Train Of Ultrashort Laser Pulses Research Articles

Discrete Illumination-Based Compressed Ultrafast Photography for High-Fidelity Dynamic Imaging.

Compressed ultrafast photography (CUP) can capture irreversible or difficult-to-repeat dynamic scenes at the imaging speed of more than one billion frames per second, which is obtained by compressive sensing-based image reconstruction from a compressed 2D image through the discretization of detector pixels. However, an excessively high data compression ratio in CUP severely degrades the image reconstruction quality, thereby restricting its ability to observe ultrafast dynamic scenes with complex spatial structures. To address this issue, a discrete illumination-based CUP (DI-CUP) with high fidelity is reported. In DI-CUP, the dynamic scenes are loaded into an ultrashort laser pulse train with controllable sub-pulse number and time interval, thus the data compression ratio, as well as the overlap between adjacent frames, is greatly decreased and flexibly controlled through the discretization of dynamic scenes based on laser pulse train illumination, and high-fidelity image reconstruction can be realized within the same observation time window. Furthermore, the superior performance of DI-CUP is verified by observing femtosecond laser-induced ablation dynamics and plasma channel evolution, which are hardly resolved in the spatial structures using conventional CUP. It is anticipated that DI-CUP will be widely and dependably used in the real-time observations of various ultrafast dynamics.

Coherent laser cooling with trains of ultrashort laser pulses

Coherent laser cooling with trains of ultrashort laser pulses

Two-dimensional laser-induced periodic surface structures formed on crystalline silicon by GHz burst mode femtosecond laser pulses

Femtosecond laser pulses with GHz burst mode that consist of a series of trains of ultrashort laser pulses with a pulse interval of several hundred picoseconds offer distinct features in material processing that cannot be obtained by the conventional irradiation scheme of femtosecond laser pulses (single-pulse mode). However, most studies using the GHz burst mode femtosecond laser pulses focus on ablation of materials to achieve high-efficiency and high-quality material removal. In this study, we explore the ability of the GHz burst mode femtosecond laser processing to form laser-induced periodic surface structures (LIPSS) on silicon. It is well known that the direction of LIPSS formed by the single-pulse mode with linearly polarized laser pulses is typically perpendicular to the laser polarization direction. In contrast, we find that the GHz burst mode femtosecond laser (wavelength: 1030 nm, intra-pulse duration: 220 fs, intra-pulse interval time (intra-pulse repetition rate): 205 ps (4.88 GHz), burst pulse repetition rate: 200 kHz) creates unique two-dimensional (2D) LIPSS. We regard the formation mechanism of 2D LIPSS as the synergetic contribution of the electromagnetic mechanism and the hydrodynamic mechanism. Specifically, generation of hot spots with highly enhanced electric fields by the localized surface plasmon resonance of subsequent pulses in the bursts within the nanogrooves of one-dimensional LIPSS formed by the preceding pulses creates 2D LIPSS. Additionally, hydrodynamic instability including convection flow determines the final structure of 2D LIPSS.

Reliable determination of pulse-shape instability in trains of ultrashort laser pulses using frequency-resolved optical gating

We describe a reliable approach for determining the presence of pulse-shape instability in a train of ultrashort laser pulses. While frequency-resolved optical gating (FROG) has been shown to successfully perform this task by displaying a discrepancy between the measured and retrieved traces for unstable trains, it fails if its pulse-retrieval algorithm stagnates because algorithm stagnation and pulse-shape instability can be indistinguishable. So, a non-stagnating algorithm—even in the presence of instability—is required. The recently introduced Retrieved-Amplitude N-grid Algorithmic (RANA) approach has achieved extremely reliable (100%) pulse-retrieval in FROG for trains of stable pulse shapes, even in the presence of noise, and so is a promising candidate for an algorithm that can definitively distinguish stable and unstable pulse-shape trains. But it has not yet been considered for trains of pulses with pulse-shape instability. So, here, we investigate its performance for unstable trains of pulses with random pulse shapes. We consider trains of complex pulses measured by second-harmonic-generation FROG using the RANA approach and compare its performance to the well-known generalized-projections (GP) algorithm without the RANA enhancements. We show that the standard GP algorithm frequently fails to converge for such unstable pulse trains, yielding highly variable trace discrepancies. As a result, it is an unreliable indicator of instability. Using the RANA approach, on the other hand, we find zero stagnations, even for highly unstable pulse trains, and we conclude that FROG, coupled with the RANA approach, provides a highly reliable indicator of pulse-shape instability. It also provides a typical pulse length, spectral width, and time-bandwidth product, even in cases of instability.

Modeling the propagation of a high-average-power train of ultrashort laser pulses.

We investigate the interpulse thermal interaction of a train of ultrashort laser pulses and develop a model to describe the isobaric heating of air by a train of pulses undergoing filamentation. We calculate the heating of air from a single laser pulse and the resulting refractive index perturbation encountered by subsequent pulses, and use this to simulate the propagation of a high-power pulse train. The simulations show deflection of laser filaments by the thermal refractive index consistent with previous experimental measurements.

Dual trace inter-pulse interferometer for measurement of phase stability of ultra short laser pulse train.

A dual trace intra-pulse and inter-pulse spatio-spectral interferometer has been set up to study the temporal stability of a ∼200fs duration laser pulse train from a cw mode-locked laser oscillator. Simultaneous recording of twin interferograms helps identify the phase error in inter-pulse interferograms due to the diagnostic setup kept in a standard laboratory environment. Applicability of inter-pulse tilted pulse-front interferograms has been demonstrated to constitute an alternative inexpensive method for visual detection and estimation of phase slippage and pulse repetition frequency of an ultra short pulse train. The effect of pump beam intensity on the repetition rate of pulses due to accumulated intra-cavity non-linear phase shifts is also presented.

Induced birefringence in glass: depletion and enhancement by orthogonal-polarized femtosecond pulses

We show spectacular and different effect of the sequential and simultaneous writing of two perpendicularly polarized ultrashort laser pulse trains on profile and magnitude of induced optical retardation inside fused silica glass. Clear birefringence was observed in the region exposed to linearly polarized pulse train radiation. It found out that the induced birefringence is erasable. It means that, when the sample is irradiated again with pulse train having perpendicular polarization, the induced birefringence is vanished and can be totally erased by optimizing the pulse energy. However, in a simultaneous writing approach, a contradictory result was observed. When the glass substrates were simultaneously (i.e. with an accuracy better than the pulse duration) exposed to two beams with perpendicular polarization the induced birefringence not only remained but also enhanced. Discussion and study on the results of interaction of polarized single ultrashort laser pulse and sequential laser beams (which spatially overlapped) having different polarizations and also change of energy ratio of simultaneously writing pulse trains helps us to analyze different results of the simultaneous interaction of two orthogonally polarized ultrashort laser pulse trains with transparent material. Our results provide pieces of evidence for further understanding the physical mechanism of creation of the birefringence using ultrashort laser pulses. Additionally, they provide the ability to manipulate the transient electron dynamics to control the profile and tailor of the induced birefringence.

Detection and elimination of pulse train instabilities in broadband fibre lasers using dispersion scan

We use self-calibrating dispersion scan to experimentally detect and quantify the presence of pulse train instabilities in ultrashort laser pulse trains. We numerically test our approach against two different types of pulse instability, namely second-order phase fluctuations and random phase instability, where the introduction of an adequate metric enables univocally quantifying the amount of instability. The approach is experimentally demonstrated with a supercontinuum fibre laser, where we observe and identify pulse train instabilities due to nonlinear propagation effects under anomalous dispersion conditions in the photonic crystal fibre used for spectral broadening. By replacing the latter with an all-normal dispersion fibre, we effectively correct the pulse train instability and increase the bandwidth of the generated coherent spectrum. This is further confirmed by temporal compression and measurement of the output pulses down to 15 fs using dispersion scan.

Filamentation of an ultrashort laser pulse train in air

Filamentation of an ultrashort laser pulse train in air

Effect of multi-pulses train on the thermomechanical response of a metal thin film

ABSTRACTUltrafast thermoelasticity based on hyperbolic two-step heat conduction model with electron-phonon interaction was applied to investigate the thermomechanical coupling responses of an immense homogeneous, isotropic gold film, which was irradiated by various ultrashort laser pulse trains. The solution to the problem was obtained by solving finite element governing equations. For the same energy injection, a new micromachining technique using laser pulse trains can offer certain advantages compared with a single laser pulse micromachining, such that the laser pulse train is provided with the advantages of long and short laser pulses. The effects of the pulse number and the pulse separation times for a laser pulse train on ultrafast thermomechanical coupling responses, i.e., the temperature, displacement as well as stress, are presented graphically and analyzed. It was found from the numerical results that the expected machining results (or thermomechanical responses) may be obtained by using pulse train technology with optimal pulse number and pulse separation time.

Ultrafast thermomechanical responses of a copper film under femtosecond laser trains: a molecular dynamics study

Using molecular dynamics (MD) simulation, the ultrafast thermomechanical coupling responses of an immense homogeneous, isotropic copper film with the thickness of 1 μm, which is irradiated by various ultra-short laser pulse trains, are investigated. For the same energy injection, the effect of laser pulse trains is studied and it is observed from the numerical results that the pulse train technology may improve the ultrafast thermomechanical responses of the film significantly. By thoroughly analysing temperature, stress and displacement (strain) of the film, the thermomechanical coupling characteristics between stress, displacement (strain) and temperature are presented perfectly at the atomic scale. It is found that the lattice temperature of the region under tensile stress (or compressive stress) is lower (or higher) than the lattice temperature of the surrounding region, and the positive and negative strain (tensile or compressive) is related to tensile and compressive stress very well.

Ultrafast electronic state conversion at room temperature utilizing hidden state in cuprate ladder system.

Photo-control of material properties on femto- (10−15) and pico- (10−12) second timescales at room temperature has been a long-sought goal of materials science. Here we demonstrate a unique ultrafast conversion between the metallic and insulating state and the emergence of a hidden insulating state by tuning the carrier coherence in a wide temperature range in the two-leg ladder superconductor Sr14-xCaxCu24O41 through femtosecond time-resolved reflection spectroscopy. We also propose a theoretical scenario that can explain the experimental results. The calculations indicate that the holes injected by the ultrashort light reduce the coherence among the inherent hole pairs and result in suppression of conductivity, which is opposite to the conventional photocarrier-doping mechanism. By using trains of ultrashort laser pulses, we successively tune the carrier coherence to within 1 picosecond. Control of hole-pair coherence is shown to be a realistic strategy for tuning the electronic state on ultrafast timescales at room temperature.

Efficient optical trapping of CdTe quantum dots by femtosecond laser pulses.

The development in optical trapping and manipulation has been showing rapid progress, most of it is in the small particle sizes in nanometer scales, substituting the conventional continuous-wave lasers with high-repetition-rate ultrashort laser pulse train and nonlinear optical effects. Here, we evaluate two-photon absorption in optical trapping of 2.7 nm-sized CdTe quantum dots (QDs) with high-repetition-rate femtosecond pulse train by probing laser intensity dependence of both Rayleigh scattering image and the two-photon-induced luminescence spectrum of the optically trapped QDs. The Rayleigh scattering imaging indicates that the two-photon absorption (TPA) process enhances trapping ability of the QDs. Similarly, a nonlinear increase of the two-photon-induced luminescence with the incident laser intensity fairly indicates the existence of the TPA process.

Pulse train induced rotational excitation and orientation of a polar molecule

We investigate theoretically the rotational excitation and field free molecular orientation of polar HBr molecule, interacting with train of ultrashort laser pulses. By adjusting the number of pulses, pulse period and the intensity of the pulse, one can suppress a population while simultaneously enhancing the desired population in particular rotational state. We have used train of laser pulses of different shaped pulse envelopes. The dynamics and orientation of molecules in the presence of pulse train of different shapes is studied and explained.

Thermal refraction in ionic liquids induced by a train of femtosecond laser pulses

The Z-scan technique is used to characterize the nonlinear refraction induced by a train of ultrashort laser pulses (80 fs, repetition rate 80.75 MHz) for a set of ionic liquids as a function of their structural parameters such as cation, anion type and alkyl chain length in the cation. The nonlinear refractive index change is originated by linear absorption processes which cause a nonlocal and inhomogeneous increase of temperature in the irradiated region of the sample that behaves as a thermal lens. The thermal refraction, according to the thermal lens model, is mainly related to three physical properties of the medium, which are the absorption at the excitation wavelength, the thermal conductivity and the thermo-optic coefficient. We explore the influence of anions and cations on these properties and on the thermal lens strength. The cationic influence was the object of study for the ILs based on the 1-butyl-3-methyl-imidazolium cation. The anionic influence was studied for the ILs based on the bis(trifluoromethylsulfonyl)imide anion. The obtained results show that both cation and anion affect significantly the thermal lens strength. The thermal refraction is 1.6 times higher in 1-butyl-3-methyl-imidazolium chloride than in 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide, and 3.3 times higher in trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide than in 1-butylpyridinium bis(trifluoromethylsulfonyl)imide. Optical absorption at 810 nm is influenced by both ionic parts, whereas the anion hardly determines the magnitude of the thermal conductivity. • We measure the thermal lens effect in a set of ionic liquids with Z-scan. • We study the cation (anion) influence when combined with [Tf 2 N] − ([BMIM] + ). • We identify the anions/cations that enhance and minimize the thermal lens effect. • We provide absorption coefficients and, when possible, thermal conductivities. • We calculate, when possible, the thermo-optic coefficients.

Angular distributions of CH3I fragment ions under the irradiation of a single pulse and trains of ultrashort laser pulses

The angular distribution of CH3I is investigated experimentally using a single Fourier transform-limited laser pulse and a pulse train, where a 90-fs 800-nm linearly polarized laser field with a moderate intensity of 2.8×1013 W/cm2 is used. The dynamic alignment is demonstrated in a single pulse experiment. Moreover, a pulse train is used to optimize the molecular alignment, and the alignment degree is almost identical to that with the single pulse. The results are analysed by using chirped femtosecond laser pulses, and it demonstrates that the structure of pulse train rather than its effective duration is crucial to the molecular alignment.

Detecting optically synthesized quasi-monochromatic sub-terahertz phonon wavepackets by ultrafast x-ray diffraction

We excite an epitaxial SrRuO3 thin film transducer by a pulse train of ultrashort laser pulses, launching coherent sound waves into the underlying SrTiO3 substrate. Synchrotron-based x-ray diffraction (XRD) data exhibiting separated sidebands to the substrate peak evidence the excitation of a quasi-monochromatic phonon wavepacket with sub-THz central frequency. The frequency and bandwidth of this sound pulse can be controlled by the optical pulse train. We compare the experimental data to combined lattice dynamics and dynamical XRD simulations to verify the coherent phonon dynamics. In addition, we observe a lifetime of 130 ps of such sub-THz phonons in accordance with the theory.

Control of Molecular Rotation with a Chiral Train of Ultrashort Pulses

Trains of ultrashort laser pulses separated by the time of rotational revival (typically, tens of picoseconds) have been exploited for creating ensembles of aligned molecules. In this work we introduce a chiral pulse train--a sequence of linearly polarized pulses with the polarization direction rotating from pulse to pulse by a controllable angle. The chirality of such a train, expressed through the period and direction of its polarization rotation, is used as a new control parameter for achieving selectivity and directionality of laser-induced rotational excitation. The method employs chiral trains with a large number of pulses separated on the time scale much shorter than the rotational revival (a few hundred femtosecond), enabling the use of conventional pulse shapers.

A universal scheme for the generation of ultrashort laser pulse trains by Kerr nonlinearphotonic crystal ultrafast all-optical switching

We propose a universal way to generate ultrashort pulse trains in arbitrary frequencyregimes by means of Kerr nonlinear photonic crystal ultrafast all-optical switching. When acontinuous laser light passes through a nonlinear photonic crystal with its band gap edgematching the laser light wavelength and pumped by a high-intensity femtosecond pulselaser, the ultrafast optical switching can lead to ultrashort pulse trains of transmissionlight. Simulation results show that the duration of the generated seed pulses can be asshort as 12, 25, 54 and 68 fs for incident laser wavelengths of 0.633, 1.55, 5.12, and10.6 µm, respectively, and the signal-to-background ratio contrast is about3 × 103, which is very suitable for the subsequent amplification of these seed pulses.

Controlling the photoluminescence of gallium arsenide with trains of ultrashort laser pulses

A train of three equally spaced femtosecond laser pulses is used to induce electron recombination in GaAs(100). Photoluminescence from the ${E}_{1}$ transition is observed when the spacing between the pulses is an integer multiple of the longitudinal-optical phonon period. This effect is also controlled by varying the relative optical phase and the polarization plane of the pulses. These observations are explained by a mechanism in which hot electrons excited in a plasma collide with the underlying lattice to produce coherently excited phonons, which scatter carriers into the L valley, where they recombine.