The possibility of steady-state population inversion in a small sample of strongly driven two-level emitters such as quantum dots (QDs) in microcavities, and its utilization towards amplification of light and acoustic waves, is investigated theoretically. We find that inversion and absorption spectra of photons and phonons crucially depend on the interplay between the intrinsic vacuum and phonon environments. The absorption profiles of photons and phonons show marked novel features such as gain instead of transparency and absorption reversed to gain, respectively. Furthermore, we report collectivity-induced substantial enhancement of inversion and pronounced gain in the photon and phonon absorption spectra for a subwavelength-sized QD ensemble.
The possibility of steady-state population inversion in a small sample of strongly driven two-level emitters such as quantum dots (QDs) in microcavities, and its utilization towards amplification of light and acoustic waves, is investigated theoretically. We find that inversion and absorption spectra of photons and phonons crucially depend on the interplay between the intrinsic vacuum and phonon environments. The absorption profiles of photons and phonons show marked novel features such as gain instead of transparency and absorption reversed to gain, respectively. Furthermore, we report collectivity-induced substantial enhancement of inversion and pronounced gain in the photon and phonon absorption spectra for a subwavelength-sized QD ensemble.
I theoretically demonstrate all-optical modulation and amplification in a one-dimensional photonic-crystal (PC) cavity with a large number of two-level quantum dots (QD's). Populations in the ground and excited states of two-level QD's with large transition dipole moments are greatly changed by an external laser, and this affects the electrical susceptibility of the QD's. In other words, all-optical modulation, in which light (the signal light) is modulated by light (the external laser), becomes possible. Moreover, a large contrast of the electromagnetic local densities of states in the PC cavity enables steady-state population inversion in two-level systems, which is generally considered impossible. The output signal power is more than twice as large as the input power at certain frequencies.
This thesis is devoted to theoretically study the systems of single quantum dots (QDs) embedded in various photonic nanostructures, in close relation with experimental data. The word single is essential: due to their nanometer size, QDs have mainly been studied as ensembles. It is only recently that the experimental progress, in particular in the domain of QDs growth, has allowed to address single QDs. This helped to observe that the confined states in a QD resemble those of an atom, leading in turn to the macro-atom picture for QDs. It opened the path to the study of cavity quantum electrodynamics (CQED) in the solid state, as the analogous of a system of an atom in an electromagnetic cavity. However, already at the level of the QD (i.e. without a solid state cavity), the macro-atom picture has been proven to be oversimplified. Various effects related to the solid state environment of the QD have been experimentally and theoretically evidenced, like the influence of acoustic and optical phonons. These are channels of decoherence for the otherwise well isolated and confined QD levels. When adding the cavity, these effects are possibly enhanced, which can lead to very large deviations from the CQED ideal picture. Therefore the clear understanding of these effects is of essential importance, especially in view of the application of solid state systems to quantum information science. In Chapter 2, we address the radiation-matter coupling between two QDs in a nanocavity. This field of research combines the high quality of light resonators to the nonlinearities of the semiconductor material. It is promising in view of manipulating and transferring single long-lived excitations, an essential goal for quantum information science. Then, we study the deviations of a QD from the idealized macroatom picture. First, in Chapter 3, we include the influence of acoustic phonons on the light emitted by a QD-cavity system. We show that the QD modified susceptibility leads to the qualitative explanation of the cavity feeding effects, which is one of the most striking deviations of the QD-cavity system from the ideal CQED picture. In addition to phonons are extended two-dimensional states populated by the non resonant pumping laser in the QD's wetting layer (WL). In Chapters 4 and 5 we build a Monte-Carlo model of the excitation-decay mechanism in a QD-cavity system. We use it to describe the influence of WL states on the dynamics of the QD, leading in turn to a non-CQED behavior. This approach is particularly successful in the case of large QD-cavity detuning, where it predicts both time-dependent and spectral characteristics of the emitted light. Finally, in Chapter 6, we discuss our conclusions, emphasizing in particular that the knowledge of the actual solid state environment of the QD leads to a better control over the parameters allowing to reach the CQED regime. We also discuss the possible extensions of this work, in particular in relation to the analysis of the stimulated emission regime.
The eect of acoustic phonons on dierent light-induced excitations of a semiconductor quantum dot is investigated.Resonant excitation of the quantum dot leads to the Rabi oscillations, which are damped due to the phonon interaction.When the excitation frequency is detuned, an occupation can only occur due to phonon absorption or emission processes.For frequency-swept excitations a population inversion is achieved through adiabatic rapid passage, but the inversion is also damped by phonons.For all three scenarios the inuence of the phonons depends non-monotonically on the pulse area.
Abstract A major challenge in generating single photons with a single emitter is to excite the emitter while avoiding laser leakage into the collection path. Ideally, any scheme to suppress this leakage should not result in a loss in the efficiency of the single-photon source. Here, we investigate a scheme in which a single emitter, a semiconductor quantum dot, is embedded in a microcavity. The scheme exploits the splitting of the cavity mode into two orthogonally-polarised modes: one mode is used for excitation, the other for collection. By linking the experiment to theory, we show that the best population inversion is achieved with a laser pulse detuned from the quantum emitter. The Rabi oscillations exhibit an unusual dependence on pulse power. Our theory describes them quantitatively, enabling us to determine the absolute population inversion. By comparing the experimental results with our theoretical model, we determine a population inversion of 98 % − 5 % + 1 % for optimal laser detuning. The Rabi oscillations depend on the sign of the laser-pulse detuning, a phenomenon arising from the non-trivial effect of phonons on the exciton dynamics. The exciton–phonon interaction is included in the theory and gives excellent agreement with all the experimental results.
Semiconductor quantum dots (QDs) in photonic nanocavities provide monolithic, robust platforms for both quantum information processing and cavity quantum electrodynamics (QED). An inherent feature of such solid-state cavity QED systems is the presence of electron-phonon interactions, which distinguishes these systems from conventional atomic cavity QED. Understanding the effects of electron-phonon interactions on these systems is indispensable for controlling and exploiting the rich physics that they exhibit. Here we investigate the effects of electron-phonon interactions on a QD-based cavity QED system. When the QD and the cavity are off-resonance, we observe phonon-assisted cavity mode emission that strongly depends on the temperature and cavity-detuning. When they are on-resonance, we observe an asymmetric vacuum Rabi doublet, the splitting of which narrows with increasing temperature. These experimental observations can be well reproduced using a cavity QED model that includes electron-acoustic-phonon interactions. Our work provides significant insight into the important but hitherto poorly understood mechanism of non-resonant QD-cavity coupling and into the physics of various cavity QED systems utilizing emitters coupled to phonons, such as nitrogen-vacancy centres in diamond and colloidal nanocrystals.
Electromagnetically induced phase grating is theoretically investigated in the driven two-level quantum dot exciton system at the presence of the exciton-phonon interactions. Due to the phonon-induced coherent population oscillation, the dispersion and absorption spectra are sharply changed and the phase modulation is enhanced via the high refractive index with nearly-vanishing absorption, which could effectively diffract a weak probe light into the first-order direction with the help of a standing-wave control field. Moreover, the diffraction efficiency of the grating can be easily manipulated by controlling the Huang-Rhys factor representing the exciton-phonon coupling, the intensity and detuning of the control field, and the detuning of the probe field. The scheme we present has potential applications in the photon devices for optical-switching and optical-imaging in the micro-nano solid-state system.