The linewidth of a conventional laser is due to fluctuations in the laser field due to spontaneous emission and described by the Schalow-Townes formula. In addition to that, in a semiconductor laser there is a contribution arising from fluctuations in the refractive index induced by carrier density fluctuations. The later are quantitatively described by the linewidth enhancement or alpha factor [C. H. Henry, IEEE J. Quantum Electron. 18 (2), 259 (1982), W. W. Chow, S. W. Koch and M. Sargent III, Semiconductor-Laser Physics, Springer-Verlag (1994), M.F. Pereira Jr et al, J. Opt. Soc. Am. B10, 765 (1993). In this paper we investigate the alpha factor of quantum cascade lasers under actual operating conditions using the Nonequilibrium Greens Functions approach [A. Wacker et a, IEEE Journal of Sel. Top. in Quantum Electron.,19 1200611, (2013), T. Schmielau and M.F. Pereira, Appl. Phys. Lett. 95 231111, (2009)]. The simulations are compared with recent results obtained with different optical feedback techniques [L. Jumpertz et al, AIP ADVANCES 6, 015212 (2016)].
Regardless of all the success of Mid Infrared Quantum Cascade Lasers (QCLs), they still do not operate at room temperature in the THz range. The main temperature degrading mechanism for THz QCLs is not known in time of writing this abstract and it is still a topic of debate by the community [S. Khanal et al, J. Opt. 16 094001, 2014]. This is a challenge to theory and it is crucial to treat all possible scattering channels with the same mathematical footing. A summary of different methods for simulating these structures is found in [C. Jirauschek et al, Appl. Phys. Rev. 1 011307, 2014]. In this work we include and study the effects of electron-electron scattering via the Single Plasmon Pole Approximation (SPPA). In this approximation we capture both the static limit as well as dynamic effects. This gives an energy dependent (non-local in time) interaction beyond the Hartree-Fock approximation. This has been studied in a similar model with promising results [T. Schmielau and M.F. Pereira, Appl. Phys. Lett. 95 231111, 2009], and with this work we want to adapt the idea into the model described in Ref. [A. Wacker et a, IEEE Journal of Sel. Top. in Quantum Electron.,19 1200611, 2013]. We start by summarizing the theory underlying the SPPA and we show how it is implemented in the context of our formalism, by showing good agreement with the results for a four well quantum cascade laser [M. Amanti et al, New J. Phys. 11 125022, 2009].
Superlattices are artificial structures with a wide range of applications and open possibilities for controlling and study transport and optical [M.F. Pereira Jr., Phys. Rev. B 52, (1995)] properties of semiconductors. In this work, we start from the full Nonequilibrium Greens Functions approach [A. Wacker et a, IEEE Journal of Sel. Top. in Quantum Electron.,19 1200611, (2013),T. Schmielau and M.F. Pereira, Appl. Phys. Lett. 95 231111, (2009)] to obtain Voltage-Current curves and compare them with experiments. By adjusting the numerical solutions of the corresponding Dyson equations to a simple model, analytical solutions are given for the nonlinear response of a biased superlattice under sub-THz radiation. The frequency multiplication process leading to multiple harmonicgeneration is described. This hybrid approach leads to predictive simulations and may have important application for a new generation of devices where the superlattices are used as both sources and detectors and may be particular useful for high resolution transient spectroscopy [A.A. Yablokov et at, IEEE Transactions on THz Science and Technology 5, 845 (2015)].
The two main design schemes for Terahertz quantum cascade lasers, based on tunnelling and scattering injection,
respectively, are theoretically compared. We apply our simulation package based on the non-equilibrium Green’s
function technique. Our results provide a good description of the gain degradation with temperature. Thermal
backfilling contributes to decrease of population inversion in both cases. However, the dropping inversion cannot
account for the total reduction of gain.
The gain profile of a quantum cascade laser is strongly influenced by the lifetime of the carriers in the upper and
lower laser state. The quantitative description of gain within the concept of nonequilibrium Green's functions
allows for a detailed understanding of various features affecting the gain spectrum: Compensation effects between
scattering processes in the upper and lower laser level, reduction of gain due to coherences between nearly
degenerate upper laser states, and dispersive gain without inversion.
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 consider a semiconductor superlattice biased into the regime of negative differential conductivity and driven by an additional GHz ac voltage, and find frequency-locked or quasiperiodic propagating field domains. With increasing driving frequency, the complex impedance exhibits strong variations of its amplitude and phase. An anomalous phase shift appears as a result of phase synchronization of the traveling domains.