Volume 9, Issue 7 - Special Issue: 2D materials for nanophotonics: from fundamentals to applications, I. Fundamentals; Guest Editors: Xiantao Jiang, Meng Zhang, Xi Ling, Pablo Alonso González, Han Zhang

Transition metal dichalcogenides are two-dimensional semiconductors with strong in-plane covalent and weak out-of-plane interactions, resulting in exfoliation into monolayers with atomically thin thickness. This creates a new era for the exploration of two-dimensional physics and device applications. Among them, MoS 2 is stable in air and easily available from molybdenite, showing tunable band-gaps in the visible and near-infrared waveband and strong light-matter interactions due to the planar exciton confinement effect. In the single-layer limit, monolayer MoS 2 exhibits direct band-gaps and bound excitons, which are fundamentally intriguing for achieving the nanophotonic and optoelectronic applications. In this review, we start from the characterization of monolayer MoS 2 in our group and understand the exciton modes, then explore thermal excitons and band renormalization in monolayer MoS 2 . For nanophotonic applications, the recent progress of nanoscale laser source, exciton-plasmon coupling, photoluminescence manipulation, and the MoS 2 integration with nanowires or metasurfaces are overviewed. Because of the benefits brought by the unique electronic and mechanical properties, we also introduce the state of the art of the optoelectronic applications, including photoelectric memory, excitonic transistor, flexible photodetector, and solar cell. The critical applications focused on in this review indicate that MoS 2 is a promising material for nanophotonics and optoelectronics.

Photonic computing and neuromorphic computing are attracting tremendous interests in breaking the memory wall of traditional von Neumann architecture. Photonic memristors equipped with light sensing, data storage, and information processing capabilities are important building blocks of optical neural network. In the recent years, two-dimensional materials (2DMs) have been widely investigated for photonic memristor applications, which offer additional advantages in geometry scaling and distinct applications in terms of wide detectable spectrum range and abundant structural designs. Herein, the recent progress made toward the exploitation of 2DMs beyond graphene for photonic memristors applications are reviewed, as well as their application in photonic synapse and pattern recognition. Different materials and device structures are discussed in terms of their light tuneable memory behavior and underlying resistive switching mechanism. Following the discussion and classification on the device performances and mechanisms, the challenges facing this rapidly progressing research field are discussed, and routes to realize commercially viable 2DMs photonic memristors are proposed.

The discovery of graphene, the first two-dimensional (2D) material, has caused an upsurge, as this kind of material revealed a tremendous potential of application in areas such as energy storage, electronics, and gas separation. MXenes are referred to as a family of 2D transition metal carbides, carbonitrides, and nitrides. After the synthesis of Ti 3 C 2 from Ti 3 AlC 2 in 2011, about 30 new compositions have been reported. These materials have been widely discussed, synthesized, and investigated by many research groups, as they have many advantages over traditional 2D materials. This review covers the structures of MXenes, discusses various synthesis routines, analyzes the properties, especially optical and electronic properties, and summarizes their applications and potential, which may give readers an overview of these popular materials.

Monoelemental two-dimensional materials are well known as Xenes. The representatives graphene and phosphorene have received considerable attention because of their outstanding physical properties. In recent years, the family members of Xenes have greatly increased, and the emerging ones are gaining more and more interest. In this review, we mainly focus on the recently developed Xenes in groups IIIA, VA, and VI. Comprehensive discussions of the latest progress are given in the aspects of basic physical properties and intriguing applications in photonics, optoelectronics, energy, and biomedicines.

The discovery of graphene has intrigued the significant interest in exploring and developing the two-dimensional layered materials (2DLMs) for the photonics application in recent years. Unlike the isotropic graphene, a number of 2DLMs possess the in-plane anisotropic crystal structure with low symmetry, enabling a new degree of freedom for achieving the novel polarization-dependent and versatile ultrafast photonic devices. In this review article, we focus on the typical anisotropic 2DLMs including BP, ReS 2 , ReSe 2 , SnS, and SnSe and summarize the recent development of these anisotropic 2DLMs in the pulsed laser and the optical switch applications. First, we introduce the fabrication methods as well as the material characterization of the anisotropic 2DLMs by analyzing the polarized Raman configuration. Second, we discuss the anisotropic nonlinear optical properties of the anisotropic 2DLMs and concentrate on the anisotropic nonlinear absorption response. Next, we sum up state of the art of the anisotropic 2DLMs in the application of pulse lasers and optical switches. This review ends with perspectives on the challenge and outlook of the anisotropic 2DLMs for ultrafast photonics applications.

Differing from its bulk counterparts, atomically thin two-dimensional transition metal dichalcogenides that show strong interaction with light are considered as new candidates for optoelectronic devices. Either physical or chemical strategies can be utilized to effectively tune the intrinsic electronic structures for adopting optoelectronic applications. This review will focus on the different tuning strategies that include its physics principles, in situ experimental techniques, and its application of various optoelectronic devices.

Over the past decade, the plasmonics of graphene and black phosphorus (BP) were widely recognized as promising media for establishing linear and nonlinear light-matter interactions. Compared to the conventional metals, they support significant light-matter interaction of high efficiency and show undispersed optical properties. Furthermore, in contrast to the conventional metals, the plasmonic properties of graphene and BP structure can be tuned by electrical and chemical doping. In this review, a deep attention was paid toward the second- and third-order nonlinear plasmonic modes of graphene and BP. We present a theoretical framework for calculating the lifetime for surface plasmons modes of graphene and BP assisted by the coupled mode theory. The effect of the Fermi energy on the second-order and third-order nonlinear response is studied in detail. We survey the recent advances in nonlinear optics and the applications of graphene and BP-based tunable plasmonic devices such as light modulation devices, switches, biosensors, and other nonlinear photonic devices. Finally, we highlight a few representative current applications of graphene and BP to photonic and optoelectronic devices.

Two-dimensional (2D) materials such as graphene, black phosphorus, and transition metal dichalcogenides have attracted significant research attention due to their novel properties and wide range of applications in electronic and optoelectronic devices. In particular, investigation of twist-controlled 2D materials has attracted tremendous attention due to their excellent properties such as smooth heterointerfaces, highly gate-tunable bandgaps, and ultrafast carrier transport. Twist-controlled 2D materials combined with their fascinating electronic structures have also indicated their outstanding potential in electronic and optoelectronic applications. In this review, the recent developments in twisted 2D materials are summarized, covering aspects related to their fabrication, twist angle-dependent optical properties, and optoelectronic applications. The photodetectors and orientation-dependent van der Waals junctions are introduced and discussed systematically. Finally, we deliver a summary and outlook to provide a guideline for the future development of this rapidly growing field.

A range of new 2D materials have recently been reported, including topological insulators, transition-metal dichalcogenides, black phosphorus, MXenes, and metal-organic frameworks, which have demonstrated high optical nonlinearity and Pauli blocking for widespread use as saturable absorbers in pulsed lasers. 2D materials are emerging as a promising platform for ultrashort-pulse fiber laser technology. This review presents a catalog of the various pulsed laser applications based on the series of emerging 2D materials. In addition, novel optical devices using layered materials, such as optical modulators, optical switches, and all-optical devices, are also included. It is anticipated that the development of 2D materials will intensify in the future, providing potentially new and wide-ranging efficacy for 2D materials in ultrafast photonic technology.

Two-dimensional (2D) organic-inorganic hybrid perovskites feature characteristics of inherent quantum-well structures and intriguing optoelectronic properties, and have therefore attracted enormous research attention for their optical applications in light emitting, sensing, modulation, and telecommunication devices. The low-cost and solution-processed fabrications as well as alternative organic spacer cations endue 2D hybrid perovskites with higher tunability in optical and photonic applications. In particular, they demonstrate distinguished nonlinear optical characters such as second-harmonic generation (SHG), two-photon absorption (2PA), and saturable absorption (SA) under the excitation of laser pulses. Here, we discuss the construction of the various sorts of 2D hybrid perovskites with different structural features. We have also highlighted some representative properties and applications of these 2D hybrid perovskites in both linear and nonlinear optical regimes.

Monolayer transitional metal dichalcogenides (TMDCs), a new class of atomically thin semiconductor, respond to optical excitation strongly with robust excitons, which stem from the reduced screening in two dimensions. These excitons also possess a new quantum degree of freedom known as valley spin, which has inspired the field of valleytronics. The strongly enhanced Coulomb interaction allows the exciton to bind with other particles to form new excitonic states. However, despite the discovery of trions, most of the excitonic states in monolayer TMDCs remain elusive until recently, when new light was shed into the fascinating excitonic fine structures with drastically improved sample quality through boron nitride encapsulation. Here, we review the latest research progress on fine structures of excitonic states in monolayer TMDCs, with a focus on tungsten-based TMDCs and related alloy. Many of the new excitonic complexes inherit the valley degree of freedom, and the valley-polarized dark excitonic states are of particular interest because of their long lifetime and possible long valley coherence time. The capability of resolving the excitonic fine structures also enables the investigation of exciton–phonon interactions. The knowledge of the interlayer between excitons and other particles not only advances our understanding of many-body effects in the monolayer TMDCs but also provides guidance on future applications based on TMDCs.

Two-dimensional (2D) layers of transition metal carbides, nitrides, or carbonitrides, collectively referred to as MXenes, are considered as the new family of 2D materials for the development of functional building blocks for optoelectronic and photonic device applications. Their advantages are based on their unique and tunable electronic and optical properties, which depend on the modulation of transition metal elements or surface functional groups. In this paper, we have presented a comprehensive review of MXenes to suggest an insightful perspective on future nanophotonic and optoelectronic device applications based on advanced synthesis processes and theoretically predicted or experimentally verified material properties. Recently developed optoelectronic and photonic devices, such as photodetectors, solar cells, fiber lasers, and light-emitting diodes are summarized in this review. Wide-spectrum photodetection with high photoresponsivity, high-yield solar cells, and effective saturable absorption were achieved by exploiting different MXenes. Further, the great potential of MXenes as an electrode material is predicted with a controllable work function in a wide range (1.6–8 eV) and high conductivity (~10 4 S/cm), and their potential as active channel material by generating a tunable energy bandgap is likewise shown. MXene can provide new functional building blocks for future generation nanophotonic device applications.

Förster resonance energy transfer (FRET)–based sensing has been steadily gaining popularity in the areas of biochemical analysis, environmental monitoring, and disease diagnosis in the past 20 years. Two-dimensional (2D) nanomaterials are extensively used as donors and acceptors in the FRET sensing because of their attractive optical and chemical properties. In this review, we first present the FRET theory and calculations to give readers a better understanding of the FRET phenomenon. Then, we discuss the recent research advances in using 2D nanomaterials as donors and acceptor in FRET sensing. Finally, we summarize the existing challenges and future directions of 2D nanomaterials in the FRET sensing applications.

Due to their novel electronic and optical properties, atomically thin layered two-dimensional (2D) materials are becoming promising to realize novel functional optoelectronic devices including photodetectors, modulators, and lasers. However, light–matter interactions in 2D materials are often weak because of the atomic-scale thickness, thus limiting the performances of these devices. Metallic nanostructures supporting surface plasmon polaritons show strong ability to concentrate light within subwavelength region, opening thereby new avenues for strengthening the light–matter interactions and miniaturizing the devices. This review starts to present how to use metallic nanostructures to enhance light–matter interactions in 2D materials, mainly focusing on photoluminescence, Raman scattering, and nonlinearities of 2D materials. In addition, an overview of ultraconfined acoustic-like plasmons in hybrid graphene–metal structures is given, discussing the nonlocal response and quantum mechanical features of the graphene plasmons and metals. Then, the review summarizes the latest development of 2D material–based optoelectronic devices integrated with plasmonic nanostructures. Both off-chip and on-chip devices including modulators and photodetectors are discussed. The potentials of hybrid 2D materials plasmonic optoelectronic devices are finally summarized, giving the future research directions for applications in optical interconnects and optical communications.

Plasmonic excitations, consisting of collective oscillations of the electron gas in a conductive film or nanostructure coupled to electromagnetic fields, play a prominent role in photonics and optoelectronics. While traditional plasmonic systems are based on noble metals, recent work has established graphene as a uniquely suited materials platform for plasmonic science and applications due to several distinctive properties. Graphene plasmonic oscillations exhibit particularly strong sub-wavelength confinement, can be tuned dynamically through the application of a gate voltage, and span a portion of the infrared spectrum (including mid-infrared and terahertz (THz) wavelengths) that is not directly accessible with noble metals. These properties have been studied in extensive theoretical and experimental work over the past decade, and more recently various device applications are also beginning to be explored. This review article is focused on graphene plasmonic nanostructures designed to address a key outstanding challenge of modern-day optoelectronics – the limited availability of practical, high-performance THz devices. Graphene plasmons can be used as a means to enhance light–matter interactions at THz wavelengths in a highly tunable fashion, particularly through the integration of graphene resonant structures with additional nanophotonic elements. This capability is ideally suited to the development of THz optical modulators (where absorption is switched on and off by tuning the plasmonic resonance) and photodetectors (relying on plasmon-enhanced intraband absorption or rectification of charge-density waves), and promising devices based on these principles have already been reported. Novel radiation mechanisms, including light emission from electrically excited graphene plasmons, are also being explored for the development of compact narrowband THz sources.

We experimentally investigated the soliton collisions between soliton molecules and deuterogenic solitons spontaneously generated on the continuous wave (cw) noise background in an ultrafast erbium-doped fiber laser mode locked with MoS 2 saturable absorber (SA). The dynamics of the soliton collisions were observed using the time-stretch dispersion Fourier transform technique. The noise-induced deuterogenic solitons first undergo spectral broadening and wavelength shifting, then collide successively with a soliton molecule and eventually vanish. Within the simple collision framework, the spectral-temporal dynamics of soliton collision would help to unveil the self-stabilization mechanism of the soliton molecules in consideration of dispersive wave shedding. This nonlinear dynamics is similar to the soliton rain, except that complex condensed soliton phase is substituted with a soliton molecule.

Unlike traditional two-dimensional layered materials, tellurium (Te) shows its one-dimensional van der Waals structure and triggers intensive researches. Through a density functional theory coupled with the nonequilibrium Green’s function calculation, we found that bilayer tellurene has a broad photoelectric response over the range from the visible to the near-infrared region. Besides, bilayer tellurene photodetector presents an ultra-strong anisotropic photo-responsivity and an ultra-high extinction ratio (~2812 at the photon energy of 3.4 eV) because of its non-layer/chain structure, which is superior to the antimonene (~145) and phosphorene/blue phosphorene bilayers (~240). Time-domain first principle study further reveals that the ultra-strong anisotropy comes from the transition of Te 5 p bonding orbitals along or perpendicular to the chain directions. With these excellent optoelectronic merits, bilayer tellurene may become a promising candidate for next-generation photoelectronic devices.

Surface plasmon resonance (SPR) sensors have been applied in a wide range of applications for real-time and label-free detection. In this article, by covering the topological insulators nanosheets on the surface of the noble metal (Au), the sensitivity of the SPR sensor is greatly enhanced because of the strong interaction of light with Au–bismuth selenide (Bi 2 Se 3 ) heterostructure. It is shown that the sensitivity of proposed SPR sensors depends on the concentration of Bi 2 Se 3 solution or the thickness of the coated Bi 2 Se 3 film. The optimised sensitivity (2929.1 nm/RIU) and figure of merit (33.45 RIU −1 ) have been obtained after three times drop-casting, and the enhancement sensitivity of proposed sensors is up to 51.97% compared to the traditional Au–SPR sensors. Meanwhile, the reflection spectrum is simulated by using the method of effective refractive index, and the reason for the increase of sensitivity is analysed theoretically. For researching the application of modified SPR sensor, heavy metal detection is employed to detect in the last part. Our proposed SPR sensors have potential applications in heavy metal detections and biosensing.

Along with the studies of the properties of optical microcavities, the optical nonlinear effects of the material and the interaction between light and matter are becoming important topics. For example, the chaotic effect of the optical field may appear under the strong laser pumping due to the nonlinear effects. In this study, the chaotic phenomenon of photonic superposed states is investigated in an optomagnonical cavity. Through the dynamic evolution of two-mode photons in the cavity, we found that the energy of the system could be transferred between different modes, and the chaotic behaviour can be tuned by the coupling and the dissipation.

Two-dimensional tin sulfide (SnS), as a black phosphorus-analogue binary semiconductor, has received considerable attention in photonics and optoelectronics. Herein, the third-order nonlinearity susceptibility Im χ 3 is enhanced from  −(6.88 ± 0.10) × 10 −14 esu to  −(15.90 ± 0.27) × 10 −14 esu by the size-related quantum confinement in layered SnS nanosheets. Due to the energy level alignment, a phonon-bottleneck effect is observed, which leads to a prolonged carrier lifetime. These results provide a platform for actively tuning the linear and nonlinear optics, and pave the way for designing SnS-based tunable and anisotropic optoelectronic devices.

All-optical modulators avoid the conversion from external electronic signals to optical signals and thus have the potential to achieve an energy-efficient high-speed photonic system. Phosphorene recently debuted as an attractive material that exhibits outstanding high electron mobility, strong light-matter interaction and modifiable bandgap, making it ideal for all-optical modulators. In this paper, by incorporating a phosphorene and silicon-based micro-ring resonator (MRR), we first propose and experimentally demonstrate a unique phosphorene-integrated all-optical modulator in telecommunications. By utilizing a phosphorene thin film with an average thickness of 22 nm as the absorption material, the rise time of only 479 ns and decay time of 113 ns are achieved, which is the fastest reported response time in the family of phosphorene modulators. The corresponding 3 dB bandwidth is larger than 2.5 MHz, and it exhibits a low-loss performance benefited from its finite bandgap. The proposed phosphorene/MRR hybrid modulator may have potential in the applications of all-optical interconnections.

MoS 2 , as a typical representative of two-dimensional semiconductors, has been explored extensively in applications of optoelectronic devices because of its adjustable bandgap. However, to date, the performance of the fabricated photodetectors has been very sensitive to the surrounding environment owing to the large surface-to-volume ratio. In this work, we report on large-scale, high-performance monolayer MoS 2 photodetectors covered with a 3-nm Al 2 O 3 layer grown by atomic layer deposition. In comparison with the device without the Al 2 O 3 stress liner, both the photocurrent and responsivity are improved by over 10 times under 460-nm light illumination, which is due to the tensile strain induced by the Al 2 O 3 layer. Further characterization demonstrated state-of-the-art performance of the device with a responsivity of 16.103 A W −1 , gain of 191.80, NEP of 7.96 × 10 −15 W Hz −1/2 , and detectivity of 2.73 × 10 10 Jones. Meanwhile, the response rise time of the photodetector also reduced greatly because of the increased electron mobility and reduced surface defects due to the Al 2 O 3 stress liner. Our results demonstrate the potential application of large-scale strained monolayer MoS 2 photodetectors in next-generation imaging systems.

Upconverting luminescent nanoparticles are photostable, nonblinking, and low chemically toxic fluorophores that are emerging as promising fluorescent probes at the single molecule level. High luminescence intensity upconversion nanoparticles (UCNPs) have previously been achieved by doping with high amounts of rare-earth ions using high excitation power (>2.5 MW/cm 2 ). However, such particles are inadequate for in vitro live-cell imaging and single-particle tracking, as high excitation power can cause photodamage. Here, we compared UCNP luminescence intensities with different dopant concentrations and presented more efficient (about seven times) UCNPs at low excitation power by increasing the concentrations of Yb 3+ and Tm 3+ dopants (NaYF 4 : 60% Yb 3+ , 8% Tm 3+ ) and adding a core-shell structure.

Two-dimensional (2D) organic-inorganic hybrid perovskites have attracted growing attention recently due to their naturally formed quantum-well structure, unique photoelectric properties and better environmental stability compared to three-dimensional perovskites. The reduced screening and enhanced Coulomb interaction in 2D perovskites result in the formation of excitonic complexes. While the properties of free excitons have been well investigated, studies on biexcitons remain elusive. Here, we report on the biexcitons in 2D (iso-BA) 2 PbI 4 (BA=C 4 H 9 NH 3 ) crystals. The biexciton emission can be observed under a very low excitation power density of 6.4 W/cm 2 at 78 K. The biexciton exhibits a large biexciton binding energy of 46 meV due to the large exciton binding energy of (iso-BA) 2 PbI 4 . Furthermore, the biexcitons exhibit a favorable polarization orientation, resulting in different anisotropy between biexcitons and excitons. Our findings would motivate more studies on biexcitons in 2D perovskites and pave the way for exploiting the many-body physics for biexciton lasing and optical storage devices.

Germanium selenide nanoplates (GeSe NPs) are considered to have broadband nonlinear optical (NLO) properties and great potential for applications in nanophotonic devices. In this work, we systematically studied the NLO response of GeSe NPs by the open-aperture Z-scan technique. GeSe NPs exhibit strong saturable absorption at wavelengths of 400, 800, and 1064 nm with different pulse durations. Furthermore, we investigated the excited carrier dynamics of GeSe NPs by the non-degenerate pump-probe technique. The fast and slow relaxation times at different wavelengths of 800, 871, 1064, and 1100 nm were components with lifetimes of about 0.54–1.08 and 52.4–167.2 ps, respectively. The significant ultrafast NLO properties of GeSe NPs imply their potential in the development of nanophotonic devices. Here, we designed and fabricated the all-optical diode by means of the GeSe/C 60 tandem structure and demonstrated that the saturable absorption behavior of GeSe NPs can be used to fabricate a photonic diode, which exhibits nonreciprocal transmission of light similar to that of an electron diode.

The combination of 2D materials and surface plasmon can produce some novel optical phenomena that have attracted much attention. Illuminated by light with different polarization states, the field distribution around the plasmonic structure can control the light-matter interaction. The interaction between graphene and light can be strongly enhanced by employing radially polarized beams in a nanocavity. Here, we study the selectively enhanced vibration of graphene in a coupled plasmonic gold nanocavity with a radially polarized optical field, and the coupling and enhancing mechanisms are investigated both experimentally and numerically. By focusing a radially polarized beam, a high z component of a localized near field in the nanocavity is provided to strongly enhance the interaction between graphene and light, which can be used to enhance the vibrational signal of the interlayer. For the in-plane vibration of graphene, a similar enhancement is obtained with a linearly and radially polarized optical field. A plasmonic nanocavity is used to enhance the vibration of graphene, which provides potential applications in studying the out-of-plane vibration mode and exploring the mechanism of the interlayer coupling of 2D materials.

The design of sulfur hosts with polar, sulfurphilic, and conductive network is critical to lithium-sulfur (Li-S) batteries whose potential applications are greatly limited by the lithium polysulfide shuttle effect. Mxenes, possessing layered-stacked structures and high electrical conductivities, have a great potential in sulfur hosts. Herein, sulfur nanoparticles uniformly decorated on titanium carbide Mxene (S@Ti 3 C 2 T x Mxene) are synthesized via a hydrothermal method and then utilized as a cathode for lithium-sulfur batteries. This unique architecture could accommodate sulfur nanoparticles expansion during cycling, suppress the shuttling of lithium polysulfide, and enhance electronical conductivity. Consequently, the S@Mxene with a high areal sulfur loading (∼4.0 mg cm −2 ) exhibits a high capacity (1477.2 mAh g −1 ) and a low capacity loss per cycle of 0.18% after 100 cycles at 0.2 C. This work may shed lights on the development of high performance sulfur-based cathode materials for Li-S batteries.

Pump fluence and probe wavelength-dependent ultrafast carrier dynamics and optical nonlinear absorption in black phosphorus nanosheets are investigated by transient absorption spectroscopy and open-aperture Z scan techniques. The decay time becomes longer with larger wavelengths under pump wavelengths of both 400 nm and 800 nm excitation. For 800 nm excitation, pump fluence-dependent lifetime shows complex behaviors, which might be due to the competition between the linear absorption and two photon absorption. For 400 nm excitation, an additional decaying channel is observed at a larger pump fluence, which is explained by an effective subband structure. In open-aperture Z scan measurements, strong saturation absorption is observed in the visible region over a broad band from 450 nm to 700 nm. The saturation intensity shows an increasing trend with increase in the wavelength. Also, the saturation intensities under different pulse widths and solvents are discussed in detail. Our results show that black phosphorus nanosheets have great potential in future ultrathin optoelectronic devices.

Indium selenide (InSe) film, an emerging two-dimensional chalcogenide semiconductor, has recently attracted growing interests in optoelectronics. However, its nonlinear characteristics and application potentials in mid-infrared (IR) region remain open, which is a very attractive but undeveloped spectral region currently. In this work, it is demonstrated that InSe film possesses excellent nonlinear absorption properties in 3- to 4-μm band. Saturable absorption measurements of InSe film at 2.8 and 3.5 μm show very low saturation energy fluences and moderate modulation depths. Pump–probe measurements at 3 and 4 μm indicate that InSe film has ultrafast responses in mid-IR region. Furthermore, the application of InSe film in mid-IR pulsed laser is demonstrated, and stable Q-switching operation of fiber laser at 2.8 μm is realized. These results show that InSe film is a promising saturable absorber for mid-IR pulsed laser.

Black phosphorus (BP) shows great potential in electronic and optoelectronic applications; however, maintaining the stable performance of BP devices over temperature is still challenging. Here, a novel BP field-effect transistor (FET) fabricated on the atomic layer deposited AlN/SiO 2 /Si substrate is demonstrated. Electrical measurement results show that BP FETs on the AlN substrate possess superior electrical performance compared with those fabricated on the conventional SiO 2 /Si substrate. It exhibits a large on-off current ratio of 5 × 10 8 , a low subthreshold swing of <0.26 V/dec, and a high normalized field-effect carrier mobility of 1071 cm 2 V −1 s −1 in the temperature range from 77 to 400 K. However, these stable electrical performances are not found in the BP FETs on SiO 2 /Si substrate when the temperature increases up to 400 K; instead, the electrical performance of BP FETs on the SiO 2 /Si substrate degrades drastically. Furthermore, to gain a physical understanding on the stable performance of BP FETs on the AlN substrate, low-frequency noise analysis was performed, and it revealed that the AlN film plays a significant role in suppressing the lattice scattering and charge trapping effects at high temperatures.

Visually monitoring of the residual morphology and quantitatively determining the degradation degree of hydrogels applied in tissue repair therapy in a real-time and noninvasive manner were a crucial technological mean. Despite conventional organic fluorescent molecules commonly used as probe to capture the real-time clues of the labeled hydrogels, they still encounter obstacles, including intrinsic photobleaching, cytotoxicity, and unknown interference factor of degradation caused by the change from polymer structure of hydrogels, thus making it difficult to accurately obtain the information of the hydrogels in vivo . To address the hard nut, we designed the multifunctional hydrogel system with a real-time quantitative aggregation-induced emission fluorescent detection and photoacoustic imaging tracking based on tetraphenylethene (TPE) that possesses the trait of aggregation-induced emission and low photobleaching, bound on the surface of mesoporous dopamine microspheres (MPDAs), and subsequently loaded into the photo-crosslinked injectable hydrogels. In vitro results showed that MPDA-TPE had good compatibility, emitted strong fluorescence when embedded in hydrogels, and maintained stable fluorescence property unless the hydrogels were degraded. Meanwhile, a mathematical formula for the kinetic degradation of hydrogels was established between gravitational and visual degradation in vitro , which can be used to predict in vivo degradation. Furthermore, MPDA possessed the clear photoacoustic imaging effect to provide more accurate clues. The designed hydrogel system holds a potential role in prediction of the in vivo degradation of implanted materials in an accurate, convenient, and real-time noninvasive manner and is a meaningful treatment aid in tissue engineering.

The strategy to improve the photocatalytic removal efficiencies towards organic pollutants is still a challenge for the novel Sillen–Aurivillius perovskite type Bi 4 NbO 8 Cl. Herein, we report carbon-supported TiO 2 /Bi 4 NbO 8 Cl (C-TiO 2 /Bi 4 NbO 8 Cl) heterostructures with enhanced charge separation efficiency, which were fabricated via molten-salt flux process. The carbon-supported TiO 2 particles were derived from MXene Ti 3 C 2 precursors, and attached on plate-like Bi 4 NbO 8 Cl, acting as electron-traps to achieve supressed recombination of photo-induced charges. The improved charge separation confers C-TiO 2 /Bi 4 NbO 8 Cl heterostructures superior photocatalytic performance with 53% higher than pristine Bi 4 NbO 8 Cl, towards rhodamine B removal with the help of photo-induced holes. Moreover, the C-TiO 2 /Bi 4 NbO 8 Cl heterostructures can be expanded to deal with other water contaminants, such as methyl orange, ciprofloxacin and 2,4-dichlorophenol with 44, 25 and 13% promotion, respectively, and thus the study offers a series of efficient photocatalysts for water purification.

Mid-infrared (mid-IR) optical spectroscopy of molecules is of large interest in physics, chemistry, and biology. However, probing nanometric volumes of molecules is challenging because of the strong mismatch of their mid-infrared absorption and scattering cross-sections with the free-space wavelength. We suggest overcoming this difficulty by nanofocusing acoustic graphene plasmon polaritons (AGPs) – oscillations of Dirac charge carriers coupled to electromagnetic fields with extremely small wavelengths – using a taper formed by a graphene sheet above a metallic surface. We demonstrate that due to the appreciable field enhancement and mode volume reduction, the nanofocused AGPs can efficiently sense molecular fingerprints in nanometric volumes. We illustrate a possible realistic sensing sсenario based on AGP interferometry performed with a near-field microscope. Our results can open new avenues for designing tiny sensors based on graphene and other 2D polaritonic materials.

Plasmonic nanocavities comprised of metal film-coupled nanoparticles have emerged as a versatile nanophotonic platform benefiting from their ultrasmall mode volume and large Purcell factors. In the weak-coupling regime, the particle-film gap thickness affects the photoluminescence (PL) of quantum emitters sandwiched therein. Here, we investigated the Purcell effect-enhanced PL of monolayer MoS 2 inserted in the gap of a gold nanoparticle (AuNP)–alumina (Al 2 O 3 )–gold film (Au Film) structure. Under confocal illumination by a 532 nm CW laser, we observed a 7-fold PL peak intensity enhancement for the cavity-sandwiched MoS 2 at an optimal Al 2 O 3 thickness of 5 nm, corresponding to a local PL enhancement of ∼350 by normalizing the actual illumination area to the cavity’s effective near-field enhancement area. Full-wave simulations reveal a counterintuitive fact that radiation enhancement comes from the non-central area of the cavity rather than the cavity center. By scanning an electric dipole across the nanocavity, we obtained an average radiation enhancement factor of about 65 for an Al 2 O 3 spacer thickness of 4 nm, agreeing well with the experimental thickness and indicating further PL enhancement optimization. Our results indicate the importance of configuration optimization, emitter location and excitation condition when using such plasmonic nanocavities to modulate the radiation properties of quantum emitters.