Exciton relaxation and level repulsion in GaAs/AlxGa1-xAs quantum wires

V groove GaAs/AlGaAs quantum wires are investigated by spatially resolved photoluminescence spectroscopy using a low-temperature scanning near- field optical microscope. The spectra along the wires feature several sharp emission lines, which are understood as the emission from exciton states localized in inhomogeneities of the confining potential. It is expected that these exciton states spatially overlap and that their energies are correlated, which leads to level repulsion. The statistical analysis of the spectra in terms of autocorrelation functions clearly reveals this effect. In order to model photoluminescence rather than absorption spectra, we perform detailed simulations of exciton relaxation and luminescence kinetics in a disordered one-dimensional system, taking into account the realistic structure of the facets at the bottom of the V groove in our disorder model. Combining near- field measurements and numerical simulations we show that disorder prevents the full relaxation of excitons towards the local ground states in the dips of the disordered potential. As a consequence, luminescence from excited states is observed. We identify in the spatial and energy correlation between localized states the origin of the observed level repulsion and discuss the role played by the two facets at the bottom of the V groove in this mechanism. This analysis also highlights the relevance of the broad photoluminescence background observed in this and in analogous experiments. We propose as explanation for the origin of this broad background the strong exciton-acoustic phonon coupling that results in phonon sidebands in the spectrum of each localized state.

Near-field spectroscopy and theoretical modeling of disordered semiconductor quantum wires

Andrea Feltrin

In this thesis work, we report scanning near-field optical photoluminescence spectroscopy measurements with at best 200 nm spatial resolution performed on disordered semiconductor V-groove AlGaAs/GaAs quantum wires. In order to interpret the results of these measurements, we develop two different theoretical models. In the first part we study the relationship between disorder and exciton thermalization processes. To do this, we measure the temperature evolution of the excitonic photoluminescence emission lines in near-field spectra at low excitation intensity. The emission peaks in these spectra arise from the radiative recombination of quantum wire excitons localized in potential dips of the confinement potential, due to monolayer width fluctuations along the quantum wire direction. We observe in a non negligible fraction of the measured spectra an increase of the emission line intensities on the low energy side of the spectrum for increasing temperature, which is in contrast with the picture of a thermalized exciton population. In addition to this, the statistical analysis of hundreds of such near-field spectra in terms of autocorrelation functions reveals a shoulder at about 2 meV, which we interpret as a level repulsion feature, related to the energetic and spatial correlation of excitonic states in a disordered system. We develop a theoretical model for the simulation of near-field photoluminescence spectra. In order to do this we model the disordered potential starting from the real interface structure of V-groove quantum wires. The Schr®odinger equation for the exciton is then solved in the center of mass approximation. We calculate the intrinsic radiative recombination rates and the phonon scattering rates, necessary to solve the Boltzmann equation for the exciton population. The statistical analysis of the simulated spectra shows a similar feature in the autocorrelation function. However a good agreement can be obtained only if the diagonal exciton phonon interaction is taken into account, which leads to a modification of the excitonic spectral lineshape and to the appearance of a broad photoluminescence emission background. The temperature dependence of the simulated spectra shows very similar features to the ones observed experimentally. The model for the simulation of steady state near-field photoluminescence spectra is improved in order to simulate time resolved far-field photoluminescence. We study the temperature dependence of the photoluminescence decay time for different disorder configurations and relate it to the exciton population dynamics. We show that at the lowest temperatures the pinning of the decay time at values of several hundreds of picoseconds corresponds to a non thermalized exciton population. On the other hand the square root like temperature dependence, expected in an ideal quantum wire, corresponds to a thermalized exciton population. We also discuss the limits of extracting the intrinsic exciton radiative lifetime from this square like dependence. Taking into account dissociated electron hole states in our model modifies the temperature behaviour, leading to a quasi exponential temperature dependence of the photoluminescence decay time. In the second part, we study the relationship between disorder and biexciton binding energies in quantum wires. We succeed in isolating several extended quantum wire domains, where one single exciton peak dominates the photoluminescence spectrum at low excitation intensity. By increasing the excitation we systematically observe in these large domains the appearance of an additional line in the spectrum, which we attribute to the radiative recombination of the biexciton. The biexcitonic binding energy slightly increases from 2.1 meV for the spatially most extended peaks to 2.3 meV for biexcitons associated to excitons whose localization length is smaller than the experimental resolution. We also observe quantum wire domains with more than one excitonic peak, indicating shorter exciton localization lengths in the order of tens of nanometers. In these domains the appearance of the biexcitonic emission line depends on the number of excitonic peaks observed at low excitation intensity in a near-field photoluminescence spectrum. In spectra containing a few exciton emission lines, biexciton peaks appear associated to each exciton peak. The binding energies show larger variations and range between 2 meV and 2.6 meV. We observe also one biexciton peak appearing on the high energy side of an exciton peak with a negative binding energy of -0.9 meV. On the contrary in spectra containing a larger number of exciton peaks, either only a few additional biexciton peaks appear or no biexcitonic emission lines at all are observed upon increasing the excitation intensity. In order to explain these results we solve the Schr®odinger equation for four one dimensional particles interacting through the Coulomb potential and confined in single particle potential boxes of variable size and barrier height. The results show that the biexciton binding energy first increases as the dip size decreases, reaches a maximum and finally decreases for the smallest sizes as a result of the Coulomb repulsion between the holes in the biexciton. However our model does not account for the disapperance of the biexciton lines, nor for the experimentally measured negative biexciton binding energy.

EPFL2004

Level repulsion of localised excitons observed in near-field photoluminescence spectra

Andrea Crottini, Benoît Marie Joseph Deveaud, Reda Idrissi Kaitouni, Xiao Wang

GaAs/GaAlAs quantum wires grown by modulated flow rate metalorganic chemical vapour deposition were investigated by spatially resolved photoluminescence spectroscopy using a scanning near-field optical microscope. It was found that the wires decompose into a series of regions that emit luminescence of varying intensity. The spectra of these regions feature several narrow emission lines, which means that there is a series of more or less localised exciton states inside each region. It is expected that these exciton states are very close to each other and are correlated, which leads to level repulsion. The mean autocorrelation function taken from a series of near-field spectra clearly reveals this level repulsion, which amounts to roughly 2 meV.

2002

Photosensitivity and luminescence of chromium and bismuth doped fibers

Christian Ban

Optical fiber materials with broad-band gain in the visible or in the telecommunication window are of great interest for optical communication or the biomedical domain in order to built integrated tunable lasers, amplifiers, ultra-short pulse lasers, or broadband light sources. Possible activator ions in compact solid-state host materials for the generation of broadband luminescence, gain, and laser emission are transition-metal (TM), bismuth and rare-earth (RE) ions. Emission lines in TM and bismuth exhibit luminescence with typically several hundred of nanometers of spectral bandwidth. This is a much broader range than can be obtained from the emission of RE ions, which is restricted to less than hundred nanometers. Chromium and bismuth-doped fibers are of great interest as they are potential candidates for broadband light sources and tunable lasers. The idea of this work was to investigate the photosensitivity and luminescence properties of Cr and Bi-doped silica optical fibers in collaboration with the Fiber Optic Research Center at the Russian Academy of Sciences (FORC-RAS) who fabricated the fibers and glass samples. The photosensitivity of Cr-and Bi-doped fibers with different dopant concentrations was investigated using an ArF excimer laser emitting at 193 nm. No refractive index change was observed in pristine Cr-doped fibers. After H2-loading, refractive index change up to 2.82×10-3 was obtained in Cr-Ga-doped fiber. The hydrogen loaded annealed fiber showed an index change of 1.4×10-3. The Bi-Al-doped fibers showed permanent photo-induced index changes up to 2.0×10-4 (highest Bi concentration). Using H2-loading index changes up to 2.2×10-3 were measured without reaching saturation. Such index changes are almost comparable to index changes in H2-loaded Ge-silicate fibers and allow fabricating very strong Bragg grating fiber laser reflectors directly in the doped fibers. Stress measurements were performed to distinguish between color center and compaction that contribute to the photo-induced refractive index change. Stress changes in SMF-28 standard telecom optical fibers irradiated by a femto-second laser emitting at 264 nm showed a contribution of compaction of 70% for non hydrogen loaded fibers whereas no stress change was observed in hydrogen loaded fiber for the same photo-induced refractive index. No stress change indicates the absence of compaction leaving color centers solely responsible for the refractive index change. Irradiated annealed H2-loaded Ga-Cr-doped fiber showed no stress change as well. Although generally assumed for H2-loaded fibers, these are the only two examples that prove a color center based photosensitivity responsible for fiber Bragg gratings reported so far. In contrary, in H2-loaded Bi-Al-doped fibers the refractive index change is related to color centers and compaction. A contribution of compaction of about 41 % and 11 %, was evaluated for the pristine and the H2-loaded Bi-Al-doped fiber. Strategies to increase the luminescence, i.e. the number of active centers include e.g. temperature annealing (Cr3+ to Cr4+), changing concentrations of dopants (Bi) and/or the co-dopants (Al, P, Ge), and fiber fabrication conditions. In this work new strategies were followed: UV irradiation (193 nm), hydrogen loading, and H2-loading followed by UV irradiation. Measurements of absorption and luminescence of Bi-doped fibers with different treatments were performed. The absorption was measured from 360 to 1700 nm. The spectrum was decomposed into 8 Bi-related and 1 OH band. The irradiation increased the absorption by ∼100 dB/m and induced two supplementary absorption bands. UV irradiation combined with hydrogen loading induced three additional bands as compared to the pristine fiber. The luminescence was investigated using visible and infra-red pump lasers. The emission takes place mainly from 600 to 900 nm (3 bands) and from 900 to 1300 nm (3 bands). The luminescence shape and intensities of the pristine and irradiated fibers were similar. H2-loading combined with irradiation increased the ratio of NIR to visible fluorescence intensity depending on the pump wavelength. A side luminescence measurement showed 18 and 13 dB luminescence increase of the 1130 nm and 1390 nm bands. This luminescence enhancement might be due to an increase of the number of active bismuth centers. Up to 1.8 dB/m on-off gain was observed in the visible band around 700 nm under 676 nm excitation in the pristine fiber. However, the absorption increases simultaneously. As a result the net-gain was always negative with a trend towards a positive value below 700 nm. High reflective fiber gratings and a narrow band transmission filter were fabricated in a single mode fiber with cut-off around 920 nm. All gratings were used to realize a Bi-all-fiber laser operating at 1179 nm with a reduced linewidth of < 100 pm. Narrow linewidth lasers have a great potential for frequency doubling to the yellow.