High Saturation Intensity Research Articles

Few-layer NbTe2 nanosheets-based saturable absorber for generating 176 fs pulse at 1 μm in ultrafast fiber laser

Ultrafast fiber lasers play an increasingly vital role in diverse fields of science and industry. In this study, a novel NbTe2 nanosheet-based saturable absorber (SA) was prepared by using liquid phase exfoliation and flame brush technology for the first time. Using a twin balance measurement system, the nonlinear optical absorption property was checked. It was found that the fabricated NbTe2 nanosheets-based SA has excellent saturable absorption properties with a large modulation depth of 5.3 %, a very high saturation intensity of 1.11 GW/cm2, and a low non-saturation loss of 63.6 %. After inserting the NbTe2 nanosheets-based SA into a ring-cavity ytterbium-doped fiber laser (YDFL), a stable soliton mode-locking state was achieved. A 176-fs ultrafast soliton pulse train was successfully generated at 1029.76 nm with the repetition rater of 3.097 MHz. Thanks to the unique and very high saturation intensity of NbTe2 nanosheets, the proposed YDFL system achieved a maximum peak power of 96.3 kW and a single pulse energy of 14.09 nJ under 370 mW pump power. These findings indicate that NbTe2 nanosheets have great potential for generating ultra-short optical pulses with higher peak power in mode-locked fiber lasers.

Single-Photon Emission from Point Defects in Hexagonal Boron Nitride Induced by Plasma Treatment.

Solid-state quantum emitters are gaining significant attention for many quantum information applications. Hexagonal boron nitride (h-BN) is an emerging host material for generating bright, stable, and tunable single-photon emission with narrow line widths at room temperature. In this work, we present a facile and efficient approach to generate high-density single-photon emitters (SPEs) in mechanically exfoliated h-BN through H- or Ar-plasma treatment followed by high-temperature annealing in air. It is notable that the postannealing is essential to suppress the fluorescence background in photoluminescence spectra and enhance emitter stability. These quantum emitters exhibit excellent optical properties, including high purity, brightness, stability, polarization degree, monochromaticity, and saturation intensity. The effects of process parameters on the quality of quantum emitters were systematic investigated. We find that there exists an optimal plasma power and h-BN thickness to achieve a high SPE density. This work offers a practical avenue for generating SPEs in h-BN and holds promise for future research and applications in quantum photonics.

Gold nanoparticles enhanced femtosecond nonlinear optical properties of sodium borate oxide glasses

We present results from our experimental measurements on the effect of annealing and intensity–dependent femtosecond (MHz) third–order nonlinear optical (NLO) attributes of gold nanoparticles (NPs) embedded sodium borate glass specimens. A melt–quench procedure was used to fabricate the gold nanoparticle–loaded sodium antimony containing borate glasses, which were then subjected to different schedules of thermal treatment at 500 °C. The highest saturation intensity and nonlinear refraction magnitude were observed for the 7–hour annealed glass specimen (i.e., for NBAu–7 glass). Thus, on the NBAu–7 glass, intensity–sensitive NLO measurements were performed. The saturable absorption behavior that was detected at lower peak intensities switched to reverse saturable absorption during the exposure of NBAu–7 glass with high fluence laser pulses. While the sign of the nonlinear refraction remained unchanged, the magnitude increased. The methodology examined in this work may enable the fabrication of borate–based glasses loaded with gold NPs with high saturation intensities and robust nonlinear refractive index magnitudes to be used in optical switching operations. The results of the current study support that the NBAu–7 glass is advantageous for optical switching functions when exposed to low–energy laser pulses. In contrast, the same glass benefits optical limiting functions when exposed to high–fluence laser pulses.

Efficient and enhanced optical switches based on saturation absorption via composite of 2D materials

In this study, linear and nonlinear optical absorption has been investigated in a composite consisting of thin film wrapped nano-sphere dielectrics. This type of plasmonic coupled hybridized nano-elements can lay the groundwork for many functional devices. The graphene-based coupled plasmonic structure could benefit from superior plasmons and new coupled plasmon modes. By taking advantage of theoretical concepts based on quasi-static approximation and experimental data, it is shown that achieving high modulation depth (∼94%) is possible. Apart from the high modulation depth, saturation intensity, linear and nonlinear absorption can be controlled through adjusting optical and geometrical parameters of the composite. This model proves excellent saturable absorber characteristics particularly its outstanding nonlinear optical properties, which show outstanding potential in manufacturing high-performance and new functional optoelectronic devices.

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.

High-responsivity operation of quantum cascade detectors at 9 µm.

Quantum cascade detectors (QCDs) are devices operating at zero external bias with a low dark-current. They show linear detection and high saturation intensities, making them suitable candidates for heterodyne detection in long-wave infrared (LWIR) free space optical communication systems. We present an approach to mitigate the performance limitation at long wavelengths, by a comparison of similar single and multi-period QCDs for optimizing their responsivity and noise behaviour. Our InGaAs/InAlAs/InP ridge QCDs are designed for operation at λ = 9.124 µm. Optical waveguide simulations support the accurate optical characterization. A detailed device analysis reveals room-temperature responsivities of 111 mA/W for the 15-period and 411 mA/W for the single-period device.

Adaptive Unsupervised-Shadow-Detection Approach for Remote-Sensing Image Based on Multichannel Features

Shadow detection is an essential research topic in the remote-sensing domain, as the presence of shadow causes the loss of ground-object information in real areas. It is hard to define specific threshold values for the identification of shadow areas with the existing unsupervised approaches due to the complexity of remote-sensing scenes. In this study, an adaptive unsupervised-shadow-detection method based on multichannel features is proposed, which can adaptively distinguish shadow in different scenes. First, new multichannel features were designed in the hue, saturation, and intensity color space, and the shadow properties of high hue, high saturation, and low intensity were considered to solve the insufficient feature-extraction problem of shadows. Then, a dynamic local adaptive particle swarm optimization was proposed to calculate the segmentation thresholds for shadows in an adaptive manner. Finally, experiments performed on the Aerial Imagery dataset for Shadow Detection (AISD) demonstrated the superior performance of the proposed approach in comparison with traditional unsupervised shadow-detection and state-of-the-art deep-learning methods. The experimental results show that the proposed approach can detect the shadow areas in remote-sensing images more accurately and efficiently, with the F index being 82.70% on the testing images. Thus, the proposed approach has better application potential in scenarios without a large number of labeled samples.

Atomically thin gallium telluride nanosheets: A new 2D material for efficient broadband nonlinear optical devices

Here, we experimentally demonstrate the nonlinear optical properties of atomically thin two-dimensional layered gallium telluride (GaTe). We performed open aperture and close aperture Z-scan measurements in the femtosecond regime for the spectral range of 520–700 nm to study the broadband nonlinear absorption and nonlinear refraction. Interestingly, exfoliated GaTe displays strong saturable absorption and high negative Kerr nonlinearity in this spectral range. We observed a high nonlinear refraction coefficient, n2I, of −(7.61±0.07)×10−1 cm2/GW with near band edge excitation at 700 nm. Additionally, it shows a significant nonlinear absorption coefficient, β, of −(18.02±0.20)×104 cm/GW at an intensity of 40 GW/cm2 with high modulation depth and low saturation intensity. Transient absorption spectroscopy measurement is also performed to investigate the carrier dynamics of exfoliated GaTe and to identify the physical mechanisms responsible for the optical nonlinearity, such as Pauli blocking. The measured nonlinear optical data of the exfoliated GaTe presented in this Letter will pave the way for potential application in ultrafast photonic devices like optical switching, passive Q-switching, and mode-locking, due to high Kerr nonlinearity and saturable absorption in the femtosecond regime.

Super-Resolution Imaging With Lanthanide Luminescent Nanocrystals: Progress and Prospect.

Stimulated emission depletion (STED) nanoscopy has overcome a serious diffraction barrier on the optical resolution and facilitated new discoveries on detailed nanostructures in cell biology. Traditional fluorescence probes employed in the super-resolution imaging approach include organic dyes and fluorescent proteins. However, some limitations of these probes, such as photobleaching, short emission wavelengths, and high saturation intensity, still hamper the promotion of optical resolution and bio-applications. Recently, lanthanide luminescent probes with unique optical properties of non-photobleaching and sharp emissions have been applied in super-resolution imaging. In this mini-review, we will introduce several different mechanisms for lanthanide ions to achieve super-resolution imaging based on an STED-like setup. Then, several lanthanide ions used in super-resolution imaging will be described in detail and discussed. Last but not least, we will emphasize the future challenges and outlooks in hope of advancing the next-generation lanthanide fluorescent probes for super-resolution optical imaging.

Millimeter-Deep Detection of Single Shortwave-Infrared-Emitting Polymer Dots through Turbid Media.

Fluorescence imaging at longer wavelengths, especially in the shortwave-infrared (SWIR: 1000-1700 nm) region, leads to a substantial decrease in light attenuation, scattering, and background autofluorescence, thereby enabling enhanced penetration into biological tissues. The limited selection of fluorescent probes is a major bottleneck in SWIR fluorescence imaging. Here, we develop SWIR-emitting nanoparticles composed of donor-acceptor-type conjugated polymers. The bright SWIR fluorescence of the polymer dots (primarily attributable to their large absorption cross-section and high fluorescence saturation intensity (as high as 113 kW·cm-2)) enables the unprecedented detection of single particles as small as 14 nm through millimeter-thick turbid media. Unlike most SWIR-emitting nanomaterials, which have an excited-state lifetime in the range of microseconds to milliseconds, our polymer dots exhibit a subnanosecond excited-state lifetime. These characteristics enable us to demonstrate new time-gated single-particle imaging with a high signal-to-background ratio. These findings expand the range of potential applications of single-particle deep-tissue imaging.

The effect of Nd2O3 content on the properties and structure of Nd3+ doped TeO2–MgO–Na2O- glass

Abstract Understanding the influence of rare-earth ions (REIs) in tellurite glass is crucial for the development of efficient solid state laser. In this work, tellurite glasses containing Nd2O3 (0–2.0 mol%) were prepared via melt-quenching method to unveil the effect of different Nd3+composition in the environment of tellurite glass. The prepared glasses were characterized using several techniques to determine their physical, thermal, structural, optical properties and the lasing performance. XRD analysis confirmed the amorphous nature of the proposed glasses. Raman analysis shows that the glass with high content of Nd3+ experienced changes in network structure. Seven well-defined absorption bands from UV–Vis absorption spectra shows the transition from the ground state (4I3/2) to several excited states of Nd3+. The negative bonding parameter proved the ionic character possessed by Nd3+and ligands. The Judd-Ofelt intensity parameters (Ω2, Ω4, Ω6) and radiative properties were determined by Judd-Ofelt theory. The photoluminescence spectra (PL) revealed the lasing potency of the prepared glass in the infrared region at 0.87 μm, 1.06 μm, and 1.30 μm due to efficient emissions from 4F3/2 → 4I9/2, 4F3/2 → 4I11/2, and 4F3/2 → 4I13/2 transition respectively. The glass containing 0.5 mol% of Nd2O3 exhibited high stimulated emission cross-section (3.63 × 10−20 cm2), high quantum efficiency (57.47%), longer decay lifetime (177.61 μs), high optical gain (11.20 × 10−24 cm2) and low saturation intensity Is (2.91 × 108 W/m2) for 4F3/2 → 4I11/2 transition, revealing its suitability for 1.06 μm solid-state laser.

Unleashing the potential of Ti2CTx MXene as a pulse modulator for mid-infrared fiber lasers

The strong light–matter interaction in two dimensional (2D) materials portends an untapped potential for fiber lasers in the long sought-after terahertz to mid-infrared spectral range. Here, the broadband nonlinear optical properties of zero-gap 2D Ti2CTx MXene—a class of 2D transition metal carbides that exhibit high laser damage threshold—is discussed. Using the open aperture Z-scan method, for the first time the broadband saturable absorption properties of Ti2CTx in the 800–1560 nm spectral range are demonstrated. Specifically, it is shown that the high absorption and low saturation intensity of Ti2CTx make it an ideal saturable absorber for mode-locked fiber lasers operating at 1565 and 1051 nm, which exhibited pulse widths of 5.3 ps and 164 ps, respectively. Additionally, a Ti2CTx saturable absorber was used as a Q-switcher to realize a pulsed fiber laser operating at 2.8 µm, which showed excellent nonlinear absorption performance in the mid-infrared regime. This study significantly broadens the scope of materials available for non-linear optical applications in the near and mid-infrared regimes.

Iron Nanoparticles for Low-Power Local Magnetic Hyperthermia in Combination with Immune Checkpoint Blockade for Systemic Antitumor Therapy.

Magnetic hyperthermia (MHT) utilizing heat generated by magnetic nanoparticles under alternating magnetic field (AMF) is an effective local tumor ablation method but can hardly treat metastatic tumors. In this work, we discover that pure iron nanoparticles (FeNPs) with high magnetic saturation intensity after being modified by biocompatible polymers are stable in aqueous solution and could be employed as a supereffective MHT agent to generate sufficient heating under a low-power AFM. Effective MHT ablation of tumors is then achieved, using either locally injected FeNPs or intravenously injected FeNPs with the help of locally applied tumor-focused constant magnetic field to enhance the tumor accumulation of those nanoparticles. We further demonstrate that the combination of FeNP-based MHT with local injection of nanoadjuvant and systemic injection of anticytotoxic T-lymphocyte antigen-4 (anti-CTLA4) checkpoint blockade would result in systemic therapeutic responses to inhibit tumor metastasis. A robust immune memory effect to prevent tumor recurrence is also observed after the combined MHT-immunotherapy. This work not only highlights that FeNPs with appropriate surface modification could act as a supereffective MHT agent but also presents the great promises of combining MHT with immunotherapy to achieve long-lasting systemic therapeutic outcome after local treatment.

High saturation intensity in InAs/GaAs quantum dot solar cells and impact on the realization of the intermediate band concept at room-temperature

High optical saturation intensity at room temperature is reported for an ensemble of undoped quantum dots. The non-linearity of the light-generated-current under resonant excitation from the valence band to the intermediate band is shown to be made up of two components: a background two-photon absorption term and a resonant optical saturation term. It is argued that the solar intensity is much lower than the saturation intensities involved for the first and second transitions in the intermediate band solar cell under 1-sun illumination and therefore prevents exciting an appreciable amount of population in the terminal level that can be ionized to the continuum and generate an appreciable additional current. This additional current is required for enhancing the energy conversion efficiency of a solar cell based on the intermediate band concept. Operating at cryogenic temperatures leads to a reduction in the saturation intensity but it might not be sufficient for increasing the energy conversion efficiency, unless concentrated sun light, and/or high density of quantum dots, and/or quantum dots with a lifetime more comparable to the radiative lifetime are used. The conclusions of this paper are also expected to apply to other quantum dot systems.

Fabrication of High Performance Magnetic Rubber from NBR and Fe3O4via in Situ Compatibilization with Zinc Dimethacrylate

Fe3O4-based magnetic rubbers are desirable because they take advantage of high elasticity, large elongations, and high magnetic saturation intensity of each component. However, direct compounding of rubbers and Fe3O4 usually resulted in low mechanical properties which made the magnetic rubbers unable to satisfy practical applications. In this paper, we prepared nitrile butadiene rubber (NBR)/Fe3O4-based magnetic rubbers with good mechanical properties by means of in situ compatibilization using zinc dimethacrylate (ZDMA). Our strategy was based on the polymerization of ZDMA during a peroxide-induced vulcanization, in which the polymerized ZDMA (PZDMA) brought massive Zn2+ ion pairs into NBR. Zn2+ ion pairs not only increased the polarity of NBR but also strongly interacted with Fe3O4 nanoparticles. Because of the graft-PZDMA, the Zn2+ ion pairs finally strengthened interactions between Fe3O4 nanoparticles and NBR molecules. We hope this report laid an applied foundation for design of high performance magn...

Solvent effect on the nonlinear absorption of 5,10-A2B2meso substituted porphyrins

The effect of the solvent on the nonlinear absorptive properties of two series of 5,10-A2B2 porphyrins was investigated with an open Z-scan technique in the ns time regime. The recorded responses, which varied between compounds and solvents, were fitted to a four-level model where the one-photon excited state absorption is followed by a two-photon process arising from the higher excited states. For most of the compounds the positive nonlinear absorption in toluene was stronger than that in DMF and chloroform. This was attributed to enhanced two-photon absorption in toluene. For DMF and chloroform the solvent effects were most likely to be compound specific. It was demonstrated that the high saturation intensity of two-photon absorption shifts the RSA/SA turnover into a higher fluence range, which is desirable for optical limiting applications. This saturation intensity of two-photon absorption varied between compounds and solvents. Additionally, nonlinear scattering contributed strongly to the open Z-scan responses for many compounds in chlorobenzene and chloroform-chlorobenzene solutions. This was associated with the photodegradation of chlorobenzene.

High Photocatalytic Activity of Fe3O4‐SiO2‐TiO2 Functional Particles with Core‐Shell Structure

This paper describes a novel method of synthesizing Fe3O4‐SiO2‐TiO2 functional nanoparticles with the core‐shell structure. The Fe3O4 cores which were mainly superparamagnetic were synthesized through a novel carbon reduction method. The Fe3O4 cores were then modified with SiO2 and finally encapsulated with TiO2 by the sol‐gel method. The results of characterizations showed that the encapsulated 700 nm Fe3O4‐SiO2‐TiO2 particles have a relatively uniform size distribution, an anatase TiO2 shell, and suitable magnetic properties for allowing collection in a magnetic field. These magnetic properties, large area, relative high saturation intensity, and low retentive magnetism make the particles have high dispersibility in suspension and yet enable them to be recovered well using magnetic fields. The functionality of these particles was tested by measuring the photocatalytic activity of the decolouring of methyl orange (MO) and methylene blue (MB) under ultraviolet light and sunlight. The results showed that the introduction of the Fe3O4‐SiO2‐TiO2 functional nanoparticles significantly increased the decoloration rate so that an MO solution at a concentration of 10 mg/L could be decoloured completely within 180 minutes. The particles were recovered after utilization, washing, and drying and the primary recovery ratio was 87.5%.

Effect of the active medium temperature on the operation of fast-flow CO2 lasers

A new formula is obtained for estimating the output power of fast-flow CO2 lasers. It is shown that a higher specific output power of these CO2 lasers in comparison with sealed-off CO2 lasers is caused mainly by the higher saturation intensity of the former. It is concluded that the temperature of the laser active medium affects almost only the charge stability and that, under stable discharge conditions, the same output power can be obtained at different temperatures of the active medium.

Large cavity quantum cascade lasers with InP interstacks

Multicore quantum cascade lasers with large cavities show low optical losses, high saturation intensity, and low beam divergence. We present a four active core laser based on InGaAs–AlInAs material system emitting at 10.5 μm. To improve thermal conductance, InP interstacks were used to separate active regions. Selective lateral etching of the active regions was used to reduce optical losses. Peak powers up to 4.6 W, average powers up to 310 mW, and wall-plug efficiencies up to 4.6% were measured at 300 K. A far field with full width at half maximum of 18.7° and 49.8° in the lateral and in the growth direction, respectively, was observed.

Influence of magnetic quantum numbermand spin-angular symmetry on sequential double detachment in strong laser fields

We model sequential double detachment of ${\mathrm{Ag}}^{\ensuremath{-}}$, ${\mathrm{C}}^{\ensuremath{-}}$, ${\mathrm{I}}^{\ensuremath{-}}$, and ${\mathrm{Al}}^{\ensuremath{-}}$ irradiated by $1064\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ light by using a strong-field model and compare the theoretical double-detachment yields with experimental ones. The symmetry of the ${\mathrm{Al}}^{\ensuremath{-}}$ and ${\mathrm{C}}^{\ensuremath{-}}$ ground states ensures that the outermost electrons have different magnetic quantum numbers $m$. The difference in detachment rate for electrons with different $m$ values leads to a 50% higher saturation intensity than expected from application of the strong-field model with ionization rates for $m=0$ electrons only. For ${\mathrm{I}}^{\ensuremath{-}}$, emission of two $m=0$ electrons requires that the ${\mathrm{I}}^{+}$ ion is left in an excited state, and this leads to an increase of 20% in the saturation intensity. Good agreement is obtained with experiment for ${\mathrm{Ag}}^{\ensuremath{-}}$, ${\mathrm{C}}^{\ensuremath{-}}$, and ${\mathrm{I}}^{\ensuremath{-}}$, but a large discrepancy is observed for ${\mathrm{Al}}^{\ensuremath{-}}$. The calculations thus indicate that rearrangement of electrons over the available $m$ values may not occur for double detachment at low intensities. Finally, we explain why ${\mathrm{C}}^{\ensuremath{-}}$ and ${\mathrm{Al}}^{\ensuremath{-}}$ are well suited for further investigations on the effect of the distribution over $m$ values on sequential double detachment.