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Theoretical investigation of electro-absorption modulators in the mid-infrared range (>∼2 μm) is performed using asymmetric intra-step quantum wells based on Ge1−η1Snη1/Ge1−η2Snη2 heterostructures with SiGeSn outer barriers. After exploring the parameter space of the Sn content difference and width ratio of the intra-layers, a linear and much larger Stark shift is realized, compared to that of a square quantum well, without an increase of the total structure width. A modulator based on an optimized intra-step quantum well structure with a total well width of 12 nm is theoretically predicted to have both a larger peak shift per unit applied field and a larger absorption change than a 12 nm square quantum well device. By analyzing the device performance based on the two figures of merit: (1) absorption change per applied field and (2) absorption change per applied field squared, and taking 10 dB extinction ratio, a 44% higher bandwidth per volt and 46% lower power consumption per bit are achieved in intra-step than in a square well. Although the swing voltage for a square quantum well can be reduced by using a larger on-set applied field and performance could be improved, we found that the intra-step quantum well using zero on-set still retains its advantages when compared to the square quantum well which uses a 0.5 V on-set voltage.

Quantum cascade lasers (QCLs) have broken the spectral barriers of semiconductor lasers and enabled a range of applications in the mid-infrared (MIR) and terahertz (THz) regimes. However, until recently, generating ultrashort and intense pulses from QCLs has been difficult. This would be useful to study ultrafast processes in MIR and THz using the targeted wavelength-by-design properties of QCLs. Since the first demonstration in 2009, mode-locking of QCLs has undergone considerable development in the past decade, which includes revealing the underlying mechanism of pulse formation, the development of an ultrafast THz detection technique, and the invention of novel pulse compression technology, etc. Here, we review the history and recent progress of ultrafast pulse generation from QCLs in both the THz and MIR regimes.

A direct bandgap can be engineered in Ge-rich group-IV alloys by increasing Sn content and by introducing tensile strain in GeSn. Here, we combine these two routes in quantum well (QW) structures and systematically analyze the properties of SiGeSn/GeSn quantum wells for a range of Sn content, strain, and well width values, within realistic boundaries. Using the k Á p method, and including L-valley within the effective mass method, we find that 13-16 nm is a preferred range of well widths to achieve high gain for tensile-strained SiGeSn/GeSn quantum wells. Within the range of the well widths, a loss ridge caused by inter-valence band absorption and free carrier absorption is found in the region of parameter space where Sn content and strain in the well are related as Sn(%) % À7:71ε xx (%) þ 17:13. Limited by a practical strain boundary of 1.7%, for a 14 nm quantum well, we find that 7:5 + 1% Sn and 1 + 0:2% strain is a promising combination to get a good net gain for photon transition energy higher than ∼0.42 eV. A maximum utilization of strain is preferred to obtain the best gain with lower energies (<0.42 eV). By comparing these designs with a compressive strain example, an engineered tensile structure shows a better performance, with a low threshold current density (1.42 kA/cm 2 ). Finally, the potential benefit of p-doping of the tensile-strained GeSn QW is also discussed.