Discover the most talked about and latest scientific content & concepts.

Concept: Ultrashort pulse


We demonstrate experimentally ultrafast all-optical switching in subwavelength nonlinear dielectric nanostructures exhibiting localized magnetic Mie resonances. We employ amorphous silicon nanodisks to achieve strong self-modulation of femtosecond pulses with a depth of 60\% at picojoule-per-disk pump energies. In the pump-probe measurements, we reveal that switching in the nanodisks can be governed by pulse-limited 65-fs-long two-photon absorption being enhanced by a factor of 80 with respect to the unstructured silicon film. We also show that undesirable free-carrier effects can be suppressed by a proper spectral positioning of the magnetic resonance, making such a structure the fastest all-optical switch operating at the nanoscale.

Concepts: Photon, Fundamental physics concepts, Nuclear magnetic resonance, Magnetic moment, Capacitor, Resonance, Amorphous silicon, Ultrashort pulse


An open challenge in the important field of femtosecond laser material processing is the controlled internal structuring of dielectric materials. Although the availability of high energy high repetition rate femtosecond lasers has led to many advances in this field, writing structures within transparent dielectrics at intensities exceeding 10(13) W/cm(2) has remained difficult as it is associated with significant nonlinear spatial distortion. This letter reports the existence of a new propagation regime for femtosecond pulses at high power that overcomes this challenge, associated with the generation of a hollow uniform and intense light tube that remains propagation invariant even at intensities associated with dense plasma formation. This regime is seeded from higher order nondiffracting Bessel beams, which carry an optical vortex charge. Numerical simulations are quantitatively confirmed by experiments where a novel experimental approach allows direct imaging of the 3D fluence distribution within transparent solids. We also analyze the transitions to other propagation regimes in near and far fields. We demonstrate how the generation of plasma in this tubular geometry can lead to applications in ultrafast laser material processing in terms of single shot index writing, and discuss how it opens important perspectives for material compression and filamentation guiding in atmosphere.

Concepts: Optics, Fundamental physics concepts, Optical fiber, Solid, Plasma, Dielectric, Lasers, Ultrashort pulse


Experimental methods for ultrafast microscopy are advancing rapidly. Promising methods combine ultrafast laser excitation with electron based imaging, or rely on super-resolution optical techniques to enable probing of matter on the nano-femto scale. Among several actively developed methods ultrafast time-resolved photoemission electron microscopy provides several advantages, among which the foremost are that time resolution is limited only by the laser source, and it is immediately capable for probing of coherent phenomena in solid state materials and surfaces. Here we present recent progress in interference imaging of plasmonic phenomena in metal nanostructures enabled by combining a broadly tunable femtosecond laser excitation source with a low-energy electron microscope.

Concepts: Electron, Electron microscope, Optics, Light, Coherence, Microscope, Scanning tunneling microscope, Ultrashort pulse


Ultrafast thermomechanical responses and spallation behaviours of monocrystal copper films irradiated by femtosecond laser pulse are investigated using molecular dynamics simulation (MDS). Films with 〈100〉, 〈110〉 and 〈111〉 crystal orientations along the thickness direction were studied. The results show that the crystal orientation has a significant effect on femtosecond laser-induced thermomechanical responses and spallation behaviors of monocrystal copper films. The discrepancy between normal stresses in copper films with different crystal orientation leads to distinct differences in lattice temperature. Moreover, the copper films with different crystal orientations present distinct spallation behaviors, including structural melting (atomic splashing) and fracture. The melting depth of 〈100〉 copper film is lower than that of 〈110〉 and 〈111〉 copper films for the same laser intensity. The dislocations and slip bands are formed and propagate from the solid-liquid interface of 〈110〉 and 〈111〉 copper films, while these phenomena do not appear in 〈100〉 copper film. Additionally, numerous slip bands are generated in the non-irradiated surface region of copper films due to reflection of mechanical stress. These slip bands can finally evolve into cracks (nanovoids) with time, which further result in the fracture of the entire films.

Concepts: Molecular dynamics, Solid, Materials science, Behavior, Human behavior, Normality, Film, Ultrashort pulse


We present measurements of the pulse length of ultracold electron bunches generated by near-threshold two-photon photoionization of a laser-cooled gas. The pulse length has been measured using a resonant 3 GHz deflecting cavity in TM110 mode. We have measured the pulse length in three ionization regimes. The first is direct two-photon photoionization using only a 480 nm femtosecond laser pulse, which results in short (∼15 ps) but hot (∼10(4 )K) electron bunches. The second regime is just-above-threshold femtosecond photoionization employing the combination of a continuous-wave 780 nm excitation laser and a tunable 480 nm femtosecond ionization laser which results in both ultracold (∼10 K) and ultrafast (∼25 ps) electron bunches. These pulses typically contain ∼10(3) electrons and have a root-mean-square normalized transverse beam emittance of 1.5 ± 0.1 nm rad. The measured pulse lengths are limited by the energy spread associated with the longitudinal size of the ionization volume, as expected. The third regime is just-below-threshold ionization which produces Rydberg states which slowly ionize on microsecond time scales.

Concepts: Electron, Light, Volume, Laser, Units of measurement, Length, Atomic physics, Ultrashort pulse


A simultaneously high-precision, wide-range, and ultrafast time-resolution one-shot 3D shape measurement method is presented. Simultaneous times of flight from multiple positions to a target encoded in a chirped optical frequency comb can be obtained from spectral interferometry. We experimentally demonstrate a one-shot imaging profile measurement of a known step height of 480 µm with µm-level accuracy. We further demonstrate the extension of the dynamic range by measuring in one shot a large step height of 3 m while maintaining high accuracy using the accurate pulse-to-pulse separation of the optical frequency comb. The proposed method with its large dynamic range and measurement versatility can be applied to a broad range of applications, including microscopic structures, objects with large size or aspect ratio, and ultrafast time-resolved imaging. This study provides a powerful and versatile tool for 3D measurement, where various ranges of measurement performances can be tailored to demand.

Concepts: Optics, Measurement, Units of measurement, Nonlinear optics, Dimensional analysis, Lasers, Ultrashort pulse, Frequency comb


A novel pulse characterization method is presented, favorably combining interferometric frequency-resolved optical gating (FROG) and time-domain ptychography. This new variant is named ptychographic-interferometric frequency-resolved optical gating (πFROG). The measurement device is simple, bearing similarity to standard second-harmonic FROG, yet with a collinear beam geometry and an added bandpass filter in one of the correlator arms. The collinear beam geometry allows tight focusing and circumvents possible geometrical distortion effects of noncollinear methods, making πFROG especially suitable for the characterization of unamplified few-cycle pulses. Moreover, the direction-of-time ambiguity afflicting most second-order FROG variants is eliminated. Possible group delay dispersion of pulses leads to a characteristic tilt in the πFROG traces, allowing the detection of uncompensated dispersion without a retrieval. Using nanojoule, three-cycle pulses at 800 nm, the πFROG method is tested, and the results are compared with spectral phase interferometry for direct electric field reconstruction measurements. Measured pulse durations agree within a fraction of a femtosecond. As a further test, the πFROG measurements are repeated with added group delay dispersion, and found to accurately reproduce the dispersion computed with Sellmeier equations.

Concepts: Optics, Measurement, Test method, Nonlinear optics, Ruler, Ultrashort pulse, Frequency-resolved optical gating, Spectral phase interferometry for direct electric-field reconstruction


Hydrogen production from water based on photoelectrochemical (PEC) reactions is feasible to solve the urgent energy crisis. Herein, hierarchical 3D self-supporting WO3 micro-nano architectures in situ grown on W plates are successfully fabricated via ultrafast laser processing hybrid with thermal oxidation. Owing to the large surface area and efficient interface charge transfer, the W plate with hierarchical porous WO3 nanoparticle aggregates has been directly employed as the photoanode for excellent PEC performance, which exhibits a high photocurrent density of 1.2 mA cm(-2) at 1.0 V vs. Ag/AgCl (1.23 V vs. RHE) under AM 1.5 G illumination and reveals excellent structural stability during long-term PEC water splitting reactions. The nanoscale and microscale features can be facilely tuned by controlling the laser processing parameters and the thermal oxidation conditions to achieve improved PEC activity. The presented hybrid method is simple, cost-effective and controllable for large-scale fabrication, which should provide a new and general route that how the properties of conventional metal oxides can be improved via hierarchical 3D micro-nano configurations.

Concepts: Water, Hydrogen, Oxide, Electrolysis, In situ, Surface area, Hydrogen production, Ultrashort pulse


We compare a femtosecond laser induced modification in silica matrices with three different degrees of porosity. In single pulse regime, the decrease of substrate density from fused silica to high-silica porous glass and to silica aerogel glass results in tenfold increase of laser affected region with the formation of a symmetric cavity surrounded by the compressed silica shell with pearl like structures. In multi-pulse regime, if the cavity produced by the first pulse is relatively large, the subsequent pulses do not cause further modifications. If not, the transition from void to the anisotropic structure with the optical axis oriented parallel to the incident polarization is observed. The maximum retardance value achieved in porous glass is twofold higher than in fused silica, and tenfold greater than in aerogel. The polarization sensitive structuring in porous glass by two pulses of ultrafast laser irradiation is demonstrated, as well as no observable stress is generated at any conditions.

Concepts: Optics, Light, Optical fiber, Sol-gel, Glass, Nonlinear optics, Fused quartz, Ultrashort pulse


We present a method by which the spectral intensity of an ultrafast laser pulse can be accumulated at selected frequencies by a controllable amount. Using a 4-f pulse shaper we modulate the phase of the frequency components of a femtosecond laser. By inducing femtosecond filamentation with the modulated pulse, we can concentrate the spectral amplitude of the pulse at various frequencies. The phase mask applied by the pulse shaper determines the frequencies for which accumulation occurs, as well as the intensity of the spectral concentration. This technique provides a way to obtain pulses with adjustable amplitude using only phase modulation and the nonlinear response of a medium. This provides a means whereby information which is encoded into spectral phase jumps may be decoded into measurable spectral intensity spikes.

Concepts: Optics, Fundamental physics concepts, Phase, Wavelength, Modulation, Sound, Nonlinear optics, Ultrashort pulse