Ultrafast exciton fluid flow in an atomically thin MoS2 semiconductor

Strong light-matter coupling in van der Waals materials

In recent years, two-dimensional (2D) van der Waals materials have emerged as a focal point in materials research, drawing increasing attention due to their potential for isolating and synergistically combining diverse atomic layers. Atomically thin transition metal dichalcogenides (TMDs) are one of the most alluring van der Waals materials owing to their exceptional electronic and optical properties. The tightly bound excitons with giant oscillator strength render TMDs an ideal platform to investigate strong light-matter coupling when they are integrated with optical cavities, providing a wide range of possibilities for exploring novel polaritonic physics and devices. In this review, we focused on recent advances in TMD-based strong light-matter coupling. In the foremost position, we discuss the various optical structures strongly coupled to TMD materials, such as Fabry-Perot cavities, photonic crystals, and plasmonic nanocavities. We then present several intriguing properties and relevant device applications of TMD polaritons. In the end, we delineate promising future directions for the study of strong light-matter coupling in van der Waals materials.

A 3D printable near-infrared triggered hydrogel with MoS2 as the crosslink center for tissue repair

Near-infrared (NIR) light, compared with ultraviolet (UV) light, has a stronger tissue penetration ability and is widely used in the medical field. However, few hydrogels can be triggered by NIR. Here, a modular polymer-nanosheet (metal disulfide) (PNS) hydrogel system was proposed, which can be photo-crosslinked through photothermal conversion under NIR light. MoS2, a transition-metal dichalcogenide, was used as a crosslink center in PNS hydrogels. Mo and S (from thiolated polymers), which are essential for gelation, were discovered to have new bonds. Furthermore, 3D printing of NIR-triggered PNS hydrogels was achieved conceptually with masked NIR. Moreover, multiple hydrogels and metal disulfides were applicable in this modular gelation system. This study indicated that these PNS hydrogels have great potential in many smart biomedical applications, including wearable sensors, noninvasive in vivo 3D bioprinting, and tissue repair substitutes.

Infrared Interlayer Excitons in Twist-Free MoTe2/MoS2 Heterobilayers

Excitonic devices based on interlayer excitons in van der Waals heterobilayers are a promising platform for advancing photoelectric interconnection telecommunications. However, the absence of exciton emission in the crucial telecom C-band has constrained their practical applications. Here, this limitation is addressed by reporting exciton emission at 0.8 eV (1550 nm) in a chemically vapor-deposited, strictly aligned MoTe2/MoS2 heterobilayer, resulting from the direct bandgap transitions of interlayer excitons as identified by momentum-space imaging of their electrons and holes. The decay mechanisms dominated by direct radiative recombination ensure constant emission quantum yields, a basic demand for efficient excitonic devices. The atomically sharp interface enables the resolution of two narrowly-splitter transitions induced by spin-orbit coupling, further distinguished through the distinct Landé g-factors as the fingerprint of spin configurations. By electrical control, the double transitions coupling into opposite circularly-polarized photon modes, preserve or reverse the helicities of the incident light with a degree of polarization up to 90%. The Stark effect tuning extends the emission energy range by over 150 meV (270 nm), covering the telecom C-band. The findings provide a material platform for studying the excitonic complexes and significantly boost the application prospects of excitonic devices in silicon photonics and all-optical telecommunications.

Transient Nanoscopy of Exciton Dynamics in 2D Transition Metal Dichalcogenides

The electronic and optical properties of 2D transition metal dichalcogenides are dominated by strong excitonic resonances. Exciton dynamics plays a critical role in the functionality and performance of many miniaturized 2D optoelectronic devices; however, the measurement of nanoscale excitonic behaviors remains challenging. Here, a near-field transient nanoscopy is reported to probe exciton dynamics beyond the diffraction limit. Exciton recombination and exciton-exciton annihilation processes in monolayer and bilayer MoS2 are studied as the proof-of-concept demonstration. Moreover, with the capability to access local sites, intriguing exciton dynamics near the monolayer-bilayer interface and at the MoS2 nano-wrinkles are resolved. Such nanoscale resolution highlights the potential of this transient nanoscopy for fundamental investigation of exciton physics and further optimization of functional devices.

Demonstration of Two-Dimensional Exciton Complex Palette

Exciton complexes in two-dimensional semiconductors, encompassing bright and dark excitons, biexcitons, and defect-bound excitons, have shown significant potential across a wide range of research areas. These applications range from exploring quantum many-body phenomena to developing nonclassical light sources and quantum transport devices. To fully leverage their dynamic and interactive properties and extend the capabilities of excitonic devices, realizing systematic engineering and mixing of the exciton complexes are crucial. Unlike conventional material methods, which often lead to undesired changes in the electronic band structure and binding energy, optical methods provide a means to manipulate the radiative decay dynamics of individual exciton complexes in a purely environmental manner. Here, we employ a specialized photonic platform, analogous to an artist's palette, to arrange and mix exciton complexes on an identical two-dimensional transition metal dichalcogenide medium. Essentially, a gradient thickness mirror (GTM) continuously tunes the local distribution of optical vacuum field interference. The GTM platform enables us to create and examine five distinct compositions of the exciton complexes of the WSe2 monolayer and their contributions to the photoluminescence spectrum. Moreover, the exciton complex palette facilitates the observation of dark and defect-bound excitons, even at high temperatures of 70 K, and its performance can be further managed by simple postprocessing manipulations.

Recent progress of exciton transport in two-dimensional semiconductors

Spatial manipulation of excitonic quasiparticles, such as neutral excitons, charged excitons, and interlayer excitons, in two-dimensional semiconductors offers unique capabilities for a broad range of optoelectronic applications, encompassing photovoltaics, exciton-integrated circuits, and quantum light-emitting systems. Nonetheless, their practical implementation is significantly restricted by the absence of electrical controllability for neutral excitons, short lifetime of charged excitons, and low exciton funneling efficiency at room temperature, which remain a challenge in exciton transport. In this comprehensive review, we present the latest advancements in controlling exciton currents by harnessing the advanced techniques and the unique properties of various excitonic quasiparticles. We primarily focus on four distinct control parameters inducing the exciton current: electric fields, strain gradients, surface plasmon polaritons, and photonic cavities. For each approach, the underlying principles are introduced in conjunction with its progression through recent studies, gradually expanding their accessibility, efficiency, and functionality. Finally, we outline the prevailing challenges to fully harness the potential of excitonic quasiparticles and implement practical exciton-based optoelectronic devices.

Searching for Unconventional Superfluid in Excitons of Monolayer Semiconductors

It is well known that two-dimensional (2D) bosons in homogeneous space cannot undergo real Bose-Einstein condensation, and the superfluid to normal phase transition is Berezinskii-Kosterlitz-Thouless (BKT) type, associated with vortex-antivortex pair unbinding. Here we point out a 2D bosonic system whose low energy physics goes beyond conventional paradigm of 2D homogeneous bosons, i.e., intralayer excitons in monolayer transition metal dichalcogenides. With intrinsic valley-orbit coupling and valley Zeeman energy, exciton dispersion becomes linear at small momentum, giving rise to a series of novel features. The critical temperature of Bose-Einstein condensation of these excitons is nonzero, suggesting true long-range order in 2D homogeneous system. The dispersion of Goldstone mode at long wavelength has the form ϵ(q)∼sqrt[q], in contrast to conventional linear phonon spectrum. The vortex energy deviates from the usual logarithmic form with respect to system size, but instead has an additional linear term. Superfluid to normal phase transition is no longer BKT type for system size beyond a characteristic scale, without discontinuous jump in superfluid density. With the recent experimental progress on exciton fluid at thermal equilibrium in monolayer semiconductors, our work points out an experimentally accessible system to search for unconventional 2D superfluids beyond BKT paradigm.