Resumen de Exciton-Polariton condensates in patterned microcavities
This PhD thesis presents experimental studies of exciton-polariton condensates in GaAs-based microcavities that have been patterned with different structures considering the feasibility of their designs for polaritonic-based circuits. The strong light-matter coupling in the cavities gives rise to fascinating new effects that make polaritons appropriate candidates for nonlinear optical technologies: their bosonic nature, together with their light effectivemass, facilitates their condensation in a macroscopic coherent phase that presents high similarities to Bose-Einstein condensates; their excitonic part leads to strong polariton-polariton Coulomb interactions; thanks to their photonic content, polaritons can be easily created and manipulated at will through optical excitation sources. Furthermore, the wavefunction of the photons emitted from the cavity is an exact copy of the intracavity polariton wavefunction and therefore, using simple optical techniques, it is possible to gain access to important information, such as the phase, wave vector or the spin of these quasiparticles, to name just a few.
In more detail, this thesis is organized as follows:
Chapter 1 presents the physics behind excitons-polaritons, with emphasis on the creation of polariton condensates in semiconductor microcavities. The semiconductor quasiparticles involved in the formation of polaritons, photons and excitons, are described individually. A general description of the propagation of light in microcavities and its interaction with excitons confined in the system is discussed. Special attention is also given to the concepts behind the Bose-Einstein condensation. Finally, the theoretical description for interference between condensates in momentum-space and the emergence of Josephson oscillations are presented.
Chapter 2 describes the experimental techniques used during the thesis. The imaging techniques for real- and momentum-space are presented, describing in detail the excitation and detection conditions. Additionally, the chapter gathers the description of the different samples used in the experiments, all of them based on differently patterned GaAs microcavities.
In chapter 3 we present a study of the temperature effect on the coherence of traveling polariton condensates. We obtain interference fringes in momentum-space as a result of the interference between polariton condensates propagating with the same speed. In a similar fashion, we obtain interference fringes in real-space when condensates, traveling in opposite directions, meet. The fringes are analyzed through two different methods, obtaining the visibility of the fringes with the first one and the fraction of condensed to uncondensed polaritons with the second. Both methods evidence a gradual decay of the condensates’ coherence with increasing temperature, and allow to obtain the critical temperature for the Bose-Einstein-like condensate phase transition. We compare our experimental findings with theoretical models, developed for atomic condensates, to describe the condensates’ coherence fading with temperature.
In chapter 4 we report a detailed study of several coupler devices consisting of two parallel planar waveguides where a deviation of 45º at both ends of the structures has been introduced. We have characterized the photonic landscape experienced by polaritons along the couplers, which reveals that an additional discretization of the polaritons’ wave vectors is introduced when the orientation of the waveguide is changed. As a consequence of the introduction of the 45º deviation in the waveguide, we find a deceleration when polariton condensates turn at the bends of the circuits where they propagate. We have found that, for certain coupler’s parameters, the tunneling of polaritons between the two arms of the device is allowed, giving rise to the direct observation of Josephson oscillations. Finally, we study the coupler’s sensitivity to linear polarization to investigate the possibility of benefiting from the spin degree of freedom. A peculiar oscillating behavior in the linear polarization at the output terminal is found.
In chapter 5, we study a compact counter-directional polariton router which operates as a polaritonic resonant tunnel diode. The device implements the means to control the propagation direction of polariton condensates making use of a photonic microdisk potential, which couples two lithographically defined waveguides and reverses the condensate’s propagation direction. The device can feasibly be scaled to larger logic architectures without the requirement for any active external control parameters. Additionally, we investigate the ultrafast dynamics of the device via time-resolved photoluminescence measurements.
In chapter 6 we report the realization of a synthetic magnetic field for polaritons in a honeycomb lattice made of coupled semiconductor micropillars. First, we describe theoretically how to implement a gauge field, engineering the lattice parameters. A strong synthetic field is induced by introducing an uniaxial strain in the lattice, giving rise to the formation of Landau levels at the Dirac points. We report direct evidences of polaritonic Landau levels in samples with different strain gradients, observing also the localization of the n=0 Landau level wavefunction in one sublattice.
