Femtosecond quantum optics with semiconductor nanostructures: single cycles of light, electrons, and photons

Abstract

Recent advances of femtosecond semiconductor physics at the limits of single electrons and photons down to sub-cycle time scales are presented. The first part deals with ultrafast measurements on single-electron systems: The transient quantum dynamics in a single CdSe/ZnSe quantum dot is investigated via femtosecond transmission spectroscopy. A two-color Er:fiber laser with excellent noise performance is key to these first resonant pump-probe measurements on a single-electron system. We have observed ultrafast bleaching of an electronic transition in a single quantum dot due to instantaneous Coulomb renormalization. Since we were also able to invert the two-level system, optical gain due to a single electron has been detected. By using π-pulses for probing, we could deterministically add or remove a single photon to or from a femtosecond light pulse, leading to non-classical states of the light field. In order to optimize electron-photon coupling, nanophotonic concepts like dielectric microresonators and metal optical antennas are explored. In the second part of the paper, we present multi-terahertz measurements on low-energy excitations in semiconductors. These studies lead towards a future time-domain quantum optics on a time scale of single cycles of light: Intense multiterahertz fields of order MV/cm are used to coherently promote optically dark and dense para excitons in Cu2O from the 1s into the 2p state. The nonlinear field response of the intra-excitonic degrees of freedom is directly monitored in the time domain via ultrabroadband electro-optic sampling. The experimental results are analyzed with a microscopic many-body theory, identifying up to two internal Rabi cycles. Subsequently, intersubband cavity polaritons in a quantum well waveguide structure are optically generated within less than one cycle of light by a femtosecond near-infrared pulse. Mid-infrared probe transients trace the non-adiabatic switch-on of ultrastrong light-matter coupling and the conversion of bare photons into cavity polaritons directly in the time domain.