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We report the improvement of quantum cascade lasers emitting at λ∼5 μm by introducing AlAs blocking barriers together with strain-compensating InAs layers into the active regions. The blocking barriers are designed to selectively prevent electrons in the initial laser state from tunneling out of the active region, while maintaining the high tunneling probabilities of the electrons in the final laser states. Adopting blocking barriers, the maximum peak power per facet at 77 K (300 K) is increased from 285 (30 mW) to 900 mW (240 mW), and the maximum operation temperature in pulsed mode has been improved from 320 to 350 K with respect to a reference sample without blocking barriers.

We point out the difference between the two mostly used methods for calculating the effective refractive index of mid-infrared quantum cascade lasers, namely by solving the Maxwell's wave equation and by analyzing the frequency spacing of the longitudinal modes of the Fabry-Perot cavity. The effective refractive indices obtained by these methods are shown to be different, as one refers to the phase effective refractive index while the other refers to the group effective refractive index, respectively. Dispersion relationships for these two effective refractive indices are deduced for mid-infrared quantum cascade lasers, which show an increase in group refractive index and a decrease in phase refractive index with wavelength. Experiments are conducted to obtain the group effective refractive indices of quantum cascade lasers emitting at various wavelengths, and good agreement has been achieved between the experimental data and theory.

Employing a “slightly diagonal” active region design for the quantum cascade lasers compared to a reference sample based on the conventional vertical transition design [R. Kohler et al., Appl. Phys. Lett. 76, 1092 (2000)], we have improved the maximum operation temperature, room-temperature maximum peak power per facet, and room-temperature slope efficiency from 320 K, 200 mW, and 570 mW/A to higher than 360 K, 3.2 W, and 2200 mW/A, respectively, for the device size of 16 μm×3 mm with as-cleaved facets operated in pulsed mode.

We report the measurements of dispersive gain (simultaneous coexistence of gain and losses on a single intersubband transition) in a quantum cascade laser. Broadband transmission spectra through the waveguide of a λ∼4.7μm In0.53Ga0.47As∕AlAs0.56Sb0.44∕InP quantum cascade laser have been studied at a bias below laser threshold and at different temperatures. For a certain range of current, and at temperatures higher than about 150K, the transmission spectra show clear dispersive gain/loss behavior with the possibility for intersubband gain to be observed even without global population inversion between laser levels.

The authors present a technique for broadband study of the optical waveguides used for midinfrared quantum cascade lasers. This method allows insight into the waveguide properties and helps in improving the waveguide design. Using this method the waveguide losses in the whole range of waveguide transparency of about 1.8–9μm have been found for a λ∼4.4μm In0.53Ga0.47As∕AlAs0.56Sb0.44∕InP quantum cascade laser. The polarization dependence of waveguide losses in a quantum cascade laser is discussed.

High peak-power operation of quaternary-barrier GaInAs∕AlGaAsSb quantum-cascade lasers emitting at λ∼3.6–3.8μm is reported. With as-cleaved facets, the lasers (18μm×2.7mm) emit a maximum peak power of 8.2W per facet at 77K. When applying a high-reflection coating to the back facet, a maximum peak power emitted from the front facet of 10.5W at 77K has been obtained for an 18μm×2.0mm device. The lasers operate in pulsed mode up to 340K, and their emission wavelength shifts from 3.65μm at 77Kto3.79μm at 300K.

Short-wavelength (λ∼4μm) single-mode distributed-feedback GaInAs∕AlAsSb quantum-cascade lasers operating in pulsed mode up to room temperature (300K) have been demonstrated by etching an index-coupled first-order distributed-feedback grating into the upper GaInAs separate confinement layer. The temperature-dependent wavelength shift of the distributed-feedback lasers is −0.14cm−1∕K (0.238nm∕K). For devices with a size of 18μm×2.9mm mounted epilayer-up with as-cleaved facets, a maximum peak power per facet of 840mW has been achieved at 77K and 4mW at 300K. The characteristic temperature T0 of the threshold current density is 105K.

We discuss the possible limitations on realizing short-wavelength GaInAs∕AlAsSb quantum-cascade lasers and consequently demonstrate room-temperature (Tmax=310K) short-wavelength (λ∼3.7–3.9μm) GaInAs∕AlAsSb quantum-cascade lasers based on triple-well vertical-transition active regions. For a device with the size of 14μm×3.0mm with as-cleaved facets, the maximum peak powers per facet are around 31mW at 300K and 17mW at 310K. The characteristic temperature of the laser is 170K in the temperature range between 220 and 310K.

Above room-temperature (T⩾400K) operation of GaInAs∕AlAsSb-based quantum-cascade lasers has been demonstrated. The lasers are based on vertical-transition active regions and consist of 25 periods of Ga0.47In0.53As∕AlAs0.56Sb0.44 active∕injection regions grown lattice-matched on InP substrates by molecular-beam epitaxy. They emit at a wavelength of λ∼4.5μm. For a device with the size of 18μm×2.8mm mounted substrate-side down with as-cleaved facets, a maximum peak power per facet of 750mW has been achieved at 300K and remains as high as 30mW at 400K. The characteristic temperature T0 of the threshold current density is 171K in the temperature range between 280K and 400K.

Quaternary-barrier-containing GaInAs∕AlGaAsSb quantum-cascade lasers, motivated by reducing the barrier height compared to that in GaInAs∕AlAsSb quantum-cascade lasers, have been demonstrated. The design of these quaternary-barrier-containing lasers is based on triple-quantum-well vertical-transition active regions, and their fabrication relies on molecular-beam-epitaxial growth of Ga0.47In0.53As∕AlGaAs1−xSbx (x close to 0.45) heterostructures on n-InP substrates. Including twenty-five periods of active regions and injection regions, the quantum-cascade lasers operate up to T⩾400K in pulsed mode, with an emission wavelength of about 4.9 μm at room temperature. The characteristic temperature T0 of the threshold current density is 169 K in the temperature range between 280 and 400 K.