Strain engineering in single-layer semiconducting transition metal dichalcogenides aims to tune their bandgap energy and to modify their optoelectronic properties by the application of external strain. In this paper we study transition metal dichalcogenides monolayers deposited on polymeric substrates under the application of biaxial strain, both tensile and compressive. We can control the amount of biaxial strain applied by letting the substrate thermally expand or compress by changing the substrate temperature. After modelling the substrate-dependent strain transfer process with a finite elements simulation, we performed micro-differential spectroscopy of four transition metal dichalcogenides monolayers (MoS2, MoSe2, WS2, WSe2) under the application of biaxial strain and measured their optical properties. For tensile strain we observe a redshift of the bandgap that reaches a value as large as 94 meV/% in the case of single-layer WS2 deposited on polypropylene. The observed bandgap shifts as a function of substrate extension/ compression follow the order WS2 < WSe2 < MoS2 < MoSe2.
Monolayer transition metal dichalcogenides such as MoS2, WS2, MoSe2, WSe2, and MoTe2 received a lot of attention recently due to their atomic thickness in combination with an optical band gap in the visible or infrared. These properties render them a promising material class for new opto-electronic devices. Strong spin-orbit coupling together with the absence of inversion symmetry leads to an emission of polarized photoluminescence after excitation with circularly polarized light. Therefore, the K and K’ valley of the semiconductor can be selectively addressed by left and right handed circularly polarized light, which is interesting for valleytronic applications. To understand the mechanisms governing the creation and destruction of valley polarization, time-resolved experiments are necessary. While stationary photoluminescence experiments show nearly perfect valley polarization, indicating very slow intervalley scattering processes, first valley-selective pump-probe experiments yielded a strong signal immediately after optical excitation in both the pumped and unpumped valley, suggesting a small valley polarization. To understand this behavior, we performed a joint experiment-theory study on the time-resolved valley dynamics in atomically thin WS2. We find strong intervalley Coulomb coupling governing the dynamics in the atomically thin semiconductor. Our results are also applicable to the other transition metal dichalcogenides MoS2, MoSe2, and WSe2, where strong intervalley Coulomb coupling is expected.
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.
We present an experimental study of electron transport in electrically driven quantum cascade laser structures. Ultrafast quantum transport from the injector into the upper laser subband is investigated by mid-infrared pump-probe experiments directly monitoring the femtosecond saturation and subsequent recovery of electrically induced optical gain. For low current densities, low lattice temperatures, and low pump pulse intensities the charge transport is dominantly coherent, i.e., we observe pronounced gain oscillations upon excitation giving evidence for a coherent electron motion between the injector and the upper laser subband. Increasing either the current density, the lattice temperature, or the pump pulse intensity the gain recovery shows an additional slow incoherent component which essentially follows the pump-initiated heating and subsequent cooling of the carrier gas in the injector.