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We show that mid-infrared transmission spectroscopy of a quantum cascade laser provides clear-cut information on changes in charge location at different bias. Theoretical simulations of the evolution of the gain/absorption spectrum for a ϳ 7.4 m InGaAs/AlInAs/InP quantum cascade laser have been compared with the experimental findings. Transfer of electrons between the ground states in the active region and the states in the injector goes hand in hand with a decrease of discrete intersubband absorption peaks and an increase of broad, high-energy absorption toward the continuum delocalized states above the barriers.

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 Lambda ~4.7 µm 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 150 K, 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

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A broadband midinfrared laser operating up to 340K in pulsed mode is presented. It was realized by incorporating seven different quantum cascade active regions with 0.2μm increments to emit at different wavelengths. Despite the relatively large wavelength increments, a continuous lasing spectrum is obtained from 7.7to8.4μm, with significant improvements in threshold current and temperature performance compared with previously reported broadband quantum cascade lasers. A full temperature dependent characterization was performed, and the threshold current density at 300K was 6.5kA∕cm2, with peak output power of over 100mW. A characteristic temperature of T0=192K was obtained.

Publisher's Note: "High-peak-power strain-compensated GaInAs/AlInAs quantum cascade lasers (λ4.6 μm) based on a slightly diagonal active region design" [Appl.We report on the midinfrared broadband transmission spectroscopy measurements of a ϳ 4.3 m strain compensated In 0.64 Ga 0.36 As/ Al 0.58 In 0.42 As/ InP quantum cascade laser. A detailed experimental analysis of the electronic distribution for bias values below the laser threshold is presented, highlighting the effects of the design with strongly diagonal laser transition. A marked voltage induced Stark shift is observed for the diagonal laser transition while the vertical intersubband transitions involved higher energy levels remained nearly bias independent. We also demonstrate the direct observation of intersubband transitions originating from the ground level of the injector miniband to the level confined above the AlInAs barriers.

We report the realisation of spectroscopic broadband transmission experiments on quantum cascade lasers (QCLs) under continuous wave operating conditions for drive currents up to laser threshold. This technique allows, for the first time, spectroscopic study of light transmission through the waveguide of QCLs in a very broad spectral range (λ~1.5-12 µm), limited only by the detector response and by interband absorption in the materials used in the QCL cladding regions. Waveguide transmittance spectra have been studied for both TE and TM polarization, for InGaAs/InAlAs/InP QCLs with different active region designs emitting at 7.4 and 10µm. The transmission measurements clearly show the depopulation of the lower laser levels as bias is increased, the onset and growth of optical amplification at the energy corresponding to the laser transitions as current is increased towards threshold, and the thermal filling of the second laser level and decrease of material gain at high temperatures. This technique also allows direct determination of key parameters such as the exact temperature of the laser core region under operating conditions, as well as the modal gain and waveguide loss coefficients.

We report broadband quantum cascade lasers emitting over a range of 6 to 8 µm at 77K and having J th =4.6kA/cm 2 at 300K. Superluminescent diodes utilizing the same design will also be described.