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The field of ultrafast science has experienced significant growth over the last decade, largely attributed to advancements in optical and laser technologies such as chirped-pulse amplification and high-harmonic generation. The distinctive characteristics of intense ultrafast free-electron lasers (FELs) have introduced novel prospects for investigating molecular dynamics, as well as providing an opportunity to gain deeper insights into nonlinear processes in materials. Therefore, high-power ultrafast FELs can be widely used for both fundamental research and practical applications. This study presents a novel approach for producing high-power femtosecond FEL pulses, utilizing chirped-pulse amplification in echo-enabled harmonic generation. Chirped seed pulses are employed to induce frequency-chirped energy modulation in the electron beam. The generated FEL pulse, which inherits the chirped frequency, can be compressed through the gratings in the off-plane mount geometry to provide ultraintense ultrafast pulses. The numerical modeling results indicate that peak power exceeding 20 GW and a pulse duration in the order of several femtoseconds can be achieved. [ABSTRACT FROM AUTHOR]

X-ray free-electron lasers (FELs) provide cutting-edge tools for fundamental researches to study nature down to the atomic level at a time-scale that fits this resolution. A precise knowledge of temporal information of FEL pulses is the central issue for its applications. Here we proposed and demonstrated a novel method to determine the FEL temporal profiles online. This robust method, designed for ultrafast FELs, allows researchers to acquire real-time longitudinal profiles of FEL pulses as well as their arrive times with respect to the external optical laser with a resolution better than 6 fs. Based on this method, we can also directly measure various properties of FEL pulses and correlations between them online. This helps us to further understand the FEL lasing processes and realize the generation of stable, nearly fully coherent soft X-ray laser pulses at the Shanghai Soft X-ray FEL facility. This method will enhance the experimental opportunities for ultrafast science in various areas.

With the growth of laser technologies in photonics, the application of ultrashort laser becomes promising and prospective for volume phase elements fabrication in transparent materials. Femtosecond laser writing (FLW) was applied for polarized-sensitive volume phase gratings (VPGs) inscription in a nanoporous silicate matrix (NPSM). The VPGs formed had the thickness L = 54 ± 0.5 μm and 77 ± 0.5 μm, and showed diffraction efficiency of 8% and 14% respectively. VPGs in an NPSM demonstrated polarization sensitivity resulting in different diffraction efficiencies from 2 to 14%. Moreover, the impregnation of the NPSM plate with distilled water, 50% acetone and 10% acetone was investigated. The achieved results provide evidence for a possibility of a VPG in NPSM to be applied as a sensor. [ABSTRACT FROM AUTHOR]

Fully integrated supercontinuum sources on‐chip are critical to enabling applications such as portable and mechanically stable medical imaging devices, chemical sensing, and light detection and ranging. However, the low efficiency of current supercontinuum generation schemes prevents full on‐chip integration. Herein, a scheme where the input energy requirements for integrated supercontinuum generation are drastically lowered by orders of magnitude is presented, for bandwidth generation of the order of 500–1000 nm. Through sign‐alternating dispersion in a CMOS‐compatible silicon nitride waveguide, an efficiency enhancement by factors reaching 2800 is achieved. It is shown that the pulse energy requirement for large‐bandwidth supercontinuum generation at high spectral power (e.g., 1/e level) is lowered from nanojoules to 6 picojoules. The lowered pulse energy requirements enable that chip‐integrated laser sources, such as mode‐locked heterogeneously or hybrid‐integrated diode lasers, can be used as a pump source, enabling fully integrated on‐chip high‐bandwidth supercontinuum sources.

The non-linear propagation of the intense near-infrared (NIR) driving field in crystals poses a challenge and can offer an opportunity to control the spectral properties of harmonics in solids. Here, we have investigated the non-linear propagation effects in wide bandgap dielectrics such as Magnesium Oxide (MgO), Chromium (Cr) doped MgO (Cr: MgO), Sapphire (Sa) crystals, and fused silica (FS). Furthermore, we have generated second and third harmonics (TH) and measured the linear polarization dependence of harmonics in these thin solids to explore the non-linear response at a strong field. We observe spectral shifts and broadening of the driving field spectrum which is imprinted on the harmonics. We attribute these effects to the strong photoionization, generation of free-carrier density, and self-phase modulation effects. This work shows the sensitivity to control the spectral profile of harmonics by manipulating the driving field, showing the possibility of new tailored solid-state ultraviolet sources for optical diagnostics. [ABSTRACT FROM AUTHOR]

Interest of the research in terahertz (THz) wave has been strongly motivated by its wide applications in the fields of physics, chemistry, biology, and engineering. Developing efficient and reliable THz source is of uttermost priority in these researches. Numerous attempts have been made in fulfilling the THz generation. Greatly benefited from the progress of the ultrafast pulses, the laser-induced-plasma is one of the auspicious tools to provide desirable THz waves, owing to its superiorities in high power threshold, intense THz signal, and ultrawide THz spectrum. This paper reviews the physics and progress of the THz generation from the laser-induced plasmas, which are produced by gas, liquid, and solid. The characteristics of the emitted THz waves are also included. There are many complicated physical processes involved in the interactions of laser-plasma, making various laser-plasma scenarios in the THz generations. In view of this, we will only focus on the THz generation classified by physical mechanisms. Finally, we discuss a perspective on the future of THz generation from the laser-induced plasma, as well as its involved challenges.

The field of ultrafast science has experienced significant growth over the last decade, largely attributed to advancements in optical and laser technologies such as chirped-pulse amplification and high-harmonic generation. The distinctive characteristics of intense ultrafast free-electron lasers (FELs) have introduced novel prospects for investigating molecular dynamics, as well as providing an opportunity to gain deeper insights into nonlinear processes in materials. Therefore, high-power ultrafast FELs can be widely used for both fundamental research and practical applications. This study presents a novel approach for producing high-power femtosecond FEL pulses, utilizing chirped-pulse amplification in echo-enabled harmonic generation. Chirped seed pulses are employed to induce frequency-chirped energy modulation in the electron beam. The generated FEL pulse, which inherits the chirped frequency, can be compressed through the gratings in the off-plane mount geometry to provide ultraintense ultrafast pulses. The numerical modeling results indicate that peak power exceeding 20 GW and a pulse duration in the order of several femtoseconds can be achieved.

High‐power laser sources are widely used in industrial precision processing and act as a new platform for strong‐field physics research using peak power over petawatt. This review focuses on realizing high‐energy solid‐state disk and slab systems and the nonlinear‐suppression strategies for high‐power fiber systems using the functional fibers. First, the implementations and enabling technologies of the solid‐state lasers for increasing peak power from gigawatt to petawatt are reviewed. Then the mechanisms and suppression strategies of the deterioration effects (including stimulated Raman scattering, stimulated Brillouin scattering, and transverse mode instability) in various fiber amplifiers are analyzed. At the same time, the mechanism and achievements of the current functional fibers are introduced. Finally, the challenges and perspectives of high‐power solid‐state and fiber amplifiers are summarized. [ABSTRACT FROM AUTHOR]

Ultrafast laser pulses spatially shaped as Bessel beams in dielectrics create high aspect ratio plasma channels whose relaxation can lead to the formation of nanochannels. We report a strong enhancement of the nanochannel drilling efficiency with illumination by double pulses separated by a delay between 10 and 500 ps. This enables the formation of nanochannels with diameters down to 100 nm. Experimental absorption measurements demonstrate that the increase of drilling efficiency is due to an increase of the confinement of the energy deposition. Nanochannel formation corresponds to a drastic change in absorption of the second pulse, demonstrating the occurrence of a phase change produced by the first pulse. This creates a highly absorbing, long-living state. Our measurements show that it is compatible with the semi-metallization of warm dense glass which takes place within a timescale of

Ultrafast laser pulses spatially shaped as Bessel beams in dielectrics create high aspect ratio plasma channels whose relaxation can lead to the formation of nanochannels. We report a strong enhancement of the nanochannel drilling efficiency with illumination by double pulses separated by a delay between 10 and 500 ps. This enables the formation of nanochannels with diameters down to 100 nm. Experimental absorption measurements demonstrate that the increase of drilling efficiency is due to an increase of the confinement of the energy deposition. Nanochannel formation corresponds to a drastic change in absorption of the second pulse, demonstrating the occurrence of a phase change produced by the first pulse. This creates a highly absorbing, long-living state. Our measurements show that it is compatible with the semi-metallization of warm dense glass which takes place within a timescale of <10 ps after the first laser pulse illumination. [ABSTRACT FROM AUTHOR]

A method is shown for preventing temporal broadening of ultrafast optical pulses in highly dispersive and fluctuating media for arbitrary signal-pulse profiles. Pulse pairs, consisting of a strong-field control-pulse and a weak-field signal-pulse, co-propagate, whereby the specific profile of the strong-field pulse precisely compensates for the dispersive phase in the weak pulse. A numerical example is presented in an optical system consisting of both resonant and gain dispersive effects. Here, we show signal-pulses that do not temporally broaden across a vast propagation distance, even in the presence of dispersion that fluctuates several orders of magnitude and in sign (for example, within a material resonance) across the pulse’s bandwidth. Another numerical example is presented in normal dispersion telecom fiber, where the length at which an ultrafast pulse does not have significant temporal broadening is extended by at least a factor of 10. Our approach can be used in the design of dispersion-less fiber links and navigating pulses in turbulent dispersive media. Furthermore, we illustrate the potential of using cross-phase modulation to compensate for dispersive effects on a signal-pulse and fill the gap in the current understanding of this nonlinear phenomenon.

Optical parametric chirped-pulse amplification (OPCPA) provides an efficient route to push ultrafast pulses toward ultrahigh peak powers. However, in high average power regime, the unavoidable thermal effects in nonlinear crystals will intrinsically destroy the phase-matching (PM) condition, which fundamentally limit average power scaling. Here, we present a theoretical design of high average power OPCPA based on simultaneous temperature and wavelength insensitive PM. By regulating the temperature set for PM as well as the wavelength of interacting waves in lithium triborate crystal, the noncollinear angles for temperature insensitive PM and wavelength insensitive PM will coincide and thus ensures simultaneous temperature and wavelength insensitive PM condition. We investigate the performances of the proposed PM scheme in comparison with traditional wavelength insensitive noncollinear PM. Because of the larger temperature bandwidth, higher average power can be anticipated in OPCPA with the proposed PM scheme. Besides, the large spectral bandwidth also allows the amplification of sub-10 fs pulses in the visible spectral range. The proposed PM scheme will provide a promising approach in generating ultrashort pulses with high peak and average powers.

A new monochromator with high spectral resolution in the extreme ultraviolet (XUV) has been developed for high-order laser harmonics selection. The system has three optical elements—a cylindrical (or spherical) focusing mirror, a uniform-line-spaced plane grating, and a plane mirror. The last element is required to maintain the focus on a fixed vertical slit when the grating subtended angle is changed in order to minimize the spectral defocusing aberration. The parameters of the focusing mirror are determined to introduce a coma that compensates for the coma given by the grating. The possibility of using two interchangeable gratings made the set-up optimized for a broad energy range of 12–50 eV. As a design test case, the set-up has been applied to a selection of the discrete spectral lines emitted by a gas-discharge lamp as the XUV source, obtaining a resolving power E/ Δ E > 3000. [ABSTRACT FROM AUTHOR]

A new monochromator with high spectral resolution in the extreme ultraviolet (XUV) has been developed for high-order laser harmonics selection. The system has three optical elements—a cylindrical (or spherical) focusing mirror, a uniform-line-spaced plane grating, and a plane mirror. The last element is required to maintain the focus on a fixed vertical slit when the grating subtended angle is changed in order to minimize the spectral defocusing aberration. The parameters of the focusing mirror are determined to introduce a coma that compensates for the coma given by the grating. The possibility of using two interchangeable gratings made the set-up optimized for a broad energy range of 12−50 eV. As a design test case, the set-up has been applied to a selection of the discrete spectral lines emitted by a gas-discharge lamp as the XUV source, obtaining a resolving power E/ Δ E > 3000.

We experimentally demonstrate the second harmonic generation (SHG) of infrared femtosecond pulses using a BIBO crystal placed in an external ring cavity, synchronized with an input mode-locked laser at 78 MHz. A frequency doubling efficiency of 53% is achieved which is, to the best of our knowledge, the highest value ever reported for a low energy input beam of 1.4 nJ/pulse. Theoretical analysis of cavity related issues such as design, fundamental mode characteristics and fidelity against misalignments are also presented. The modeling of SHG cavity enables us to estimate the cavity losses and the mode matching visibility. Such synchronized SHG cavities in pulse domain, having higher SHG conversion efficiencies compared to their continuous wave counterparts, may find potential applications in scientific areas such as in photonics, and in quantum optics. [ABSTRACT FROM AUTHOR]

Black phosphorus, or BP, has found a lot of applications in recent years including photonics. The most recent studies have shown that the material has an excellent optical nonlinearity useful in many areas, one of which is in saturable absorption for passive mode-locking. A direct interaction scheme for mode-locking, however, has a potential to optically cause permanent damage to the already delicate material. Evanescent field interaction scheme has already been proven to be a useful method to prevent such danger for other 2-dimensional nanomaterials. In this report, we have utilized the evanescent field interaction to demonstrate that the optical nonlinear characteristics of BP is sufficiently strong to use in such an indirect interaction method. The successful demonstration of the passive mode-locking operation has generated pulses with the pulse duration, repetition rate, and time bandwidth product of 2.18 ps, 15.59 MHz, and 0.336, respectively. [ABSTRACT FROM AUTHOR]

A method is shown for preventing temporal broadening of ultrafast optical pulses in highly dispersive and fluctuating media for arbitrary signal-pulse profiles. Pulse pairs, consisting of a strong-field control-pulse and a weak-field signal-pulse, co-propagate, whereby the specific profile of the strong-field pulse precisely compensates for the dispersive phase in the weak pulse. A numerical example is presented in an optical system consisting of both resonant and gain dispersive effects. Here, we show signal-pulses that do not temporally broaden across a vast propagation distance, even in the presence of dispersion that fluctuates several orders of magnitude and in sign (for example, within a material resonance) across the pulse's bandwidth. Another numerical example is presented in normal dispersion telecom fiber, where the length at which an ultrafast pulse does not have significant temporal broadening is extended by at least a factor of 10. Our approach can be used in the design of dispersion-less fiber links and navigating pulses in turbulent dispersive media. Furthermore, we illustrate the potential of using cross-phase modulation to compensate for dispersive effects on a signal-pulse and fill the gap in the current understanding of this nonlinear phenomenon. [ABSTRACT FROM AUTHOR]

X-ray free-electron lasers (FELs) provide cutting-edge tools for fundamental researches to study nature down to the atomic level at a time-scale that fits this resolution. A precise knowledge of temporal information of FEL pulses is the central issue for its applications. Here we proposed and demonstrated a novel method to determine the FEL temporal profiles online. This robust method, designed for ultrafast FELs, allows researchers to acquire real-time longitudinal profiles of FEL pulses as well as their arrive times with respect to the external optical laser with a resolution better than 6 fs. Based on this method, we can also directly measure various properties of FEL pulses and correlations between them online. This helps us to further understand the FEL lasing processes and realize the generation of stable, nearly fully coherent soft X-ray laser pulses at the Shanghai Soft X-ray FEL facility. This method will enhance the experimental opportunities for ultrafast science in various areas., Competing Interests: The authors declare that they have no conflicts of interest in this work., (© 2022 The Authors. Publishing Services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd.)

Optimizing and measuring the femtosecond pulse duration of compression are very difficult in the case of online and vacuum in a multi-petawatt laser facility. Therefore, a method is presented in this study to determine the optimum point of pulse duration by simulation calculation and experimental measurements. Using the method in the multi-petawatt laser facility, the residual dispersion is completely eliminated, and a near Fourier-transform-limited pulse duration can be attained. These results are beneficial in finding an optimized pulse duration and guarantying the repeatability of the laser physical experiment results in multi-petawatt femtosecond laser facility. [ABSTRACT FROM AUTHOR]

Photonic integrated circuits (PICs) are notable for their enhanced functionalities with material flexibilities to find applications in wearable high‐speed data management systems. Due to the miniaturized dimensions of PICs, the employment of a nanomaterial having significant optical nonlinearity is critical. Here, it is demonstrated that a polymer waveguide can be harmonized with nonlinear graphene to form ultrashort laser pulses. The graphene works as nonlinear saturable absorber on the polymer waveguide prepared with a perfluorinated acrylic resin. The evanescent field of a laser propagating through the waveguide interacts with graphene to induce intracavity intensity modulation for femtosecond‐scale pulse formation. The laser output is characterized quantitatively as the central wavelength, spectral width, repetition rate, extinction ratio, and pulse duration, which are 1553.32 nm, 10.21 nm, 4.18 MHz, 76.03 dB, and 874 fs, respectively. Stable operation is verified over 3 h. A graphene–polymer waveguide saturable absorber is demonstrated ensuring the nonlinear evanescent field interaction of a laser propagating through the polymer waveguide with chemical vapor deposition‐grown graphene. A passively mode‐locked fiber laser is constructed to yield an ultra‐short pulsed laser output characterized with center wavelength, repetition rate, and pulse duration of 1553.32 nm, 4.18 MHz, and 874 fs, respectively. [ABSTRACT FROM AUTHOR]