S. Hein

Experimental evidence on quantum well–quantum dash energy transfer in tunnel injection structures for 1.55μm emission

Here comes a report on the investigation of the energy transfer in InP-based tunnel injection structures, consisting of InAs∕InAlGaAs quantum dashes (QDashes) and an InGaAs∕InAlGaAs quantum well (QW), designed for 1.55μm emission at room temperature. Temperature dependent photoluminescence excitation (PLE) spectroscopy was used to experimentally confirm that the carriers created in the well reach the quantum dash layer by the tunneling through a thin InAlAs∕InAlGaAs barrier and recombine there radiatively. A measurable QW-QDash energy transfer has been detected up to 130K. The electronic structure of the whole complex system obtained by modulation spectroscopy exhibits full conformity with the PLE measurement results.

Carrier trapping and luminescence polarization in quantum dashes

We study experimentally and theoretically polarization-dependent luminescence from an ensemble of quantum-dot-like nanostructures with a very large in-plane shape anisotropy (quantum dashes). We show that the measured degree of linear polarization of the emitted light increases with the excitation power and changes with temperature in a non-trivial way, depending on the excitation conditions. Using an approximate model based on the k.p theory, we are able to relate this degree of polarization to the amount of light hole admixture in the exciton states which, in turn, depends on the symmetry of the envelope wave function. Agreement between the measured properties and theory is reached under assumption that the ground exciton state in a quantum dash is trapped in a confinement fluctuation within the structure and thus localized in a much smaller volume of much lower asymmetry than the entire nanostructure.

Exciton and biexciton emission from a single InAs/InP quantum dash

Molecular beam epitaxy grown InAs/InGaAlAs/InP quantum dashes designed for the 1.5 μm range were investigated by microphotoluminescence spectroscopy. The exciton and biexciton emission from a single quantum dash was detected revealing a biexciton binding energy of about 0.4 meV. The dependence of the photoluminescence intensity versus the excitation power density was determined and analyzed using the three level rate equation model, which allowed to confirm that the observed lines originate from the same single quantum dash.

The impact of p-doping on the static and dynamic properties of 1.5μm quantum dash lasers on InP

p-type modulation doping in the range of 0–100 acceptors per quantum dash (QDash) has been carried out to investigate the impact on QDash lasers on (100) InP. The differential gain was found to increase more than 50% for doping concentrations of 50 acceptors per QDash for constant cavity length lasers. However, this benefit is overcompensated by enhanced gain compression and enlarged thermal heating due to high internal losses in highly p-doped devices. The maximum modulation bandwidth of 8GHz in continuous wave operation at room temperature is, therefore, obtained for a moderate p-doping level of 10 holes per QDash.

Columnar quantum dashes for an active region in polarization independent semiconductor optical amplifiers at 1.55μm

Here comes a report on the optical properties of InP based InAs columnar quantum dashes, which are proposed as an alternative for columnar quantum dots in semiconductor optical amplifiers construction since they offer convenient spectral tuning over 1.55μm together with a very broad and high gain. Electronic structure details are investigated by photoreflectance and photoluminescence and analyzed by comparison with effective mass calculations. Columnar quantum dash emission from the cleaved edge is examined by polarization resolved photoluminescence showing a transition of the dominant polarization from transverse electric to transverse magnetic with an increase in the quantum dash vertical dimension.

Height-driven linear polarization of the surface emission from quantum dashes

The influence of the nanostructure height on the polarization of the surface emission was systematically investigated for In(Ga)As/InP quantum dashes. Polarization-resolved photoluminescence experiment was compared to theoretical considerations based on the multiband k·p theory and an analytical formula relating the polarization anisotropy to the nano-object geometry was derived. Substantial in-plane structure shape asymmetry induces a pronounced degree of linear polarization in surface emission, which depends strongly not only on the lateral aspect ratio but also on the nanostructure height. Additionally, strongly linearly polarized surface emission (up to 90%) was demonstrated for columnar quantum dashes by combining the in-plane elongation with a significantly increased height.

Modulation Bandwidth and Linewidth Enhancement Factor of High-Speed 1.55-

InAs-InGaAlAs-InP quantum-dash lasers have been fabricated showing continuous-wave operation up to 100degC with a characteristic temperature of 88 K between 25degC and 85degC and output powers above 27 mW at room temperature (RT). The small-signal modulation bandwidth of 10 GHz at RT amounts still to 4 GHz at 85degC. The linewidth enhancement factor above threshold is evaluated by means of the frequency-modulation/amplitude-modulation method in dependence on modulation frequency and drive current, exhibiting a value of 2.5 slightly above threshold.

\mu

InAs columnar quantum dash (CQDash) structures on (100) InP have been realized by gas source molecular beam epitaxy for stacking numbers of up to 24. Laser devices show low threshold current densities between 0.73 and 3.5 kA/cm2, dependent on the CQDash orientation within the cavity. Photoluminescence and electroluminescence measurements confirm a strong relationship between the polarization degree of the emission and the orientation of the CQDashes. Eventually, the polarization of the CQDash emission could be changed from predominantly transverse electric to transverse magnetic by simply altering the dash alignment relative to the light propagation axis.

m Quantum-Dash Lasers

We report on polarization independent 1.55 µm room temperature edge emission from an InGaAs immersion layer, being a quasi-two-dimensional surrounding of columnar quantum dashes formed during the close stacking self assembled growth. Calculations performed in an 8 band kp model revealed that the in-plane strain distribution across the immersion layer induces a significant heavy hole–light hole valence states mixing leading to equal transverse electric and transverse magnetic components of the optical transition intensity.

Orientation dependent emission properties of columnar quantum dash laser structures

InAs Columnar Quantum Dash (CQDash) structures on (100) InP for polarization insensitive semiconductor optical amplifiers have been realized. Photoluminescence and electroluminescence measurements confirm a strong relationship between the polarization degree of the emission and the orientation of the CQDashes. The polarization properties of the CQDash emission could thus be changed from predominantly transverse electric to transverse magnetic by adjusting the dash orientation relative to the light propagation axis.