Ground State Absorption Cross Section Research Articles

Crossover from Reverse Saturable to Saturable Absorptions in Two-Dimensional Tungsten Disulfide

The Tungsten disulfide atomic layer has enormous scientific merits that play a key role in the nonlinear absorption process. Four distinct excitonic peaks in the absorption spectra are crucial for identifying the polarity of nonlinear absorption by virtue of the resonant or non-resonant excitation process. The excitation source of 532 nm is enough for the one-photon optical transition at both low-energy excitonic peaks and two-photon transition at both higher-energy excitonic peaks. The Tungsten disulfide atomic layer exhibited the reverse saturable (or positive nonlinear) absorption through the two-step excitation process which could be possible due to the optical transition at the higher energy excitonic peaks. In addition, the polarity of the nonlinear absorption coefficient and its interchange have been analytically studied with the aid of a magnitude of excited-state absorption cross-section and Ground-state absorption cross-section. Besides, the various possible factors affecting the Z-scan results are discussed.

Investigations of 2D PtS2's Saturable Absorption Characteristic and Its Optimization to OPO's Operation.

A 6.2 nm-thickness platinum disulfide (PtS2) film was prepared by electron beam evaporation with post vulcanization. The nonlinear transmittance was measured by power scanning method and the modulation depth is fitted to be 13%. Based on the transmittance curve, saturable absorption parameters of PtS2 are calculated with inhomogeneously broadening mechanism, including 6.4298 × 10−19 cm−2 ground-state absorption cross-section, 2.5927 × 10−19 cm−2 excited-state absorption cross-section, and 1.043 ms excited-state lifetime. The PtS2 film combined with active time management was implemented to modulate the fundamental light of optical parametric oscillator (OPO). Owing to the nonlinear absorption property of PtS2, the operation of Q-switched OPO was optimized in both the experiment and dynamical theory. In particular, the conversion efficiency was experimentally improved by 13.2%. The pump-to-signal conversion efficiency went up to 3.29%, which is the highest conversion value reported so far. The theoretical values fit the experiment well, which are from the Gaussian rate equations with PtS2’s saturable-absorption characteristic.

Measurement of absorption cross sections of Co2+:MgAl2O4 crystals with transition of Co2+ ions along two orthogonal directions.

The nonlinear transmission curves of the Co2+:MgAl2O4 crystal at 1.5 µm are experimentally measured by the pump-probe technique. The measurement results demonstrate that the transition into the near-infrared absorption band is polarized not only along the C3 local symmetry axis, but also along the direction perpendicular to the C3 axis, arising from the distortion of oxygen tetrahedrons about the Co2+ ions along the four C3 local symmetry axes. The obtained saturation curves are analyzed using a theoretical model taking into account the transition of Co2+ ions along two orthogonal directions. The ground-state absorption cross sections for polarization parallel and perpendicular to the C3 axis are calculated to be 3.4×10-19cm2 and 2.6×10-20cm2, respectively. The excited-state absorption cross section for polarization parallel to the C3 axis is estimated to be4.2×10-20cm2.

Crystal structure elucidation, experimental and computational optical properties of novel acceptor-donor-acceptor organic material: A suitable candidate for nonlinear applications

A new organic nonlinear optical (NLO) material, 2-(4-chlorophenoxy)-N'-[(1E)-1-(4-fluorophenyl) ethylidene] acetohydrazide (4CP4FPEA), has been synthesized and single crystals were grown using DMF as solvent. The compound was characterized by 1H NMR and single X-ray diffraction analyses. Analysis of intermolecular interactions was carried out by mapping Hirshfeld surfaces over 2D fingerprint plots, dnorm and curvedness using Crystal Explorer. Open aperture Z-Scan experimental curve showed that the 4CP4FPEA molecule exhibits lower transmittance at the focus at 532 nm wavelength. 4CP4FPEA exhibits good optical limiting behavior at the wavelength 532 nm. Estimated excited state absorption cross-section (σex) and ground state absorption cross-section (σg) are found to be 3.59 × 10−17cm2 and 1.9 × 10−21 cm2 respectively (σg <<< σex) indicating that nonlinear absorption is due to Reverse Saturable Absorption. Semi-empirical calculations were implemented to calculate static and frequency dependent mean polarizability, first-order hyperpolarizability and second-order hyperpolarizability using time-dependent Hartree–Fock (TDHF) method. We have observed enhancement in nonlinear optical property compared to our previous reported molecule 2-(4-chlorophenoxy)-N'-[(1E)-1-(4-methylphenyl) ethylidene] acetohydrazide (4CP4MPEA).

Synthesis, structure and spectroscopy of Fe2+:MgAl2O4 transparent ceramics and glass-ceramics

We report on a comparative study of a transparent Fe:MgAl2O4 (spinel) ceramics and a transparent nanophase Fe:MgAl2O4-based glass-ceramics. The 0.1 mol% Fe:MgAl2O4 ceramics was synthesized by hot pressing (at 1500 °C/50 MPa) of powders obtained by the sol-gel method using LiF as a sintering aid. The Fe:MgAl2O4 ceramic is a single-phase material (cubic structure, sp. gr. Fd3‾m, a = 8.083 Å) with a mean grain size of ~50 μm. The ceramic exhibits a broadband transparency of 0.2–6.0 μm and a high in-line transmission at ~1 μm of 74.4%. The iron ions are presented in the ceramics in the single state of Fe2+ species in tetrahedral (Td) sites. A broad absorption band spanning from ~1.2 to 3.7 μm assigned to the 5E → 5T2 (5D) transition of Fe2+ ions in Td sites is observed, corresponding to a ground-state absorption cross section of 0.28 × 10−18 cm2 at 1.90 μm. The glass-ceramics were prepared by secondary two-stage heat-treatments of the magnesium aluminosilicate glass nucleated by titanium oxide and doped with 0.1 mol% FeO. Transparent Fe:MgAl2O4-based glass-ceramics obtained at the temperature of the second stage of 800–1000 °C were multi-phase materials containing two crystalline nanophases, i.e., spinel (mean size: 3.7–7.4 nm) and magnesium aluminotitanate solid solution (mean size: 6.4–20.6 nm), as well as residual silica-rich glass. Glass-ceramics obtained at the temperature of the second stage of 1050 °C were transparent and based on Fe-doped sapphirine. For glass-ceramics, absorption has a much more complex character as it is caused by interplay of iron and titanium ions in different valence states, coordination sites and locations. The iron ions enter the spinel nanocrystals but unlike the ceramic, in the form of both VIFe2+ and IVFe2+ species. The developed ceramics and glass-ceramics are promising for saturable absorbers of mid-infrared (2–3 μm) lasers.

Transparent glass-ceramics based on Co2+-doped γ-GaxAl2−xO3 spinel nanocrystals for passive Q-switching of Er lasers

Transparent glass-ceramics (GCs) based on Co2+-doped γ-GaxAl2−xO3 spinel nanocrystals were developed in the lithium aluminogallosilicate glass system with TiO2 as a nucleating agent. The initial glass contains spinel nanocrystals with sizes less than 3 nm. Secondary heat-treatments in the temperature range of 680–800 °C result in fabrication of transparent GCs based on γ-GaxAl2−xO3 crystals enriched in aluminum with cubic spinel structure (sp. gr. Fd3m) and a mean size up to 5.5 nm. At higher temperatures, GCs become translucent and then opaque due to additional crystallization of lithium aluminosilicate solid solutions (ss) with β-quartz structure and lithium aluminogallium silicate ss with β-spodumene (keatite) structure. The introduction of gallium oxide improves the temperature stability of spinel crystals that are not decomposed at high heat-treatment temperatures when aluminosilicate crystals evolve. The Co2+ ions in GCs enter the spinel crystals predominantly in tetrahedral sites. Transparent GCs feature broadband absorption (1.15–1.7 μm) owing to the 4A2(4F) → 4T1(4F) transition of IVCo2+ species in spinel nanocrystals. They exhibit saturable absorption and high ground-state absorption (GSA) cross-section σGSA of 3.1 ± 0.4 × 10−19 cm2 at 1.54 μm. Glass-ceramic based on γ-GaxAl2−xO3 crystals is employed as a saturable absorber of an eye-safe diode-side-pumped Er3+,Yb3+:glass laser delivering 1.1 mJ/42 ns pulses at 1535 nm.

Linear and non-linear optical properties of transparent glass-ceramics based on Co2+-doped Zn(Al,Ga)2O4 spinel nanocrystals

Transparent glass-ceramics containing Co2+:Zn(Al,Ga)2O4 spinel nanocrystals with sizes of 6-11 nm were prepared on the basis of the glass of the ZnO-Al2O3-Ga2O3-SiO2 system nucleated by TiO2. The absorption and luminescence properties of these glass-ceramics are defined mainly by tetrahedrally coordinated Co2+ ions located in Zn(Al,Ga)2O4 nanocrystals with spinel structure. Co2+ ions in glass-ceramics exhibit absorption saturation at 1.54 µm (the 4A2(4F)→4T1(4F) transition of Co2+ ions in tetrahedrally coordinated sites). This absorption band is shifted to higher wavelengths by 20 nm as compared with glass-ceramics with no Ga2O3 addition. The concentration of Co2+ ions in Zn(Al,Ga)2O4 nanocrystals changes in a complex way with rise in heat-treatment temperature. Characteristic times of luminescence decay, values of ground-state absorption cross-sections, and absorption saturation recovery time were determined. The developed glass-ceramics are promising as saturable absorbers for 1.6 µm erbium lasers.

Crystal structure, linear and nonlinear optical properties of three thiophenyl chalcone derivatives: A combined experimental and computational study

Abstract Three thiophenyl chalcone derivatives, (E)-1-(5-bromo-2-thienyl)-3-(4-bromophenyl)prop-2-en-1-one (B5B2SC), (E)-1-(5-bromo-2-thienyl)-3-(3-chlorophenyl)prop-2-en-1-one (3C5B2SC), and (E)-1-(5-bromo-2-thienyl)-3-(2-methoxyphenyl)prop-2-en-1-one (2M5B2SC), were successfully synthesized and crystallized. The structures were confirmed by experimental and theoretical FTIR and 1H NMR spectra. The crystal structures were determined by single-crystal XRD method. The B5B2SC, 3C5B2SC and 2M5B2SC crystallize in the monoclinic space groups P 21/c (centrosymmetric), P 2 (non-centrosymmetric) and triclinic space group P 1 ‾ (centrosymmetric), respectively. All these structures adopt trans configuration with respect to C6═C7 double bond. The chalcones have high thermal stability, a wide transparency in the visible region and small optical band gaps. The 3C5B2SC exhibits powder second-harmonic generation (SHG) intensity of about 1.7 times higher than Urea. From continuous wave (CW) laser (532 nm) Z-scan experimental data analyses, the third-order NLO parameters such as nonlinear absorption coefficient (β = ~10−4 cm/W), nonlinear refractive index (n2= ~10−9 cm2/W), third-order NLO susceptibility (χ(3) = ~10−7 esu), molecular hyperpolarizability (γh = ~10−27 esu), ground state absorption cross section (σg = ~10−22 cm2) and excited state cross sections (σex= ~10−20 cm2) were determined. The chalcones also exhibited optical limiting and all optical switching behaviour for CW laser operating at 532 nm wavelengths. The electric dipole moment, polarizability, hyperpolarizabilty, HOMO-LUMO energy gap, global chemical reactivity descriptors (GCRD), molecular electrostatic potentials (MEP) and natural bond orbital (NBO) analysis were computed theoretically (DFT). The results indicate that these three chalcones with thiophene moiety are potential materials for optical device applications.

Energy transfer upconversion in Nd:YAG at cryogenic temperatures

The precise characterisation of the ground state absorption cross section around 800 nm for Nd:YAG from room temperature to liquid nitrogen temperature is presented. These results enabled the measurement of the energy transfer upconversion macroparameter over the same temperature range for 0.3at.%- and 0.6at.%-doped samples via a simple automated z-scan technique. The main absorption cross section peak at 808 nm is found to increase from (6.90 ± 0.30) pm2 at the highest, to (42.30 ± 2.10) pm2 at the lowest temperatures. Over the same range, the energy transfer upconversion parameter increases from (21.5 ± 2.3) 10−18 cm3/s to (52.6 ± 2.5) 10−18 cm3/s and from (36.0 ± 2.8) 10−18 cm3/s to (65.7 ± 1.9) 10−18 cm3/s, for the 0.3at.%- and 0.6at.%-doped crystals, respectively. Although energy transfer upconversion is known to limit room temperature operation on the 946 nm transition for this laser, we demonstrate that when the crystal is cooled to liquid nitrogen temperature, despite a twofold increase in the macroparameter, it has a negligible effect on performance.

Transition-metal-doped saturable absorbers for passive Q-switching of visible lasers

We experimentally investigate crystals doped with tetravalent chromium or divalent cobalt as saturable absorbers for passive Q-switching of visible solid-state lasers. The recovery time of the ground-state and excited-state absorption cross sections are determined by pump-probe and Z-scan measurements, respectively. We provide saturation intensities, useful wavelength ranges of the investigated materials, and advices to realize passive Q-switching of visible lasers using these crystals as saturable absorbers.

Passively Q-switched near-infrared lasers with bismuthene quantum dots as the saturable absorber

Bismuthene quantum dots (Bi-QDs) were synthesized via the liquid phase exfoliation (LPE) method for the passive Q-switching operation in near-infrared (NIR) region. The nonlinear optical properties of the prepared Bi-QDs were investigated by the open-aperture Z-scan technology. The modulation depths were 18.1% and 5.1% at 1.06 and 1.34 µm, respectively. Based on the Bi-QDs saturable absorber, passively Q-switched Nd:GdVO4 lasers operating at 4F3/2 → 2I11/2 and 4F3/2 → 2I13/2 transitions were demonstrated, showing the wideband optical modulation in NIR regime. For the 4F3/2 → 2I13/2 transition lasing at 1.34 µm, the shortest pulse duration of 155 ns was obtained with a repetition rate of 457 kHz. With respect to the 4F3/2 → 2I11/2 transition at 1.06 µm, the minimum pulse duration was 150 ns with a repetition rate of 424 kHz, leading to a single pulse energy of 261 nJ and a peak power of 1.68 W. In addition, the ground state absorption cross section and the excited state absorption cross section of Bi-QDs were also investigated for the first time. The impact of the excited state lifetime on the output parameters was numerically stimulated by the coupled rate equations. Our work confirmed that the trap state in Bi-QDs played an important role in the pulse generation mechanism.

Third-order nonlinear optical properties of WTe2 films synthesized by pulsed laser deposition

The prominent third-order nonlinear optical properties of WTe2 films are studied through the Z-scan technique using a femtosecond pulsed laser at 1030 nm. Open-aperture (OA) and closed-aperture (CA) Z-scan measurements are performed at different intensities to investigate the nonlinear absorption and refraction properties of WTe2 films. OA Z-scan results show that WTe2 films always hold a saturable absorption characteristic without transition to reverse saturable absorption. Further, a large nonlinear absorption coefficient β is determined to be −3.37×103 cm/GW by fitting the OA Z-scan curve at the peak intensity of 15.603 GW/cm2. In addition, through the slow saturation absorption model, the ground state absorption cross section, excited state absorption cross section, and absorber’s density were found to be 1.4938×10−16 cm2, 1.2536×10−16 cm2, and 6.2396×1020 cm−3, respectively. CA Z-scan results exhibit a classic peak–valley shape of the CA Z-scan signal, which reveals a self-defocusing optical effect of WTe2 films under the measured environment. Furthermore, a considerable nonlinear refractive index value n2 can be obtained at −1.629×10−2 cm2/GW. Ultimately, the values of the real and imaginary parts of the third-order nonlinear susceptibility of WTe2 are roughly estimated to be −5.93×10−9 and −1.01×10−8 esu with the help of first-principles calculations. WTe2 possesses sizeable third-order nonlinear coefficients compared with other materials, which suggests that it has great potential to improve performance in nonlinear optical devices.

Experimental and dynamical study of a dual Q-switched intracavity OPO based on few-layer MoSe2 SA.

A 2.5nm-thickness molybdenum diselenide (MoSe2) saturable absorber (SA) is prepared by electron beam evaporation (EBE) method. Applying the prepared MoSe2 SA to an acousto-optic (AO) Q-switched fundamental laser, a dual-loss-modulated intra-cavity optical parametric oscillator (IOPO) has been experimentally realized. The signal-pulse train from this IOPO has 0.0053 standard deviation (SD) of pulse amplitude. When the MoSe2 SA is applied to IOPO, the signal pulse is compressed by maximum 68%, the peak power increases by 274%, and the nonlinear conversion increases by 12.6%. To solve the established rate equation of IOPO, the ground-state and excited-state absorption cross section of MoSe2 are rationally estimated to be 1.04×10-18cm-2 and 6.25×10-19cm-2 from the measured transmittance curve, and the excited-state lifetime is 275.6µs. The numerical solution of the equations fits the experimental data well.

A simple way to prepare Co:MgAl2O4 transparent ceramics for saturable absorber

Co-doped magnesium aluminate (Co:MgAl2O4) transparent ceramics were obtained by pre-sintering in air and hot isostatic pressing (HIP) with commercial spinel and CoO powders as raw materials. The influences of the pre-sintering temperature on the microstructure and optical quality of the 0.05 at% Co:MgAl2O4 ceramics were studied. It is found that the relative density of Co:MgAl2O4 ceramics increases from 96.8% at 1500 °C to 98.7% at 1600 °C and then decreases with further increasing the pre-sintering temperature. The average grain size of Co:MgAl2O4 ceramics increase from 4.6 μm to 19.2 μm with the increase of pre-sintering temperature from 1500 to 1700 °C. For the sample pre-sintered at 1600 °C for 5 h and HIP post-treated at 1800 °C for 3 h, the average grain size is about 23.0 μm and the near in-line transmittance reaches 75% at the wavelength range of 1700–2400 nm. The ground state absorption cross section at 1540 nm of the optimal 0.05 at% Co:MgAl2O4 ceramics is calculated to be 2.9 × 10−19 cm2.

Enhanced photostability and optical nonlinearity of nickel and cobalt organometallic complexes

Nonlinear optical and limiting properties of nickel, cobalt metal-organic complexes and ligand (L) {L = N,N′-o-phenylenebis (4-hyrdoxy-salicylideneimine)} were studied using Z-scan technique using Q-switched Nd: YAG laser with nanosecond pulses at 532 nm. The results reveal that metal complexes exhibit large negative nonlinear refractive index (n2) of the order of 10−11 esu. The metal complexes displays large nonlinearity than the ligand due to effective charge delocalization between metal ion and ligand. The magnitude of ground state absorption cross section is small compared to the effective excited-state absorption cross section implying the observed nonlinearity is due to reverse saturable absorption. Further, metal-organic complexes exhibit enhanced optical limiting behaviour at nanosecond laser pulses.

Fabrication and properties of Co:MgAl2O4 transparent ceramics for a saturable absorber from coprecipitated nanopowder

AbstractThe 0.05 at.% Co:MgAl2O4 precursor was synthesized by the coprecipitation method from a mixed solution of magnesium, aluminum, and cobalt nitrates using ammonium carbonate as the precipitant. 0.05 at.% Co:MgAl2O4 transparent ceramics were successfully obtained via vacuum sintering and hot isostatic pressing (HIP) of 0.05 at.% Co:MgAl2O4 nanopowder calcined at 1100°C for 4 hours. The properties of powder and ceramics were comprehensively investigated. X‐ray diffraction (XRD) results showed that Co:MgAl2O4 nanopowder had a pure spinel phase. Also, the in‐line transmittances of the HIP posttreated Co:MgAl2O4 ceramics with the thickness of 1.2 mm were 82% at 400 nm and 84.7% at 900 nm. The average grain sizes of 0.05 at.% Co:MgAl2O4 ceramics before and after the HIP posttreatment were 11 and 28 μm, respectively. The calculated ground state absorption cross section of 0.05 at.% Co:MgAl2O4 ceramics was 2.9 × 10−19 cm2, indicating that this ceramics is a promising material applied as a saturable absorber for passive laser Q‐switches in the 1.3‐1.7 μm domain.

Nonlinear absorption and refraction in phthalocyanine-doped polymers

Lead-tetrakis-4(cumylphenoxy)phthalocyanine (PbPc) is dispersed into several common optical polymers at various doping levels. The linear and nonlinear optical properties are measured at 600 nm near the region where the excited state and ground state absorption cross sections are nearly equal. The effective nonlinear refraction coefficients observed at high concentration are as much three orders of magnitude larger than that observed for fused silica.

Characterising energy transfer upconversion in Nd-doped vanadates at elevated temperatures.

The Energy Transfer Upconversion (ETU) macroparameter is measured for Nd-doped GdVO4 and YVO4 samples at temperatures ranging from Room Temperature (RT) to 450K, by means of a simple and automated z-scan technique. Furthermore, the ground state absorption cross section into the 2H9/2 +4F5/2 energy levels is characterised for both crystals over the same range of temperatures. The 808 nm π-polarisation absorption cross section is found to decrease from (58.6 ± 0.2) pm2 to (30.9 ± 0.6) pm2 for Nd:YVO4 and (54.0 ± 0.3) pm2 to (25.7 ± 0.5) pm2 for Nd:GdVO4, from RT to 450K. Over the same range the ETU coefficient decreases from (3.2 ± 0.7) 10-16cm3/s to (1.8 ± 0.4 10-16cm3/s and (5.0 ± 0.5) 10-16cm3/s to (3.4±0.2) 10-16cm3/s for 0.6 at. % and 1 at. % Nd:YVO4 respectively, and (3.3 ± 0.5) 10-16cm3/s to (0.8 ± 0.2) 10-16cm3/s and (5.5 ± 0.5) 10-16cm3/s to (3.1 ± 0.3) 10-16cm3/s for 0.5 at.% and 1.1 at.% Nd:GdVO4.

Optical spectroscopic investigation of Ba3Tb(PO4)3 single crystals for visible laser applications

Optical spectroscopic characterization of Ba3Tb(PO4)3 single crystals grown by the Czochralski technique was performed in order to evaluate the potential for visible laser application. Powder X-ray diffraction patterns at different temperatures were recorded to calculate the thermal expansion for temperatures between 298 and 1073 K. Refractive indices in the visible spectral region were determined and fitted into a Sellmeier equation. Absorption spectra were measured in the UV and visible range to obtain the ground-state absorption cross-sections, which were used for a Judd-Ofelt analysis. Electrostatic and spin-orbit interaction parameters were fitted, allowing for more precise Judd-Ofelt calculations. The Judd-Ofelt intensity parameters were calculated and employed to study the radiative transition probabilities, branching ratios and radiative lifetime associated to the potential upper laser level 5D4. The fluorescence spectrum shows several lines in the visible range, which can be assigned to 5D4 → 7FJ (J=6,5,4,3,2,1,0) transitions of Tb3+. These data enabled the determination of the branching ratio of the transitions from the 5D4 excited state. The stimulated emission cross-section of the 5D4 → 7F5 transition was found to be 5.9 × 10−22 cm2 at a wavelength of 549.6 nm by using the Füchtbauer-Ladenburg equation. Fluorescence decay curves were recorded to obtain the fluorescence lifetime. The resulting value of 3.17 ms is in reasonable agreement with the theoretical value of 3.54 ms pointing towards a high radiative quantum efficiency of 90% despite a high doping ion density of 3.5 × 1021 cm−3 and revealing the low effect of detrimental inter-ionic processes in Tb3+.

Synthesis and characterization of 8-hydroxyquinolin-1-ium 2,2,2-trifluoroacetate and 8-hydroxyquinolin-1-ium 2,2,2-trichloroacetate by z-scan technique

The 8-hydroxyquinolin-1-ium 2,2,2-trifluoroacetate and 8-hydroxyquinolin-1-ium 2,2,2-trichloroacetate (8-HQ) salts have been synthesized and characterized by FTIR, NMR and UV–Vis absorption spectroscopic methods. The third order nonlinear optical properties (NLO) of the two 8-HQ salts have been investigated by z-scan technique. The recorded experimental data led to calculate the nonlinear optical absorption coefficient (β), the nonlinear refractive index (n2), the excited-state absorption cross section (σex) and the ground-state absorption cross section (σg) of the studied compounds. The two 8-HQ salts have been found to exhibit large optical nonlinearity. These types of materials may be considering new photonic applications.