Saturation Intensity Research Articles

A new organic molecular probe as a powerful tool for fluorescence imaging and biological study of lipid droplets.

Background: Lipid droplets (LDs) are critical organelles associated with many physiological processes in eukaryotic cells. To visualize and study LDs, fluorescence imaging techniques including the confocal imaging as well as the emerging super-resolution imaging of stimulated emission depletion (STED), have been regarded as the most useful methods. However, directly limited by the availability of advanced LDs fluorescent probes, the performances of LDs fluorescence imaging are increasingly unsatisfied with respect to the fast research progress of LDs. Methods: We herein newly developed a superior LDs fluorescent probe named Lipi-QA as a powerful tool for LDs fluorescence imaging and biological study. Colocalization imaging of Lipi-QA and LDs fluorescent probe Ph-Red was conducted in four cell lines. The LDs staining selectivity and the photostability of Lipi-QA were also evaluated by comparing with the commercial LDs probe Nile Red. The in-situ fluorescence lifetime of Lipi-QA in LDs was determined by time-gated detection. The cytotoxicity of Lipi-QA was assessed by MTT assay. The STED saturation intensity as well as the power- and gate time-dependent resolution were tested by Leica SP8 STED super-resolution nanoscopy. The time-lapse 3D confocal imaging and time-lapse STED super-resolution imaging were then designed to study the complex physiological functions of LDs. Results: Featuring with the advantages of the super-photostability, high LDs selectivity, long fluorescence lifetime and low STED saturation intensity, the fluorescent probe Lipi-QA was capable of the long-term time-lapse three-dimensional (3D) confocal imaging to in-situ monitor LDs in 3D space and the time-lapse STED super-resolution imaging (up to 500 STED frames) to track the dynamics of LDs with nanoscale resolution (37 nm). Conclusions: Based on the state-of-the-art fluorescence imaging results, some new biological insights into LDs have been successfully provided. For instance, the long-term time-lapse 3D confocal imaging has surely answered an important and controversial question that the number of LDs would significantly decrease rather than increase upon starvation stimulation; the time-lapse STED super-resolution imaging with the highest resolution has impressively uncovered the fission process of nanoscale LDs for the first time; the starvation-induced change of LDs in size and in speed has been further revealed at nanoscale by the STED super-resolution imaging. All of these results not only highlight the utility of the newly developed fluorescent probe but also significantly promote the biological study of LDs.

Application of biomass as clean energy in magnetized roasting of iron tailings

Biomass, as a clean energy source, has great potential in reducing emissions and protecting the environment. This study investigates the influence of biomass as a reducing agent on the magnetization roasting process of iron tailings, and utilizes methods such as TG, XRD, VSM to analyze the thermal decomposition of biomass, phase transition and magnetic transformation of iron tailings, in order to explain the mechanism of iron recovery through magnetization roasting. The results show that at a roasting temperature of 700 °C, roasting time of 15 min, and 10% of the poplar sawdust (PS) used, the iron grade of the concentrate obtained through weak magnetic separation is 65.2%, with iron recovery rate of 96%. The magnetic saturation intensity increases from 0.107 emu/g to 59.28 emu/g. The study demonstrates that the combination of biomass magnetization roasting and weak magnetic separation can effectively recover iron from low-grade iron tailings.

Flexible optical limiters based on Cu3VSe4 nanocrystals.

Optical limiters are greatly needed to protect eyes and sensitive optoelectronic devices such as photodetectors and sensors from laser damage, but they are currently plagued by low efficiency. In this work, we utilized Cu3VSe4 nanocrystals (NCs) to enhance laser protection performance, and they exhibit higher saturation intensity and broader nonlinear spectral response extending into the near IR region than the C60 benchmark. A flexible optical limiter goggle prototype based on the NCs significantly attenuated the incident laser beam, with Z scan and I scan measurements demonstrating a giant nonlinear absorption coefficient β value of 1.0 × 10-7 m W-1, a large optical damage threshold of 3.5 J cm-2, and a small starting threshold of 0.22 J cm-2. Transient absorption spectroscopy disclosed that the origin of the excellent nonlinearity was associated with quasi-static dielectric resonance behavior and a large TPA cross-section of 3.3 × 106 GM was measured for Cu3VSe4 NCs, suggesting the potential of intermediate bandgap (IB) semiconductors as alternatives to plasmonic noble metals for ultrafast photonics. Hence, optical limiters based on such semiconductors offer new avenues for laser protection in optoelectronic and defense fields.

Demonstration of multiple dissipative solitons with nickel-based metal organic framework saturable absorber

In this work, we have demonstrated the generation of dissipative soliton from single to multi-pulsing phenomenon. The generation of ultrashort pulses was achieved with nickel-based metal organic framework (Ni-MOF) as saturable absorber in a ring cavity erbium-doped fiber laser. The SA was fabricated by spin-coating Ni-MOF/polydimethylsiloxane composite on a tapered fiber substrate, which had an insertion loss of 2.82 dB, a modulation depth of 4.61 %, and saturation intensity of 30.12 MW/cm2. A self-started DS mode-locked pulse was observed at 1559.89 nm central wavelength with 3-dB bandwidth of 19.72 nm at threshold pump power of 33.5 mW. Its pulse width of 7.89 ps was recorded at 7.94 MHz repetition rate. The nature of pulse splitting was observed up to three pulses which were separated with 2.62 ns time spacing with the increment of pump power until 134.2 mW. The generated multiple dissipative soliton pulses realized in the laser cavity through Ni-MOF saturable absorber were stable and invariant for 1-hour stability measurement.

SiO2 Passivated Graphene Saturable Absorber Mirrors for Ultrashort Pulse Generation.

Owing to its broadband absorption, ultrafast recovery time, and excellent saturable absorption feature, graphene has been recognized as one of the best candidates as a high-performance saturable absorber (SA). However, the low absorption efficiency and reduced modulation depth severely limit the application of graphene-based SA in ultrafast fiber lasers. In this paper, a single-layer graphene saturable absorber mirror (SG-SAM) was coated by a quarter-wave SiO2 passivated layer, and a significantly enhanced modulation depth and reduced saturation intensity were obtained simultaneously compared to the SG-SAM without the SiO2 coating layer. In addition, long-term operational stability was found in the device due to the excellent isolation and protection of the graphene absorption layer from the external environment by the SiO2 layer. The high performance of the SAM was further confirmed by the construction of a ring-cavity EDF laser generating mode-locked pulses with a central wavelength of 1563.7 nm, a repetition rate of 34.17 MHz, and a pulse width of 830 fs.

Tuning Magnetic and Photophysical Properties of Luminomagnetic Metal-Organic Framework Composites in an Inverse Core-Satellite Structure.

Luminomagnetic composites have been synthesized that allow for an individual tuning of luminescence intensity, chromaticity and magnetization by combination of superparamagnetic, citrate-stabilized iron oxide nanoparticles with the luminescent MOFs 3 ∞ [Ln2 (BDC)3 (H2 O)4 ] (Ln=Eu, Tb; BDC2- =terephthalate). The components are arranged to a concept of inverse structuring compared to previous luminomagnetic composites with MOF@magnetic particle (shell@core) composition so that the luminescent MOF now acts as core and is covered by magnetic nanoparticles forming the satellite shell. Thereby, the magnetic and photophysical properties are individually tuneable between high emission intensity (1.2 ⋅ 106 cps mg-1 ) plus low saturation magnetization (6 emu g-1 ) and the direct opposite (0.09 ⋅ 106 cps mg-1 ; 42 emu g-1 ) by adjusting the particle coverage of the MOF. This is not achievable with a core-shell structure having a magnetic core and a dense MOF shell. The composition of the composites and the influence of different synthesis conditions on their properties were investigated by SEM/EDX, PXRD, magnetization measurements and photoluminescence spectroscopy.

Broadband Nonlinear Response and Ultrafast Photonics Applications in Few-Layer MBene

2D MBenes, the latest derivatives of bulk ternary or quaternary transition metal boride (MAB) phases, have gradually attracted increasing research attention as novae in the flatland. However, the fabrication of MBenes is still a great challenge at present, leading to the lack of reported MBene applications in the photonic field (e.g., ultrafast lasers). Hence, we successfully fabricated few-layer Mo4/3B2–xTz MBenes by selectively etching nanolaminated 3D parent boride (Mo2/3Y1/3)2AlB2. Density functional theory calculations were used to theoretically analyze its electronic and optical properties. By employing the Z-scan technique, the nonlinear absorption characteristics of Mo4/3B2–xTz MBenes from the visible to mid-infrared regime were revealed for the first time. Compared with mainstream 2D materials and common MXenes, Mo4/3B2–xTz has more potential as a nonlinear optical (NLO) material, such as stronger nonlinear absorption (about 100 times of graphene) and lower saturation intensity (about 1% of black phosphorus). Furthermore, by integrating Mo4/3B2–xTz as a saturable absorber (SA) into three similar gain fiber-based laser resonators, broadband stable ultrafast pulses were obtained with pulse durations of 358.7 ps (1 μm), 582.3 fs (1.5 μm), and 2.31 ps (2 μm). Innovation in MBenes as SA will allow the design of novel photonic systems and devices with broadband functionality, opening a promising paradigm for the conscious design of high-performance NLO materials.

InP/ZnSeS/ZnS Core–Shell Quantum Dots as Novel Saturable Absorbers in Mode‐Locked Fiber Lasers

AbstractIn this study, red light emitting InP/ZnSeS/ZnS quantum dots (QDs) are prepared by thermal injection method. The results show that the InP/ZnSeS/ZnS QDs saturable absorbers (SA) have good nonlinear saturable absorption properties with modulation depth of 24.2% and saturation intensity of 0.08 KW cm−2. Then the QD SAs are applied to the erbium‐doped fiber laser (EDFL) ring cavity system, and a mode‐locked laser pulse with a pulse width of 635 fs is generated. In addition, the pulse width is determined by the carrier recovery time, which is closely related to the defect density in the material. A large number of defects are eliminated in the QDs by hydrogen fluoride (HF) treatment, and the pulse width is reduced from 635 to 450 fs. Results of time‐resolved photoluminescence and ultrafast transient absorption spectroscopy (TAS) show that HF treatment indeed reduces defects in InP/ZnSeS/ZnS QDs, indicated by the decrease of the carrier recovery time. This is different from the reported 2D SA materials, which usually reduce the carrier recovery time by increasing the defect density. This study will inspire new applications of QDs in ultrafast photonics and nonlinear optics.

γ-MnO2-deposited photonic crystal fiber: Novel photonic devices for multi-state solitons generation

Transition metal oxides, especially MnO2 nanoparticles, are often used as cathode of dry batteries and catalysts. They are also promising materials for ultrafast laser applications due to their strong absorption in visible light and near-infrared wavelength range. In this work, high-concentration MnO2 nanoparticles dispersions embedded in the hole cladding of dual-hole photonic crystal fiber is fabricated as a saturable absorber with a modulation depth of 4 % and a saturation intensity of 25 MW/cm2. Strong MnO2-light interaction occurs due to enhanced evanescent-field strength (over 10 cm) of the SA could enable stable mode locking operation at 1.55 μm region. By inserting a Sagnac fiber filter in the cavity, multi-state solitons are experimentally demonstrated with identical layout, respectively, which greatly improves the versatility of this laser. This study proves that MnO2 nanoparticles possess excellent nonlinear optical properties in the near-infrared band. The simple in-line structure of the proposed nanoparticles-deposited device could pave a way for high power and all-fiber applications of photonics.

Passively Q-switched 2 µm fiber laser with WO3 saturable absorber

We have successfully demonstrated a reliable and simple passively Q-switched Thulium-doped fiber laser (TDFL) utilizing tungsten oxide (WO3) as a saturable absorber (SA). The SA film was fabricated by embedding WO3 material into Polyvinyl alcohol (PVA) host polymer. The saturable absorption and saturation intensity of the fabricated WO3-based SA are about 20 % and 0.04 MW/cm2, respectively. The Q-switched TDFL pulses operate at a central wavelength 1960.2 nm with a 3 dB bandwidth of 1.4 nm, pulse duration of 2.32 μs, maximum repetition rate of 64.27 kHz, maximum output power of 8.2 mW, and maximum pulse energy of 127.58 nJ. To our knowledge, this is the first time that WO3 has been used as a Q-switcher for operation in 2 μm region.

Impact of pedestal density gradient and collisionality on ELM dynamics

BOUT++ turbulence simulations are conducted to capture the underlying physics of small ELM characteristics achieved by increasing separatrix density via controlling strike points from vertical to horizontal divertor plates for three EAST discharges. BOUT++ linear simulations show that the most unstable modes change from high-n ideal ballooning modes to intermediate-n peeling–ballooning modes and eventually to peeling–ballooning stable plasmas in the pedestal. Nonlinear simulations show that the fluctuation is saturated at a high level for the lowest separatrix density. The ELM size decreases with increasing separatrix density, until the fraction of this energy lost during the ELM crash becomes less than 1% of the pedestal stored energy, leading to small ELMs. Simulations indicate that small ELMs can be triggered either by the marginally peeling–ballooning instability near the peak pressure gradient position inside the pedestal or by a local instability in the pedestal foot with a larger separatrix density gradient. The pedestal collisionality scan for type-I ELMs with steep pedestal density gradient shows that both linear growth rate and ELM size decrease with increasing collisionality. On the contrary, the pedestal collisionality and pedestal density width scan with a weak pedestal density gradient indicate small ELMs can either be triggered by a high-n ballooning mode or by a low-n peeling mode in a low collisionality region 0.04–0.1. The simulations indicate the weaker the linear unstable modes near marginal stability with small linear growth rate, the lower nonlinearly saturated fluctuation intensity and the smaller turbulence spreading from the linear unstable zone to stable zone in the nonlinear saturation phase, leading to small ELMs.

Photoluminescence of Nitrogen-Vacancy Centers by Ultraviolet One- and Two-Photon Excitation of Fluorescent Nanodiamonds.

Fluorescent nanodiamonds contain nitrogen-vacancy (NV) centers as quantum defects. When exposed to a continuous-wave 325 nm laser or a femtosecond 344 nm laser, the particles emit red fluorescence from NV0 centers at ∼620 nm. Power dependence measurements of the emission strength revealed a predominantly linear behavior at the laser peak intensity lower than 1 GW·cm-2, contributed mainly by photoexcitation of electrons from the valence band of diamond to the NV0 centers, followed by relaxation via electron-hole recombination. In the higher power regions, however, nonresonant two-photon interband excitation of the diamond matrix dominates the photoluminescence processes. Best fits of the experimental data to semiempirical models revealed an ionization coefficient of ∼1 cm-1 for the one-photon valence-to-defect excitation and a saturation intensity of 180 ± 60 GW·cm-2 for the two-photon interband excitation. The study provides new insight into the photoionization of NV0 centers and the interband excitation properties of diamond in the UV region.

CsPbBr3 Perovskite Nanocrystals for a Q-Switched Pulsed Fiber Laser in the C-Band Region.

All-inorganic perovskite nanocrystals have been widely reported as promising light-harvesting and light-emitting semiconductor nanomaterials. However, their nonlinear optical properties and laser applications have rarely been explored, especially for pulse laser modulation in the telecommunication C-band window. Herein, we experimentally demonstrated a passively Q-switched erbium-doped fiber laser (EDFL) operation at the C-band region using perovskite CsPbBr3 nanocrystals as a saturable absorber (SA). The broadband linear optical absorption in the 300-2000 nm range and the nonlinear optical absorption at the C-band range of around 1560 nm were discovered and investigated in CsPbBr3 nanocrystals. The CsPbBr3-based SA exhibited good saturable absorption performance with a modulation depth and saturation intensity equivalent to 19.1% and 10.9 MW/cm2, respectively. By integrating the CsPbBr3 SA into an EDFL cavity, a passively Q-switched operation with a central wavelength of 1560 nm, a threshold pump power of 60 mW, and the shortest pulse duration of 5.96 μs was achieved. In addition, such a Q-switching operation exhibited long-term stability. Our results indicate that the CsPbBr3 perovskite nanocrystals can serve as an efficient candidate for constructing pulsed lasers in the C-band or even longer NIR wavelength region.

High laser performance of an Al3+ and Nd3+-codoping in sodium-borotellurite glass for NIR broadband laser application

The main spectroscopic properties of Nd3+ ions immersed in an oxide glass with composition 59B2O3–10TeO2–30Na2O–1Al2O3:1Nd2O3 (in mol%) has been studied by X-ray diffraction, EDS, SEM, luminescence, transmission, FTIR and Raman spectroscopies. Very high values of the main spectroscopic parameters of Nd3+ ions were determined from the Ωt intensity parameters according to Judd-Ofelt formalism. The 4F3/2 → 4I11/2 NIR transition at 1060 nm of Nd3+ ions is evaluated from the relevant laser parameters reporting a very high quantum efficiency (95%), large effective bandwidth (36 nm), high laser performance quality (9.19 × 10−24 cm2·s) and low saturation intensity (2.04 × 108 W/m2). The excellent non-linear properties of Nd3+ ions are attributed to a microcrystallization inside the glass induced by Al3+ and Nd3+-codoping and a high sodium concentration which intensified the ligand field strengths around Nd3+ ions. Such microcristallization is verified by the presence of several crystalline peaks observed in the FTIR and Raman spectra and identified as Al–O and Te–O linkage vibrations in α-Al2O3 and α-TeO2 phases. As a consequence, a significant reduction of free hydroxyls is appreciated at the midinfrared region in both spectra. Whereas the presence of graphite was detected in the Raman spectrum at 2450 and 2889 cm−1. The great centrosymmetric loss of the Nd3+-sites is indicated by a high value of the Ω2 parameter (36.98 × 10−20 cm2) and confirmed by matched vibrational modes displayed in the deconvoluted Raman and FTIR spectra from 370 to 1750 cm−1 according to the Mutual Exclusion Rule for non-centrosymmetric point groups. The X-ray pattern of the Nd3+-doped oxide glass shows three different amorphous phases generated by delocalized and localized 4f-electrons indicating coexistence of ionic and covalent oxide-metal bonds as characteristic of the invert glasses. However, the compositional parameters confirmed a preferentially covalent domain of oxide-metal bonds as well as presence of crystalline phase, being both structural characteristics consistent with the Judd-Ofelt, transmission, FTIR and Raman results. Finally, the radiative properties are compared with those reported for other Nd3+-activated aluminate glasses suggesting that Al3+ and Nd3+-codoped sodium-borotellurite glass has a great potential for NIR broadband laser application.

Optical saturable absorption of conformal graphene directly synthesized on nonlinear device surfaces

Optical saturable absorption (SA) in graphene has been intensively studied during the last decades to achieve graphene-based ultrafast pulse generation. We experimentally investigate the optical nonlinearities of transfer-free graphene synthesized by the atomic carbon spraying (ACS) method that is invented for conformal and uniform graphene coating on non-planar device surfaces to enhance the interaction of lasers with the nanostructures. Its successful implementation of nonlinear operations in practical photonic devices is confirmed. The SAs of two other different graphene structures including graphene prepared by chemical vapor deposition (CVD) and reduced graphene oxides (rGOs) by electro-spraying (ES) are quantitatively evaluated as references. All three nanocrystal structures are elucidated by transmission electron microscopy and Raman analysis comparatively and systematically. We confirm in X-ray photoelectron spectroscopy analysis that the contents of sp2 bonds in ACS graphene as an indicator of the nonlinearity is 73.94 %. The measured value reaches the level of CVD graphene (76.70 %) and is much higher than 58.75 % of ES rGOs. Importantly, the ACS graphene displays a steep SA slope with saturation intensity of 32.14 MW cm−2, an order of magnitude lower than conventional ES rGOs, verifying unimpaired nonlinear properties of graphene prepared by the suggested ACS method.

The nonlinear optical property of germanium saturable absorber and its application in normal dispersion Er-doped fiber laser

Recently, germanium (Ge) has been considered as the two-dimensional (2D) material saturable absorber (SA) due to its excellent nonlinear saturable absorption properties. However, previous reports explored the nonlinear saturable absorption properties of Ge SA in anomalous dispersion Er-doped fiber (EDF) laser. To date, the mode-locked operation achieved by normal dispersion EDF laser based on Ge SA has not yet been reported until now. In this work, Ge SA with saturation intensity of 11.69 MW/cm2 and modulation depth of 6.17 % was used to demonstrate a normal dispersion EDF laser. Morphological and nonlinear saturable absorption properties of Ge were also investigated. To the best of our knowledge, the mode-locked pulse with repetition rate of 6.66 MHz were achieved in normal dispersion EDF laser based on Ge SA for the first time. Compared with previous work about Ge SA in anomalous dispersion regime, our result proves that Ge also has excellent optical performance in normal dispersion regime.

Super-resolution microscopy enabled by high-efficiency surface-migration emission depletion

Nonlinear depletion of fluorescence states by stimulated emission constitutes the basis of stimulated emission depletion (STED) microscopy. Despite significant efforts over the past decade, achieving super-resolution at low saturation intensities by STED remains a major technical challenge. By harnessing the surface quenching effect in NaGdF4:Yb/Tm nanocrystals, we report here high-efficiency emission depletion through surface migration. Using a dual-beam, continuous-wave laser manipulation scheme (975-nm excitation and 730-nm de-excitation), we achieved an emission depletion efficiency of over 95% and a low saturation intensity of 18.3 kW cm−2. Emission depletion by surface migration through gadolinium sublattices enables super-resolution imaging with sub-20 nm lateral resolution. Our approach circumvents the fundamental limitation of high-intensity STED microscopy, providing autofluorescence-free, re-excitation-background-free imaging with a saturation intensity over three orders of magnitude lower than conventional fluorophores. We also demonstrated super-resolution imaging of actin filaments in Hela cells labeled with 8-nm nanoparticles. Combined with the highly photostable lanthanide luminescence, surface-migration emission depletion (SMED) could provide a powerful mechanism for low-power, super-resolution imaging or biological tracking as well as super-resolved optical sensing/writing and lithography.

Passively mode-locked fiber laser based on GeTe as a saturable absorber.

In this work, we fabricate a saturable absorber based on GeTe with saturation intensity and modulation depth of 12.6M W/c m 2 and 7%, respectively. We obtain stable conventional soliton and stretched soliton mode-locking operation. For the conventional soliton state, the average output power increased from 0.93 to 8.70mW with the increase of pump power, and the fundamental repetition rate was 7.8351MHz. Its central wavelength and 3dB bandwidth were 1564.72 and 4.78nm, respectively. For the stretched soliton state, when the pump power was increased from 87.4 to 420.3mW, the average output power increased from 2.05 to 10.46mW. When the maximum average output power reached 10.46mW, the maximum average single-pulse energy was 0.86nJ. The experimental results show that GeTe nanosheets will have broad application potential in the field of ultrafast photonics.

Generation of soliton operating at 1.5 µm regime using Indium Selenide based saturable absorber

We successfully accomplished a soliton pulse in an Erbium-doped fiber laser (EDFL) cavity by integrating an Indium Selenide (In2Se3)-based passive saturable absorber (SA). In2Se3 film was prepared by using a common dry cast method and owns saturable intensity, modulation depth, and non-saturable loss of 1066.57 MW/cm2, 1.1 %, and 72.2 %, respectively. By implementing In2Se3 film into the EDF laser cavity, a generation of soliton with 1.880 MHz, at the threshold pump power of 83.63 mW was achieved. A narrow pulse width of 5.51 ps and remarkable temporal stability, as demonstrated by a signal noise ratio (SNR) of 78.52 dB and peak power of 1.00 kW obtained using In2Se3 as SA operating in a 1.5-μm regime proven its potential in photonic applications.

All-fiber nonlinear multimode interference saturable absorber in reflection mode

We present a reflection mode saturable absorber based on nonlinear multimode interference in a graded index multimode fiber. The saturable absorber provides a modulation depth of 12 % at a low saturation intensity of 18.5 MW/cm2. The all-fiber structure and reflection mode design of this saturable absorber makes it an ideal device for the fabrication of fiber lasers with linear resonant cavity. When inserted in a linear laser cavity comprising a thulium doped fiber as the gain medium, the saturable absorber triggers stable modelocked pulses with a center wavelength of 1905 nm and spectral width of 11.2 nm.