Tailoring the nonlinear optical performance of two-dimensional MoS2 nanofilms via defect engineering

Ingenious Modulation of Third-Order Nonlinear Optical Response of Zr-MOFs through Defect Engineering Based on a Mixed-Linker Strategy

Defect engineering plays a pivotal role in regulating electronic structure and facilitating charge transfer, yielding captivating effects on third-order nonlinear optical (NLO) properties. In this work, we utilized a mixed-linker strategy to intentionally disrupt the initial periodic arrangement of UiO-66 and construct defects. Specifically, we incorporated tetrakis(4-carboxyphenyl)porphyrin (TCPP) with an exceptionally electron-rich delocalization system into the framework of UiO-66 using a one-pot solvothermal method, ingeniously occupying the partial distribution sites of the Zr6 clusters. Compared to UiO-66, the NLO absorption and refraction performance of TCPP/UiO-66 were significantly improved. Additionally, due to the presence of nitrogen-rich sites that can accommodate metal ions in the porphyrin ring of TCPP, Co(II), Ni(II), Cu(II), and Zn(II) are introduced into TCPP/UiO-66, extending the d-π conjugation effect to further regulate the defects. The NLO absorption behavior transforms saturation absorption (SA) to reverse saturation absorption (RSA), while the refraction behavior shifts from self-defocusing to self-focusing. This work shows that defects can effectively regulate the electronic structure, while TCPP plays a crucial role in significantly enhancing electron delocalization.

Modulating the nonlinear absorption response of SnOx thin films via phase engineering

Phase (composition) is known to play a key role in determining the electronic and optical properties of amorphous oxide semiconductors. In this work, modulating the ultrafast nonlinear optical (NLO) response of SnO₂ and SnO thin films by tuning oxygen partial pressure during film sputtering is explored. Femtosecond Z-scan results demonstrate that intermediate phases have no profound impact on the two-photon absorption (TPA) response of SnO₂ and SnO films. Interestingly, the magnitude of the effective nonlinear absorption coefficient (β_eff) of both intermediate SnO_2-x and SnO_x are enhanced after the change of Sn²⁺/Sn⁴⁺ composition ratio, as measured by picosecond Z-scan technique. Femtosecond degenerate pump-probe measurements show that intermediate phases accelerate the carrier trapping and improve the defect-related carrier absorption in SnO_x (SnO-rich) film, while intermediate phase suppress the TPA response of SnO_2-x (SnO₂-rich) films, therefore carrier-induced absorption dominates the NLO behavior of SnO_2-x film on picosecond regime. Our results indicate a simple and effective way to modulate the NLO response of transparent conductive oxide SnO₂ and SnO.

Improved detectivity and response speed of MoS2 phototransistors based on the negative-capacitance effect and defect engineering

Due to the unique crystal structure, outstanding optoelectronic properties and a tunable band gap from 1.2-1.8 eV, two-dimensional molybdenum disulfide (MoS₂) has attracted extensive attention as a promising candidate for future photodetectors. In this work, a negative-capacitance (NC) MoS₂ phototransistor is fabricated by using H f _0.5 Z r _0.5 O ₂ (HZO) as ferroelectric layer and Al₂O₃ as matching layer, and a low subthreshold swing (SS) of 39 mV/dec and an ultrahigh detectivity of 3.736×10¹⁴ cmHz^1/2W^-1 are achieved at room temperature due to the NC effect of the ferroelectric HZO. Moreover, after sulfur (S) treatment on MoS₂, the transistor obtained a lower SS of 33 mV/dec, a detectivity of 1.329×10¹⁴ cmHz^1/2W^-1 and specially a faster response time of 3-4 ms at room temperature, attributed to the modulation of photogating effect induced by S-vacancy passivation in MoS₂ by the S treatment. Therefore, the combination of the defect engineering on MoS₂ and the NC effect from ferroelectric thin film could provide an effective solution for high-sensitivity phototransistors based on two-dimensional materials in the future.

Mid-Infrared Optoelectronic Devices Based on Two-Dimensional Materials beyond Graphene: Status and Trends

Since atomically thin two-dimensional (2D) graphene was successfully synthesized in 2004, it has garnered considerable interest due to its advanced properties. However, the weak optical absorption and zero bandgap strictly limit its further development in optoelectronic applications. In this regard, other 2D materials, including black phosphorus (BP), transition metal dichalcogenides (TMDCs), 2D Te nanoflakes, and so forth, possess advantage properties, such as tunable bandgap, high carrier mobility, ultra-broadband optical absorption, and response, enable 2D materials to hold great potential for next-generation optoelectronic devices, in particular, mid-infrared (MIR) band, which has attracted much attention due to its intensive applications, such as target acquisition, remote sensing, optical communication, and night vision. Motivated by this, this article will focus on the recent progress of semiconducting 2D materials in MIR optoelectronic devices that present a suitable category of 2D materials for light emission devices, modulators, and photodetectors in the MIR band. The challenges encountered and prospects are summarized at the end. We believe that milestone investigations of 2D materials beyond graphene-based MIR optoelectronic devices will emerge soon, and their positive contribution to the nano device commercialization is highly expected.

Boosting the Photocatalytic Activity and Resistance of Photostability of ZnS Nanoparticles

Defects play a vital role in improving photocatalytic performance. However, the specific influence mechanism of sulfur defects (DSS) on sulfide photocatalytic performance and stability is still unclear. In this work, an ingenious solvent-free self-overflow strategy is designed to introduce DSS into ZnS nanoparticles and explore the specific promotion mechanism of photocatalytic performance and photostability. The results indicate that the introduced DSS in ZnS nanoparticles can simultaneously boost the photocatalytic hydrogen production (PHE) performance and photostability of ZnS: the PHE rate of the defective ZnS can increase up to 21350.23 μmol·h^-1·g^-1, which is roughly 4.7 times higher than that of pristine ZnS. Both experiments and theoretical calculationsshow that the enhanced photocatalytic performance could be attributed to the change of energy band position after introducing DSS. Specifically, the introduction of DSS can raise the conduction band (CB) position of ZnS to enhance the reducing ability of photogenerated electrons. Besides, the valence band (VB) position can also be raised to boost the light absorption ability of ZnS and restrain the photocorrosion by weakening the oxidation capacity of the photogenerated holes. The ingenious strategy and interesting mechanism in this job provide a simple artful tactic to fabricate other defective sulfide photocatalysts and open up a particular path to promote the photostability of the photocatalysts.

Layer-Dependent Nonlinear Optical Properties of WS2, MoS2, and Bi2S3 Films Synthesized by Chemical Vapor Deposition

Two-dimensional (2D) layered materials have shown layer-dependent optical properties in both linear optical and nonlinear optical (NLO) regimes due to prominent interlayer coupling and quantum confinement in an atomic scale. However, the NLO properties become more complicated as both saturable absorption (SA) and reverse saturable absorption (RSA) easily happen in 2D materials, which results in a significant challenge to understand the evolution of nonlinear absorption with layers. Motivated by this, chemical vapor-deposited chalcogenide compounds (WS₂, MoS₂, and Bi₂S₃) are used to investigate the pump intensity and layer number-dependent NLO properties. The values of nonlinear absorption coefficients of these chalcogenide compounds increase with the pump intensity by an 800 nm femtosecond laser, which can be described by an empirical power law function. The SA process due to the large transition probability of the ground state readily takes place in thick samples, while RSA occurs easily in thin samples due to the two-photon absorption (TPA). The transition from TPA to SA is deduced to occur at 13L-WS₂, 15L-MoS₂, and 5L-Bi₂S₃, which is related to the layer-dependent band gaps. Our results provide an efficient way to tune optical nonlinearities with a controlled layer number and to design corresponding NLO devices such as optical switches and saturable absorbers.

Unveiling and engineering of third-order optical nonlinearities in NiCo2O4 nanoflowers

In this Letter, we demonstrate for the first time, to the best of our knowledge, NiCo₂O₄ (NCO) as a novel nonlinear optical material with straightforward potential applications in optical limiting. For the 532 nm nanosecond laser, excited state absorption (ESA) and free-carrier absorption give rise to large ESA coefficient (β_ESA) and positive nonlinear n₂. On the other hand, when excited with the 800 nm femtosecond laser, two-photon absorption (TPA) takes place, and bound carriers induce strong negative n₂. The values of β and n₂ obtained for NCO are found to be higher compared to other conventional transition metal oxides and, therefore, are promising for optics and other photonics applications.

Unveiling the dimension-dependence of femtosecond nonlinear optical properties of tellurium nanostructures

Low dimensional tellurium is currently of great interest for potential electronic applications due to the experimentally observed Weyl fermions and the excellent carrier mobility, on/off ratios and current-carrying capacity in devices. However, the optical properties of Te nanostructures are not well explored, especially in the field of nonlinear optics. Here, we prepared a series of Te nanostructures by electrochemical exfoliation and liquid phase exfoliation methods, including one-dimensional (1D) Te nanowires (NWs), quasi-1D Te nanorods (NRs), zero-dimensional (0D) Te nanodots (NDs) and two-dimensional (2D) Te nanosheets (NSs). Femtosecond Z-scan measurements reveal unique dimension-dependent nonlinear optical (NLO) properties. 1D Te NWs and quasi-1D Te NRs exhibited higher saturable absorption behavior than 0D Te nanostructures, while the 2D Te NSs are a high performance optical limiting material. Ultrafast transient absorption spectroscopy revealed the dimension-dependent exciton dynamics. The reverse saturable absorption of 2D Te NSs is derived from faster exciton relaxation and stronger excited state absorption. This work paves the way for the design of saturable absorbers with high performance and broadens the application of 2D Te in the field of laser protection and other novel ultrafast photonics.

Tunable nonlinear optical responses and carrier dynamics of two-dimensional antimonene nanosheets

Sb nanosheets, also known as antimonene, have received ever-growing consideration as a promising new type of two-dimensional (2D) material due to their many attractive properties. However, how their nonlinear optical (NLO) properties are affected by their nanosheet structure and measurement conditions remains unclear. Herein, we report a successful size-selective production method for Sb nanosheets, which is based on a combination of lithium ion intercalation, solvent exfoliation and size selection centrifugation. This high-yield and size-selective preparation method enables fundamental investigation on the relation of the intrinsic optical properties of Sb nanosheets. Nanosecond Z-scan measurements revealed a unique size-dependent broadband NLO response. When the average size is reduced from 3 micrometers to 50 nanometers, the Sb nanosheets exhibit a clear transition from saturable absorption to reversed saturable absorption. Ultrafast transient absorption spectroscopic investigation indicated that exciton cooling is significantly faster in a small nanosheet than in large ones, revealing that the different exciton relaxation dynamic plays key roles in the distinct size-tunable nonlinear optical response. This work paves new ways towards the mass production and practical application of antimonene.

Vertically-oriented MoS2 nanosheets for nonlinear optical devices

Transition metal dichalcogenides such as MoS₂ represent promising candidates for building blocks of ultra-thin nanophotonic devices. For such applications, vertically-oriented MoS₂ (v-MoS₂) nanosheets could be advantageous as compared to conventional horizontal MoS₂ (h-MoS₂) given that their inherent broken symmetry would favor an enhanced nonlinear response. However, the current lack of a controllable and reproducible fabrication strategy for v-MoS₂ limits the exploration of this potential. Here we present a systematic study of the growth of v-MoS₂ nanosheets based on the sulfurization of a pre-deposited Mo-metal seed layer. We demonstrate that the sulfurization process at high temperatures is driven by the diffusion of sulfur from the vapor-solid interface to the Mo seed layer. Furthermore, we verify an enhanced nonlinear response in the resulting v-MoS₂ nanostructures as compared to their horizontal counterparts. Our results represent a stepping stone towards the fabrication of low-dimensional TMD-based nanostructures for versatile nonlinear nanophotonic devices.

Enhanced two-photon absorption and two-photon luminescence in monolayer MoS2 and WS2 by defect repairing

In this work, we investigated the nonlinear optical properties of monolayer MoS₂ and WS₂ modulated by defect engineering via chemical treatment. The results demonstrate that the two-photon luminescence (TPL) and two-photon absorption (TPA) coefficient were remarkably improved after the repair of sulfur vacancies for both monolayer MoS₂ and WS₂. After the chemical treatment, the nonradiative relaxation path dominant in pristine monolayer MoS₂ is significantly alleviated, resulting in enhanced TPL. Our work affords an effective way to tailor the nonlinear absorption, luminescence and relaxation properties of sulfur-based two-dimensional metal dichalcogenides by defect engineering.

Nonlinear optical properties of MoS2-WS2 heterostructure in fiber lasers

As a saturable absorption material, the heterostructure with the van der Waals structure has been paid much attention in material science. In general, the heterogeneous combination is able to neutralize, or even exceed, the individual material's advantages in some aspects. In this paper, which describes the magnetron sputtering deposition method, the tapered fiber is coated by the MoS₂-WS₂ heterostructure, and the MoS₂-WS₂ heterostructure saturable absorber (SA) is fabricated. The modulation depth of the prepared MoS₂-WS₂ heterostructure SA is measured to be 19.12%. Besides, the theoretical calculations for the band gap and carrier mobility of the MoS₂-WS₂ heterostructure are provided. By employing the prepared SA, a stable and passively erbium-doped fiber laser is implemented. The generated pulse duration of 154 fs is certified to be the shortest among all fiber lasers based on transition mental dichalcogenides. Results in this paper provide the new direction for the fabrication of ultrafast photon modulation devices.

Mode-Locked Er-Doped Fiber Laser by Using MoS2/SiO2 Saturable Absorber

The two-dimensional (2D) layered material MoS₂ has attracted numerous attentions for electronics and optoelectronics applications. In this work, a novel type of MoS₂-doped sol-gel glass composite material is prepared. The nonlinear optical properties of prepared MoS₂/SiO₂ composite material are measured with modulation depth (ΔT) of 3.5% and saturable intensity (I_sat) of 20.15 MW/cm². The optical damage threshold is 3.46 J/cm². Using the MoS₂/SiO₂ composite material as saturable absorber (SA), a passive mode-locked Er-doped fiber (EDF) laser is realized. Stable conventional soliton mode-locking pulses are successfully generated with a pulse width of 780 fs at the pump power of 90 mW. In the pump power range of 100-600 mW, another stable mode-locking operation is obtained. The pulse width is 1.21 ps and the maximum output power is 5.11 mW. The results indicate that MoS₂/SiO₂ composite materials could offer a new way for optical applications.