Doeon Lee

Information on quantum states pervades the visible spectrum of the ubiquitous Au144(SR)60 gold nanocluster.

Absorption spectra of very small metal clusters exhibit individual peaks that reflect the discreteness of their localized electronic states. With increasing size, these states develop into bands and the discrete absorption peaks give way to smooth spectra with, at most, a broad localized surface-plasmon resonance band. The widely accepted view over the last decades has been that clusters of more than a few dozen atoms are large enough to have necessarily smooth spectra. Here we show through theory and experiment that for the ubiquitous thiolate cluster compound Au144(SR)60 this view has to be revised: clearly visible individual peaks pervade the full near-IR, VIS and near-UV ranges of low-temperature spectra, conveying information on quantum states in the cluster. The peaks develop well reproducibly with decreasing temperature, thereby highlighting the importance of temperature effects. Calculations using time-dependent density-functional theory indicate the contributions of different parts of the cluster-ligand compound to the spectra.

Selectively tunable optical Stark effect of anisotropic excitons in atomically thin ReS2.

The optical Stark effect is a coherent light–matter interaction describing the modification of quantum states by non-resonant light illumination in atoms, solids and nanostructures. Researchers have strived to utilize this effect to control exciton states, aiming to realize ultra-high-speed optical switches and modulators. However, most studies have focused on the optical Stark effect of only the lowest exciton state due to lack of energy selectivity, resulting in low degree-of-freedom devices. Here, by applying a linearly polarized laser pulse to few-layer ReS2, where reduced symmetry leads to strong in-plane anisotropy of excitons, we control the optical Stark shift of two energetically separated exciton states. Especially, we selectively tune the Stark effect of an individual state with varying light polarization. This is possible because each state has a completely distinct dependence on light polarization due to different excitonic transition dipole moments. Our finding provides a methodology for energy-selective control of exciton states.

Ultrafast quantum beats of anisotropic excitons in atomically thin ReS 2

Quantum beats, periodic oscillations arising from coherent superposition states, have enabled exploration of novel coherent phenomena. Originating from strong Coulomb interactions and reduced dielectric screening, two-dimensional transition metal dichalcogenides exhibit strongly bound excitons either in a single structure or hetero-counterpart; however, quantum coherence between excitons is barely known to date. Here we observe exciton quantum beats in atomically thin ReS2 and further modulate the intensity of the quantum beats signal. Surprisingly, linearly polarized excitons behave like a coherently coupled three-level system exhibiting quantum beats, even though they exhibit anisotropic exciton orientations and optical selection rules. Theoretical studies are also provided to clarify that the observed quantum beats originate from pure quantum coherence, not from classical interference. Furthermore, we modulate on/off quantum beats only by laser polarization. This work provides an ideal laboratory toward polarization-controlled exciton quantum beats in two-dimensional materials.Quantum beats are periodic oscillations originating from the superposition of coherent quantum states. Here, the authors observe exciton quantum beats in the optical spectrum of atomically thin ReS2, and modulate the intensity of the quantum beat signal by means of light polarisation.

Publisher Correction: Ultrafast quantum beats of anisotropic excitons in atomically thin ReS 2

In the originally published HTML and PDF versions of this article, Figs. 3g and 4d contained typesetting errors affecting the way the data points were displayed. This has now been corrected in the HTML and PDF versions.

Generation, transport, and detection of valley-coupled spin-polarized electrons in WSe 2 -graphene-topological insulator heterostructure devices

We report a novel hybrid heterostructure device that merges the valleytronic and spintronic functionality. We sequentially generate, transport and detect the valley-coupled spin-polarized electrons via non-local photogalvanic effects from WSe<inf>2</inf>-graphene-Bi<inf>1,5</inf>Sb<inf>0.5</inf>Te<inf>1.7</inf>Se<inf>1.3</inf>.