2D Nb_(2)CT_(x) MXene/MoS_(2) heterostructure construction for nonlinear optical absorption modulation

Two-dimensional(2D)nonlinear optical mediums with high and tunable light modulation capability can significantly stimulate the development of ultrathin,compact,and integrated optoelectronics devices and photonic elements.2D carbides and nitrides of transition metals(MXenes)are a new class of 2D materials with excellent intrinsic and strong light-matter interaction characteristics.However,the current understanding of their photo-physical properties and strategies for improving optical performance is insufficient.To address this issue,we rationally designed and in situ synthesized a 2D Nb_(2)C/MoS_(2) heterostructure that outperforms pristine Nb2C in both linear and nonlinear optical performance.Excellent agreement between experimental and theoretical results demonstrated that the Nb_(2)C/MoS_(2) inherited the preponderance of Nb_(2)C and MoS_(2) in absorption at different wavelengths,resulting in the broadband enhanced optical absorption characteristics.In addition to linear optical modulation,we also achieved stronger near infrared nonlinear optical modulation,with a nonlinear absorption coefficient of Nb_(2)C/MoS_(2) being more than two times that of the pristine Nb_(2)C.These results were supported by the band alinement model which was determined by the X-ray photoelectron spectroscopy(XPS)experiment and first-principal theory calculation.The presented facile synthesis approach and robust light modulation strategy pave the way for broadband optoelectronic devices and optical modulators.

Ultrafast nonlinear absorption and nonlinear refraction in few-layer oxidized black phosphorus

We experimentally investigated the nonlinear optical response in few-layer oxidized black phosphorus(OBP) by the femtosecond Z-scan measurement technique, and found that OBP not only possesses strong ultrafast saturable absorption but also a nonlinear self-defocusing effect that is absent in black phosphorus(BP). The saturable absorption property originates mainly from the direct band structure, which is still maintained in OBP. The emergence of self-defocusing might originate from the combined consequences of the oxygen-induced defects in BP. Our experimental findings might constitute the first experimental evidence on how to dynamically tune its nonlinear property, offering an inroad in tailoring its optical properties through chemical modification(oxidation, introducing defects, etc.). The versatile ultrafast nonlinear optical properties(saturable absorption and self-defocusing) imply a significant potential of the layered OBP in the development of unprecedented optoelectronic devices, such as mode lockers, optical switches, laser beam shapers, and wavelength converters.

Two-photon absorption and stimulated emission in poly-crystalline Zinc Selenide with femtosecond laser excitation

The optical nonlinearity in polycrystalline zinc selenide(ZnSe),excited with 775 nm,1 kHz femtosecond laser pulses was investigated via the nonlinear transmission with material thickness and the Z scan technique.The measured two photon absorption coefficientβwas intensity dependent,inferring that reverse saturated absorption(RSA)is also relevant dur-ing high intensity excitation in ZnSe.At low peak intensity I<5 GW cm^(-2),we findβ=3.5 cm GW^(-1) at 775 nm.The spec-tral properties of the broad blueish two-photon induced fluorescence(460 nm-500 nm)was studied,displaying self-ab-sorption near the band edge while the upper state lifetime was measured to be τ_(e)~3.3 ns.Stimulated emission was ob-served when pumping a 0.5 mm thick polycrystalline ZnSe sample within an optical cavity,confirmed by significant line narrowing fromΔλ=11 nm(cavity blocked)toΔλ=2.8 nm at peak wavelength λ_(p)=475 nm while the upper state life-time also decreased.These results suggest that with more optimum pumping conditions and crystal cooling,polycrystal-line ZnSe might reach lasing threshold via two-photon pumping atλ=775 nm.

Realization of high-performance tri-layer graphene saturable absorber mirror fabricated via a one-step transfer process

Graphene,as a saturable absorber(SA),has attracted much attention for its application in ultrashort pulse fiber lasers due to its ultrafast interband carrier relaxation and ultra-broadband wavelength operation.Nevertheless,during the stacking process of monolayer graphene layer,the induced nonuniform contact at the interface of graphene layers deteriorate the device performance.Herein,we report the fabrication of graphene saturable absorber mirrors(SAMs)via a one-step transfer process and the realization of the much enlarged modulation depth and the much reduced nonsaturable loss with tri-layer graphene(TLG)than single-layer graphene(SLG)due to the improved uniform contact at the interface.Moreover,the operation of 1550 nm mode-locked Er-doped fiber laser with the TLG SAM exhibits excellent output characteristics of the maximum output power of 9.9 mW,a slope efficiency of 2.4%and a pulse width of 714 fs.Our findings are expected to pave the way toward high-performance ultrashort pulse fiber lasers based on graphene SAs.

Thickness dependency of nonlinear optical properties in ITO/Sn composite films

In this study,a batch of indium tin oxide(ITO)/Sn composites with different ratios was obtained based on the principle of thermal evaporation by an electron beam.The crystalline structure,surface shape,and optical characterization of the films were researched using an X-ray diffractometer,an atomic force microscope,a UV-Vis-NIR dual-beam spectrophotometer,and an open-hole Z-scan system.By varying the relative thickness ratio of the ITO/Sn bilayer film,tunable nonlinear optical properties were achieved.The nonlinear saturation absorption coefficientβmaximum of the ITO/Sn composites is−10.5×10^(−7)cm/W,approximately 21 and 1.72 times more enhanced compared to monolayer ITO and Sn,respectively.Moreover,the improvement of the sample nonlinear performance was verified using finite-difference in temporal domain simulations.

Nonlinear optical absorption of few-layer molybdenum diselenide （MoSe2） for passively mode-locked soliton fiber laser [Invited]

In this paper,both nonlinear saturable absorption and two-photon absorption（TPA） of few-layer molybdenum diselenide（MoSe2） were observed at 1.56 μm wavelength and further applied to mode-locked ultrafast fiber laser for the first time to our knowledge.Few-layer MoSe2 nanosheets were prepared by liquid-phase exfoliation method and characterized by x ray diffractometer,Raman spectroscopy,and atomic force microscopy.The obtained fewlayer MoSe2 dispersion is further composited with a polymer material for convenient fabrication of MoSe2 thin films.Then,we investigated the nonlinear optical（NLO） absorption property of the few-layer MoSe2 film using a balanced twin-detector measurement technique.Both the saturable absorption and TPA effects of the few-layer MoSe2 film were found by increasing the input optical intensity.The saturable absorption shows a modulation depth of 0.63% and a low nonsaturable loss of 3.5%,corresponding to the relative modulation depth of 18%.The TPA effect occurred when the input optical intensity exceeds 260 MW∕cm2.Furthermore,we experimentally exploit the saturable absorption of few-layer MoSe2 film to mode lock an all-fiber erbium-doped fiber laser.Stable soliton mode locking at 1558 nm center wavelength is achieved with pulse duration of 1.45 ps.It was also observed that the TPA process suppresses the mode-locking operation in the case of higher optical intensity.Our results indicate that layered MoSe2,as another two-dimensional nanomaterial,can provide excellent NLO properties（e.g.,saturable absorption and TPA） for potential applications in ultrashort pulse generation and optical limiting.

Direct detection of the transient superresolution effect of nonlinear saturation absorption thin films

Using a strong nonlinear saturation absorption effect is one technique for breaking through the diffraction limit. In this technique, formation of a dynamic and reversible optical pinhole channel and transient superresolution is critical. In this work, a pump–probe transient detection and observation–experimental setup is constructed to explore the formation process directly. A Ge2Sb2Te5 thin film with strong nonlinear saturation absorption is investigated. The dynamic evolution of the optical pinhole channel is detected and imaged, and the transient superresolution spot is directly captured experimentally. Results verify that the superresolution effect originates from the generation of an optical pinhole channel and that the formation of the optical pinhole channel is dynamic and reversible. A good method is provided for direct detection and observation of the transient process of the superresolution effect of nonlinear thin films.

Nonlinear and saturable absorption properties of PbS nanocrystalline thin films

The structural,morphological,optical,and nonlinear optical properties of a lead sulfde(PbS) thin film grown by chemical bath deposition(CBD) are investigated by X-ray difraction(XRD),scanning electron microscope(SEM),ultraviolet-visible(UV-Vis),and open aperture Z-scan experiments.The band gap energy of the PbS nanocrystalline film is 1.82 eV,higher than that of bulk PbS at 300 K.The nonlinear absorption properties of the film are investigated using the open aperture Z-scan technique at 1064 nm and pulse durations of 4 ns and 65 ps.Intensity-dependent switching of the film from nonlinear absorption to saturable absorption is observed.The nonlinear absorption coefficient increases monotonically with increasing pulse duration from 65 ps to 4 ns.

Phase-modulated quantum-sized TMDs for extreme saturable absorption

Two-dimensional semiconductors such as transition metal dichalcogenides(TMDs)have attracted much interest in the past decade.Herein,we present an all-physical top-down method for the scalable production of the intrinsic TMD quantum sheets(QSs).The phases of the TMDs(e.g.,2H-MoSe_(2),2H-WSe_(2),and Td-WTe2)remain stable during the transformation from bulk to QSs.However,phase transition(from Td to 2H)is detected in MoTe2.Such phase-modulation by size-reduction has never been reported before.The TMD QSs can be well dispersed in solvents,resulting in remarkable photoluminescence with excitation wavelength-,concentration-,and solvent-dependence.Meanwhile,the TMD QSs can be readily solution-processed into hybrid thin films,which demonstrate exceptional nonlinear saturation absorption(NSA).Notably,2H-MoTe2 QSs in poly(methyl methacrylate)show extremely high NSA performance with(absolute)modulation depth up to 46.6%and saturation intensity down to 0.81 MW·cm^(−2).Our work paves the way towards quantum-sized TMDs.

Fast electrochemical activation of the broadband saturable absorption of tungsten oxide nanoporous film

The on-demand modulation of defects in materials for the effective modulation of optical nonlinearity is desirable,while it remains a great challenge.In this work,we demonstrate that electrochemical activation is a facile and convenient approach to modulating the broadband third-order nonlinear absorption of nanoporous tungsten oxide(WO3-x)thin film.The film does not exhibit optical nonlinearity at the initial state,while shows a distinct saturable absorption under an applied voltage of-2.5 V with the excitation of 515,800,and 1,030 nm laser.The nonlinear absorption coefficient(β_(eff))is-766.38±6.67 cm·GW^(-1) for 1,030 nm laser,-624.24±17.15 cm·GW-1 for 800 nm laser,and-120.70±11.49 cm·GW^(-1) for 515 nm laser,and the performance is competitive among inorganic saturable absorbers.The activation is accomplished in 2 min.The performance enhancement is ascribed to the formation of abundant in-gap defect states because of the reduction of the tungsten atoms,and a Pauli-blocking effect occurs during the excitation of in-gap defect states.The small feature size of WO_(3-x)(-12 nm)enables the effective and fast introduction and removal of the defects in porous film,and accordingly the fast and broadband modulation of optical nonlinearity.Our results suggest a controllable,effective,and convenient approach to tuning the nonlinear absorption of materials.

Phase/size dual controlled 2D semiconductor In_(2)X_(3)(X=S,Se,Te)for saturable absorption modulation

The production of two-dimensional nanosheets(2D NSs)with all sizes(1-100 nm)and few(<10)layers is highly desired but far from satisfactory.Herein,we report an all-physical top-down method to produce indium chalcogenide(In2X3(X=S,Se,Te))NSs with wide-range(150-3.0 nm)controlled sizes.The method combines silica-assisted ball-milling and sonication-assisted solvent exfoliation to fabricate multiscale NSs with varying distributions,which are then precisely separated by cascade centrifugation.Multiple characterization techniques reveal that the as-produced In2X3 NSs are intrinsic and defect-free and remainβ-phase during the whole process.The redispersions of In2X3 NSs exhibit prominent excitation wavelength-,solvent-,concentration-,and size-dependent photoluminescence.The NSs-poly(methyl methacrylate)(PMMA)hybrid thin films demonstrate strong size effects in nonlinear saturation absorption.The absolute modulation depths of 35.4%,43.3%,47.2%and saturation intensities of 1.63,1.05,0.83 MW·cm^(−2)(i.e.,163,105,and 83 nJ·cm^(−2))are derived for the In_(2)S_(3),In_(2)Se_(3),and In2Te3 quantum sheets,respectively.Our method paves the way for mass production and full exploration of full-scale 2D NSs.

Third-order nonlinear optical responses of CuO nanosheets for ultrafast pulse generation

As a representative transition metal oxide,cupric oxide(CuO)has attracted a lot of interest in miscel-laneous fields.In the present work,the prominent third-order nonlinear optical(NLO)features of CuO nanosheets were demonstrated via the typical Z-scan technique at 1 m m.The open-aperture(OA)Z-scan measurements at different intensities clearly showed that CuO nanosheets possessed the saturable ab-sorption(SA),reverse saturable absorption(RSA)and optical limiting behaviors.The closed-aperture(CA)Z-scan technology was implanted to investigate the nonlinear refractive index and the third-order nonlinear susceptibility.In virtue of the strong nonlinear optical absorption properties of CuO nano-sheets,a stable all-solid-state mode-locked laser with the as-prepared CuO nanosheets as saturable absorber at 1.06 m m was realized for the first time.Our work confirmed that CuO nanosheets exhibited outstanding NLO properties,which might stimulate the development in ultrafast photonics and nonlinear optics.

Affirming nonlinear optical coefficient constancy from z-scan measurement

Goodness of fit is demonstrated for theoretical calculation of z-scan data based on beams propagating in the nonlinear medium and the Fresnel–Kirchhoff diffraction integral in experiments with high nonlinear refraction and absorption. The constancy of nonlinear optical parameters is achieved regardless of sample thickness and laser intensity, which clarifies the physical significance of optical parameters. We have obtained γ = 2.0 × 10-19 m2/W and β = 5.0 × 10-13 m/W for carbon disulfide excited by a pulsed laser at 800 nm with pulse duration of 35 fs,which are independent of sample thickness and laser intensity. Affirming constancy of the extracted parameters to the incident light intensity may become a practice to verify the goodness of the z-scan experiment.

Optical Properties of Silver Nanoplates Synthesized by Photoinduced Method

Silver nanoplates were synthesized in aqueous solution by photoinduced chemical reduction method with tungsten lamp as light source.The growth process was analyzed and characterized.The linear absorption spectra showed that,along with the growth process,the surface plasmon resonance of silver seed nanoparticles at 395 nm decreased gradually,while a new plasmon band at 740 nm corresponding to silver nanoplates appeared and increased gradually.Z-scan technique was used to explore the nonlinear optical properties of silver nanoplates.The results displayed that with the reaction time increases from 0 h to 24 h,the value of nonlinear absorption(NLA) coefficient and the value of nonlinear refraction(NLR) index of the products increased from 0 to 3.167 cm/GW and from 0.64×10 4 to 6.83×10 4 cm 2 /GW,respectively.