Philipp Tonndorf

Ultrafast Coulomb-Induced Intervalley Coupling in Atomically Thin WS2

Monolayers of semiconducting transition metal dichalcogenides hold the promise for a new paradigm in electronics by exploiting the valley degree of freedom in addition to charge and spin. For MoS2, WS2, and WSe2, valley polarization can be conveniently initialized and read out by circularly polarized light. However, the underlying microscopic processes governing valley polarization in these atomically thin equivalents of graphene are still not fully understood. Here, we present a joint experiment-theory study on the ultrafast time-resolved intervalley dynamics in monolayer WS2. Based on a microscopic theory, we reveal the many-particle mechanisms behind the observed spectral features. We show that Coulomb-induced intervalley coupling explains the immediate and prominent pump-probe signal in the unpumped valley and the seemingly low valley polarization degrees typically observed in pump-probe measurements compared to photoluminescence studies. The gained insights are also applicable to other light-emitting monolayer transition metal dichalcogenides, such as MoS2 and WSe2, where the Coulomb-induced intervalley coupling also determines the initial carrier dynamics.

Nanoscale Positioning of Single-Photon Emitters in Atomically Thin WSe2

Single-photon emitters in monolayer WSe2 are created at the nanoscale gap between two single-crystalline gold nanorods. The atomically thin semiconductor conforms to the metal nanostructure and is bent at the position of the gap. The induced strain leads to the formation of a localized potential well inside the gap. Single-photon emitters are localized there with a precision better than 140 nm.

Selective Raman modes and strong photoluminescence of gallium selenide flakes on sp2 carbon

Two-dimensional materials awakened a strong interest in the scientific and technological communities due to their exceptional properties that can be tuned by the material thickness and chemistry. In order to correlate optical properties with crystallographic structure and morphology, in this work, the authors aim at studying GaSe nanoflakes deposited on highly ordered pyrolytic graphite by means of atomic force microscopy, Raman, and photoluminescence (PL) spectroscopies. The authors found that the basal plane of the flakes can be attributed to the e-phase expected for bulk samples grown by the Bridgman method. However, a strong difference in the Raman spectra was systematically found at the edge of our GaSe flakes. Forbidden Raman modes located around 250 cm−1 were selectively observed at specific locations. These modes could not be directly attributed to the e-phase observed in the basal plane or in the bulk. The atomic force microscopy investigations show that high topographical features characterize t...

On-Chip Waveguide Coupling of a Layered Semiconductor Single-Photon Source

Fully integrated quantum technology based on photons is in the focus of current research, because of its immense potential concerning performance and scalability. Ideally, the single-photon sources, the processing units, and the photon detectors are all combined on a single chip. Impressive progress has been made for on-chip quantum circuits and on-chip single-photon detection. In contrast, nonclassical light is commonly coupled onto the photonic chip from the outside, because presently only few integrated single-photon sources exist. Here, we present waveguide-coupled single-photon emitters in the layered semiconductor gallium selenide as promising on-chip sources. GaSe crystals with a thickness below 100 nm are placed on Si3N4 rib or slot waveguides, resulting in a modified mode structure efficient for light coupling. Using optical excitation from within the Si3N4 waveguide, we find nonclassicality of generated photons routed on the photonic chip. Thus, our work provides an easy-to-implement and robust light source for integrated quantum technology.

Single-photon emitters in GaSe

Single-photon sources are important building blocks for quantum technology. Recently, non-classical light emitters have been found in the transition metal dichalcogenide WSe2 [1].

Deterministic positioning of single-photon emitters in monolayer WSe 2 on the nanoscale

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 WSe2. 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 WSe2 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).

Ultrafast Coulomb intervalley interaction in monolayer WS 2

We reveal the ultrafast intervalley dynamics in monolayer WS2 with a spectrally-resolved ultrafast pump-probe experiment and a microscopic theory. We find strong intervalley Coulomb coupling in the atomically thin semiconductor.

Single photon emission from localized excitons in monolayer WSe 2

We observe stable and narrowband single photon emission from localized quantum emitters in a WSe 2 monolayer. Photoluminescence excitation spectroscopy reveals that the emission originates from single excitons trapped in a local potential well.