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In 3 D + time numerical simulations, we study the wavelength scaling law for the energy of terahertz (THz) radiation emitted from a two-color femtosecond filament, which forms during cofocusing into air the fundamental and second harmonics of the laser pulse. In our simulations, the central wavelength of the fundamental harmonic varied from 0.8 to 8 μ m and the numerical aperture varied from 0.006 to 0.03. While the harmonics and supercontinuum development are not extreme, so the harmonics spectra are clearly separated, the energy of the generated THz radiation is proportional to the oscillation energy of the electrons, which grows as the squared pump wavelength, and the total number of free electrons in the filament, which decreases quasi-exponentially with the pump wavelength. As a result, the scaling law for the THz energy on the pump wavelength is nonmonotonic with the maximum at 1.6–4 μ m depending on the focusing conditions. [ABSTRACT FROM AUTHOR]

Femtosecond laser filamentation is a unique nonlinear optical phenomenon when high-power ultrafast laser propagation in all transparent optical media. During filamentation in the atmosphere, the ultrastrong field of 1013–1014 W/cm2 with a large distance ranging from meter to kilometers can effectively ionize, break, and excite the molecules and fragments, resulting in characteristic fingerprint emissions, which provide a great opportunity for investigating strong-field molecules interaction in complicated environments, especially remote sensing. Additionally, the ultrastrong intensity inside the filament can damage almost all the detectors and ignite various intricate higher order nonlinear optical effects. These extreme physical conditions and complicated phenomena make the sensing and controlling of filamentation challenging. This paper mainly focuses on recent research advances in sensing with femtosecond laser filamentation, including fundamental physics, sensing and manipulating methods, typical filament-based sensing techniques and application scenarios, opportunities, and challenges toward the filament-based remote sensing under different complicated conditions. [ABSTRACT FROM AUTHOR]

Manipulation of ferroelectric polarization and associated domain is at the heart of ferroelectric‐based applications. Polarization switching is typically achieved by a static or pulsed electric field, which however suffers from limitations associated with the requirement of circuitry access and the sluggish switching time. To circumvent abovementioned challenges, significant effort has been devoted to optical control of ferroelectric polarization, which can provide tremendous potential to develop ferroelectric devices with more freedom and to extend transient operation down to picosecond time scale. In this review, both the phenomena and the mechanism of optical control over polarization and associated domain evolution are explored in various kinds of ferroelectrics, including oxide perovskites, van der Waals ferroelectrics, and ferroelectric heterostructures, etc. All‐optical fabrication of 3D nonlinear photonic crystals based on femtosecond laser‐induced ferroelectric domain inversion is presented. In particular, some intriguing examples of terahertz field‐induced polarization reversal and paraelectric‐to‐ferroelectric phase transition are highlighted. The review concludes with a brief summary of the related branches, and discusses the challenges as well as the potential directions of this emerging field. [ABSTRACT FROM AUTHOR]

Recent reports on sub‐terahertz (THz) generation from a laser‐irradiated wire might have evaded the most essential contents of the wire radiation dynamics. Here, the origin of terahertz generation from a metal wire is revisited and a comprehensive diagnosis of the terahertz radiation from a 100 µm‐diameter tungsten wire irradiated by an intense femtosecond laser is implemented. For the first time, the long‐neglected but more efficient high‐frequency terahertz radiation is experimentally observed of which the spectra, polarization, tunability, and wire‐length‐dependent intensification are investigated comprehensively. A new picture of the wire radiation dynamics is presented to reveal the origin of the extreme terahertz surface wave, its evolution, and radiation mechanism. This extremely intense and ultrashort half‐cycle surface wave is spontaneously induced by a laser‐driven transient charge‐separation field and evolves into a multi‐cycle surface wakefield structure as it propagates along the wire owing to the self‐interaction between the surface wave and its carrier, the surface current. By manipulating the coupling and transport dynamics of the surface wave on the wire, tunable and intensified THz radiation covering a wide range from 0.1 to 20 THz has been realized, paving the way for broad applications such as terahertz acceleration, bio‐medicine, nonlinear THz science and beyond. [ABSTRACT FROM AUTHOR]

Femtosecond laser filament induced breakdown spectroscopy is a promising technique that can be used in the remote sensing of air pollutants. Raising the laser energy saturation point below which the laser filament fluorescence increases exponentially with the incident laser energy will significantly improve the sensing sensitivity and detection distance. In this work, a saturation point raising scheme is proposed in which a quarter-circle π phase plate is used to form four optical filaments in air. The extrapolation based on the experimental results infers that the employment of the quarter-circle π phase plate and its induced multiple filaments can lead to the increase of the laser energy saturation point by four times, which is proportional to the number of sub-regions in the phase plate. [ABSTRACT FROM AUTHOR]

• •Multi-wavelength ultrashort annular pulses are generated by femtosecond Bessel beam. • •The interference of dual Bessel beams enhances the annular beam's intensity. • •The cascaded resonant four-wave mixing is the annular beams' generation mechanism. • •Ultrashort annular beam may have important applications in imaging and communication. Double femtosecond Bessel laser beams (fs-BLBs) are used to interact with UV-visible silica glass and ultrashort annular beams (ABs) with six different peak wavelengths in the visible and near-infrared range are simultaneously generated while the super-continuum emission is totally suppressed. Experimental results show that as the time interval Δ t between double fs-BLBs gradually approaches zero, the generation efficiency of the ABs increases and reaches the maximum at Δ t = 0. When Δ t equals ±18 fs, the intensity of the annular beam with a central wavelength of 605 nm decreases to half of its maximum. Compared with the case using single femtosecond Bessel laser beam, the ABs' generation efficiency can be greatly enhanced due to the interference of double fs-BLBs. It is considered that the four-wave mixing (FWM) and corresponding cascaded parametric processes are responsible for the generation of these ABs, which can be confirmed by the numerical simulations. The colorful ABs have important applications in the imaging and communication fields, such as the stimulated emission depletion microscopy (STED), the high speed 3D live fluorescence imaging, and the multiplexing optical communications. [ABSTRACT FROM AUTHOR]

The sixth-generation wireless communication will exploit the radio band with frequencies higher than 90 GHz, reaching terahertz (THz) band, to achieve huge signal bandwidths. However, the cost-effective fabrication methods of the key components in THz band, which can compromise large scale, high precision, and high efficiency, remain great challenges at present. In this work, we have developed a high throughput fabrication method based on the femtosecond laser filament direct writing. The ability of fabricating large-scale THz elements with high precision and fast speed has been demonstrated by fabricating 100 × 100 mm2 spiral zone plates (SZPs), which can convert the Gaussian THz beam into vortex beam. The performance of the obtained THz vortex beam is in good agreement with the theoretical predictions. The fabrication method reported here has promising applications in fabricating various kinds of THz elements on substrates with both flat and curved surfaces. [ABSTRACT FROM AUTHOR]

• The transmittance of the hydrogel in visible light band is not much different from that of the human cornea. • The mechanical stiffness of the hydrogel can be enhanced after two-photon CXL. • The hydrogel's microstructure offers insights into the reasons about improvement in the mechanical stiffness. • The hydrogel materials may prove a good candidate for artificial cornea implants. Corneal disease seriously threatens the quality of human vision and even causes blindness, but the shortage of corneal donors is insufficient to meet the demand for corneal transplantation. Artificial corneas prepared by hydrogel materials have the characteristics of high-water content, good biocompatibility and low cytotoxicity, is an effective way to treat corneal diseases. Although hydrogel technology has advanced over the past few years, their application often suffers from limitations in mechanical properties and complicated fabrication procedures. Here, a novel high-transparent and stiffness-enhanced bio-adhesive hydrogel composed of porcine type I collagen is developed by near infrared femtosecond laser based two-photon collagen crosslinking (CXL). Transmission spectra show that the transparency of the prepared hydrogel in the visible band is not less than 90 %, and the mechanical stiffness can be enhanced by 265.27 % ± 27.08 % without significantly reducing transparency after two-photon CXL. Additionally, the hydrogel's microstructure offers insights into the reasons contributing to the observed improvement in the mechanical stiffness. Overall, this work presents a generalizable preparation strategy for hydrogel materials with high light transmission, mechanical strength, and also potential of biocompatibility and bioadhesion in terms of biocompatibility, that might be an excellent candidate for artificial cornea implants and will foster potential advances in soft materials. [ABSTRACT FROM AUTHOR]

Abstract: Femtosecond laser filamentation is particularly interesting for remote sensing pollutant in the atmosphere. In this work, we investigate the local shot-to-shot stability of the filament induced fluorescence of nitrogen in air. It is found that the root-mean square fluctuation of the fluorescence signal is at least one order of magnitude lower than that of the linear propagation case. In practice, it would contribute to improve the robustness of long distance spectroscopic analysis of the fluorescence of pollutant molecules inside the filament. We further point out that this unique property of filament induced fluorescence spectroscopy is because of the intensity clamping, a profound phenomenon of filamentation. [Copyright &y& Elsevier]

Terahertz (THz) wave generated by femtosecond laser filamentation can spatially constrained inside the plasma channels. Our simulation results show that this constraint to THz wave will gradually weaken with the increasing THz frequency. At the same time, results also indicate that the higher plasma density possesses the stronger constraint. In addition, the model field radius and width of THz wave will become larger with the increasing plasma channel radius. Based on the simulation results we find the changes of refractive index at THz wave play an important role on THz wave propagation in plasma channels. [ABSTRACT FROM AUTHOR]

Highlights • The alcoholic liquids are observed by single beam time-resolved eclipsed Z-scan. • A simple method to extract the purely optical nonlinearities is presented. • The n 2 is 2.04 × 10−16 and 1.72 × 10−16 cm2/W for methanol and ethanol, respectively. • The self-focusing effects are masked by thermal effects after ∼10 μs irradiation. Abstract The experiments and applications of nonlinear optical liquids are often complicated by the presence and competition of the fast self-focusing and slow self-defocusing effects. As a result, the time-resolved measurements of liquids nonlinearities are very crucial. In this work, the temporal evolution of optical nonlinearities due to the accumulation of thermal effect for alcoholic liquids—frequently adpoted as the solvent of many organic nonlinear molecules and used in many nonlinear optical experiments—under high-repetition-rate femtosecond lasers is demonstrated using single beam time-resolved eclipsed Z-scan, that is, the combination of single beam time-resolved Z-scan and eclipsed Z-scan methods. The pure self-focusing nonlinearities arising from molecular reorientation of alcoholic liquids can be precisely extracted by a simple data processing. It can be concluded that the characteristic time of the accumulation of thermal nonlinear optical effects is 479.28 and 601.62 μs, and the fast self-focusing effects will be completely masked by the thermal nonlinear optical effects after t = 17.35 and 22.25 μs of laser irradiation for methanol and ethanol, respectively. The “combined” Z-scan technique can also have an extensive application for other advanced materials. [ABSTRACT FROM AUTHOR]

• Filament-induced aerosol fluorescence emission appears a strong angular dependence. • The backward angular distribution of the aerosol fluorescence can be attributed to Mie scattering. • The divergence angle of aerosol fluorescence radiation is affected by the particle size distribution. Backward angular distributions of the sodium fluorescence in NaCl aerosols induced by femtosecond laser filamentation in air with a distance of 10 m are measured. The recorded signals appear as strong angular dependence. Further study indicates that it could be associated with the Mie scattering of the filamentation-induced "aerosol lasing" in the forward direction. These results will benefit the remote-sensing of air pollutants by femtosecond laser filamentation-based LIDAR (light detection and ranging). [ABSTRACT FROM AUTHOR]

• Acoustic method was adopted to quantitatively measure the long-distance filamentation. • A simple physical model was presented to analyze the spatial distribution of filament. • The onset and length of filament can be conveniently obtained from the model. • The increase of the filament length can be obtained for longer pulse with more energy. The promising application of femtosecond laser filamentation in atmospheric remote sensing brings imperative demand for diagnosing and controlling the spatiotemporal dynamics of long-distance filamentation. Here, the acoustic method was adopted to quantitatively diagnose the long-distance filamentation of femtosecond pulses manipulated by energy and temporal domain. The onset and length of filament can be conveniently obtained from a simple analytic formula based on reasonable approximation and further demonstrated by the experimental results. The influence of pulse energy and initial pulse chirp under three different focal lengths (∼10 m, 20 m, and 30 m) on the filamentation in the air were studied experimentally. These findings provide a guiding significance for the optimal control of the long-distance propagation of filament, thereby laying a firm foundation for femtosecond laser-based atmospheric remote sensing. [ABSTRACT FROM AUTHOR]

• A graphene foam-based MHz ultrasonic source is excited by femtosecond laser. • Its bandwidth is 100 times larger than that by the piezoelectric transducer. • The ultrasonic source has a dipole-like pressure distribution. The performance of a photo-thermo-acoustic (PTA) ultrasonic source can be significantly improved by utilizing graphene-based materials. Graphene is an excellent PTA material owing to its wide electromagnetic absorption spectrum, low heat capacity per unit area, and high thermal conductivity. In this study, a broadband graphene-foam-based PTA ultrasonic source covering the frequency range of 50 kHz to 1.8 MHz was excited by a near-infrared femtosecond laser beam. The lower and upper frequency limits of the acoustic wave that can be detected in the experiments were determined by the responsivity of the microphone and the attenuation of the acoustic wave in air, respectively. The experimental results show that the sound pressure of this ultrasonic source was independent of the laser polarization and incident angle. The peak-to-peak magnitude of the ultrasonic wave was proportional to laser energy when the single-pulse energy of the femtosecond laser varied from 0.4 to 1.0 mJ. Therefore, the sound intensity of the ultrasonic source could be easily controlled by modulating the laser energy. The experimental results also show that the ultrasonic wave emitted from the graphene foam had a dipole-like acoustic pressure distribution, and its principal emission direction was normal to the sample surface, regardless of the laser incidence angle. This characteristic may benefit future applications in directed message transfer/acquisition and nondestructive testing/imaging. [ABSTRACT FROM AUTHOR]

• Large birefringence of THz wave inside a femtosecond laser filament was observed. • The birefringence is attributed to the strong confinement of THz wave inside the filament. • THz wave with arbitrary polarization orientation, ellipticity and chirality can be produced. Large birefringence of THz wave inside a femtosecond laser filament in air has been found in this work. By varying the laser pulse energy, the filament length is changed, inducing variation in the phase delay between two perpendicularly polarized THz pulses. The birefringence could be explained by the delayed rotational molecular wave packet of air molecules inside the femtosecond laser-induced filament. The newly discovered THz birefringence is essentially attributed to the strong confinement of the THz pulse inside the filament. This phenomenon has opened a new way to freely control the polarization of THz pulses emitted from an optical filament and significantly benefits polarization-sensitive THz spectroscopy and control of matter. [ABSTRACT FROM AUTHOR]

• An analytic model is proposed to predict the final nanoparticle size distribution. • The model explains the power law size distribution with a clear physical picture. • The nanodroplets' evaporation is the origin of the power law size distribution. • The final size distribution and generation process of nanoparticles are bridged. Near-infrared femtosecond laser is employed to ablate the copper target in vacuum (10−4 Pa). The ejected nanoparticles are collected by a silicon substrate. The 3D profiles of the deposited nanoparticles indicate before resolidification on the substrate they are nanodroplets generated by some phase separation process, such as phase explosion or cavitation. To study the nanoparticles' generation dynamics and size distribution, the commonly used methods are the molecular dynamics simulation and the Monte Carlo method. However, the former cannot predict the particle size distribution at the final state and the latter cannot provide a clear physical picture. In this paper, taking both the nanoparticles' generation and ejection into account, an analytical thermodynamic model based on the nanodroplets' evaporation is proposed. In this model, the nanoparticles' sizes are redistributed due to the nanodroplets' evaporation during the flight from the target to the substrate, which provides a clear physical picture and successfully explains the physical origin of the final power law particle size distribution on the substrate. The model bridges the gap between the final size distribution and the thermodynamic generation process of nanoparticles, which will benefit the size control of the ablation-generated nanoparticles. [ABSTRACT FROM AUTHOR]