Van der Waals (vdW) ferroelectrics offer a unique platform for exploring light–matter interactions and developing advanced optoelectronic devices. However, the excited‐state properties of these materials, particularly those involving self‐trapped excitons (STEs), remain largely unexplored. Here, we report the observation of STEs emission in the vdW ferroelectric CuInP 2 S 6 (CIPS), which is activated by in‐plane lattice distortions induced by Cu deficiencies. The STEs exhibit a broad photoluminescence (PL) range from 500–700 nm, a large Stokes shift of ≈ 0.7–0.8 eV, and a nanosecond‐scale lifetime. Temperature‐dependent PL measurements reveal strong electron–phonon coupling, characterized by a high Huang–Rhys factor of 8.54 and an exciton binding energy of 318 meV. Using excited‐state‐sensitive Goos–Hänchen (GH) shift spectroscopy, we identify polarization‐dependent optical responses of STEs, with a distinct displacement peak emerging only under s ‐polarized illumination. Furthermore, the optical conductivity extracted from GH spectra exhibits nonlinear dispersion near the energy levels of the self‐trapped states, indicating modified dielectric behavior in the excited state. The coexistence of ferroelectricity and visible‐light activity endows Cu‐deficient CIPS with strong potential for broadband light emission, photodetection, and neuromorphic optoelectronic applications.

Large optical anisotropy is of paramount importance for efficient light manipulation in optoelectronic devices. Van der Waals layered materials, exhibiting large structural contrast between in-plane and out-of-plane directions, are inherently anisotropic in 3D space. The measurements of their optical constants were limited to the 2D planes. Here, the reflectance spectra of layered MoS2, NbOCl2, and WTe2 crystals are measured directly from their edge and basal surfaces to compare their out-of-plane and in-plane optical constants in the range of 500–1000 nm. The results indicate that their out-of-plane refractive indices are smaller than the in-plane refractive indices. The out-of-plane extinction coefficients of MoS2 and NbOCl2 are zero but nonzero for WTe2, as confirmed by the transient reflection spectroscopies. The nonzero extinction coefficient of WTe2 arises from the symmetry of the transition dipole moment and density of states, which are determined by the crystal structure. Meanwhile, compared to their in-plane optical constants, the out-of-plane optical constants of MoS2 and NbOCl2 exhibit less dispersion, whereas WTe2 exhibits enhanced out-of-plane dispersion around 2.14 eV. This enhancement is attributed to significant increases in optical transition probability resulting from a larger density of states. These optical parameters indicate giant birefringence (e.g., >1.8 for MoS2, >0.6 for NbOCl2, and >0.5 for WTe2) and linear dichroism (e.g., up to 100

Van der Waals (vdW) ferroelectrics offer a unique platform for exploring light-matter interactions and developing advanced optoelectronic devices. However, the excited-state properties of these materials, particularly those involving self-trapped excitons (STEs), remain largely unexplored. Here, we report the observation of STEs emission in the vdW ferroelectric CuInP2S6 (CIPS), which is activated by in-plane lattice distortions induced by Cu deficiencies. The STEs exhibit a broad photoluminescence (PL) range from 500-700 nm, a large Stokes shift of approximate to 0.7-0.8 eV, and a nanosecond-scale lifetime. Temperature-dependent PL measurements reveal strong electron-phonon coupling, characterized by a high Huang-Rhys factor of 8.54 and an exciton binding energy of 318 meV. Using excited-state-sensitive Goos-H & auml;nchen (GH) shift spectroscopy, we identify polarization-dependent optical responses of STEs, with a distinct displacement peak emerging only under s-polarized illumination. Furthermore, the optical conductivity extracted from GH spectra exhibits nonlinear dispersion near the energy levels of the self-trapped states, indicating modified dielectric behavior in the excited state. The coexistence of ferroelectricity and visible-light activity endows Cu-deficient CIPS with strong potential for broadband light emission, photodetection, and neuromorphic optoelectronic applications.

Optical beam shifts, such as the Goos–Hänchen (GH) shift and the Imbert–Fedorov (IF) shift, are fundamental optical phenomena. However, because these shifts are so minute, direct measurement is challenging, and obtaining an optical shift spectrum is even more difficult. Here, we successfully obtained GH and IF shift spectra using a beam displacement amplification technique. The optical shift spectrum of graphene shows very good agreement with the predicted results. Meanwhile, we observed two distinct peaks in the optical shift spectrum of WS 2 , corresponding to the A- and B-exciton transitions at the K -point in the Brillouin zone. More importantly, when graphene is stacked on WS 2 to form a van der Waals heterostructure, the A-exciton characteristic of WS 2 increases by an order of magnitude, which is essentially different from absorption and fluorescence spectra. Furthermore, refractive index change can be precisely captured by GH shift spectroscopy, which proves the optical shift spectrum to be an ideal candidate for a highly sensitive biosensor. Optical shift spectroscopy could enable new applications for nanophotonic devices and provide a platform for the study of intrinsic properties of two-dimensional materials, especially for van der Waals heterostructure.

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.

Two-dimensional material nanochannels with molecular-scale confinement can be constructed by Van der Waals assembly and show unexpected fluid transport phenomena. The crystal structure of the channel surface plays a key role in controlling fluid transportation, and many strange properties are explored in these confined channels. Here, we use black phosphorus as the channel surface to enable ion transport along a specific crystal orientation. We observed a significant nonlinear and anisotropic ion transport phenomenon in the black phosphorus nanochannels. Theoretical results revealed an anisotropy of ion transport energy barrier on the black phosphorus surface, with the minimum energy barrier along the armchair direction approximately ten times larger than that along the zigzag direction. This difference in energy barrier affects the electrophoretic and electroosmotic transport of ions in the channel. This anisotropic transport, which depends on the orientation of the crystal, may provide new approaches to controlling the transport of fluids.

Ultrafast optical nonlinearities of N,N -dimethylformamide (DMF) are studied by using polarized light at 400 nm. Both nonlinear refraction (NLR) and stimulated Rayleigh-wing scattering (SRWS) depend on the polarization state of incident beam, while two-photon absorption (TPA) changes negligibly with polarization state. The polarization dependence of SRWS originates from that of NLR via self-focusing effect. Third-order susceptibility elements of DMF were determined, and a method to distinguish the multi-photon absorption signal from SRWS in Z-scan is provided. These results are helpful for the nonlinear optical research of the novel materials dissolved in DMF.

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.

Twisted van der Waals structures exhibit a variety of unusual electrical and optical phenomena and could provide a powerful means for designing nanodevices with tunable chiral properties. However, programming intrinsic chiral properties of the film on the atomic scale remains a great challenge due to the limitations of fabrication and measurement techniques. Here, we report a highly tunable large optical activity of twisted anisotropic two-dimensional (2D) materials, including black phosphorus (BP), ReS2, PdSe2, and α-MoO3, by varying the twist angle between the stacked layers. The chirality can be deliberately tailored through the engineering of the symmetry, band structure, and anisotropy of 2D materials, demonstrating the high tunability of the chirality. The results show the highest thickness-normalized ellipticity value (13.8 deg μm-1, twisted ReS2) and ellipticity value (1581 mdeg, twisted BP) among the systems based on 2D materials. It is also shown that the chiroptical response exists in an extremely large spectral range from the visible to the infrared. Furthermore, the twisted ReS2 enabled spin-selective control of the information transformation. These results show that highly controllable chirality in twisted 2D anisotropic materials has considerable potential in on-chip polarization optics, nano-optoelectronics, and biology.

Rhenium diselenide（ReSe2） has gathered much attention due to its low symmetry of lattice structure, which makes it possess in-plane anisotropic optical, electrical as well as excitonic properties and further enables ReSe 2 have an important application in optoelectronic devices. Here, we report the thickness-dependent exciton relaxation dynamics of mechanically exfoliated few-layer ReSe 2 flakes by using time-resolved pump–probe transient transmission spectroscopies. The results reveal two thickness-dependent relaxation processes of the excitons. The fast one correlates with the exciton formation（i.e., the conversion of hot carriers to excitons）, while the slow one is attributed to the exciton recombination dominated by defect-assisted exciton trapping besides photon emission channel. The decrease of scattering probability caused by defects leads to the increase of fast lifetime with thickness, and the increase of slow lifetime with thickness is related to the trap-mediated exciton depopulation induced by surface defects. Polarization-dependent transient spectroscopy indicates the isotropic exciton dynamics in the two-dimensional（2D） plane. These results are insightful for better understanding of excitonic dynamics of ReSe 2 materials and its application in future optoelectronic and electronic devices.

We study the Goos–Hänchen (GH) effect in mechanically exfoliated anisotropic two-dimensional (2D) black phosphorus and rhenium disulfide by using a beam displacement amplification technique. We observed anisotropic GH shifts along different crystal orientations of two 2D materials, and they exhibit different anisotropy characteristics. It is because the GH shift is mainly affected by the phase shift of light in the 2D materials, which is related to the real part of the refractive index, while the absorption effect related to the imaginary part of the refractive index has negligible influence on the GH shift. The GH shift can be applied to explore the anisotropy of the real part of the refractive index of a 2D material. Our results provide an important way to reveal the anisotropy mechanism of 2D materials.

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.

The PL emission and carrier relaxation of CuInP2S6 vary greatly with temperature under phase transition, while the two-photon absorption changes slightly with temperature.

Layered quaternary metal thio/selenophosphates exhibit composition-dependent physical properties and have valuable applications in optoelectronic devices. Exploring their optical properties and photoexcited carrier dynamics is a prerequisite for their device design. Here, the ultrafast photocarrier dynamics and nonlinear absorption of a bulk AgInP2S6 crystal, a new member of this material family, are investigated by employing photoluminescence, time-resolved transient optical spectroscopies, and intensity-dependent transmission. The AgInP2S6 exhibits defect-assisted photoluminescence with more effective excitation under defect-state absorption and two-photon absorption at 800 nm. After initial intraband relaxation, the interband-excited carriers naturally accumulate at the conduction band minimum or valance band maximum, and then most of the nonequilibrium carriers relax via interband nonradiative recombination while the rest are transferred to the defect bands. Following that, the nonequilibrium carriers in the defect bands will first relax to the bottoms of the defect bands and finally recombine there. As compared with its analogue CuInP2S6, the substitution of Cu with Ag results in the variation of the two-photon absorption coefficient as well as the increase in carrier relaxation times due to the accompanying defect change. These findings provide microscopic insights into the optical response process of AgInP2S6 and valuable references for developing its optoelectronic devices.

The optical signals (such as Raman scattering, absorption, reflection) of van der Waals heterostructures (vdWHs) are very important for structural analysis and the application of optoelectronic devices. However, there is still a lack of research on the effect of each layer of two-dimensional materials on the optical signals of vdWHs. Here, we investigated the contribution from different layers to the optical signal of vdWHs by using angle-resolved polarized Raman spectroscopy (ARPRS) and angle-dependent reflection spectroscopy. A suitable theoretical model for the optical signal of vdWHs generated by different layers was developed, and vdWHs stacked by different two-dimensional (2D) materials were analyzed. The results revealed a strong dependence of the relative strengths of the optical signals of the upper and lower layers on the thicknesses of 2D materials and the SiO2 layer on the Si/SiO2 substrate. Interestingly, on the 285 nm SiO2/Si substrate, the contribution to the optical signal by the underlying 2D material was much greater than that by the upper layer. Furthermore, optical signals originating from different layers of twisted black phosphorus (BP) for different twist angles were studied. There is great significance for optical spectroscopy to study vdWHs, as well as the development of better twisted 2D materials and moiré physics.

Strain engineering is the most effective method to break the symmetry of the graphene lattice and achieve graphene band gap tunability. However, a critical strain (>20%) is required to open the graphene band gap, and it is very difficult to achieve such a large strain. This limits the development of experimental research and optoelectronic devices based on graphene strain. In this work, we report a method for preparing large-strain graphene superlattices via surface energy engineering. The maximum strain of the curved lattice could reach 50%. In particular, our pioneering work reports the behavior of an ultrafast (as short as 6 ps) photoresponse in a strained folded graphene superlattice. The photocurrent map shows a large increase (up to 10(2)) of the photoresponsivity in the tensile graphene lattice, which is generated by the interaction between the strained and pristine graphene. Through Raman spectroscopy, Kelvin probe force microscopy, and high-resolution transmission electron microscopy, we demonstrate that the ultrathreshold strain in the graphene bends triggers the opening of the graphene band gap and results in a unique photovoltaic effect. This work deepens the understanding of the strain-induced change of the photoelectrical properties of graphene and proves the potential of strained graphene as a platform for the generation of novel highspeed, miniaturized graphene-based photodetectors.

The Goos-Hlinchen (GH) shift and Imbert-Fedorov (IF) shift caused by light-matter and spin-orbit interactions can reveal the intrinsic properties of nanomaterials. We propose a beam displacement amplification technique (BDAT) that can break the optical diffraction limit in beam displacement measurements. The displacement resolution of the BDAT is 4 nm, and the detection size is 5 mu m, which is very suitable for the displacement measurement of mechanically exfoliated two-dimensional (2D) materials with a thickness on the scale of nanometers. With the help of the BDAT, we measured the GH shift and IF shift of graphene with different thicknesses. We found that the s-polarized light has a strong absorption effect in graphene with a thickness of approximately 15 nm, causing abnormal GH and IF shifts. This abnormal GH shift combined with the BDAT can be applied to detect changes in the refractive index, with a sensitivity of up to 9.5 x 10(-8) per reflective index unit. The BDAT holds promise as the most widespread means of displacement measurement, uncovering the properties of 2D materials and enhancing their application potential.

Recently, palladium diselenide (PdSe2) has emerged as a promising material with potential applications in electronic and optoelectronic devices due to its intriguing electronic and optical properties. The performance of the device is strongly dependent on the charge-carrier dynamics and the related hot phonon behavior. Here, we investigate the photoexcited-carrier dynamics and coherent acoustic phonon (CAP) oscillations in mechanically exfoliated PdSe2 flakes with a thickness ranging from 10.6 nm to 54 nm using time-resolved non-degenerate pump-probe transient reflection (TR) spectroscopy. The results imply that the CAP frequency is thickness-dependent. Polarization-resolved transient reflection (PRTR) measurements reveal the isotropic charge-carrier relaxation dynamics and the CAP frequency in the 10.6 nm region. In addition, the deformation potential (DP) mechanism dominates the generation of the CAP. Moreover, a sound velocity of 6.78 × 103 m s-1 is extracted from the variation of the oscillation period with the flake thickness and the delay time of the acoustic echo. These results provide insight into the ultrafast optical coherent acoustic phonon and optoelectronic properties of PdSe2 and may open new possibilities for PdSe2 applications in THz-frequency mechanical resonators.

Seeking controllable and efficient surface dopant molecules for transition‐metal dichalcogenides (TMDCs) is highly valuable for fully understanding TMDCs properties and their applications to relevant devices. The general doping effect of solvents on TMDCs are explored. By selecting suitable solvents with optimized relevant factors, controllable n‐doping of molybdenum disulfide (MoS 2 ) is obtained on the same device with the sheet density of electrons increased from 2.3 × 10 11 to 6.4 × 10 12 , 9.7 × 10 12 , and 1.6 × 10 13 by use of dimethylsulfoxide, N , N ‐dimethylformamide, and N ‐methyl‐pyrrolidone (NMP), respectively. The doping principle is explained by charge‐donating characteristics of molecule and dipole interaction. After doping by NMP, the contact resistance is reduced by four times, and the on/off current ratio of fabricated top‐gated MoS 2 transistors is increased by 3 orders of magnitude. This work can guide the selection of suitable solvents for effective doping of two‐dimensional materials and advance the development of precise controllable electronic and optoelectronic devices.

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.