| 1. |
- Nelander, Rikard, et al.
(författare)
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Fingerprints of spatial charge transfer in Quantum Cascade Lasers
- 2007
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 102, s. 1-113104
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Tidskriftsartikel (refereegranskat)abstract
- 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 lambda~7.4 µm 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.
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| 2. |
- Nelander, Rikard
(författare)
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Lineshape in Quantum Cascade Lasers - Temperature, Screening and Broadening
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation deals with the optical properties of Quantum Cascade Lasers (QCLs). Studies are performed using transport calculations employing the theory of Non-Equilibrium Green's Functions, and screening is studied within the Random Phase Approximation. Focus is put on the temperature dependent effects in THz QCLs. After a popular science summary and a presentation and discussion of the theory and the methods used in this work, five original papers are presented. Paper I investigates the evolution of the optical spectra as a function of bias for a mid-infrared QCL. It is found that most spectral features can be explained by a shift of the electrons from the active region to the injection region of the laser. Paper II investigates the gain spectra of a THz QCL for increasing temperature. It is found that increased broadening, with constant population inversion, decrease the peak gain at low temperatures. The increased broadening is addressed to enhanced scattering due to reduced screening at elevated temperatures. Paper III investigates the impact of acoustic phonon scattering on the gain spectra for THz QCLs. It is found that at the temperatures of interest, acoustic phonon scattering has a marginal effect on the gain properties of THz QCLs. In Paper IV different aspects of the gain spectra are summarized with focus on the theoretical description using Non-Equilibrium Green's Functions and density matrix theory. Here, the focus lies on dispersive gain and correlations effects. Paper V investigates how the subband structure and the different subband temperatures affect screening. If is found that screening has a strong impact on scattering for increasing temperature, and that isotropic Debye screening is an excellent approximation for THz QCLs at temperatures of interest.
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| 3. |
- Nelander, Rikard, et al.
(författare)
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Temperature degradation of the gain transition in terahertz quantum cascade lasers - the role of acoustic phonon scattering
- 2009
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Ingår i: physica status solidi (c). - : Wiley. - 1610-1634 .- 1862-6351 .- 1610-1642. ; 6:2, s. 579-582
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Konferensbidrag (refereegranskat)abstract
- The temperature degradation in terahertz quantum cascade lasers is investigated with the focus on the role acoustic phonon scattering. A self-energy describing the electron-acoustic phonon interaction is derived and used to study the importance in transport and optical properties. We observe a strong degradation of the gain peak with temperature. Despite the very small coupling to electrons, the exclusion of acoustic phonon scattering alters the current and peak gain in the order of 5%. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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| 4. |
- Nelander, Rikard, et al.
(författare)
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Temperature dependence and screening models in quantum cascade structures
- 2009
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 106:6
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Tidskriftsartikel (refereegranskat)abstract
- Different screening models in quantum cascade lasers are compared by calculating the contribution of intrasubband impurity scattering to the optical linewidth as a function of temperature. We find a strong impact of impurity scattering, which increases substantially with temperature. A simple isotropic bulk screening model works well if the screening length is of the order of or longer than the period length of the cascade structure. (C) 2009 American Institute of Physics. [doi:10.1063/1.3226072]
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| 5. |
- Nelander, Rikard, et al.
(författare)
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Temperature dependence of the gain profile for THz quantum cascade lasers
- 2008
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 92:8, s. 1-081102
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Tidskriftsartikel (refereegranskat)abstract
- We study the rapid decrease of peak gain in resonant-phonon THz Quantum Cascade Lasers with increasing temperature. The effect of various microscopic scattering processes on the gain profile as a function of temperature is discussed. We argue that increased broadening, prim arily due to increased impurity scattering, and not diminishing population inversion, is the main reason for the reduction of peak gain.
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| 6. |
- Pereira, M. F., Jr., et al.
(författare)
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Characterization of intersubband devices combining a nonequilibrium many body theory with transmission spectroscopy experiments
- 2007
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Ingår i: Journal of Materials Science: Materials in Electronics. - : Springer Science and Business Media LLC. - 0957-4522 .- 1573-482X. ; 18:7, s. 689-694
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we apply a microscopic nonequilibrium many body Keldysh Green's functions approach to an analysis of complex intersubband optical materials and devices. The calculated absorption/gain spectra are in very good agreement with transmission spectroscopy measurements of quantum cascade laser structures operating in the mid-infrared (midIR). The very good agreement reached between theoretical results and experimental measurements benchmarks the predictive power of our algorithms and its potential as a design tool for efficient intersubband designs.
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| 7. |
- Revin, D.G., et al.
(författare)
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Probing the electronic and optical properties of quantum cascade lasers under operating conditions
- 2006
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Ingår i: Proceedings of SPIE, the International Society for Optical Engineering. - : SPIE. ; 6386, s. 1-63860
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 8. |
- Wacker, Andreas, et al.
(författare)
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Simulation of gain in quantum cascade lasers
- 2009
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Ingår i: Proceedings of SPIE, the International Society for Optical Engineering. - : SPIE. ; 7230, s. 72301-72301
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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