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Ultrafast lasers have significantly contributed to the advancement of research and technology; however, high‐intensity lasers bring potential risks to precision instruments and the human eye. Developing optical limiting (OL) devices capable of reasonably controlling laser energy to an acceptable energy level is imperative. Nevertheless, achieving both exceptional OL performance and broad‐spectrum laser intensity tunability proves highly challenging. In this work, the synthesis of a porphyrin‐based 2D polymer (Por‐2DP) using a template‐assisted self‐coupling polymerization, constructing a 2D framework through the carbon‐carbon single bond coupling of porphyrin monomer is presented. The resultant Por‐2DP demonstrates remarkable broadband nonlinear absorption performance, spanning the visible and near‐infrared spectral regions under an ultrafast pulse laser (35 fs) for the first time. OL thresholds at excitation wavelengths of 515, 800, and 1550 nm are determined to be 1.44, 0.48, and 0.54 mJ cm−2, respectively, surpassing reported 2D materials to date. Additionally, the Por‐2DP exhibits prominent photostability, enabling sustained operation under intense light conditions for a long time, thereby enhancing OL practical applicability. This study not only introduces a novel OL material and device but also promotes the application of innovative 2D organic polymers in NLO field. [ABSTRACT FROM AUTHOR]

A Yb:YAG thin‐disk regenerative amplifier with 3 kHz repetition rate pumps a second‐order nonlinear mixing scheme providing femtosecond transients of maximum electric field strength beyond 330 MV cm−1 at a center frequency of 45 THz. This value surpasses field conditions present at submolecular dimensions of matter. The inherent competition between efficiency and bandwidth in parametric downconversion is overcome with a third‐order nonlinear step of self‐phase modulation (SPM). The high repetition rate, versatility, beam quality, and passive phase stability of our system support advanced spectroscopic approaches like electro‐optic sampling (EOS). With this technique, solitonic self‐compression and octave‐spanning supercontinua directly on a subcycle scale are studied. [ABSTRACT FROM AUTHOR]

The optical filament-based radioxenon sensing can potentially overcome the constraints of conventional detection techniques that are relevant for nuclear security applications. This study investigates the spectral signatures of pure xenon (Xe) when excited by ultrafast laser filaments at near-atmosphericpressure and in short and loose-focusing conditions. The two focusing conditions lead to laser intensity differences of several orders of magnitude and different plasma transient behavior. The gaseous sample was excited at atmospheric pressure using ∼7 mJ pulses with a 35 fs pulse duration at 800 nm wavelength. The optical signatures were studied by time-resolved spectrometry and imaging in orthogonal light collection configurations in the ∼400 nm (VIS) and ∼800 nm (NIR) spectral regions. The most prominent spectral lines of atomic Xe are observable in both focusing conditions. An on-axis light collection from an atmospheric air–Xe plasma mixture demonstrates the potential of femtosecond filamentation for the remote sensing of noble gases. [ABSTRACT FROM AUTHOR]

Strong ultrafast nonlinear optical (NLO) responses in near‐infrared (NIR) regions and favorable solution processability are two urgent requirements for practical NLO applications, especially for optical limiting (OL) applications. However, simultaneously optimizing these properties remains significant challenging. In this study, the successful synthesis of a phthalocyanine‐based 2D polymer (Pc‐2DP) is reported; its structure features extended π‐electrons delocalization space constructed by topologically integrating the phthalocyanine units into the 2D framework. The as‐prepared few‐layered Pc‐2DP not only exhibits power‐ and wavelength‐dependent NLO properties toward 35 fs ultrafast pulsed laser, but it also demonstrates excellent ultrafast reverse saturable absorption (RSA) responses in NIR regions. The nonlinear absorption coefficients of Pc‐2DP dispersion reach up to 0.12 and 0.19 cm GW−1 at 800 and 1550 nm, respectively, which are much higher than those of the typical MXene (Ti3C2TX) and molybdenum disulfide with hexagonal structure (2H‐MoS2). The obtained NLO devices exhibit remarkable OL performance at 800 nm and, particularly, in the telecommunication region (1300 to 1550 nm), which is highly desired but seldom reported. The study demonstrates that the as‐prepared Pc‐2DP featuring a highly conjugated aromatic framework is a very promising OL material and that this research provides a new strategy for developing high‐performance 2D organic NLO polymers. [ABSTRACT FROM AUTHOR]

The generation of power- and wavelength-scalable few optical cycle pulses remains one of the major challenges in modern laser physics. Over the past decade, the development of table-top optical parametric chirped pulse amplification-based systems was progressing at amazing speed, demonstrating excellent performance characteristics in terms of pulse duration, energy, peak power and repetition rate, which place them at the front line of modern ultrafast laser technology. At present, table-top optical parametric chirped pulse amplifiers comprise a unique class of ultrafast light sources, which currently amplify octave-spanning spectra and produce carrier-envelope phase-stable, few optical cycle pulses with multi-gigawatt to multi-terawatt peak powers and multi-watt average powers, with carrier wavelengths spanning a considerable range of the optical spectrum. This article gives an overview on the state of the art of table-top optical parametric chirped pulse amplifiers, addressing their relevant scientific and technological aspects, and provides a short outlook of practical applications in the growing field of ultrafast science.

The quest for ever-shorter optical pulses has been ongoing for over half a century. Although few-cycle pulses have been generated for nearly 40 years, pulse lengths below the single-cycle limit have remained an elusive goal for a long time. For this purpose, optical waveform synthesizers, generating high-energy, high-average-power pulses via coherent combination of multiple pulses covering different spectral regions, have been recently developed. They allow unprecedented control over the generated optical waveforms, spanning an extremely broad spectral range from ultraviolet to infrared. Such control allows for steering strong-field interactions with increased degrees of freedom. When driving high-harmonic generation, tailored waveforms can produce bright attosecond pulse trains and even isolated attosecond pulses with tunable spectra up to the soft X-ray range. In this paper recent progress on parametric and hollow-core fiber waveform synthesizers is discussed. Newly developed seeding schemes; absolute, relative, and spectral phase measurement; and control techniques suitable for synthesizers are described. The progress on serial and parallel waveform synthesis based on Ti:sapphire and Ytterbium laser systems and their latest applications in high-harmonic generation in gaseous and solid media, attosecond science, and laser wakefield acceleration is discussed. [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]

Advances in the generation of the shortest optical laser pulses down to the sub-cycle regime promise to break new ground in ultrafast science. In this work, we theoretically demonstrate the potential scaling capabilities of soliton self-compression in hollow capillary fibers with a decreasing pressure gradient to generate near-infrared sub-cycle pulses in very different dispersion and nonlinearity landscapes. Independently of input pulse, gas and fiber choices, we present a simple and general route to find the optimal self-compression parameters which result in high-quality pulses. The use of a decreasing pressure gradient naturally favors the self-compression process, resulting in shorter and cleaner sub-cycle pulses, and an improvement in the robustness of the setup when compared to the traditional constant pressure approach. [ABSTRACT FROM AUTHOR]

The optical filament-based radioxenon sensing can potentially overcome the constraints of conventional detection techniques that are relevant for nuclear security applications. This study investigates the spectral signatures of pure xenon (Xe) when excited by ultrafast laser filaments at near-atmosphericpressure and in short and loose-focusing conditions. The two focusing conditions lead to laser intensity differences of several orders of magnitude and different plasma transient behavior. The gaseous sample was excited at atmospheric pressure using ∼7 mJ pulses with a 35 fs pulse duration at 800 nm wavelength. The optical signatures were studied by time-resolved spectrometry and imaging in orthogonal light collection configurations in the ∼400 nm (VIS) and ∼800 nm (NIR) spectral regions. The most prominent spectral lines of atomic Xe are observable in both focusing conditions. An on-axis light collection from an atmospheric air–Xe plasma mixture demonstrates the potential of femtosecond filamentation for the remote sensing of noble gases.

The influence of probe laser chirp on hybrid femtosecond/picosecond pure-rotational coherent anti-Stokes Raman scattering is studied theoretically and experimentally. Experiments of N2(hybrid fs/ps pure-rotational coherent anti-Stokes Raman scattering) are carried out using an in-house built second harmonic bandwidth compressor, results with different probe chirps of are reported. The experimental spectra are fitted with and without considering probe chirp. Including the chirp improves the fit to experimental spectra at all probe delays. The effect of probe pulse chirp is evaluated through a quantitative analysis of matching residuals.

The thin-film post-compression technique has the ability to reduce the pulse duration in PW-class lasers, increasing the peak power. Here, the nonlinear response of an increasingly available optical thermoplastic demonstrates enhanced spectral broadening, with corresponding shorter pulse duration compared to fused silica glass. The thermoplastic can be used close to its damage threshold when refreshed using a roller mechanism, and the total amount of material can be varied by folding the film. As a proof-of-principle demonstration scalable to 10-PW, a roller mechanism capable of up to 6 passes through a sub-millimeter thermoplastic film is used in vacuum to produce two-fold post-compression of the pulse. The compact design makes it an ideal method to further boost ultrahigh laser pulse intensities with benefits to many areas, including driving high energy acceleration. [ABSTRACT FROM AUTHOR]

We present ultra-broadband flat-topped supercontinuum laser sources with programmable repetition rate, spectral bandwidth, and wavelength regime. Supercontinuum sources with multiband coverage can be achieved by merging high-repetition-rate electro-optic frequency comb techniques with optical line-by-line pulse shaping. Because nonlinear broadening in the optical wave-breaking regime does not provide the required flatness and spectral bandwidth to the comb-based sources, we iteratively manipulate the spectral amplitude and phase of the optical pulse shaper. We implemented here a smooth flat-topped supercontinuum with a 97 nm bandwidth at 10 dB and programmable repetition rate up to 50 GHz by applying a bandwidth-limited quasi-super-Gaussian apodised pulse train to a highly nonlinear medium with programmable super-Gaussian and phase coefficients. This work opens new possibilities for generating robust and coherent multiple optical carriers with further C-bands in optical networks. We believe that the supercontinuum sources based on electro-optic frequency comb and pulse shaping techniques enable the provision of coherent flat-topped optical sources, at least from the L- to S-band with a programmable repetition rate and spectral bandwidth. [ABSTRACT FROM AUTHOR]

We demonstrate a new modality of ultrafast laser spectroscopy based on high‐harmonic‐driven stimulated Raman scattering (hi‐SRS) and incoherent laser‐plasma emission (iLPE). The spectra of high‐order harmonics driven by sub‐100‐fs pulses in the mid‐infrared are shown to display well‐resolved Raman‐gain, Raman‐loss, and iLPE features, enabling a chemically specific, single‐beam, single‐shot spectroscopy of gases and plasmas within a spectral range stretching from the near‐infrared to vacuum ultraviolet. We present a theoretical framework based on a suitably adapted nonlinear evolution equation that enables a closed‐form description of hi‐SRS as an ultrafast nonlinear‐optical phenomenon. We show that the content of hi‐SRS as a physical process and as a method of spectroscopy goes well beyond standard SRS, offering a powerful resource for spectroscopy, remote sensing, and ultrafast optical science. Standing out as particularly relevant for nitrogen‐containing gas systems, including atmospheric air, is a multimodal spectroscopic probe that combines ultrafast hi‐SRS via the second positive system of molecular nitrogen with iLPE via the γ band of nitrogen monoxide. [ABSTRACT FROM AUTHOR]

The thin-film post-compression technique has the ability to reduce the pulse duration in PW-class lasers, increasing the peak power. Here, the nonlinear response of an increasingly available optical thermoplastic demonstrates enhanced spectral broadening, with corresponding shorter pulse duration compared to fused silica glass. The thermoplastic can be used close to its damage threshold when refreshed using a roller mechanism, and the total amount of material can be varied by folding the film. As a proof-of-principle demonstration scalable to 10-PW, a roller mechanism capable of up to 6 passes through a sub-millimeter thermoplastic film is used in vacuum to produce two-fold post-compression of the pulse. The compact design makes it an ideal method to further boost ultrahigh laser pulse intensities with benefits to many areas, including driving high energy acceleration.

Nano-crystals were formed in the exposed regions of photo-thermo-refractive glass undergoing irradiation with zeroth order chirp-controlled ultrafast laser Bessel beams and subsequent heat treatment. Effects of various writing powers, pulse durations and heat treatment time on the distribution and the size of the nano-crystals were investigated. The results show that nano-crystals’ distribution depended on the laser power density spatial shape, while the size of the nano-crystals is quasi-independent. However, the average diameter of the nano-crystals was affected by the heat treatment time, decreasing from 175 to 105 nm with the time halved. In addition, using crystallographic characterization by X-ray diffraction, the nano-crystal composition in the laser-exposed regions was detected to be sodium fluoride.

The effects of water vapor ionization on the nonlinear propagation of femtosecond laser pulses with a 248 nm wavelength are numerically investigated in this paper. It is found that ionization of H2O molecules plays a significant role in air ionization, which seriously affects the dynamic and energy deposition of filamentation. The propagation of femtosecond pulses in air with different humidity levels are compared. The total number of electrons and total deposited pulse energy increase with the humidity increases. However, they tend to be saturated in high humidity conditions. Results presented here are conducive to characterizing the long-range propagation of filaments under atmospheric conditions. [ABSTRACT FROM AUTHOR]

The nonlinear propagation of femtosecond Airy laser filaments in water is numerically investigated in this paper. We mainly consider the influences of confinement parameter, pulse duration and beam waist on the deposited energy of filaments. The values of confinement parameter are found to have a significant impact on the temporal and spatial dynamics of the pulse. The characteristics of energy deposition also differ widely for Airy beams with different confinement parameters. The less the confinement parameter, the more energy deposited by filamentation. However, the relative deposited energy evolves relatively small changes under different confinement parameters. We also found the longer pulse duration and longer beam waist are beneficial to the total deposited energy increase. [ABSTRACT FROM AUTHOR]

Intense ultrashort laser pulses at 1.55 μm are generated by a home-built light source and compressed to 20 fs by multiple plate continuum (MPC) and Fourier pulse shaping. The entire system consists of three building blocks, responsible for seed generation, optical parametric chirped pulse amplification (OPCPA), and nonlinear compression. A commercial Yb:KGW amplifier pumps a difference frequency generator and an optical parametric amplifier (OPA) to generate 1 kHz, 44 fs, 10 μJ, carrier-envelope phase stable seed pulses at 1.55 μm. A grating–lens pair stretches the seed pulses to 50 ps by introducing a great amount of normal group delay dispersion (GDD). After that, the chirped pulse energy is boosted to 5 mJ by two OPA stages pumped by a commercial Nd:YAG amplifier (30 mJ, 85 ps). To compress the amplified pulses, a grating pair compressor providing an adequate amount of anomalous GDD is employed. The OPCPA system eventually delivers 3 mJ and 80 fs pulses, limited by the phase-matching bandwidth of nonlinear crystals. To further compress the pulse duration, pulses are sent to an MPC stage, which consisted of nine pieces of 200-μm-thick quartz plates. The OPCPA output spectrum is broadened by four times and compressed to 20 fs (3.6 carrier cycles) by a femtosecond Fourier-transform pulse shaper. The pulses are characterized by polarization gating cross-correlation frequency-resolved optical gating. Due to the high pondermotive force, an intense and long-wavelength laser system like this is useful in seeding a MeV-level electron accelerator and generating coherent soft X-ray high-harmonic generation. [ABSTRACT FROM AUTHOR]

Highlights • Experiment for radially polarized pulses through noble gas-filled hollow-core fibers. • A radially polarized supercontinuum covering from 540 nm to 1070 nm is obtained. • The supercontinuum polarization state is spectrally analyzed with optical filters. • The method can be extended to other kinds of cylindrical vector beams. Abstract Spectral broadening of a radially polarized (RP) pulse in a gas-filled hollow-core fiber (HCF) is experimentally studied. Broad spectrum covering from 540 nm to 1070 nm is obtained. It is found that the output spectrum preserves the RP state after nonlinear propagation in HCF, both for the components in the input pulse and for the newly generated spectral portion. This method shows great potential in generating RP sources with unprecedented spectral width, and may also be extended to other kinds of cylindrical vector beams. [ABSTRACT FROM AUTHOR]

Abstract We present investigations on the extent of spectral broadening occurring during the propagation of laser pulses in a Thin-Disk Multipass Amplifier (TDMPA) with the aim to exploit this nonlinear effect for pulse compression. The demonstrated compression of 2.05 mJ-pulses to a duration of about 136 fs FWHM at an average power of 409 W and a peak power of 9 GW is in good agreement with numerical pulse propagation simulations. Our spectral broadening and compression scheme introduced additional losses below 2%, is scalable to average powers of >1 kW with multi-mJ pulses, and strongly enhances the capabilities of ultrafast TDMPAs towards shorter pulses and higher peak powers. [ABSTRACT FROM AUTHOR]

The crucial role of nonlinear propagation effects in the self-guiding of femtosecond laser pulses required accurate representation of nonlinearities to describe them. In this paper, an improved theoretical model has been proposed to study the height distribution of the atmospheric nonlinear refractive index. The results show that the revised model obviously improves accurate estimation of nonlinear index at the long wavelength band. Based on the model, we also found the atmospheric nonlinear refractive index differs much from the lower atmosphere to the upper atmosphere. Our results are essential for engineering applications based on the long-distance ultrashort laser pulses' transmission in diverse atmosphere. [ABSTRACT FROM AUTHOR]

Single-shot carrier envelope phase (CEP) measurement is a challenge in the research field of ultrafast optics. We theoretically investigate how an intense terahertz pulse modulates second harmonic emission (SH) from a gas plasma induced by a few-cycle laser pulse (FCL). Results show that the modulation quantity of SH intensity has a cosinoidal dependence on the CEP of FCL pulses, based on which we propose a low energy, all-optical method for single-shot CEP measurements via using a known intense terahertz pulse. Moreover, we propose an experimental realization. [ABSTRACT FROM AUTHOR]

This paper presents the design of a W-type index chalcogenide fiber for mid-infrared beyond 10 μm. The main advantages of the W-type index fiber are the possibility to control the chromatic dispersion with a larger possible core area for single-mode operation compared to a common step-index fiber and to enhance the fiber nonlinearity. Our fiber design consists of Ge15Sb15Se70 glass core, Ge20Se80 glass inner cladding, and Ge20Sb5Se75 glass outer cladding. The optical mode distribution of the chalcogenide fiber is numerically calculated by a finite element method based on edge element analysis. With 6-μm core diameter and 12-μm inner cladding diameter, the proposed fiber design exhibits flat anomalous dispersion in the wavelength range (4.3–6.5 μm) with a peak of about 7 ps/(nm·km). Also, it offers a high nonlinear coefficient of 0.12 W−1m−1 at a pump wavelength of 6.3 μm. The simulation results show that our proposed fiber design provides a highly coherent Mid-IR SC extending from 3.7 to 12 μm pumped at low peak pump power of 1 kW. [ABSTRACT FROM AUTHOR]

With a notable exception of space–time variable swap, pulse evolution equations of ultrafast optics are mathematically similar to the equations of quantum mechanics and beam dynamics. The Lagrangian structure of these equations is, however, much less intuitive. Here, we show that such a structure can be identified via a path-integral analysis of the pulse evolution equation, offering useful insights into the Lagrangian underpinning of ultrafast nonlinear optics. The Euler–Lagrange equation applied to the optical analog of the Lagrange function leads to a second-order differential equation that parallels in its structure second Newton's law. Stationary-phase space–time paths found by solving such an equation reveal the hidden inner workings behind a vast class of field-waveform transformations in ultrafast optics, including the buildup of nonlinear phase, modulation instabilities, and soliton breather dynamics. • The Lagrangian structure of optical-waveform-evolution equations is identified via a path-integral analysis of the pertinent pulse evolution equation, offering useful insights into the Lagrangian underpinning of ultrafast nonlinear optics. • The Euler–Lagrange equation applied to the optical analog of the Lagrange function leads to a second-order differential equation that parallels in its structure second Newton's law; • Stationary-phase space–time paths found by solving such an equation reveal the hidden inner workings behind a vast class of field-waveform transformations in ultrafast optics. [ABSTRACT FROM AUTHOR]

Nonlinear ultrashort pulse propagation in a mode-locked Yb:YAG laser with a highly nonlinear intra-cavity medium is analyzed using a nonlinear Schrodinger equation. The output spectra are extended by the increased laser intensity, and spectral bandwidths wider than those of the gain medium are achieved. Moreover, pulse widths are shortened by increased laser intensity to considerably less than those of the gain medium. The simulation results qualitatively agree with the experimental results.

We propose and demonstrate a phase retrieval method using a novel variant of the dispersion scan (d-scan) technique via both simulations and experimental measurements on a femtosecond fiber laser. The method combines a map of group-delay d-scan second-harmonic generation (SHG) spectra together with the fundamental spectrum as inputs. In order to ensure that the technique is robust when pulse-to-pulse fluctuations are present, we use only the wavelengths of the resulting SHG peaks, and avoid the areas with low signal in the borders of the data trace. Simulations confirmed that phase-retrieval is accurate even with high levels of laser fluctuation. The tradeoff is that pulses that have abruptly changing or highly modulated phase profiles, are not retrievable. The d-scan uses the compressor gratings intrinsic to fiber chirped-pulse amplification (CPA) systems, so the method is low cost. For experimental verification, we used a 470-fs ytterbium-fiber CPA system. However, the method is applicable to measurements on fiber-based femtosecond laser systems, which generally have an order of magnitude less bandwidth compared with the Ti:sapphire lasers used to test earlier variants of the d-scan approach. [ABSTRACT FROM AUTHOR]

We demonstrate and analyze a series of experiments in traditional and soft condensed matter using coherent optical spectroscopy and microscopy with ultrafast time resolution. We show the capabilities of resolving both real and imaginary parts of the third-order nonlinearity in the vicinity of Raman resonances from a medium probed within microscopic volumes with an equivalent spectral resolution of better than 0.1 cm−1. We can differentiate between vibrations of various types within unit cells of crystals, as well as perform targeted probes of areas within biological tissue. Vibrations within the TiO6 octahedron and the ones for the Ti-O-P intergroup were studied in potassium titanyl phosphate crystal to reveal a multiline structure within targeted phonon modes with closely spaced vibrations having distinctly different damping rates (~0.5 ps−1 versus ~1.1 ps−1). We also detected a 1.7–2.6 ps−1 decay of C-C stretching vibrations in fat tissue and compared that with the corresponding vibration in oil. [ABSTRACT FROM AUTHOR]

We report on the design and performance of a velocity map imaging (VMI) spectrometer optimized for experiments using high-intensity extreme ultraviolet (XUV) sources such as laser-driven high-order harmonic generation (HHG) sources and free-electron lasers (FELs). Typically exhibiting low repetition rates and high single-shot count rates, such experiments do not easily lend themselves to coincident detection of photo-electrons and -ions. In order to obtain molecular frame or reaction channel-specific information, one has to rely on other correlation techniques, such as covariant detection schemes. Our device allows for combining different photo-electron and -ion detection modes for covariance analysis. We present the expected performance in the different detection modes and present the first results using an intense high-order harmonic generation (HHG) source. [ABSTRACT FROM AUTHOR]

The dynamic of electron densities in matter upon the interaction with an intense, few-cycle electric field of light causes variety of nonlinear phenomena. Capturing the spatiotemporal dynamics of electrons calls for isolated attosecond pulses in the X-ray regime, with sufficient flux to allow for: (i) attosecond pump–attosecond probe spectroscopy; or (ii) four-dimensional imaging. Light field synthesizers generate arbitrary sub-cycle, non-sinusoidal waveforms. They have a great potential to overcome the limitations of current laser sources and to extend attosecond pulses towards the X-ray regime. In this paper, we show theoretically how the achievable high-energy, high-power waveforms from current light field synthesizers can be optimized to enhance the harmonic yield at high photon energies and can serve as a promising source for scaling the photon energies of attosecond pulses. We demonstrate that the simulated optimized, non-sinusoidal waveform in this work can increase the photon flux of keV, attosecond pulses by five orders of magnitude compared to the achievable flux from longer wavelength sources and at similar photon energies. [ABSTRACT FROM AUTHOR]

Laser-induced nonadiabatic alignment in nitrogen and mixtures of nitrogen with Ar gas was investigated using the ultrafast optical Kerr effect in the collinear pump-probe setup. We find that dissipative media produce desired alignment characteristics in distinct manners to decoherence and to population relaxation. The results show that the intense laser alignment can be used as a control tool of molecular dynamics in dissipative environments. [ABSTRACT FROM AUTHOR]

By performing the femtosecond-pulsed -scan experiments at a wavelength of 780nm, we report the observation of simultaneous third- and fifth-order refractive nonlinearities in the film of a side-chain azobenzene copolymer synthesized by free-radical polymerization method. Theoretically, we extend two continuous wave -scan theories suitable for the excitation of ultrafast laser pulses. First one is valid only for the situation of the sample with the pure third-order refractive nonlinearity. Second one is applicable when the sample exhibits the simultaneous third- and fifth-order refractive nonlinearities. Both -scan theories allow the precise analysis of femtosecond-pulsed -scan experimental data. Using the femtosecond time-resolved optical Kerr effect measurements, we estimate the nonlinear response time of the sample to be in the sub-picosecond regime. The physical mechanisms of the observed nonlinearity are also discussed. The high nonlinear figure of merit with ultrafast response in this polymer is desirable in nonlinear photonic devices. [ABSTRACT FROM AUTHOR]

Benefiting from ultrafast temporal resolution, broadband spectral bandwidth, as well as high peak power, passively mode-locked fibre lasers have attracted growing interest and exhibited great potential from fundamental sciences to industrial and military applications. As a nonlinear system containing complex interactions from gain, loss, nonlinearity, dispersion, etc., ultrafast fibre lasers deliver not only conventional single soliton but also soliton bunching with different types. In analogy to molecules consisting of several atoms in chemistry, soliton molecules (in other words, bound solitons) in fibre lasers are of vital importance for in-depth understanding of the nonlinear interaction mechanism and further exploration for high-capacity fibre-optic communications. In this Review, we summarize the state-of-the-art advances on soliton molecules in ultrafast fibre lasers. A variety of soliton molecules with different numbers of soliton, phase-differences and pulse separations were experimentally observed owing to the flexibility of parameters such as mode-locking techniques and dispersion control. Numerical simulations clearly unravel how different nonlinear interactions contribute to formation of soliton molecules. Analysis of the stability and the underlying physical mechanisms of bound solitons bring important insights to this field. For a complete view of nonlinear optical phenomena in fibre lasers, other dissipative states such as vibrating soliton pairs, soliton rains, rogue waves and coexisting dissipative solitons are also discussed. With development of advanced real-time detection techniques, the internal motion of different pulsing states is anticipated to be characterized, rendering fibre lasers a versatile platform for nonlinear complex dynamics and various practical applications. [ABSTRACT FROM AUTHOR]

The effects of water vapor ionization on the nonlinear propagation of femtosecond laser pulses with a 248 nm wavelength are numerically investigated in this paper. It is found that ionization of H2O molecules plays a significant role in air ionization, which seriously affects the dynamic and energy deposition of filamentation. The propagation of femtosecond pulses in air with different humidity levels are compared. The total number of electrons and total deposited pulse energy increase with the humidity increases. However, they tend to be saturated in high humidity conditions. Results presented here are conducive to characterizing the long-range propagation of filaments under atmospheric conditions.

We present a reliable and original experimental technique based on the analysis of beam self-trapping to measure ultrafast optical nonlinearities in planar waveguides. The technique is applied to the characterization of Ge–Sb–Se chalcogenide films that allow Kerr induced self-focusing and soliton formation. Linear and nonlinear optical constants of three different chalcogenide waveguides are studied at 1200 and 1550 nm in femtosecond regime. Waveguide propagation loss and two photon absorption coefficients are determined by transmission analysis. Beam broadening and narrowing results are compared with simulations of the nonlinear Schrödinger equation solved by BPM method to deduce the Kerr n 2 coefficients. Kerr optical nonlinearities obtained by our original technique compare favorably with the values obtained by Z-scan technique. Nonlinear refractive index as high as ( 69 ± 11 ) × 1 0 − 18 m 2 ∕ W is measured in Ge 12 . 5 Sb 25 Se 62 . 5 at 1200 nm with low nonlinear absorption and low propagation losses which reveals the great characteristics of our waveguides for ultrafast all optical switching and integrated photonic devices. [ABSTRACT FROM AUTHOR]

A high-power femtosecond Yb:fiber system is seeded by a phase-locked Er:fiber source and drives an ultra-broadband optical parametric amplifier that operates at 10 MHz repetition rate. The resulting pulses display precise control of the carrier-envelope phase. Their 8.3 fs temporal duration corresponds to 2.3 optical cycles of the 1100 nm carrier wavelength. Focusing 200 nJ of pulse energy into widegap materials generates optical harmonics up to fifth order. Even in a perturbative regime, strong effects of the carrier-envelope phase on the emitted spectra are observed. [ABSTRACT FROM AUTHOR]

We present an optical system based on two toroidal mirrors in aWolter configuration to focus broadband extreme ultraviolet (XUV) radiation. Optimization of the focusing optics alignment is carried out with the aid of an XUV wavefront sensor. Back-propagation of the optimized wavefront to the focus yields a focal spot of 3.6 × 4.0 µm² full width at half maximum, which is consistent with ray-tracing simulations that predict a minimum size of 3.0 × 3.2 µm². This work is important for optimizing the intensity of focused high-order harmonics in order to reach the nonlinear interaction regime. [ABSTRACT FROM AUTHOR]

We show that low-threshold Risken–Nummedal–Graham–Haken (RNGH) instabilities in quantum cascade lasers (QCLs) occur due to a combined effect of the carrier coherence and population grating induced in the gain medium. QCLs with a few mm long cavity exhibit intermittent RNGH self-pulsations while regular self-pulsations are possible in short-cavity QCLs, with the cavity length of 100 μm or smaller. Transient behavior to RNGH self-pulsations in short-cavity QCLs shows features resembling cooperative superradiance which opens a practical way of achieving ultrashort pulse production regimes in the mid-infrared spectral range. [ABSTRACT FROM PUBLISHER]

We investigated numerically the possibility of all normal dispersion fiber design for near-infrared supercontinuum generation based on a standard air-silica microstructure. The design procedure includes finding of target dispersion profile and subsequent finding of appropriate geometrical fiber design by inverse dispersion engineering. It was shown that the tailoring of dispersion profile could increase the spectral width of generated supercontinuum while maintaining perfect spectral flatness. Conditions necessary for wide and flat supercontinuum generation as well as restrictions imposed by chosen materials were discussed. As a result of design and optimization procedure, an air-silica design was found providing normal dispersion up to \text3\;\mu \textm. Simulation results with 10 nJ, 100 fs pulses demonstrate supercontinuum generation up to 1.3 octave; whereas pumping with 30 nJ, 100 fs pulses could provide 1.8 octavse supercontinuum. [ABSTRACT FROM PUBLISHER]

The classic Raman coupled equations, which describe the principles of the interaction between the incident and generated laser, have important applications in numerous domains. However, the classic Raman equations ignored the off-axis generated laser energy, which cannot be ignored in certain circumstances. We improved the classic equations by considering the off-axis laser energy of the stimulated Raman scattering. A single-pass multiwavelength potassium gadolinium tungstate crystal Raman generator pumped by a picosecond Nd:YAG laser was realized. On the basis of theoretical calculation and experimental verification, the improved equations produced numerical results for the output spectrum and output laser conversion efficiency that have a better agreement with the experimental results than the classic equations [ABSTRACT FROM AUTHOR]

A novel scheme of blind frequency resolved optical gating (Blind-FROG) measurement for optical arbitrary waveforms, based on sum frequency effect in periodically poled lithium niobate (PPLN) optical waveguide, is proposed. The proposed scheme is very simple, low cost and good noise performance. Furthermore, it can simultaneously measure two arbitrary, potentially very complex pulses. Through theoretical analysis and numerical simulation, the performance of the scheme is studied. The results show that, the accuracy of measurement degrades with the increase of the two optical arbitrary waveforms complexity; Nevertheless, the smaller the polarization periodic mismatching is, the higher the measurement accuracy becomes; And the accuracy of the complete measurement system also can be improved by increasing the number of iterations. [ABSTRACT FROM AUTHOR]

We demonstrate strong enhancements of signal yield and image contrast in third-harmonic microscopy by appropriate choice of driving laser wavelength to modulate the phase-matching conditions of the conversion process by dispersion control. Tuning the laser wavelength in the range of 1010 – 1350 nm at samples containing interfaces with water and glass, we obtained large signal enhancements up to a factor of 19, and improvements in the image contrast by an order of magnitude. The effect is most pronounced at interfaces with media of small and/or not too different nonlinear optical susceptibilities, e.g., as it is the case in typical samples in harmonic microscopy. Beyond the demonstration of this new variant of third-harmonic microscopy, our findings are also of relevance to a proper choice of laser systems for harmonic microscopy setups. [ABSTRACT FROM AUTHOR]

With an approach of controlling the nonlinearity of medium rather than the light field, the effect of field-free molecular alignment on filamentation and resulting white-light generation is studied. This is done by measuring the rotational wavepacket evolution of nitrogen molecules after passing of a femtosecond laser pump pulse by observing the nonlinear propagation dynamics of a variably delayed filament-producing probe pulse. [ABSTRACT FROM AUTHOR]

If the laser-induced grating in a close-to-collinear two-beam geometry comprises only a few grooves, then the phase between the grooves of the grating and its envelope is decisive to nonlinear effects. Here it is shown that the groove-envelope phase, which can be altered through the delay between the pulses on a subcycle timescale, determines the interferences that arise in self-diffraction. The shape of the interference pattern changes as the laser intensity approaches the damage threshold in a thin dielectric solid, and simulations show that the shape depends on the mechanisms that lead to self-diffraction. [ABSTRACT FROM AUTHOR]

Kerr lens mode locking strongly depends upon intensity. Low pulse intensity weakens the nonlinear Kerr lens, and eventually mode locking is not supported. We demonstrate efficient mode-locking in a Ti:Sapphire laser oscillator with record-low intra-cavity pulse intensity (18 GW/cm2) and intra-cavity average power (340 mW). Using an additional nonlinear window in the cavity, the Kerr nonlinearity can be enhanced by an order of magnitude compared to the standard Ti:Sapphire cavity, allowing mode locking with an output coupler reflectivity as low as R = 55% and record-low intra-cavity pulse intensity. Our results provide an important new possibility to control mode-locking in cavities with very low intra-cavity pulse intensity, such as semiconductor laser oscillators and high-repetition rate mode-locked lasers. [ABSTRACT FROM AUTHOR]