M. Gurioli

Thermal ionization of excitons in GaAs/AlGaAs quantum well structures

Photoluminescence and photoluminescence excitation spectra have been performed on GaAs/AlGaAs quantum well structures in the temperature range 4–300 K. Sharp exciton resonances are present up to room temperature and can be ascribed to localized excitons for T≤50–70 K and to free excitons at higher values of T. Nevertheless, a line‐shape analysis of the PL spectra clearly shows the presence of band‐to‐band recombination. A fit based on a simple statistical model reproduces with high accuracy the photoluminescence spectrum line shape and allows to evaluate the relative densities of excitons and free carriers generated by the excitondissociation. We find that the ratios of the relative densities can be interpreted on the basis of the law of mass action for describing the thermal equilibrium between excitons,electrons, and holes.

Evidence of intermolecular π-stacking enhancement of second-harmonic generation in a family of single chain magnets

The second-order nonlinear optical properties of a family of rare-earth-based single chain magnets are presented (using a fundamental field at wavelength λ = 766 nm), together with an analysis of the origin of the second-harmonic generation (SHG) process. By studying the fundamental crystal symmetries and the geometrical arrangement of the constituting elements of the system in the unit cell we show that the main contribution to the nonlinear process arises from intermolecular π-stacking interactions.

Engineering of light confinement in strongly scattering disordered media

Disordered photonic materials can diffuse and localize light through random multiple scattering, offering opportunities to study mesoscopic phenomena, control light-matter interactions, and provide new strategies for photonic applications. Light transport in such media is governed by photonic modes characterized by resonances with finite spectral width and spatial extent. Considerable steps have been made recently towards control over the transport using wavefront shaping techniques. The selective engineering of individual modes, however, has been addressed only theoretically. Here, we experimentally demonstrate the possibility to engineer the confinement and the mutual interaction of modes in a two-dimensional disordered photonic structure. The strong light confinement is achieved at the fabrication stage by an optimization of the structure, and an accurate and local tuning of the mode resonance frequencies is achieved via post-fabrication processes. To show the versatility of our technique, we selectively control the detuning between overlapping localized modes and observe both frequency crossing and anti-crossing behaviours, thereby paving the way for the creation of open transmission channels in strongly scattering media.

Low density GaAs∕AlGaAs quantum dots grown by modified droplet epitaxy

Low temperature photoluminescence spectroscopy is used to analyze the effects of the Ga coverage and of the postgrowth thermal annealing on the electronic properties of low density (≈1×109cm−2) self-assembled GaAs∕AlGaAs quantum dots (QDs) grown by modified droplet epitaxy (MDE). We demonstrate that with the MDE method it is possible to obtain low density and high efficiency QD samples with high photoluminescence efficiency. Large modifications of the photoluminescence band, which depend on Ga coverage and thermal annealing, are found and quantitatively interpreted by means of a simple model based on the Al-Ga interdiffusion.

Ultra-narrow emission from single GaAs self-assembled quantum dots grown by droplet epitaxy

We realized ultra-narrow excitonic emission from single GaAs/AlGaAs quantum dots (QDs) grown by a refined droplet epitaxy technique. We found that uncapped quantum dots can be annealed at 400 degrees C without major changes in their morphology, thus enabling an AlGaAs capping layer to be grown at that temperature. Consequently, we demonstrate a fourfold reduction of the linewidth of the emission together with an increased recombination lifetime, compared to the conventional droplet epitaxial QDs. The averaged linewidth of neutral excitons measured by micro-photoluminescence on single quantum dots was around 35 microeV.

Modified droplet epitaxy GaAs/AlGaAs quantum dots grown on a variable thickness wetting layer

We show that the use of modified droplet epitaxy allows to tune the wetting layer thickness in GaAs quantum dot structures. Morphological observations demonstrate that the wetting layer at the base of the dots can be controlled or even removed by changing the surface stoichiometry of substrates before droplet formations. Spectroscopical measurements show that the variation of the wetting layer thickness strongly influences the optical properties of the dots. The experimental transition energies of the dots well agree with a theoretical model based on effective mass approximation.

Role of the wetting layer in the carrier relaxation in quantum dots

We present picosecond time resolved photoluminescence measurements of GaAs/AlGaAs quantum dot structures—grown by modified droplet epitaxy—where no wetting layer is connecting the dots. We find a fast carrier relaxation time (30 ps) to the dot ground state, which becomes even faster for increasing the photogenerated carrier injection. This shows that the two–dimensional character of the wetting layer is not relevant in determining the quantum dot capture, in contrast with the conclusions of several models so far presented in literature. We discuss the role of the barrier states as well as the possibility of Auger processes involving the zero-dimensional levels of the quantum dots.

Tuning of photonic crystal cavities by controlled removal of locally infiltrated water

We present a spectral tuning mechanism of photonic crystal microcavities based on microfluidics. The microinfiltration with water of one or few cavity holes and its subsequent controlled evaporation allow us to tune the cavity resonances in a spectral range larger than 20 nm, with subnanometer accuracy, and we also observe that the addition of water in the microcavity region improves its quality factor Q.

Near-field imaging of coupled photonic-crystal microcavities

We report by means of near-field microscopy on the coupling between two adjacent photonic crystal microcavities. Clear-cut experimental evidence of the spatial delocalization of coupled-cavity optical modes is obtained by imaging the electromagnetic local density of states. We also demonstrate that it is possible to design photonic structures with selective coupling between different modes having orthogonal spatial extensions

Spectral diffusion and line broadening in single self-assembled GaAs∕AlGaAs quantum dot photoluminescence

We experimentally and theoretically investigate the photoluminescence broadening of different excitonic complexes in single self-assembled GaAs∕AlGaAs quantum dots. We demonstrate that the excitonic fine-structure splitting leads to a sizable line broadening whenever the detection is not resolved in polarization. The residual broadening in polarized measurements is systematically larger for the exciton with respect to both the trion and the biexciton recombination. The experimental data agree with calculations of the quantum confined Stark effect induced by charge defects in the quantum dot (QD) environment, denoting the role of the QD spectator carrier rearrangement in reducing the perturbation of the fluctuating environment.