P. Podemski

Photoreflectance-probed excited states in InAs∕InGaAlAs quantum dashes grown on InP substrate

Photoreflectance (PR) measurements have been performed on InAs∕In0.53Ga0.23Al0.24As quantum dashes (QDashes) molecular-beam epitaxy grown on InP substrate. The PR features related to all relevant parts of the structure have been detected, including the ground and excited state optical transitions in QDashes. QDash ground state transition shifts from 1.5 to almost 2μm with the increase in the thickness of InAs layer, corresponding to the increase in the average size of the dashes. Excited state transitions have been clearly observed at the energy of about 150meV above the ground state transition energy.

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.

Thermal quenching of photoluminescence from InAs∕In0.53Ga0.23Al0.24As∕InP quantum dashes with different sizes

InAs∕In0.53Ga0.23Al0.24As quantum dashes (QDashes) have been investigated in a temperature-dependent photoluminescence (PL) experiment. It has been observed that the increase of the QDash size shifts the emission peak to the longer wavelength and leads to a slower thermal quenching of PL. This thermal quenching can be described by a standard expression with two characteristic activation energies which are attributed to the electron escape and the heavy-hole escape to the conduction and valence continua in QDash barriers, respectively. The found activation energies are in the range of 21–64 and 121–223meV for the first and the second quenching mechanism, respectively.

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.

Photoluminescence from a single InGaAs epitaxial quantum rod

Microphotoluminescence (μ-PL) experiment has been performed on a structure with InGaAs/GaAs epitaxial quantum rods (quantum dots with the aspect ratio as high as 4.1) grown by depositing short-period InAs/GaAs superlattice by molecular beam epitaxy on GaAs substrate. The exciton and biexciton emission from a single quantum rod has been detected via the excitation power dependence of the μ-PL spectra. The origin of the single rod lines has been confirmed by a rate equation model. For a number of quantum rods within the investigated ensemble, the biexciton binding energy has been determined to be in the range of 1.0–2.2meV.

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.

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.

On the tunnel injection of excitons and free carriers from In0.53Ga0.47As∕In0.53Ga0.23Al0.24As quantum well to InAs∕In0.53Ga0.23Al0.24As quantum dashes

The authors investigate the efficiency of exciton and free carrier injections from quantum well (QW) to quantum dashes (QDashes) in tunnel-injection structures composed of In0.53Ga0.47As∕In0.53Ga0.23Al0.24As QW and InAs∕In0.53Ga0.23Al0.24As QDashes with ground state transitions separated by the energy close to the energy of LO phonon. As far as the thermal energy is smaller than the binding energy of QW excitons, almost all of the carriers created in QW tunnel to QDash. The injection process is much less efficient for free carriers because the energy difference between energy levels in QW and QDashes is not favorable for tunneling of electrons and holes separately.

On the applicability of a few level rate equation model to the determination of exciton versus biexciton kinetics in quasi-zero-dimensional structures

Hereby, we present a few level rate equation model in a context of the interpretation of excitation power dependent exciton and biexciton emission intensity from single quantum-dot-like structures. We emphasize that it not only allows identifying the excitonic and biexcitonic emission from one quasi-zero-dimensional object, but gives also an insight into the kinetics of the carriers confined in the system (both the internal dynamics of the exciton within its fine structure and the relative exciton to biexciton lifetimes ratio), the regime of the confinement itself and the importance of the higher energy levels occupation. Eventually, there are presented and discussed examples of the rate equation model application for an analysis of the experimental data for several kinds of epitaxial nanostructures.

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.