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Layered materials exhibit different electronic and phonon properties along in-plane and out-of-plane directions; existing studies focus on their in-plane behaviors, and the influence of such anisotropies on the dynamics of photocarriers and phonons is unknown. Here, we fabricate layered PdSe2 crystals with flat edge surfaces and compare the time-resolved ultrafast spectroscopies on their basal and edge surfaces. Pronounced differences in the transient reflection spectroscopies reveal the inconsistent photocarrier and phonon dynamics behaviors on the two surfaces: the slow hot carrier relaxation process is accelerated and the thermoelasticity-induced longitudinal coherent acoustic phonon oscillation completely vanishes on the edge surface, as compared with the basal surface. Theoretical analysis reveals that the inconsistent hot carrier dynamics originate from the anisotropic properties of low-energy phonons in PdSe2, and the absence of phonon oscillation on the edge surface results from the wavevector-limited sensitivity of acoustic B1u mode. Moreover, polarization-dependent spectroscopies indicate the diverse optical anisotropies beyond the in-plane of PdSe2. This work provides a new method to explore unique physical properties and modulate the optical anisotropy of layered materials. [ABSTRACT FROM AUTHOR]

Direct growth of large-area uniform graphene films on insulating materials could facilitate the applications of graphene in optoelectronic devices. The ultrafast photocarrier relaxation and saturable absorption of direct chemical vapor deposition-grown graphene glasses, with lots of defects, are studied by using femtosecond time-resolved pump–probe and Z-scan techniques at 800 nm. We find that both the relaxation times associated with hot carrier cooling and the hot phonon effect are greatly suppressed in these defect-rich graphene glasses, which further leads to the increase of both saturation intensity and anisotropy in transient optical response for graphene glasses as compared with defect-free graphene. And, both the suppression effect and saturation intensity increase with the thickness of graphene film. The dominance of defect-assisted carrier-acoustic phonon scattering (i.e., supercollision, the collision of a carrier with both an acoustic phonon and defects) in the cooling process of hot carriers is responsible for the suppression of relaxation time. [ABSTRACT FROM AUTHOR]

The substrate effect is an important issue in the properties of two-dimensional transition metal dichalcogenides (2D TMDs). Quantitatively determining the dependence of the photoluminescence (PL) emission properties and the excitonic behavior of single-layer 2D materials in a specific dielectric environment would provide helpful guidance for the rational design of substrates for high performance 2D TMD PL emission devices. Here, using a WSe2 monolayer on different substrates as a model system, it is demonstrated that the PL emission intensities can drastically change depending on the substrate effect. From the analysis of the excitonic behavior, the results reveal that the spectral weight between the neutral and charged excitons in the PL spectra is significantly modified by the substrate types, and the weight factor is dependent on the laser excitation density. The charged exciton binding energy is obviously negatively related to the substrate dielectric constant. Furthermore, the change trends of the binding energy of the monolayer WSe2 on different substrates are basically the same for the increase of the excitation density. These results suggest that the choice of the substrate plays a significant role in the modulation of the PL properties and exciton states of atomically thin WSe2; hence, substrate engineering should be carefully considered in the design of future 2D devices. [ABSTRACT FROM AUTHOR]

As two important members of two-dimensional (2D) transition metal dichalcogenides, ReS2 and ReSe2 have gained interest for optoelectronic and photonic applications. The key to the application of the 2D materials to the optoelectronic devices is to understand the interaction between light and matter. Here, we report that the chemical vapor deposition-grown few-layer ReS2 and ReSe2 display saturable absorption under 400 nm pulse excitation, measured through intensity dependent transmission and confirmed by ΔT/T spectroscopy. ΔT/T spectroscopy substantiates the coexistence of saturable absorption and excited state absorption at 800 nm for both ReS2 and ReSe2. Two time constants are extracted from time-resolved spectroscopy; the short time constant of 10–20 ps is associated with the relaxation of hot carriers and exciton formation, and the long time constant of 70–100 ps is assigned to exciton lifetime. The polarization dependence of ΔT/T reveals that the initial distribution of photoexcited carriers centered at excitation state is anisotropic, and this initial anisotropy loses rapidly with carrier relaxation. The nonequilibrium carriers scattered far away from excitation state are fully isotropic in the entire relaxation process. These findings provide fundamental information for using the two materials in ultrafast optoelectronic and photonic devices. [ABSTRACT FROM AUTHOR]

As one of the representatives of emerging metallic transition-metal dichalcogenides, niobium ditelluride (NbTe2) has attracted intensive interest recently due to its distorted lattice structure and unique physical properties. Here, we report on the ultrafast carrier dynamics in NbTe2 measured using time-resolved pump–probe transient reflection spectroscopy. A thickness-dependent carrier relaxation time is observed, exhibiting a clear increase in the fast and slow carrier decay rates for thin NbTe2 flakes. In addition, pump-power-dependent measurements indicate that the carrier relaxation rates are power-independent and the peak amplitude of the transient reflectivity increases linearly with the pump power. The isotropic relaxation dynamics in NbTe2 is also verified by performing polarization-resolved pump–probe measurements. These results provide insight into the light–matter interactions and charge-carrier dynamics in NbTe2 and will pave the way for its applications to photonic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

Tunable optical properties could expand the functionalities of optoelectronic devices, so exploring materials with tunable optical properties is important for the development of high-performance optoelectronic devices. CuInP2S6 (CIPS) is a ferroelectric layered material with a Curie temperature of 315 K. The low phase transition temperature provides the possibility of conveniently modulating the optical properties of CIPS for device applications via temperature control. Here, we study the temperature-tunable photoluminescence (PL) and ultrafast nonlinear absorption of CIPS under ferroelectric–paraelectric phase transition, and reveal the temperature-dependence mechanism from a photoexcited carrier relaxation process. We observe broadband PL emission around 592 nm from defect-states and significant two-photon absorption at 800 nm, and time-resolved PL and transient absorption spectroscopies indicate that the carrier relaxation process consists of rapid intraband scattering, interband nonradiative recombination, charge transfer from the conduction band minimum to defect-bands and subsequent rapid intra-defect-band relaxation and slow defect-state assisted recombinations. As the temperature increases, the PL emission intensity and excitation efficiency decrease significantly due to enhanced nonradiative recombination at defect-states; meanwhile, the two-photon absorption coefficient increases and the carrier relaxation times are unchanged although phonon-scattering enhances the nonradiative recombination process. Our findings reveal the temperature behavior of the CIPS optical response under phase transition and pave the way for designing CIPS-based temperature-tunable optoelectronic devices. [ABSTRACT FROM AUTHOR]

Glucose homeostasis of adipocytes could be regulated by immune-adipose crosstalk. In order to investigate the effects of Lymphotoxin-like inducible protein that competes with glycoprotein D for herpesvirus entry on T cells (LIGHT) on glucose metabolism, we performed the present study. Our results showed that LIGHT deficiency improved glucose tolerance and enhanced glucose consumption of inguinal white adipose tissue (iWAT) under high fat diet. Consistently, Light overexpression could inhibit glucose uptake during the process of white adipogenesis. Mechanistically, LIGHT interacted with lymphotoxin-β receptor (LTβR) to attenuate AKT pathway leading to downregulation of glucose transporter-4 (GLUT4) expression, which resulted in glucose uptake inhibition. In summary, our findings revealed LIGHT-LTβR-AKT-GLUT4 axis as a regulator of glucose uptake in adipose tissue, which suggested the pivotal role of LIGHT in maintaining glucose homeostasis. • LIGHT attenuates glucose uptake of adipocytes without receding glucose utilization. • LIGHT downregulates GLUT4 expression to inhibit glucose uptake. • LIGHT interacts with LTβR to suppress GLUT4 expression via AKT signaling pathway. [ABSTRACT FROM AUTHOR]