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Optical logic gates based on nonlinear optical property of material with ultrafast response speed and excellent computational processing power can break the performance bottleneck of electronic transistors. As one of the layered 2D materials, Ta2NiS5 exhibits high anisotropic mobility, exotic electrical response, and intriguing optical properties. Due to the low-symmetrical crystal structures, it possesses in-plane anisotropic physical properties. The optical absorption information of Ta2NiS5 is investigated by anisotropic linear absorption spectra, femtosecond laser intensity scanning (I-scan), and non-degenerate pump-probe technology. The I-scan results show a distinct maximum of ∼4.9 % saturable absorption (SA) and ∼4 % reverse saturable absorption (RSA) at different polarization directions of the incident laser. And, these unique nonlinear optical (NLO) properties originate from the anisotropic optical transition probability. Furthermore, the novel Ta2NiS5-based all-optical logic gates are proposed by manipulating the NLO absorption processes. And, the all-optical OR and NOR logic gates possess an ultrafast response speed approaching 1.7 THz. Meanwhile, an all-optical information transmission method with higher security and accuracy is achieved, which has promising potential to avoid the disclosure of information. This work provides a new path for designing versatile and novel optical applications based on Ta2NiS5 materials. [ABSTRACT FROM AUTHOR]

Optical logic gates based on nonlinear optical property of material with ultrafast response speed and excellent computational processing power can break the performance bottleneck of electronic transistors. As one of the layered 2D materials, Ta2NiS5 exhibits high anisotropic mobility, exotic electrical response, and intriguing optical properties. Due to the low-symmetrical crystal structures, it possesses in-plane anisotropic physical properties. The optical absorption information of Ta2NiS5 is investigated by anisotropic linear absorption spectra, femtosecond laser intensity scanning (I-scan), and non-degenerate pump-probe technology. The I-scan results show a distinct maximum of ∼4.9 % saturable absorption (SA) and ∼4 % reverse saturable absorption (RSA) at different polarization directions of the incident laser. And, these unique nonlinear optical (NLO) properties originate from the anisotropic optical transition probability. Furthermore, the novel Ta2NiS5-based all-optical logic gates are proposed by manipulating the NLO absorption processes. And, the all-optical OR and NOR logic gates possess an ultrafast response speed approaching 1.7 THz. Meanwhile, an all-optical information transmission method with higher security and accuracy is achieved, which has promising potential to avoid the disclosure of information. This work provides a new path for designing versatile and novel optical applications based on Ta2NiS5 materials.

The investigation of the nonlinear optical (NLO) properties of polyaniline (PANI) and polyaniline doped with multi-walled carbon nanotube (PANI-MWCNT) in both solution and film forms was conducted using the open-aperture z-scan approach. A Q-switched resonant Nd: YAG laser with a wavelength of 532 nm and two different fluences was utilized in this study. Based on the z-scan experiments, it was observed that the NLO behaviour of PANI and PANI-MWCNT composites exhibited a transition from reverse saturable absorption (RSA) to saturable absorption (SA) when the samples changed from a liquid to a film state. Two photon absorption (TPA) and thermally induced nonlinear scattering are the main mechanisms behind the RSA behaviour of the materials in solution form. The reason for SA behaviour of these materials in film form is attributed as the ground-state free electron bleaching in the conduction band due to the increased laser intensity. Due to increased structural disorder and the defect states or trap levels created by the dopants in the PANI system, the NLO properties of the PANI-MWCNT composite in both solution and film forms have significantly improved compared to those of pure PANI. Urbach tail analysis and carbon cluster determination revealed the presence of defect states in the composites system. The composite of PANI and MWCNT was verified using Fourier transform infrared (FTIR) spectroscopy. The functional elements present in the composites are verified by X-ray photoelectron spectroscopy. The NLO parameters of the samples, viz., the nonlinear absorption coefficient (β), the imaginary part of the nonlinear susceptibility χi(3) , and the figure of merit (FOM) of PANI-MWCNT, exhibited a notable enhancement in their values over PANI. Moreover, the PANI-MWCNT film demonstrated superior SA performance and the PANI-MWCNT solution demonstrated better OL performance compared to that of the PANI film and solution respectively. Also the effects of dopant induced structural modifications and lattice disorder on the NLO behaviour of these materials are correlated with the obtained NLO parameters. The tunable NLO properties accomplished by controlling the structural phase and dopant-induced defects enhance the possibility of these nanostructures for applications in optical limiting, optical switching, optical modulation, optical pulse compression and laser pulse narrowing.

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 35 fs, 800 nm laser excitation. The result shows the thicker Sb2Se3 film (∼ 50 nm) performs a larger third-order NLO susceptibility (Im χ (3)) approximately − 4. 5 9 × 1 0 − 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 χ (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. [ABSTRACT FROM AUTHOR]

In our work, a multi-layer topological insulator (TI) Bi2Se3 thin film was prepared by the chemical vapor deposition method (CVD), and its saturable absorption and damage characteristics were experimentally studied. The results show that when the wavelength is 1064 nm, the saturable absorption parameters of TI: Bi2Se3 film, including modulation depth αs, non-saturable loss αns, and saturation power intensity Isat, increase with the increase in film thickness, and the damage threshold is inversely proportional to the film thickness. The thicker the film layer, the lower the damage threshold. Among them, modulation depth αs is up to 51.2%, minimum non-saturable loss αns is 1.8%, maximum saturation power intensity Isat is 560.8 kW/cm2, and the damage threshold is up to 909 MW/cm2. The influence of the controllable thickness of TI: Bi2Se3 film on passive Q-switching and mode-locking performance of laser is discussed and analyzed when TI: Bi2Se3 film is prepared by the CVD method as a saturable absorber (SA). Finally, the performance of TI: Bi2Se3 thin film applied to nanosecond laser isolation at the 1064 nm band is simulated and analyzed. It has the natural advantage of polarization independence, and the maximum isolation can reach 16.4 dB. [ABSTRACT FROM AUTHOR]

Lead-free double perovskites (DPs) have received a lot of attention owing to their good stability and environmental friendliness. They have been widely used in the field of optoelectronics because of their high quantum efficiency and long carrier migration distance. However, the application of lead-free DPs in ultrafast photonics is rarely explored. Cs2AgBiBr6, a typical lead-free double perovskite, has advantages such as toxicity free of Pb, high stability, and strong nonlinear absorption, showing great potential application in ultrafast photonics. However, to date, there is no reported research on the application of Cs2AgBiBr6 in ultrafast optical devices. Herein, we prepared Cs2AgBiBr6 nanocrystals, tested their ultrafast nonlinear absorption properties, and further prepared them as a saturable absorber (SA) to achieve stable mode-locking pulses in Er-doped fiber lasers. The results show that it has a low saturation intensity (0.75 MW/cm2), resulting in a low starting threshold power (50 mW) for mode-locking. The obtained pulsed fiber laser shows a pulse width as narrow as 452 fs. In addition, the Cs2AgBiBr6 SA has unique advantages in long-term stability and it was found that this Cs2AgBiBr6 SA can still work well after 10 months. In general, our results prove that Cs2AgBiBr6 is an excellent nonlinear optical material and has good application potential in ultrafast photonics. [ABSTRACT FROM AUTHOR]

We present the design and simulations of a novel integrated device concept enabling a frequency conversion of a broad signal. The solution is based on a hybrid silicon–graphene photonic chip, which could be used for controlled spectrometry in low-cost devices. The device is based on a silicon-on-insulator (SOI) platform on which an arrayed waveguide grating (AWG) is designed for operation at the center wavelength of λ = 1800 nm. The AWG is spectrally separating one broad input signal to thirty-two-output channels with a channel spacing of 2.72 nm. The output signals are well separated and uniform with the extinction ratio and the standard deviation of 10.00 dB and 0.04, respectively. The 3 dB channel width is 1.34 nm, which is suitable for sensing applications with significant accuracy. After spacial and spectral separation, each output signal is then converted to one signal at 1480 nm wavelength through a graphene-based saturable absorber scheme. Therefore, the device allows the detection of each separated signal with a simple near-infrared camera on which the outputs are imaged using conventional optics, leading to a classical pixel/wavelength correspondence. Crossed-waveguide couplers are designed to combine the controlling signal at 1480 nm to each channel waveguide of the AWG. The combination of the signals saturates the graphene layer at the output waveguides, allowing the pass of the controlling wavelength. This device can be applied as a spectrometer in environmental sensing and monitoring with high efficiency and low cost. [ABSTRACT FROM AUTHOR]

High‐entropy alloys (HEAs) possess superior mechanical, thermal, and electronic properties, which attract great interest in the fields of photothermal, electrocatalysis, and magnetic applications. Herein, the nonlinear optical properties of FeCoNiAlTi HEA with a large specific surface area for the first time is investigated. Owing to the strong d–d interaction, the interband transition near the Fermi level could be enhanced in HEAs, leading to broadband optical absorption. Excitation with the picosecond laser source, the FeCoNiAlTi HEA possesses saturable absorption with a large modulation depth of 7.61% and 30.47% at 1 and 1.5 µm, respectively. Furthermore, at the higher excitation level at 1.5 µm, the saturable absorber exhibits a typical reverse saturable absorption property. Subsequently, the FeCoNiAlTi HEA on the tapered fiber first serves as the saturable absorber in the Yb‐doped fiber laser and the Er‐doped fiber laser, leading to the noise‐like pulses at 1 µm, and the stable Q‐switching, conventional soliton, and dissipative soliton resonance mode‐locking operations at 1.5 µm. This work confirms that the enhanced saturable absorption in HEAs can endow the high‐performance ultrashort pulse generation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

All‐optical modulation plays a key role in next‐generation optical processing and has attracted enormous attention worldwide. With extraordinary optoelectronic characteristics and friendly integration compatibility with various nanostructures, graphene shows great potential for ultrafast and energy‐efficient all‐optical modulation. Here, high‐efficiency on‐chip all‐optical modulation is experimentally demonstrated based on ultra‐thin silicon/graphene hybrid waveguides, which are complementary‐metal‐oxide‐semiconductor‐compatible and easy to fabricate. Owing to the enhanced light‐graphene interaction enabled by the ultra‐thin silicon photonic platform, the optical nonlinear absorption in graphene is greatly enhanced and a modulation depth of >2 dB is achieved with a saturation threshold of 0.9 pJ per pulse for a 50‐µm‐long modulator. The measured modulation efficiency is as high as 0.052 dB µm−1. Furthermore, the proposed all‐optical modulator has the potential to operate at a bandwidth of hundreds of gigahertz. The present hybrid integration of graphene on ultra‐thin silicon photonic waveguides paves the way toward the applications of on‐chip ultrafast and energy‐efficient all‐optical information processing. [ABSTRACT FROM AUTHOR]

In this paper, we used the liquid phase stripping method to manufacture MoS2–SnSe2 heterojunction saturable absorber (SA). The scanning electron microscope, the atomic force microscope, and the Raman spectrometer were used to characterize the MoS2–SnSe2 heterojunction SA. The Q‐switched mode‐locked (QSML) Nd:GGG laser with MoS2–SnSe2 heterojunction SA was presented. The maximum output power of the QSML Nd:GGG laser was 236.4 mW with a repetition rate of 83.3 MHz and pulse width of 447 ps. In short, the MoS2–SnSe2 heterojunction nanosheets present excellent saturable absorption characteristics. [ABSTRACT FROM AUTHOR]

The high‐quality germanene nanosheets (GeNS) were fabricated by a liquid‐phase exfoliated technique and used as the saturable absorber (SA) to generate the harmonic mode‐locked (HML) pulses in an Er‐doped fiber laser. The investigation on the nonlinear absorption effect of the GeNS was carried out systemically in this work. On the basis of the GeNS SA, the third‐ and fourth‐order HML pulses with repetition rates of 5.275 and 7.031 MHz were achieved, corresponding to the minimum pulse width of 3.04 and 3.21 ps, respectively. The research reveals that the GeNS has great capability for ultrafast pulses modulation and can be used as an outstanding nonlinear optical device to expand its applications in nonlinear optics. [ABSTRACT FROM AUTHOR]

This paper focuses the study of nonlinear absorption (NLA) features of anthraquinone dye namely, acid blue 129 with different polar solvents including ethanol, methanol, acetone, 1-proponal and DMSO. The acid blue 129 dye in polar solvents is characterized by UV–Visible absorption and fluorescence study. The NLA measurements of acid blue 129 dye is examined via open aperture (OA) Z-scan method employing at 650 nm wavelength. The OA Z-scan curve of the experimental dye exhibits both saturable absorption (SA) and reveres saturable absorption (RSA) signature, which results from negative and positive NLA coefficient (β). The order of ground and excited state absorption cross-section is measured to be 10˗22 m2 and 10˗18 m2, respectively and proved the existence of RSA in acid blue 129 dye. The order of NLA coefficient and imaginary component of the third-order nonlinear susceptibility of acid blue 129 dye in polar solvents is calculated to be 10˗3 cm/W and 10˗5 esu, respectively. The multi-parameter scale viz., Kamlet-Abboud-Taft is used to measure the influence of solvent parameters on solute molecule. The results from the experiments suggest that the acid blue 129 is a suitable candidate for nonlinear optical (NLO) applications. [ABSTRACT FROM AUTHOR]

Optical waveguides covered with thin films, which transmittance can be controlled by external action, are widely used in various applications from optical modulators to saturable absorbers. It is natural to suggest that the losses through such a waveguide will be proportional to the absorption coefficient of the covering material. In this letter, we demonstrate that under certain conditions, this simple assumption fails. Instead, we observe that the reduction of the material loss of the film can lead to an increase in the propagation losses through the waveguide. For this, we use a side polished fiber covered with a single-walled carbon nanotube thin film whose absorption can be attenuated either by a short pulse illumination (due to absorption saturation) or with electrochemical gating. For the films thicker than 50 nm, we observe saturable absorption to turn into optical limiting with nonmonotonic dependence on the incident power. With a numerical simulation, we identify that this nontrivial behavior comes from mode reshaping due to changes in the absorption coefficient of the covering film. We demonstrate the applicability of the observed effect by fabricating the device which nonlinear optical response can be controllably switched between saturable absorbing and optical limiting. Finally, we utilize an analytical approach to predict the required parameters and corresponding nontrivial shapes of the nonlinear absorbance curves. These results provide new perspectives for engineering complex reconfigurable nonlinear optical responses and transmittance dependences of nanomaterial covered waveguides.

Upconverter/MoS2 heterostructure thin films fabricated by hydrothermal technique are confirmed by X‐ray diffraction and Raman studies. Field‐emission scanning electron microscopy (FESEM) and high‐resolution transmission electron microscopy (HRTEM) reveal the nanobelt structured MoS2 mingled with trapezoid structure fluoride upconverters and sphere‐shaped oxide upconverters. The films exhibit green (H11/22 to I15/24) and red (F9/24 to I15/24) emission. The influence of upconverters on the nonlinear absorption of MoS2 is exploited by Z‐scan technique using nanopulsed laser. Estimated ground‐state absorption cross section is higher than the excited‐state absorption cross section of the samples and it confirms the occurrence of saturable absorption. CeO2:Yb,Er@MoS2 exhibits stronger saturable absorption due to synergetic effects like higher linear absorption coefficient at 532 nm, stronger green emission, and reduced size. Results pave way for the design of optical switches, Q‐switches, and mode lockers using upconverter integrated MoS2 thin films. [ABSTRACT FROM AUTHOR]

The next generation of photonics and nano-optical devices may be based on two-dimensional(2D) transition metal dichalcogenides (TMDs). In this research, molybdenum diselenide () nanosheets, as one important member of TMDs, have been synthesized by the solvothermal method and characterized through XRD patterns, SEM, and TEM images. Nanosheets were found to have a hexagonal phase based on XRD patterns and the crystallinity percentage is 24.8 %. The lattice constants of the hexagonal phase of are calculated as a= 3.08 ºA, c= 13.72 ºA. The calculated average value of the crystallite size, dislocation density, and micro strain are 21.935 nm, 2.138 nm-2 and 9.070, respectively. A few layers of nanosheets without wrinkles were observed on TEM and SEM. Next, the synthesized nanosheets were employed to prepare thin films with three different thicknesses using the spin coating method. By employing a continuous wave (CW) Nd : YAG laser at 532 nm via a Z-scan approach, this study investigates how thin film thickness affects the thermal nonlinear optical (NLO) responses of nanosheets. The magnitude of NLO coefficients of the prepared thin films decreased with increasing film thickness. It is observed that the prepared thin films possess saturable absorption (SA) as well as the self-focusing effect. Saturable absorbers and mode-locking devices can be developed with thin films because of their improved NLO properties. [ABSTRACT FROM AUTHOR]

Amorphous-Ge (α-Ge) or free-standing nanoparticles (NPs) synthesized via hydrogen-free plasma-enhanced chemical vapor deposition (PECVD) were applied as transmissive or reflective saturable absorbers, respectively, for starting up passively mode-locked erbium-doped fiber lasers (EDFLs). Under a threshold pumping power of 41 mW for mode-locking the EDFL, the transmissive α-Ge film could serve as a saturable absorber with a modulation depth of 52–58%, self-starting EDFL pulsation with a pulsewidth of approximately 700 fs. Under a high power of 155 mW, the pulsewidth of the EDFL mode-locked by the 15 s-grown α-Ge was suppressed to 290 fs, with a corresponding spectral linewidth of 8.95 nm due to the soliton compression induced by intra-cavity self-phase modulation. The Ge-NP-on-Au (Ge-NP/Au) films could also serve as a reflective-type saturable absorber to passively mode-lock the EDFL with a broadened pulsewidth of 3.7–3.9 ps under a high-gain operation with 250 mW pumping power. The reflection-type Ge-NP/Au film was an imperfect mode-locker, owing to their strong surface-scattered deflection in the near-infrared wavelength region. From the abovementioned results, both ultra-thin α-Ge film and free-standing Ge NP exhibit potential as transmissive and reflective saturable absorbers, respectively, for ultrafast fiber lasers. [ABSTRACT FROM AUTHOR]

The family of 2D transition metal carbides, nitrides, and carbonitrides (MXenes) has attracted an enormous amount of attention due to their tunable optical, electronic, electrochemical, and mechanical properties. Recently, a new branch of MXenes materials research has emerged that is exploring and engineering the intrinsic optical response of MXenes, resulting in compact nonlinear optical (NLO) devices. As a novel 2D materials system, MXenes not only exhibit common advantages of classical 2D materials for NLO applications but also demonstrate their unique superiority, such as high yield and scalable synthesis, good stability, switchable NLO response, etc. Here, a fundamental overview of MXenes nonlinear optics is provided, covering everything from MXenes synthesis to linear and NLO properties and NLO applications. The synthesis method and its influence on the MXenes structures and morphology are discussed, which dominated the linear optics of MXenes. Then, the third‐order NLO properties and carrier dynamics of MXenes, from basic theory to experimental results, are elaborated. Their NLO applications, including ultrashort laser pulse, single‐frequency laser generation, all‐optical phase modulation, wavelength modulation, and passive photonic diodes, are also highlighted. Finally, the current challenges and an outlook for future MXenes NLO research are proposed. [ABSTRACT FROM AUTHOR]

Featured with high thermal decomposition temperature and layered structure, violet phosphorus (VP) offers an unparalleled stable allotrope of phosphorus to demonstrate the optoelectronic device and photonics elements with high performance at the nanoscale. Here, we report few-layer and hundreds of nanometer-sized VP with robust stability in different solvents and ambient conditions by ultrasound-assisted liquid phase exfoliation approach. For the first time, the ultrafast carrier dynamics and third-order nonlinear optical response of VP were investigated. Sub-picosecond timescale ultrafast carrier dynamic and ultrafast nonlinear saturable absorption of VP were demonstrated. Our findings demonstrated that VP possessed a promising potential for use in ultrafast nonlinear photonic applications such as saturable absorbers and optical switches. [ABSTRACT FROM AUTHOR]

The donor-acceptor interaction of a charge-coupled heterostructure encompassing a metal and an amorphous semiconductor subjected to a laser field has many potential applications in the realm of nonlinear optics. In this work, we fabricate an electron donor gold nanoparticle (AuNP) and acceptor amorphous Ge24Se76 heterostructure on a quartz substrate using a sequential thermal evaporation technique. In this charge coupled heterostructure, we demonstrate the ultrafast third-order nonlinear absorptive and refractive response, and their sign reversal compared to pristine Ge24Se76. Enhanced optical nonlinearity in these heterostructures of varying plasmonic wavelengths is due to charge transfer, verified by the Raman spectroscopy. Further, the ultrafast transient absorption measurements support the thesis of charge transfer in the AuNP/Ge24Se76 heterostructure. These findings open up exciting opportunities for developing novel device technologies with far-reaching applications in nonlinear optics.

We present the design and simulations of a novel integrated device concept enabling a frequency conversion of a broad signal. The solution is based on a hybrid silicon–graphene photonic chip, which could be used for controlled spectrometry in low-cost devices. The device is based on a silicon-on-insulator (SOI) platform on which an arrayed waveguide grating (AWG) is designed for operation at the center wavelength of λ = 1800 nm. The AWG is spectrally separating one broad input signal to thirty-two-output channels with a channel spacing of 2.72 nm. The output signals are well separated and uniform with the extinction ratio and the standard deviation of 10.00 dB and 0.04, respectively. The 3 dB channel width is 1.34 nm, which is suitable for sensing applications with significant accuracy. After spacial and spectral separation, each output signal is then converted to one signal at 1480 nm wavelength through a graphene-based saturable absorber scheme. Therefore, the device allows the detection of each separated signal with a simple near-infrared camera on which the outputs are imaged using conventional optics, leading to a classical pixel/wavelength correspondence. Crossed-waveguide couplers are designed to combine the controlling signal at 1480 nm to each channel waveguide of the AWG. The combination of the signals saturates the graphene layer at the output waveguides, allowing the pass of the controlling wavelength. This device can be applied as a spectrometer in environmental sensing and monitoring with high efficiency and low cost.

Nonlinear optical materials, especially two-dimensional materials, are anticipated to reveal broadband optical nonlinearity for future miniaturized photonic applications. Herein, we report a physical vapor deposition method to produce β-In _2 Se _3 thin film and investigate the broadband nonlinear absorption ( β ) and refraction (n _2 ) characteristics. The β-In _2 Se _3 semiconductor shows an excellent optical nonlinearity with large β in 10 ^2 cm GW ^−1 scale and n _2 in 10 ^−12 cm ^2 W ^−1 scale from visible to NIR wavelengths, which are superior to those of metal carbides and nitrides (MXenes) and metal-organic frameworks. This excellent optical nonlinearity makes β-In _2 Se _3 a promising candidate for advanced nanophotonic devices and beyond.

The propagation of light in the epsilon-near-zero (ENZ) region of materials exhibits intriguing linear and nonlinear optical phenomenon that have been extensively exploited for a plethora of applications. Here, we show that the optical properties as well as the ENZ wavelength of magnetron-sputtered indium tin oxide (ITO) thin films could be judiciously engineered. The measurement of nonlinear optical properties reveals that the control of deposition conditions allows for the tuning of absorptive optical nonlinearity between saturable absorption and reverse saturable absorption. The ENZ wavelength for the ITO film is deduced as around 1553 nm. We obtain the highest third-order nonlinear absorption coefficient and imaginary part of third-order nonlinear susceptibility for the ITO thin film through Z-scan method as −50.56 cm/GW and ∼38 × 10−14 e.s.u. at 1050 nm, and −64.50 cm/GW and ∼45 × 10−14 e.s.u. at 1550 nm, respectively. We demonstrate further that the strong saturable absorption of the ITO thin film enables Q-switched pulse laser generation in ∼1050 and ∼1550 nm regions with tunable repetition rates and pulse energies. The present results suggest the great application potential of the ITO thin film in the field of nonlinear optical devices.

Metafibers expand the functionalities of conventional optical fibres to unprecedented nanoscale light manipulations by integrating metasurfaces on the fibre tips, becoming an emerging light-coupling platform for both the nanoscience and fibre optics communities. Current metafibers remain proof-of-concept demonstrations that mostly explore isolated bare fibres owing to the lack of standard interfaces with universal fibre networks. Here, we develop methodologies for fabricating well-defined plasmonic metasurfaces directly on the end facets of commercial single -mode fibre jumpers using standard planar technologies and provide the first demonstration of their practical applications in the nonlinear plasmonic regime. Featuring plug-and-play connections with fibre circuitry and arbitrary metasurface landscapes, the metafibers with tunable plasmonic resonances are implemented into fibre laser cavities, yielding all-fibre sub-picosecond (minimum 513 fs) soliton mode locked lasers at optical wavelengths of 1.5 μm and 2 μm, demonstrating their unusual polarimetric nonlinear transfer functions and superior saturation absorption responses. The nanofabrication process flow is compatible with existing cleanroom technologies, offering metafibers an avenue to become a regular member of functionalised fibre components. This work paves the way toward the next generation of ultrafast lasers, optical frequency combs, and ultracompact ‘all-in-fibre’ optical systems.

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. [ABSTRACT FROM AUTHOR]

Third‐order nonlinear optical (NLO) absorption properties and ultra‐fast carrier dynamics of organic 4‐N,N‐dimethylamino‐4'‐N'‐methyl‐stilbazolium tosylate (DAST) at the wavelength of 520 nm are investigated by femtosecond open‐aperture Z‐scan technique and transient absorption spectroscopy, respectively. Z‐scan measurements indicate that DAST solution exhibits ultra‐high nonlinear saturable absorption (SA) responses with a figure of merit of 4.57 × 10−13 esu cm and a saturation strength of 3.20 GW cm−2, far superior to those of the most known 2D SA materials under similar excitations. Transient absorption results reveal that the outstanding SA performances of the solution‐phase DAST are rooted in ultra‐fast ground‐state bleaching based on the Pauli blocking effect. The subsequent excited state carrier relaxation processes are dominated by rapid intraband carrier cooling and defect‐assisted interband Auger recombination. Moreover, high‐quality DAST−polyvinyl alcohol composite films prepared by solution casting exhibit similar ultra‐high SA responses and reliable long‐term stability without any degradation of their SA responses even after exposure to air moisture for 100 days. These findings establish an experimental and theoretical foundation for the development of high‐performance ultra‐fast optoelectronic devices based on organic NLO materials in the future. [ABSTRACT FROM AUTHOR]

Objective: The first-order absorption is a common model used in Pharmacokinetics. The absorption of some drugs follows carrier mediated transport. It has been proposed that the amount of drug available may saturate the transport mechanism resulting in an absorption slower than the one predicted by the first-order model. Saturable absorption has been modeled at the differential equation level by substituting the constant rate absorption by a Hill kinetics absorption. However, its exact solution is so far unknown. The goal of this is to know the exact solution of different Hill kinetic absorption models. Methods: We start defining different absorption models and increasing then their complexity. The simplest case is the first-order absorption model and the most complex will be a generalized Hill kinetic absorption model. The differential equation of each model is integrated. Results: The complexity of the models their solutions may be not expressed in a close-form, or in term of elementary functions. We obtain and discuss the exact solutions of the different Hill kinetics absorption models. To do that, the solutions are studied according to the possible values of the free parameters of the models. We show the differences between models through simulations. Conclusions: The knowledge of closed-form solutions allows to illustrate the differences between the different absorption models and minimizes the errors of numerical integration. [ABSTRACT FROM AUTHOR]

Measuring the absorption coefficient and analyzing the saturable absorption property of erbium-doped fiber (EDF) of different lengths at different values of input power is necessary for designing amplifiers and lasers with optimum features. Absorption coefficient for shorter lengths such as 1 m results in a perfect match between the experimentally obtained data and the trend expected for a homogeneously broadened system. This was not the case for longer lengths such as 4.6 m, 11 m and 25 m of EDF. The characteristic equation for saturable absorption needs modification at longer lengths of EDF. The deviation in the trend, from systems with homogeneous and inhomogeneous broadening, is very systematic with an increase in fiber length, input power and the operating wavelength. The influence of an increase in these three factors is similar, with a larger deviation from the standard inhomogeneous case to an increased inhomogeneity. The characteristics of saturable absorption in the presence of another signal at a lower wavelength imply a more rapid deviation from inhomogeneous broadening, but towards homogeneous broadening. [ABSTRACT FROM AUTHOR]

Compatible with existing processing technology, chemical vapor deposition method is used to synthesize ZrS2 and HfS2 films a a large scale. The nonlinear optical properties are characterized by Z‐scan measurement with femtosecond pulses at 800 nm. The results show that saturable absorption happens in ZrS2 owing to the larger ground state absorption than the excited state absorption, while reverse saturable absorption appears in HfS2 due to the two‐photon absorption. The figure of merit values of ZrS2 (≈4.30 ± 0.12 × 10−15 esu cm) and HfS2 (≈6.0 ± 1.4 × 10−15 esu cm) are much larger than those of MoS2 and graphene in ultrafast nonlinear optical performance at the wavelength of 800 nm. [ABSTRACT FROM AUTHOR]

Nonlinear optics is an important research direction with various applications in laser manufacturing, fabrication of nanostructure, sensor design, optoelectronics, biophotonics, quantum optics, etc. Nonlinear optical materials are the fundamental building blocks, which are critical for broad fields ranging from scientific research, industrial production, to military. Nanoparticles demonstrate great potential due to their flexibility to be engineered and their enhanced nonlinear optical properties superior to their bulk counterparts. Synthesis of nanoparticles by laser ablation proves to be a green, efficient, and universal physical approach, versatile for fast one-step synthesis and potential mass production. In this review, the development and latest progress of nonlinear optical nanoparticles synthesized by laser ablation are summarized, which demonstrates its capability for enhanced performance and multiple functions. The theory of optical nonlinear absorption, experimental process of laser ablation, applications, and outlooks are covered. Potential for nanoparticle systems is yet to be fully discovered, which offers opportunities to make various types of next-generation functional devices.

• A suitable length of unpumped erbium-doped fiber in an erbium-doped fiber laser reduces the laser linewidth. • Saturable absorption at the signal wavelength in the unpumped fiber causes the reduction in linewidth of the laser. • The interplay of cavity length, cavity loss and gain decide the optimal length of the fiber for laser linewidth reduction. • The laser linewidth reduction is sensitive to the location of the unpumped fiber in the cavity. • Passive loss such as by bending the fiber cannot cause laser linewidth reduction as obtained by saturable absorption. A standard erbium-doped fiber ring laser is constructed and studied for its lasing features. An additional length of unpumped erbium-doped fiber inserted in the ring cavity increases the cavity loss as well as the cavity length, and decides the new central wavelength of lasing. Under optimum conditions, the saturable absorption effect in the added length of unpumped erbium-doped fiber introduces more loss for the longitudinal modes of lower power while leaving the longitudinal modes of higher power undisturbed, thus narrowing the laser linewidth. The reduction in the 3 dB linewidth of the lasing peak from 0.170 nm to 0.070 nm is demonstrated using an additional unpumped erbium-doped fiber length of 85 cm at a low pump power of 60 mW. As a result of this reduction in linewidth, the quality factor increases from 9.2 x 103 to 2.2 x 104 and the number of longitudinal modes reduces from ∼ 1400 to ∼ 580 within the central lasing peak. The distinct reduction of spectral width and the consequent increase in the quality factor of the laser output are brought out through the experimental data. Unlike a linear loss element introduced in the ring cavity, the narrowing of linewidth due to the saturable absorber is not accompanied with a loss of power at the central wavelength. In addition, it is found to be dependent on the location where the saturable absorber is present in the ring cavity, thus emphasizing the role of selective absorption in the ring cavity. [ABSTRACT FROM AUTHOR]

In this paper, we explored the nonlinear optical (NLO) properties of Bismuth (Bi) doped titanium dioxide (TiO 2) nanoparticle (NP) colloids in ethanol, with laser pulses of duration ∼ 150 femtoseconds (fs). The Bi-doped TiO 2 NPs were synthesized via the chemical route, sol-gel method. The increased photo response of TiO 2 upon doping it with metals has become a burning issue to extend the applications of metal-doped TiO 2 NPs as integral parts in solar cells, catalysts, phototherapy, etc. The bandgap of anatase TiO 2 NPs was observed to be reduced to 2.89 eV upon doping Bi. The modified band structure of the Bi-doped TiO 2 NPs demonstrates novel chemical, mechanical, and optical properties. NLO studies were conducted on Bismuth-doped TiO2 NPs submerged in ethanol employing femtosecond laser input pulses with incoming wavelengths 700, 750, 800, 850, 900, and 950 nm. It has been detected that open aperture (OA) studies performed at an input peak intensity of 100 MW/cm2 on Bi-doped TiO 2 NPs in ethanol were exhibiting complex NLO behavior, i.e., reverse saturable absorption (RSA) in saturable absorption (SA) and SA in RSA with mostly effective two-photon absorption (1 + 1) coefficients. Particularly, at ∼800 nm, the behavior observed was so complex, i.e., RSA in SA in RSA in SA, with an effective 3 PA (2 + 1) co-efficient (γ) ∼6.5 × 10−24 m3/W2. The closed aperture (CA) studies at ∼37 MW/cm2 exhibited self-defocusing effects, i.e., an intensity-dependent refractive index (n 2) with a negative signature. [Display omitted] • 1.Bandgap decreased from 3.2 eV to 2.98 eV upon doping of 2.5 % Bi. TiO 2 NPs'. • Bi-doped TiO 2 NPs in ethanol demonstrated complex behavior RSA in SA in RSA. • A complex behavior RSA in SA in RSA in SA at 800 nm. [ABSTRACT FROM AUTHOR]

A flexible, self-standing, defect-free and robust nanofilm was innovatively fabricated via utilization of an interfacially confined dynamic condensation technique. The structure and thickness tunable nanofilm depicted remarkable broadband nonlinear optical properties. [Display omitted] • Flexible, self-standing, defect-free nanofilms were fabricated via interfacially confined dynamic condensation. • The nanofilms show excellent nonlinear optical properties including broadband saturable absorption (720 nm to 1700 nm) and large nonlinear absorption coefficient (−10.1 cm/MW). • The nonlinear optical properties can be tuned by changing either the incident pulse intensity or the nanofilm thickness. • A four-level model provided an insight into the possible underlying physical processes. Interfacial nanofilms with nonlinear optical (NLO) properties were prepared via confined dynamic condensation of 4,4′-methylenedianiline (MDA) with the synthesized 2,3-bis(4-(bis(4-formylphenyl)amino)phenyl)fumaronitrile (BTFA). Investigated using the open-aperture Z-scan technique, BTFA showed reverse saturable absorption ascribed to the synergetic mechanisms of two-photon and excited-state absorption. In contrast, the as-prepared nanofilms demonstrated broadband saturable absorption within the spectral range of 720∼1700 nm. The characteristics of nonlinear absorption coefficient (β) decreased along with increasing the incident pulse intensity. Taking advantage of the flexibility and post-machinability properties, the folding layers of the nanofilms offered the feasibility to fine-tune the specific NLO responses. The optimal β value was found to be −10.1 cm/MW for eight-layer nanofilm as well as the normalized transmittance increased up to 35-fold at 800 nm. Utilized as a conceptual saturable absorber, the representative modulation depth and saturation intensity were observed to be around 2.4% and 7.37 GW/cm2 at 800 nm, respectively, comparable to traditional two-dimensional (2D) materials. Aiming to clarify the possible underlying physical processes, a four-level model was employed to illustrate the fast relaxation of the excited states. Present work demonstrates that proper design of building blocks combined with interfacially confined dynamic condensation enables rational development of high-performance NLO materials. [ABSTRACT FROM AUTHOR]

The development of nonlinear optical (NLO) devices is limited to the mid‐infrared range due to the lack of broadband NLO response. As an indirect band gap semiconductor, BiOCl has a band gap of ≈3.3 eV. The special anisotropic lamellar structure makes it exhibit excellent electrical and optical properties; however, its NLO saturable absorption effect is rarely reported. In this study, the NLO response, optical properties, and applications of BiOCl crystals grown by chemical vapor phase transport method are studied in detail. The transmission spectrum indicates the BiOCl crystal has a broadband transparent region (0.4–11 µm). The saturable absorption of BiOCl is characterized by the Z‐scan method with 532 and 1030 nm pulsed lasers. With BiOCl as the saturable absorber (SA), watt‐level passively Q‐switched pulsed laser based on Yb:LuAG single crystal fiber (SCF) is realized. This is the first report of BiOCl as SA for passively Q‐switched all‐solid‐state pulsed laser, and the obtained average power is the highest value of passively Q‐switched SCF pulsed lasers reported so far. The high transmittance, high laser damage threshold, wideband NLO response, and environmental stability of BiOCl indicate that BiOCl crystal is a potential candidate for NLO applications. [ABSTRACT FROM AUTHOR]

We improved the three‐dimensional spatial resolution of laser scanning transmission microscopy by exploiting the saturated absorption of dye molecules. The saturated absorption is induced by the high‐intensity light irradiation and localises the signal within the centre of the focal spot. Our numerical calculation indicates that the spatial resolution in transmission imaging is significantly improved for both lateral and axial directions using nonlinear transmitted signals induced by saturated absorption. We experimentally demonstrated the improvement of the three‐dimensional resolution by observing fine structures of stained rat kidney tissues, which were not able to be visualised by conventional laser scanning transmission microscopy. [ABSTRACT FROM AUTHOR]

Single-stage processing of high-quality transparent functional polycrystalline ceramics is desirable but challenging. In the present work, spark plasma sintering (SPS) was employed for fabrication of Co2+:MgAl 2 O 4 saturable absorbers for laser passive Q-switching. Densification of commercial MgAl 2 O 4 powders, doped via co-precipitation, was carried out by conventional SPS and high-pressure SPS (HPSPS) under pressures of 60 and 400 MPa, respectively. The presence of LiF, a common sintering additive, was detrimental to optical properties as it promoted reaction of cobalt with sulfur impurities and the formation of Co 9 S 8 inclusions. Densification by HPSPS without LiF allowed to obtain highly transparent Co2+:MgAl 2 O 4. The optical properties of samples, with doping concentrations varying between 0.01 and 0.1 at.% Co2+, were assessed and saturable absorption was demonstrated at ~1.5 µm wavelength, exhibiting ground-state (σ gs) and excited (σ es) cross-sections of 3.5×10-19 and 0.8×10-19 cm2, respectively. Thus, it was established that HPSPS is an effective method to fabricate transparent Co2+:MgAl 2 O 4 ceramics. • Commercial MgAl 2 O 4 nano-powders were doped with Co2+ by co-precipitation. • Highly transparent 0.01–0.1 at% Co2+:MgAl 2 O 4 ceramics were fabricated by HPSPS. • Conventional SPS was not a viable method to produce Co2+:MgAl 2 O 4 ceramics. • LiF sintering additive induced reaction of Co with S which formed Co 9 S 8 inclusions. • HPSPS-processed Co2+:MgAl 2 O 4 can serve as laser passive Q-switch medias. [ABSTRACT FROM AUTHOR]

Ultrafast charge transfer in van der Waals heterostructures can effectively engineer the optical and electrical properties of two-dimensional semiconductors for designing photonic and optoelectronic devices. However, the nonlinear absorption conversion dynamics with the pump intensity and the underlying physical mechanisms in a type-II heterostructure remain largely unexplored, yet hold considerable potential for all-optical logic gates. Herein, two-dimensional ReSe 2 /ReS 2 heterostructure is designed to realize an unusual transition from reverse saturable absorption to saturable absorption (SA) with a conversion pump intensity threshold of approximately 170 GW/cm 2 . Such an intriguing phenomenon is attributed to the decrease of two-photon absorption (TPA) of ReS 2 and the increase of SA of ReSe 2 with the pump intensity. Based on the characterization results of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, femtosecond transient absorption spectrum, Kelvin probe force microscopy, and density functional theory calculation, a type-II charge-transfer-energy level model is proposed combined with the TPA of ReS 2 and SA of ReSe 2 processes. The results reveal the critical role of ultrafast interfacial charge transfer in tuning the unusual nonlinear absorption and improving the SA of ReSe 2 /ReS 2 under different excitation wavelengths. Our finding deepens the understanding of nonlinear absorption physical mechanisms in two-dimensional heterostructure materials, which may further diversify the nonlinear optical materials and photonic devices.

Metal–organic frameworks (MOFs) are widely explored owing to their excellent nonlinear optical (NLO) properties. However, researchers have mainly focused on designing new structures of MOF crystals to adjust the NLO response while ignoring the influence of the number of metal–ligand coordination bonds on the NLO properties of MOFs. In this study, the influence of the coordination numbers of MOFs on their NLO properties is studied for the first time. Herein, MOFs with different coordination numbers, using trifluoroacetic acid as an auxiliary agent, are synthesized and their NLO properties are tested using the Z‐scan technique. The results reveal that the NLO absorption properties of MOFs with high coordination numbers can be effectively modulated from saturable absorption to reverse saturable absorption by increasing the excitation intensity. First‐principles calculations show that a change in the coordination numbers leads to a change in the charge transfer from metal to ligand, thereby resulting in different NLO responses. The MOFs with a high coordination number have potential applications in novel optical switches or logic gates. These results provide a reference for the precise adjustment of the NLO properties of MOFs. [ABSTRACT FROM AUTHOR]

Abstract Antimonene, a new member of monoelemental two‐dimensional crystals composed of antimony atoms, was predicted to possess high stability, as well as outstanding electrical and optical properties. Here, we report on the experimental results of productive, high‐quality synthesis of antimonene quantum dots (AQDs) via liquid‐phase exfoliation. The obtained AQDs exhibited excellent nonlinear saturable absorption. The modulation depth and the saturable intensity were found to be around 3.9% and 10.02 GW/cm2, respectively. By using the AQDs saturable absorber in the 639 nm Pr:YLF laser, stable Q‐switching pulsed laser was realized. The maximum output power was 83 mW and the corresponding pulse width was 255 ns with a pulse repetition rate of 227 kHz. These results suggest that AQDs are a promising candidate for nonlinear optical applications.

Bismuth telluride nanoparticles (NPs) attract attention due to the growth of sensitivity of the Bi2Te3 NPs-containing registrars in the near- and mid-infrared ranges. We describe the synthesis and characterization of these structures and analyze the low-order nonlinear optical properties of the colloidal suspensions and thin films containing Bi2Te3 NPs using 1064 and 532 nm, 10 ns pulses. Colloidal Bi2Te3 NPs demonstrate saturable absorption and positive nonlinear refraction (saturation intensity 7 × 108 W cm−2, nonlinear absorption coefficient β SA = −7 × 10−8 cm W−1, nonlinear refractive index γ = 9 × 10−12 cm2 W−1), while at stronger excitation by 532 nm, 10 ns pulses the reverse saturable absorption dominates over other nonlinear optical processes. We achieved significant growth of the nonlinear optical parameters of the thin films containing these NPs (film thickness l = 60 nm, β SA = −1.2 × 10−4 cm W−1, γ = 5 × 10−7 cm2 W−1 in the case of 532 nm probe radiation and β = −5 × 10−5 cm W−1, γ = 6 × 10−8 cm2 W−1 in the case of 1064 nm probe radiation) compared with colloidal Bi2Te3 NPs and discuss the observed peculiarities of the nonlinear response of Bi2Te3 nanostructures.

Nonlinear optics (NLO) is an extremely critical research area. In recent years, covalent organic frameworks (COFs) have attracted much attention as a potential NLO material. In this study, a series of COF materials with different D-A effects were successfully synthesized based on the excellent electron-donating properties of the triphenylamine group and were prepared into D-A COFs\KBr sheet materials that can be used for third-order NLO by the KBr−pressing method. The results of the open-aperture Z-scan technique showed that all COFs\KBr sheets exhibited saturable absorption (SA) behavior at 3 μJ, with nonlinear absorption coefficients (β) of −1.64, −1.23, and −2.02 × 10−9 mW−1 for PT-PA , PT-BD , and PT-TZ , respectively. Theoretical calculations show that the electron transport distance of PT-PA is shorter than that of PT-BD , while the introduction of S atoms makes the D-A effect of PT-TZ stronger than that of PT-BD , and thus the SA properties of PT-PA and PT-TZ are better than those of PT-BD. These works indicate that the COFs\KBr sheets have a potential application as high-performance NLO materials and also provide an option for COFs powder application in the field of third-order NLO. In this study, we successfully prepared three COF/KBr composite films for Z-scan experiments using potassium bromide compression method. In experiments, COF materials exhibit significant donor-acceptor (D-A) interactions. By shortening the charge transport distance in the COF structure, or introducing sulfur (S) atoms to enhance the electron donor capacity, we effectively improve the optical nonlinear properties of COF materials. [Display omitted] • Three kinds of D-A COF/KBr sheets were prepared by the potassium bromide pressing method. • Due to varying electron donors and electron transport distances, COFs' electronic structures significantly alter. • PT-TZ/KBr sheet have a high nonlinear absorption coefficient of −2.02 × 10−9 m W−1. • Improve the NLO properties of COFs by reducing charge transport distance or incorporating S to boost electron donation. [ABSTRACT FROM AUTHOR]

Pharmacokinetic data of gabapentin (GBP) in community-dwelling elderly patients show a significant effect of advanced age on GBP pharmacokinetics due to altered renal function. However, there are no data in elderly nursing home (NH) patients to evaluate gabapentin absorption and elimination. Our objective was to characterize the pharmacokinetics of GBP in elderly nursing home patients maintained on GBP therapy. This was a prospective pharmacokinetic study in elderly nursing home patients (≥60 years) receiving GBP for the management of chronic pain or epilepsy from seven nursing homes. Pharmacokinetic parameters were estimated by nonlinear mixed-effects modeling. A one-compartment model described the data and clearance (CL) was associated with estimated glomerular filtration rate (eGFR) (p

Amorphous-Ge (α-Ge) or free-standing nanoparticles (NPs) synthesized via hydrogen-free plasma-enhanced chemical vapor deposition (PECVD) were applied as transmissive or reflective saturable absorbers, respectively, for starting up passively mode-locked erbium-doped fiber lasers (EDFLs). Under a threshold pumping power of 41 mW for mode-locking the EDFL, the transmissive α-Ge film could serve as a saturable absorber with a modulation depth of 52–58%, self-starting EDFL pulsation with a pulsewidth of approximately 700 fs. Under a high power of 155 mW, the pulsewidth of the EDFL mode-locked by the 15 s-grown α-Ge was suppressed to 290 fs, with a corresponding spectral linewidth of 8.95 nm due to the soliton compression induced by intra-cavity self-phase modulation. The Ge-NP-on-Au (Ge-NP/Au) films could also serve as a reflective-type saturable absorber to passively mode-lock the EDFL with a broadened pulsewidth of 3.7–3.9 ps under a high-gain operation with 250 mW pumping power. The reflection-type Ge-NP/Au film was an imperfect mode-locker, owing to their strong surface-scattered deflection in the near-infrared wavelength region. From the abovementioned results, both ultra-thin α-Ge film and free-standing Ge NP exhibit potential as transmissive and reflective saturable absorbers, respectively, for ultrafast fiber lasers.

• Nano-deposits at different deposition temperatures have hexagonal wurtzite structures with (1 0 1) plane. • Deposits prepared at 723 K have higher crystallinity with few dislocations' density. • Films deposited at 723 K demonstrate enhanced electrical properties. • Photoluminescence studies indicate near-white light emission across all thin films. • The total nonlinear susceptibility decreases as the deposition temperature rises. This study reports the influence of deposition temperature (573 K-773 K) on the microstructural, linear and nonlinear optical properties of zinc oxide nano thin films synthesized using the chemical spray pyrolysis method. Synthesized films are polycrystalline, exhibiting a dominant (1 0 1) orientation, for which the crystallite size is found to increase with deposition temperature up to 723 K followed by a decrease. Concurrently, the deposit prepared at 723 K showed improved electrical properties, i.e. higher charge carrier concentration and mobility. Optical studies reveal increased transmittance in the visible region and a gradual increase of optical bandgap with deposition temperature. The photoluminescence studies show a near-white light emission for all thin films, however, the density of defect states was comparatively higher for thin films deposited at lower temperatures. A saturable absorption behaviour was observed due to the presence of defects. The thin films show a negative refractive index with self-defocusing phenomena. The total nonlinear susceptibility is found to decrease with increasing deposition temperature, and the solution-processed functional thin films thus have potential implications for nonlinear optical applications such as nonlinear optical switching, optical memory management, and saturable absorbers. [ABSTRACT FROM AUTHOR]

The ultrafast optical nonlinearity of an azonaphthalene derivative, hydroxy naphthol blue (HNB) disodium salt, is investigated using transient absorption spectra (TAS) and Z-scan technique. Reverse saturable absorption (RSA) is observed within the resonant absorption band, which is usually dominated by ground-state bleaching (GSB) and stimulated emission (SE) in most organic compounds. This observation indicates a remarkably strong excited-state absorption (ESA) in HNB. Global analysis of TAS suggests an ultrafast conversion from saturable absorption (SA) to RSA, which is assigned to the internal conversion from locally excited (LE) state to the first singlet (S 1) state. Further Z-scan study verifies the above results and reveals another intensity-depend SA-RSA transition, indicating increased S 1 population under strong laser irradiation. The calculated value of third-order nonlinear absorption coefficient at 650 nm, 190 fs is determined as 1.23 × 10−12 m/W. These studies demonstrate significant potential of azonaphthalene in ultrafast laser protection if further modulation of excited-state absorption can be achieved. • The ultrafast optical nonlinearity of HNB is studied by TAS and Z-scan. • RSA in resonant absorption region is observed due to extremely strong ESA on first excited state. • Mechanism of intensity-depend SA-RSA transition is investigated. [ABSTRACT FROM AUTHOR]