Heiko Kissel

Interdot carrier transfer in asymmetric bilayer InAs∕GaAs quantum dot structures

Transient photoluminescence from a series of asymmetric InAs quantum-dot bilayers with a GaAs barrier layer thickness varying from 30 to 60 monolayers between the quantum-dot planes is investigated. The interdot carrier transfer process is analyzed. In the framework of a three-level system, interdot carrier transfer times between 200 and 2500ps are derived and compared with similar data from the literature. Within the semiclassical Wentzel–Kramers–Brillouin approximation, the observed “transfer time-barrier thickness-relation” supports nonresonant tunneling as the microscopic carrier transfer mechanism.

Carrier transfer in self-assembled coupled InAs/GaAs quantum dots

Photoluminescence (PL) spectra and time-resolved PL data from AlGaAs/GaAs superlattice structures containing thin InAs layers of about 1–3 monolayer grown on semi-insulating (001)-oriented GaAs substrates at lowered temperatures are studied. The size distribution of InAs quantum dots (QDs) among different families (modes) is controlled by variation of growth temperature and/or growth interruption. We demonstrate the stabilization of the PL magnitude caused by strong coupling between different modes and the full width at half maximum of “large size” QD modes within a certain temperature interval (50–150 K) due to feeding of the radiative transitions from nonradiative decay and carrier transfer arising from decaying excitonic states of the small size QD modes. Strong competition between different channels of ground state relaxation leads to an oscillating dependence of the PL transient for the small size QD mode. Efficient inter- and intramode tunneling rules out “bottleneck restrictions” for the PL. The pa...

Photoluminescence study of carrier transfer among vertically aligned double-stacked InAs/GaAs quantum dot layers

Photoluminescence (PL) properties of self-organized quantum dots (QDs) in a vertically aligned double-layer InAs/GaAs QD structure are studied as a function of temperature from 10 to 290 K. The QDs in a sample with a 1.8 ML InAs seed layer and a second 2.4 ML InAs layer are found to self-organize in pairs of unequal sized QDs with clearly discernible ground-states transition energy. The unusual temperature behavior of the PL for such asymmetrical QD pairs provides clear evidence for carrier transfer from smaller to larger QDs by means of a nonresonant multiphonon-assisted tunneling process in the case of interlayer transfer and through carrier thermal emission and recapture within one layer.

Staircase-like spectral dependence of ground-state luminescence time constants in high-density InAs/GaAs quantum dots

Time-resolved photoluminescence (PL) from InAs/GaAs quantum dots with a bimodal size distribution is used to investigate the dynamic carrier-transfer processes which couple transfer between similarly sized quantum dots and between quantum dots in different size categories. The relationship between the decay time and the emission energy appears staircaselike and the energetic positions of the steps as well as the shape can be correlated to the shape of the steady-state PL emission through a rate-equation theory. These results show how transient PL can be used to investigate the dynamics of carrier transfer in quantum-dot systems.

Optical detection of asymmetric quantum-dot molecules in double-layer InAs/GaAs structures

Self-assembled quantum dots (QDs) in double-layer InAs/GaAs structures are studied by resonant photoluminescence and photoluminescence excitation spectroscopy. A weakly correlated (50%) double-layer system with an array of vertically coupled QDs (asymmetric quantum-dot molecules) was formed in a structure consisting of the 1.8-monolayer-thick first and the 2.4-monolayer-thick second InAs layers separated by 50 monolayers of GaAs. The nature of discrete quantum states in this system was studied and resonances corresponding to vertically coupled QDs were clearly observed for the first time.

Photoluminescence linewidths from multiple layers of laterally self-ordered InGaAs quantum dots

Laterally ordered multilayered arrays of InGaAs quantum dots are investigated by photoluminescence as a function of high index GaAs substrates. Different laser wavelengths are used to investigate the photoluminescence from quantum dots layer-by-layer. High optical quality is demonstrated for laterally ordered quantum dot arrays. GaAs(511)B is identified as the optimum high index substrate for growth of InGaAs∕GaAs multilayered quantum dots, demonstrating strong photoluminescence with a narrow full width at half maximum linewidth of 23meV in spite of the potential for misfit dislocations.

High-power, high-brightness and low-weight fiber coupled diode laser device

New solid-state laser devices, especially fiber laser systems, require increasingly higher optical pump power provided by fiber-coupled diode laser modules. In particular for defense technology, robust but lightweight high-power diode laser sources with high brightness are needed. We have developed a novel diode laser device combining high power, high brightness, wavelength stabilization and low weight, which becomes more and more important for a multitude of applications. Heart of the device is a specially tailored laser bar, which epitaxial and lateral structure is designed such that only standard fast- and slow-axis collimator lenses are required to couple the beam into a 200 μm fiber with numerical aperture of 0.22. In this paper we present a detailed characterization of the new diode laser device with up to 775 W of optical power coupled into a 200 μm, NA 0.22 fiber. One important feature of the device is a lightweight design due to a special housing optimized for low weight. In addition we present results of a diode laser device with 675 W of optical output power and improved spectral quality, which is ensured over a wide range of temperature and current by means of volume holographic gratings for wavelength stabilization. For this device an overall efficiency of more than 42.5 % has been achieved. Furthermore we present a compact diode laser source with 230 W of optical power coupled into a 200 μm, NA 0.22 fiber. This diode laser device is optimized with regard to highest efficiency and yields an overall electro-optical efficiency of more than 50 %.

Advances in performance and beam quality of 9xx-nm laser diodes tailored for efficient fiber coupling

The impact of new direct-diode and fiber laser systems on industrial manufacturing drives the demand for highbrightness diode laser pump sources suitable for simple fiber coupling with high efficiency. Within the German funded project HEMILAS laser mini-bars with different bar geometries and small fill factors were investigated. We present results on 9xx nm bars with tailored beam parameter products for simplified coupling to fibers with core diameters of 200μm and 300μm with a numerical aperture of 0.22 and compare beam quality parameters, brightness, conversion efficiency, and thermal performance of different bar designs. Optimized epitaxy structures yield conversion efficiency maxima above 66%. The slow axis divergence angle of mini-bars with a fill factor of 10% featuring five 100μm wide and 4mm long emitters based on this epitaxy structure stays below 7°, which corresponds to a beam parameter product of 15mm mrad, up to very high output power of over 45W. This result was achieved for mounting on actively cooled submounts using hard solder. A similar bar with 5mm cavity length and using soft soldering reached an output power of 60W at the same beam parameter product. At 4mm cavity length, no COMD failures were observed up to currents exceeding the thermal rollover and the maximum output cw power was 95W.

Tailoring of high-temperature photoluminescence in InAs/GaAs bilayer quantum dot structures

Temperature-dependent photoluminescence is investigated in bilayer InAs∕GaAs quantum dot structures with constant InAs deposition θ1 in the seed layer, but variable deposition θ2 in both the second layer and the GaAs spacer layer. It is shown that interlayer coupling, leading to the formation of asymmetric quantum dot pairs, strengthens the high-temperature photoluminescence and strongly influences carrier relaxation channels. We report that radiative recombination and carrier capture efficiency by the quantum dots in the second layer can be tailored using the deposition θ2 and the GaAs spacer thickness.

Cavity-enhanced thermal emission from semiconductor lasers

The thermal emission properties of GaAs-based semiconductor diode lasers are studied with a calibrated setup for quantitative measurements of spectrally resolved emittances. Emission spectra are measured for a stack of high-power diode-laser bars and for n-doped bulk GaAs reference samples and are compared to a calibrated blackbody radiator. The thermal radiation measurements are analyzed with a formalism providing the directional spectral emittance, transmittance, and reflectance under normal incidence. The thermal emission properties are related to the underlying materials properties such as absorption coefficients and carrier concentrations. The thermal emission from the semiconductor lasers displays a pronounced enhancement compared to the bulk reference samples. The metallic contacts and the heavily p-doped layers of the diodes are identified as the origin of this cavity enhancement. The semitransparent nature of the semiconductor materials influences the results of thermal infrared imaging substanti...