Broadband nonlinear optical response in multi-layer black phosphorus: an emerging infrared and mid-infrared optical material

Application of black phosphorus nanodots to live cell imaging

BACKGROUND

Black phosphorus (BP) has emerged as a novel class of nanomaterials owing to its unique optical and electronic properties. BP, a two-dimensional (2D) nanomaterial, is a structure where phosphorenes are stacked together in layers by van der Waals interactions. However, although BP nanodots have many advantages, their biosafety and biological effect have not yet been elucidated as compared to the other nanomaterials. Therefore, it is particularly important to assess the cytotoxicity of BP nanodots for exploring their potentials as novel biomaterials.

METHODS

BP nanodots were prepared by exfoliation with a modified ultrasonication-assisted solution method. The physicochemical properties of BP nanodots were characterized by transmission electron microscopy, dynamic light scattering, Raman spectroscopy, and X-ray diffractometry. In addition, the cytotoxicity of BP nanodots against C2C12 myoblasts was evaluated. Moreover, their cell imaging potential was investigated.

RESULTS

Herein, we concentrated on evaluating the cytotoxicity of BP nanodots and investigating their cell imaging potential. It was revealed that the BP nanodots were cytocompatible at a low concentration, although the cell viability was decreased with increasing BP nanodot concentration. Furthermore, our results demonstrated that the cells took up the BP nanodots, and the BP nanodots exhibited green fluorescence.

CONCLUSIONS

In conclusion, our findings suggest that the BP nanodots have suitable biocompatibility, and are promising candidates as fluorescence probes for biomedical imaging applications.

CVD-grown MoSe2 with high modulation depth for ultrafast mode-locked erbium-doped fiber laser

Two-dimensional materials have been widely used as optical modulator materials in mode-locked fiber lasers. In terms of the performance of the fiber laser, one with an ultrashort pulse and high stability has great commercial value. Herein, the MoSe₂ grown by the chemical vapor deposition (CVD) method with high modulation depth, quality lattice structure and uniformity is successfully applied in a mode-locked erbium-doped fiber laser. The pulse duration and signal-to-noise ratio of the laser are 207 fs and 85 dB, respectively. The multifarious performance comparisons indicate that the CVD-based MoSe₂ saturable absorber with the tapered fiber structure has unique advantages not only in the generation of ultrashort pulses, but also in the optimization of laser stability.

Few-layer Ti3C2Tx (T = O, OH, or F) saturable absorber for a femtosecond bulk laser

Few-layered titanium carbide (Ti₃C₂T_x), a novel two-dimensional (2D) Van der Waals material in the MXene family, was fabricated with a liquid-phase method and applied as a saturable absorber for a continuous-wave mode-locked femtosecond bulk laser. Pulses as short as 316 fs with a repetition rate of 64.06 MHz and maximum output power of 0.77 W were achieved at the central wavelength of 1053.2 nm, demonstrating the first known, to the best of our knowledge, application of MXene in an all-solid-state laser. Considering the flexible band gap for different surface functional groups of Ti₃C₂T_x, these results may promote the development of ultrafast photonics and further applications of 2D optoelectronic layered materials in the infrared and mid-infrared regions.

Reverse Saturable Absorption Induced by Phonon-Assisted Anti-Stokes Processes

In materials showing reverse saturable absorption (RSA), optical transmittance decreases at intense laser irradiation. One approach to application of these materials is to protect the sensors or human eyes from laser damage. To date, research has mainly concentrated on thin films and suspensions of graphite and its nanostructure (including nanotubes, graphene, and graphene oxides), which are mainly used as an optical limiter for nanosecond laser pulses. Moreover, thin individual pieces of semiconductor usually exhibit increased transmittance due to saturable absorption when the laser energy (E_laser ) is higher than the band gap (E_B ). Here, it is shown that indirect gap semiconductor WSe₂ exhibits high RSA on exposure to a femtosecond laser under E_laser > E_B near band gap excitation, which is attributed to the longitudinal optical phonon-assisted anti-Stokes transition by the annihilation of phonons and the absorption of photons. An optical limiting threshold (≈21.6 mJ cm^-2 ) lower than those reported for other optical-limiting materials currently for femtosecond laser at 800 nm is observed.

Size effect of WSe2 on red passively Q-switched fiber laser output performance

Transition metal dichalcogenides (TMDs) possess a direct bandgap in the visible frequency range and can be applied as attractive visible saturable absorbers (SAs). In this paper, a new TMDs tungsten selenide (WSe₂)-based red Q-switcher is fabricated and successfully used for Q-switched pulse generation in a red Pr³⁺-doped ZBLAN fiber laser. The passive Q-switching fiber laser at 635.2 nm generates a stable pulse with pulse duration of 504 ns and average output power of 4.91 mW as well as a tunable pulse repetition rate of 131.9-260.4 kHz. Furthermore, the size effect of WSe₂ on the 635 nm passive Q-switching output performance is also investigated. Our work will provide certain significance for the optimization of passive Q-switching red fiber lasers based on TMDs.

Prolonged photo-carriers generated in a massive-and-anisotropic Dirac material

Transient electron-hole pairs generated in semiconductors can exhibit unconventional excitonic condensation. Anisotropy in the carrier mass is considered as the key to elongate the life time of the pairs, and hence to stabilize the condensation. Here we employ time- and angle-resolved photoemission spectroscopy to explore the dynamics of photo-generated carriers in black phosphorus. The electronic structure above the Fermi level has been successfully observed, and a massive-and-anisotropic Dirac-type dispersions are confirmed; more importantly, we directly observe that the photo-carriers generated across the direct band gap have the life time exceeding 400 ps. Our finding confirms that black phosphorus is a suitable platform for excitonic condensations, and also open an avenue for future applications in broadband mid-infrared BP-based optoelectronic devices.

Acetone improves the topographical homogeneity of liquid phase exfoliated few-layer black phosphorus flakes

Liquid phase exfoliation of 2D materials has issues related to the sorption of the solvent, the oxidation of the sample during storage, and the topographical inhomogeneity of the exfoliated material. N-methyl-2-pyrrolidone (NMP), a common solvent for black phosphorus (BP) exfoliation, has additional drawbacks like the formation of by-products during sonication and poor solvent volatility. Here we demonstrate an improvement in the topographical homogeneity (i.e. thickness and lateral dimensions) of NMP-exfoliated BP flakes after resuspension in acetone. The typical size of monolayers and bilayers stabilised in acetone was 99.8 ± 27.4 nm and 159.1 ± 57 nm, respectively. These standard deviations represent a threefold improvement over those of the NMP-exfoliated originals. Phosphorene can also be exfoliated directly in acetone by very long ultrasonication. The product suspension enjoys the same dimensional homogeneity benefits, which confirms that this effect is an intrinsic property of the acetone-BP system. The quality and stability of the exfoliated flakes was checked by XRD, TEM, electron diffraction and Raman spectroscopy. Thermal expansion coefficients of the [Formula: see text] B_2g and [Formula: see text] Raman modes were calculated for drop-casted samples as -0.018 28 cm^-1 K^-1, -0.030 56 cm^-1 K^-1 and -0.032 19 cm^-1 K^-1, respectively. The flakes withstand 20 min in O₂ flow at 373 K without lattice distortion.

Low-Cost Black Phosphorus Nanofillers for Improved Thermoelectric Performance in PEDOT:PSS Composite Films

In recent years, two-dimensional black phosphorus (BP) has seen a surge of research because of its unique optical, electronic, and chemical properties. BP has also received interest as a potential thermoelectric material because of its high Seebeck coefficient and excellent charge mobility, but further development is limited by the high cost and poor scalability of traditional BP synthesis techniques. In this work, high-quality BP is synthesized using a low-cost method and utilized in a PEDOT:PSS film to create the first ever BP composite thermoelectric material. The thermoelectric properties are found to be greatly enhanced after the BP addition, with the power factor of the film, with 2 wt % BP (36.2 μW m^-1 K^-2) representing a 109% improvement over the pure PEDOT:PSS film (17.3 μW m^-1 K^-2). A simultaneous increase of mobility and decrease of the carrier concentration is found to occur with the increasing BP wt %, which allows for both Seebeck coefficient and electrical conductivity to be increased. These results show the potential of this low-cost BP for use in energy devices.

Synthesis of Black Phosphorus Quantum Dots with High Quantum Yield by Pulsed Laser Ablation for Cell Bioimaging

Black phosphorus quantum dots (BPQDs), with an average diameter of about 6 nm and a height of about 1.1 nm, are successfully synthesized by means of a pulsed laser ablation (PLA) method in isopropyl ether (IPE) solvent. The photoluminescence PL quantum yield of the as-prepared sample is as high as 20.7 %, which is 3 times that of BPQDs prepared by means of probe ultrasonic exfoliation (approximately 7.2 %). The stable and blue-violet PL emission of the BPQDs is observed. It can be elucidated that electrons transit from the LUMO energy level to the HOMO energy level, as well as energy levels below the HOMO (H1 and H2). In addition, BPQDs are also utilized in bioimaging in HeLa cells, showing an intense and stable PL signal and excellent biocompatibility. Hence, this work indicates that the obtained BPQDs with high quantum yield and stable PL emission have great potential for biomedical applications, including biolabeling, bioimaging, and drug delivery.

Passive mode locking resulting from weak polarization dependence based on evanescent field interaction with a monolayer graphene absorber

Weak polarization dependence based on evanescent field interaction with a monolayer graphene absorber is demonstrated. By covering single-layer graphene on the top of a fiber taper, the symmetry of the light field propagation was destroyed. Polarization-dependent loss (PDL) of 2 dB was obtained, and no saturable absorption was observed. Passive mode locking was achieved by using the graphene-covered fiber taper in a fiber laser. It is found that the weak PDL down to 2 dB is capable for achieving mode locking in a fiber laser. Therefore, the claim of saturable absorption of any graphene or two-dimensional materials in a fiber laser needs to be made with care.

Stable passively Q-switched erbium-doped fiber laser incorporating a PbS quantum dots polystyrene composite film based saturable absorber

We demonstrated a passively Q-switched erbium-doped fiber laser (EDFL) using PbS quantum dots in polystyrene films (QDPFs) as saturable absorbers (SAs). Compared to other SAs, PbS QDPFs have advantages of broad absorption range, high quantum yield, low cost, and facile preparation. We have successfully generated stable Q-switched pulses with an average output power of 40.19 mW, a single pulse energy of 586.1 nJ, a repetition rate of 68.04 kHz, a pulse width of 3.9 μs, and a signal-to-noise ratio of 50.5 dB under 660 mW pump power. The output of the EDFL has been monitored for 5 consecutive hours under laboratory conditions to show stable operation of the laser system.

First-Principles Investigation of Black Phosphorus Synthesis

Black phosphorus (BP) is a layered semiconductor with outstanding properties, making it a promising candidate for optoelectronic and other applications. BP synthesis is an intriguing task largely due to the insufficient understanding of the synthesis mechanism. In this work, we use density functional theory calculations to examine BP and its precursor red phosphorus as they are formed from P₄ building blocks. Our results suggest that, without external effects such as pressure or addition of a catalyst, the precursor is energetically favored in the initial steps of the synthesis, even though BP is the more stable allotrope. The higher energy of BP is dictated by its 2D geometry that gives rise to the higher number of high-energy strained bonds at the edge compared to the 1D geometry of red phosphorus. The elucidated BP formation pathway provides a natural explanation for the effectiveness of the recently discovered Sn/I catalyst used in BP synthesis.

Black phosphorus Q-switched and mode-locked mid-infrared Er:ZBLAN fiber laser at 3.5 μm wavelength

With the proposal of dual-wavelength pumping (DWP) scheme, DWP Er:ZBLAN fiber lasers at 3.5 μm have become a fascinating area of research. However, limited by the absence of suitable saturable absorber, passively Q-switched and mode-locked fiber lasers have not been realized in this spectral region. Based on the layer-dependent bandgap and excellent photoelectric characteristics of black phosphorus (BP), BP is a promising candidate for saturable absorber near 3.5 μm. Here, we fabricated a 3.5-μm saturable absorber mirror (SAM) by transferring BP flakes onto a Au-coated mirror. With the as-prepared BP SAM, we realized Q-switching and mode-locking operations in the DWP Er:ZBLAN fiber lasers at 3.5 μm. To the best of our knowledge, it is the first time to achieve passively Q-switched and mode-locked pulses in 3.5 μm spectral region. The research results will not only promote the development of 3.5-μm pulsed fiber lasers but also open the photonics application of two-dimensional materials in this spectral region.

Three-dimensional Dirac semimetal thin-film absorber for broadband pulse generation in the near-infrared

In this Letter, the transient nonlinear absorption of three-dimensional (3D) topological Dirac semimetal Cd₃As₂ thin film was characterized in the near-infrared band. By performing broadband pump-probe measurements, we experimentally proved that molecular beam epitaxy (MBE) grown Cd₃As₂ exhibits strong and tunable saturable absorption effects across 1-2 μm. By further inserting the Cd₃As₂ film into the cavities of Tm- and Er-doped fiber lasers, we obtained stable mode-locked operations at 1.96 and 1.56 μm. Our results experimentally establish that Cd₃As₂ is a promising broadband saturable absorber (SA) for pulsed lasers in the infrared range.

Ultra-broadband nonlinear optical response of two-dimensional h-BN nanosheets and their hybrid gel glasses

Hydrophilic hydroxylated hexagonal boron nitride nanosheets (BNNSs), also called 'white graphene', exhibit high water solubility, compatibility, dispersity with physical and/or chemical effects on the gel matrix, and unexpected but outstanding near-infrared adsorption and nonlinear optical limiting properties. These properties mean that hydroxylated BNNSs are suitable for practical applications in solidification and devices. Hydroxylated BNNSs are doped into an organically modified silicate matrix through a facile sol-gel process. The resulting BNNS-organically modified silicate glass hybrids demonstrate various solid structures (glass, coating, and film), good uniformity, thermostability, toughness, and high linear transmittance in the visible and near-infrared regions. Hybrid glasses showed large optical limiting effects and high doping concentrations without the addition of a compatibiliser, and their limiting thresholds and concentration exceed those of the corresponding suspensions by factors of 1.3-2 and 20-130, respectively. The optical limiting performance of BNNSs is almost equal to or may exceed those of graphene oxide and graphene in solutions and doped glasses. The combined mechanisms of nonlinear absorption, refraction, and scattering are deduced. A solid organically modified silicate hybrid transparent system doped with BNNSs is a promising candidate material for optical limiters and nonlinear optical devices. This system could be an efficient solid-state optical limiter in the visible and long-wavelength near-infrared regions (532-2000 nm).

Er-doped Q-switched fiber laser with a black phosphorus/polymethyl methacrylate saturable absorber

A stable Q-switched Er-doped fiber (EDF) laser is successfully obtained by using a black phosphorus (BP)/polymethyl methacrylate (PMMA) film as the saturable absorber (SA). To avoid the oxidization of nanomaterials, the BP nanoparticles are fabricated via a liquid-phase exfoliation method and then embedded into a PMMA film that possesses excellent optical transparency in the selected spectrum range. The modulation depth (MD) of the BP/PMMA film SA is 14.3% and the saturable intensity (I_sat) is 6.9  MW/cm². By inserting the BP/PMMA film into the EDF laser cavity, we achieve the stable passive Q-switching operation over the wavelength range from 1561.21 nm to 1564.16 nm. The repetition rate increases from 10.348 kHz to 30.098 kHz, and the pulse duration decreases from 25.01 μs to 2.98 μs by altering the pump power from 9 mW to 90 mW. The maximum single pulse energy is 283.91 nJ. To the best of our knowledge, 283.91 nJ is the largest single pulse energy among the Q-switched fiber lasers with BP as the Q-switcher at the 1.5 μm wavelength region. The experimental results evidently show that the BP/PMMA film SA can work as a promising Q-switcher for large pulse energy fiber lasers.

Recent progress in 2D group-VA semiconductors: from theory to experiment

This review provides recent theoretical and experimental progress in the fundamental properties, electronic modulations, fabrications and applications of 2D group-VA materials.

Metal-assisted exfoliation of few-layer black phosphorus with high yield

We introduce a metal-assisted exfoliation method to produce few-layer black phosphorus with the lateral size larger than 50 μm and the area 100 times larger than those exfoliated using the normal “scotch-tape” technique.

Optical nonlinearity engineering of a bismuth telluride saturable absorber and application of a pulsed solid state laser therein

Saturable absorbers (SAs) have interesting applications for the realization of pulsed lasers in various wavelengths of fiber and solid-state lasers. Topological insulators (TIs) have been recently discovered to feature saturable absorption due to their unique band structure. In this study, high-purity layers of Bi₂Te₃ thin film SA have been successfully prepared using the spin coating-coreduction approach (SCCA). Compared with the typical method of preparing SAs, the SCCA can be used to prepare topological insulator saturable absorbers (TISAs) with high optical quality, large area consistency, and controllable thickness, which is critical for pulsed lasers. To the best of our knowledge, this study is the first observation and discussion of clear thickness-dependent optical nonlinearity. In this study, a Q-switched bulk Nd:YAG laser is demonstrated and investigated using the prepared TISA as the absorber. The timing jitter and amplitude fluctuation of the stable pulse laser indicated that the SCCA is suitable for fabricating a Bi₂Te₃ SA. Furthermore, the SCCA enables the establishment of a pulsing laser through saturation intensity engineering.

High-peak-power mode-locking pulse generation in a dual-loss-modulated laser with BP-SA and EOM

In this Letter, a high-quality black phosphorus saturable absorber (BP-SA) with a multilayer structure was prepared, and its corresponding characteristics were investigated. Based on the BP-SA and an electro-optic modulator (EOM), a dual-loss-modulated mode-locked laser with a controllable repetition rate and high peak power was realized, which has the repetition rate of EOM and the mode-locking pulse width in the Q-switching and mode-locking laser. The output performances versus the pump power were measured. The maximum pulse peak power of 3.89 MW was obtained with the minimum pulse duration of 119 ps. To the best of our knowledge, this is the highest pulse peak power with BP-SA reported.

Passivation of black phosphorus saturable absorbers for reliable pulse formation of fiber lasers

Black phosphorus (BP) has attracted increasing attention due to its unique electrical properties. In addition, the outstanding optical nonlinearity of BP has been demonstrated in various ways. Its functionality as a saturable absorber, in particular, has been validated in demonstrations of passive mode-locked lasers. However, normally, the performance of BP is degraded eventually by both thermal and chemical damage in ambient conditions. The passivation of BP is the critical issue to guarantee a stable performance of the optical devices. We quantitatively characterized the mode-locked lasers operated by BP saturable absorbers with diversified passivation materials such as polydimethylsiloxane (PDMS) or Al₂O₃, considering the atomic structure of the materials, and therefore the hydro-permeability of the passivation layers. Unlike the BP layers without passivation, we demonstrated that the Al₂O₃-passivated BP layer was protected from the surface oxidation reaction in the long-term, and the PDMS-passivated one had a short-term blocking effect. The quantitative analysis showed that the time-dependent characteristics of the pulsed laser without passivation were changed with respect to the pulse duration, spectral width, and time-bandwidth product displaying 550 fs, 2.8 nm, and 0.406, respectively. With passivation, the changes were limited to <43 fs, <0.3 nm, and <0.012, respectively.

Metal-Ion-Modified Black Phosphorus with Enhanced Stability and Transistor Performance

Black phosphorus (BP), a burgeoning elemental 2D semiconductor, has aroused increasing scientific and technological interest, especially as a channel material in field-effect transistors (FETs). However, the intrinsic instability of BP causes practical concern and the transistor performance must also be improved. Here, the use of metal-ion modification to enhance both the stability and transistor performance of BP sheets is described. Ag⁺ spontaneously adsorbed on the BP surface via cation-π interactions passivates the lone-pair electrons of P thereby rendering BP more stable in air. Consequently, the Ag⁺ -modified BP FET shows greatly enhanced hole mobility from 796 to 1666 cm² V^-1 s^-1 and ON/OFF ratio from 5.9 × 10⁴ to 2.6 × 10⁶ . The mechanisms pertaining to the enhanced stability and transistor performance are discussed and the strategy can be extended to other metal ions such as Fe³⁺ , Mg²⁺ , and Hg²⁺ . Such stable and high-performance BP transistors are crucial to electronic and optoelectronic devices. The stability and semiconducting properties of BP sheets can be enhanced tremendously by this novel strategy.

Dual-band and polarization-independent infrared absorber based on two-dimensional black phosphorus metamaterials

Two-dimensional (2D) black phosphorus (BP) with direct band gap, bridges the characteristics of graphene with a zero or near-zero band gap and transition metal dichalcogenides with a wide band gap. In the infrared (IR) regime, 2D BP materials can attenuate electromagnetic energy due to losses derived from its surface conductivity. This paper proposes an IR absorber based on 2D BP metamaterials. It consists of multi-layer BP-based nano-ribbon pairs, each formed by two orthogonally stacked nano-ribbons. The multi-layer BP metamaterials and bottom gold mirror together form a Fabry-Perot resonator that could completely inhibit light transmission to create strong absorption through the BP metamaterials. Unlike previously reported BP metamaterial absorbers, this new structure can operate at two frequency bands with absorption > 90% in each owning to the first-order and second-order Fabry-Perot resonant frequencies. It is also polarization independent due to the fourfold rotational structural symmetry. To our best knowledge, this is the first report on using BP metamaterials in an absorber that operates independent of polarization and in dual bands.

Observation of Third-order Nonlinearities in Graphene Oxide Film at Telecommunication Wavelengths

All-optical switches have been considered as a promising solution to overcome the fundamental speed limit of the current electronic switches. However, the lack of a suitable third-order nonlinear material greatly hinders the development of this technology. Here we report the observation of ultrahigh third-order nonlinearity about 0.45 cm²/GW in graphene oxide thin films at the telecommunication wavelength region, which is four orders of magnitude higher than that of single crystalline silicon. Besides, graphene oxide is water soluble and thus easy to process due to the existence of oxygen containing groups. These unique properties can potentially significantly advance the performance of all-optical switches.

Passively mode-locked 1.34 μm bulk laser based on few-layer black phosphorus saturable absorber

By using few-layer black phosphorus (BP) as saturable absorber, an efficient mode-locked Nd:GdVO₄ bulk laser operating at 1.34 μm was realized. An average output power of 350 mW was achieved with a slope efficiency of 15%. The corresponding mode-locking pulse repetition rate, pulse duration and pulse energy were 58.14 MHz, 9.24 ps and 3.0 nJ, respectively. To the best of our knowledge, the pulse width is the shortest among the mode-locked 1.34 μm neodymium lasers ever obtained with other two-dimensional materials saturable absorber. The results clearly indicate the few-layered BP is a kind of promising saturable absorber for ultrafast 1.34 μm lasers.

Two-Dimensional Transition Metal Dichalcogenides and Their Charge Carrier Mobilities in Field-Effect Transistors

Two-dimensional (2D) materials have attracted extensive interest due to their excellent electrical, thermal, mechanical, and optical properties. Graphene has been one of the most explored 2D materials. However, its zero band gap has limited its applications in electronic devices. Transition metal dichalcogenide (TMDC), another kind of 2D material, has a nonzero direct band gap (same charge carrier momentum in valence and conduction band) at monolayer state, promising for the efficient switching devices (e.g., field-effect transistors). This review mainly focuses on the recent advances in charge carrier mobility and the challenges to achieve high mobility in the electronic devices based on 2D-TMDC materials and also includes an introduction of 2D materials along with the synthesis techniques. Finally, this review describes the possible methodology and future prospective to enhance the charge carrier mobility for electronic devices.

Black phosphorus flakes covered microfiber for Q-switched ytterbium-doped fiber laser

We demonstrate a passively Q-switched ytterbium-doped fiber laser based on black phosphorus (BP) flakes covered microfiber. The BP saturable absorber is fabricated by sandwiching a microfiber between two pieces of polydimethylsiloxane supported BP flakes film, which is prepared by the mechanical exfoliation method. In this case the BP flakes can be well protected from the action of air and moisture. By incorporating BP flakes covered microfiber into a ytterbium-doped ring fiber laser, stable and reliable Q-switched operation at 1064 nm can be realized via interaction between few-layers BP flakes and the evanescent field of the laser. The laser allows Q-switched pulse generation with a repetition rate in the range of 26-76 kHz and a pulse duration in the range of 5.5-2.0 μs, by varying the pump power from 38 mW to 100 mW.

Fundamental and harmonic mode-locking at 2.1 μm with black phosphorus saturable absorber

We report mode-locking in holmium-doped all-fiber laser based on black phosphorus saturable absorber. The generated solitons are centered at 2094 nm with bandwidth reaching 4.2 nm and pulse duration of 1.3 ps. In harmonic mode-locking, up to 10th harmonic (290 MHz) was obtained. Properties of black phosphorus saturable absorber are investigated. Our findings validate black phosphorus suitability for ultrafast applications in mid-infrared.

All-optical tunable microfiber knot resonator with graphene-assisted sandwich structure

We demonstrate an all-optical tunable microfiber knot resonator (MFKR) by direct light-graphene interaction using external vertical incidence pump laser. The 1530 nm CW pump source is employed to irradiate the sample, which can achieve the performance modulation of MFKR including transmission loss, extinction ratio, and resonant wavelength by the saturable absorption, photo-thermal, and optical Kerr effects, respectively. Compared with the MFKR with only the bottom graphene film, the tunable ranges of transmission loss and extinction ratio are increased by 69 and 125 times, respectively, which can induce a remarkable amplitude tuning. The resonant wavelength of MFKR occurs a red-shift under the irradiation of the pump light, and the red-shift range can exceed one free spectral range (FSR), which means the resonant wavelength could be tuned in the full wavelength range of the transparent window of optical fiber. It is promising for the device to be applied as an all-optical modulator, tunable optical filter, etc.

All-optical phase shifter and switch near 1550nm using tungsten disulfide (WS2) deposited tapered fiber

All-optical phase shifters and switches play an important role for various all-optical applications including all-optical signal processing, sensing and communication. In this paper, we demonstrate a fiber all-optical phase shifter using few-layer 2D material tungsten disulfide (WS₂) deposited on a tapered fiber. WS₂ absorbs injected 980 nm pump (control light) and generates heat, which changes the refractive index of both WS₂ and tapered fiber due to thermo-optic effect and achieves a maximum phase shift of 6.1π near 1550 nm. The device has a loss of 3.7 dB. By constructing a Mach-Zehnder interferometer with WS₂ based phase shifter in one arm, an all-optical switch is also obtained with an extinction ratio of 15 dB and a rise time of 7.3 ms. This all fiber low-cost and compact optical phase shifter and switch demonstrates the potential of 2D transition metal dichalcogenides for all-optical signal processing devices.

Ultrafast nonlinear optical response in solution dispersions of black phosphorus

We report the spatial self-phase modulation (SSPM) effect for solution dispersions of black phosphorus (BP). The experimental results suggest that this concentration-dependent coherent light diffraction is due to the ultrafast and large third-order optical nonlinearity of BP. The third-order nonlinear susceptibility of BP has been simply obtained about 10^-19 m²/V² by analyzing the experimental results. The fast relaxation time during dynamic relaxation is obtained as 0.13 ps. Our experimental results imply novel potential application of BP in ultrafast nonlinear phase modulation devices based on their nonlinear optical response.

Emerging Trends in Phosphorene Fabrication towards Next Generation Devices

The challenge of science and technology is to design and make materials that will dominate the future of our society. In this context, black phosphorus has emerged as a new, intriguing two-dimensional (2D) material, together with its monolayer, which is referred to as phosphorene. The exploration of this new 2D material demands various fabrication methods to achieve potential applications- this demand motivated this review. This article is aimed at supplementing the concrete understanding of existing phosphorene fabrication techniques, which forms the foundation for a variety of applications. Here, the major issue of the degradation encountered in realizing devices based on few-layered black phosphorus and phosphorene is reviewed. The prospects of phosphorene in future research are also described by discussing its significance and explaining ways to advance state-of-art of phosphorene-based devices. In addition, a detailed presentation on the demand for future studies to promote well-systemized fabrication methods towards large-area, high-yield and perfectly protected phosphorene for the development of reliable devices in optoelectronic applications and other areas is offered.

Few-Layer Black Phosphorus Carbide Field-Effect Transistor via Carbon Doping

Black phosphorus carbide (b-PC) is a new family of layered semiconducting material that has recently been predicted to have the lightest electrons and holes among all known 2D semiconductors, yielding a p-type mobility (≈10⁵ cm² V^-1 s^-1 ) at room temperature that is approximately five times larger than the maximum value in black phosphorus. Here, a high-performance composite few-layer b-PC field-effect transistor fabricated via a novel carbon doping technique which achieved a high hole mobility of 1995 cm² V^-1 s^-1 at room temperature is reported. The absorption spectrum of this material covers an electromagnetic spectrum in the infrared regime not served by black phosphorus and is useful for range finding applications as the earth atmosphere has good transparency in this spectral range. Additionally, a low contact resistance of 289 Ω µm is achieved using a nickel phosphide alloy contact with an edge contacted interface via sputtering and thermal treatment.

Rapid and Large-Area Characterization of Exfoliated Black Phosphorus Using Third-Harmonic Generation Microscopy

Black phosphorus (BP) is a layered semiconductor that recently has been the subject of intense research due to its novel electrical and optical properties, which compare favorably to those of graphene and the transition metal dichalcogenides. In particular, BP has a direct bandgap that is thickness-dependent and highly anisotropic, making BP an interesting material for nanoscale optical and optoelectronic applications. Here, we present a study of the anisotropic third-harmonic generation (THG) in exfoliated BP using a fast scanning multiphoton characterization method. We find that the anisotropic THG arises directly from the crystal structure of BP. We calculate the effective third-order susceptibility of BP to be ∼1.64 × 10^-19 m² V^-2. Further, we demonstrate that multiphoton microscopy can be used for rapid, large-area characterization indexing of the crystallographic orientations of many exfoliated BP flakes from one set of multiphoton images. This method is therefore beneficial for samples of areas ∼1 cm² in future investigations of the properties and growth of BP.

Ultraviolet saturable absorption and ultrafast carrier dynamics in ultrasmall black phosphorus quantum dots

Understanding the photoexcited carrier-relaxation actions in ultrasmall black phosphorus quantum dots (BPQDs) will play a crucial role in the fields of electronics and optoelectronics. Herein, we report the ultraviolet (UV) saturable absorption and ultrafast photoexcited carrier-relaxation dynamics of BPQDs. The ultrasmall BPQDs are synthesized using a facile liquid-exfoliation method and possess a diameter of 3.8 ± 0.6 nm and a thickness of 1.5 ± 0.4 nm. Femtosecond open-aperture (OA) Z-scan measurements showed typical saturable absorption properties in the UV band. A negative nonlinear optical (NLO) absorption coefficient of -(1.4 ± 0.3) × 10^-3 cm GW^-1 and a saturable intensity of 6.6 ± 1.3 GW cm^-2 were determined. Using a degenerate pump-probe technique, an ultrafast photoexcited carrier-recombination time was observed in the range of 216-305 fs, which was 3 orders of magnitude faster than that of BP nanosheets. Such an ultrafast relaxation component may be attributable to the edge- and step-mediated recombination and was confirmed by our density functional theory (DFT) calculations. This work provides fundamental insight into the underlying mechanism of the photoexcited carrier relaxation dynamic action in BPQDs which can enable UV photonic devices.

Broadband ultrafast nonlinear absorption and ultra-long exciton relaxation time of black phosphorus quantum dots

Black phosphorus (BP) has recently attracted significant attention for its brilliant physical and chemical features. The remarkable strong light-matter interaction and tunable direct wide range band-gap make it an ideal candidate in various application regions, especially saturable absorbers. In this paper, ultrasmall black phosphorus quantum dots (BPQDs), a unique form of phosphorus nanostructures, with average size of 5.7 ± 0.8 nm are synthesized. Compared with BP nanosheets (BPNs) with similar thickness, the ultrafast nonlinear optical (NLO) absorption properties and excited carrier dynamics are investigated in wide spectra. Beyond the saturation absorption (SA), giant two photon absorption (TPA) is observed in BPQDs. BPQDs exhibit quite different excitation intensity and wavelength dependent nonlinear optical (NLO) response from BPNs, which is attributed to the quantum confinement and edge effects. The BPQDs show broadband photon-induced absorption (PIA) under the probe wavelength from 470 nm to 850 nm and a fast and a slow decay time are obtained as long as 92 ± 10 ps and 1100 ± 100 ps, respectively. The substantial independence for ultra-long time scales of pump intensity and temperature reveals that the carrier recombination mechanism may be attributed to a defect-assisted Auger capture process. These findings will help to develop optoelectronic and photonic devices operating in the infrared and visible wavelength region.

Emerging Low-Dimensional Materials for Nonlinear Optics and Ultrafast Photonics

Low-dimensional (LD) materials demonstrate intriguing optical properties, which lead to applications in diverse fields, such as photonics, biomedicine and energy. Due to modulation of electronic structure by the reduced structural dimensionality, LD versions of metal, semiconductor and topological insulators (TIs) at the same time bear distinct nonlinear optical (NLO) properties as compared with their bulk counterparts. Their interaction with short pulse laser excitation exhibits a strong nonlinear character manifested by NLO absorption, giving rise to optical limiting or saturated absorption associated with excited state absorption and Pauli blocking in different materials. In particular, the saturable absorption of these emerging LD materials including two-dimensional semiconductors as well as colloidal TI nanoparticles has recently been utilized for Q-switching and mode-locking ultra-short pulse generation across the visible, near infrared and middle infrared wavelength regions. Beside the large operation bandwidth, these ultrafast photonics applications are especially benefit from the high recovery rate as well as the facile processibility of these LD materials. The prominent NLO response of these LD materials have also provided new avenues for the development of novel NLO and photonics devices for all-optical control as well as optical circuits beyond ultrafast lasers.

Few-layered MoS2 as a saturable absorber for a passively Q-switched Er:YAG laser at 1.6 μm

The passively Q-switched Er:YAG laser at 1.6 μm was achieved with a YAG-based MoS₂ saturable absorber (SA) for the first time. The saturable absorption properties of the MoS₂ SA near 1.6 μm were investigated. Under an absorbed pump power of 8.09 W, an average output power of 1.08 W with a pulse duration of 1.138 μs and a repetition rate of 46.6 kHz was obtained, corresponding to an optical conversion efficiency of 40.67%. The pulse energy and peak power were calculated to be 23.08 μJ and 20.28 W, respectively.

Carbon Nanotube Mode-Locked Thulium Fiber Laser With 200 nm Tuning Range

We demonstrated a mode-locked thulium/holmium (Tm/Ho) fiber laser continuously tunable across 200 nm (from 1860 nm to 2060 nm), which to the best of our knowledge represents the widest tuning range ever achieved for a passively mode-locked fiber laser oscillator. The combined use of a broadband carbon nanotube (CNT) saturable absorber and a diffraction grating mirror ensures ultra-broad tuning range, superb stability and repeatability, and makes the demonstrated laser a highly practical source for spectroscopy, imaging and optical communications. The laser emits <5 ps pulses with an optical spectral bandwidth of ∼3 nm across the full tuning range. Our results indicate that carbon nanotubes can be an excellent saturable absorber for achieving gain-bandwidth-limited tunable operation for 2 μm thulium fiber lasers.

Raman Sensitive Degradation and Etching Dynamics of Exfoliated Black Phosphorus

Layered black phosphorus has drawn much attention due to the existence of a band gap compared to the widely known graphene. However, environmental stability of black phosphorus is still a major issue, which hinders the realization of practical device applications. Here, we spatially Raman map exfoliated black phosphorus using confocal fast-scanning technique at different time intervals. We observe a Raman intensity modulation for , B_2g, and modes. This Raman modulation is found to be caused by optical interference, which gives insights into the oxidation mechanism. Finally, we examine the fabrication compatible PMMA coating as a viable passivation layer. Our measurements indicate that PMMA passivated black phosphorus thin film flakes can stay pristine for a period of 19 days when left in a dark environment, allowing sufficient time for further nanofabrication processing. Our results shed light on black phosphorus degradation which can aid future passivation methods.

High-repetition-rate all-fiber femtosecond laser with an optical integrated component

We demonstrate a high-repetition-rate all-fiber soliton pulse laser mode-locked by the nonlinear polarization rotation technique. The laser cavity is effectively shortened by incorporating an optical integrated component possessing the hybrid functions of a polarization-dependent isolator, a wavelength-division multiplexer, and an output coupler. Resultant output soliton pulses have a fundamental repetition rate of 384 MHz, a 3-dB spectral bandwidth of 25.2 nm, and a dechirped pulse duration of 115 fs. By using an external power amplification and pulse recompression system, the average output power of the laser is boosted to 207 mW. The amplified pulses have a 2.33-ps duration, which is recompressed to 340 fs. Numerical simulations reproduce the generation of high-repetition-rate soliton pulses in the fiber laser. Such a simple and low-cost high-repetition-rate fiber laser is a potential laser source for various practical applications.

Tin diselenide as a new saturable absorber for generation of laser pulses at 1μm

The nonlinear optical responses of two-dimensional (2D) few-layer tin diselenide (SnSe₂) have been investigated in this work. By applying the Z-scan technique, the saturable absorption of SnSe₂ thin film has been observed at a wavelength of ~1μm, which enables SnSe_2, a new saturable absorber for pulsed laser generation. A detailed exploration of the nonlinear absorption of SnSe₂ film and their dependence on excitation pulse energy is carried out. Our results show that saturation intensity of the nonlinear absorption is ~23.78 GW/cm² and corresponding nonlinear absorption coefficients are at a range from -13596 to -2967.3 cm/GW. By employing the SnSe₂-coated mirror as a saturable absorber, the passively Q-switched lasing has been achieved on a crystalline waveguide platform at ~1 μm.

Infrared absorber based on sandwiched two-dimensional black phosphorus metamaterials

As a direct band gap two-dimensional (2D) semiconductor material, black phosphorus (BP) bridges the characteristics of graphene, with a zero or near-zero band gap, and transition metal dichalcogenides, with a wide band gap. In the infrared (IR) regime, 2D BP materials can harvest electromagnetic energy due to losses derived from its surface conductivity. In this paper, we propose an IR absorber design comprising 2D BP metamaterials sandwiched between dielectric layers. The multilayered sandwich-like absorber structure is mounted on a full reflective gold mirror, which forms a Fabry-Perot resonator to strengthen light-matter interactions. Harvested surface plasmons are excited around the 2D BP metamaterial edges, and the incident IR light can be efficiently dissipated by increasing the number of layers of the sandwich-like structure (NLSS). The physical absorption mechanism can be attributed to the destructive interference from the metamaterials, which can be enhanced with increasing NLSS. Here, a phase difference of about 180° is obtained between the directly reflected wave from the first interface and the emergent wave derived from the superposition of the multiple reflections among the resonator, and the amplitude of the emergent wave is steadily reduced to a value close to that of the directly reflected wave with increasing NLSS for incident transverse-magnetic polarized IR illumination.

Nonlinear Black Phosphorus for Ultrafast Optical Switching

The outstanding electronic and optical properties of black phosphorus (BP) in a two-dimensional (2D) but unique single-layer puckered structure have opened intense research interest ranging from fundamental physics to nanoscale applications covering the electronic and optical domains. The direct and controllable electronic bandgap facilitating wide range of tunable optical response coupled with high anisotropic in-plane properties made BP a promising nonlinear optical material for broadband optical applications. Here, we investigate ultrafast optical switching relying on the optical nonlinearity of BP. Wavelength conversion for modulated signals whose frequency reaches up to 20 GHz is realized by four-wave-mixing (FWM) with BP-deposited D-shaped fiber. In the successful demonstration of the FWM based wavelength conversion, performance parameter has been increased up to ~33% after employing BP in the device. It verifies that BP is able to perform efficient optical switching in the evanescent field interaction regime at very high speed. Our results might suggest that BP-based ultra-fast photonics devices could be potentially developed for broadband applications.

Electric- and magnetic-field dependence of the electronic and optical properties of phosphorene quantum dots

Recently, black phosphorus quantum dots were fabricated experimentally. Motivated by these experiments, we theoretically investigate the electronic and optical properties of rectangular phosphorene quantum dots (RPQDs) in the presence of an in-plane electric field and a perpendicular magnetic field. The energy spectra and wave functions of RPQDs are obtained numerically using the tight-binding approach. We find edge states within the band gap of the RPQD which are well separated from the bulk states. In an undoped RPQD and for in-plane polarized light, due to the presence of well-defined edge states, we find three types of optical transitions which are between the bulk states, between the edge and bulk states, and between the edge states. The electric and magnetic fields influence the bulk-to-bulk, edge-to-bulk, and edge-to-edge transitions differently due to the different responses of bulk and edge states to these fields.

Tunable Broadband Nonlinear Optical Properties of Black Phosphorus Quantum Dots for Femtosecond Laser Pulses

Broadband nonlinear optical properties from 500 to 1550 nm of ultrasmall black phosphorus quantum dots (BPQDs) have been extensively investigated by using the open-aperture Z-scan technique. Our results show that BPQDs exhibit significant nonlinear absorption in the visible range, but saturable absorption in the near-infrared range under femtosecond excitation. The calculated nonlinear absorption coefficients were found to be (7.49 ± 0.23) × 10⁻³, (1.68 ± 0.078) × 10⁻³ and (0.81 ± 0.03) × 10⁻³ cm/GW for 500, 700 and 900 nm, respectively. Femtosecond pump-probe measurements performed on BPQDs revealed that two-photon absorption is responsible for the observed nonlinear absorption. The saturable absorption behaviors observed at 1050, 1350 and 1550 nm are due to ground-state bleaching induced by photo-excitation. Our results suggest that BPQDs have great potential in applications as broadband optical limiters in the visible range or saturable absorbers in the near-infrared range for ultrafast laser pulses. These ultrasmall BPQDs are potentially useful as broadband optical elements in ultrafast photonics devices.

High performance of a passively Q-switched mid-infrared laser with Bi2Te3/graphene composite SA

We report passively Q-switched ∼2 and ∼3  μm mid-infrared (MIR) solid-state lasers with a self-assembly solvothermal-synthesized Bi₂Te₃/graphene heterostructure saturable absorber (SA) for the first time. Based on the oxidation resistance and high thermal conductivity of graphene, and large modulation depth of Bi₂Te₃ nanosheets, two high-performance Q-switching lasers were realized. One is a Tm:YAP laser with a maximum average output power of 2.34 W and a pulse width of 238 ns at ∼2  μm. The corresponding maximum pulse peak power was 91 W, which was much improved in comparison with the pure graphene-based Tm laser. The other one is an Er:YSGG laser producing a pulse width of 243 ns, which is the shortest among the 2D SAs-based ∼3  μm solid-state lasers, as far as we know. Our results indicate that such a composite Bi₂Te₃/graphene material is a promising SA for generating high-performance mid-infrared pulse lasers.

Diode-pumped mode-locked Tm:LuAG laser at 2 μm based on GaSb-SESAM

Mode-locking of a directly diode-pumped Tm:LuAG laser is demonstrated using GaSb-based semiconductor saturable absorber mirrors (SESAMs). Stable and self-starting mode-locked operation was realized, generating pulses as short as 13.6 ps at 2024 nm with a maximum output power of 98 mW. Two GaInAs-based SESAMs were used for comparison with the operation based upon the use of the GaSb SESAM; in this case, longer pulses with durations of 27 ps and 34 ps were obtained under the same experimental conditions. Our work sets a new record in pulse duration for mode-locked Tm:LuAG lasers and confirms that lattice-matched GaSb-based SESAMs are beneficial for mode-locked solid-state lasers in the 2 μm range.

Broadband atomic-layer MoS2 optical modulators for ultrafast pulse generations in the visible range

Visible lasers are a fascinating regime, and their significance is illustrated by the 2014 Noble prizes in physics and chemistry. With the development of blue laser diodes (LDs), the LD-pumped solid-state visible lasers become a burgeoning direction today. Constrained by the scarce visible optical modulators, the solid-state ultrafast visible lasers are rarely realized. Based on the bandgap structure and optoelectronic properties of atomic-layer MoS₂, it can be proposed that MoS₂ has the potential as a visible optical modulator. Here, by originally revealing layer-dependent nonlinear absorption of the atomic-layer MoS₂ in the visible range, broadband atomic-layer MoS₂ optical modulators for the visible ultrafast pulse generation are developed and selected based on the proposed design criteria for novel two-dimensional (2D) optical modulators. By applying the selected MoS₂ optical modulators in the solid-state praseodymium lasers, broadband mode-locked ultrafast lasers from 522 to 639 nm are originally realized. We believe that this Letter should promote the development of visible ultrafast photonics and further applications of 2D optoelectronic materials.

Ultrafast nonlinear optical properties of thin-solid DNA film and their application as a saturable absorber in femtosecond mode-locked fiber laser

A new extraordinary application of deoxyribonucleic acid (DNA) thin-solid-film was experimentally explored in the field of ultrafast nonlinear photonics. Optical transmission was investigated in both linear and nonlinear regimes for two types of DNA thin-solid-films made from DNA in aqueous solution and DNA-cetyltrimethylammonium chloride (CTMA) in an organic solvent. Z-scan measurements revealed a high third-order nonlinearity with n₂ exceeding 10⁻⁹ at a wavelength of 1570 nm, for a nonlinarity about five orders of magnitude larger than that of silica. We also demonstrated ultrafast saturable absorption (SA) with a modulation depth of 0.43%. DNA thin solid films were successfully deposited on a side-polished optical fiber, providing an efficient evanescent wave interaction. We built an organic-inorganic hybrid all-fiber ring laser using DNA film as an ultrafast SA and using Erbium-doped fiber as an efficient optical gain medium. Stable transform-limited femtosecond soliton pulses were generated with full width half maxima of 417 fs for DNA and 323 fs for DNA-CTMA thin-solid-film SAs. The average output power was 4.20 mW for DNA and 5.46 mW for DNA-CTMA. Detailed conditions for DNA solid film preparation, dispersion control in the laser cavity and subsequent characteristics of soliton pulses are discussed, to confirm unique nonlinear optical applications of DNA thin-solid-film.