Single-mode tunable laser emission in the single-exciton regime from colloidal nanocrystals

Whispering-gallery-mode resonators have been extensively used in conjunction with different materials for the development of a variety of photonic devices. Among the latter, hybrid structures, consisting of dielectric microspheres and colloidal core/shell semiconductor nanocrystals as gain media, have attracted interest for the development of microlasers and studies of cavity quantum electrodynamic effects. Here we demonstrate single-exciton, single-mode, spectrally tuned lasing from ensembles of optical antenna-designed, colloidal core/shell CdSe/CdS quantum rods deposited on silica microspheres. We obtain single-exciton emission by capitalizing on the band structure of the specific core/shell architecture that strongly localizes holes in the core, and the two-dimensional quantum confinement of electrons across the elongated shell. This creates a type-II conduction band alignment driven by coulombic repulsion that eliminates non-radiative multi-exciton Auger recombination processes, thereby inducing a large exciton–bi-exciton energy shift. Their ultra-low thresholds and single-mode, single-exciton emission make these hybrid lasers appealing for various applications, including quantum information processing

Transport of indirect excitons in ZnO quantum wells

We report on spatially- and time-resolved emission measurements and observation of transport of indirect excitons in ZnO/MgZnO wide single quantum wells

Hybrid nanomaterials for novel photonic devices

This PhD thesis investigates the optical properties of colloidal semiconductor nanocrystals and evaluates concepts regarding the development of novel photonic devices. Spectroscopic studies of the exciton dynamics in colloidal lead sulfide (PbS) quantum dots (QDs) by tuning the temperature are presented. The lowest exciton splitting for a range of PbS QDs sizes is calculated and a transfer of the oscillator strength from dark to bright states as the size increases is demonstrated. Hybrid structures with PbS QDs deposited on silicon substrates were also studied in order to explore whether excitons can be created in this material by means of resonant energy transfer.Furthermore, elongated asymmetric cadmium selenide/cadmium sulfide (CdSe/CdS) quantum rods are used as gain medium for the development of whispering gallery mode microlasers. Single-mode operation of hybrid lasers based on colloidal CdSe/CdS core/shell QRs in silica microspheres is for the first time reported. Laser-emission tunability over a range of 2.1 nm is also demonstrated, by heating the microsphere cavity with a 3.5 µm laser.In the last part of this thesis, unstructured and micro structured LiNbO3 are presented as excellent substrates for cell culture. Two commonly used neuron-like cells have been successfully proliferated and differentiated on both polar (±z) faces of LiNbO3 crystal substrates. Spatially selective attachment of neuron-like cells onto the domain engineered micro-structured substrates is also shown, providing the opportunity for the development of functional materials for the study of neuronal networks

Size- and temperature-dependent carrier dynamics in oleic acid capped PbS quantum dots

In this work, we investigate the temperature dependence of the photoluminescence decay and integrated photoluminescence of oleic acid capped PbS quantum dots with diameters ranging from 2.3 to 3.5 nm over a broad temperature range (6–290 K). All the investigated samples exhibit similar behavior, consisting of three different temperature regimes. The low-temperature regime (<180 K) is characterized by an increase in the average decay rate and a decrease in integrated photoluminescence. The intermediate regime (∼180–250 K) is described by an enhancement in the photoluminescence intensity and a decrease in the average decay rate. The high-temperature regime (>250 K) is governed by quenched photoluminescence intensity and acceleration in the average lifetimes. We propose a three-level system, composed of bright, dark, and surface states, which describes the observed photoluminescence dynamics of oleic acid capped PbS QDs

Dipolar exciton fluids in (Al,Ga)N/GaN quantum wells

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Transport of Indirect Excitons in Polar GaN/AlGaN Quantum Wells.

International audienc

Transport of dipolar excitons in (Al,Ga)N/GaN quantum wells.

International audienceWe investigate the transport of dipolar indirect excitons along the growth plane of polar (Al,Ga)N/GaN quantum well structures by means of spatially and time-resolved photoluminescence spectroscopy. The transport in these strongly disordered quantum wells is activated by dipole-dipole repulsion. The latter induces an emission blue shift that increases linearly with exciton density, whereas the radiative recombination rate increases exponentially. Under continuous, localized excitation, we observe continuously decreasing emission energy, as excitons propagate away from the excitation spot. This corresponds to a steady-state gradient of exciton density,measured over several tens of micrometers. Time-resolved microphotoluminescence experiments provide information on the dynamics of recombination and transport of dipolar excitons.We account for the ensemble of experimental results by solving the nonlinear drift-diffusion equation. Quantitative analysis suggests that in such structures, exciton propagation on the scale of 10 to 20 μm is mainly driven by diffusion, rather than by drift, due to the strong disorder and the presence of nonradiative defects. Secondary exciton creation, most probably by the intense higher-energy luminescence, guided along the sample plane, is shown to contribute to the excitonemission pattern on the scale up to 100 μm. The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination

Nonlinear optical spectroscopy of indirect excitons in coupled quantum wells

International audienceIndirect excitons in coupled quantum wells are long-living quasiparticles, explored in the studies of collective quantum states. We demonstrate that, despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons, and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons but also one of free electrons by two orders of magnitude