Publications
This study proposes a method to obtain the ratio of the fundamental TE/TM optical response of a quantum-confined system from the measurement of the polarization dependence of the edge photovoltage spectrum, by correcting for polarization-dependent features of the experimental system. When applied to compressive- and tensile-strained InGaP quantum well structures, the results are in excellent agreement with known ratios of the band-edge matrix elements. This method is of particular value in the study of quantum dot systems where the polarization behaviour is difficult to predict.
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This work demonstrates polariton lasing in a bulk ZnO planar microcavity under non-resonant optical pumping at a small negative detuning (δ ≈ −1/6 of the 130 meV vacuum Rabi splitting) and at a temperature of 120 K. The strong coupling regime is maintained at lasing threshold, since the coherent nonlinear emission from the lower polariton branch (LPB) occurs at zero in-plane wavevector, well below the uncoupled cavity mode. The contribution of multiple localized polariton modes above threshold and the non-thermal polariton statistics indicate that the system operates in a far-from-equilibrium regime, likely related to the moderate photon lifetime and in-plane photonic disorder in the cavity.
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This work reports a considerable improvement in the optical quality factor (Q) of GaN-based microdisk (μ-disk) resonators embedding GaN quantum dots (QDs) grown on AlN and AlGaN barriers. Room-temperature photoluminescence (PL) spectroscopy reveals a large number of high-Q whispering gallery modes (WGMs) spanning a wide spectral range (2.6 to 3.4 eV), enabling identification of different radial mode families through comparison with simulations. GaN/AlN QD-based μ-disks demonstrate record-high Q values (Q > 7000 for a 5 μm diameter disk and Q ≈ 5000 for a 2 μm disk), representing the state of the art for nitride photonic structures. The superior performance is attributed to the high etching quality and to the comparatively lower sub-bandgap absorption of QDs with respect to quantum wells.
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This study investigates self-assembled, catalyst-free GaN micropillars (μpillars) grown on (0001) sapphire substrates by metalorganic vapour phase epitaxy (MOVPE). Transmission electron microscopy (TEM) and KOH chemical etching reveal the systematic coexistence of two domains of opposite polarity (Ga-polar and N-polar) within each single micropillar, originating at the micropillar/substrate interface during nucleation and propagating along the entire length of the pillar. Dislocations are generated at the wire/substrate interface but bend after several hundreds of nanometers, resulting in dislocation-free micropillars of several tens of micrometers in length. Spatially resolved cathodoluminescence (CL) and microphotoluminescence (μPL) reveal large optical property differences between the two polarity domains, attributed to unequal incorporation of impurities, dopants and vacancies depending on polarity.
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This work demonstrates the control of polarization and dipole moment in semiconductor nanostructures through nanoscale engineering of shape and composition. Rodlike (columnar) quantum dot (CQD) nanostructures, elongated along the growth direction, are obtained by molecular beam epitaxial growth. By varying the aspect ratio and the compositional contrast between the rod and the surrounding matrix, the polarization of the dominant interband transition is rotated from transverse-electric (TE) to transverse-magnetic (TM), and the dipole moment is modified — producing a radical change in the voltage dependence of absorption spectra. These results open the way toward optimization of quantum dot optical amplifiers and electro-optical modulators.
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