We review the history, development, design principles, experimental operating characteristics, and specialized architectures of interband cascade lasers for the mid-wave infrared spectral region. We discuss the present understanding of the mechanisms limiting the ICL performance and provide a perspective on the potential for future improvements. Such device properties as the threshold current and power densities, continuous-wave output power, and wall-plug efficiency are compared with those of the quantum cascade laser. Newer device classes such as ICL frequency combs, interband cascade vertical-cavity surface-emitting lasers, interband cascade LEDs, interband cascade detectors, and integrated ICLs are reviewed for the first time.

Two ten-stage interband cascade laser structures were grown by solid-source molecular beam epitaxy. The wafers were processed into both broad-area ridges for threshold characterization and Au-electroplated narrow ridges for high-temperature cw operation. Pulsed threshold current densities in the broad-area devices were as low as $3.8\text{A}/{\text{cm}}^2$ at 78 K and $590\text{A}/{\text{cm}}^2$ at 300 K. An $11\text{-}\mu \text{m}$ -wide ridge exhibited a new maximum cw operating temperature of 288 K at $\lambda =4.1\mu \text{m}$ . An even longer-wavelength device emitted at $5.1\mu \text{m}$ with a maximum cw operating temperature of 229 K.

The balance of photogeneration and recombination gives rise to an optimum bandgap for any solar cell. The radiative limit represents the lowest permissible level of recombination in a solar cell and, therefore, places an upper limit on the voltage that can be attained. Introducing additional nonradiative recombination results in a loss in voltage that can only be compensated for by moving to higher bandgaps. Consequently, the optimal bandgap for solar energy conversion will rise with increasing nonradiative recombination rate. This balance was recognized by Shockley and Queisser for single-junction solar cells and is here extended to multijunction solar cells. A rise in optimal bandgaps has been observed in simulated single-, double-, and triple-junction devices as nonradiative recombination increases. Optimal bandgaps between excellent and poor diode quality devices are shown to differ by 100s of meV under 1-sun illumination with both terrestrial and extraterrestrial spectra but exhibit no significant change at high concentration due to the dominance of the radiative component in the recombination dynamics.

Silicon integration of mid-infrared (MIR) photonic devices promises to enable low-cost, compact sensing and detection capabilities that are compatible with existing silicon photonic and silicon electronic technologies. Heterogeneous integration by bonding III-V wafers to silicon waveguides has been employed previously to build integrated diode lasers for wavelengths from 1310 to 2010 nm. Recently, Fabry-Pérot Quantum Cascade Lasers integrated on silicon provided a 4800 nm light source for mid-infrared (MIR) silicon photonic applications. Distributed feedback (DFB) lasers are appealing for many high-sensitivity chemical spectroscopic sensing applications that require a single frequency, narrow-linewidth MIR source. While heterogeneously integrated 1550 nm DFB lasers have been demonstrated by introducing a shallow surface grating on a silicon waveguide within the active region, no mid-infrared DFB laser on silicon has been reported to date. Here we demonstrate quantum cascade DFB lasers heterogeneously integrated with silicon-on-nitride-on-insulator (SONOI) waveguides. These lasers emit over 200 mW of pulsed power at room temperature and operate up to 100 °C. Although the output is not single mode, the DFB grating nonetheless imposes wavelength selectivity with 22 nm of thermal tuning.

We describe how a midwave infrared photonic integrated circuit (PIC) that combines lasers, detectors, passive waveguides, and other optical elements may be constructed on the native GaSb substrate of an interband cascade laser (ICL) structure. The active and passive building blocks may be used, for example, to fabricate an on-chip chemical detection system with a passive sensing waveguide that evanescently couples to an ambient sample gas. A variety of highly compact architectures are described, some of which incorporate both the sensing waveguide and detector into a laser cavity defined by two high-reflectivity cleaved facets. We also describe an edge-emitting laser configuration that optimizes stability by minimizing parasitic feedback from external optical elements, and which can potentially operate with lower drive power than any mid-IR laser now available. While ICL-based PICs processed on GaSb serve to illustrate the various configurations, many of the proposed concepts apply equally to quantum-cascade-laser (QCL)-based PICs processed on InP, and PICs that integrate III-V lasers and detectors on silicon. With mature processing, it should become possible to mass-produce hundreds of individual PICs on the same chip which, when singulated, will realize chemical sensing by an extremely compact and inexpensive package.

Interband cascade lasers with ten active stages were grown by molecular beam epitaxy (MBE). Devices emitting near $3.6\mu \mathrm{m}$ at room temperature operated in pulsed mode to a temperature of $330\mathrm{K}$ . The threshold current densities were the lowest to date for NRL growths, ranging from $9\mathrm{A}∕{\mathrm{cm}}^2$ at $78\mathrm{K}\text{to}1.7\mathrm{kA}∕{\mathrm{cm}}^2$ at $300\mathrm{K}$ . The voltage efficiency was $\approx 94\%$ at $140\mathrm{K}$ , and the pulsed slope efficiency was $121\mathrm{mW}∕\mathrm{A}$ at $300\mathrm{K}$ . Antimonide photodetectors with a cutoff wavelength of $12\mu \mathrm{m}$ were also grown in the same machine by MBE. The absorbing layer consisted of an $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{In}\mathrm{Sb}$ type-II superlattice, and the depletion region comprised a four-layer “W” structure with a graded energy gap in order to minimize the tunneling and generation currents. Quantum efficiencies of 30%–40% were achieved in the $8–10\mu \mathrm{m}$ band.