at maximum current was centered close to 9.2 µm.
An important aspect of the LIV curves shown in Fig. 10 is their behavior at bias values above LI curve roll over. The pulsed LI curve shows a very abrupt decrease in optical power, while the pulsed IV curve shows a sharp increase in differential resistance. This behavior in the vicinity of the roll-over condition demonstrates that carrier tunneling from the injector to the active region states other than the upper laser level is suppressed. In other words, these results indicate improved injection efficiency for the upper laser level. The devices have characteristic temperatures of T0~169K and T1~331K when estimated between 35C and 75C.
Preliminary results for MBE QCLs based on high-strain composition with large E54
Preliminary pulsed LIV characteristics for a quantum cascade laser based on high strain composition with large E54 are shown in Fig. 11. Slope efficiency for the laser stays constant throughout the entire dynamic range. As a consequence, maximum wall-plug efficiency is reached at roll-over current rather than rather than in the middle of dynamic range, which is typically observed for traditional QCLs. In addition, IV curve exhibits a negative differential resistance at bias just above roll-over. These data suggest that injection efficiency for the structure has been further improved. However, due to growth/processing related problems we could not accurately measure injection efficiency for the structure. This will be the focus of our future work.
8. MOCVD-grown DFB QC lasers at λ = 7.4, 7.8, and 9.5μm
The LWIR region of the spectrum is of particular interest for many sensing applications due to the wealth of molecules having their fundamental absorption features in this range. We have demonstrated several distributed-feedback (DFB) lasers with a buried grating obtained during the BH MOCVD process by etching through a sacrificial layer giving the desired refractive index contrast with the cladding layer so that the operating wavelength and the emission characteristics can be controlled and optimized for the specific desired target absorption feature. The typical growth and fabrication procedures are otherwise unchanged respect to the standard FP lasers.
Devices in the 7.xμm range (in particular at λ = 7.43μm and λ = 7.83μm), such as the ones illustrated in Figs. 12 and 13, yield CW power output in single wavelengths close to 300mW in the best cases, and with efficiencies close to 6%. As for longer wavelength DFBs, we manufactured devices at λ = 9.5μm that are capable of P>150mW of continuous output power and operation up to T = 45C or more (see Fig. 14 for details). The temperature tuning coefficient is typically 0.4-0.6nm/K while the CW current tuning is about 50nm/A. All figures show spectral tuning behavior of the QCLs with temperature at fixed current.
9. Current issues and perspectives for future improvement
Great improvements can still be envisioned in this field especially as QC lasers enter the more established domain of high power lasers once covered by laser diodes alone. Several technologies can be transitioned to the Mid- or Long-Wave IR that have proven successful in the field of high power laser diodes and that can help enhance the device performances. In particular optimizing mounting and coatings for high power, scaling up emission characteristics by means of emitter arrays and beam combination techniques, optical amplifiers, broad area emitters, and so on. The push for better performances coming from the applications’ needs will be crucial in determining the development goals of the coming technology improvements in the LWIR and hence the directions that will be taken. The main issues to be overcome, besides optical materials issues for the LWIR (such as coatings, fibers, combiners, isolators, and so on), remain of fabrication and manufacturing nature: in particular dealing with the high uniformity and purity of the active material required to avoid defects over a large volume (i.e. for several mm long waveguides), and being able to manufacture low-loss, thick dielectric waveguides for high power emission.
The team at AdTech would like to acknowledge partial support from the following funding agencies: the Office for Naval Research under contract no. N00014-11-C-0252, and the Missile Defence Agency under contract no. HQ0147-11-C-7701.
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