Picosecond Laser Pulses Research Articles

Application of mid-infrared free-electron laser for structural analysis of biological materials.

A mid-infrared free-electron laser (MIR-FEL) is a synchrotron-radiation-based femto- to pico-second pulse laser. It has unique characteristics such as variable wavelengths in the infrared region and an intense pulse energy. So far, MIR-FELs have been utilized to perform multi-photon absorption reactions against various gas molecules and protein aggregates in physical chemistry and biomedical fields. However, the applicability of MIR-FELs for the structural analysis of solid materials is not well recognized in the analytical field. In the current study, an MIR-FEL is applied for the first time to analyse the internal structure of biological materials by using fossilized inks from cephalopods as themodel sample. Two kinds of fossilized inks that were collected from different strata were irradiated at the dry state by tuning the oscillation wavelengths of the MIR-FEL to the phosphoryl stretching mode of hydroxyapatite (9.6 µm) and to the carbonyl stretching mode of melanin (5.8 µm), and the subsequent structural changes in those materials were observed by using infrared microscopy and far-infrared spectroscopy. The structural variation of these biological fossils is discussed based on the infrared-absorption spectral changes that were enhanced by the MIR-FEL irradiation, and the potential use of MIR-FELs for the structural evaluation of biomaterials is suggested.

1 kHz重复频率多脉冲皮秒激光器研制及其空间碎片激光测距应用

1 kHz重复频率多脉冲皮秒激光器研制及其空间碎片激光测距应用

Surface treatment on cobalt and titanium alloys using picosecond laser pulses in burst mode

The authors report on the results of surface treatment experiments using a solid-state amplified laser source emitting laser pulses with a pulse duration of 10 ps. The laser source allows the generation of pulse trains (bursts) with an intra-burst pulse repetition rate of 80 MHz (pulse-to-pulse time interval about 12.5 ns) with up to eight pulses per burst. In this study a wavelength of 1064 nm was used to investigate both ablation of material and laser-induced surface modifications occuring in metallic implant alloys CoCrMo (cobalt-chromium-molybdenum) and TiAlV (titanium-aluminum-vanadium) in dependence of the number of pulses and fluences per pulse in the burst. By using the burst mode, a smoothing effect occurs in a certain parameter range, resulting in very low surface roughness of the generated microstructures. It is demonstrated that at fluences per pulse which are smaller than the material-specific ablation threshold, a self-organized pore formation takes place if a defined number of pulses per burst is used. Thus, the advantage of the MHz burst mode in terms of a possible surface modification is established.

Taming Ultrafast Laser Filaments for Optimized Semiconductor–Metal Welding

AbstractUltrafast laser welding is a fast, clean, and contactless technique for joining a broad range of materials. Nevertheless, this technique cannot be applied for bonding semiconductors and metals. By investigating the nonlinear propagation of picosecond laser pulses in silicon, it is elucidated how the evolution of filaments during propagation prevents the energy deposition at the semiconductor–metal interface. While the restrictions imposed by nonlinear propagation effects in semiconductors usually inhibit countless applications, the possibility to perform semiconductor–metal ultrafast laser welding is demonstrated. This technique relies on the determination and the precompensation of the nonlinear focal shift for relocating filaments and thus optimizing the energy deposition at the interface between the materials. The resulting welds show remarkable shear joining strengths (up to 2.2 MPa) compatible with applications in microelectronics. Material analyses shed light on the physical mechanisms involved during the interaction.

Saturable absorption behavior of (5,10,15,20-tetraphenylporphyrinato)-silver crystal thin film at wavelength 532 nm

A single crystal of (5,10,15,20-tetraphenylporphyrinato)-silver was obtained and its structure reported. Using direct growth on quartz substrates method, a crystal thin film was obtained and the film’s morphology was investigated by atomic force microscopy. The nonlinear absorption properties of the film were studied using open aperture [Formula: see text]-scan technique by picosecond laser pulses at different fluence at wavelength 532 nm, nonlinear saturable absorption behavior was observed. Time-dependent unrestricted Hartree-Fock with the basis set LanL2DZ was used in computing the linear and non-linear optical properties of (5,10,15,20-tetraphenylporphyrinato)-silver.

Neural Stimulation In Vitro and In Vivo by Photoacoustic Nanotransducers

Summary Neuromodulation is an invaluable approach for the study of neural circuits and clinical treatment of neurological diseases. Here, we report semiconducting polymer nanoparticles based photoacoustic nanotransducers (PANs) for neural stimulation in vitro and in vivo. Our PANs strongly absorb the nanosecond pulsed laser in the near-infrared second window (NIR-II) and generate localized acoustic waves. PANs are shown to be surface modified and selectively bind onto neurons. PAN-mediated activation of primary neurons in vitro is achieved with ten 3-ns laser pulses at 1,030 nm over a 3-ms duration. In vivo neural modulation of mouse brain activities and motor activities is demonstrated by PANs directly injected into brain cortex. With submillimeter spatial resolution and negligible heat deposition, PAN stimulation is a new non-genetic method for precise control of neuronal activities, opening up potentials in non-invasive brain modulation.

Ultraviolet resonance Raman spectroscopy with a continuously tunable picosecond laser: Application to the supramolecular ligand guanidiniocarbonyl pyrrole (GCP)

We present a UVRR spectroscopy setup which is equipped with a picosecond pulsed laser excitation source continuously tunable in the 210–2600 nm wavelength range. This laser source is based on a three-stage optical parametric amplifier (OPA) pumped by a bandwidth-compressed second harmonic output of an amplified Yb:KGW laser. It provides <15 cm−1 linewidth pulses below 270 nm, which is sufficient for resolving Raman lines of samples in condensed phase studies. For demonstrating the capability of this tunable setup for UVRR spectroscopy we present its application to the artificial ligand guanidiniocarbonyl pyrrole (GCP), a carboxylate binder used in peptide and protein recognition. A UVRR excitation study in the range 244–310 nm was performed for identifying the optimum laser excitation wavelength for UVRR spectroscopy of this ligand (λmax = 298 nm) at submillimolar concentrations (400 µM) in aqueous solution. The optimum UVRR spectrum is observed for laser excitation with λexc = 266 nm. Only in the relatively narrow range of λexc = 266–275 nm UVRR spectra with a sufficiently high signal-to-noise ratio and without severe interference from autofluorescence (AF) were detectable. At longer excitation wavelengths the UVRR signal is masked by AF. At shorter excitation wavelengths the UVRR spectrum is sufficiently separated from the AF, but the resonance enhancement is not sufficient. The presented tunable UVRR setup provides the flexibility to also identify optimum conditions for other supramolecular ligands for peptide/protein recognition.

Influence of a low-Z thin substrate on a microwire hard x-ray source driven by a picosecond laser pulse for point-projection x-ray radiography

In the point-projection hard x-ray radiography of dense matter, for example, an inertial confinement fusion implosion capsule at stagnation time, a picosecond laser driven gold microwire is used to produce a short pulse point, bremsstrahlung hard x-ray source. The microwire was held by a low-Z CH thin substrate commonly used to promote experimental performance. We explored the influence of the low-Z thin substrate on the microwire bremsstrahlung hard x-ray source via particle-in-cell and Monte Carlo simulations. It was shown that both of the microwires, with or without the low-Z thin substrate, could emit more intense hard x-ray radiation than the radiator buried in the equal-density substrate, which benefited from efficient electron recirculation. The freestanding microwire exhibited further enhanced electron recirculation compared to that with the low-Z thin substrate, while the increased hot electrons were only present for the energetic electrons of &amp;gt;1 MeV. Thus, the freestanding microwire could produce significantly more intense MeV gamma x-ray emission with respect to that with the substrate, but an ignorable increment was exhibited at the softer x-ray emission of 10–200 keV. These results provided valuable insights into the design of backlighter targets in point-projection x-ray radiography, such as a freestanding microwire being preferred in MeV gamma-ray radiography, while the microwire with the CH thin substrate could be used in the 10–200 keV hard x-ray Compton radiography of an implosion capsule.

On the proton radiography of magnetic fields in targets irradiated by intense picosecond laser pulses

Proton radiography is a common diagnostic technique in laser-driven magnetic field generation studies. It is based on measuring proton beam deflection in electromagnetic fields induced around the target with the help of radiochromic film stacks. Unraveling information recorded in experimental radiographs and extracting the field profiles is not always a straightforward task. In this paper, some aspects of data analysis by reproducing experimental radiographs in numerical simulations are described. The approach allows determining the field strength and structure in the target area for various target geometries.

An Improved High-Rate Discharging Performance of “Unbalanced” LiFePO4 Cathodes with Different LiFePO4 Loadingsby a Grid-Patterned Micrometer Size-Holed Electrode Structuring

The degradation of charging/discharging capacities in the rate-performance test of lithium iron phosphate (LFP) cathodes with different loading amounts of an active material on both sides of a current collector (i.e., “unbalanced” LFP/LFP cathodes) in a laminated cell (typically composed of anode/separator/unbalanced cathodes/separator/anode) was not observed actually at low C-rates (e.g., 0.1 C). However, the rate-performance data obtained at high C-rates (e.g., > 5 C) indicated that the imbalance of the loading amounts of an active cathode material on both sides of an Al current collector causes a significant capacity degradation. We have found that it is possible to prevent the capacity degradation observed at high C-rates by holing the unbalanced LFP/LFP cathodes in a micrometer-sized grid-patterned way (the percentages of the holed area are typically several %) using a pico-second pulsed laser: The non-holed unbalanced LFP/LFP cathodes exhibited a considerable capacity degradation at C-rates which are, for example, larger than 5C, while the holed ones showed no degradation in capacity even at high C-rates (e.g., 5-20 C). Forming micrometer-sized grid-patterned holes in the LFP/LFP cathodes leads to an improved capacity and high-rate performance of their charging/discharging processes. The improvement in the discharging performance at the holed cathodes at high C-rates can be considered to result from a facilitated transfer of Li+ ions in the discharging of the thick layer as well as the thin one through both the surface and the sidewalls (produced by forming the holes) of the LFP layer (or the LFP/current collector layers). In order to keep a high discharge capacity at high C-rates even in thick LFP layers, resulting in an improvement of the battery performance of LIBs, the optimization of the diameter of holes formed and the percentage of holed areas (i.e., opening rate) is examined now.Fig. 1 Schematic description of high-rate discharging processes on (A) non-holed, (B) through-holed and (C) non-through-holed LFP/LFP cathodes with loading ratios of 3 at low- and high-rate discharging (e.g., at 0.1 and 10 C). Figure 1

Surface modeling and component analysis of picosecond laser ablation of CVD diamond

Chemical vapor deposition diamond (CVDD) has several advantages as micro mill material. Picosecond laser machining technology is promising method to machine CVDD, but there are multiple uncertainties. This study aims to optimize the irradiation of CVDD surface via controllable prediction of CVDD surface after picosecond laser irradiation. Based on theories of Gaussian pulse beam and picosecond laser ablation material, machined surface morphology model was developed to predict surface roughness Sa and removal depth of CVDD. Then, the effects of picosecond laser parameters on the morphology, roughness Sa, removal depth, and components of CVDD surface were analyzed. Results showed that the trend of changes in roughness and removal depth based on theoretical model was approximately the same as that in experimental machined surface, with average relative errors of roughness and removal depth of 9.6% and 16.6%, respectively. In addition, removal depth of CVDD surface decreased with the increase in scanning speed and filling pitch, and increased with the increase in average laser power. The variation in surface roughness Sa was the same as that of removal depth in terms of average laser power and scanning speed, while had a concave point at a filling pitch of 7.5 μm. A decrease in overlapping area of pulse spot can reduce the aggravation of sp2 hybrid amorphous carbon defects.

Dynamics of picosecond laser ablation for surgical treatment of colorectal cancer

Endoluminal surgery for the treatment of colorectal neoplasia is typically carried out using electrocautery tools which imply limited precision and the risk of harm through collateral thermal damage to the adjacent healthy tissue. As a potential alternative, we present the successful colonic epithelial laser ablation by means of picosecond laser pulses. Laser ablation studies performed in ex-vivo colon tissue result in cavities with comparable thickness to early stage colorectal cancers. The corresponding histology sections exhibit only minimal collateral damage to the surrounding tissue and the depth of the ablation can be controlled precisely by means of the pulse energy. High-speed imaging has been used for the first time to visualize picosecond laser ablation of cancerous tissue in a clinically relevant model. This information was correlated with histopathology and optical surface profilometry revealing the dynamic nature of the laser tissue interaction and the need for temporal or spatial separation of pulses for optimum efficacy with regards to tissue removal. Overall, the application of picosecond laser pulses to ablate endoluminal bowel lesions demonstrates significantly improved precision and reduced thermal damage to the adjacent tissue in comparison to conventional procedures and hence will enable more precise surgical treatment of cancers.

Two key frontier issues on picosecond pulses generated by mode-locked fiber lasers

Narrowband dissipative soliton mode-locked fiber lasers can produce transform-limited picosecond pulses. Unfortunately, due to the limitation of allowable nonlinear phase shift for the intracavity pulse, the repetition rate of the pulses generated by such lasers cannot be reduced by increasing the cavity length; the pulse energy is only below 0.1 nJ. These seriously restrict the practical application of such picosecond pulsed fiber lasers. In this paper, we propose a method that allows the cavity length to be increased to reduce the repetition rate of the narrowband dissipative soliton picosecond fiber laser pulses by extracting the pulse energy out of the cavity with a coupler to suppress the accumulation of nonlinear phase shift of the intracavity pulses. Using this method, the laser repetition rate was successfully reduced from 35.2 MHz to 1.77 MHz, and the pulse time-frequency characteristics remained unchanged. We also propose a method to suppress spectral broadening in picosecond pulse fiber amplification based on inter-stage FBG notch filtering. By simply using the inter-stage notch filter, the output pulse spectrum width after the first-stage fiber amplifier can be narrowed, allowing the second-stage fiber amplifier to further increase the pulse energy, and also, the pulse can be reshaped to be nearly Gaussian-shaped, allowing the second-stage fiber amplifier to increase the pulse energy higher by using the Gaussian pulse characteristics of the smaller spectral broadening slope. Using this method, on the premise of keeping the RMS spectral width within 0.4 nm, after a 10 ps pulse passes through a standard single-mode fiber amplifier, the pulse energy can be increased from 0.2 nJ to more than 10 nJ.

Absorption of a nanosecond laser pulse by a picosecond laser-induced preformed aluminum plasma

The LIBS (Laser-Induced Breakdown Spectroscopy) method has already demonstrated its reliability and its robustness in many situations for the multi-elemental composition determination of samples. However, certain conditions prevent a totally satisfactory determination. For instance, the method is weakly efficient to measure with accuracy the light elements concentration in metallic matrices. Since the laser pulse used to produce the plasma contributes to its heating, using an additional pulse (double pulse configuration) provides the increase in electron temperature and density without additional ablation. A better signal-to-noise ratio and a lower limit of detection can be reached. The present paper reports the results of different experiments performed to quantify the modifications induced (1) on the electron density by the second laser pulse in a preformed aluminum plasma, and (2) on the second laser pulse itself. The related experiments have been done in the case where the plasma is produced by a picosecond laser pulse and the second laser pulse is of the nanosecond type. The electron density reaches a maximum resulting from the total ionization of the aluminum plasma volume irradiated by the second laser pulse.

Picosecond pulsed underwater laser ablation of silicon and stainless steel: Comparing crater analysis methods and analysing dependence of crater characteristics on water layer thickness

Liquid layer thickness dependence of 515 nm, 7 picosecond pulsed laser ablation of stainless steel 304 and silicon is analyzed. Ablated crater volume and diameter are compared to ablated craters in ambient air by means of a novel, objective numerical procedure. While silicon ablation under a water layer is found to be more efficient in terms of removed material volume per pulse than ablation in ambient air, an opposite trend is found for stainless steel 304. For both materials, the ablation efficiently drops when the liquid layer thickness is decreased to 1 milimeter. A probable reason for the ablation efficiency drop is persistent bubble formation.

Strong enhancement of coherent terahertz radiation by target ablation using picosecond laser pulses

We have studied the generation of terahertz (THz) radiation via the interaction of an intense femtosecond laser pulse with aluminum (Al) and plastic targets pre-ablated by a picosecond ablation pulse. The measurements of the plasma shadowgraph and 1D-MULTI simulations show a similar micrometer density scale length of the pre-ablated plasma for the two types of targets after ablation. It is found that for the Al target, the intensity of THz radiation increased obviously under the condition of ablation, and only with the appropriate ablation delay time, the cutoff energy and charge of the accelerated protons also increased. 2D-simulations using the EPOCH program indicate that the strong enhancement of THz for the Al target is due to the increased number of energetic electrons from the interaction between the main laser and the ablated plasma. However, for the plastic targets, the intensity of THz radiation was almost vanished in the case of ablation, but the accelerated protons still existed and the cutoff energy even tended to increase. However, due to the lack of ability to simulate the molecular structure, we cannot distinguish between Al and plastic targets in the simulation. The mechanism of radiation suppression by ablation of the plastic target needs further investigation.

Laser Formation of Holes in Nonmetallic Substrates

The use of a pulse picosecond laser with a wavelength of 532 nm and frequency of 15 Hz in the formation of holes with diameters up to 0.1 mm in nonmetallic substrates (silicon, ceramics, etc.) was proposed. It was found by modeling in MathCAD and COMSOL Multiphysics that the half-angle value of the light cone aperture that depends on the focusing of laser radiation exerts a significant effect on the depth and diameter of holes in laser processing. The influence of the regimes of laser processing employing nanosecond and picosecond lasers with wavelengths of 1064 and 532 nm, respectively, on the conicity and time of formation of holes in silicon substrates was investigated. The preliminary heating of the substrate to 170–200°С reduces the time of formation of the holes by 20%.

Asymmetric broadening and blue shift of the stimulated Raman scattering spectrum in water under chirped picosecond laser pulse train excitation

For the first time, we have revealed an asymmetric broadening and a 35 cm−1 blue shift (up to 3400 cm−1) of the stimulated Raman scattering OH band in water under chirped picosecond pulse train excitation (pulse duration ∼60 ps with 8 ns interval between pulses) while increasing the train length to 25 pulses. From the observed broadening and shift of the OH band centroid, a lower estimate (≥8 ns) of molecular complexes spectral states relaxation time at room temperature was obtained.

Optical reflectivity of heat-treated nanofibrous silicon thin-films induced by high energy picosecond laser pulses

In this study, selective optical properties of Si/SiO2 thin films and their dependence on heat treatment were investigated. Samples were synthesized by pulsed-laser ablation deposition of c-Si on glass substrates and annealed at different temperatures (400 °C, 1000 °C). Nanofabrication parameters were chosen to produce significantly different nanostructures ranging from agglomerated particles to nanofibers. Morphology, crystalline, and elemental characterization techniques were employed to explore the variations in visible range reflection measurements. The adverse effect of annealing temperatures on Vis-range reflection spectra to increase and decrease the intensity was confirmed. These improvements are attributed to crystalline and amorphous lattice structures and elemental composition supported by the X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) method results.

Lasing from solution-processed CsPbBr3 octahedral resonators

CsPbBr3 has been recognized as one of the most important members in the family of metal halide perovskites, which could offer high optical gain for low-threshold lasing. Despite of significant advances achieved thus far, it is still in high demand to develop new material platforms to further rationally design perovskite-based resonators. Here, CsPbBr3 microcrystals in octahedral shape are developed via a simple antisolvent approach, which exhibit single- or dual-mode lasing upon excitation with picosecond laser pulses. The experimental results reveal an unusual phenomenon that the pump threshold shows a profound dropdown of nearly one order from 65.5 down to 7.4 μJ/cm2 when the microcrystal size is varying from 20 to 4 μm. The experimental results are in contrary to numerical simulations that a larger microcrystal supports eigenmodes with a higher quality factor. We propose that such a dependence is perhaps dominated with scattering losses of different-sized microcrystals. The lowest pump threshold achieved in CsPbBr3 octahedral resonators is comparable to those supported in high-quality CsPbBr3 resonators reported before. The results suggest the potential of the developed CsPbBr3 octahedral microcrystals in creating high-quality light emitters.