Saturable Absorption Mechanism Research Articles

One- and Two-Photon Photophysical Properties, Ultrafast Dynamics, and DFT Study of Three Modified Zinc Phthalocyanines.

As two-photon absorption (TPA) materials, phthalocyanine molecules have promising application prospects due to their large TPA absorption cross-section, high third-order nonlinear optical susceptibility, and ultrafast response characteristics. In this work, optical properties and the ultrafast response of three modified zinc phthalocyanine molecules (P-HPcZn, Pc-P-Pc, and (DR1)4PcZn) were analyzed. No obvious side-shoulder absorption peaks in the Q-band can be observed from the steady-state absorption spectra of the three molecules, confirming the lack of aggregation products in the solutions of our measurement. Open-aperture Z-scan results show relatively large TPA cross-section values of 136.4 and 55.3 GM for Pc-P-Pc and (DR1)4PcZn, respectively. The nonlinear optical results show that the absorption process observed under the excitation of femtosecond pulses is a reverse saturable absorption (RSA) mechanism. Up-conversion fluorescence spectra of (DR1)4PcZn in THF solution indicate that the fluorescence emission mechanism is TPA. In the study of ultrafast dynamics, the transient absorption spectra were investigated and the decay lifetime of the dynamic traces corresponding to some representative probe wavelengths was obtained through data fitting with a multi-exponential function. Finally, the charge transfer and excited state properties of the modified zinc phthalocyanine molecules were discussed in depth by the DFT method. The energy gaps of P-HPcZn, Pc-P-Pc, and (DR1)4PcZn are 2.16, 1.39, and 2.13 eV, respectively. The results indicate that the Pc-P-Pc of donor-acceptor-donor (D-A-D) structure has the smallest energy gap as well as the best charge transfer properties.

Inverse Saturable Absorption Mechanism in Mode-Locked Fiber Lasers with a Nonlinear Amplifying Loop Mirror

From the perspective of the differential phase delay experienced by the two counterpropagating optical fields, the self-starting of the mode-locked fiber laser with a non-linear amplifying loop mirror (NALM) is theoretically studied. Although it is generally believed that NALM shows a saturable absorption effect on both continuous wave (CW) light and pulses, we find a counter-intuitive fact that cross-phase modulation (XPM) leads to opposite signs of differential non-linear phase shifts (NPSs) in these two cases, resulting in inverse saturable absorption (ISA) during the pulse formation process. The ISA is not helpful for the self-starting of laser mode-locking and can be alleviated by introducing a non-reciprocal phase shifter into the fiber loop. These results are helpful for optimizing the design of NALM and lowering the self-starting threshold of the high-repetition-rate mode-locked fiber laser.

Graded-index multimode fiber in nonlinear absorbing loop mirror as saturable absorber for noise-like pulse generation in 2 μm region

This investigation explores the potential use of graded-index multimode fiber (GIMF) in a nonlinear absorbing loop mirror (NAbLM) as an artificial saturable absorber for ultrashort pulse generation. The saturable absorption mechanism of GIMF-NAbLM was based on a combination of nonlinear phase shift and multimodal interference that was satisfied by the asymmetric positioning of the GIMF in the single-mode fiber loop mirror. The proposed saturable absorber was integrated in a mode-locked thulium-doped fiber laser with figure-of-eight cavity. A noise-like pulse of 164 fs spike pulse duration at 1950 nm central wavelength was attained in the net anomalous dispersion with a 10 m long GIMF. The GIMF-NAbLM structure demonstrates immense prospects as an effective saturable absorption device for generating stable ultrashort pulse in the 2 µm region due to its simplicity.

Nonlinear optical property measurements of rhenium diselenide used for ultrafast fiber laser mode-locking at 1.9\xa0\u03bcm

An experimental investigation into the nonlinear optical properties of rhenium diselenide (ReSe2) was conducted at a wavelength of 1.9 μm using the open-aperture and closed-aperture Z-scan techniques for the nonlinear optical coefficient (β) and nonlinear refractive index (n2) of ReSe2, respectively. β and n2 measured at 1.9 μm were ~ − 11.3 × 103 cm/GW and ~ − 6.2 × 10–2 cm2/GW, respectively, which to the best of our knowledge, are the first reported measurements for ReSe2 in the 1.9-μm spectral region. The electronic band structures of both ReSe2 and its defective structures were also calculated via the Perdew–Becke–Erzenhof functional to better understand their absorption properties. A saturable absorber (SA) was subsequently fabricated to demonstrate the usefulness of ReSe2 for implementing a practical nonlinear optical device at 1.9 μm. The 1.9-μm SA exhibited a modulation depth of ~ 8% and saturation intensity of ~ 11.4 MW/cm2. The successful use of the ReSe2-based SA for mode-locking of a thulium–holmium (Tm–Ho) co-doped fiber ring cavity was achieved with output pulses of ~ 840 fs at 1927 nm. We believe that the mode-locking was achieved through a hybrid mechanism of saturable absorption and nonlinear polarization rotation.

Tunneling-induced optical limiting in quantum dot molecules

We present a convenient way to obtain an optical power limiting behavior in a quantum dot molecule system, induced by an interdot tunneling. Also, the effect of system parameters on the limiting performance is investigated; interestingly, the tunneling rate can affect the limiting performance of the system so that the threshold of the limiting behavior can be a function of the input voltage, allowing the optimization of the limiting action. Furthermore, by investigating the absorption of the probe field, it is demonstrated that the optical limiting is due to a reverse saturable absorption mechanism; indeed, analytical results show that this mechanism is based on a cross-Kerr optical nonlinearity induced by the tunneling. Additionally, the limiting properties of the system are studied by using a Z-scan technique.

Structural and femtosecond third-order nonlinear optical properties of electron donor – acceptor substituted chalcones: An experimental and computational approach

Structural and third-order nonlinear optical (NLO) properties of two D-π-A-π-A [ (2E)-3-(3-chlorophenyl)-1-(3-nitrophenyl)prop-2-en-1-one (3C3NC) and (2E)-3-(4-fluorophenyl)-1-(4-nitrophenyl)prop-2-en-1-one (FNC)] and one D-π-A-π-D [(2E)-1-(9-anthryl)-3-(4-fluorophenyl) prop-2-en-1-one (FANC)] type chalcone derivatives were investigated experimentally as well as theoretically. All the three chalcones are centrosymmetric (CS) and crystallized in P21/c space group. In crystal packing, all compounds exhibit C–H⋯O hydrogen bonds and π···π interactions. From UV–Vis–NIR absorption spectra, the cut-off wavelength and optical band gaps of the chalcones were evaluated. The thermal stability and the melting point of the chalcones were determined from the TGA/DTA techniques. The ultrafast third-order NLO properties of the three chalcones in solution (0.01 M) were evaluated by Z-scan technique using femtosecond (fs) pulses from Ti: Sapphire laser (∼150 fs, 80 MHz, 800 nm). Third-order nonlinear absorption (NLA) coefficient (β ∼ 10−9 cmW−1), nonlinear refraction (NLR) index (n2∼10−14 cm2W−1), susceptibility (χ(3) ∼ 10−12 esu) and molecular hyperpolarizability (γh ∼ 10−31 esu) were unambiguously determined. The chalcones satisfy the condition to observe reverse saturable absorption (RSA) mechanism (σex > σg). Further, the larger values of σex/σg (∼2–3) indicate that the chalcones would exhibit remarkable optical limiting property. The estimated optical limiting (OL) threshold values are in the order of μJ/cm2. Further, all the chalcones have satisfied the one-photon and two-photon figures of merit conditions (W > 1 and T < 1) for the optical switching applications. In addition to experimental findings, the optimized geometry, HOMO-LUMO energy gap, NLO parameters such as electronic dipole moment, polarizability, first and second hyperpolarizabilities of all the three chalcones were determined theoretically (DFT) for different wavelength at B3LYP/6-311 + G (d,p) basic set. The experimentally determined second-order susceptibilities (Z-scan technique) are reasonably in agreement with the computational values (TD-DFT). The MEP surface revealed that the 3C3NC, FNC and FANC molecules have some potential sites for electrophilic and nucleophilic attack. The experimental and computational NLO data clearly suggest that the nonlinearity of chalcone molecules greatly depends on the electron donor and acceptor strengths of the substituents. Further, all the chalcones are potential materials for optical limiting and optical switching applications.

The effects of intersystem and reverse intersystem crossing on the optical limiting properties of C60

By presenting a theoretical steady-state rate equation analysis, we demonstrate an efficient optical power limiting in a fullerene system, which can be attributed to a reverse saturable absorption mechanism. The limiting behavior, characterized by means of the sample’s transmittance, is explained via the population redistribution among singlet and triplet levels. In addition, the effects of intersystem crossing and reverse intersystem crossing rates on the limiting behavior are investigated. Our numerical results show how these rates alter the optical limiting performance; indeed, the intersystem crossing has a constructive role in decreasing the optical limiting threshold. In addition, a theoretical investigation of the z-scan technique is employed to further characterize the power limiting properties.

Excited state absorption and relaxation dynamics in a series of heptamethine dyes under femtosecond and nanosecond excitations

We show that the near-infrared-absorbing polymethine cyanine dyes exhibit nonlinear absorption (NLA) in the visible region, where they have negligible or very low linear absorption. In chloroform solvent, the studied cyanines were found to be photo-unstable and exhibited photo-degradation under prolonged exposures as compared to methanol and dimethyl sulphoxide solvents, where they were stable. Excitation with femtosecond (fs) pulses in the visible region (400–750 nm) exhibits excited state absorption (ESA) as characterized by positive differential absorption (ΔOD) in transient absorption signal. Non-resonant excitations in the visible region are governed by reverse saturable absorption (RSA) mechanism leading to ESA. Excitation with nanosecond pulses also shows NLA owing to ESA from triplet states as compared to that from singlet states under fs excitations. Single beam Z-scan studies were performed with nanosecond (ns) pulses to evaluate ground (σgr) and excited state (σex) absorption cross-sections, which confirms the mechanism of RSA as σex were found to be greater than σgr. The population relaxation from higher excited singlet states shows ultrafast behavior with multi-exponential decay components. The fast time component decay varies from a few hundreds of fs (τ1) to some hundreds of picoseconds (τ2), and the long-time decay is in the ns domain (τ3).

Defects-induced nonlinear saturable absorption mechanism in europium-doped ZnO nanoparticles synthesized by facile hydrothermal method

In the present work, the effect of europium (Eu) dopant on the third-order nonlinear optical (NLO) responses of ZnO nanoparticles (NPs) is investigated. The NPs are synthesized by facile hydrothermal technique using saponin as a capping agent. The samples are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), Energy-dispersive spectroscopy (EDS), UV–visible spectroscopy and Room-temperature photoluminescence spectroscopy (RTPL). The synthesized NPs exhibit wurtzite crystal structure with good crystallinity. Williamson–Hall analysis of XRD pattern revealed the presence of dopant induced compressive strain in the crystal lattice of Eu doped ZnO NPs. The capping of saponin on the surface of NPs is confirmed by FTIR study. A significant decrease in the optical band gap of Eu-doped ZnO NPs attributed to the presence of extended defect states below the conduction band. An enhancement in the blue luminescence is observed for the highest doping level (5 at. wt% of Eu), and this is attributed to defects-related emission in the NPs. The third-order NLO properties of the NPs are explored by a single beam Z-scan technique by employing continuous-wave laser of wavelength 532 nm. The open aperture Z-scan curves showed a remarkable switching from reverse saturable absorption to saturable absorption in 5 at. wt% Eu-doped ZnO NPs is caused due to the ground-state bleaching effect. Further, the closed aperture Z-scan curves exhibit the positive nonlinear refractive index in the 5 at. wt% Eu-doped ZnO NPs attributed to the thermo-optical effects.

Photophysical and nonlinear optical characteristics of pyridyl substituted phthalocyanine - Detonation nanodiamond conjugated systems in solution

In this study photophysical, nonlinear absorption and optical limiting properties of detonation nanodiamonds (DNDs)-phthalocyanine nanoconjugate systems containing: 2,9(10),16(17),23(24)-tetrakis-(4-pyridyloxy) phthalocyaninato (H2TPPc), 2,9(10),16(17),23(24)-tetrakis-(4-pyridyloxy) phthalocyanato zinc(II) (ZnTPPc) and 2,9(10),16(17),23(24)-tetrakis-(4-pyridyloxy) phthalocyanato silicon(IV) hydroxide (Si(OH)2TPPc), were investigated in dimethylsulfoxide solution. Pcs were non-covalently linked to nanondiamonds (also covalently linked for Si(OH)2TPPc) and investigated using 532 nm laser excitation at 10 ns pulses for their optical limiting properties. Complexes that have higher triplet state absorption also possessed enhanced nonlinear optical behaviour following reverse saturable absorption mechanism. Superior optical performance is observed when the Pcs had a central metal with axial ligands conjugated to DNDs in solution. Nanoconjugate of DNDs-Si(OH)2TPPc and respective Pc in solution gave the highest imaginary third-order susceptibility (Im[X(3)]) and hyperpolarizability (γ) at 2.91 × 10−8 and 3.17 × 10−8 esu and 3.88 × 10−28 and 4.22 × 10−28 esu, respectively, with Ilim value of 0.47 and 0.39 J·cm−2.

Photo-induced excitonic structure renormalization and broadband absorption in monolayer tungsten disulphide.

Atomically thin transition metal dichalcogenides (TMDCs) have emerged as a new class of two-dimensional (2D) material for novel optoelectronic applications. In particular, 2D TMDCs are viewed as intriguing and appealing materials to construct Q-switching and mode-locked modulators, due to their broadband saturable absorption even of photon energy below their excitonic energies. However, the dynamics and mechanism of saturable absorption inside TMDCs has yet to be investigated. In this paper, the relaxation dynamics of monolayer tungsten disulphide (WS2) was investigated considering different excitonic transitions. WS2 illustrates dramatic changes in optical responses when excited by intense laser pulses, which are characterized by the broadband photo-induced nonresonance absorption and the giant excitonic bands renormalization process. The experimental results show that strong photo-induced restructuring of excitonic bands has picosecond lifetime and full recovery of optical responses takes hundreds of picosecond. Additionally, our observations reveal that heavy renormalization and overlap of excitonic bands are induced by strong many-body Coulomb interactions. Moreover, the broadband absorption feature of WS2 opens up new applications in broadband saturable absorbers and ultrafast photonic devices.

Influence of Ga doping ratio on the saturable absorption mechanism in Ga doped ZnO thin solid films processed by sol–gel spin coating technique

In the present study, the nonlinear optical properties of sol–gel spin coated gallium doped zinc oxide (GZO) thin solid films are explored with nanosecond laser pulses using the z-scan technique. The higher doping ratios of Ga result in a large redshift of the energy gap (0.38 eV) due to the existence of enhanced grain boundary defects in GZO films. A positive nonlinear absorption coefficient is observed in undoped 1 at.wt.% GZO and 2 at.wt.% GZO films, and a negative nonlinear absorption coefficient in 3 at.wt.% GZO film. Fewer defects in undoped 1% GZO and 2% GZO films resulted in reverse saturable absorption (RSA), whereas a saturable absorption (SA) mechanism is observed in 3% GZO films and is attributed to the enhanced defect concentration in the band structure of GZO. However, all the films showed a self-defocusing mechanism, derived by a closed aperture z-scan technique. The present work sheds light on the defect mechanism involved in the observed nonlinear properties of GZO films.

Retraction: Third order nonlinear properties in the 1,1′-Methylenedipyridinium bis[tetrachloridoaurate(III)] using Z-Scan technique

The third-order nonlinear optical properties of the 1,1'-Methylenedipyridinium bis[tetrachloridoaurate(III)] Dimethyl sulfoxide (DMSO) have been studied using Z-scan technique. Measurements were performed with a CW diode laser at 635 nm wavelength and 16.4 mW power. The nonlinear optical absorption coefficient (β) and nonlinear refraction (n2) were found at 7.45×10-4 cm/W and 1.08×10-8 cm2/W, respectively. The nonlinear absorption coefficient β decrease with increase in on-axis input intensity I0. The cross-section of the excited state (σex) was found to be larger than the cross-section of the ground state (σg). This feature confirmed that the reverse saturable absorption (RSA) mechanism was responsible for the optical limiting effect. The results suggest that this material is considered as a promising candidate for future optical devices applications.

Nonlinear optical study of 1-(carboxymethyl)-8-hydroxyquinolin-1-ium chloride and 1-(carboxymethyl)quinolin-1-ium chloride salts by Z-scan technique

Z-scan technique was used to investigate the nonlinear optical properties of 1-(carboxymethyl)-8-hydroxyquinolin-1-ium chloride and 1-(carboxymethyl)quinolin-1-ium chloride salts. The new 1-(carboxymethyl)-8-hydroxyquinolin-1-ium chloride and 1-(carboxymethyl)quinolin-1-ium chloride salts were synthesized and characterized using UV–visible, FTIR and NMR measurements and the characterization spectra confirm the expected molecular structure of the prepared salts. Measurements were performed with a CW Diode laser at 635nm wavelength and 26mW power. The nonlinear optical absorption coefficient (β) and nonlinear refractive index (n2) of the 1-(carboxymethyl)-8-hydroxyquinolin-1-ium chloride was affected by OH group. The excited-state absorption cross sections (σex) and the ground -state absorption cross sections (σg) were calculated for the two studied compounds. It was found that the σex is larger than the σg, indicating that the reverse saturable absorption mechanism (RSA) is the dominating mechanism for the observed absorption nonlinearities.Our results suggest that this material should be considered as a promising candidate for future optical devices applications.

Semiconductor Diode Laser Mode-Locking by a Waveguide Array

We demonstrate theoretically that robust mode-locking can be achieved in an edge-emitting semiconductor diode laser with a waveguide array architecture. The waveguide array provides an ideal saturable absorption mechanism for initial start-up from noise, as well as pulse shaping and stabilization. One element in the waveguide array is forward-biased to generate gain, while nonlinear coupling to the other elements results in an effective saturable absorber. This new approach can be integrated directly with standard edge-emitting designs.

Z-scan measurements of single walled carbon nanotube doped acetylenedicarboxylic acid polymer under CW laser

Z-scan measurements of single walled carbon nanotube (SWCNT) doped with acetylenedicarboxylic acid (ADC) polymer are performed using a CW diode laser at 635nm wavelength with 17mW power. The nonlinear absorption coefficient (β), nonlinear refractive index (n2), the real and imaginary parts of the third-order nonlinear optical susceptibility (Re χ3), (Im χ3) of the investigated samples are calculated. It was found that the β values decrease with increase in on-axis input intensity I0. Also, these values are found to be proportional with sample concentrations. The excited-state absorption cross sections were calculated to be at σex=5.08×10−14cm2 for the (SWCNT) and at 15.1×10−14cm2 for the ADC polymer. It was found that the σex is larger than ground-state absorption cross sections, indicating that the reverse saturable absorption mechanism (RSA) is the dominating mechanism for the observed absorption nonlinearities.

Nonlinear optical properties of zinc oxide doped bismuth thin films using Z-scan technique

ZnO doped Bi thin films were grown on glass substrates by spray ultrasonic technique. This paper presents the effect of Bi doping concentration on structural and nonlinear optical properties of zinc oxide thin films. These thin films were characterized by X-ray diffractometer technique. XRD analysis revealed that the ZnO:Bi thin films indicated good preferential orientation along c-axis perpendicular to the substrate. The nonlinear optical properties such as nonlinear absorption coefficient (β) and third order nonlinear susceptibility (Imχ(3)) are investigated. The calculations have been performed with a Z scan technique using Nd:YAG laser emitting 532nm. The reverse saturable absorption (RSA) mechanism was responsible for the optical limiting effect. The results suggest that this material considered as a promising candidate for future optical device applications.

Optical nonlinearity investigation of the tetracarbonyl(2,2′-bipyridyl)molybdenum(0) using Z-scan technique

Abstract The third order nonlinear optical properties of the Tetracarbonyl(2,2′-bipyridyl)molybdenum(0) in CHCl 3 solution have been studied using Z-scan technique. Measurements were performed with a CW Diode laser at 635 nm wavelength and 15 mW power. The nonlinear optical absorption coefficient ( β ) and nonlinear refraction ( n 2 ) were found at 2.86 × 10 −3 cm/W and 4.28 × 10 −7 cm 2 /W respectively. The nonlinear absorption coefficient β values decrease with increase in on-axis input intensity I 0 . The reverse saturable absorption (RSA) mechanism was responsible for the optical limiting effect. The results suggest that this material is considered as a promising candidate for future optical devices applications.

Nonlinear dynamics and optical power limiting of nanoseconds pulses in naphthalocyanines and phthalocyanines with central metals gallium and indium

Abstract The optical dynamics of octa-(4-tert-butylphenoxy) substituted naphthalocyanines and phthalocyanines with different central metals (Ga, In) were investigated for 532 nm–7 ns laser pulses. In this work, we used numerical theoretical method with two-dimensional paraxial field equation and rate equations solving by Crank–Nicholson method. A five-level model was proposed to simulate the dynamical processes and the prominent optical limiting mechanism with long pulses is the sequential (singlet–singlet) × (triplet–triplet) two-photon absorption. The theoretical results indicate that both naphthalocyanines and phthalocyanines exhibit good optical limiting performances, which is attributed mainly to the reverse saturable absorption mechanism. The naphthalocyanines with larger difference of absorption cross sections and faster rate of the intersystem crossing transition between triplet and singlet states exhibit better optical limiting behaviour.

Investigation of optical nonlinearity of 8-hydroxyquinolinium 2-chloroacetate and 8-hydroxyquinolinium (Z)-3-carboxyacrylate salts by Z-scan technique

A novel two organic salts: 8-hydroxyquinolinium 2-chloroacetate and 8-hydroxyquinolinium (Z)-3-carboxyacrylate were synthesized and characterized using UV–visible, FTIR and NMR measurements. The characterization spectra confirm the expected molecular structure of the prepared salts. The Z-scan technique is used to achieve optical nonlinearity measurements with a CW Diode laser at 635nm wavelength and 14.5mW power. The nonlinear optical absorption coefficient (β) and nonlinear refraction (n2) of the two studied compounds were affected by Cl group more than OH. The reverse saturable absorption (RSA) mechanism was responsible for the optical limiting effect. The results suggest that this material considers as a promising candidate for future optical devices applications.