The free-electron laser FELBE at the Helmholtz-Zentrum Dresden Rossendorf enables experiments with spectral,
temporal, and, by means of near-field microscopy, also high spatial resolution. FELBE delivers picosecond IR and THz
pulses in a wavelength range from 4 μm to 280 μm. Here we review the potential of the laser and focus on two highlight
pump-probe experiments. In the first experiment, the relaxation dynamics in self assembled InGaAs quantum dots at
energies below the Reststrahlen band is studied. Long intradot relaxation times (1.5 ns) are found for level separations of
14 meV (3.4 THz), decreasing very strongly to ~ 2 ps at 30 meV (7 THz). The results are in very good agreement with
our microscopic theory of the carrier relaxation process, taking into account polaron decay via acoustic phonons. In the
second experiment, the relaxation dynamics in graphene is investigated at photon energies E = 20 - 250 meV. For
excitations below the energy of the optical phonon (G mode), the relaxation times are more than one order of magnitude
longer as compared to the relaxation times observed for near infrared excitation.
We report the first realization of short wavelength (λ ~ 3.05 - 3.6 μm) lattice matched In<sub>0.53</sub>Ga<sub>0.47</sub>As/AlAs<sub>0.56</sub>Sb<sub>0.44</sub>/InP
quantum cascade lasers (QCLs). The highest-performance device (λ ~ 3.6μm) displays pulsed laser action for
temperatures up to 300 K. The shortest wavelength QCL (λ ≈ 3.05 μm) operates in pulsed mode at temperatures only up
to 110 K. The first feasibility study of the strain compensated InGaAs/AlAsSb/InP QCLs (λ ~ 4.1 μm) proves that the
lasers with increased indium fractions in the InGaAs quantum wells of 60 and 70% display no degradation compared
with the lattice matched devices having identical design. This strain compensated system, being of particular interest for
QCLs at λ <~ 3.5μm, provides increased energy separation between the Γ and X conduction band minima in the quantum
wells, thus decreasing possible carrier leakage from the upper laser levels by intervalley scattering. We also demonstrate
that the performance of strain compensated InGaAs/AlAsSb QCLs can be improved if AlAsSb barriers in the QCL
active region are replaced by AlAs layers. The introduction of AlAs is intended to help suppress compositional
fluctuations due to inter diffusion at the quantum well/barrier interfaces.
We report measurements on a series of quantum dot infrared photodetectors grown with different combinations of
monolayer thicknesses (2.2. 2.55 and 2.9 ML) and quantum dot layer sheet doping densities (6×10<sup>10</sup> cm<sup>-2</sup> and 12×10<sup>10</sup>
cm<sup>-2</sup>). The dark current and noise current were higher in devices grown with sheet doping density of 12×10<sup>10</sup> cm<sup>-2</sup>. At a
given bias voltage the dark current and the noise current was found to be lowest in devices having 2.55 ML and sheet
doping density of 6×10<sup>10</sup> cm<sup>-2</sup>. This combination gives a sheet doping density to dot density ratio of approximately unity.
Highest gain was achieved in devices with 2.55 ML and sheet doping density of 6×10<sup>10</sup> cm<sup>-2</sup>.
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.
Here we present an engineering study showing how altering various aspects of the growth parameters of an InAs dot within an InGaAs well (DWELL) QDIP affects its performance. Amongst our findings, we show capability to control the absorption wavelength both during and after growth by altering the size of the dots and via the quantum confined Stark effect respectively. The addition of AlGaAs current blocking layers is shown to reduce deleterious dark current by over two orders of magnitude.
In this paper we present single mode quantum cascade lasers (QCLs) based on the GaAs and the InP material systems. We show results for first- and second-order distributed feedback (DFB) QC lasers with surface gratings. The InP based lasers are grown by metalorganic vapor phase epitaxy (MOVPE) and show single mode continuous wave emission up to 200 K. In pulsed operation we achieved single mode surface emission peak output powers exceeding 1 Watt at room temperature. The presented GaAs/AlGaAs laser features an air/AlGaAs waveguide, combined with a second-order distributed feedback grating. That laser shows 3 Watts of single mode output power via the surface at 78 K.
We report MOVPE-grown quantum cascade lasers with operating wavelengths between λ~7.5-9.5μm with threshold current densities as low as 2.4kA/cm2 at room temperature. Seven wafers grown for operation at ~9μm show a variation of just 3% in the superlattice periods obtained from X-ray analysis, and laser emission is observed from all wafers with a ~5meV spread of emission energies. Multimode Fabry-Perot and singlemode distributed feedback lasers have been fabricated, operating at λ~7.8μm at room temperature, corresponding with absorption lines in the infrared spectra of methane. In addition, we have produced a strain compensated MOVPE-grown quantum cascade laser operating at λ~4.5μm.