Showing papers on "Femtosecond published in 2012"
TL;DR: By guiding a mid-infrared femtosecond laser in a high-pressure gas, ultrahigh harmonics can be generated that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to more than 1.6 kilo–electron volts, allowing, in principle, the generation of pulses as short as 2.5 attoseconds.
Abstract: High-harmonic generation (HHG) traditionally combines ~100 near-infrared laser photons to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms adds constructively. Here, we show that by guiding a mid-infrared femtosecond laser in a high-pressure gas, ultrahigh harmonics can be generated, up to orders greater than 5000, that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to more than 1.6 kilo-electron volts, allowing, in principle, the generation of pulses as short as 2.5 attoseconds. The multiatmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, further enhancing the x-ray yield. Finally, the x-ray beam exhibits high spatial coherence, even though at high gas density the recolliding electrons responsible for HHG encounter other atoms during the emission process.
1,475 citations
SLAC National Accelerator Laboratory1, Max Planck Society2, Arizona State University3, Cornell University4, State University of New York System5, Johns Hopkins University Applied Physics Laboratory6, European Synchrotron Radiation Facility7, University of Gothenburg8, University of Hamburg9, University of Lübeck10, Uppsala University11
TL;DR: Serial femtosecond crystallography (SFX) is applied using an x-ray free-electron laser (XFEL) to obtain high-resolution structural information from microcrystals of the well-characterized model protein lysozyme, demonstrating the immediate relevance of SFX for analyzing the structure of the large group of difficult-to-crystallize molecules.
Abstract: Structure determination of proteins and other macromolecules has historically required the growth of high-quality crystals sufficiently large to diffract x-rays efficiently while withstanding radiation damage. We applied serial femtosecond crystallography (SFX) using an x-ray free-electron laser (XFEL) to obtain high-resolution structural information from microcrystals (less than 1 micrometer by 1 micrometer by 3 micrometers) of the well-characterized model protein lysozyme. The agreement with synchrotron data demonstrates the immediate relevance of SFX for analyzing the structure of the large group of difficult-to-crystallize molecules.
764 citations
TL;DR: In this paper, the formation of laser-induced periodic surface structures (LIPSS) in different materials (metals, semiconductors, and dielectrics) upon irradiation with linearly polarized fs-laser pulses (τ,∼ 30-150 fs, λ, ∼, ∼ 800 nm) in air environment is studied experimentally and theoretically.
Abstract: The formation of laser-induced periodic surface structures (LIPSS) in different materials (metals, semiconductors, and dielectrics) upon irradiation with linearly polarized fs-laser pulses (τ ∼ 30–150 fs, λ ∼ 800 nm) in air environment is studied experimentally and theoretically. In metals, predominantly low-spatial-frequency-LIPSS with periods close to the laser wavelength λ are observed perpendicular to the polarization. Under specific irradiation conditions, high-spatial-frequency-LIPSS with sub-100-nm spatial periods (∼λ/10) can be generated. For semiconductors, the impact of transient changes of the optical properties to the LIPSS periods is analyzed theoretically and experimentally. In dielectrics, the importance of transient excitation stages in the LIPSS formation is demonstrated experimentally using (multiple) double-fs-laser-pulse irradiation sequences. A characteristic decrease of the LIPSS periods is observed for double-pulse delays of less than 2 ps.
653 citations
TL;DR: The generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40 nanometres, is reported by coupling a high-power near-infrared frequency comb to a robust femtosecond enhancement cavity, and the absolute frequency of the argon transition has been determined by direct frequency comb spectroscopy.
Abstract: By coupling a high-power, high-repetition-rate near-infrared frequency comb to a femtosecond optical cavity, a frequency comb operating in the extreme-ultraviolet spectral range has been produced, by high harmonic generation, and provides high-resolution spectroscopy in this spectral region. Laser-based optical frequency combs, so called because they emit evenly spaced spectral lines, are used in precision spectroscopy and other applications requiring accurate measurements, such as atomic clocks. Efforts to extend this capability to shorter wavelengths in the extreme ultraviolet — which would open up exciting new applications, including searches for variation in fundamental constants — have lacked sufficient power for the purpose until now. Jun Ye and co-workers demonstrate a new approach, using a high-power, high-repetition pulsed infrared laser coupled into an optical cavity, to produce an improved extreme UV comb. In a first precision spectroscopy demonstration, they use direct frequency comb spectroscopy to determine argon and neon atomic transitions with ultra-high precision. The development of the optical frequency comb (a spectrum consisting of a series of evenly spaced lines) has revolutionized metrology and precision spectroscopy owing to its ability to provide a precise and direct link between microwave and optical frequencies1,2. A further advance in frequency comb technology is the generation of frequency combs in the extreme-ultraviolet spectral range by means of high-harmonic generation in a femtosecond enhancement cavity3,4. Until now, combs produced by this method have lacked sufficient power for applications, a drawback that has also hampered efforts to observe phase coherence of the high-repetition-rate pulse train produced by high-harmonic generation, which is an extremely nonlinear process. Here we report the generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40 nanometres, by coupling a high-power near-infrared frequency comb5 to a robust femtosecond enhancement cavity. These combs are powerful enough for us to observe single-photon spectroscopy signals for both an argon transition at 82 nanometres and a neon transition at 63 nanometres, thus confirming the combs’ coherence in the extreme ultraviolet. The absolute frequency of the argon transition has been determined by direct frequency comb spectroscopy. The resolved ten-megahertz linewidth of the transition, which is limited by the temperature of the argon atoms, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. Owing to the lack of continuous-wave lasers, extreme-ultraviolet frequency combs are at present the only promising route to extending ultrahigh-precision spectroscopy to the spectral region below 100 nanometres. At such wavelengths there is a wide range of applications, including the spectroscopy of electronic transitions in molecules6, experimental tests of bound-state and many-body quantum electrodynamics in singly ionized helium and neutral helium7,8,9, the development of next-generation ‘nuclear’ clocks10,11,12 and searches for variation of fundamental constants13 using the enhanced sensitivity of highly charged ions14.
417 citations
TL;DR: Measurements indicate that current X-ray free-electron laser technology should enable structural determination from submicrometre protein crystals with atomic resolution, and the shortest apparent pulse lengths occur at the highest resolution.
Abstract: Researchers describe a mechanism capable of compressing fast and intense X-ray pulses through the rapid loss of crystalline periodicity. It is hoped that this concept, combined with X-ray free-electron laser technology, will allow scientists to obtain structural information at atomic resolutions.
309 citations
TL;DR: The extension of broadband degenerate OPO operation further into mid-infrared by synchronously pumps a 500-μm-long crystal of orientation patterned GaAs providing broadband gain centered at 4.1 µm is reported.
Abstract: We report the extension of broadband degenerate OPO operation further into mid-infrared. A femtosecond thulium fiber laser with output centered at 2050 nm synchronously pumps a 500-μm-long crystal of orientation patterned GaAs providing broadband gain centered at 4.1 µm. We observe a pump threshold of 17 mW and output bandwidth extending from 2.6 to 6.1 µm at the −30 dB level. Average output power was 37 mW. Appropriate resonator group dispersion is a key factor for achieving degenerate operation with instantaneously broad bandwidth. The output spectrum is very sensitive to absorption and dispersion introduced by molecular species inside the OPO cavity.
277 citations
TL;DR: It is shown that all-optical switching with polarized femtosecond laser pulses in ferrimagnetic GdFeCo is subjected to a threshold fluence absorbed in the magnetic layer, independent of either the excitation wavelength or the polarization of the laser pulse.
Abstract: Using magneto-optical microscopy in combination with ellipsometry measurements, we show that all-optical switching with polarized femtosecond laser pulses in ferrimagnetic GdFeCo is subjected to a threshold fluence absorbed in the magnetic layer, independent of either the excitation wavelength or the polarization of the laser pulse. Furthermore, we present a quantitative explanation of the intensity window in which all-optical helicity-dependent switching (AO-HDS) occurs, based on magnetic circular dichroism. This explanation is consistent with all the experimental findings on AO-HDS so far, varying from single- to multiple-shot experiments. The presented results give a solid understanding of the origin of AO-HDS, and give novel insights into the physics of ultrafast, laser controlled magnetism.
270 citations
06 May 2012
TL;DR: Ultrafast spectroscopy and quantum-dynamics simulations of an artificial supramolecular light-harvesting system give strong evidence that the quantum-correlated wavelike motion of electrons and nuclei governs the ultrafast electronic charge transfer.
Abstract: Summary form only given. In artificial light harvesting systems the conversion of light into charges or chemical energy happens on the femtosecond time scale and is thought to involve the incoherent jump of an electron from the optical absorber to an electron acceptor. Here we investigate the primary process of electronic charge transfer dynamics in a carotene-porphyrin-fullerene triad, a prototypical elementary component for an artificial light harvesting system combining coherent femtosecond spectroscopy and first-principles quantum dynamics simulations. Our experimental and theoretical results provide strong evidence that the driving mechanism of the photoinduced current generation cycle is a quantum-correlated wavelike motion of electrons and nuclei on a timescale of few tens of femtoseconds. We furthermore highlight the fundamental role played by the interface between the light-absorbing chromophore and the charge acceptor in triggering the coherent wavelike electron-hole splitting.
259 citations
TL;DR: A review of femtosecond filamentation can be found in this paper with emphasis on the collective work of filamentation nonlinear optics, which is a new hot subject of current debate.
Abstract: This is a review of some recent development in femtosecond filamentation science with emphasis on our collective work. Previously reviewed work in the field will not be discussed. We thus start with a very brief description of the fundamental physics of single filamentation of powerful femtosecond laser pulses in air. Intensity clamping is emphasized. One consequence is that the peak intensity inside one or more filaments would not increase significantly even if one focuses the pulse at very high peak power even up to the peta-watt level. Another is that the clamped intensity is independent of pressure. One interesting outcome of the high intensity inside a filament is filament fusion which comes from the nonlinear change of index of refraction inside the filament leading to cross beam focusing. Because of the high intensity inside the filament, one can envisage nonlinear phenomena taking place inside a filament such as a new type of Raman red shift and the generation of very broad band supercontinuum into the infrared through four-wave-mixing. This is what we call by filamentation nonlinear optics. It includes also terahertz generation from inside the filament. The latter is discussed separately because of its special importance to those working in the field of safety and security in recent years. When the filamenting pulse is linearly polarized, the isotropic nature of air becomes birefringent both electronically (instantaneous) and through molecular wave packet rotation and revival (delayed). Such birefringence is discussed in detailed. Because, in principle, a filament can be projected to a long distance in air, applications to pollution measurement as well as other atmospheric science could be earned out. We call this filamentation atmospheric science. Thus, the following subjects are discussed briefly, namely, lightning control, rain making, remote measurement of electric field, microwave guidance and remote sensing of pollutants. A discussion on the higher order Kerr effect on the physics of filamentation is also given. This is a new hot subject of current debate. This review ends on giving our view of the prospect of progress of this field of filamentation in the future. We believe it hinges upon the development of the laser technology based upon the physical understanding of filamentation and on the reduction in price of the laser system.
240 citations
TL;DR: Shine a light: a circular dichroism effect in the ±10 % regime on randomly oriented chiral molecules in the gas phase is demonstrated and the enantiomers are distinguished.
Abstract: Shine a light: a circular dichroism effect in the ±10 % regime on randomly oriented chiral molecules in the gas phase is demonstrated. The signal is derived from images of photoelectron angular distributions produced by resonance-enhanced multiphoton ionization and allows the enantiomers to be distinguished. To date, this effect could only be generated with a synchrotron source. The new tabletop laser-based approach will make this approach far more accessible.
234 citations
TL;DR: The first time-resolved femtosecond serial X-ray crystallography results from an irreversible photo-chemical reaction at the Linac Coherent Light Source are obtained.
Abstract: We demonstrate the use of an X-ray free electron laser synchronized with an optical pump laser to obtain X-ray diffraction snapshots from the photoactivated states of large membrane protein complexes in the form of nanocrystals flowing in a liquid jet. Light-induced changes of Photosystem I-Ferredoxin co-crystals were observed at time delays of 5 to 10 µs after excitation. The result correlates with the microsecond kinetics of electron transfer from Photosystem I to ferredoxin. The undocking process that follows the electron transfer leads to large rearrangements in the crystals that will terminally lead to the disintegration of the crystals. We describe the experimental setup and obtain the first time-resolved femtosecond serial X-ray crystallography results from an irreversible photo-chemical reaction at the Linac Coherent Light Source. This technique opens the door to time-resolved structural studies of reaction dynamics in biological systems.
TL;DR: An ultrafast thin disk laser that generates an average output power of 275 W, which is higher than any other modelocked laser oscillator, and opens a new avenue for power scaling femtosecond oscillators to the kW level.
Abstract: We present an ultrafast thin disk laser that generates an average output power of 275 W, which is higher than any other modelocked laser oscillator. It is based on the gain material Yb:YAG and operates at a pulse duration of 583 fs and a repetition rate of 16.3 MHz resulting in a pulse energy of 16.9 μJ and a peak power of 25.6 MW. A SESAM designed for high damage threshold initiated and stabilized soliton modelocking. We reduced the nonlinearity of the atmosphere inside the cavity by several orders of magnitude by operating the oscillator in a vacuum environment. Thus soliton modelocking was achieved at moderate amounts of self-phase modulation and negative group delay dispersion. Our approach opens a new avenue for power scaling femtosecond oscillators to the kW level.
TL;DR: Singlet fission was demonstrated to play a predominant role in the excited state relaxation of the rubrene crystal in contrast to rubrene in solution.
Abstract: The excited state dynamics of rubrene in solution and in the single crystal were studied by femtosecond pump–probe spectroscopy under various excitation conditions. Singlet fission was demonstrated to play a predominant role in the excited state relaxation of the rubrene crystal in contrast to rubrene in solution. Upon 500 nm excitation, triplet excitons form on the picosecond time scale via fission from the lowest excited singlet state. Upon 250 nm excitation, fission from upper excited singlet states is observed within 200 fs.
TL;DR: Graphene oxide (GO) thin films on glass and plastic substrates were found to display interesting broadband nonlinear optical properties, which were significantly enhanced upon partial reduction by using laser irradiation or chemical reduction methods.
Abstract: Graphene oxide (GO) thin films on glass and plastic substrates were found to display interesting broadband nonlinear optical properties. We have investigated their optical limiting activity for femtosecond laser pulses at 800 and 400 nm, which could be tuned by controlling the extent of reduction. The as-prepared GO films were found to exhibit excellent broadband optical limiting behaviors, which were significantly enhanced upon partial reduction by using laser irradiation or chemical reduction methods. The laser-induced reduction of GO resulted in enhancement of effective two-photon absorption coefficient at 400 nm by up to ∼19 times and enhancement of effective two- and three-photon absorption coefficients at 800 nm by ∼12 and ∼14.5 times, respectively. The optical limiting thresholds of partially reduced GO films are much lower than those of various previously reported materials. Highly reduced GO films prepared by using the chemical method displayed strong saturable absorption behavior.
TL;DR: The femtosecond erbium-doped all-fiber lasers mode-locked with graphene oxide demonstrated are comparable with those of graphene saturable absorbers and the superiority of easy fabrication and hydrophilic property of graphene oxide will facilitate its potential applications for ultrafast photonics.
Abstract: We demonstrated the femtosecond erbium-doped all-fiber lasers mode-locked with graphene oxide, which can be conveniently obtained from natural graphite by simple oxidation and ultra-sonication process. With proper dispersion management in an all-fiber ring cavity, the laser directly generated 200 fs pulses at a repetition rate of 22.9 MHz and the average output power was 5.8 mW. With the variation of net cavity dispersion, output pulses with pulse width of 0.2~3 ps were obtained at a repetition rate of 22.9~0.93 MHz. These results are comparable with those of graphene saturable absorbers and the superiority of easy fabrication and hydrophilic property of graphene oxide will facilitate its potential applications for ultrafast photonics.
TL;DR: In this article, the authors exploit frequency-resolved optical gating to directly investigate plasmon response times of different antenna geometries resonant in the near-infrared.
Abstract: Plasmonic nanoantennas are efficient devices to concentrate light in spatial regions much smaller than the wavelength. Only recently, their ability to manipulate photons also on a femtosecond time scale has been harnessed. Nevertheless, designing the dynamical properties of optical antennas has been difficult since the relevant microscopic processes governing their ultrafast response have remained unclear. Here, we exploit frequency-resolved optical gating to directly investigate plasmon response times of different antenna geometries resonant in the near-infrared. Third-harmonic imaging is used in parallel to spatially monitor the plasmonic mode patterns. We find that the few-femtosecond dynamics of these nanodevices is dominated by radiative damping. A high efficiency for nonlinear frequency conversion is directly linked to long plasmon damping times. This single parameter explains the counterintuitive result that rod-type nanoantennas with minimum volume generate by far the strongest third-harmonic emission as compared to the more bulky geometries of bow-tie-, elliptical-, or disk-shaped specimens.
TL;DR: A new method for increasing the resolution of direct femtosecond laser writing by multiphoton polymerization, based on quencher diffusion is presented, and it is shown that radical inhibition is responsible for the increased resolution.
Abstract: We present a new method for increasing the resolution of direct femtosecond laser writing by multiphoton polymerization, based on quencher diffusion. This method relies on the combination of a mobile quenching molecule with a slow laser scanning speed, allowing the diffusion of the quencher in the scanned area and the depletion of the multiphoton-generated radicals. The material we use is an organic–inorganic hybrid, while the quencher is a photopolymerizable amine-based monomer which is bound on the polymer backbone upon fabrication of the structures. We use this method to fabricate woodpile structures with a 400 nm intralayer period. This is comparable to the results produced by direct laser writing based on stimulated-emission-depletion microscopy, the method considered today as state-of-the-art in 3D structure fabrication. We optically characterize these woodpiles to show that they exhibit well-ordered diffraction patterns and stopgaps down to near-infrared wavelengths. Finally, we model the quencher ...
TL;DR: Volume and live multicolor imaging of 'Brainbow'-labeled tissues as well as simultaneous three-color fluorescence and third-harmonic imaging of fly embryos are reported.
Abstract: We achieve simultaneous two-photon excitation of three chromophores with distinct absorption spectra using synchronized pulses from a femtosecond laser and an optical parametric oscillator. The two beams generate separate multiphoton processes, and their spatiotemporal overlap provides an additional two-photon excitation route, with submicrometer overlay of the color channels. We report volume and live multicolor imaging of 'Brainbow'-labeled tissues as well as simultaneous three-color fluorescence and third-harmonic imaging of fly embryos.
TL;DR: This work presents experimental and theoretical work that demonstrates that the nanobubble formation results from the generation of a nanoscale plasma around the particle due to the enhanced near-field rather than from the heating of the particle.
Abstract: The generation of nanobubbles around plasmonic nanostructures is an efficient approach for imaging and therapy, especially in the field of cancer research. We show a novel method using infrared femtosecond laser that generates ≈800 nm bubbles around off-resonance gold nanospheres using 200 mJ/cm2 45 fs pulses. We present experimental and theoretical work that demonstrate that the nanobubble formation results from the generation of a nanoscale plasma around the particle due to the enhanced near-field rather than from the heating of the particle. Energy absorbed in the nanoplasma is indeed more than 11 times the energy absorbed in the particle. When compared to the usual approach that uses nanosecond laser to induce the extreme heating of in-resonance nanoparticles to initiate bubble formation, our off-resonance femtosecond technique is shown to bring many advantages, including avoiding the particles fragmentation, working in the optical window of biological material and using the deposited energy more effi...
TL;DR: An electrospun liquid microjet has been developed that delivers protein microcrystal suspensions at flow rates of 0.14-3.1 µl min(-1) to perform serial femtosecond crystallography (SFX) studies with X-ray lasers, extending SFX to biological samples that necessitate minimal sample consumption.
Abstract: An electrospun liquid microjet has been developed that delivers protein microcrystal suspensions at flow rates of 0.14-3.1 mu l min(-1) to perform serial femtosecond crystallography (SFX) studies w ...
TL;DR: The first single-shot images of ferromagnetic, nanoscale spin order taken with femtosecond x-ray pulses are presented, opening new ways to combine ultrafast laser spectroscopy with sequential snapshot imaging on a single sample, generating a movie of excited state dynamics.
Abstract: We present the first single-shot images of ferromagnetic, nanoscale spin order taken with femtosecond x-ray pulses. X-ray-induced electron and spin dynamics can be outrun with pulses shorter than 80 fs in the investigated fluence regime, and no permanent aftereffects in the samples are observed below a fluence of 25 mJ/cm{sup 2}. Employing resonant spatially-muliplexed x-ray holography results in a low imaging threshold of 5 mJ/cm{sup 2}. Our results open new ways to combine ultrafast laser spectroscopy with sequential snapshot imaging on a single sample, generating a movie of excited state dynamics.
TL;DR: This paper presents a comprehensive review of optofluidic devices for biological analysis fabricated by femtosecond laser processing, which has been used to determine the functions of living microorganisms, determine the concentrations of liquid samples, detect and manipulate single cells, and rapidly screen algae populations.
Abstract: Femtosecond laser direct writing is a promising technique for fabricating optofluidic devices since it can modify the interior of glass in a spatially selective manner through multiphoton absorption. The chemical properties of laser-irradiated regions in glass are modified allowing them to be selectively etched by subsequent wet etching using aqueous solutions of etchants such as hydrofluoric (HF) acid. This technique can be used to directly form three-dimensional microfluidic systems. The two-step process can also be used to fabricate free-space optical components such as micromirrors and microlenses inside glass. In addition, femtosecond laser direct writing can alter the optical properties of a substrate to create a wide range of micro-optical components inside glass, including optical waveguides, Mach-Zehnder interferometers, and optical attenuators. The unique ability of femtosecond laser direct writing to simultaneously alter the chemical and optical properties of glass opens up a new avenue for fabricating a variety of optofluidic microchips for biological analysis. Optofluidic microchips fabricated using femtosecond lasers have been used to determine the functions of living microorganisms, determine the concentrations of liquid samples, detect and manipulate single cells, and rapidly screen algae populations. This paper presents a comprehensive review of optofluidic devices for biological analysis fabricated by femtosecond laser processing.
TL;DR: It is found that the thermalization of the surface states is initially determined by interband scattering from the bulk conduction band, lasting for about 0.5 ps; this establishes a correlation between the nanoscale thickness of the bulk charge reservoir and the evolution of the ultrafast carrier dynamics in the surface Dirac cone.
Abstract: We discuss the ultrafast evolution of the surface electronic structure of the topological insulator Bi$_2$Te$_3$ following a femtosecond laser excitation. Using time and angle resolved photoelectron spectroscopy, we provide a direct real-time visualisation of the transient carrier population of both the surface states and the bulk conduction band. We find that the thermalization of the surface states is initially determined by interband scattering from the bulk conduction band, lasting for about 0.5 ps; subsequently, few ps are necessary for the Dirac cone non-equilibrium electrons to recover a Fermi-Dirac distribution, while their relaxation extends over more than 10 ps. The surface sensitivity of our measurements makes it possible to estimate the range of the bulk-surface interband scattering channel, indicating that the process is effective over a distance of 5 nm or less. This establishes a correlation between the nanoscale thickness of the bulk charge reservoir and the evolution of the ultrafast carrier dynamics in the surface Dirac cone.
TL;DR: It is shown that three-dimensional structural information can be retrieved from multiple electron diffraction patterns of aligned molecules from multiple femtosecond laser pulses.
Abstract: Imaging isolated molecules in three dimensions with atomic resolution is important for elucidating complex molecular structures and intermediate states in molecular dynamics. This goal has so far remained elusive due to the random orientation of molecules in the gas phase. We show that three-dimensional structural information can be retrieved from multiple electron diffraction patterns of aligned molecules. The molecules are aligned impulsively with a femtosecond laser pulse and probed with a femtosecond electron pulse two picoseconds later, when the degree of alignment reaches a maximum.
TL;DR: A robust laser design employing all normal dispersion polarisation maintaining fibres operating at 1 μm, mode-locked using a nonlinear amplifying fibre loop mirror that provides an additional degree of freedom allowing easy control over the pulse parameters.
Abstract: We report on a new design for a passively mode locked fibre laser employing all normal dispersion polarisation maintaining fibres operating at 1 μm The laser produces linearly polarized, linearly chirped pulses that can be recompressed down to 344 fs Compared to previous laser designs the cavity is mode-locked using a nonlinear amplifying fibre loop mirror that provides an additional degree of freedom allowing easy control over the pulse parameters This is a robust laser design with excellent reliability and lifetime
TL;DR: It is experimentally demonstrated a passively mode-locked femtosecond laser by using a graphene-based saturable absorber mirror (graphene SAM) in the spectral region of 2 μm.
Abstract: We experimentally demonstrated a passively mode-locked femtosecond laser by using a graphene-based saturable absorber mirror (graphene SAM) in the spectral region of 2 μm. The graphene SAM was fabricated by transferring chemical-vapor-deposited, high-quality, and large-area graphene on a highly reflective plane mirror. Stable mode-locked laser pulses as short as 729 fs were obtained with a repetition rate of 98.7 MHz and an average output power of 60.2 mW at 2018 nm.
TL;DR: In this paper, an experimental proof and full characterization of laser generation in molecular nitrogen in an argon-nitrogen gas mixture remotely excited at a distance above 2 m in a femtosecond laser filament was reported.
Abstract: We report an experimental proof and full characterization of laser generation in molecular nitrogen in an argon-nitrogen gas mixture remotely excited at a distance above 2 m in a femtosecond laser filament. Filamentation experiments performed with near-infrared, 1-\ensuremath{\mu}m-wavelength and midinfrared, 4-\ensuremath{\mu}m-wavelength short-pulse laser sources show that mid-IR laser pulses enable radical enhancement of filamentation-assisted lasing by N${}_{2}$ molecules. Energies as high as 3.5 \ensuremath{\mu}J are achieved for the 337- and 357-nm laser pulses generated through the second-positive-band transitions of N${}_{2}$, corresponding to a 0.5$%$ total conversion efficiency from midinfrared laser energy to the energy of UV lasing.
TL;DR: Laser-induced ultrafast demagnetization in [Co/Pd](30) multilayer films is studied, which, for the first time, achieves a spatial resolution better than 100 nm by using femtosecond soft X-ray pulses.
Abstract: Femtosecond magnetization phenomena have been challenging our understanding for over a decade Most experiments have relied on infrared femtosecond lasers, limiting the spatial resolution to a few micrometres With the advent of femtosecond X-ray sources, nanometric resolution can now be reached, which matches key length scales in femtomagnetism such as the travelling length of excited 'hot' electrons on a femtosecond timescale Here we study laser-induced ultrafast demagnetization in [Co/Pd]30 multilayer films, which, for the first time, achieves a spatial resolution better than 100 nm by using femtosecond soft X-ray pulses This allows us to follow the femtosecond demagnetization process in a magnetic system consisting of alternating nanometric domains of opposite magnetization No modification of the magnetic structure is observed, but, in comparison with uniformly magnetized systems of similar composition, we find a significantly faster demagnetization time We argue that this may be caused by direct transfer of spin angular momentum between neighbouring domains
TL;DR: In this paper, two-photon polymerization and femtosecond laser multiphoton ablation are combined to create 3D structures in a photocurable polymer and cut voids in the polymer.
Abstract: An international team of researchers has combined two well-known fabrication techniques to produce intricate three-dimensional nanostructures. Yong Fen Lu and co-workers say that their novel approach could be used to make nanosized devices that are difficult to produce through either technique alone. Both techniques employ a femtosecond laser. The first — two-photon polymerization—is used to ‘draw’ 3D structures in a photocurable polymer. The second—femtosecond laser multiphoton ablation—is used to cut voids, such as channels, in the polymer. Combining these two techniques enables the efficient fabrication of complex three-dimensional nanostructures, such as integrated optical circuits and lab-on-a-chip devices.
TL;DR: Off-axis phase-matched terahertz generation in long air-plasma filaments produced by femtosecond two-color laser focusing is observed, and the phase-matching condition observed here provides a simple method for scalable terAhertzgeneration in elongated plasmas.
Abstract: We observe off-axis phase-matched terahertz generation in long air-plasma filaments produced by femtosecond two-color laser focusing. Here, phase matching naturally occurs due to off-axis constructive interference between locally generated terahertz waves, and this determines the far-field terahertz radiation profiles and yields. For a filament longer than the characteristic two-color dephasing length, it emits conical terahertz radiation in the off-axis direction, peaked at 4-7° depending on the radiation frequencies. The total terahertz yield continuously increases with the filament length, well beyond the dephasing length. The phase-matching condition observed here provides a simple method for scalable terahertz generation in elongated plasmas.