Colouration by total internal reflection and interference at microscale concave interfaces.

Many physical phenomena create colour: spectrally selective light absorption by pigments and dyes1,2, material-specific optical dispersion3 and light interference4-11 in micrometre-scale and nanometre-scale periodic structures12-17. In addition, scattering, diffraction and interference mechanisms are inherent to spherical droplets18, which contribute to atmospheric phenomena such as glories, coronas and rainbows19. Here we describe a previously unrecognized mechanism for creating iridescent structural colour with large angular spectral separation. Light travelling along different trajectories of total internal reflection at a concave optical interface can interfere to generate brilliant patterns of colour. The effect is generated at interfaces with dimensions that are orders of magnitude larger than the wavelength of visible light and is readily observed in systems as simple as water drops condensed on a transparent substrate. We also exploit this phenomenon in complex systems, including multiphase droplets, three-dimensional patterned polymer surfaces and solid microparticles, to create patterns of iridescent colour that are consistent with theoretical predictions. Such controllable structural colouration is straightforward to generate at microscale interfaces, so we expect that the design principles and predictive theory outlined here will be of interest both for fundamental exploration in optics and for application in functional colloidal inks and paints, displays and sensors.

Temperature-Dependent Excitonic Light Manipulation with Atomically Thin Optical Elements.

Monolayer 2D semiconductors, such as WS2, exhibit uniquely strong light-matter interactions due to exciton resonances that enable atomically thin optical elements. Similar to geometry-dependent plasmon and Mie resonances, these intrinsic material resonances offer coherent and tunable light scattering. Thus far, the impact of the excitons' temporal dynamics on the performance of such excitonic metasurfaces remains unexplored. Here, we show how the excitonic decay rates dictate the focusing efficiency of an atomically thin lens carved directly out of exfoliated monolayer WS2. By isolating the coherent exciton radiation from the incoherent background in the focus of the lens, we obtain a direct measure of the role of exciton radiation in wavefront shaping. Furthermore, we investigate the influence of exciton-phonon scattering by characterizing the focusing efficiency as a function of temperature, demonstrating an increased optical efficiency at cryogenic temperatures. Our results provide valuable insights into the role of excitonic light scattering in 2D nanophotonic devices.

Hidden phonon highways promote photoinduced interlayer energy transfer in twisted transition metal dichalcogenide heterostructures.

Vertically stacked van der Waals (vdW) heterostructures exhibit unique electronic, optical, and thermal properties that can be manipulated by twist-angle engineering. However, the weak phononic coupling at a bilayer interface imposes a fundamental thermal bottleneck for future two-dimensional devices. Using ultrafast electron diffraction, we directly investigated photoinduced nonequilibrium phonon dynamics in MoS2/WS2 at 4° twist angle and WSe2/MoSe2 heterobilayers with twist angles of 7°, 16°, and 25°. We identified an interlayer heat transfer channel with a characteristic timescale of ~20 picoseconds, about one order of magnitude faster than molecular dynamics simulations assuming initial intralayer thermalization. Atomistic calculations involving phonon-phonon scattering suggest that this process originates from the nonthermal phonon population following the initial interlayer charge transfer and scattering. Our findings present an avenue for thermal management in vdW heterostructures by tailoring nonequilibrium phonon populations.

Photoluminescence upconversion in monolayer WSe2 activated by plasmonic cavities through resonant excitation of dark excitons.

Anti-Stokes photoluminescence (PL) is light emission at a higher photon energy than the excitation, with applications in optical cooling, bioimaging, lasing, and quantum optics. Here, we show how plasmonic nano-cavities activate anti-Stokes PL in WSe2 monolayers through resonant excitation of a dark exciton at room temperature. The optical near-fields of the plasmonic cavities excite the out-of-plane transition dipole of the dark exciton, leading to light emission from the bright exciton at higher energy. Through statistical measurements on hundreds of plasmonic cavities, we show that coupling to the dark exciton leads to a near hundred-fold enhancement of the upconverted PL intensity. This is further corroborated by experiments in which the laser excitation wavelength is tuned across the dark exciton. We show that a precise nanoparticle geometry is key for a consistent enhancement, with decahedral nanoparticle shapes providing an efficient PL upconversion. Finally, we demonstrate a selective and reversible switching of the upconverted PL via electrochemical gating. Our work introduces the dark exciton as an excitation channel for anti-Stokes PL in WSe2 and paves the way for large-area substrates providing nanoscale optical cooling, anti-Stokes lasing, and radiative engineering of excitons.