Femtosecond Regime Research Articles

Supercontinuum Generation Assisted by Wave Trapping in Dispersion-Managed Integrated Silicon Waveguides

Compact chip-scale comb sources are of significant interest for many practical applications. Here, we experimentally study the generation of supercontinuum (SC) in an axially varying integrated waveguide. We show that the local tuning of the dispersion enables the continuous blue shift of dispersive waves thanks to their trapping by the strongly compressed pump pulse. This mechanism provides new insight into supercontinuum generation in a dispersion varying integrated waveguide. Pumped close to 2.2 $\mu$m in the femtosecond regime and at a pulse energy of $\sim$ 4 pJ, the output spectrum extends from 1.1 $\mu$m up to 2.76 $\mu$m and shows good coherence properties. Octave-spanning SC is also observed at input energy as low as $\sim$ 0.9 pJ. We show that the supercontinuum is more robust against variations of the input pulse parameters and is also spectrally flatter in our numerically optimized waveguide than in fixed-width waveguides. This research demonstrates the potential of dispersion varying waveguides for coherent SC generation and paves the way for integrated low power applications, such as chip-scale frequency comb generation, precision spectroscopy, optical frequency metrology, and wide-band wavelength division multiplexing in the near-infrared.

Graphene Capacitor-Based Electrical Switching of Mode-Locking in All-Fiberized Femtosecond Lasers.

Effective high-capacity data management necessitates the use of ultrafast fiber lasers with mode-locking-based femtosecond pulse generation. We suggest a simple but highly efficient structure of a graphene saturable absorber in the form of a graphene/poly(methyl methacrylate) (PMMA)/graphene capacitor and demonstrate the generation of ultrashort pulses by passive mode-locking in a fiber ring laser cavity, with simultaneous electrical switching (on/off) of the mode-locking operation. The voltage applied to the capacitor shifts the Fermi level of the graphene layers, thereby controlling their nonlinear light absorption, which is directly correlated with mode-locking. The flexible PMMA layer used for graphene transfer also acts as a dielectric layer to realize a very simple but effective capacitor structure. By employing the graphene capacitor on the polished surface of a D-shaped fiber, we demonstrate the switching of the mode-locking operation reversibly from the femtosecond pulse regime to a continuous wave regime of the ring laser with an extinction ratio of 70.4 dB.

Experimental dispersion of the third-order optical susceptibility of graphene oxide

We experimentally determined the dispersion of third-order optical susceptibility χ(3) of graphene oxide (GO) in the visible region (450 - 750 nm) by combining spectroscopic ellipsometry and ultrafast pump and probe spectroscopy in the femtosecond regime. In order to mitigate the damage of wide-spectrum laser to photonic devices, GO has become a promising material for optical limiting (OL) devices. However, there is no report about the χ(3) dispersion of GO, which is a complex quantity that directly corresponds to nonlinear refraction and absorption and is a crucial parameter for the manipulation and application of its OL properties. Here, we identified that the linear optical response of GO shows a flat dispersion in the visible region. In contrast, its nonlinear optical response exhibits saturable absorption (SA) at the short wavelength and reverse saturable absorption (RSA) at the long wavelength. These results propel the application of GO in the broadband OL devices based on the RSA behavior. In addition, by controlling the fraction of sp2 and sp3 hybridizations, it also provides opportunities to tailor the NLO properties and OL performance of GO.

Experimental and Theoretical Determination of the Effective Penetration Depth of Ultrafast Laser Radiation in Stainless Steel

This study intends to present a simple two-temperature model (TTM) for the fast calculation of the ablation depth as well as the corresponding effective penetration depth for stainless steel by considering temperature-dependent material parameters. The model is validated by a comparison of the calculated to the experimentally determined ablation depth and the corresponding effective penetration depth in dependence on the pulse duration (200 fs up to 10 ps) and the fluence. The TTM enables to consider the interaction of pulsed laser radiation with the electron system and the subsequent interaction of the electrons with the phonon system. The theoretical results fit very well to the experimental results and enable the understanding of the dependence of the ablation depth and of the effective penetration depth on the pulse duration. Laser radiation with a pulse duration in the femtosecond regime results in larger ablation depths compared to longer-pulsed laser radiation in the picosecond regime. Analogously to the ablation depth, larger effective penetration depths are observed due to considerably higher electron temperatures for laser radiation with pulse durations in the femtosecond regime.

Self-frequency-doubling Yb:CNGS lasers operating in the femtosecond regime

We study Y b : C a 3 N b G a 3 S i 2 O 14 (Yb:CNGS) lasers in the mode-locked regime, focusing on their self-frequency-doubling potential. We characterize the laser performance in configurations without and with phase-matching for second-harmonic generation. In the former case, the laser generates 47 fs pulses at a central wavelength of 1052 nm with an average output power of 45 mW at 78.4 MHz. These pulses are further compressed extracavity down to 40 fs, which indicates that the employed crystal is one of the most promising novel gain media regarding ultrafast laser operation. Furthermore, we demonstrate that in a configuration close to phase-matching, the laser emits 40 mW of fundamental radiation at 1046 nm with 100 fs pulse duration simultaneously producing 66 mW of second-harmonic centered at 525 nm. In order to better describe the self-frequency-doubling phenomenon, we compare our experimental observations obtained in both configurations with results of numerical modeling.

Single-step printing of metallic nanoparticles in 2D micropatterns

This work demonstrates a single-step method for synthesis and printing of metallic nanoparticles (NPs) in 2D micropatterns. The method is based on femtosecond laser writing, enabling fast and high precision deposition of silver, gold, or copper NPs in space-selective areas, which in general is not achieved by the traditional chemical routes or laser ablation of metal. Such finds were accomplished by employing laser-induced forward transfer in the femtosecond pulse regime (fs-LIFT). The results are promising for application, since metallic NPs, with a lognormal diameter distribution averaging between ~ 4 and 30 nm, allow exploring plasmon bands throughout the visible spectrum. The mechanisms behind NP formation by fs-LIFT are discussed based on the thermomechanical response of the material. An estimative based on the heat transfer suggests the occurrence of mechanical fragmentation rather than material vaporization. The main result addressed herein, however, is the ability to deposit silver, gold, and copper nanoparticles in selected regions, supporting the development of NP-based devices via one-step processing and their application in sensors and photonic circuits.

Pulse-duration dependence of laser-induced modifications inside silicon.

The advent of ultrafast infrared lasers provides a unique opportunity for direct fabrication of three-dimensional silicon microdevices. However, strong nonlinearities prevent access to modification regimes in narrow gap materials with the shortest laser pulses. In contrary to surface experiments for which one can always define an energy threshold to initiate modifications, we establish that some other threshold conditions inevitably apply on the pulse duration and the numerical aperture for focusing. In an experiment where we can vary continuously the pulse duration from 4 to 21 ps, we show that a minimum duration of 5.4 ps and a focusing numerical aperture of 0.85 are required to successfully initiate modifications. Below and above thresholds, we investigate the pulse duration dependence of the conditions applied in matter. Despite a modest pulse duration dependence of the energy threshold in the tested range, we found that all pulse durations are not equally performing to achieve highly reproducible modifications. Taken together with previous reports in the femtosecond and nanosecond regimes, this provides important guidelines on the appropriate conditions for internal structuring of silicon.

Two-plasmon-decay induced fast electrons in intense femtosecond laser–solid interactions

The nonlinear coupling of intense laser pulse with plasma leads to excitation of several parametric instabilities, featuring plasmon, acoustic or electromagnetic modes. Specifically, the two-plasmon decay (TPD) instability, relevant to the generation of fast electrons, originates at the quarter critical surface, where the incident photon decays into a pair of electron plasmon waves. Although well explored by nanosecond lasers, the TPD instability is rarely seen in femtosecond laser–plasmas, mainly due to steep plasma profiles and the ultrashort duration of the driving pulse. Our experiments show TPD boosting of fast electrons in the femtosecond regime using low intensity contrast and temporally stretched laser pulses. The fast electron spectrum and 3ω0/2 harmonic emission from plasma show significant enhancement for stretched pulses (120 fs) in comparison to transform-limited short pulses (30 fs). The generation of fast electrons is found to be linked with the growth of 3ω0/2 harmonic. The effect of longer femtosecond pulses on TPD growth is observed using several fast electron diagnostics such as electron kinetic energy spectra, hard x-ray emission, rear side plasma emission, Cherenkov emission, and time-space resolved shadowgrams up to laser intensity of 1.5 × 1018 W cm−2. We, thus, provide robust and unambiguous demonstration of TPD instability driven generation of fast electrons in femtosecond laser–plasma interactions.

Controlling energy transfer from intense ultrashort light pulse to crystals: A comparison study in attosecond and femtosecond regimes

The energy exchange between photons and electrons has been investigated theoretically by ab initio approach based on time-dependent density functional theory. Using diamond as a concrete example, three types of resonance and cancellation in the transfer of energy are theoretically observed, that allows one to gain a useful independent insight into the interaction processes of attosecond light pulses with matter. Our results demonstrate the linearity in energy transfer from intense attosecond light pulses to solids, in contrast to the nonlinearity in energy transfer from intense femtosecond light pulses to solids as expected from the conventional point of view, opening new perspectives for attoscience.

Fundamental investigations of ultrashort pulsed laser ablation on stainless steel and cemented tungsten carbide

An ultrashort pulse laser, capable of varying the pulse duration from 0.2 ps up to 10 ps, is used to study the ablation characteristics of stainless steel and cemented tungsten carbide. In addition to the influence of pulse duration, the number of pulses and the wavelength are examined for their influence on the ablation process. By determining the ablation diameter of generated cavities, the ablation threshold of the materials is calculated as a function of the number of pulses, the pulse duration, and the wavelength. The experimentally determined ablation thresholds tend to agree with calculated values. Due to the incubation effect, the ablation threshold decreases with increasing the number of pulses. In this context, the incubation factor (0.81@1030 nm and 0.80@515 nm for stainless steel, 0.90@1030 nm and 0.77@515 nm for cemented tungsten carbide) for the investigated materials is determined. On the basis of the measured ablated volume, the effective penetration depth (a reduction in a range from about 12 nm and 15 nm to 6 nm for stainless steel and in a range from 22 nm and 32 nm to 11 nm and 13 nm for cemented tungsten carbide by increasing the pulse duration from 0.2 ps to 10 ps) of the energy is calculated and it is proven that in the femtosecond regime the penetration depth increases compared with the picosecond regime. In consequence, the efficiency of the ablation process is increased by using shorter laser pulses.

Femtosecond Laser-Induced Damage Characterization of Multilayer Dielectric Coatings

The laser-induced damage threshold (LIDT) of optical components is one of the major constraints in developing high-power ultrafast laser systems. Multi-layer dielectric (MLD) coatings-based optical components are key parts of high-power laser systems because of their high damage resistance. Therefore, understanding and characterizing the laser-induced damage of MLD coatings are of paramount importance for developing ultrahigh-intensity laser systems. In this article, we overview the possible femtosecond laser damage mechanisms through damage morphologies in various MLD optical coatings tested in our facility. To evaluate the major contributions to the coating failure, different LIDT test methods (R-on-1, ISO S-on-1 and Raster Scan) were carried out for a high reflective hybrid Ta2O5/HfO2/SiO2 MLD mirror coating at a pulse duration of 37 fs. Different LIDT test methods were compared due to the fact that each test method exposes the different underlying damage mechanisms. For instance, the ISO S-on-1 test at a higher number of laser pulses can bring out the fatigue effects, whereas the Raster Scan method can reveal the non-uniform defect clusters in the optical coating. The measured LIDT values on the sample surface for the tested coating in three test methods are 1.1 J/cm2 (R-on-1), 0.9 J/cm2 (100k-on-1) and 0.6 J/cm2 (Raster Scan) at an angle of incidence of 45 deg. The presented results reveal that the performance of the tested sample is limited by coating defects rather than fatigue effects. Hence, the Raster Scan method is found to be most accurate for the tested coating in evaluating the damage threshold for practical applications. Importantly, this study demonstrates that the testing of different LIDT test protocols is necessary in femtosecond regime to assess the key mechanisms to the coating failure.

Pulse duration dependence of ablation threshold for fused silica in the visible femtosecond regime

In this study, we investigated the variation of the femtosecond pulse duration of the threshold fluence of fused silica by a single-shot irradiation at a wavelength of 400 nm. The single-shot threshold fluence was obtained from the relationship between the crater area based on the crater shape and the irradiation fluence. The crater shape was divided into two affected regimes depending on the pulse duration and the laser fluence. The crater depth and each threshold fluence for the corresponding two affected regimes were also measured. We have found that shorter pulse duration provides more energy-efficient results.

Standing growth mechanism and ultrafast nonlinear absorption properties of WS2 films

Tungsten disulfide (WS2) films were successfully prepared by magnetron sputtering. The structure, morphology and standing growth mechanism of WS2 films under different sputtering conditions (sputtering time and substrate temperature) were studied. The scanning electron microscopy (SEM) showed that the sputtering time reached 1.5 h or the substrate temperature reached 250 °C, the morphology of WS2 reveal a “sickle” texture on the surface. The standing growth mechanism was demonstrated by SEM results. X-ray diffraction (XRD) patterns showed that only (101) diffraction peak existed in WS2 films. The nonlinear absorption in WS2 films were characterized by Z-scan technique in the femtosecond regimes at 800 nm. The results showed that the WS2 films have two-photon absorption process, and the transition from saturable absorption to reverse saturable absorption was observed. The nonlinear absorption coefficients of the WS2 films at different sputtering time were 6.386 × 10−10 (for 2 h), 9.704 × 10−10 (for 1.5 h) and −9.759 × 10−10 m/W (for 1 h), respectively. The nonlinear absorption coefficients of the WS2 films at different substrate temperature were −6.275 × 10−10 (for 250 °C), 5.430 × 10−9 (for 300 °C) and 3.899 × 10−9 m/W (for 350 °C), respectively. The standing WS2 films have excellent nonlinear optical performance and simple synthesis route, which provide a good prospect for the application of two-dimensional materials in nonlinear optics.

(Invited) Lanthanide Doped Nanoparticles and Their Applications

Since approximately 2000, lanthanide doped nanoparticles have received significant attention due to their interesting luminescent properties. At the core of this interest is their ability to convert near-infrared excitation light (typically 980 or 800 nm) to higher energies spanning the ultraviolet, visible and near-infrared regions of the spectrum through a multiphoton process known as upconversion. Upconversion differs from conventional multiphoton excitation in other materials where no real intermediate states are present necessitating the use of ultrafast lasers (in the femtosecond regime) for simultaneous excitation to the upper emitting state. The lanthanide ions possess a multitude of 4f electronic states that have long lifetimes (micro- to millisecond) thus act as population reservoirs in the upconversion process. Hence, upconversion occurs through real, long-lived intermediate states through a sequential photon absorption process. This eliminates the need for ultrafast excitation and as a result, upconversion can be observed using inexpensive, continuous wave diode lasers. Upconversion luminescence can be exploited for a number of applications in nanomedicine, theranostics, photovoltaics, photocatalysis, as well as many others.While their upconversion luminescence has been studied in great detail, lanthanide doped nanoparticles can also emit in the near-infrared through a direct Stokes (down-shifted) luminescence process. Of particular importance is that these near-infrared emissions lie within the biological windows where biological tissues are optically transparent. For obvious reasons, a great deal of the work on the nanomedicine and theranostics applications of lanthanide doped nanoparticles has shifted to their near-infrared luminescence properties.In this presentation, we will demonstrate the synthesis of these nanoparticles, establish how changing their nanoscale architecture can affect their luminescence properties (both upconversion and near-infrared), and discuss their potential applications, particularly in nanomedicine.

Lax pair, Darboux-dressing transformation and localized waves of the coupled mixed derivative nonlinear Schrödinger equation in a birefringent optical fiber

Under investigation in this paper is a coupled mixed derivative nonlinear Schrödinger equation, which can describe the pulse propagation in the femtosecond or picosecond regime of a birefringent optical fiber. Based on a Lax pair, we provide a Darboux transformation, and obtain the breather wave solutions. Then, a novel vector rogue wave solution is constructed by using Darboux-dressing transformation and asymptotic expansion. Those solutions exhibit some elastic and collision behaviors between a localized wave and a bright–dark soliton. Our works can be of importance in the theoretical experiments of the rogue wave generation mechanism and propagation trajectory.

Influence of the pulse duration at near-infrared wavelengths on the laser-induced material removal of hot-dipped galvanized steel

Hot-dipped galvanized steel is processed with short- and ultrashort-pulsed lasers in air at near-infrared wavelengths with pulse durations ranging from 350 fs to 241 ns. The morphology of the ablated craters (processed over a range of laser fluence levels and a number of laser pulses) is analyzed by confocal laser scanning microscopy and scanning electron microscopy. The ablation threshold of galvanized steel is found to increase with laser pulse durations following a simple power Fth=A.τB law. Longer pulse durations in the nanosecond regime, as compared to pulse durations in the picosecond and femtosecond regime, result in higher ablation efficiency and energy penetration depth at the cost of surface quality.

Morphology study and threshold measurement of laser induced damage of nano-porous antireflective silica thin films in nano- and femtosecond pulse regimes

Laser induced damage threshold (LIDT) of nano-porous antireflective silica thin films is measured using 10 ns laser pulses at λ = 532 nm. The thin films are prepared by dip-coating of BK7 substrates and then drying them by three different heating methods, to see how their LIDT and transmission are affected. Furthermore, using SEM imaging, the morphology of the laser induced damages is inspected and compared to those of similar samples, previously irradiated by femtosecond laser pulses at λ = 800 nm. The images evidently show that in the regions damaged by nanosecond pulses, the silica nanoparticles are melted and fused to each other, while in the samples irradiated by femtosecond pulses, the silica nanoparticles are sputtered and dispersed around, in the damaged area. The SEM images clearly demonstrate the different damage mechanisms involved in the nanosecond and femtosecond regimes of interaction.

Design Rules for Laser‐Treated Icephobic Metallic Surfaces for Aeronautic Applications

AbstractIce accretion on external aircraft surfaces due to the impact of supercooled water droplets can negatively affect the aerodynamic performance and reduce the operational capability and, therefore, must be prevented. Icephobic coatings capable of reducing the adhesion strength of ice to a surface represent a promising technology to support thermal or mechanical ice protection systems. Icephobicity is similar to hydrophobicity in several aspects and superhydrophobic surfaces embody a straightforward solution to the ice adhesion problem. Short/ultrashort pulsed laser surface treatments are proposed as a viable technology to generate superhydrophobic properties on metallic surfaces. However, it has not yet been verified whether such surfaces are generally icephobic under representative icing conditions. This study investigates the ice adhesion strength on Ti6Al4V, an alloy commonly used for aerospace components, textured by means of direct laser writing, direct laser interference patterning, and laser‐induced periodic surface structures laser sources with pulse durations ranging from nano‐ to femtosecond regimes. A clear relation between the spatial period, the surface microstructure depth, and the ice adhesion strength under different icing conditions is investigated. From these observations, a set of design rules can be defined for superhydrophobic surfaces that are icephobic, too.

Membrane Protein Dynamics Revealed by X-Ray Scattering with a Femtosecond Free-Electron Laser

The recent development of ultrafast X-ray free-election lasers (XFELs) gives a new avenue to experimentally investigate protein dynamics in the picosecond and femtosecond time regimes. The XFEL beam of the Linac Coherent Light Source (LCLS) has a very high peak X-ray brightness, which is key for probing elusive ultra-fast structural changes during the biological function of protein molecules. With XFEL probes, femtosecond time-resolved small- and wide-angle X-ray scattering (TR-SWAXS) experiments afford unprecedented opportunities in membrane biophysics. Here we describe the application of XFEL technology in pump-probe, TR-SWAXS experiments to structurally interrogate the conformational changes of membrane proteins, with an emphasis on G-protein-coupled receptors. Rhodopsin in detergent micelles was delivered to the X-ray beam using micro-jet technology. Time-resolved data were generated by recording X-ray scattering following the pump-laser illumination (light) as well as X-ray scattering without pump-laser illumination (dark). The 2D X-ray scattering patterns were radially integrated to generate 1D scattering profiles versus momentum transfer vector (Q). On-the-fly data analysis using the OnDA software package revealed the light-triggered “protein quake” of rhodopsin during early stages of its activation (within 10 ps). Double-difference profiles of rhodopsin-opsin solution scattering from −200 fs to 100 ps emphasized the structural effects of photoactivation. The signal from retinal after light excitation propagates as a shock wave throughout the protein and into the solvent. Theoretical molecular dynamics (MD) simulations provide a molecular interpretation and show that the calculated radius of gyration (Rg) of rhodopsin increases upon light excitation. Pump-probe experiments of photoactive proteins using an XFEL bring structural biology to a new level that will enable high-resolution structural movies of biomolecules in action. Establishing whether there is a single activation step or an energy landscape will be significant to understanding the functional GPCR activation mechanisms.

Enhancement of ultrafast nonlinear optical response of zinc selenide nanoparticle decorated reduced graphene oxide sheets

The synergistic effect of zinc selenide (ZnSe) nanoparticle functionalized into reduced graphene oxide (RGO) sheets on nonlinear optical (NLO) properties has been investigated by single beam z-scan technique. Comprehensive measurements on nonlinear absorption (NLA) as well as nonlinear refraction (NLR) have been performed on RGO, ZnSe, and RGO-ZnSe composites at 630 nm in the femtosecond regime. Both NLA and NLR of RGO-ZnSe show an enhancement in NLO properties compared to pure RGO and ZnSe in an intensity range of 37GW/cm2 to 130GW/cm2. The enhanced optical nonlinearity of RGO-ZnSe may have been caused due to strong interlayer coupling between RGO and ZnSe, as well as the availability of a large number of NLA states in the composite. The interlayer coupling between ZnSe nanoparticles and RGO sheets has been confirmed by transmission electron microscopy, UV-Visible, and photoluminescence spectroscopy. At low input pulse intensity (∼37GW/cm2), saturation absorption dominates, whereas NLA becomes prominent in the higher intensity regime (55GW/cm2–130GW/cm2) for RGO and ZnSe. NLA is the dominant phenomenon for RGO-ZnSe in the whole experimental intensity range. Moreover, it is observed that the dispersion of RGO, ZnSe, and RGO-ZnSe in dimethylformamide exhibits positive NLR. This study indicates an enhancement in nonlinear optical response of the RGO-semiconductor composite, which is very promising for graphene based photonic device applications.