Leonid V. Keldysh

Strong coupling in a single quantum dot–semiconductor microcavity system

Cavity quantum electrodynamics, a central research field in optics and solid-state physics, addresses properties of atom-like emitters in cavities and can be divided into a weak and a strong coupling regime. For weak coupling, the spontaneous emission can be enhanced or reduced compared with its vacuum level by tuning discrete cavity modes in and out of resonance with the emitter. However, the most striking change of emission properties occurs when the conditions for strong coupling are fulfilled. In this case there is a change from the usual irreversible spontaneous emission to a reversible exchange of energy between the emitter and the cavity mode. This coherent coupling may provide a basis for future applications in quantum information processing or schemes for coherent control. Until now, strong coupling of individual two-level systems has been observed only for atoms in large cavities. Here we report the observation of strong coupling of a single two-level solid-state system with a photon, as realized by a single quantum dot in a semiconductor microcavity. The strong coupling is manifest in photoluminescence data that display anti-crossings between the quantum dot exciton and cavity-mode dispersion relations, characterized by a vacuum Rabi splitting of about 140u2009µeV.

Strong coupling in a single quantum dot semiconductor microcavity system

Properties of atom-like emitters in cavities are successfully described by cavity quantum electrodynamics (cQED). We report on cavity quantum electrodynamics (cQED) experiments in a single quantum dot semiconductor system. CQED, which is a very active research field in optics and solid state physics, can be divided into a weak and a strong coupling regime. In case of weak coupling, the spontaneous emission rate of an atom-like emitter, e.g. a single quantum dot exciton, can be enhanced or reduced compared to the value in vacuum in an irreversible emission process. In contrast, a reversible energy exchange between the emitter and the cavity mode takes place when the conditions for strong coupling are fulfilled. We investigate weak as well as strong coupling in a system based on a low density In0.3Ga 0.7As quantum dot layer placed as the active layer in a high quality planar AlAs/GaAs distributed Bragg reflector cavity grown by molecular beam epitaxy. Using electron beam lithography and deep plasma etching, micropillars with high Q-factors (up to 43.000 for 4 μm diameter) were realized from the planar cavity structure. Due to the high oscillator strength of the In0.3Ga 0.7As quantum dots together with a small mode volume in high finesse micropillar cavities it is possible to observe strong coupling characterized by a vacuum Rabi splitting of 140 μeV. The fabrication of high-Q micropillar cavities as well as conditions necessary to realize strong coupling in the present system are discussed in detail.

Coherent photonic coupling of semiconductor quantum dots

We report a new type of coupling between quantum dot excitons mediated by the strong single-photon field in a high-finesse micropillar cavity. Coherent exciton coupling is observed for two dots with energy differences of the order of the exciton-photon coupling. The coherent coupling mode is characterized by an anticrossing with a particularly large line splitting of 250 microeV. Because of the different dispersion relations with temperature, the simultaneous photonic coupling of quantum dot excitons can be easily distinguished from cases of sequential strong coupling of two quantum dots.

Interference effects in the emission spectra of quantum dots in high-quality cavities

We have investigated theoretically and experimentally the emission of (quantum dot-cavity) systems for different coupling strength and a wide range of exciton-photon mode detunings controlled by temperature variation in the range 10–45 K. Under close to resonance conditions, the radiation spectrum from the cavity emission becomes essentially dependent on the primary excitation path, which can be either via resonant quantum-dot exciton or via cavity mode. Particularly, in the case of nonresonant cavity mode excitation, the emission line becomes split into two asymmetric lines already in the weak coupling regime.

Strong exciton-photon coupling of In 0.43 Ga 0.57 As quantum dots in high quality micropillar cavities

We report on strong coupling between a discrete optical mode of a high-Q micropillar cavity and single excitons of self assembled In_0.43Ga_0.57As quantum dots and compare the results with previous studies on In_0.3Ga_0.7As quantum dots.