Femtosecond Pulse Train Research Articles

Femtosecond laser processing of fused silica and aluminum based on electron dynamics control by shaping pulse trains

The pulse train effects on femtosecond laser material processing are investigated from the viewpoint of electron dynamics on dielectrics with fused silica as a case study and metals with Al as a case study in air and water. During femtosecond laser (800 nm, 35 fs) pulse train (double pulses per train) processing of fused silica, a non-monotonic relationship between ablation size and pulse separation is observed with an abrupt rise in the range of 150–275 fs. It is assumed that this is due to the enhancement of photon–electron coupling efficiency and transition of the phase-change mechanism by adjusting the free electron density during pulse train ablation. Surface quality in Al is improved with less recast by designing the pulse energy distribution to adjust the electron/lattice temperature distribution. Furthermore, the positive effects on ablation quality by femtosecond pulse train technology are more significant in water than those in air.

CONTROLLING ABOVE-THRESHOLD DISSOCIATION BRANCHING RATIOS OF HD+ WITH FEMTOSECOND LASER PULSE TRAIN

Above-threshold dissociation (ATD) process of the molecular ions HD+ steered by a femtosecond laser pulse train (LPT) is investigated theoretically using the time-dependent quantum wave packet method. Energy-dependent distributions of ATD fragments are analyzed by using an asymptotic-flow expression in the momentum space. It is found that fragment kinetic energy spectra shift to low energy region with increasing pulse number of LPT. The photofragment branching ratio between the 1sσg and 2pσu dissociation channels is sensitive to the pulse number of LPT. The momentum distribution of the ATD fragments is discussed in detail.

High-throughput rear-surface drilling of microchannels in glass based on electron dynamics control using femtosecond pulse trains

This study proposes a rear-surface ablation enhancement approach to fabricate high-aspect-ratio microchannels by temporally shaping femtosecond laser pulse trains. In the case study of K9 glass, enhancements of up to a 56 times higher material removal rate and a three times greater maximum drilling depth are obtained by the proposed method, as compared with conventional femtosecond laser drilling at the same processing parameters. The improvements are due to the changes of photon-electron interactions by shaping femtosecond pulse train, which can effectively adjust the photon absorption and localized transient material properties by changing electron dynamics such as free electron densities.

First-principles electron dynamics control simulation of diamond under femtosecond laser pulse train irradiation

A real-time and real-space time-dependent density functional is applied to simulate the nonlinear electron–photon interactions during shaped femtosecond laser pulse train ablation of diamond. Effects of the key pulse train parameters such as the pulse separation, spatial/temporal pulse energy distribution and pulse number per train on the electron excitation and energy absorption are discussed. The calculations show that photon–electron interactions and transient localized electron dynamics can be controlled including photon absorption, electron excitation, electron density, and free electron distribution by the ultrafast laser pulse train.

All-Fiber 40GHz Femtosecond Pulse Train Source

We experimentally demonstrate an all-fiber method to generate a high-quality femtosecond pulse train with a repetition rate of 40GHz. A continuous wavelength laser together with an intensity modulator is used to generate an initial pulse train with a repetition rate of 40GHz. Highly nonlinear fiber (HNLF) together with single mode fiber is utilized to compress the initial pulse from 7.8ps to 985fs. The autocorrelation trace has a good sech 2 profile with no pedestal. This pulse source has a potential to be multiplexed to a higher repetition rate for its narrow pulse width. This method is high-quality, cost-effective and can be easily integrated with the fiber system.

Flat Supercontinuum Generation within the Telecommunication Wave Bands in a Photonic Crystal Fiber with Central Holes

Flat supercontinuum in the telecommunication wave bands of E+S+C is generated by coupling a train of femtosecond pulses generated by a mode-locked Ti:sapphire laser into the fundamental mode of a photonic crystal fiber with central holes fabricated in our lab. The pulse experiences the anomalous dispersion regime, and the soliton dynamic effect plays an important role in supercontinuum generation. The output spectrum in the wavelength range of 1360–1565 nm does not include significant ripples due to higher pump peak power, and the normalized intensity shows less fluctuation.

Formation mechanisms of sub-wavelength ripples during femtosecond laser pulse train processing of dielectrics

A new model is proposed to investigate femtosecond laser pulse train processing of dielectrics by including the laser wave properties in the photon particle properties based plasma model. In the case studies, the pulse duration is 50 fs and the pulse delays are 0, 25, 50 and 75 fs. Both the laser wave–particle duality and transient localized changes of material properties are considered in the proposed model, and the formation mechanism of sub-wavelength ripples is revealed. This study shows that the interference between surface plasmons and laser field plays a key role in the formation of sub-wavelength ripples for which the excitation of surface plasmons is necessary. The predicted period of sub-wavelength ripples is in agreement with the experiments.

Femtosecond pulses and dynamics of molecular photoexcitation: RbCs example

We investigate the dynamics of molecular photoexcitation by unchirped femtosecond laser pulses using RbCs as a model system. This study is motivated by a goal of optimizing a two-color scheme of transferring vibrationally-excited ultracold molecules to their absolute ground state. In this scheme the molecules are initially produced by photoassociation or magnetoassociation in bound vibrational levels close to the first dissociation threshold. We analyze here the first step of the two-color path as a function of pulse intensity from the low-field to the high-field regime. We use two different approaches, a global one, the 'Wavepacket' method, and a restricted one, the 'Level by Level' method where the number of vibrational levels is limited to a small subset. The comparison between the results of the two approaches allows one to gain qualitative insights into the complex dynamics of the high-field regime. In particular, we emphasize the non-trivial and important role of far-from-resonance levels which are adiabatically excited through 'vertical' transitions with a large Franck-Condon factor. We also point out spectacular excitation blockade due to the presence of a quasi-degenerate level in the lower electronic state. We conclude that selective transfer with femtosecond pulses is possible in the low-field regime only. Finally, we extend our single-pulse analysis and examine population transfer induced by coherent trains of low-intensity femtosecond pulses.

Ultrafast solid-liquid-vapor phase change of a thin gold film irradiated by femtosecond laser pulses and pulse trains

Effects of different parameters on the melting, vaporization and resolidification processes of thin gold film irradiated by femtosecond pulses and pulse train were systematically studied. The classical two-temperature model was adopted to depict the non-equilibrium heat transfer in electrons and lattice. The melting and resolidification processes, which was characterized by the solid-liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, was obtained by considering the interfacial energy balance and nucleation dynamics. Vaporization process which leads to ablation was described by tracking the location of liquid-vapor interface with an iterative procedure based on energy balance and gas kinetics law. The parameters in discussion included film thickness, laser fluence, pulse duration, pulse number, repetition rate, pulse train number, etc. Their effects on the maximum lattice temperature, melting depth and ablation depth were discussed based on the simulation results.

First-principles calculations of the electron dynamics during femtosecond laser pulse train material interactions

This Letter presents first-principles calculations of nonlinear electron–photon interactions in crystalline SiO2 ablated by a femtosecond pulse train that consists of one or multiple pulses. A real-time and real-space time-dependent density functional method (TDDFT) is applied for the descriptions of electrons dynamics and energy absorption. The effects of power intensity, laser wavelength (frequency) and number of pulses per train on the excited energy and excited electrons are investigated.

Acousto‐optic materials for special applications with ultra‐short optical pulses

AbstractWe report here new acousto‐optic designs dedicated for applications in the edge of today's research: generation and spatially scanning of highly stable femtosecond laser pulse trains possibly with variable repetition rate, to provide optimal circumstances for a variety of applications: pump‐probe experiments, laser spectroscopy, laser microscopy. The design of new interaction geometries and device configurations serves the fulfillment of criteria like minimization of material and angular dispersion and maximization of diffraction efficiency, acoustic bandwidth and optical phase stability. We summarize experimental results obtained during pulse picking a femtosecond optical pulse train, actively stabilizing a laser oscillator with 20 MHz pulse repetition rate and implementing an acousto‐optic scanner for two‐photon laser microscope using dedicated devices with parameters optimized for the given application (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Broad and ultra-flattened supercontinuum generation in the visible wavelengths based on the fundamental mode of photonic crystal fibre with central holes

By coupling a train of femtosecond pulses with 100 fs pulse width at a repetition rate of 76 MHz generated by a mode-locked Ti: sapphire laser into the fundamental mode of photonic crystal fibre (PCF) with central holes fabricated through extracting air from the central hole, the broad and ultra-flattened supercontinuum (SC) in the visible wavelengths is generated. When the fundamental mode experiences an anomalous dispersion regime, three phases in the SC generation process are primarily presented. The SC generation (SCG) in the wavelength range from 470 nm to 805 nm does not emerge significant ripples due to a higher pump peak power and the corresponding mode fields at different wavelengths are observed using Bragg gratings. The relative intensity fluctuations of output spectrum in the wavelength ranges of 530 nm to 640 nm and 543 nm to 590 nm are only 0.028 and 0.0071, respectively.

Critical phenomena in periodic stratified media due to time-varying dielectric permittivity functions

The critical phenomena in a one-dimensional periodic stratified medium with time-varying dielectric permittivity functions are theoretically investigated for both TM and TE waves by solving the time-dependent electric displacement equation and the magnetic field equation, respectively. It is found that there exists a critical value of the varying amplitude when the dielectric permittivity function is changing with time. When the varying amplitude is below or at the critical value, the periodic structure can reflect an incident plane wave into a periodic femtosecond pulse train with respect to time. On the other hand, when the varying amplitude is much above the critical value, the reflectivity is descending more rapidly with time, which means that, at one moment, the incident light is transmitted thoroughly and the periodic structure is becoming transparent. The effect of the layer number on the reflectivity is also presented. Especially at the critical value, the pulse train generated is periodic with almost 100% peaks and zero troughs for both TM and TE waves when the layer number is up to 11. These interesting conclusions could find potential realistic applications in the future.

Synchronized time- and high-field-resolved all-optical pump–probe magneto-optical setup based on a strong alternating magnetic field and its application in magnetization dynamics of high coercivity magnetic medium

An alternating magnetic field (AMF) apparatus is developed and composed of an electromagnet and driving power supply. The structure of the electromagnet and configuration of the driving supply are described in detail. The apparatus can produce a peak magnetic field up to 9000 Oe and above under a small driving power at its resonance frequency of 1.14 kHz. Based on synchronization between the AMF and the femtosecond laser pulse train, a photomagnetic synchronized time- and high-field-resolved all-optical pump-probe magnetic-optical setup is developed. This setup has the ability to reinitialize any magnetic states between two successive laser pulses so that irreversible magnetization reversal dynamics can be studied. Dynamic Kerr hysteresis loops and magnetization reversal dynamics of high coercivity ferromagnetic TbFeCo and FePt films are demonstrated using this setup, showing importance of this synchronized time- and high-field-resolved all-optical pump-probe magnetic-optical setup in the research of ultrafast magnetization dynamics.

Shot noise limited characterization of ultraweak femtosecond pulse trains

Ultrafast science is inherently, due to the lack of fast enough detectors and electronics, based on nonlinear interactions. Typically, however, nonlinear measurements require significant powers and often operate in a limited spectral range. Here we overcome the difficulties of ultraweak ultrafast measurements by precision time-domain localization of spectral components. We utilize this for linear self-referenced characterization of pulse trains having ∼ 1 photon per pulse, a regime in which nonlinear techniques are impractical, at a temporal resolution of ∼ 10 fs. This technique does not only set a new scale of sensitivity in ultrashort pulse characterization, but is also applicable in any spectral range from the near-infrared to the deep UV.

Modeling of Ultrafast Phase Change Processes in a Thin Metal Film Irradiated by Femtosecond Laser Pulse Trains

Ultrashort laser pulses can be generated in the form of a pulse train. In this paper, the ultrafast phase change processes of a 1 μm free-standing gold film irradiated by femtosecond laser pulse trains are simulated numerically. A two-temperature model coupled with interface tracking method is developed to describe the ultrafast melting, vaporization, and resolidification processes. To deal with the large span in time scale, variable time steps are adopted. A laser pulse train consists of several pulse bursts with a repetition rate of 0.5–1 MHz. Each pulse burst contains 3–10 pulses with an interval of 50 ps–10 ns. The simulation results show that with such configuration, to achieve the same melting depth, the maximum temperature in the film decreases significantly in comparison to that of a single pulse. Although the total energy depositing on the film will be lifted, more energy will be transferred into the deeper part, instead of accumulating in the subsurface layer. This leads to lower temperature and temperature gradient, which is favorable in laser sintering and laser machining.

Enhancement of third-harmonic generation in nonlocal spatial solitons

We report on the observation of Type I third-harmonic generation induced by a train of femtosecond laser pulses in nematic liquid crystals. We find that as the average power of the train is increased, the frequency conversion process is enhanced as a consequence of the tight confinement of the pulses into a nonlocal spatial soliton.

Absolute distance measurement system using a femtosecond laser as a modulator

The generation of broadband microwave frequency comb from a femtosecond pulse train by direct photodetection opens the possibility for high-accuracy length measurements of long distances. We demonstrate a relatively simple realization of this measurement principle: an electronic distance measurement system based on a time-of-flight approach, driven by a femtosecond fibre laser source as a modulator. By the evaluation of the phase shifts of two distinct comb frequencies, a coarse and a fine measurement of the absolute distance can be performed. The range of the measurement system is demonstrated up to a length of 100 m. The experimental comparison of the femtosecond laser system with a conventional reference counting interferometer shows a precision better than ±10 µm at 100 m, corresponding to a relative measurement uncertainty of 1 × 10−7 L. The limiting factors for the measurement uncertainty of the system are theoretically investigated and shown to be of the same order of magnitude.

Broadband emission spectrum dynamics of large water droplets exposed to intense ultrashort laser radiation

We report on experiments on the interaction of a gigawatt femtosecond laser pulse train with hanging isolated millimeter-sized water droplets. A transparent droplet experienced explosive boiling-up and emitted light in the visible spectrum as a result of laser-induced plasma formed inside the droplet volume. The droplet emission spectra showed remarkable broadening, depending on the laser power. The role of pulse self-phase modulation in measured spectral broadening when the pulse propagates through the droplet is discussed.

High energy femtosecond supercontinuum light generation in large mode area photonic crystal fiber

A large mode area photonic crystal fiber (LMA PCF) with an effective area of 180 μm 2 is used to generate a high energy, micro-joule range, flat, octave spanning supercontinuum (SC) extending from ~ 600 nm to ~ 1720 nm. A train of femtosecond pulses from a widely-tunable parametric amplifier pumped by a Ti:Sapphire regenerative amplifier system are coupled into a 20 cm length of LMA PCF generating a SC of 1.4 μJ energy. We present an experimental study of the high energy SC as a function of the input power and the pumping wavelength. The spectrum obtained at a pump wavelength of 1260 nm presents spectral flatness variation less than 12 dB over more than 1.1 octave bandwidth. The physical processes behind the SC formation are described in the normal and the anomalous dispersion regions. Our experimental results are successfully compared with the numerical solution of the nonlinear Schrödinger equation.