Schematic representation of a 4-level QCL-active region. Black arrows show relevant non-radiative scattering paths and injection efficiencies. The dashed arrow represents the lasing transition.
A portion of the conduction band profile of the structure S1 under 100 kV/cm electric field. Layer composition and thickness are indicated in the original publication by Semtsiv et al. 20 Solid lines represent the moduli-squared of relevant Wannier-Stark states. The dashed arrow represents the lasing transition. Relevant states are labeled in analogy to Figure 1 .
Conduction band profile of the structure S2 under 76 kV/cm electric field. The layer thickness in nm from left to right starting from the widest active region quantum well are 5.0/1.0/4.2/2.1/3.8/1.5/3.4/1.3/3.0/1.1/2.6/0.9/2.3/ 0.8 /0.7/ 0.8 /0.7/2.3/0.9/0.7/0.9/0.7/2.0/1.3/0.7/1.3/0.7/1.8/0.9. AlAs layers are in bold, layers are in roman, and layers are in italics. Underlined layers are doped to . Solid lines represent the moduli-squared of relevant Wannier-Stark states. The dashed arrow represents the lasing transition. Relevant states are labeled in analogy to Figure 1 .
Threshold current density as a function of reciprocal resonator length at different heat sink temperatures for structure S1. The solid lines illustrate the expected linear 1/L-dependence at different heat sink temperatures.
(a) Modal gain coefficient (solid squares) and (solid dots) as a function of heat sink temperature for structures S1 (a) and S2 (b). The solid lines represent polynomial fits to the data. The calculated waveguide loss due to free-carrier-absorption is also shown (dashed lines).
Measured threshold current density (solid dots) for two samples of the studied QCL-structures as a function of heat sink temperature. The solid lines represents the expected threshold current density without the thermally activated leakage current.
Experimental values for the escape current density at laser threshold (solid dots) for structures S1 (a) and S2 (b) as a function of heat sink temperature. Fitting this current with Eq. (10) , the experimental points can be reconstructed. The several lines represent best fits to the data for different activation energies , 120, 140 (solid line), 160, and 180 meV, in the case of structure S1, and , 60, 70 (solid line), 80, and 90 meV, for structure S2.
Calculated form factors for the studied QCL-structures as a function of LO-phonon momentum in the direction of confinement. A mean lattice constant a = 5.94 Å has been used for graphical representation purposes. The notations of Figures 2 and 3 are used for structure S1 (a), and S2 (b), respectively.
Voltage-current and light-output (a) characteristics for a 25 μm × 3.0 mm sample of structure S1. Twice the single facet output is considered. The device was driven with 300 ns × 10 kHz current pulses at different heat sink temperatures. The temperature dependence of the power efficiency is also shown (b).
Threshold current density (solid dots) and total external differential quantum efficiency (solid squares) as a function of heat sink temperature for the sample of Figure 9 (structure S1). The dashed lines represent exponential fits resulting in characteristic temperatures T 0 and T 1 as high as 175 K and 550 K, respectively, for this sample.
Characteristic temperature T 0 as a function of the resonator length L for both structures studied in this paper. Expected values for T 0 (solid lines) can be extracted from a numerical analysis of the functional dependence of on L. Experimental data are included as well (solid dots).
Energy differences of subband's minima for structures S1 and S2 (in meV) obtained following intersubband spacing calculations within the effective mass approximation. For a schematic representation of the energy levels, see Figures 2 and 3 .
Fitting parameters for the determination of the thermal escape current density in the studied QCL-structures. The last column represents the averaged scattering time calculated at assuming and a typical subband's concentration of charge carriers .
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