Showing papers on "Femtosecond published in 2020"

Reversible 3D laser printing of perovskite quantum dots inside a transparent medium

Abstract: The three-dimensional (3D) patterning of semiconductors is potentially important for exploring new functionalities and applications in optoelectronics1,2. Here, we show that it is possible to write on demand 3D patterns of perovskite quantum dots (QDs) inside a transparent glass material using a femtosecond laser. By utilizing the inherent ionic nature and low formation energy of perovskite, highly luminescent CsPbBr3 QDs can be reversibly fabricated in situ and decomposed through femtosecond laser irradiation and thermal annealing. This pattern of writing and erasing can be repeated for many cycles, and the luminescent QDs are well protected by the inorganic glass matrix, resulting in stable perovskite QDs with potential applications such as high-capacity optical data storage, information encryption and 3D artwork. Luminescent CsPbBr3 quantum dots can be written into glass using femtosecond laser pulses and thermal annealing, and erased by further femtosecond laser irradiation. The resulting quantum dot patterns could prove useful for data storage, decoration or security purposes.

Observation of extremely efficient terahertz generation from mid-infrared two-color laser filaments

Abstract: 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.

Ultrabroadband nonlinear optics in nanophotonic periodically poled lithium niobate waveguides

Abstract: Quasi-phase-matched interactions in waveguides with quadratic nonlinearities enable highly efficient nonlinear frequency conversion. In this paper, we demonstrate the first generation of devices that combine the dispersion engineering available in nanophotonic waveguides with quasi-phase-matched nonlinear interactions available in periodically poled lithium niobate (PPLN). This combination enables quasi-static interactions of femtosecond pulses, reducing the pulse energy requirements by several orders of magnitude compared to conventional devices, from picojoules to femtojoules. We experimentally demonstrate two effects associated with second harmonic generation (SHG). First, we observe efficient quasi-phase-matched SHG with $ {\lt} {100}\;{\rm fJ}$<100fJ of pulse energy. Second, in the limit of strong phase-mismatch, we observe spectral broadening of both harmonics with as little as 2 pJ of pulse energy. These results lay a foundation for a new class of nonlinear devices, in which coengineering of dispersion with quasi-phase-matching enables efficient nonlinear optics at the femtojoule level.

Experimental observation of the liquid-liquid transition in bulk supercooled water under pressure

Abstract: We prepared bulk samples of supercooled liquid water under pressure by isochoric heating of high-density amorphous ice to temperatures of 205 ± 10 kelvin, using an infrared femtosecond laser. Because the sample density is preserved during the ultrafast heating, we could estimate an initial internal pressure of 2.5 to 3.5 kilobar in the high-density liquid phase. After heating, the sample expanded rapidly, and we captured the resulting decompression process with femtosecond x-ray laser pulses at different pump-probe delay times. A discontinuous structural change occurred in which low-density liquid domains appeared and grew on time scales between 20 nanoseconds to 3 microseconds, whereas crystallization occurs on time scales of 3 to 50 microseconds. The dynamics of the two processes being separated by more than one order of magnitude provides support for a liquid-liquid transition in bulk supercooled water.

Controlling free electrons with optical whispering-gallery modes

Abstract: Free-electron beams are versatile probes of microscopic structure and composition1,2, and have revolutionized atomic-scale imaging in several fields, from solid-state physics to structural biology3. Over the past decade, the manipulation and interaction of electrons with optical fields have enabled considerable progress in imaging methods4, near-field electron acceleration5,6, and four-dimensional microscopy techniques with high temporal and spatial resolution7. However, electron beams typically couple only weakly to optical excitations, and emerging applications in electron control and sensing8–11 require large enhancements using tailored fields and interactions. Here we couple a free-electron beam to a travelling-wave resonant cavity mode. The enhanced interaction with the optical whispering-gallery modes of dielectric microresonators induces a strong phase modulation on co-propagating electrons, which leads to a spectral broadening of 700 electronvolts, corresponding to the absorption and emission of hundreds of photons. By mapping the near-field interaction with ultrashort electron pulses in space and time, we trace the lifetime of the the microresonator following a femtosecond excitation and observe the spectral response of the cavity. The natural matching of free electrons to these quintessential optical modes could enable the application of integrated photonics technology in electron microscopy, with broad implications for attosecond structuring, probing quantum emitters and possible electron–light entanglement. The coupling between light and relativistic free electrons is enhanced through phase matching of electrons with optical whispering-gallery modes in dielectric microspheres and through extended modal lifetimes.

Ultralow-loss geometric phase and polarization shaping by ultrafast laser writing in silica glass

Abstract: Polarization and geometric phase shaping via a space-variant anisotropy has attracted considerable interest for fabrication of flat optical elements and generation of vector beams with applications in various areas of science and technology. Among the methods for anisotropy patterning, imprinting of self-assembled nanograting structures in silica glass by femtosecond laser writing is promising for the fabrication of space-variant birefringent optics with high thermal and chemical durability and high optical damage threshold. However, a drawback is the optical loss due to the light scattering by nanograting structures, which has limited the application. Here, we report a new type of ultrafast laser-induced modification in silica glass, which consists of randomly distributed nanopores elongated in the direction perpendicular to the polarization, providing controllable birefringent structures with transmittance as high as 99% in the visible and near-infrared ranges and >90% in the UV range down to 330 nm. The observed anisotropic nanoporous silica structures are fundamentally different from the femtosecond laser-induced nanogratings and conventional nanoporous silica. A mechanism of nanocavitation via interstitial oxygen generation mediated by multiphoton and avanlanche defect ionization is proposed. We demonstrate ultralow-loss geometrical phase optical elements, including geometrical phase prism and lens, and a vector beam convertor in silica glass.

Time-resolved serial femtosecond crystallography at the European XFEL.

Abstract: The European XFEL (EuXFEL) is a 3.4-km long X-ray source, which produces femtosecond, ultrabrilliant and spatially coherent X-ray pulses at megahertz (MHz) repetition rates. This X-ray source has been designed to enable the observation of ultrafast processes with near-atomic spatial resolution. Time-resolved crystallographic investigations on biological macromolecules belong to an important class of experiments that explore fundamental and functional structural displacements in these molecules. Due to the unusual MHz X-ray pulse structure at the EuXFEL, these experiments are challenging. Here, we demonstrate how a biological reaction can be followed on ultrafast timescales at the EuXFEL. We investigate the picosecond time range in the photocycle of photoactive yellow protein (PYP) with MHz X-ray pulse rates. We show that difference electron density maps of excellent quality can be obtained. The results connect the previously explored femtosecond PYP dynamics to timescales accessible at synchrotrons. This opens the door to a wide range of time-resolved studies at the EuXFEL.

Femtosecond-to-millisecond structural changes in a light-driven sodium pump.

Abstract: Light-driven sodium pumps actively transport small cations across cellular membranes1. These pumps are used by microorganisms to convert light into membrane potential and have become useful optogenetic tools with applications in neuroscience. Although the resting state structures of the prototypical sodium pump Krokinobacter eikastus rhodopsin 2 (KR2) have been solved2,3, it is unclear how structural alterations over time allow sodium to be translocated against a concentration gradient. Here, using the Swiss X-ray Free Electron Laser4, we have collected serial crystallographic data at ten pump-probe delays from femtoseconds to milliseconds. High-resolution structural snapshots throughout the KR2 photocycle show how retinal isomerization is completed on the femtosecond timescale and changes the local structure of the binding pocket in the early nanoseconds. Subsequent rearrangements and deprotonation of the retinal Schiff base open an electrostatic gate in microseconds. Structural and spectroscopic data, in combination with quantum chemical calculations, indicate that a sodium ion binds transiently close to the retinal within one millisecond. In the last structural intermediate, at 20 milliseconds after activation, we identified a potential second sodium-binding site close to the extracellular exit. These results provide direct molecular insight into the dynamics of active cation transport across biological membranes.

High-speed femtosecond laser plasmonic lithography and reduction of graphene oxide for anisotropic photoresponse

Abstract: Micro/nanoprocessing of graphene surfaces has attracted significant interest for both science and applications due to its effective modulation of material properties, which, however, is usually restricted by the disadvantages of the current fabrication methods. Here, by exploiting cylindrical focusing of a femtosecond laser on graphene oxide (GO) films, we successfully produce uniform subwavelength grating structures at high speed along with a simultaneous in situ photoreduction process. Strikingly, the well-defined structures feature orientations parallel to the laser polarization and significant robustness against distinct perturbations. The proposed model and simulations reveal that the structure formation is based on the transverse electric (TE) surface plasmons triggered by the gradient reduction of the GO film from its surface to the interior, which eventually results in interference intensity fringes and spatially periodic interactions. Further experiments prove that such a regular structured surface can cause enhanced optical absorption (>20%) and an anisotropic photoresponse (~0.46 ratio) for the reduced GO film. Our work not only provides new insights into understanding the laser-GO interaction but also lays a solid foundation for practical usage of femtosecond laser plasmonic lithography, with the prospect of expansion to other two-dimensional materials for novel device applications.

Three-Dimensional Laser-Assisted Patterning of Blue-Emissive Metal Halide Perovskite Nanocrystals inside a Glass with Switchable Photoluminescence

Abstract: The high-precision patterning of metal halide perovskites (MHPs) is of paramount importance for their device application. Here, we demonstrate the femtosecond (fs)-laser-assisted formation of three...

VSe2 nanosheets for ultrafast fiber lasers

Abstract: In this work, we investigate VSe2 for generating femtosecond and large energy mode-locked laser pulses for the first time. Two types of VSe2 based saturable absorbers (SAs), microfiber–VSe2 and VSe2/polyvinyl alcohol (PVA), are prepared, which exhibit strong nonlinear optical characteristics with a modulation depth (ΔT) of 22.5% and 1.849%, respectively. In contrast to other transition metal dichalcogenides (TMDCs), theoretical calculations show that VSe2 is a metallic TMDC SA. With the implementation of microfiber–VSe2, conventional soliton mode-locking Er-doped fiber (EDF) laser is demonstrated with an ultrashort pulse duration of 910 fs. Based on VSe2/PVA, a large energy EDF laser is established. The maximum single pulse energy is 25.57 nJ. This study suggests that VSe2 possesses application potential in ultrafast photonics and provides a valuable strategy for the development of TMDC based devices with desirable optoelectronic properties.

High-energy hybrid femtosecond laser system demonstrating 2 × 10 PW capability

Abstract: We report on a two-arm hybrid high-power laser system (HPLS) able to deliver 2 × 10 PW femtosecond pulses, developed at the Bucharest-Magurele Extreme Light Infrastructure Nuclear Physics (ELI-NP) Facility. A hybrid front-end (FE) based on a Ti:sapphire chirped pulse amplifier and a picosecond optical parametric chirped pulse amplifier based on beta barium borate (BBO) crystals, with a cross-polarized wave (XPW) filter in between, has been developed. It delivers 10 mJ laser pulses, at 10 Hz repetition rate, with more than 70 nm spectral bandwidth and high-intensity contrast, in the range of 1013:1. The high-energy Ti:sapphire amplifier stages of both arms were seeded from this common FE. The final high-energy amplifier, equipped with a 200 mm diameter Ti:sapphire crystal, has been pumped by six 100 J nanosecond frequency doubled Nd:glass lasers, at 1 pulse/min repetition rate. More than 300 J output pulse energy has been obtained by pumping with only 80% of the whole 600 J available pump energy. The compressor has a transmission efficiency of 74% and an output pulse duration of 22.7 fs was measured, thus demonstrating that the dual-arm HPLS has the capacity to generate 10 PW peak power femtosecond pulses. The reported results represent the cornerstone of the ELI-NP 2 × 10 PW femtosecond laser facility, devoted to fundamental and applied nuclear physics research.

Rewritable color nanoprints in antimony trisulfide films

Abstract: Materials that exhibit large and rapid switching of their optical properties in the visible spectrum hold the key to color-changing devices. Antimony trisulfide (Sb2S3) is a chalcogenide material that exhibits large refractive index changes of ~1 between crystalline and amorphous states. However, little is known about its ability to endure multiple switching cycles, its capacity for recording high-resolution patterns, nor the optical properties of the crystallized state. Unexpectedly, we show that crystalline Sb2S3 films that are just 20 nm thick can produce substantial birefringent phase retardation. We also report a high-speed rewritable patterning approach at subdiffraction resolutions (>40,000 dpi) using 780-nm femtosecond laser pulses. Partial reamorphization is demonstrated and then used to write and erase multiple microscale color images with a wide range of colors over a ~120-nm band in the visible spectrum. These solid-state, rapid-switching, and ultrahigh-resolution color-changing devices could find applications in nonvolatile ultrathin displays.

Coherent beam combining of 61 femtosecond fiber amplifiers.

Abstract: We report on the coherent beam combining of 61 femtosecond fiber chirped-pulse amplifiers in a tiled-aperture configuration along with an interferometric phase measurement technique. Relying on coherent beam recombination in the far field, this technique appears suitable for the combination of a large number of fiber amplifiers. The 61 output beams are stacked in a hexagonal arrangement and collimated through a high fill factor hexagonal micro-lens array. The residual phase error between two fibers is as low as λ/90 RMS, while a combining efficiency of ∼50% is achieved.

Breaking 50 Femtosecond Resolution Barrier in MeV Ultrafast Electron Diffraction with a Double Bend Achromat Compressor

Abstract: We propose and demonstrate a novel scheme to produce ultrashort and ultrastable MeV electron beam. In this scheme, the electron beam produced in a photocathode radio frequency (rf) gun first expands under its own Coulomb force with which a positive energy chirp is imprinted in the beam longitudinal phase space. The beam is then sent through a double bend achromat with positive longitudinal dispersion where electrons at the bunch tail with lower energies follow shorter paths and thus catch up with the bunch head, leading to longitudinal bunch compression. We show that with optimized parameter sets, the whole beam path from the electron source to the compression point can be made isochronous such that the time of flight for the electron beam is immune to the fluctuations of rf amplitude. With a laser-driven THz deflector, the bunch length and arrival time jitter for a 20 fC beam after bunch compression are measured to be about 29 fs (FWHM) and 22 fs (FWHM), respectively. Such an ultrashort and ultrastable electron beam allows us to achieve 50 femtosecond (FWHM) resolution in MeV ultrafast electron diffraction where lattice oscillation at 2.6 THz corresponding to Bismuth A(1g) mode is clearly observed without correcting both the short-term timing jitter and long-term timing drift. Furthermore, oscillating weak diffuse scattering signal related to phonon coupling and decay is also clearly resolved thanks to the improved temporal resolution and increased electron flux. We expect that this technique will have a strong impact in emerging ultrashort electron beam based facilities and applications.

Bioinspired micro/nanostructured surfaces prepared by femtosecond laser direct writing for multi-functional applications

Abstract: Femtosecond laser direct writing (FLDW) has been widely used for controllable biomimetic micro/nanostructure processing due to its specific advantages including high precision, simplicity, and compatibility for diverse materials in comparison with other methods (e.g. ion etching, sol-gel process, chemical vapor deposition, template method, and self-assembly). These biomimetic micro/nanostructured surfaces are expected to be broadly applicable in self-cleaning, oil-water separation, and fog collection, which have attracted increasing attentions ranging from academic research to industrial applications in recent years. In this review, we firstly present the inherent relationship among nature organisms, fabrication methods, micro/nanostructures and their potential applications. Thereafter, we throw a list of current fabrication strategies so as to highlight the advantages of FLDW in manufacturing bioinspired microstructured surfaces. Subsequently, we summarize a variety of typical bioinspired design (e.g. lotus leaf, pitcher plant, rice leaf, butterfly wings and so on) for diverse multifunctional micro/nanostructures through extreme femtosecond laser processing. On the basis of the principle of interfacial chemistry and geometrical optics, we present some potential applications of these functional micro/nanostructures in the fields of structural color, self-cleaning, oil-water separation, fog harvesting, underwater bubble collection, droplet directional transport and droplet/optical switch. Finally, we propose some underlying challenges and opportunities in the extreme fabrication of bioinspired micro/nanostructures by FLDW, and follow the outlook of the development in terms of femtosecond laser processing in biomimetic domains.

Interpreting the enhanced photoactivities of 0D/1D heterojunctions of CdS quantum dots /TiO2 nanotube arrays using femtosecond transient absorption spectroscopy

Abstract: Heterojunctions of CdS quantum dots (QDs)/TiO2 nanotube arrays (NTAs) were synthesized via a simple vapor transport approach. The 0D/1D QDs/NTAs electrodes exhibited 5-fold improvement efficiency for photocatalytic dye degradation and hydrogen evolution compared to pure TiO2 NTAs in a photoelectrochemistry cell, which was attributed to the faster transportation and retarded recombination of electron-hole pairs in the 0D/1D heterostructures. Femtosecond transient absorption offers deep insights into the dynamics of the charge carriers, showing that surface-trapped electrons were generated within 1.2 ps. The lifetimes for recombination of electrons at shallow trap sites with holes and deep trapped electrons with holes are prolonged to 73.2 ps and 622.6 ps, respectively. The reduced charge transfer resistance is verified by electrochemical impedance spectroscopy. Mott-Schottky plots indicated downward shifts for both the conduction band and valence band in the heterostructure. Finally, a Z-scheme photoreactive mechanism is proposed to explain the enhanced catalytic activities.

Attosecond spectroscopy of liquid water.

Abstract: Electronic dynamics in liquids are of fundamental importance, but time-resolved experiments have so far remained limited to the femtosecond time scale. We report the extension of attosecond spectroscopy to the liquid phase. We measured time delays of 50 to 70 attoseconds between the photoemission from liquid water and that from gaseous water at photon energies of 21.7 to 31.0 electron volts. These photoemission delays can be decomposed into a photoionization delay sensitive to the local environment and a delay originating from electron transport. In our experiments, the latter contribution is shown to be negligible. By referencing liquid water to gaseous water, we isolated the effect of solvation on the attosecond photoionization dynamics of water molecules. Our methods define an approach to separating bound and unbound electron dynamics from the structural response of the solvent.

Patterned arrays of assembled nanoparticles prepared by interfacial assembly and femtosecond laser fabrication

Abstract: Creating shape-defined structures of inorganic nanoparticles in a maskless and template-free fashion would advance the engineering of nanoparticle-based devices and structures with desired configurations for various applications. In this work, a novel fabrication protocol combining bottom-up interfacial assembly and subtractive laser patterning was developed for creating patterned arrays of assembled nanoparticles. A solid film of magnetic nanoparticles (10 nm, monodisperse CoFe2O4) was assembled as a nanoparticle film (thickness less than 100 nm) on liquid interface under guiding field, and it was further transferred to Si substrate followed by selective material removal using femtosecond laser pulses, producing patterned arrays (typical size of 3 μm) of assembled nanoparticles. The size, shape, and arrangement of the patterned arrays were finely regulated by adjusting the laser pulse energy and laser scanning path. The magnetization behavior and magnetic anisotropy of the patterned arrays differ from those of the nanoparticle-assembled film, as reflected by the changes of coercivity and squareness along the out-of-plane direction. The presented fabrication protocol is compatible with microelectronic fabrication techniques and can be applied to various inorganic nanoparticles.

Optical parametric amplification in gas-filled hollow-core capillary for the generation of tunable pulses in the infrared

Abstract: Ultrashort pulses in the near-infrared (NIR) to mid-infrared (MIR) are widely used for laser matter interaction experiments, e.g. the relaxation process of carrier semiconductors and chemical dynamics at the femtosecond and attosecond time scale [1,2]. Many different approaches based on nonlinear processes or laser devices can be found to generate pulses in theses spectral ranges. Recently, four wave mixing (FWM) based parametric amplification in gas-filled hollow core capillary (HCC) has been used to create a tunable source of ultrashort pulses. For example, pulses can be generated in the visible with an energy at the 10 µJ level [4] and in the near infrared at ~1.4 µm with an energy of 5 µJ and a pulse duration of 45 fs [5]. Here, we present an implementation of a scheme to generate tuneable pulses from the NIR to the MIR toward a high-power level. The general principle of the FWM process relies on the combination of two pulses: a strong pump and a weak signal which co-propagate in a gas filled HCC. According to the phase matching condition, a part of the pump energy is transferred from the pump to the signal and an idler is created. In our experiment, this process was driven by pulses from a 1 kHz, Ti: Sapphire laser (800 nm, 120 fs) in combination with a weak continuum tunable from 420 to 650 nm (the signal) obtained by focusing a part of the 800nm laser into a 5 mm thick Sapphire plate. The relative delay between the pump and the seed pulses was controlled by a translation stage. Both beams were focused in a 30 cm long argon filled HCC with an inner core diameter of 150 µm. In parallel, we firstly achieved numerical simulations to predict the optimal pressure when the three waves propagate in the fundamental modes. From the computed total phase mismatch (Figure 1.a), we determine that tunable pulses in the near/mid infrared with high gain can be obtained from a pressure < 2 bar. Figure 1.b-c shows the experimental spectrum for a pressure of ~2 bar and an energy in the capillary of 146 µJ, when the pump pulse and the continuum signal are temporally overlapped. The tunability was obtained by changing the relative delay between the signal and the pump with the translation stage. In this condition, the idler is found to be tuneable from 1µm to 1.3 µm. Others simulations and experiments are in progress to extend the bandwidth toward the mid-infrared. a) b) c) Fig. 1 (a) Total phase mismatch in a Ar filled HCC. The core diameter is 150 µm. The pressure is tuned from 0.5 to 2 bar. The pump energy is 250 uJ.at 800nm b) Continuum spectrum (black line). Amplified spectrum for several delays, (color lines). c) Infrared spectrum generated by the FWM for a pressure of 2 bar. To summarize, we have shown that FWM based parametric amplification in gas filled hollow core capillary is an efficient method to generate tuneable pulses in the infrared band with a promising potentiality to reach the mid infrared. References [1] B.

Spectral absorption control of femtosecond laser-treated metals and application in solar-thermal devices.

Abstract: Direct femtosecond (fs) laser processing is a maskless fabrication technique that can effectively modify the optical, electrical, mechanical, and tribological properties of materials for a wide range of potential applications. However, the eventual implementation of fs-laser-treated surfaces in actual devices remains challenging because it is difficult to precisely control the surface properties. Previous studies of the morphological control of fs-laser-processed surfaces mostly focused on enhancing the uniformity of periodic microstructures. Here, guided by the plasmon hybridisation model, we control the morphology of surface nanostructures to obtain more control over spectral light absorption. We experimentally demonstrate spectral control of a variety of metals [copper (Cu), aluminium (Al), steel and tungsten (W)], resulting in the creation of broadband light absorbers and selective solar absorbers (SSAs). For the first time, we demonstrate that fs-laser-produced surfaces can be used as high-temperature SSAs. We show that a tungsten selective solar absorber (W-SSA) exhibits excellent performance as a high-temperature solar receiver. When integrated into a solar thermoelectric generation (TEG) device, W-SSA provides a 130% increase in solar TEG efficiency compared to untreated W, which is commonly used as an intrinsic selective light absorber.

Femtosecond dual-comb Yb:CaF2 laser from a single free-running polarization-multiplexed cavity for optical sampling applications.

Abstract: Dual optical frequency combs are an appealing solution to many optical measurement techniques due to their high spectral and temporal resolution, high scanning speed, and lack of moving parts. However, industrial and field-deployable applications of such systems are limited due to a high-cost factor and intricacy in the experimental setups, which typically require a pair of locked femtosecond lasers. Here, we demonstrate a single oscillator which produces two mode-locked output beams with a stable repetition rate difference. We achieve this via inserting two 45°-cut birefringent crystals into the laser cavity, which introduces a repetition rate difference between the two polarization states of the cavity. To mode-lock both combs simultaneously, we use a semiconductor saturable absorber mirror (SESAM). We achieve two simultaneously operating combs at 1050 nm with 175-fs duration, 3.2-nJ pulses and an average power of 440 mW in each beam. The average repetition rate is 137 MHz, and we set the repetition rate difference to 1 kHz. This laser system, which is the first SESAM mode-locked femtosecond solid-state dual-comb source based on birefringent multiplexing, paves the way for portable and high-power femtosecond dual-combs with flexible repetition rate. To demonstrate the utility of the laser for applications, we perform asynchronous optical sampling (ASOPS) on semiconductor thin-film structures with the free-running laser system, revealing temporal dynamics from femtosecond to nanosecond time scales.

Intelligent control of mode-locked femtosecond pulses by time-stretch-assisted real-time spectral analysis.

Abstract: Mode-locked fiber lasers based on nonlinear polarization evolution can generate femtosecond pulses with different pulse widths and rich spectral distributions for versatile applications through polarization tuning. However, a precise and repeatable location of a specific pulsation regime is extremely challenging. Here, by using fast spectral analysis based on a time-stretched dispersion Fourier transform as the spectral discrimination criterion, along with an intelligent polarization search algorithm, for the first time, we achieved real-time control of the spectral width and shape of mode-locked femtosecond pulses; the spectral width can be tuned from 10 to 40 nm with a resolution of ~1.47 nm, and the spectral shape can be programmed to be hyperbolic secant or triangular. Furthermore, we reveal the complex, repeatable transition dynamics of the spectrum broadening of femtosecond pulses, including five middle phases, which provides deep insight into ultrashort pulse formation that cannot be observed with traditional mode-locked lasers. A method providing greater control over very short duration light pulses generated by systems called mode-locked fiber lasers (MLFLs) will enhance the use of the light in such diverse applications as atomic clocks, radars, optical computing, measuring systems and astronomy. Researchers in China led by Lilin Yi at Shanghai Jiao Tong University developed apparatus and software algorithms allowing automatic ‘intelligent control’ over the generation of MLFL light pulses. The system can manipulate key aspects of the frequency range and composition of the pulses – technically their ‘spectral width’ and ‘spectral shape’ – more effectively than previously possible. The procedure also yields new technical insights into the factors determining the nature of the generated light pulses. The researchers believe their low-cost and portable system will find widespread application in research and industry.

Omnidirectional iridescence via cylindrically-polarized femtosecond laser processing

Abstract: We report the femtosecond (fs) laser fabrication of biomimetic omnidirectional iridescent metallic surfaces exhibiting efficient diffraction for practically any angle of light incidence. Such diffractive behavior is realized by means of mul-ti-directional low-spatial-frequency, laser-induced periodic surface structures (LSFL) formed upon exploiting the cylindrical symmetry of a cylindrical vector (CV) fs field. We particularly demonstrate that the multi-directional gratings formed on stainless steel surface by a radially polarized fs beam, could mimic the omnidirectional structural coloration properties found in some natural species. Accordingly, the fabricated grating structures can spatially disperse the incident light into individual wavelength with high efficiency, exhibiting structural iridescence at all viewing angles. Analytical calculations using the grating equation reproduced the characteristic variation of the vivid colors observed as a function of incident angle. We envisage that our results will significantly contribute to the development of new photonic and light sensing devices.

Octave-spanning coherent supercontinuum generation in an AlGaAs-on-insulator waveguide

Abstract: We demonstrate supercontinuum generation over an octave spaning from 1055 to 2155 nm on the highly nonlinear aluminum gallium arsenide (AlGaAs)-on-insulator platform. This is enabled by the generation of two dispersive waves in a 3-mm-long dispersion-engineered nano-waveguide. The waveguide is pumped at telecom wavelengths (1555 nm) with 3.6 pJ femtosecond pulses. We experimentally validate the coherence of the generated supercontinuum around the pump wavelength (1450–1750 nm), and our numerical simulation shows a high degree of coherence over the full spectrum.

Structural dynamics in proteins induced by and probed with X-ray free-electron laser pulses.

Abstract: X-ray free-electron lasers (XFELs) enable crystallographic structure determination beyond the limitations imposed upon synchrotron measurements by radiation damage. The need for very short XFEL pulses is relieved through gating of Bragg diffraction by loss of crystalline order as damage progresses, but not if ionization events are spatially non-uniform due to underlying elemental distributions, as in biological samples. Indeed, correlated movements of iron and sulfur ions were observed in XFEL-irradiated ferredoxin microcrystals using unusually long pulses of 80 fs. Here, we report a femtosecond time-resolved X-ray pump/X-ray probe experiment on protein nanocrystals. We observe changes in the protein backbone and aromatic residues as well as disulfide bridges. Simulations show that the latter’s correlated structural dynamics are much slower than expected for the predicted high atomic charge states due to significant impact of ion caging and plasma electron screening. This indicates that dense-environment effects can strongly affect local radiation damage-induced structural dynamics. The local X-ray-induced dynamics that occur in protein crystals during serial femtosecond crystallography (SFX) measurements at XFELs are not well understood. Here the authors performed a time-resolved X-ray pump X-ray probe SFX experiment, and they observe distinct structural changes in the disulfide bridges and peptide backbone of proteins; complementing theoretical approaches allow them to further characterize the details of the X-ray induced ionization and local structural dynamics.

Sub-cycle atomic-scale forces coherently control a single-molecule switch

Abstract: Scanning probe techniques can leverage atomically precise forces to sculpt matter at surfaces, atom by atom. These forces have been applied quasi-statically to create surface structures1–7 and influence chemical processes8,9, but exploiting local dynamics10–14 to realize coherent control on the atomic scale remains an intriguing prospect. Chemical reactions15–17, conformational changes18,19 and desorption20 have been followed on ultrafast timescales, but directly exerting femtosecond forces on individual atoms to selectively induce molecular motion has yet to be realized. Here we show that the near field of a terahertz wave confined to an atomically sharp tip provides femtosecond atomic-scale forces that selectively induce coherent hindered rotation in the molecular frame of a bistable magnesium phthalocyanine molecule. Combining lightwave-driven scanning tunnelling microscopy21–24 with ultrafast action spectroscopy10,13, we find that the induced rotation modulates the probability of the molecule switching between its two stable adsorption geometries by up to 39 per cent. Mapping the response of the molecule in space and time confirms that the force acts on the atomic scale and within less than an optical cycle (that is, faster than an oscillation period of the carrier wave of light). We anticipate that our strategy might ultimately enable the coherent manipulation of individual atoms within single molecules or solids so that chemical reactions and ultrafast phase transitions can be manipulated on their intrinsic spatio-temporal scales. The near field of a terahertz wave confined to a scanning probe tip provides femtosecond atomic-scale forces that coherently modulate the switching probability of a molecule between two stable adsorption geometries.