Design Optimization and Realization Of Interband Cascade Lasers Beyond The Sweet-Sport Operation Wavelength

The main objective of the work was to optimize designs of epitaxially grown mid-infrared interband cascade lasers (ICLs) of different wavelengths based on GaSb and InAs substrates. This implies design, growth and characterisation of low threshold current lasers as well as the theoretical and experimental comparisons of lasers with different claddings and/or growths on different substrates. ICLs are portable photonic devices emitting in mid-infrared region, featuring low threshold current and threshold power densities and high voltage efficiencies. The light emission is based on interband recombination. The active region of an ICL is cascaded, thus it exhibits multiple photon generation for a recombination of a single electron and hole. A high number of stages contributes to lower threshold carrier density to obtain lasing; however, it also increases the overall voltage, hence there is a trade-off between threshold current and threshold power density. A single stage of the active region is composed of InAs/GaInSb/InAs recombination region, hole injector and electron injector. The detailed principle of an ICL is explained in section 3.3. In addition to active region, waveguides of the ICLs play an important role in the performance of the ICL. The main purpose of waveguides is to optically confine the mode to the active region and to separate the mode from the highly doped substrate to avoid the optical losses. In addition, it is important that waveguides provide a sufficient heat dissipation from the active region for a stable laser operation. For these reasons, materials of low refractive index and high thermal conductivity are desirable when designing the waveguides of an ICL. Plasmon-enhanced InAs(Sb) claddings are particularly interested materials as with higher doping, their refractive index can substantially reduce, however on expense of higher optical absorption losses due to high doping. The optical absorption losses are limiting the wavelength range of ICL operation at room temperature. The influences of different waveguide designs on performance of ICLs are discussed in the first three published works listed in the end of this dissertation. In the first work, the two 6-stages GaSb-based ICLs emitting at 5.0 μm of the same active region and geometry, but different cladding species were grown. One ICL was featuring quaternary bulk Al0.85Ga0.15As0.07Sb0.93 of lower refractive index of nr = 3.30 and the other, the commonly used AlSb/InAs superlattice (SL) claddings of nr = 3.39. The lower refractive index provided 14.4 % higher optical mode confinement factor in the active region. The ICL with bulk claddings has shown the superior performance, as its room temperature (RT) threshold current density was 24 % lower than the ICL with SL claddings. The measured threshold power densities of the two ICLs were however comparable due to wider transition layers and insufficient doping of the cap layer of the ICL with bulk claddings, that resulted in its higher series resistance. Plasmon-enhanced claddings have gained attention in recent years as they gradually extended the operation window of ICLs to beyond 14 μm at cryogenic temperatures. In the second work, a similar concept, but for shorter wavelengths, was applied to GaSb-based ICLs and plasmon-enhanced InAs0.915Sb0.085 lattice-matched to GaSb. The device parameters were optimized in terms of maximizing the optical confinement in the active region and minimizing the overall absorption losses. Furthermore, two ICLs emitting at 4.6 μm with the same 12-stages active region and hybrid SL plasmon-enhanced claddings were grown on InAs and GaSb substrates. It is shown that due to lower refractive index of outer n+-InAs0.915As0.085 layer (nr = 2.88) than of n+-InAs (nr = 3.10) and due to higher refractive index of GaSb separate confinement layers (SCLs) than of InAs SCLs, the ICL grown on GaSb substrate has 3.8 % higher optical confinement factor in the active region. Experimentally, the GaSb-based ICL has performed with 15.3 % lower threshold current density, but also 12.8 % higher threshold voltage, resulting in comparable threshold power densities. The measured absorption losses in GaSb-based and InAs-based ICLs of 5.8 cm-1 and 4.6 cm-1 are reasonably matching with the calculated values of 6.4 cm-1 and 4.9 cm-1 Despite the lower optical mode confinement in highly doped plasmon-enhanced claddings in GaSb-based ICL, the lower mobility in the GaSb than in InAs SCLs resulted in higher total free carrier absorption (FCA) in GaSb-based ICL. It is shown that the calculated threshold gains of the two ICLs are equal due to compensation of higher optical confinement in the active region with the higher absorption losses, and vice-versa. Finally, in the third work a high performance 8-stages ICL emitting at 5.2 μm and featuring the hybrid cladding design, was realized. Due to the low refractive index of n+-InAs0.915As0.085 (nr = 2.70), the cladding design projected 11.2 % higher optical mode overlap with the active region than the ICL of the same active region and SL claddings. The measured RT threshold current density of Jth = 242 A/cm2 is the lowest reported value in pulsed mode for wavelengths longer than 5 μm. The threshold power density of Pth = 840 W/cm2 was matching the lowest reported values at similar wavelengths. The threshold current density of continuous wave (cw) operation at RT was higher than pulsed operation was suggesting, Jth = 861 A/cm2. The maximum measured output power was 6.1 mW, with the wallplug efficiency of 1.4 %, which is in the range of reported values in literature. In addition to variations and optimizations of cladding designs, an alternative active region is discussed for long wavelength emission. A design composed of AlSb/InAs/GaInSb/InAs1-xSbx /GaInSb/InAs/AlSb was proposed for a recombination region of long wavelength ICL with a focus on wavelength of 7.5 μm. The symmetrical triple V-shaped well design has shown 18.9 % higher wavefunction overlap than the standard W-shaped well of the same wavelength. For this comparison, both W-shaped and triple V-shaped designs were optimized for every layer thickness and arsenic fraction ratio of the middle well (x). In addition, the calculated valence intersubband absorption (VISA) loss of the V-shaped structure was notably lower than for W-shaped QW regardless of bias applied. However, it is shown that despite the higher wavefunction overlap and lower absorption, application of triple V-shaped wells in ICLs can be challenging due to high bias sensitivity of the coupling of ground states of electron InAs(Sb) and hole GaInSb QWs.