Ultrashort Laser Research Articles

Multi-slicing matrix method for calculating s-polarization reflectance modulation induced by picosecond acoustic pulses for stratified media

Ultra short pulse lasers with sub-picosecond pulse duration can generate ultrasonic acoustic pulses with wavelengths in the sub-micrometer range. The reflectance modulation induced by the acoustic pulses can be detected by a probe laser. Once a method of calculating the acoustic pulse propagation and the reflectance modulation is established, it will be possible to extract information about the internal structures of a medium from measurements of the time-sequential reflectance modulation by fitting the calculations to experimental results. Thus, a multi-slicing matrix method of calculating the reflectance modulation with an arbitrary incident angle of the probe laser for s-polarization is proposed here. The method based on the Abeles transfer matrix method applicable to stratified media is mathematically validated and is shown to reproduce the analytical formula derived using the Born approximation. The method only uses matrix multiplications, which makes the calculation algorithm simple and is easy to code with matrix manipulation software. The thickness of a stratified layer of an amorphous carbon stacked on a substrate of silicon is demonstrated to be measured with the method.

Effects of Cr3+ ion concentration on the spectral characterization in Cr3+:Ca0.93Mg1.07Si2O6 crystals

Abstract Four Cr3+:Ca0.93Mg1.07Si2O6 crystals with Cr concentrations in the range of 0.3–2.0 at. % were successfully grown by Czochralski technique. The structure of the crystal was determined by the single-crystal X-ray diffraction method. The absorption spectra, fluorescence spectra and fluorescence decay curve were measured. There are two broad absorption bands with peaks at around 446 nm and 650 nm, respectively. With the increasing of the Cr3+ ions concentrations in the crystals, the fluorescence spectra showed a large redshift, with the emission peaks shifted from 783 nm to 986 nm. It is found that the two crystallographic Mg sites in the structure lead to the two different emission bands. The crystal field splitting (Dq), Racah parameters Dq/B and the decay curves were analyzed to distinguish the origins of the two peaks. The results show that the Cr3+:Ca0.93Mg1.07Si2O6 crystals have unusual spectroscopic features and may have a potential application for tunable and ultrashort lasers.

Edge shaping of CVD diamond coated cutting tool with ultrashort pulse laser

Edge shaping of CVD diamond coated cutting tool with ultrashort pulse laser

Progress on a High-Power, Ultrashort Laser

Progress on a High-Power, Ultrashort Laser

Light absorption by surface nanoholes and nanobumps

This paper deals with a numerical investigation of the energy deposition induced by ultrafast laser interaction with nanostructures. We calculate and analyze the intensity near-field reactive and radiative patterns resulted from the interference of the incident light with light scattered by individual subwavelength holes and bumps on the surface of metallic and dielectric materials. The role of light polarization, optical material properties, collective effects and nature of the imperfections in localized energy absorption is elucidated. The results open new perspectives in precise light manipulation by surface inhomogeneities and well-controlled surface nanostructuring by ultrashort laser.

Near infrared broadband emission of a new bismuth-doped Gd0.1Y0.9AlO3 crystal

A Bi doped Gd0.1Y0.9AlO3 (GYAP) crystal with high optical quality was successfully grown by the Czochralski method. An intense and broadband Near-IR (NIR) 900–1700 nm emission in Bi:GYAP crystal was observed for the first time. By analyzing the emission and excitation spectra of the Bi:GYAP crystal, the optical properties were researched in details. It is found that the two broadband NIR luminescence emission mainly peaking at 1005 and 1565 nm are ascribed to two different Bi+ related centers distinct sites. Moreover, the visible emission peaking at 430, 470, 515 and 630 nm are also obtained under the excitation at 250 nm, which show that there exist multiple valence states of luminescence in the mixed crystal. All these results show that Bi:GYAP crystal may become an attractive host for developing wide tuning range and ultrashort lasers at around NIR wavelength.

Single-Shot Terahertz Time Profiling Using Curved Wavefront

We report an improved single-shot electro-optic sampling (EOS) based detection scheme to measure the time profile of a terahertz (THz) pulse using inherent curvature of a converging wavefront. A simple addition to conventional multishot EOS technique enables single-shot recording of THz time profile. While the temporal resolution of the proposed method is limited only by the duration of the probe laser pulse, the spatial resolution can be externally controlled with ease. Our method is easy to implement and has the potential to provide better signal-to-noise ratio compared to other single-shot methods. An interesting spin-off of the proposed technique is that the quintessential space-to-time calibration process itself reveals the spatial profile of the THz radiation. The technique is simple yet versatile and capable of characterizing THz radiation emitted by high-power THz sources employing ultrashort intense lasers and modern-day particle accelerators. As a proof of principle demonstration, we provide measurement and characterization of the THz pulse emitted by two-color laser-produced plasma in ambient atmosphere using 45 fs Ti:sapphire laser pulses. Our method faithfully reproduces the THz waveform measured by conventional scanning EOS technique.

Static Coherent States Method: One- and Two-Electron Laser-Induced Systems with Classical Nuclear Dynamics

In this report, we introduce the static coherent states (SCS) method for investigating quantum electron dynamics in a one- or two-electron laser-induced system. The SCS method solves the time-dependent Schrödinger equation (TDSE) both in imaginary and real times on the basis of a static grid of coherent states (CSs). Moreover, we consider classical dynamics for the nuclei by solving their Newtonian equations of motion. By implementing classical nuclear dynamics, we compute the electronic-state potential energy curves of H2+ in the absence and presence of an ultra-short intense laser field. We used this method to investigate charge migration in H2+. In particular, we found that the charge migration time increased exponentially with inter-nuclear distance. We also observed substantial charge localization for sufficiently long molecular bonds.

Influence of Femtosecond Laser Parameters and Environment on Surface Texture Characteristics of Metals and Non-Metals – State of the Art

Enhancing the surface functionality by ultrashort pulsed laser texturing has received the considerable attention from researchers in the past few decades. Femtosecond lasers are widely adopted since it provides high repeatability and reproducibility by minimizing the heat affected zone (HAZ) and other collateral damages to a great extent. The present paper reports some recent studies being made worldwide on femtosecond laser surface texturing of metals, ceramics, polymers, semiconductors, thinfilms and advanced nanocomposites. It presents the state of the art knowledge in femtosecond laser surface texturing and the potential of this technology to improve properties in terms of biological, tribological and wetting performance. Since the texture quality and functionality are enhanced by the proper selection of appropriate laser parameters and ambient conditions for individual application, reporting the influence of laser parameters on surface texture characteristics assume utmost importance.

Near-field marking of gold nanostars by ultrashort pulsed laser irradiation: experiment and simulations

Quantitative measurements of the electric near-field distribution of star-shaped gold nanoparticles have been performed by femtosecond laser ablation. Measurements were carried out on and off the plasmon resonance. A detailed comparison with numerical simulations of the electric fields is presented. Semi-quantitative agreement is found, with slight systematic differences between experimentally observed and simulated near-field patterns close to strong electric-field gradients. The deviations are attributed to carrier transport preceding ablation.

Phase-matched harmonic generation in gas-filled waveguides in the vicinity of a multiphoton resonance

We investigate phase-matched harmonic generation of ultrashort (picosecond) laser pulses, tuned in the vicinity of a five-photon resonance in argon, confined in a hollow-core optical waveguide. This combines resonance enhancements and tight spatial confinement to increase the conversion efficiency toward vacuum-ultraviolet radiation pulses. We demonstrate that appropriate choice of the gas pressure maintains optimal phase-matching conditions also in the presence of inevitable dynamic level shifts at high intensities. Moreover, we reveal the considerable contribution of higher-order transversal waveguide modes to the total conversion efficiency and investigate the role of cascading frequency conversion processes. Finally, we study additional signal enhancements by buffer gas admixtures. The experimental data are compared with numerical simulations, taking higher transversal waveguide modes and cascade frequency conversion into account, identifying also the potential of quasi-phase matching by polarization mode beating. Our investigations show that proper choice of experimental parameters enables significant resonance enhancements in the conversion efficiency of harmonic generation.

Thin water film assisted glass ablation with a picosecond laser

Ultrashort lasers have become a promising tool for fast and high quality cutting of glasses. However, growing industry constantly seek for higher production throughput maintaining excellent process quality. In most cases, glass processing speed is limited to avoid clogging the laser-ablated channel, minimize crack generation, and heat accumulation effects in the glass material. Studies have shown that the introduced water layer onto the surface of the workpiece can significantly improve laser throughput and processing quality.This paper investigates laser ablation of soda-lime glass sheets in ambient air and water-assisted conditions. Glass cutting and single scan groove ablation were investigated in spite of ablation efficiency and scribe morphology. A 532 nm laser working at 100 kHz repetition rate was utilized in the experiments. Results showed that the applied thin flowing water film on the sample’s surface improved the glass ablation efficiency by 12 times.

Design and experimental study on high quality beam and high stability power of laser

High-performance lasers with high-quality beam laser and high-stability power are widely used for laser machining, laser precision measuring, etc. Reflection curved mirrors are widely used in lasers to provide several small intracavity focal spots and reduce the dispersion and volume of the laser. The primary disadvantage of using reflection curved mirrors in folded resonators is that relatively large angles of incidence deform the circular transverse pattern of the output beam and limit laser performance. In addition, in high power lasers or ultra short pulse lasers, the gain medium thermal lens focal length fluctuation is the primary cause of the instability of laser output power. This paper focuses on beam quality and power stability of laser, and an effective method of solving the two problems, i.e., astigmatism and instability power of laser, is presented. The laser resonator with high-quality laser beam and high-stability power is very easy and intuitive to design by this method, in which the resonator transform circle graphic theory is used and the thermal lens and astigmatism compensation is taken into account. The theoretical investigation shows that the astigmatism in two terminal arms of the folded laser resonator can be successfully eliminated by using this method, and the experimental measurements of the pattern of the laser output beam show that the deformations of spot intensity profiles in the two terminal arms can be simultaneously compensated for completely in the cavity, which is in good agreement with the analytical prediction. When the focal length of the laser crystal thermal lens varies, the variations of radii of spots not only at some key position, but also at all locations of the laser resonance designed by this method, are overtly smaller than the variation of the normal resonant cavity. The stability of the output laser power of the laser is better than that of a universal laser resonator under the same external conditions.

Surface processing and modification of CVD diamond with ultrashort pulse laser

Surface processing and modification of CVD diamond with ultrashort pulse laser

超短脉冲光纤激光相干合成(特邀)

超短脉冲光纤激光相干合成(特邀)

Influence of initial surface roughness of titanium on shape of periodic surface nanostructures produced with ultrashort pulsed laser

Influence of initial surface roughness of titanium on shape of periodic surface nanostructures produced with ultrashort pulsed laser

Femtosecond laser machining for characterization of local mechanical properties of biomaterials: a case study on wood

The standard preparation technique for micro-sized samples is focused ion beam milling, most frequently using Ga+ ions. The main drawbacks are the required processing time and the possibility and risks of ion implantation. In contrast, ultrashort pulsed laser ablation can process any type of material with ideally negligible damage to the surrounding volume and provides 4 to 6 orders of magnitude higher ablation rates than the ion beam technique. In this work, a femtosecond laser was used to prepare wood samples from spruce for mechanical testing at the micrometre level. After optimization of the different laser parameters, tensile and compressive specimens were produced from microtomed radial-tangential and longitudinal-tangential sections. Additionally, laser-processed samples were exposed to an electron beam prior to testing to study possible beam damage. The specimens originating from these different preparation conditions were mechanically tested. Advantages and limitations of the femtosecond laser preparation technique and the deformation and fracture behaviour of the samples are discussed. The results prove that femtosecond laser processing is a fast and precise preparation technique, which enables the fabrication of pristine biological samples with dimensions at the microscale.

Optimal control of high harmonics for the generation of double attosecond pulses by using a chirped laser pulse

An efficient scheme for the optimization of ultrashort femtosecond pulse shapes interacting with an atom to control high harmonics spectrum and double attosecond pulse generation is presented. The time-dependent Schrödinger equation of one-dimensional hydrogen atom is solved numerically to obtain electric field emission. The genetic algorithm optimization method is used to control the phase and amplitude of ultrashort excitation laser pulses to generate the desired attosecond-shaped pulses. An appropriate cost function is introduced for genetic algorithm optimization of double attosecond pulse generation. It is shown that the relative intensity of two generated pulses, their delay time and duration can be controlled in this approach. Finally, the parameters of the optimized emitted attosecond pulse are compared with those of desired pulses, and the underlying physical mechanisms are discussed in detail.

Demonstration of arbitrary temporal shaping of picosecond pulses in a radially polarized Yb-fiber MOPA with > 10 W average power.

High precision surface processing has an unmet demand for picosecond pulses with arbitrary temporal profiles in radial polarization states and at high average powers. Here, simultaneous spatial and arbitrary temporal shaping of chirped 10 - 100 picoseconds pulses is demonstrated with an Yb-doped fiber laser system generating an output power of more than 10 W at 40 MHz repetition frequency. The closed-loop control algorithm carves the pulses using a commercial, rugged, and fiberized optical pulse shaper placed at the front end of the system and uses feedback from the output pulse shapes for optimization. Arbitrary complex temporal profiles were demonstrated using a dispersive Fourier transform based technique and limits set by the system were investigated. Pulse shaping in the spatial domain was accomplished using an S-waveplate, fabricated in-house, to change the linearly polarized fundamental mode into a doughnut mode with radial polarization. This was amplified in a final-stage few-mode large-mode area fiber amplifier. Placing both temporal and spatial shaping elements before the power-amplifier avoids complex and potentially lossy conversion of the spatial mode profile at the output and provides an efficient route for power-scaling. The use of properly oriented quarter- and half-wave plates, which have both low loss and high power handling capability, enabled the output to be set to pure radial or azimuthal polarization states. Using commercial off-the-shelf components, our technique is able to immediately enhance the versatility of ultrashort fiber laser systems for high precision material processing and other industrial applications.

Selective photoreceptor changes after ultrashort pulse laser exposure in the infrared

Selective photoreceptor changes after ultrashort pulse laser exposure in the infrared