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The study of conductive oxides has gained momentum within the photonics community due to their unique linear and nonlinear optical properties. Despite recent experiments reporting on high harmonic generation from thin films, the optical/electronic behavior of these compounds at the nanoscale is still not fully understood due to the lack of a suitable theoretical model. In the present work, aluminum zinc oxide is excited near its epsilon‐near‐zero crossing point using incident femtosecond pulses having peak power densities in the 1 TW cm−2 range. A relatively efficient frequency up‐conversion including even and odd harmonics up to the seventh order is observed. A hydrodynamic‐Maxwell theoretical approach is adopted, capable of simultaneously taking into account linear and nonlinear dispersions, nonlocal effects, surface, magnetic, and bulk nonlinearities in a spectral region that spans over two and a half octaves from the UV to the NIR region. The study enables a deeper understanding of the fundamental material parameters regulating optical nonlinearities, providing important insights to engineer this class of materials for applications in sensing, ultra‐fast physics, and spectroscopy. [ABSTRACT FROM AUTHOR]

In this paper, the Kundu–Eckhaus equation which describes the propagation of ultrashort pulses or femtosecond light pulses in nonlinear optics is investigated using the new mapping method. As a result, different structures of chirped solitary and periodic waves were yielded. The dynamic of the soliton and kink soliton chirp waves were illustrated from which we can conclude that the sign of the Raman parameter identifies the chirp wave function direction. [ABSTRACT FROM AUTHOR]

The generation of terahertz radiation via laser-induced plasma from two-color femtosecond pulses in air has been extensively studied due to its broad emission spectrum and significant pulse energy. However, precise control over the temporal properties of these ultra-broadband terahertz pulses, as well as the measurement of their polarization state, remain challenging. In this study, we review our latest findings on these topics and present additional results not previously reported in our earlier works. First, we investigate the impact of chirping on the fundamental wave and the effect of manipulating the phase difference between the fundamental wave and the second-harmonic wave on the properties of generated terahertz pulses. We demonstrate that we can tune the time shape of terahertz pulses, causing them to reverse polarity or become bipolar by carefully selecting the correct combination of chirp and phase. Additionally, we introduce a novel technique for polarization characterization, termed terahertz unipolar polarimetry, which utilizes a weak probe beam and avoids the systematic errors associated with traditional methods. This technique is effective for detecting polarization-structured terahertz beams and the longitudinal component of focused terahertz beams. Our findings contribute to the improved control and characterization of terahertz radiation, enhancing its application in fields such as nonlinear optics, spectroscopy, and microscopy. [ABSTRACT FROM AUTHOR]

A key objective of the paper is to study the dynamical system for the two types of Sasa–Satsuma equations, namely; Sasa–Satsuma equation and higher-order Sasa–Satsuma equation. A Sasa–Satsuma equation is used to describe the propagation of femtosecond pulses through optical fiber systems. The bifurcation and chaotic characteristic of the Sasa–Satsuma equation and higher-order Sasa–Satsuma equation that arises in fluid dynamics and nonlinear optics are studied. For both models, by using the theory of planar dynamical system the bifurcation and chaotic characteristic of the Sasa–Satsuma equation and higher-order Sasa–Satsuma equation that arises in fluid dynamics and nonlinear optics are studied. For a better understanding of these dynamical behaviors, phase portraits in 2D and 3D figures are dawn. For both equations, the equilibrium points and their effects on the bifurcation behavior are analyzed. Moreover, from the presented results, both models have different dynamical behavior. [ABSTRACT FROM AUTHOR]

Many scientific and technological applications require the generation of broadly tunable femtosecond light pulses. Optical parametric amplifiers (OPAs) exploit second-order nonlinear interactions to convert a high-power fixed wavelength pulse (the pump) into a tunable pulse (the signal). This paper reviews the principles of OPAs and highlights their capability to generate few-optical-cycle pulses with high energy and carrier-envelope-phase stability. [ABSTRACT FROM AUTHOR]

This paper investigates a higher-order modified nonlinear Schrödinger equation with higher-order dispersion and self-steepening effects, which can be used to study the dynamics of asymmetric and steepened optical pulse transmission in optical fibres. The modulation instability of the plane-wave solution has been analysed, and the state transition of the rogue waves under the higher-order dispersion effect is presented. The findings demonstrate that the self-steepening effect may also produce an asymmetric unstable frequency band. Combined with the modulation instability, the rogue wave, rational soliton and mixed interaction of localized waves have a quantitative relationship with plane-wave parameters. Various exact solutions can be accurately located and obtained according to the generalized Darboux transformation. The asymptotic analysis of rational solutions demonstrates the state transition of higher-order rogue waves. This paper illustrates the significance of modulation instability for studying the integrable systems by the Darboux transformation. It is a new guidance to solve the difficulty of exact excitation of asymmetric localized waves in complex integrable systems. The results have prospective applications and references for the emergence, amplification and compression of asymmetric pulses in optical experiments. [ABSTRACT FROM AUTHOR]

Coherent correlation between two distant broadband single photons can be realized by using the nonlinear optical method, which provides a key technology for spread‐spectrum communication in the quantum network. Here, sum‐frequency generation (SFG) between two broadband single photons is experimentally demonstrated by using chirp technology. The results show that the efficiency of the SFG is 6.51 × 10−8, and the visibility of coherent correlation of two distant broadband photons of the telecommunications band is 93.7%. This technique can also be used for other quantum applications, such as quantum entanglement swapping and quantum communication. [ABSTRACT FROM AUTHOR]

The effect of Fe defects on carrier recombination and two-photon induced ultrafast exciton dynamics in GaN crystals were investigated using femtosecond transient absorption spectroscopy. The absorption kinetics exhibited completely different characteristics under different nonequilibrium carrier concentrations and distributions. The carrier recombination mechanisms under different excitation conditions can be interpreted by the model of Fe defect-related energy level. The absorption under one-photon excitation originates from the intraband transition of free holes, and the capture of holes by Fe2+ leads to the decay of the absorption. The ultrafast excitonic state [Fe2+,hVB] formation time under two-photon excitation results in a rapid absorption decay (<20 ps), while the long tail of the absorption response could be attributed to long-lived excitonic state (>100 ns). These findings are crucial to the applications of GaN for ultrafast optoelectronics and integrated nonlinear optics. [ABSTRACT FROM AUTHOR]

High harmonic generation (HHG) in semiconductors has been extensively studied recently in the high-intensity limit using middle infrared (mid-IR) femtosecond laser pulses resulting in emission spectra of self-phase modulated harmonics resting on top of a broadband continuum. In this report, a different approach to HHG in polycrystalline zinc selenide (poly-ZnSe) was explored utilizing a relatively low power regime (1–40 GW/cm2) and much longer (30 ps) mid-IR laser pulses. Through a combination of low power, picosecond excitation, and narrowband (<10 nm full width at half maximum) mid-IR excitation, the nonlinear optical effects in poly-ZnSe could be isolated and studied independently. From the clearly distinguishable HHG peaks, harmonic conversion efficiencies of 10−4–10−12 for second to ninth harmonic in poly-ZnSe were measured, and the relationship between the Nth harmonic intensity and excitation intensity (I0) was found to follow a power law, I0x with x ≤ N/2, as a result of the random quasi-phase matching process. [ABSTRACT FROM AUTHOR]

Fiber Bragg grating (FBG) relative humidity (RH) sensors are fabricated in commercially available polyimide (PI)-coated optical fibers with diameters of 50 and 125 μm. Infrared (800 nm) femtosecond pulse duration laser pulses and a phase mask are used to inscribe Type-I and Type-II FBGs directly through the protective polyimide coatings of both 50 and 125 μm diameter fibers without typical fiber processing such as hydrogen loading, cryogenic storage, stripping, recoating or annealing. The devices are then evaluated for their performance as humidity sensors. At telecom wavelengths, the 50 μm diameter fiber devices with a 10 μm thick PI coating had a wavelength shift of the Bragg resonance at a constant temperature of 2.7 pm/%RH, whereas the 125 μm diameter fiber devices with a 17 μm thick PI coating had a wavelength shift of 1.8 pm/%RH. The humidity sensors in the 50 µm diameter fiber demonstrated a more rapid response time to small changes in humidity and a weaker hysteresis when compared to the 125 µm diameter fiber devices. No modification to the PI coatings was observed during fabrication. No difference in RH sensitivity was observed for Type-I devices when compared with Type-II devices with the same fiber. The applicability of this approach for fabricating distributed RH sensing arrays with hundreds of sensing elements on a single fiber is discussed. [ABSTRACT FROM AUTHOR]

Femtosecond light pulses carrying optical angular momentums (OAMs), possessing intriguing properties of helical phase fronts and ultrafast temporal profiles, enable many applications in nonlinear optics, strong-field physics, and laser micromachining. While generation of OAM-carrying ultrafast pulses and their interactions with matters are intensively studied in experiments, 3D characterization of ultrafast OAM-carrying light beams in spatiotemporal domain has, however, proved difficult to achieve. Conventional measurement schemes rely on the use of a reference pulsed light beam, which needs to be well characterized in its phase front and to have sufficient overlap and coherence with the beam under test, largely limiting practical applications of these schemes. Here a self-referencing set-up is demonstrated based on a tilted interferometer that can be used to measure complete spatiotemporal information of OAM-carrying femtosecond pulses with different topological charges. Through scanning one interferometer arm, the spectral phase over the pulse spatial profile can be obtained using the tilted interference signal, and the temporal envelope of the light field at one particular position around its phase singularity can be retrieved simultaneously, enabling 3D beam reconstruction. This self-referencing technique, capable of measuring spatiotemporal ultrafast optical-vortex beams, may find many applications in fields of nonlinear optics and light-matter interactions. [ABSTRACT FROM AUTHOR]

Third-order nonlinear optical (NLO) properties of indium tin oxide (ITO) thin film were studied using high repetition rate (80 MHz), femtosecond (100 fs), and near-infrared (NIR) (750–820 nm) laser pulses. An ITO thin film was prepared using an RF magnetron sputtering system. The film thickness was determined using a scanning electron microscope (SEM), while the linear optical properties of the thin film were measured using a UV–Vis spectrophotometer. Nonlinear absorption (NLA) studies of ITO thin film using an open aperture Z-scan revealed a combination of nonlinear phenomena: reverse saturable absorption (RSA) and saturable absorption (SA). A saturation model has been used to explain the observed saturation of two-photon absorption (2PA) at high incident powers. The ITO film's NLA properties were found to be excitation power and wavelength dependent. The NLA coefficient was shown to be inversely proportional to the excitation power and wavelength. Furthermore, NLA measurements of ITO thin films show that the transition from RSA to RSA-SA-RSA switching behavior was influenced by ITO thickness. Because of their high third-order nonlinear optical responses, these ITO thin films are ideal candidates for photonic applications. [ABSTRACT FROM AUTHOR]

This work reports a series of new organic electro‐optic salt crystals possessing unusual orthogonal dipole‐coupling assembly that act as dimple‐free THz generators. In the new crystals, optically anisotropic aromatic anions introducing highly electronegative fluorinated substituents are orthogonally assembled with optically anisotropic nonlinear optical benzothiazolium cations. These orthogonal cation–anion assembled crystals exhibit strong suppression of intrinsic THz phonon modes along the polar axis, which results in a wide THz absorption‐free range from 1.5 to 4.0 THz without substantial absorption peaks. This is sharply in contrast to previous benchmark crystals with parallel cation–anion assembly; for example, in analogous crystals, several strong absorption peaks appear in this frequency range. This absence of strong absorption peaks is attributed to the orthogonal cation–anion dipole coupling assembly, the formation of fluorinate‐induced strong interionic interactions, and a high packing ability of uniaxial anions. With top‐level macroscopic optical nonlinearity, new orthogonal‐dipole assembled crystals make dimple‐free THz wave generation possible in the range of about 1.5–4.0 THz, which is very beneficial for characterizing signature peaks related to molecular phonon motions of analytes in THz spectroscopy. Therefore, orthogonal molecular assembly of optically highly anisotropic molecules provides an efficient way to substantially reduce and control intrinsic THz phonon modes. [ABSTRACT FROM AUTHOR]

We report two-photon absorption in ruby in an experiment simple enough to reproduce in an undergraduate optics course. The high peak power of a femtosecond (fs) pulse from an unamplified titanium:sapphire laser is sufficient to drive a two-photon transition to ruby's broad absorption bands, with subsequent emission at 694 nm from the metastable state. This experiment uses pulses with tens of nanojoules rather than the tens or hundreds of millijoules required for nanosecond experiments, and can be performed with trivial instrumentation – eliminating photomultiplier tubes in favor of simple, inexpensive photodiodes with no amplification or signal processing. A two-photon measurement provides excellent educational value for undergraduate students and expands the repertoire of pedagogical optics experiments in an interesting direction. [ABSTRACT FROM AUTHOR]

The Lugiato–Lefever equation is a cubic nonlinear Schrödinger equation (NLSEs) that occurs as a model in nonlinear optics and includes damping, detuning, and driving. On the basis of a model of coupled nonlinear NLSEs, the trapping behaviour of two chirped solitons creating a bound state in a single-mode birefringent fibre is explored. The positive initial chirp is critical for managing the soliton trapping threshold amplitude without creating excessive pulse broadening. In this paper, the Jacobi elliptic function (JE) technique, which is one of the efficient integration procedures, is used to investigate chirped elliptic and solitary wave solitons (SWS) to NLSEs with generalized Longitudinal Lugiato Lefever (GLLL) equation. As a result, the bright, single, dark, bell, kink soliton, and other solutions of the governing model are found. In further, we demonstrate successful results in 3D, 2D and contour structures. [ABSTRACT FROM AUTHOR]

In this study, we experimentally investigated the nonlinear optical properties of Au nanoparticles (Au NPs) that were prepared in pure distilled water using the laser ablation method. The Au NPs were prepared using a nanosecond Nd:YAG laser with an ablation time of 5 or 10 min at a constant laser energy of 100 mJ. The structure and the linear optical properties of the Au NPs were investigated using a transmission electron microscope (TEM) and UV-visible spectrophotometer analysis, respectively. The TEM measurements showed that the average size of the Au NPs varied from 20.3 to 14.1 nm, depending on the laser ablation time. The z-scan technique was used to investigate the nonlinear refractive index ( n 2 ) and nonlinear absorption coefficient (γ) of the Au NPs, which were irradiated at different excitation wavelengths that ranged from 740 to 820 nm and at different average powers that ranged from 0.8 to 1.6 W. The Au NP samples exhibited a reverse saturable absorption (RSA) behavior that increased when the excitation wavelength and/or incident laser power increased. In addition, the Au NPs acted as a self-defocusing material whenever the excitation wavelength or incident power were modified. [ABSTRACT FROM AUTHOR]

Ionization-induced multiwave mixing is attracting much interest nowadays due to the possibility of generating short pulses of secondary radiation over a very wide spectral range, from terahertz to far ultraviolet. This paper presents an analytical method for calculating the amplitudes of arbitrary spectral components of free electron currents arising under the action of multicolor ionizing laser pulses. We show that this method can be used to obtain the dependences of characteristics of a frequency-tunable third harmonic of the intense component of a three-color pulse obtained in an optical parametric generator. The obtained results are in good agreement with quantum mechanical calculations. [ABSTRACT FROM AUTHOR]

A series of "push–pull" porphyrins, MTPP(MN)(TPA)2 (M = 2H, CuII, NiII, and ZnII), having triphenylamine (TPA) and dicyanovinyl (DCN) groups at antipodal positions were synthesized and characterised by UV-Vis, fluorescence and NMR spectroscopic techniques, MALDI-TOF mass spectrometry, cyclic voltammetry, DFT, and elemental analysis, which were then further utilized for third-order nonlinear optical measurements under mild conditions using femtosecond laser pulses. Remarkably, MTPP(MN)(TPA)2 (M = 2H, CuII, NiII, and ZnII) exhibited 21–48 nm and 38–80 nm bathochromic shifts in B and Qx(0,0) bands as compared to the corresponding MTPPs (M = 2H, CuII, NiII, and ZnII); the results are consistent with the effect of enhanced resonance due to TPA and –I effect of DCN moieties. In cyclic voltammetry, the push–pull porphyrins exhibited a cathodic shift (0.13–0.51 V) in their first oxidation potential as compared to the precursor owing to the presence of electron-donating TPA groups. The third-order nonlinear optical responses were recorded using a single-beam femtosecond Z-scan technique to retrieve information about the nonlinear absorption and nonlinear refraction of the samples. The two-photon absorption coefficients (β) are in the range of 0.87 × 10−13 to 4.28 × 10−13 m W−1 and the nonlinear refractive index (n2) in the range of 1.21 × 10−19 to 7.36 × 10−19 m2 W−1. The ultrafast absorption dynamics of the ground-state bleaching (GSB) and photo-induced absorption (PIA) are monitored by femtosecond broadband transient absorption studies. The strong nonlinearity of these push–pull porphyrins makes them potential candidates for nonlinear optical and photonic device applications. [ABSTRACT FROM AUTHOR]

We compared the experimental two-photon absorption spectra of two hydrazone derivatives of chalcone (HDC-1 and HDC-2) and two pyrazoline derivatives of dibenzylideneacetone and chalcone (PD-1 and PD-2). The compounds were synthesized by the Claisen-Schmidt reaction and characterized using NMR, FTIR, and UV–Vis. Using femtosecond pulses, the Z-scan technique recorded two-photon absorption cross-section (σ 2 P A ) spectra of the investigated compounds, which presented values of σ 2 P A , ranging from ∼12 GM to ∼50 GM. Such σ 2 P A difference is generally associated with the conjugated chain length and the different functional and peripherical groups used. For example, increasing the π -chain and peripherals donor groups p-OCH 3 from HDC-1 to HCD-2 led to a change in the σ 2 P A magnitude (σ H D C − 1 2 P A ∼ 35 G M , σ H D C − 2 2 P A ∼ 50 G M). The modification on the pyrazoline derivatives, i.e., increasing the conjugated chain and adding p-OCH 3 in both phenyls in the PD-2 compared to compound PD-1 doubled the σ 2 P A magnitude (σ P D − 1 2 P A ∼ 12 G M , σ P D − 2 2 P A ∼ 22 G M). According to the obtained results and considering the investigated compounds' short π-chain, we could claim that these classes of organic molecules promise nonlinear optical materials for optical limiting due to optical transparency ranging from 450 nm for pyrazolines to 550 nm for hydrazone derivatives. • Short π-Chain Organic Compounds: Hydrazones have N=N bonds and aromatic rings; pyrazolines have a 5-member nitrogen ring. • Z-Scan Technique: using femtosecond pulses under 100 μW to accurately characterize the two-photon absorption spectra. • Molecular Structure Impact: affect their two-photon absorption, ranging from 12-50 GM. • Potential Photonic Applications: Hydrazones and pyrazolines show potential in optical limiting devices. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate enhanced spectral broadening of femtosecond optical pulses after propagation through silicon-on-insulator (SOI) nanowire waveguides integrated with two-dimensional (2D) graphene oxide (GO) films. Owing to the strong mode overlap between the SOI nanowires and the GO films with a high Kerr nonlinearity, the self-phase modulation (SPM) process in the hybrid waveguides is significantly enhanced, resulting in greatly improved spectral broadening of the femtosecond optical pulses. A solution-based, transfer-free coating method is used to integrate GO films onto the SOI nanowires with precise control of the film thickness. Detailed SPM measurements using femtosecond optical pulses are carried out, achieving a broadening factor of up to ~4.3 for a device with 0.4-mm-long, 2 layers of GO. By fitting the experimental results with the theory, we obtain an improvement in the waveguide nonlinear parameter by a factor of ~3.5 and in the effective nonlinear figure of merit (FOM) by a factor of ~3.8, relative to the uncoated waveguide. Finally, we discuss the influence of GO film length on the spectral broadening and compare the nonlinear optical performance of different integrated waveguides coated with GO films. These results confirm the improved nonlinear optical performance of silicon devices integrated with 2D GO films. [ABSTRACT FROM AUTHOR]

In this study, the single- and multiple-pulse laser-induced surface damage characteristics of the Ge 8 As 23 S 69 chalcogenide film under the femtosecond laser irradiation were experimentally investigated. The 1-on-1 and S-on-1 methods were utilized to measure the femtosecond laser damage by using single and multiple pulses, respectively. The femtosecond laser-induced damage threshold (LIDT) was obtained using linear regression method by measuring the damage morphology under different pulse energies and pulse numbers of the femtosecond laser. Results show that the LIDT of the film under single-pulse radiation is higher than that under multiple-pulse radiation because of accumulation and defect effects. For the single-pulse radiation, LIDT is attributed to the field enhancement effect, multiphoton ionization, and avalanche ionization. The single-pulse damage threshold of Ge 8 As 23 S 69 films is 232.548 mJ/cm2. For the multiple-pulse radiation, the LIDT of the film decreases with increasing pulse number due to accumulation and defect effects. [ABSTRACT FROM AUTHOR]

Multi‐photon absorption (MPA) has attracted interest for applications exploiting the tight spatial control of interaction volume and long wavelength excitation. However, a deficiency of molecules exhibiting higher‐order nPA (n‐photon absorption, n>2) and a lack of structure–property studies to define the key structural characteristics needed to optimize higher‐order MPA performance have hindered practical development. We herein report the syntheses of second‐ and third‐generation metallodendrimers and assessment of their nonlinear absorption, together with those of zero‐ and first‐generation analogues. We report the first 5PA and 6PA data for an organometallic. The largest dendrimer exhibits exceptional three‐, four‐, five‐ and six‐photon absorption to femtosecond‐pulsed light. The systematically varied compounds highlight the crucial role of metal‐to‐oligo(phenyleneethynylene) charge transfer in promoting outstanding MPA activity. [ABSTRACT FROM AUTHOR]

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 800 nm. 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. 5 0 × 1 0 − 7 and 6. 3 9 × 1 0 − 7 cm2/GW, respectively. The results show that the bimetallic nanoparticle has potential possibility in nonlinear photonic applications. [ABSTRACT FROM AUTHOR]

Nanohybrids composed of carbon dots and gold nanoparticles are of considerable interest for luminescence imaging, photocatalysis, luminescence sensing and nonlinear optics. We describe a method for controlled growth of Au nanoparticles on the carbon nanodots by photocatalytic reduction of Au(III) ions under blue light irradiation. The resulting Au–carbon dots nanohybrids exhibit enhanced nonlinear photoluminescence under near-infrared femtosecond laser excitation with intensity which sharply depends on the gold-to-carbon dots volume ratio. Bright broadband photoluminescence of nanohybrids makes them attractive for luminescent imaging. [ABSTRACT FROM AUTHOR]

Supercontinuum generation has been extensively studied in optical waveguides pumped with femtosecond pulses, while there are few reports using picosecond pump pulses with a small spectral power variation. In this paper, we numerically demonstrate the supercontinuum generation in a dispersion-engineered silicon nitride waveguide pumped with 10-ps pulses at 1550 nm wavelength. A supercontinuum from 656 to 3867 nm (2.5 octaves) is enabled by uniquely tailored dispersion. The spectrum has a power variation of < 15 dB and shows a very flat pedestal varying by < 5 dB. The spectra generated from different dispersion profiles are compared and analyzed, which verifies the unique role of dispersion tailoring in flattened supercontinuum generation. To the best of our knowledge, it is the first study on such broad and flat supercontinuum generation on a chip using picosecond pump pulses, which would be highly desirable for many applications. [ABSTRACT FROM AUTHOR]

Light generated in optical fibers due to various nonlinear processes often has a broad spectrum and is commonly considered as a potentially convenient source of seed for tunable wavelength laser systems and many other applications. The nonlinear processes usually considered when discussing this spectrum broadening are coherent.In this paper we present experimental and theoretical investigation of a partially coherent nonlinear phenomenon occurring at the same time — ultraviolet–visible light (375 nm–500 nm) generation in a short photonic crystal fiber pumped by ∼110fs duration and 1028 nm wavelength pulses.

Thin-disk multipass amplifiers represent one of the most powerful approaches to scale the average and peak powers of ultrafast laser systems. The present paper presents the amplification of picosecond and femtosecond pulses to average powers exceeding 2 and 1 kW, respectively. Second-harmonic generation in lithium-triborate crystals with powers higher than 1.4 kW and 400 W at a wavelength of 515 nm with picosecond and femtosecond pulse durations, respectively, are also reported. Furthermore, third-harmonic generation was demonstrated with output powers exceeding 250 W at a wavelength of 343 nm. Finally, processing of silicon, metals, and polycrystalline diamond with fs pulses at an average power of 1 kW is presented to demonstrate removal rates that are improved by orders of magnitude as compared to state-of-the-art techniques. [ABSTRACT FROM AUTHOR]

The investigation of two-dimensional (2D) nonlinear optical materials offers a promising way to construct the high-performance optical devices in fundamental and industrial applications because of their rich distinct optoelectronic properties. Herein, by utilizing the liquid exfoliation method, vanadium disulfide (VS2) nanosheets are prepared and the thickness is measured to be 3.16 nm. In addition, we have fabricated the VS2-based optical device and the nonlinear optical property is characterized with modulation depth of 23.97%. By using VS2 as saturable absorber, a high stable passively mode-locking Er-doped fiber laser is obtained with pulse duration of 169 fs and the largest average output power of 70.5 mW. The slope efficiency is up to 7.9%. In comparison to recent results of mode-locking fiber lasers with 2D materials, the VS2-based fiber laser demonstrates better performance. To the best of our knowledge, this is the first example of using VS2 for generating femtosecond mode-locked laser pulse. Our experimental results not only reveal VS2 ultrafast photonics application, but also advance the high-performance applications for information science and nonlinear optics. [ABSTRACT FROM AUTHOR]

A synchronously pumped optical parametric oscillator based on periodically poled potassium titanyl phosphate (PPKTP) crystal delivering femtosecond nanojoule energy pulses in the 1.45 μm–1.8 μm spectral range is used as pump source for continuum generation in conventional non-zero dispersion shifted optical fiber. Detailed numerical simulations and experiments employing cross-correlation frequency-resolved optical gating (XFROG) reveal soliton and dispersive wave generation dynamics and can help to estimate limits of how far the pump wavelength can be shifted from zero-dispersion wavelength (ZDW) to still obtain sufficient spectrum broadening. We also demonstrate the applicability of recently introduced fiber dispersion measurement technique based on XFROG trace analysis to conventional optical fibers. Combination of this efficient parametric device with standard non-zero dispersion shifted low cost and low loss optical fiber results in efficient continuum generation with output/input average power ratio greater than 50%.

Femtosecond laser pulses have been extensively employed to achieve 3D micro‐ and nanofabrication in various materials for a broad variety of applications. LiNbO3 crystal is a multifunctional material due to its combination of a number of excellent properties. By using femtosecond laser direct writing, various structures have been fabricated in LiNbO3 crystals based on the refractive index changes, surface morphology modifications, or domain structures to realize intriguing applications in integrated photonics. Herein, a state‐of‐the‐art review is provided on the femtosecond laser processing of LiNbO3 crystals, including the fabrication of diverse photonic structures and applications in nonlinear optics. To conclude, a brief outlook with a few potential spotlights is presented. [ABSTRACT FROM AUTHOR]

Laser lift‐off (LLO) is commonly applied to separate functional thin films from the underlying substrate, in particular light‐emitting diodes (LEDs) on a gallium nitride (GaN) basis from sapphire. By transferring the LED layer stack to foreign carriers with tailored characteristics, for example, highly reflective surfaces, the performance of optoelectronic devices can be drastically improved. Conventionally, LLO is conducted with UV laser pulses in the nanosecond regime. When directed to the sapphire side of the wafer, absorption of the pulses in the first GaN layers at the sapphire/GaN interface leads to detachment. In this work, a novel approach towards LLO based on femtosecond pulses at 520 nm wavelength is demonstrated for the first time. Despite relying on two‐photon absorption with sub‐bandgap excitation, the ultrashort pulse widths may reduce structural damage in comparison to conventional LLO. Based on a detailed study of the laser impact as a function of process parameters, a two‐step process scheme is developed to create freestanding InGaN/GaN LED chips with up to 1.2 mm edge length and ≈5 μm thickness. The detached chips are assessed by scanning electron microscopy and cathodoluminescence, revealing similar emission properties before and after LLO. [ABSTRACT FROM AUTHOR]

Extreme nonlinear interactions of THz electromagnetic fields with matter are the next frontier in nonlinear optics. However, reaching this frontier in free space is limited by the existing lack of appropriate powerful THz sources. Here, we experimentally demonstrate that two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm allows one to generate ultrashort sub-cycle THz pulses with sub-milijoule energy and THz conversion efficiency of 2.36%, resulting in THz field amplitudes above 100 MV cm−1. Our numerical simulations predict that the observed THz yield can be significantly upscaled by further optimizing the experimental setup. Finally, in order to demonstrate the strength of our THz source, we show that the generated THz pulses are powerful enough to induce nonlinear cross-phase modulation in electro-optic crystals. Our work paves the way toward free space extreme nonlinear THz optics using affordable table-top laser systems. Powerful terahertz pulses are generated during the nonlinear propagation of ultrashort laser pulses in gases. Here, the authors demonstrate efficient sub-cycle THz pulse generation by using two-color midinfrared femtosecond laser filaments in ambient air. [ABSTRACT FROM AUTHOR]

An interaction of laser pulse, containing a few cycles, with substance is a modern problem, attracting attention of many researches. The frequency conversion is a key problem for a generation of such pulses in various ranges of frequencies. Adequate description of such pulse interaction with a medium is based on a slowly evolving wave approximation (SEWA), which has been proposed earlier for a description of propagation of the laser pulse, containing a few cycles, in a medium with cubic nonlinear response. Despite widely applicability of the frequency conversion for various nonlinear optics problems solutions, SEWA has not been applied and developed for a theoretical investigation of the frequency doubling process until present time. In this study the set of generalized nonlinear Schrödinger equations describing a second harmonic generation of the super-short femtosecond pulse is derived. The equations set contains terms, describing the pulses self-steepening, and the second order dispersion (SOD) of the pulse, a diffraction of the beam as well as mixed derivatives. We propose the transform of the equations set to a type, which does not contain both the mixed derivatives and time derivatives of the nonlinear terms. This transform allows us to derive the integrals of motion of the problem: energy, spectral invariants and Hamiltonian. We show the existence of two specific frequencies (singularities in the Fourier space) inherent to the problem. They may cause an appearance of non-physical absolute instability of the problem solution if the spectral invariants are not taken into account. Moreover, we claim that the energy preservation at the laser pulses propagation may not occur if these invariants do not preserve. Developed conservation laws, in particular, have to be used for developing of the conservative finite-difference schemes, preserving the conservation laws difference analogues, and for developing of adequate theory of the modulation instability of the laser pulses, containing a few cycles. [ABSTRACT FROM AUTHOR]

Understanding the two-photon spectral properties of photoinitiators is crucial for the advancements and applications of photopolymerization (PP) techniques, specifically focusing on two-photon polymerization (TPP) for three-dimensional printing (3DP). In this work, we have investigated a specific photoinitiator designated as TECEB, a three-branched carbazole-based compound used in TPP at visible wavelengths. The lack of detailed two-photon absorption (TPA) data for TECEB is now fulfilled with this one- and two-photon absorption investigation from the experimental and theoretical points of view. Femtosecond pulses were used to acquire the TPA cross-section (σ (2) ) spectra via the Z-scan technique. Using the Gaussian software package, quantum chemical calculations (QCC) were conducted with 36 different combinations, comprising 6 chemistry models and 6 basis sets. TECEB's TPA results clearly show that this compound has a strong potential to be used as polymeric solutions in 3DP-TPP, mainly due to its TPA cross section in the spectral region between 500 nm and 550 nm, with values up to 250 GM. • In-depth Analysis : Our investigation comprehensively explains TECEB's two-photon absorption (TPA) properties. Through a combined experimental and theoretical approach, we were able to elucidate the mechanisms governing TPA's behavior. • Significance of TECEB : Our research underscores TECEB's pivotal role as a Two-Photon Polymerization (TPP) initiator. Its unique molecular structure and distinct electronic properties translate to a TPA cross-section suitable for commercial TPP applications. • Methodological Rigor : From a methodological standpoint, we tested 36 combinations of 6 chemistry models and 6 basis sets to simulate OPA and TPA spectra. Our findings indicate that CAM-B3LYP/6-311+G(2d,p) offers the optimal balance in the reproducibility of both OPA and TPA spectra. • Practical Implications : The insights gleaned from our research are instrumental in advancing photopolymerization techniques. As 3D printing using TPP gains momentum in various fields, our findings will have broad implications, catalyzing the development of intricate, high-resolution 3D structures. [ABSTRACT FROM AUTHOR]

A recent study conducted by scientists at EPFL and the Max Planck Institute has successfully brought nonlinear optical phenomena into a transmission electron microscope (TEM). The study focused on "Kerr solitons," waves of light that maintain their shape and energy as they move through a material. The researchers generated dissipative Kerr solitons inside a photonic microresonator, which allowed them to probe soliton dynamics and perform ultrafast modulation of electron beams. This breakthrough could lead to advancements in electron microscopy and the development of nonlinear photonic devices. [Extracted from the article]

Time and frequency asymmetries in ultrashort chirped laser fields might appear as a consequence of dispersive propagation, pulse shaping techniques, or generation of auxiliary light pulses needed in nonlinear optics. Here, we try to find an answer to the question of how to solve analytically coherent anti‐Stokes Raman scattering (CARS) under asymmetric conditions of chirped femtosecond laser pulses. The approach breaks in two parts. One for the field amplitudes and the other for the phases. The former revolves around Gaussian dependences that, besides being rather common in ultrashort laser physics, can be arranged and mixed to reproduce spectrally asymmetric laser amplitudes. The latter is limited to field phases with cubic frequency dependence (i.e., second‐order chirp) whose asymmetry is simulated by adding a linear term to the quadratic phase. Both approximations for amplitudes and phases of the three laser pulses are mandatory to guarantee the solution to the complexity posed by the CARS problem. Comparisons with known experimental and numerical results support the validity of the model. [ABSTRACT FROM AUTHOR]

• The work is the first comprehensive studies of nonlinear optical properties. • In this work we studied the possibility of self-compression of fs laser pulses. • The present study is useful for ample of applications. Nonlinear optical properties of soda-lime glasses irradiated by 100 fs laser pulses with intensities ranging from 80 to 182 GW/cm2 and wavelengths from 780 to 870 nm were investigated by the Z-scan technique. Saturated three photon absorption (3PA) was observed and the dependence of the 3PA coefficient on the intensity and the irradiation wavelengths. The measured value of the nonlinear refractive index was found to be intensity dependent and wavelength independent. The estimated nonlinear optical properties were in good agreement with the experimental results. The self-compression of femtosecond laser pulses in a soda-lime glass was evaluated numerically taken into account the interplay between group velocity dispersion (GVD) and self-phase modulation (SPM) in the soda-lime. A numerical simulation of the transmission of a 200 fs laser pulse through soda-lime glass of 1.2 cm thickness showed that the interplay between anomalous GVD and SPM lead to a significant compression of the pulse to 8.5 fs i.e. by a factor of 23. [ABSTRACT FROM AUTHOR]

Low-frequency molecular rotations recorded in a region of 0–150 cm–1 with a high "signal/noise" ratio in the ultrafast optical Kerr effect (OKE), which is a third-order nonlinear optical response upon nonresonant excitation of a liquid by femtosecond laser pulses, are analyzed in detail. It is shown that the reduced Raman spectral density (RSD) derived from experimental data using the well-known deconvolution procedure is ambiguous due to the problem of separating the orientational and librational contributions to the total OKE signal. This fact significantly limits the reliability of information derived about molecular motions in a liquid. For the example of an ultrafast OKE in benzonitrile, it is shown that, in the range of 0–300 fs, rotational responses cannot be considered independent, and the application of a number of additional criteria resulting from the assumption of their correlation allows this ambiguity in deriving the RSD function from experimental data to be removed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Investigate the optical nonlinearities of SQ-640 under variouswavelengthsand pulse widths. • Displayself-focusing at visible wavelength andself-defocusing at NIR wavelengths. • Discusstheoriginof optical nonlinearitiesvia transient absorption and refraction dynamics. • The results suggest SQ-640 is a potential material for NLR based devices. The abundant nonlinear optical responses of a squaraine derivative (2, 4-bis[4-(diethylamino)-2-hydroxyphenyl]squaraine, SQ-640) are explored at visible and near-infrared (NIR) regions. The sign and magnitude of the third-order (intensity depend) nonlinear absorption and refraction for SQ-640 are determined under different pluses with Z-scan method. The results of this work show reverse saturable absorption (RSA) of SQ-640 at visible and NIR region and its potential for optical limiting (OL) application. Moreover, SQ-640 shows self-focusing at visible wavelength while shows self-defocusing at NIR wavelengths. The transient refraction dynamics indicate that positive excited-state refraction causes self-focusing at visible region while negative nonlinear refraction at NIR region mainly originates from bound electron response (Kerr effect) and negative excited state refraction. Besides, the nonlinear refraction index of SQ-640 under 532 nm ranges from ∼ 10-19 m2/W at femtosecond pulses to ∼ 10-16 m2/W at nanosecond pulses and the second order hyperpolarizability is about 4 × 10-28 esu at nanosecond case. The Kerr refraction coefficient is about −6.54 × 10-19 m2/W at 700 nm under femtosecond laser pulses. The obtained OL thresholds at 532 nm are ∼ 26.8 mJ/cm2 for picosecond pulses and ∼ 85.6 mJ/cm2 for nanosecond pulses. The enriching information extracted from NLR in SQ-640 may offer an optional perspective to understand the structure–property relationships in these compounds. The results of optical nonlinearities in SQ-640 gained from this study may be of assistance to its better applications in laser protection and optical switching. [ABSTRACT FROM AUTHOR]

Nanofibers consisting of organic nonlinear optical l -Phenylalanine (LPA) nanocrystals embedded in poly-ethylene-oxide (PEO) polymer were synthesized by the electrospinning technique. The preferred crystallographic orientation of the organic molecules that crystallized inside the nanofibers was demonstrated through X-ray diffraction and calculations show that the average crystallite size is around 28 nm. SEM images clearly show the formation of smooth fibers with average diameter of 246 nm. The emission of green light from the nanofiber powder sample approved the second harmonic generation (SHG) under the excitation of Nd: YAG laser (1064 nm). Further the generation of second harmonic from the fibers was confirmed by nonlinear microscope imaging and power dependent SHG studies, performed using femtosecond fiber laser (1040 nm). The nonlinear absorption and its power limiting characteristics of LPA-PEO electro-spun nanofibers were studied utilizing the Z-scan method employing Q switched Nd: YAG laser as an excitation source (532 nm, 5 ns, 10 Hz). Open aperture Z-scan measurement (with intensity 1.23 × 1012 W/m2 and 2.4623 × 1012 W/m2) revealed the reverse saturable absorption property of LPA-PEO nanofibers. The existence of a sequential two-photon absorption process involving real resonant intermediate states has been demonstrated by the intensity dependent two-photon absorption coefficient. The usefulness of LPA optical fibers as power limiters is ensured by their lower optical limiting threshold than other optical limiting materials. • Nonlinear optical studies for LPA-PEO nanofibers synthesized by Electrospinning technique and investigate how the nonlinear optical properties alter as we move from bulk to lower dimensions. • The high nonlinear absorption coefficient and low limiting threshold is found in the case of fiber sample that raises its suitability for power limiting application. • Second harmonic generation efficiency of LPA-PEO fibers is found to be 1.78 times than that of KDP. • Nonlinear microscope imaging was performed with fibers to confirm the SHG in fibers form. • The nonlinear absorption of LPA that has been identified nanofibers may be useful for a number of applications, including power limiters, optical sensors, etc. [ABSTRACT FROM AUTHOR]

Environmentally friendly quantum dots (QDs) with high two-photon absorption coefficients in the near-infrared region are important for nonlinear optics (NLO) applications. In this work, CuLaSe 2 and ZnCuLaSe 2 QD are successfully synthesized and characterized using UV–Vis, Photoluminescence and HRTEM analysis. Next, the third-order NLO properties of these materials are studied using the femtosecond Z-scan method. The samples showed efficient nonlinear responses. The nonlinear absorption coefficient (β), the nonlinear refractive index (n 2) and the third-order nonlinear susceptibility (χ (3) ) values for all samples are deduced and discussed in detail. The results reported here pave the way towards building CuLaSe 2 and ZnCuLaSe 2 QDs-based nonlinear photoelectric devices operating in the near-infrared field. • CuLaSe 2 and ZnCuLaSe 2 QD are successfully synthesized and characterized. • Third-order NLO properties of CuLaSe 2 and ZnCuLaSe 2 QDs have been investigated using femtosecond Z-scan analysis for the first time. • The third-order optical properties of CuLaSe 2 QDs are far better than that of ZnCuLaSe 2 QDs. • The extrapolated values of β, n 2 and χ (3) are found to be on the order of 10−11 cm/W, 10−16 cm2/W and 10−14 esu, respectively. • The results suggest that ZnCuLaSe 2 and CuLaSe 2 QDs can be potential candidates for future applications in NLO. [ABSTRACT FROM AUTHOR]

We demonstrate a cascaded, two-stage, fiber-based optical parametric amplifier (OPA) in which the signal pulses are generated in the first stage via spontaneous degenerate four-wave mixing (DFWM) mechanism and subsequently are used as the seed pulse for the stimulated DFWM in the second stage. A compact femtosecond fiber laser provides the pump pulse with 220-fs duration at 20-MHz repetition rate. Large-mode area (LMA) photonic crystal fibers (PCFs) with zero dispersion wavelength at 1054 nm and a dispersion slope of 0.20 ps/km/nm 2 provide the parametric gain. Pulses with 2-mW average power in spectral range from 850 to 1000 nm are obtained from the first-stage OPA, and they are further amplified to 23.4 mW in the second stage OPA, corresponding to 10.7-dB parametric gain. The output pulse duration and spectral bandwidth are closely related to the temporal delay between the signal and pump pulses in the second stage. With an optimized time delay, pulses with >1 nJ energy and <200 fs duration are obtained at the spectral range from 850 to 1000 nm. [ABSTRACT FROM AUTHOR]

A new method is proposed to systematically measure the phase-intensity information of chirped pulses that is based on the chirp-coefficient bisection iteration (CBI) concept. Through the CBI procedure with measured spectrum and temporal intensity profiles (or intensity autocorrelations), spectral and temporal amplitude-phase information can be rapidly retrieved. We experimentally verified that our method has high precision for nanosecond- and picosecond-level pulses and low precision for femtosecond-level pulses. Our proposed method does not require a sophisticated setup and has the advantage of accurate determination of temporal and spectral chirp coefficients with various orders. It also has lower cost, simple operation, in particular covers a wider measurement range than the main current methods. Moreover, the retrieved waveforms can reveal both the pulse shape and the actual intensity with spectral and temporal chirped coefficients of various orders, which can be directly used in various pulse propagation analyses such as chirped pulse amplification. [ABSTRACT FROM AUTHOR]

Featured Application: Supercontinuum generation in novel infrared fibers from high-repetition-rate femtosecond mode-locked laser pulses provides mid-infrared radiation with very attractive features such as high coherence and spectral power density over an ultra-broad spectral range, thus opening new prospects for remote sensing applications, in particular through mid-infrared spectroscopy. Mid-infrared supercontinuum generation from 1.5 to 4.5 µm with sub-nanojoule femtosecond pump pulses is demonstrated by using a short segment of dispersion-engineered step-index tellurite fiber with very low OH content. Distinct group-velocity dispersion regimes in a simple design of step-index tellurite fiber are also reported, which allows to choose the nonlinear pulse propagation regime according to the required tailoring of the supercontinuum source. Numerical simulations based on the generalized nonlinear Schrödinger equation are used to determine optimized fiber parameters before experimental demonstrations. We also analyse the coherence properties of the resulting supercontinuum sources. [ABSTRACT FROM AUTHOR]

The syntheses of oligo(p-phenylene ethynylene)s (OPEs) end-functionalized by a nitro acceptor group and with a ligated ruthenium unit at varying locations in the OPE chain, namely, trans-[Ru{(C≡C-1,4-C6H4)nNO2}(C≡CR)(dppe)2] (dppe=1,2-bis(diphenylphosphino)ethane; n=1, R=1,4-C6H4C≡C-1,4-C6H4C≡CPh, 1,4-C6H4NEt2; n=2, R=Ph, 1,4-C6H4C≡CPh, 1,4-C6H4C≡C-1,4-C6H4C≡CPh, 1,4-C6H4NO2, 1,4-C6H4NEt2; n=3, R=Ph, 1,4-C6H4C≡CPh), are reported. Their electrochemical properties were assessed by cyclic voltammetry, their linear optical properties and quadratic and cubic nonlinear optical properties were assayed by UV/Vis/NIR spectroscopy, hyper-Rayleigh scattering studies employing nanosecond pulses at 1064 nm, and broad spectral range Z-scan studies employing femtosecond pulses, respectively, and their linear optical properties and vibrational spectroscopic behavior in the formally RuIII state was examined by UV/Vis/NIR and IR spectroelectrochemistry, respectively. The potentials of the metal-localized oxidation processes are sensitive to alkynyl-ligand modification, but this effect is attenuated on n-bridge lengthening. Computational studies employing time-dependent density functional theory were undertaken on model complexes, with a 2D scan revealing a soft potential-energy surface for intra-alkynyl-ligand aryl-ring rotation; this is consistent with the experimentally observed blueshift in optical absorption maxima. Quadratic optical nonlinearities are significant and cubic NLO coefficients for these small complexes are small. The optimum length of the alkynyl ligands and the ideal metal location in the OPE to maximize the key coefficients have been defined. [ABSTRACT FROM AUTHOR]