The excitonic insulator (EI) is a more than 60-year-old theoretical proposal that is still elusive. It is a purely quantum phenomenon involving the spontaneous generation of excitons in quantum mechanics and the spontaneous condensation of excitons in quantum statistics. At this point, the excitons represent the ground state rather than the conventional excited state. Thus, the scarcity of candidate materials is a key factor contributing to the lack of recognized EI to date. In this review, we begin with the birth of EI, presenting the current state of the field and the main challenges it faces. We then focus on recent advances in the discovery and design of EIs based on the first-principles Bethe-Salpeter scheme, in particular the dark-exciton rule guided screening of materials. It not only opens up new avenues for realizing excitonic instability in direct-gap and wide-gap semiconductors, but also leads to the discovery of novel quantum states of matter such as half-EIs and spin-triplet EIs. Finally, we will look ahead to possible research pathways leading to the first recognized EI, both theoretically and computationally.

Electron-hole interactions play a crucial role in determining the optoelectronic properties of materials, and in low-dimensional systems this is especially true due to the decrease of screening. In this review, we focus on one unique quantum phase induced by the electron-hole interaction in two-dimensional systems, known as "exciton insulators" (EIs). Although this phase of matter has been studied for more than half a century, suitable platforms for its stable realization remain scarce. We provide an overview of the strategies to realize EIs in accessible materials and structures, along with a discussion on some unique properties of EIs stemming from the band structures of these materials. Additionally, signatures in experiments to distinguish EIs are discussed.

Two-dimensional (2D) transition metal dichalcogenides (TMDs), endowed with exceptional light-matter interaction strength, have become a pivotal platform in advanced optoelectronics, enabling atomically precise control of excitonic phenomena and offering transformative potential for engineering next-generation optoelectronic devices. In contrast to the narrowband absorption characteristics of conventional band-edge excitons, which are limited by the bandgap energy, high-energy excitons not only demonstrate broad momentum matching capability in the ultraviolet regime due to band nesting effects, but also exhibit distinct absorption peak signatures owing to robust excitonic stabilization under 2D confinement. These unique photophysical properties have established such systems as a prominent research frontier in contemporary exciton physics. This review primarily outlines the distinctive physical characteristics of high-energy excitons in TMDs from the perspectives of band structure, excitonic characteristics, and optical properties. Subsequently, we systematically delineate cutting-edge developments in TMD-based photonic devices exploiting high-energy excitonic band-nesting phenomena, with dedicated emphasis on the strategic engineering of nanoscale heterostructures for tailored optoelectronic functionality. Finally, the discussion concludes with an examination of the challenges associated with the design of high-energy exciton devices and their potential future applications.

Transition metal dichalcogenide (TMDC) monolayers provide an ideal platform for exciton and valley-spintronics exploration due to their unique properties. Integrating TMDC monolayers with conventional semiconductors allows for harnessing the unique properties of both materials. This strategy holds great promise for the development of advanced optoelectronics and spintronic devices. In this work, we investigated exciton and valley dynamics in WSe$_{2}$/GaAs heterostructure by employing the femtosecond pump-probe ultrafast spectroscopy. Facilitated by the charge transfer within the heterostructure, it was found that the exciton of WSe$_{2}$ exhibited much longer lifetime of nanosecond than that of the WSe$_{2}$ monolayer counterpart. Especially, a significantly long valley lifetime up to $\sim 2.7 $ ns was observed for trions of WSe$_{2}$ in the heterostructure even under the off-resonant excitation, which is found to be associated with the resident electrons accumulated at the interface resulting from the charge transfer and resultant interfacial electric field. Our results provide fundamental references for conventional semiconductor-integrated TMDC heterostructures that have great potential for designing novel optoelectronic and spintronic devices.

CdSe nanoplatelets (NPLs) are promising candidates for on-chip light sources, yet their performance is hindered by surface defects and inefficient optical gain. Herein, we demonstrate that CdSeS crown passivation significantly enhances the photophysical property of CdSe NPLs. Laser spectroscopy techniques reveal suppressed electronic and hole trapping at lateral surfaces, leading to a 4.2-fold increase in photoluminescence quantum yield and a shortened emission lifetime from 13.5 to 4.8 ns. In addition, amplified spontaneous emission is achieved under nanosecond pulse pumping, with thresholds of 0.75 to 0.16 mJ/cm$^{2}$ for CdSe and CdSe/CdSeS NPLs, respectively. By integrating CdSe/CdSeS NPLs with high-refractive-index SiO$_{2}$ scatters, coherent random lasing is realized at a threshold of 0.21 mJ/cm$^{2}$. These findings highlight the critical role of lateral surface passivation in optimizing optical gain and pave the way for low-cost, multifunctional nanophotonic devices.

We investigate electronic structures and excitonic properties of monolayer SiP$_2$ within the framework of first-principles GW plus Bethe-Salpeter equation (GW-BSE) calculations. Within the G$_0$W$_0$ approximation, monolayer SiP$_2$ is identified as a direct-gap semiconductor with an electronic gap of 3.14 eV, and the excitons exhibit a hybrid-dimensional character similar to that of the bulk counterpart. The optical absorption spectra reveal pronounced excitonic effects with strong anisotropy: the first bright exciton has a binding energy of 840 meV under x-polarized light, compared with 450 meV under y-polarized light. We further analyze the symmetry origins of the polarization-dependent optical selection rules through group theory. This binding energy difference arises from the intrinsic nature of the excitons: flat-band excitons under x-polarized light and conventional excitons localized at a single $\bm{k}$ point under y-polarized light. Our work enhances the understanding of excitonic behavior in monolayer SiP$_2$ and highlights its potential for polarization-sensitive and directionally tunable optoelectronic applications.

Recent breakthroughs in hot carrier (HC) cooling dynamics within halide perovskite nanocrystals (NCs) have positioned them as promising candidates for next-generation optoelectronic applications. Therefore, it is of great importance to systematically summarize advances in understanding and controlling HC relaxation mechanisms. Here, we offer an overview of advances in the understanding of the HC cooling process in perovskite NCs, with a focus on influences of excitation energy, excitation intensity, composition, size, dimensionality, doping, and core-shell structure on the HC cooling times. Finally, we propose suggestions for future investigations into the HC cooling process in perovskite NCs.

Microcavity exciton-polaritons, formed by strong light-matter coupling, are essential for realizing Bose-Einstein condensation and low-threshold lasing. Such polaritonic lasing and condensation have been demonstrated in III-V semiconductors at liquid helium temperatures. However, the complex fabrication of these microcavities and operating temperatures limit their room-temperature practical application. Here, we experimentally realize room-temperature exciton-polariton condensation and polaritonic lasing in a CsPbBr$_{3}$ perovskite planar microcavity fabricated by the pressing process. Angle-resolved photoluminescence spectra demonstrate the strong light-matter coupling and the formation of exciton-polaritons in such a pressed microcavity. Above the critical threshold, mass polaritons accumulating at the bottom of dispersion lead to a narrow emission linewidth and pronounced blueshift, further reinforcing the Bose-Einstein condensation and polaritonic lasing in this system. Our results offer a feasible and effective approach to investigate exciton-polariton condensation and polariton lasing at room temperature.

Two-dimensional (2D) transition-metal dichalcogenide (TMD) monolayers based on become a promising platform to study photonics and optoelectronics. Electrically controlling the excitonic properties of TMD monolayers can be realized in different devices. In this work, we realize the strong coupling between the excitons of WS$_2$ monolayers and a photonic cavity mode in a liquid crystal microcavity. The formed exciton polaritons can be electrically tuned by applying voltage to the microcavity. Our work offers a way to study exciton-polariton manipulation based on TMD monolayers by electrical methods at room temperature.

Understanding interlayer charge transfer is crucial for elucidating interface interactions in heterostructures. As the layer number can significantly influence the interface coupling and band alignment, the charge transfer behaviors can be largely regulated. Here, we constructed two-dimensional (2D) heterostructures consisting of monolayer WS$_{2}$ and few-layer InSe to investigate the impact of InSe thickness on exciton dynamics. We performed photoluminescence (PL) spectroscopy and lifetime measurements on pristine few-layer InSe and the heterostructures with different InSe thicknesses. For pristine InSe layers, we found a non-monotonic layer dependence on PL lifetime, which can be attributed to the interplay between the indirect-to-direct bandgap transition and surface recombination effects. For heterostructures, we demonstrated that the type I band alignment of the heterostructure facilitates electron and hole transfer from monolayer WS$_2$ to InSe. As the InSe layer number increases, the reduction in conduction band minimum (CBM) enhances the driving force for charge transfer, thereby improving the transfer efficiency. Furthermore, we fabricated and characterized a WS$_{2}$/InSe optoelectronic device. By analyzing bias voltage dependent PL spectra, we further demonstrated that the trions in WS$_{2}$ within the heterostructure are positively charged ($X^+$), and their emission intensity can be efficiently modulated by applying different biases. This study not only reveals the layer-dependent characteristics of band alignment and interlayer charge transfer in heterostructures but also provides valuable insights for the applications of 2D semiconductors in optoelectronic devices.

Insight into exciton dynamics of two-dimensional (2D) transition metal dichalcogenides (TMDs) is critical for the optimization of their performance in photonic and optoelectronic devices. Although current researches have primarily concentrated on the near-resonant excitation scenario in 2D TMDs, the case of excitation energies resonating with high-energy excitons or higher energies has yet to be fully elucidated. Here, a comparative analysis is conducted between high-energy excitation (360 nm) and near-resonant excitation (515 nm) utilizing transient absorption spectroscopy to achieve a comprehensive understanding of the exciton dynamics within monolayer WS$_{2}$. It is observed that the high-energy C-exciton can be generated via an up-conversion process under 515 nm excitation, even the energy of which is less than that of the C-exciton. Furthermore, the capacity to efficiently occupy band-edge A-exciton states leads to longer lifetimes for both the C-excitons and the A-excitons under conditions of near-resonant excitation, accompanied by an augmented rate of radiative recombination. This study provides a paradigm for optimizing the performance of 2D TMDs-based devices by offering valuable insights into their exciton dynamics.