Polariton

Résumé

Les polaritons sont des quasiparticules issues du couplage fort entre une onde lumineuse et une onde de polarisation électrique. Plusieurs cas de figure sont possibles :

L'onde de polarisation est un phonon optique, c’est-à-dire essentiellement l'oscillation mécanique de deux atomes de charge opposée à l'intérieur d'un cristal. Les polaritons phononiques ont été beaucoup étudiés par la spectroscopie Raman dans les années 1970 - 80 et ont permis de mesurer la constante diélectrique à haute fréquence dans les semiconducteurs.

L'onde de polarisation est un plasmon dans un métal, c’est-à-dire essentiellement l'oscillation mécanique collective du nuage électronique d'un métal. Aux longueurs d'onde susceptibles de supporter des plasmons, l'onde lumineuse ne peut pas pénétrer dans le métal, cependant le couplage peut avoir lieu à l'interface métal/air (ou métal/diélectrique en général). Une telle onde de surface est ainsi appelée plasmon polariton de surface.

L'onde de polarisation est

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Interaction of Confined Polaritons in Microcavity Structures

Morteza Navadeh Toupchi

A polariton is a quasiparticle formed from the coupling of a confined photon in a cavity to electronic excitation, like exciton in a semiconductor. This dissertation reports on series of experiments in confined polariton interaction by design, fabrication, and characterization of semiconductor microcavity structures operating in strong or weak coupling regime. In the first part of the thesis, we mainly concentrate on the optical study of the 2D microcavity sample, including spin-dependent lower-upper polariton cross interactions by pump-probe spectroscopy technique, supported by theoretical analyses and numerical simulations based on Gross-Pitaevskii equations. In particular, we present a scattering resonance behavior via an exciton molecule (biexciton) when polaritons from both the upper and lower branches with anti-parallel spins are involved through a polaritonic cross Feshbach resonance. This demonstration will permit the control of the polariton interbranch scattering. The second part of the thesis is dedicated to the design and fabrication of the potentials where the photonic part of polaritons is confined laterally by adjusting the thickness of the cavity layer locally in so-called mesa structures. By engineering a periodic lattice of mesas on a two-dimensional microcavity, it is possible to couple confined polariton modes of nearby mesas to establish an optical lattice analogous to the crystalline semiconductorsâ electronic band structures. We especially demonstrate the localization of light with a lasing mode at the edge of the Brillouin zone in a two-dimensional triangular lattice. We produce a self-trapping of light by optically inducing a local breaking of the strong-coupling regime of excitons to photons. In the weak coupling regime, we control the confined modes by the shape of the generated defect. We also reveal a controllable localization degree and experimental signature of the Anderson localization in microcavity polaritons by inducing positional disorder in the triangular lattice. The last part is devoted to the fabrication of sub-micron size mesas to enhance polariton interaction by confining them tightly and discuss the quantum correlation of polaritons by a Hanbury Brown and Twiss (HBT) setting toward polariton blockade.

EPFL2021

Chargement

Two-dimensional (2D) materials are atomically thin crystals with exceptional mechanical, electrical and optical properties. Their unique characteristics originating from quantum confinement in the vertical dimension have attracted a strong interest for scientific research as well as technological applications. Moreover, the weak van der Waals coupling between layers facilitates the stacking of different materials to form heterostructures with new functionalities. This thesis focuses on semiconductor transition metal dichalcogenides (TMDCs), with a direct bandgap in the monolayer limit. The reduced dielectric screening leads to the formation of tightly bound electron-hole pairs (excitons) that are stable at room temperature and dominate the optical spectrum. The first part of the thesis explores the use of TMDC monolayers for developing transparent and tunable exciton light emitting diodes (LEDs) and photodetectors (PDs). The high modulation frequencies and integration capabilities with photonic structures make them promising candidates as optical/electrical interconnects. The properties of TMDC monolayers are determined by those of carriers close to the conduction and valence band extrema, located at the K and -K valleys in reciprocal space. The broken inversion symmetry and strong spin-orbit coupling links the valley and spin degrees of freedom, and enables optical excitation and detection of spin-valley accumulation with circularly polarized light. In the second part of the thesis, we explore the use of electric and magnetic fields to control the spin-valley dynamics of single carriers and excitons in TMDC-based optoelectronic devices. The spin-valley accumulation is probed by photoluminescence, electroluminescene and magneto-optical Kerr rotation.The third part of the thesis focuses on the integration of TMDCs in planar optical cavities. Their large oscillator strength induces a strong exciton-photon coupling, which results in the formation of polaritons. The observed polaritons propagate over long-distances and inherit the spin-valley properties of constituent excitons. By tuning the semiconductor carrier density, we achieve electrical control of exciton-photon coupling and polariton transport. Finally, we demonstrate a cavity integrated polariton light emitting diode where the location, directionality and polarization of emitted light can be electrically controlled at room temperature. Altogether, this thesis investigates the properties of charge carriers and excitons in 2D materials to realize practical optoelectronic devices where the spin-valley degree of freedom can be encoded in the polarization of light.

EPFL2022

Chargement

The present Ph.D. thesis consists in a series of experiments carried out in the Laboratory of Quantum Optoelectronics under the direction of professor Benoît Deveaud-Plédran between April 2006 and April 2010. We study the effect of lateral confinement on the dynamics of microcavity polaritons according to two important subjects. On the one hand, we study the polariton relaxation, on the other hand, we study the role of the spin in polariton mutual interactions. We first introduce microcavity polaritons, their optical properties and present the polariton pseudospin formalism. In our sample, polaritons are trapped through their photonic component in cylindrical extensions of the cavity length called mesas. Polariton relaxation is studied in the linear and nonlinear regimes. In the linear regime, we demonstrate that polariton interactions with acoustic phonons are enhanced under lateral confinement. Thermalization, which is forbidden in planar microcavities, is facilitated by confinement, and is very efficient in small diameter mesas. We develop for the first time a comprehensive model of polariton relaxation dynamics under confinement, and highlight the role of polariton states with large exciton content. This work will be profitable to the design of future samples dedicated to the study of Bose-Einstein condensation. In the nonlinear regime, we study the impact of polariton-polariton collisions on the spatial dynamics of microcavity polaritons. In the low-density regime, when the mesa is excited in a coherent superposition of the three lowest energy states, the polariton dynamics is characterized by dipole oscillations. In the high-density regime, we observe a continuous damping of the dipole oscillation. This is due to multiple parametric scattering processes, which redistribute the energy to the benefit of the ground state. Moreover, we demonstrate that the collisional damping is more efficient for collisions between polaritons of the same spin than between polaritons of opposite spins. We investigate in detail the influence of polariton spin in the interactions between polaritons. We first consider the case of planar polaritons. We observe that optical bistability is strongly reduced, or even suppressed, when the system contains polaritons of opposite spins. This results from a pairing of polaritons of opposite spins to form biexciton states. We demonstrate the polarization control of the different optical instability regimes. We then study the case of confined polaritons. As the system is cleaner (we work with a single polariton energy level), we achieve full optical control over the spinor interactions. We demonstrate the first realization of multi-valued switching with a coherent spin ensemble in the solid state. This is a very important step in the research on spin manipulation for the development of spin optoelectronic devices. Finally, we show the influence of the polariton spin on the coupling between planar and confined polaritons and on the control of the polariton fluid dynamics. All our experiments are confirmed by theoretical models. We propose perspectives for the next studies on the dynamics of polaritons and on the coherent spin manipulation in microcavities.

EPFL2010

Chargement