Nonlinear Saturable Absorption Research Articles

Filled Skutterudites for Broadband Saturable Absorbers

Filled skutterudites (FSs), which are known to be good thermoelectric materials, have been intensively investigated in the field of condensed matter physics in the past decades, due to their large variety of electronic and magnetic properties. However, to the best of the authors' knowledge, there has been no previous investigation of the optical properties of FSs. The nonlinear optical saturable absorption property of FSs is investigated in this work for the first time, and the results are reported here. More specifically, Co4Sb12 is chosen as a binary skutterudite system, and indium ions are added as a filling element to form a FS of In0.2Co4Sb12. The material properties are systematically investigated using a series of measurements. Density functional theory calculations of the electronic band structures of InCo4Sb12 and In0.25Co4Sb12 are conducted for better understanding of their energy band structures. It is also demonstrated that a broadband saturable absorber (SA) that can simultaneously operate at 1.5 and 1.9 µm can readily be implemented by depositing particles of In0.2Co4Sb12 onto fiber ferrules. The efficacy of the prepared SA is tested by incorporating the device into rare earth ion‐doped fiber ring cavities, to show its capability for both 1.5 and 1.9 µm operation.

Phosphorene quantum dot saturable absorbers for ultrafast fiber lasers

We fabricate ultrasmall phosphorene quantum dots (PQDs) with an average size of 2.6 ± 0.9 nm using a liquid exfoliation method involving ultrasound probe sonication followed by bath sonication. By coupling the as-prepared PQDs with microfiber evanescent light field, the PQD-based saturable absorber (SA) device exhibits ultrafast nonlinear saturable absorption property, with an optical modulation depth of 8.1% at the telecommunication band. With the integration of the all-fiber PQD-based SA, a continuous-wave passively mode-locked erbium-doped (Er-doped) laser cavity delivers stable, self-starting pulses with a pulse duration of 0.88 ps and at the cavity repetition rate of 5.47 MHz. Our results contribute to the growing body of work studying the nonlinear optical properties of ultrasmall PQDs that present new opportunities of this two-dimensional (2D) nanomaterial for future ultrafast photonic technologies.

Q-switched fiber laser based on saturable absorption of ferroferric-oxide nanoparticles

We demonstrate that ferroferric-oxide (Fe3O4) nanoparticles exhibit nonlinear saturable absorption property at 1.55 μm, and fabricate two filmy saturable absorbers by embedding the nanoparticles into a polyvinyl alcohol (PVA) film or polyimide (PI) film separately. In the Fe3O4-PVA (Fe3O4-PI) Q-switched fiber laser, the pulse repetition rate increases from 8.5 kHz (5.5 kHz) to 28 kHz (49 kHz) and the pulse duration decreases from 23.5 μs (47 μs) to 6 μs (3.5 μs) by varying the pump power from 25 mW (23 mW) to 150 mW (650 mW). Experiment results indicate that PI-based saturable absorbers can afford larger powers than PVA-based saturable absorbers, which can be attributed to the higher fusion point of the PI film. The Fe3O4-PI saturable absorber exhibits features of high damage threshold, low cost, and good flexibility, which could be applied in fields of near-infrared pulse generation and frequency conversions.

Investigation on Third-Order Nonlinear Optical Properties of Four Transition Metal Complexes Thin Films by Z-Scan Technique

In this work, four new transition metal complexes, [Zn(PTP)]12(1,4-NDA)12·2H2O (1), [Zn(PTP) (4,4′-ODA)]2 (2), [Ni(PTP)(4,4′-ODA)]2 (3) and [Ni(FTP)]2Cl4 (4), have been synthesized by the reaction between a metal salt, Zn(II) or Ni(II), a rigid polycarboxylic ligand, naphthalene-1,4-dicarboxylic acid (1,4-NDA) or 4,4′-oxydibenzoic acid (4,4′-ODA) and a co-ligand, 4′-phenyl-2,2′:6′,2″-terpyridine (PTP) or 4′-(furan-2-yl)-2,2′:6′,2″-terpyridine (FTP). Complex 1 based on {Zn(PTP)(1,4-NDA)}n intertwined chains characterized by three different configurations which form a chiral chain structure. Complexes 2 and 3 present a structure made of discrete rings containing two central metal atoms. In Complex 4, two complex Ni(FTP)2+ ions are found to be connected with the μ2–Cl bridge to form a binuclear cluster. The third-order nonlinear optical properties of each complex thin film were evaluated by the Z-scan technique, indicating a strong third-order nonlinear reverse saturable absorption effect, with a high absorption coefficient β 3.89 × 10−6 for 1, 40.50 × 10−6 for 2, 4.76 × 10−6 for 3, and 1.37 × 10−6 m W−1 for 1, respectively. Interestingly, Complex 1 exhibits an effective nonlinear refractive effect with a refractive coefficient of 7.16 × 10−13 m2 W−1. The third-order NLO susceptibilities χ (3) of each complexes were calculated.

Nonlinear Saturable and Polarization-induced Absorption of Rhenium Disulfide

Monolayer of transition metal dichalcogenides (TMDs), with lamellar structure as that of graphene, has attracted significant attentions in optoelectronics and photonics. Here, we focus on the optical absorption response of a new member TMDs, rhenium disulphide (ReS2) whose monolayer and bulk forms have the nearly identical band structures. The nonlinear saturable and polarization-induced absorption of ReS2 are investigated at near-infrared communication band beyond its bandgap. It is found that the ReS2-covered D-shaped fiber (RDF) displays the remarkable polarization-induced absorption, which indicates the different responses for transverse electric (TE) and transverse magnetic (TM) polarizations relative to ReS2 plane. Nonlinear saturable absorption of RDF exhibits the similar saturable fluence of several tens of μJ/cm2 and modulation depth of about 1% for ultrafast pulses with two orthogonal polarizations. RDF is utilized as a saturable absorber to achieve self-started mode-locking operation in an Er-doped fiber laser. The results broaden the operation wavelength of ReS2 from visible light to around 1550 nm, and numerous applications may benefit from the anisotropic and nonlinear absorption characteristics of ReS2, such as in-line optical polarizers, high-power pulsed lasers, and optical communication system.

Generators of High-Power Ultrashort Microwave Pulses with a Saturable Absorber in a Feedback Circuit

We study theoretically the mechanism of the formation of high-power ultrashort microwave pulses, which is based on the effect of passive mode locking in electron generators, whose feedback circuit includes a nonlinear saturable absorber (amplitude filter). The mechanism under consideration is an analog of the well-known method of generation of ultrashort pulses, which is used widely in laser physics. Nevertheless, realization of this method in electron generators has certain specific features connected with the motion of the active medium (electron flow). It is shown that being applied to vacuum electronics, this method is sufficiently universal, and various types of both relativistic and subrelativistic amplifiers operated in wavelength ranges from the centimeter to submillimeter ones can be used for actualization of ultrashort pulse generators as active elements. In the additional section, which acts as a saturable absorber, one can use either the Kompfner absorption effect, or the effect of cyclotron resonance absorption of radiation by a rectilinear electron beam. It is demonstrated on the basis of numerical simulation that the peak power of ultrashort pulses in electron generators with passive mode locking can exceed the power achieved in the stationary generation regimes significantly and, in some cases, the power of the electron flow.

Dielectric, etching and Z-scan studies of glycine doped potassium thiourea chloride crystal

Abstract Glycine doped potassium thiourea chloride (PTC) crystal has been grown by slow solution evaporation technique. The dielectric studies have been employed to examine substantial improvement in dielectric constant and dielectric loss of glycine doped PTC crystal. The etching studies have been performed to investigate the surface quality of this crystal. The z-scan studies have been carried out at 632.8 nm to explore the third order nonlinear optical nature. The negative nonlinear refraction of glycine doped PTC crystal was found to be of 7.27 × 10−12 cm2/W. The origin of high magnitude of third order nonlinear optical susceptibility and reverse saturable nonlinear absorption have been investigated. The obtained results were explored to discuss the nonlinear optical applications of PTC crystal.

“Closer is Better and Two is Superior to One”: Third-Order Optical Nonlinearities of a Family of Porphyrin–Anderson Type Polyoxometalate Hybrid Compounds

A family of covalently bonded hybrid compounds composed of Anderson type polyoxometalate (POM) moiety and porphyrin moiety have been synthesized and thoroughly characterized. The compounds all show remarkable nonlinear reverse saturable absorption and self-defocusing effect at 532 nm with a pulse duration τ = 6 ns, rendering them promising candidate materials for device applications in photonics and optoelectronics. More importantly, it is found that the hybrid wherein POM is coupled covalently to porphyrin through shorter bridge has an NLO response superior to the hybrid wherein POM is bonded via longer bridge to porphyrin, and the hybrid having two porphyrins connected to POM shows more enhancement than the hybrid having single porphyrin fused to POM. Disclosure of the inherent structure–property relationship is expected to be instructive for exploration of new porphyrin-POM based NLO materials. Meantime, the hybrid compounds have optical-limiting thresholds lower than 1.0 J/cm2, implying their high pot...

Ultra-broadband Nonlinear Saturable Absorption for Two-dimensional Bi2TexSe3-x Nanosheets.

We report the ultra-broadband nonlinear optical (NLO) response of Bi2TexSe3−x nanosheets produced by a facile solvothermal method. Our result show that the extracted basic optical nonlinearity parameters of Bi2TexSe3−x nanosheets, αNL, Imχ(3), and FOM reach ~104 cm/GW, ~10−8 esu and ~10−13 esu cm, respectively, which are several orders of magnitude larger than those of bulk dielectrics. We further observed the excitation intensity dependence of the NLO absorption coefficient and the NLO response sensitivity. The mechanisms of those phenomena were proposed based on physical model. The wavelength dependence of the NLO response of Bi2TexSe3−x nanosheets was investigated, and we determined that the Bi2TexSe3−x nanosheets possess an ultra-broadband nonlinear saturable absorption property covering a range from the visible to the near-infrared band, with the NLO absorption insensitive to the excitation wavelength. This work provide fundamental and systematic insight into the NLO response of Bi2TexSe3−x nanosheets and support their application in photonic devices in the future.

Nonlinear optical responses in two-dimensional transition metal dichalcogenide multilayer: WS2, WSe2, MoS2 and Mo 0.5 W0.5 S2.

Synthetic two-dimensional transition metal dichalcogenides such as, tungsten disulphide (WS2), tungsten diselenide (WSe2), molybdenum disulphide (MoS2) as well as mixed molybdenum tungsten disulphide (Mo0.5W0.5S2) single crystals were grown by the chemical vapor transport method using halogens (bromine or chlorine) as transport agents. Multi- layer samples were cleaved from the single crystals, and their nonlinear optical (NLO) properties were obtained from both open aperture and closed aperture Z-scan measurements using a picosecond mode-locked Nd: YAG laser operating at a wavelength of 1064 nm, with pulse duration of 25 ps, and 20 Hz repetition rate. Both WS2 and MoS2 exhibited nonlinear saturable absorption (SA), whereas WSe2 and Mo0.5W0.5S2 showed nonlinear two-photon absorption (2PA). A large 2PA coefficient β as high as + 1.91x10-8 cm/W was obtained for the Mo0.5W0.5S2, and an index of refraction coefficient γ = -2.47x10-9 cm2/W was obtained for the WSe2 sample.

WS2 as a saturable absorber for Q-switched 2 micron lasers.

We prepared WS2 nanosheets by using the thermal decomposition method and demonstrated for the first time its nonlinear saturable absorption property at around 2 μm. With the as-prepared WS2 nanosheets as saturable absorber (SA), a passively Q-switched Tm:LuAG laser was realized successfully, and 660 ns laser pulses with an average output power of 1.08 W and pulse peak power of 26 W at a repetition rate of 63 kHz were obtained for an incident pump power of 7 W. Our experimental results definitely demonstrate that WS2 could be a kind of promising SA for solid-state 2 μm lasers.

Bound Soliton Fiber Laser Mode-Locking Without Saturable Absorption Effect

Stable bound soliton laser mode-locking is experimentally demonstrated in an environmentally stable hybrid mode-locked Er-doped fiber laser without incorporating (equivalent) nonlinear saturable absorption effects. The laser has a sigma-type cavity and exhibits a higher threshold for single to bound soliton mode-locking state transition. A nonlinear eigenstate analysis based on the master equation model and the concept of steady-state lasing gain is developed to more physically explain the observed mode-locking state transition phenomena.

Ultra-broadband nonlinear saturable absorption of high-yield MoS2 nanosheets

High-yield MoS2 nanosheets with strong nonlinear optical (NLO) responses in a broad near-infrared range were synthesized by a facile hydrothermal method. The observation of saturable absorption, which was excited by the light with photon energy smaller than the gap energy of MoS2, can be attributed to the enhancement of the hybridization between the Mo d-orbital and S p-orbital by the oxygen incorporation into MoS2. High-yield MoS2 nanosheets with high modulation depth and large saturable intensity generated a stable, passively Q-switched fiber laser pulse at 1.56 μm. The high output power of 1.08 mW can be attained under a very low pump power of 30.87 mW. Compared to recently reported passively Q-switched fiber lasers utilizing exfoliated MoS2 nanosheets, the efficiency of the laser for our passive Q-switching operation is larger and reaches 3.50%. This research may extend the understanding on the NLO properties of MoS2 and indicate the feasibility of the high-yield MoS2 nanosheets to passively Q-switched fiber laser effectively at low pump strengths.

Nonlinear optical switching and optical limiting in colloidal CdSe quantum dots investigated by nanosecond Z-scan measurement

The semiconductor nanocrystals are found to be promising class of third order nonlinear optical materials because of quantum confinement effects. Here, we highlight the nonlinear optical switching and optical limiting of cadmium selenide (CdSe) quantum dots (QDs) using nanosecond Z-scan measurement. The intensity dependent nonlinear absorption and nonlinear refraction of CdSe QDs were investigated by applying the Z-scan technique with 532nm, nanosecond laser pulses. At lower intensities, the nonlinear process is dominated by saturable absorption (SA) and it is changed to reverse saturable absorption (RSA) at higher intensities. The SA behaviour is attributed to the ground state bleaching and the RSA is ascribed to free carrier absorption (FCA) of CdSe QDs. The nonlinear optical switching behaviour and reverse saturable absorption makes CdSe QDs are good candidate for all-optical device and optical limiting applications.

Generation of “gigantic” ultra-short microwave pulses based on passive mode-locking effect in electron oscillators with saturable absorber in the feedback loop

A periodic train of powerful ultrashort microwave pulses can be generated in electron oscillators with a non-linear saturable absorber installed in the feedback loop. This method of pulse formation resembles the passive mode-locking widely used in laser physics. Nevertheless, there is a specific feature in the mechanism of pulse amplification when consecutive energy extraction from different fractions of a stationary electron beam takes place due to pulse slippage over the beam caused by the difference between the wave group velocity and the electron axial velocity. As a result, the peak power of generated “gigantic” pulses can exceed not only the level of steady-state generation but also, in the optimal case, the power of the driving electron beam.

Extending resolution of scanning optical microscopy beyond the Abbe limit through the assistance of InSb thin layers

The resolution of light imaging is required to extend beyond the Abbe limit to the subdiffraction, or even nanoscale. In this Letter, we propose to extend the resolution of scanning optical microscopy (SOM) beyond the Abbe limit as a kind of subdiffraction imaging technology through the assistance of InSb thin layers due to obvious nonlinear saturation absorption and reversible formation of an optical pinhole channel. The results show that the imaging resolution is greatly improved compared with the SOM itself. This work provides a way to improve the resolution of SOM without changing the SOM itself, but through the assistance of InSb thin layers. This is also a simple and practical way to extend the resolution of SOM beyond the Abbe limit.

Erbium-doped fiber laser passively mode locked with few-layer WSe2/MoSe2 nanosheets.

Few-layer transition-metal dichalcogenide WSe2/MoSe2 nanosheets are fabricated by a liquid exfoliation technique using sodium deoxycholate bile salt as surfactant, and their nonlinear optical properties are investigated based on a balanced twin-detector measurement scheme. It is demonstrated that both types of nanosheets exhibit nonlinear saturable absorption properties at the wavelength of 1.55 μm. By depositing the nanosheets on side polished fiber (SPF) or mixing the nanosheets with polyvinyl alcohol (PVA) solution, SPF-WSe2 saturable absorber (SA), SPF-MoSe2 SA, PVA-WSe2 SA, and PVA-MoSe2 SA are successfully fabricated and further tested in erbium-doped fiber lasers. The SPF-based SA is capable of operating at the high pump regime without damage, and a train of 3252.65 MHz harmonically mode-locked pulses are obtained based on the SPF-WSe2 SA. Soliton mode locking operations are also achieved in the fiber laser separately with other three types of SAs, confirming that the WSe2 and MoSe2 nanosheets could act as cost-effective high-power SAs for ultrafast optics.

Nonlinear optical properties and optical power limiting effect of Giemsa dye

The nonlinear optical properties of Giemsa dye in chloroform solution for different concentrations and dye mixed with poly(methylmethacrylate) (PMMA) as a dye-doped polymer film were investigated using continuous wave (CW) low power solid-state laser (SSL) operating at wavelength of 532nm as an excitation source. Using the single beam z-scan technique, the nonlinear refractive index (n2), the nonlinear absorption coefficient (β), and the third-order nonlinear optical susceptibility (χ(3)) of Giemsa dye were measured. The measurements reveal that both n2 and β are dependent on the dye concentration. The obtained results indicate that the Giemsa dye exhibits positive nonlinear saturable absorption (SA) and negative refraction nonlinearity, manifestation of self-defocusing effect. Optical power limiting characteristics of the Giemsa dye at different concentrations in solution and polymer film were studied. The observed large third-order optical nonlinearity of Giemsa dye confirms that Giemsa dye is a promising nonlinear material for the optical power limiting and photonic devices applications.

Q-switched waveguide laser based on two-dimensional semiconducting materials: tungsten disulfide and black phosphorous.

Owing to their unique properties, graphene-like two dimensional semiconducting materials, including Tungsten Disulfide (WS2) and Black Phosphorous (BP), have attracted increasing interest from basic research to practical applications. Herein, we demonstrated the ultrafast nonlinear saturable absorption response of WS2 and BP films in the waveguide structure. Through fabricating WS2 and BP films by evaporating the solutions on glass wafers. Saturable absorber films were attached onto the end-facet of the waveguide, which therefore constitutes a resonant cavity for the waveguide laser. Under a pump laser at 810 nm, we could obtain a stable Q-switched operation in the waveguide structure. This work indicated the significant potential of WS2 and BP for the ultrafast waveguide laser.

Nonlinear saturable absorption of nanoscaled Bi2Te3/PMMA composite film

In order to increase metallic state of surface and elevate dispersion of Bi2Te3 nanoparticles, we synthesized nanoscaled Bi2Te3 with a size of 91.4±2.2nm via a hydrothermal method. Bi2Te3 nanoparticles exhibit narrow distribution in size and uniform dispersion in several solvents, such as ethanol and polymethyl methacrylate (PMMA) solution. A Bi2Te3/PMMA flexible composite film was fabricated to further guarantee the uniformity of dispersion during the application. An open-aperture Z-scan technique was utilized to verify saturable absorption response of the Bi2Te3/PMMA composite film under 130fs pulse at a wavelength of 800nm. The nonlinear absorption coefficient β was fitted to be ~10−11m/W, and the value of β slightly increased as the incident laser strengthened. A modulation depth of 15.1% and a saturation intensity of 18.9GW/cm2 for the composite film were also calculated. Our investigation suggests that the nanoscaled Bi2Te3/PMMA composite film could potentially be applied in large-energy laser pulses due to its high saturable intensity, and which might enlarge the application range of topological insulator (TI) materials.