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T HE recent discovery of ultrafast spintronic THz emitters has attracted a lot of attention due to their high power, broadband emission as well as easy fabrication [1] , [2] . They are based on nanometer-thin bilayers of a magnetic (M) and a nonmagnetic (NM) metal layer and rely on the inverse spin Hall effect. The THz emission properties can be tuned by using different M or NM materials, tailoring the magnetic properties [3] , or building functional layer stacks [4] , [5] . Here, we present a spin valve THz emitter, which allows for switching of the THz emission amplitude by a low magnetic field of a few mT.

The availability of efficient and robust quantum light sources in large numbers is one of the key building blocks for quantum photonic technology. Recently, bright and stable single-photon emitters have been discovered in the 2D material hexagonal boron nitride (hBN), which efficiently emit single photons even at room-temperature [1] . The controlled positioning of nanocrystals hosting light emitters is an important technique for the bottom-up fabrication of functional nanostructures as well as for fundamental investigations on the particles themselves.

The recent discovery of spintronic THz emitters based on nanometer-thin bilayers of a ferromagnetic (FM) and a nonmagnetic metal (NM) layer has attracted a lot of attention due to their high power, gapless and broadband emission, and easy fabrication [1], [2]. The emission properties can be tuned easily e.g. by varying the FM or NM materials, the magnetic properties or the electrical conductivity. However, the microscopic mechanism governing the THz emission is presently not fully understood, which limits their design and optimization.

Monolayers of the transition metal dichalcogenides (TMDCs) MoS 2 , MoSe 2 , WS 2 , WSe 2 and MoTe 2 are a new class of atomically thin, direct band gap semiconductors with extraordinary optical and electronic properties. Due to their unique electronic structure the spin and valley (K+ or K−) of the excitons are locked, which allows for selective, optical addressing of the valleys. These properties render atomically thin TMDCs promising materials for applications in the field of “valleytronics”, where the valley index is used as a carrier for quantum information.

Single-photon sources are important building blocks for quantum technology. Recently, bright and stable single-photon emitters have been reported in the atomically thin semiconductor WSe 2 . However, the localized light sources appear at random positions at the edges of the material [1]. Here, we demonstrate the deterministic positioning of single-photon emitters in monolayer WSe 2 on the nanoscale [2]. The monolayer is placed on top of a gapped single-crystalline gold rod. The atomically thin semiconductor folds around the metal nanostructure and is bent at the position of the gap (Fig. 1a).

The physics of solid-state nanostructures is governed by the fact that materials properties change drastically when spatial dimensions are reaching the deBroglie wavelength of electrons. During the last decade, completely new functionalities have been achieved by exploiting these effects. On the other side, a comparable control in the fourth dimension, time, is still missing since it demands to manipulate optical transitions between quantum states on a few-femtosecond scale set by the oscillation period of light. We are exploring this fundamental regime of light-matter interaction. The experiments are based on recent advances of femtosecond technology such as ultrabroadband Er:fiber lasers emitting single cycles of light [1], field-resolved detection of phase-locked electromagnetic transients with a bandwidth exceeding 100 THz [2] and multi-terahertz sources delivering amplitudes in excess of 1 V/A which are comparable to inner-atomic fields [3].

Semiconductor quantum dots are promising systems for robust and scalable quantum information processing. Ultrafast sequences of coherent quantum operations may be envisioned with femtosecond light pulses, if the involved quantum states are separated by at least tens of meV. Therefore, small quantum dots with high confinement potentials are favourable. Due to their large Coulomb correlation energies, CdSe quantum dots are ideal candidates.

Semiconductor quantum dots are promising single-electron systems for solid-state quantum information processing. Ultrafast sequences of coherent operations may be envisioned with femtosecond light pulses, if the involved quantum states are separated by at least tens of meV. Therefore, small quantum dots with high confinement potentials are favourable. Due to their large Coulomb correlation energies, CdSe quantum dots are ideal candidates.

The efficient coupling of light from the far field into nanometer sized objects is one of the fundamental challenges in current nanooptics. We present two ideas to reach this goal. On the one hand, we place the nanoobject (in our case a colloidal semiconductor quantum dot) into an optical micropillar resonator (Fig. 1a). Three-dimensional light confinement is observed and we calculate the cavity modes of resonators with circular and elliptical cross section by modeling the pillar microcavity as a waveguide with an effective refractive index [1]. The concept is extended into the ultraviolet region of the electromagnetic spectrum and the first dielectric pillar microcavity with colloidal ZnO quantum dots is demonstrated [2]. On the other hand, we fabricate metal optical nanoantennas by various techniques (electron-beam lithography, focused ion beam milling, and colloidal masks). In particular, we present a tunable bowtie optical nanoantenna [3], which consists of two gold nanotriangles (Fig. 1b). The feedgap can be continuously varied by manipulation of an antenna arm with nanometer precision via an atomic force microscope. At the same time the optical response of the nanoantenna is determined via darkfield scattering spectroscopy. Exciting with ultrafast laser pulses, we investigate the nonlinear optical properties of single metal nanoantennas.

In summary, we have fabricated high-Q micropillar cavities with circular and elliptical cross section containing CdSe/ZnS quantum dots or rods in the spacer layer. We find a systematically higher Q-factor along the short axis of the elliptical pillar compared to circular ones. The spectral position of the cavity resonances have been calculated by modeling the pillar cavity as a waveguide with an effective refractive index.

We measure electron spin dynamics in GaAs quantum wells with varying electron density. Electron Lande g factor is measured to characterize disorder potential. Electron spin coherence is lost from interplay between localization and dynamical scattering.

Presents time-resolved measurements of photoinduced intersubband absorption in an undoped GaAs/AlGaAs coupled quantum well excited by an ultrashort interband optical pulse. An interband pump pulse injects electrons into the first and second subband of an undoped asymmetric double quantum well (ADQW) with a level spacing smaller than the LO phonon energy. The time evolution of the electron population in these two subbands is monitored by probing the MIR intersubband transitions to a third (empty) subband.

We observe a fundamental difference in the influence of doping electrons and optically excited electrons on the electron g-factor and spin relaxation time T/sub 2/ in undoped and n-doped bulk GaAs.