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An optical spectroscopic characterization is carried out on a reactive dye (reactive orange 1). This dye is widely applied in textile coloration. It is a potential candidate for photonics applications. Its absorption cross-section spectra are measured. A fluorescence spectroscopic characterization is undertaken by measuring the fluorescence quantum distributions and fluorescence quantum yields. The saturable absorption is studied by nonlinear transmission measurements with intense picosecond laser pulses (second harmonic pulses of a mode-locked Nd:glass laser). The nonlinear optical absorption and refraction coefficients are measured by using the top-hat Z-scan technique at a wavelength of 532 nm with 35 ps duration pulses. Reactive orange 1 has the two-photon absorption coefficient of 1.20 cm/GW and the nonlinear refraction coefficient of -7.33 × 10^-6 cm²/GW, respectively. In reactive orange 1, there occurs fast ground-state recovery by internal conversion likely via conical intersections. Low excited-state absorption and fast ground-state absorption recovery make it an ideal candidate for passive mode-locking of picosecond and femtosecond lasers as well as for fast nonlinear optical gating.

Pulses as short as 260 fs have been generated in a diode-pumped low-gain Er:Yb:glass laser by exploiting the nonlinear optical response of single-layer graphene. The application of this novel material to solid-state bulk lasers opens up a way to compact and robust lasers with ultrahigh repetition rates.

The third order nonlinear optical properties of two different sized carboxylate-modified microspheres are investigated at different pump powers. The nonlinear absorption (NLA) is measured at 532 nm using 10 Hz, 7 ns pulses from the second harmonic generated from a Q-switched Nd-YAG laser in an open aperture Z-scan setup. The nonlinear response is size-dependent, and switching from reverse saturable absorption (RSA) to saturable absorption (SA) is observed as the size increases.

Nanostructured zinc sulfide (ZnS) thin films were synthesized in polyvinyl alcohol matrix by chemical bath deposition and self-assembly techniques. ZnS nanostructured thin films show second harmonic generation (SHG) under irradiation with a pico-second Nd:YAG laser system and the second harmonic intensity is higher for self-assembled nanotree like structured ZnS thin film in comparison with that from chemical bath deposited thin film. Under nanosecond laser pulses, thin films possess good saturable absorption behavior. The optical bandgap and visible luminescence also get enhanced.

We study the nonlinear (NL) absorption and refraction of 5,10,15,20-tetraphenyl-21H, 23H-porphin cobalt(II) (CoTPP) and 5,10,15,20-tetrakis(4-methoxyphenyl)-21H, 23H-porphin cobalt(II) (MCoTPP). The measurements were carried out in solutions prepared with chlorobenzene applying the D4$\sigma$-Z-scan technique with a laser delivering single pulses in the picosecond regime at 532nm. For each compounds, the NL coefficients were measured for three concentrations. The results show that these materials with unfilled $d$ shell exhibit a saturable absorption (SA), where they have been compared to the basic structure tetraphenylporphyrin (TPP) and zinc tetraphenylporphyrin (ZnTPP) with filled $d$ shell metal in order to see the influence of the central metal ion on the NL absorption (NLA) behavior.

We investigate the optical nonlinearity of CaCu₃Ti₄O${}_{12}$ (CCTO) thin films on MgO substrates using Z-scan technique at a near-infrared wavelength of 1064nm with 25ps duration pulses. The near-infrared nonlinear saturable absorption coefficient $\beta$ and the nonlinear refraction coefficient $\gamma$ of the values $-2.36\times 10^4$cm/GW and 0.66cm²/GW, respectively, were deduced from the study of the nonlinear response at different excitation intensities. The three-level model is proposed to interpret the large optical nonlinearity in CCTO films in near-infrared spectrum, by considering the intermediate energy levels due to the defects or grain boundaries. The large optical nonlinearity and fast response at picosecond time-domain indicate that CaCu₃Ti₄O${}_{12}$ thin films are promising candidates for applications in near-infrared nonlinear photonics devices.

The study of nonlinear absorption has potential applications as saturable absorption and optical limiting character in scientific and engineering technology. In this work, we report on the Gaussian vortex beam z-scan to study the saturable absorption in the two-level model. For a given material and laser beam parameters, even though vortex beam shows less saturable absorption nature than the conventional Gaussian beam but not much appreciated. The open aperture z-scan peak is narrower for Gaussian vortex beam as compared with Gaussian beam. Also, in the presence of pico- and femto-second laser pulses, the central dark core of Gaussian vortex beam can provide a stable and accurate z-scan profile with avoiding thermal and other nonlinear optical effects at the beam waist of z-scan in the saturable absorber characterization. With including above mentioned points, we have shown how the Gaussian vortex z-scan is superior to the conventional Gaussian beam z-scan for saturable absorber characterization. Though discussed for single-photon absorption, it may extend to multi-photon absorption.

In this work, the Au@Ag bimetallic core–shell nanostructures were synthesized by a seed-mediated growth. The crystal structure, morphology, elemental composition, atomic concentration, and absorption spectrum of the as-synthesized nanoparticles were characterized by means of X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible linear absorption spectrum, respectively. The femtosecond third-order optical nonlinearities of nanoparticle dispersions were investigated by carrying out the femtosecond-pulsed $Z$-scan measurements at 800nm. The experimental results indicate that Au@Ag core–shell nanoparticles exhibit the positive refractive nonlinearity and negative absorptive nonlinearity. The third-order nonlinear refraction indexes of Au and Au@Ag nanoparticles are measured to be $2.50\times 10^{-7}$ and $6.39\times 10^{-7}$cm²/GW, respectively. The results show that the bimetallic nanoparticle has potential possibility in nonlinear photonic applications.

In this work, the nonlinear optical response and the ultrafast carrier dynamics in the visible region of NbSe₂ are investigated. The transient absorption (TA) spectra reveal the relaxation process, including electron–phonon scattering and phonon–phonon scattering. These results contribute to an in-depth understanding of the light-matter interactions and carrier dynamics in NbSe₂ and will establish a foundation for its applications in nonlinear photonic and optoelectronic devices.

Direct growth of large-area uniform graphene films on insulating materials could facilitate the applications of graphene in optoelectronic devices. The ultrafast photocarrier relaxation and saturable absorption of direct chemical vapor deposition-grown graphene glasses, with lots of defects, are studied by using femtosecond time-resolved pump–probe and Z-scan techniques at 800 nm. We find that both the relaxation times associated with hot carrier cooling and the hot phonon effect are greatly suppressed in these defect-rich graphene glasses, which further leads to the increase of both saturation intensity and anisotropy in transient optical response for graphene glasses as compared with defect-free graphene. And, both the suppression effect and saturation intensity increase with the thickness of graphene film. The dominance of defect-assisted carrier-acoustic phonon scattering (i.e., supercollision, the collision of a carrier with both an acoustic phonon and defects) in the cooling process of hot carriers is responsible for the suppression of relaxation time.

Both experimental and theoretical works have demonstrated that two-dimensional materials exhibit strong thickness-dependent electronic and linear optical properties. However, the nonlinear optical (NLO) effect with respect to the thickness still needs to be further investigated, which is of great significance to design different photonic devices. Herein, we develop the physical vapor deposition technique to prepare a series of antimony selenide (Sb₂Se${}_3)$ films. The relationship between the thickness of Sb₂Se₃ film and the NLO absorption was investigated by a $Z$-scan system under 35fs, 800nm laser excitation. The result shows the thicker Sb₂Se₃ film ($\sim$50nm) performs a larger third-order NLO susceptibility (Im${\chi }^{(3)})$ approximately $-4.59\times 10^{-8}$ esu, which can be interpreted by the stronger photon absorption, lower saturable intensity, and larger absorption cross-section of ground-state. More importantly, the Im${\chi }^{(3)}$ absolute value of thick Sb₂Se₃ film is also far larger than those of graphene, black phosphorus, transition metal dichalcogenide, MXenes, and heterostructure. This work demonstrates that Sb₂Se₃ film is an excellent saturable absorber and provides a promising application in nonlinear photonics.

We present our results from the experimental and modeling studies of picosecond (ps) and femtosecond (fs) nonlinearities of two novel corroles (a) tritolyl corrole (TTC) (b) triphenyl corrole (TPC) using the Z-scan technique. Both open and closed aperture Z-scan curves were recorded with ~2 ps/~40 fs laser pulses at a wavelength of 800 nm and nonlinear optical coefficients were extracted for both studies. Both the molecules possessed negative nonlinear refractive index (n₂) as revealed by signature of the closed aperture data in both (ps and fs) time domains. Picosecond nonlinear absorption data of TPC obtained at a concentration of 5 × 10^-4 M demonstrated complex behavior with switching from reverse saturable absorption (RSA) within saturable absorption (SA) at lower peak intensities to RSA at higher peak intensities. TTC data recorded at the similar concentration exhibited saturable absorption (SA) type of behavior at lower peak intensities to switching from RSA with in SA at higher peak intensities. At a concentration of 2.5 × 10^-4 M, the ps open aperture data at higher peak intensities illustrated effective three-photon absorption (3PA) for both the molecules. We also report the picosecond spectral dependent Z-scan studies performed at 680 nm, 700 nm, and 740 nm. Nonlinear absorption and refraction of both the samples at these three wavelengths were studied in detail. Femtosecond nonlinear absorption data of TPC and TTC demonstrated the behavior of saturable absorption (SA) at a concentration of 1 × 10^-3 M. Solvent contribution to the nonlinearity was also identified. We have also evaluated the sign and magnitude of third order nonlinearity. We discuss the nonlinear optical performance of these organic molecules.

A single crystal of (5,10,15,20-tetraphenylporphyrinato)-silver was obtained and its structure reported. Using direct growth on quartz substrates method, a crystal thin film was obtained and the film’s morphology was investigated by atomic force microscopy. The nonlinear absorption properties of the film were studied using open aperture $Z$-scan technique by picosecond laser pulses at different fluence at wavelength 532 nm, nonlinear saturable absorption behavior was observed. Time-dependent unrestricted Hartree-Fock with the basis set LanL2DZ was used in computing the linear and non-linear optical properties of (5,10,15,20-tetraphenylporphyrinato)-silver.