Design And Characterization Of Quantum Cascade Lasers

Quantum cascade laser (QCL), as a unipolar semiconductor laser based on intersubband transitions in quantum wells, covers a large portion of the Mid and Far Infrared electromagnetic spectrum. The frequency of the optical transition can be determined by engineering the layer sequence of the heterostructure. The focus of this work is on Terahertz (THz) frequency range (frequency of 1 - 10 THz and photon energy of ~ 4 - 40 meV), which is lacking of high power, coherent, and efficient narrowband radiation sources. THz QCL, demonstrated in 2002, as a perfect candidate of coherent THz source, is still suffering from the empirical operating temperature limiting factor of T [ap] h̳[omega]/kB, which allows this source to work only under a cryogenic system. Most of high performance THz QCLs, including the world record design which lased up to ~ 200 K, are based on a resonant phonon (RP) scheme, whose population inversion is always less than 50%. The indirectly-pumped (IDP) QCL, nicely implemented in MIR frequency, starts to be a good candidate to overcome the aforementioned limiting factor of RP-QCL. A rate equation (RE) formalism, which includes both coherent and incoherent transport process, will be introduced to model the carrier transport of all presented structures in this thesis. The second order tunneling which employed the intrasubband roughness and impurity scattering, was implemented in our model to nicely predict the behavior of the QCL designs. This model, which is easy to implement and fast to calculate, could help us to engineer the electron wavefunctions of the structure with optimization tools. We developed a new design scheme which employs the phonon scattering mechanism for both injecting carrier to the upper lasing state and extracting carrier from lower lasing state. Since there is no injection/extraction state to be in resonance with lasing states, this simple design scheme does not suffer from broadening due to the tunneling. Finally, three different THz IDP-QCLs, based on phonon-photon-phonon (3P) scheme were designed, grown, fabricated, and characterized. The performance of those structures in terms of operating temperature, threshold current density, maximum current density, output optical power, lasing frequency, differential resistance at threshold, intermediate resonant current before threshold, and kBT/h̳[omega] factor will be compared. We could improve the kBT/h̳[omega] factor of the 3P-QCL design from 0.9 in first iteration to 1.3 and the output optical power of the structure from 0.9 mW in first design to 3.4 mW. The performance of the structure in terms of intermediate resonant current and the change in differential resistance at threshold was improved.

Quantum cascade lasers provide some of the highest output powers available for light in the mid-infrared range (from 3 to 8 m). As many of their applications require portability, designs that have a high wall-plug efficiency are essential, and were designed and grown by others to achieve this goal. However, because a large fraction of these devices did not operate at all, very few of the standard laser measurements could be performed to determine their properties. Therefore, measurements needed to be performed that could non-destructively probe the behavior of QCLs while still providing useful information. This thesis explores these types of measurements, all of which fall into the category of device spectroscopy. Through polarization-dependent transmission and photovoltaic spectroscopy, a large portion of the quantum mechanical bandstructure could be determined, along with many of the parameters characterizing crystal growth quality. In addition, high-resolution transmission spectroscopy was used to find the properties of the QCL waveguide. In order to find the correspondence between theory and experiment, bandstructure simulations were performed using a three-band p model, and two-dimensional electromagnetic simulations were performed to describe the laser's optical properties. These simulations were found to be in relatively good agreement with the device measurements, and any discrepancies were found to be consistent with problems in the growth and fabrication.

The as yet unattained milestone of room-temperature operation is essential for establishing Terahertz Quantum Cascade Lasers (THz QCLs) as practical sources of THz radiation. Temperature performance is hypothesized to be limited by upper laser level lifetime reduction due to non-radiative scattering, particularly by longitudinal optical phonons. To address this issue, this work studies highly "diagonal" QCLs, where the upper and lower laser level wave functions are spatially separated to preserve upper laser level lifetime, as well as several other issues relevant to high temperature performance. The highly diagonal devices of this work performed poorly, but the analysis herein nevertheless suggest that diagonality as a design strategy cannot yet be ruled out. Other causes of poor performance in the lasers are identified, and suggestions for future designs are made.

In the past 20 years, mid-infrared Quantum Cascade Lasers (mid-IR QCLs) have been experiencing rapid development and have become practical mid-IR sources for a variety of applications. There is particular technological interest in high efficiency lasers designed for the midwave infrared (MWIR) atmospheric window (3-5 [mu]m) and longwave infrared (LWIR) atmospheric window (8-13 [mu]m). This work presents a systematic study over mid-IR QCLs, including theoractical modeling, device fabrication and characterization. An effective bandstructure calculation method is implemented in this work for active region modeling. A standard process for fabricating mid-IR QCLs has been developed, based on which both LWIR (~ 9 [mu]m) and MWIR (~ 4 [mu]m) QCLs have been successfully demonstrated. Comprehensive testing results are analyzed and discussed, yielding valuable information about the current device design.

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