Highly Efficient And Reliable Quantum Cascade Lasers

Mid-infrared (mid-IR) quantum cascade lasers (QCLs) have been commercially available for low power applications, however, while the desire for higher power devices is present, the efficiency and reliability are severe limitations. This work takes a multi-faceted approach to improving the reliability and efficiency of QCLs including: the identification and mitigation of failure mechanisms under high power continuous wave (CW) and quasi-continuous wave (QCW) operation, optical and thermal modeling of devices to further reduce active region heating, verification of these models using charge-coupled device (CCD) based thermoreflectance, and the introduction of interface roughness (IFR) engineered devices to reduce IFR scattering and leakage. Atom probe tomography (APT) is also employed to investigate the amount of aluminum and gallium incorporation in thin InAlAs barriers and InGaAs wells. It was found that thin layers with thicknesses less than 2 nm require an intentional aluminum or gallium overshoot in the gas phase during growth to grow the targeted compositions. This was verified when the overshoot in thin barriers resulted in the convergence of modeled and experimental emitting wavelengths. APT was also used to interrogate a few key interfaces within a 40 stage strain-compensated QCL emitting near 4.6 [mu]m. This interrogation yielded both in-plane and axial IFR parameters for barriers of high and low aluminum incorporation, and in turn high and low strain, respectively. It was found that the barrier with the highest aluminum target had a nearly 50% larger root mean square (RMS) roughness when compared to the shorter barriers. As the IFR scattering is proportional to the square of both the RMS roughness and in-plane correlation length, this finding has a significant impact on the IFR scattering and leakage. The variable IFR parameters, axial correlation length, graded interfaces, graded lattice constants, graded conduction band edge, and quaternary alloy disorder (AD) scattering have been incorporated into a scattering model. Results from this model suggest lower global lifetimes and significantly reduced transition efficiencies which results in lower IFR leakage, however, if electronic temperatures from software using non-equilibrium Green's function (NGEF) is incorporated, leakage currents remain high.

Quantum Cascade Lasers are a novel semiconductor light source with the unique property of wavelength tunability over the mid-infrared and terahertz range of frequencies. Advances since their first demonstration in 1994 have led to highly efficient designs capable of continuous room temperature operation. In lieu of increased advances in laser core efficiency, power scaling with broad area quantum cascade lasers has demonstrated enhanced continuous power. This initial work is used as a starting point for continuing advances in average brightness of quantum cascade lasers. A figure of merit calculation reliably predicts to within parts in thousands the qualitative beam profile of continuously driven and high duty cycle devices. Further, a model is developed to project performance not only in continuously driven conditions, but also in variable duty cycles. This is combined with the figure of merit calculation to guide designs for optimized average brightness.

Quantum cascade lasers (QCLs) are semiconductor lasers that emit in the mid- to far-infrared and employ intersubband transitions in multiple quantum-well structures. Conventionally, the active region of QCLs has consisted of quantum wells and barriers of fixed-alloy composition. That has led to severe carrier leakage from the upper-laser level and injector states, evidenced by strong temperature dependences of the device characteristics, which resulted in low values for wall-plug efficiency [eta]wp of CW-operating devices. We have devised in the past means for carrier-leakage suppression, and have recently derived a comprehensive carrier-leakage formalism that bridges the gap between theoretical and experimental values for the internal efficiency. Here we present a refinement of the comprehensive carrier-leakage formalism and employ it for comparing our band-engineered ~ 8 [mu]m-emitting QCL, so-called step-tapered active-region (STA), to a conventional ~ 8 [mu]m-emitting QCL. We find that the internal efficiency reaches a high value of ~ 73.6%, due to record-high injection- and laser-transition efficiencies. Experimentally we obtain a single-facet [eta]wp value of 10.6%, a record-high value for 8-11 Îơm-emitting QCLs grown by MOCVD. Then, by using both band- and interface-roughness (IFR)-scattering - engineering we designed an optimized 8.2 [mu]m-emitting STA-QCL that reaches a record-high injection efficiency of 89.5%. By minimizing the waveguide loss and raising the doping level the device reaches a record-high internal efficiency (80%) for ~ 8 [mu]m-emitting QCLs as well as a projected [eta]wp value of 11.2%. The studies are extended to devices of higher layer-interface quality, grown by two different techniques. As a result, we obtain [eta]wp values as high as 15.6 %. In addition, the optimized STA-QCL has a lower-level lifetime dominated by IFR scattering, which makes it amenable to further optimization via IFR engineering. Finally, we analyze an ~ 8 [mu]m-emitting QCLs that holds the world record [eta]wp value, primarily due to low voltages via the realization of photon-induced carrier transport. We find that the device has significant carrier leakage, and show that our optimized STA QCL can reach comparable [eta]wp values if high-quality interfaces are employed. We then derive ultimate limits for the [eta]wp value in the 7-11 [mu]m wavelength range.

The mid-infrared domain is a promising optical domain because it holds two transparency atmospheric windows, as well as the fingerprint of many chemical compounds. Quantum cascade lasers (QCLs) are one of the available sources in this domain and have already been proven useful for spectroscopic applications and free-space communications. This thesis demonstrates how to implement a private free-space communication relying on mid-infrared optical chaos and this requires an accurate cartography of non-linear phenomena in quantum cascade lasers. This private transmission is made possible by the chaos synchronization of two twin QCLs. Chaos in QCLs can be generated under optical injection or external optical feedback. Depending on the parameters of the optical feedback, QCLs can exhibit several non-linear phenomena in addition to chaos. Similarities exist between QCLs and laser diodes when the chaotic dropouts are synchronized with an external modulation, and this effect is known as the entrainment phenomenon. With a cross-polarization reinjection technique, QCLs can generate all-optical square-waves. Eventually, it is possible to trigger optical extreme events in QCLs with tilted optical feedback. All these experimental results allow a better understanding of the non-linear dynamics of QCLs and will extend the potential applications of this kind of semiconductor lasers.

Quantum cascade laser (QCL) performance continues to improve towards the requirements of applications such as infrared counter measures. However, key metrics, such as wall-plug efficiency (WPE), are still not fully met. DARPA's EMIL program continues to support progress in QCLs, and this report summarizes the Princeton team's work during Phase I of this program. Although the work systematically addressed all major facets of efficiency, the greatest advancements involved injection designs, which improved almost all efficiency components. Strain compensated QCLs with heterogeneous injectors produced low voltage defect. The active core consisted of interdigitated undoped and doped injectors followed by nominally identical optical transitions. The undoped injectors were designed with reduced voltage defect while the doped injector designs were more conventional. The measured average voltage defect was less than 79 meV. At 80 K, a 2.3 mm long, back facet high reflectance coated laser had an emission wavelength of 4.7 micrometers and output 2.3 W pulsed power with 19% peak WPE. Other QCLs emitting at 4.2 micrometers featured a low voltage defect and short injector with only four quantum wells. Devices with a voltage defect of 20 meV and a record voltage efficiency of 91% were demonstrated for pulsed operation at 180 K. Voltage efficiencies of greater than 80% were exhibited at room temperature. WPEs ranging from 21% at cryogenic temperatures to 5.3% at room temperature were achieved. Interface roughness effects were analyzed as in homogeneous broadening, explaining the temperature dependent QCL gain spectra and suggesting improved designs. Specifically, density-matrix theory revealed benefits from stronger coupling between injector and upper laser level that led to low-temperature pulsed QCLs nearing 50% WPE.

Tremendous progress has been made in the last few years in the growth, doping and processing technologies of the wide bandgap semiconductors. As a result, this class of materials now holds significant promis for semiconductor electronics in a broad range of applications. The principal driver for the current revival of interest in III-V Nitrides is their potential use in high power, high temperature, high frequency and optical devices resistant to radiation damage. This book provides a wide number of optoelectronic applications of III-V nitrides and covers the entire process from growth to devices and applications making it essential reading for those working in the semiconductors or microelectronics. Broad review of optoelectronic applications of III-V nitrides

Learn how the rapidly expanding area of mid-infrared and terahertz photonics has been revolutionized in this comprehensive overview. State-of-the-art practical applications are supported by real-life examples and expert guidance. Also featuring fundamental theory enabling you to improve performance of both existing and future devices.

This book describes the physics, fabrication technology, and applications of the quantum cascade laser.

The proposed research is a study of designing high-efficiency Mid-IR quantum cascade lasers (QCL). This thesis explores "injector-less" designs for achieving lower voltage defects and improving wall plug efficiencies through highly strain-balanced structures and minimized injector regions. This work contains experimental design work for testing and evaluating Mid-IR QCL performance, simulation work for verifying wavefunction and energy alignment, as well as, Monte Carlo transport simulations for evaluating designs, and finally measuring lasing and spontaneous emission performance for various designs.