Picosecond Pulsed Laser Source Research Articles

Nano/Microstructuring of Nickel Electrodes by Combining Direct Laser Interference Patterning and Polygon Scanner Processing for Efficient Hydrogen Production

The present work utilizes direct laser interference patterning in conjunction with a polygon scanner system to fabricate micro‐ and nanostructures on the surface of nickel foils. A picosecond pulsed laser source with an average power of ≈470 W is used to pattern line‐like structures having spatial periods of 11.0 and 25.0 μm and a maximum structure depth of up to 15 μm. The scanning speed and the number of consecutive scans, ranging from 5 to 500, are varied in order to assess their influence on the structure formation process. Furthermore, the use of ultrashort pulses permits the production of hierarchical surface structures having up to four feature size levels because of incubation effects and growth of different types of laser‐induced periodic surface structures. 2D‐fast Fourier transforms are applied to scanning electron microscopy images for analyzing the evolution of features from the nano‐ to microscale. The method proposed here permits the reduction of 22% in the overpotential of the hydrogen evolution reaction, which linearly correlates to the drop in energy required to drive this chemical reaction.

Submerged single-photon LiDAR imaging sensor used for real-time 3D scene reconstruction in scattering underwater environments.

We demonstrate a fully submerged underwater LiDAR transceiver system based on single-photon detection technologies. The LiDAR imaging system used a silicon single-photon avalanche diode (SPAD) detector array fabricated in complementary metal-oxide semiconductor (CMOS) technology to measure photon time-of-flight using picosecond resolution time-correlated single-photon counting. The SPAD detector array was directly interfaced to a Graphics Processing Unit (GPU) for real-time image reconstruction capability. Experiments were performed with the transceiver system and target objects immersed in a water tank at a depth of 1.8 meters, with the targets placed at a stand-off distance of approximately 3 meters. The transceiver used a picosecond pulsed laser source with a central wavelength of 532 nm, operating at a repetition rate of 20 MHz and average optical power of up to 52 mW, dependent on scattering conditions. Three-dimensional imaging was demonstrated by implementing a joint surface detection and distance estimation algorithm for real-time processing and visualization, which achieved images of stationary targets with up to 7.5 attenuation lengths between the transceiver and the target. The average processing time per frame was approximately 33 ms, allowing real-time three-dimensional video demonstrations of moving targets at ten frames per second at up to 5.5 attenuation lengths between transceiver and target.

Three-dimensional flower-like MoS2 strung by multi-wall carbon nanotubes: a versatile alternative for high performance optical limiters

Heterostructures based on transition-metal dichalcogenide layered materials show great potential for various applications in nonlinear photonic devices such as optical switches, mode-locking lasers, and optical limiters. Herein, we report the design and synthesis of a hierarchical heterostructure of multi-wall carbon nanotubes (MWCNTs) decorated by three-dimensional molybdenum disulfide (MoS2). The unique MoS2/MWCNT heterostructure was successfully synthesized by a simple one-pot hydrothermal method, as confirmed by field emission scanning and transmission electron microscopies, X-ray diffraction, Raman spectrum, and X-ray photoelectron spectroscopy. The nonlinear optical (NLO) and optical limiting (OL) responses of the heterostructured MoS2/MWCNT and those of its individual components were investigated by the Z-scan technique at 532-nm with nano- and picosecond pulsed-laser sources. The NLO and OL properties of the MoS2/MWCNT heterostructure were improved compared with those of MoS2 and MWCNTs individually. The OL threshold of the heterostructured MoS2/MWCNT was 0.53 J/cm2, which is lower than or comparable to those of either common transition-metal dichalcogenides or graphene-like compounds. The NLO mechanisms are attributed to nonlinear absorption and nonlinear refraction at a picosecond timescale combined with nonlinear scattering induced by the MWCNTs at a nanosecond timescale. The improvements in NLO and OL performance are also attributed to photo-induced interfacial charge transfer between MoS2 and MWCNTs in the unique heterostructured MoS2/MWCNT. We report an efficient method of fabricating novel heterostructures with controllable NLO response. The unique morphology and excellent NLO properties obtained from these MoS2/MWCNT heterostructures show great potential for future optical and photonic applications.

Fabrication of four-level hierarchical topographies through the combination of LIPSS and direct laser interference pattering on near-beta titanium alloy

Complex repetitive periodic surface patterns were produced on a near-beta Ti-13Nb-13Zr alloy, using two-beam Direct Laser Interference Patterning (DLIP) employing a picosecond-pulsed laser source with wavelengths of 355 nm, 532 nm and 1064 nm. Different types of Laser-induced periodic surface structures (LIPSS) are produced, including low and high spatial frequency LIPSS, which are observed frequently on top of the line-like DLIP microstructures, as well as quasi-periodic microstructures with periods greater than the laser wavelength. The feature size of the fabricated LIPSS features could be tuned as function of the utilized laser process parameters.

Thermal Effects in the Ablation of Bovine Cortical Bone with Pulsed Laser Sources.

Lasers have advantages as bone surgical tools over mechanical methods, but two goals should be achieved to assure its use: Similar ablation rates to those obtained with mechanical tools (1 mm3/s at least) and to avoid thermal damage, a condition that can prevent proper bone healing. We present results of cow femoral bone with a 355 nm nanosecond (ns) and a 1064 nm picosecond (ps) pulsed laser sources that allow us to discuss the influence on the process of pulse duration and the selective ablation through high energy absorption (as bone highly absorbs 355 nm radiation). The treated samples were characterized by scanning electron microscopy (SEM) and Raman spectroscopy. The evaluation of the thermal effects produced in the samples shows clear differences between both laser sources: On one hand, the ns laser allows reaching high ablation rates (around 1 mm3/s); Raman spectra show no signal of bone carbonization, but unavoidable thermal effects in the form of melted and solidified material have been observed by electron microscopy in the samples treated with this laser. On the other hand, ablation without any sign of thermal effects is obtained using the ps laser, but with lower ablation rates, (around 0.15 mm3/s).

Fabricating Laser-Induced Periodic Surface Structures on Medical Grade Cobalt–Chrome–Molybdenum: Tribological, Wetting and Leaching Properties

Hip-implants structured with anti-bacterial textures should show a low-friction coefficient and should not leach hazardous substances into the human body. The surface of a typical material used for hip-implants, namely Cobalt–Chrome–Molybdenum (CoCrMo) was textured with different types of laser-induced periodic surface structures (LIPSS)—i.e., low spatial frequency LIPSS (LSFL), hierarchical structures consisting of grooves superimposed with high spatial frequency LIPSS (HSFL) and Triangular shaped Nanopillars (TNP)—using a picosecond pulsed laser source. The effect of LIPSS on the wettability, friction, as well as wear of the structures, when slid against a polyethylene (PE) counter surface and biocompatibility was analyzed. Surfaces covered with LSFL show superhydrophobicity and grooves with superimposed HSFL, as well as TNP, show hydrophobic behavior. The coefficient of friction (CoF) of LIPSS against a polyethylene (PE) counter surface was found to be higher (ranging from 0.40 to 0.66) than the CoF of (polished) CoCrMo, which was found to equal 0.22. It was found that the samples release cobalt within biocompatible limits. Compared to polished reference surfaces, LIPSS cause higher friction of CoCrMo against PE contact. However, the wear of the PE counter surface only increased significantly for the LSFL textures. For these reasons, it is concluded that LIPSS are not suitable for a heavily loaded metal-on-plastic bearing contact.

Picosecond Pulsed Laser Ablation of Liquid Covered Stainless Steel: Ef-fect of Liquid Layer Thickness on Ablation Efficiency

Under liquid laser ablation is a material removal technique in which a focused laser beam passes through a liquid layer on top of the surface of a sample to be processed. When compared to laser ablation without a liquid layer, material (re)deposition around ablated regions, as well as the size of the Heat Affected Zone (HAZ) is decreased. In addition, the ablation efficiency of the process, in terms of the amount of material removed per pulse, can be optimized by careful variation of the height of the liquid layer: a liquid layer height variation as small as a few tens of millimeters already has a measurable effect on the amount of ablated material. In studies reported in existing literature, the required liquid layer height is typically realized by pouring a pre-defined amount of liquid on top of the sample surface. Surface tension, however, causes the air-liquid interface at the boundaries of the domain to deviate from the planar interface away from the boundaries, which affects the accuracy with which the liquid layer height can be determined. To the best of our knowledge, these accuracy issues have not been studied in previous research. Therefore, an experimental set-up is proposed which circumvents the issues of a curved free surface. Next, a 7 picosecond pulsed laser source (M2≤1.3) at a wavelength of 515nm was employed to study the efficiency of laser ablation of stainless steel for a range of liquid layer heights. Our findings provide a more detailed quantification of crater depth as a function of liquid layer height than is available through existing literature.

Study on fabrication of PCD micro-milling tool by picosecond pulsed laser

For the purpose of processing some forms of diamond, such as polycrystalline diamond (PCD) and chemical vapor deposition diamond, picosecond pulsed laser (PSPL) has the decisive advantage of wear-free material removal. This availability of picosecond pulsed laser sources enables the efficient applications of cutting diamond to generate sharp and regular cutting edge. Therefore, in this study, picosecond pulsed laser is used to fabricate polycrystalline diamond (PCD) micro tool. In order to optimize the picosecond pulsed laser processing, the effects of PSPL cutting parameters including the pulse energy, pulse pitch, and grain size of PCD on the cutting results are investigated. The results indicate the grain size has the greatest impact on the cutting edge radius and pulse energy has great effect on the processed surface of PCD. The results prove that PSPL is a better choice for precision fabrication of PCD micro tool. Finally, the fabrication of PCD micro tools by PSPL has been demonstrated.

Printing of complex free-standing microstructures via laser-induced forward transfer (LIFT) of pure metal thin films

A combined approach of laser-induced forward transfer (LIFT) and chemical etching of pure metal films is studied to fabricate complex, free-standing, 3-dimensional gold structures on the few micron scale. A picosecond pulsed laser source with 515 nm central wavelength is used to deposit metal droplets of copper and gold in a sequential fashion. After transfer, chemical etching in ferric chloride completely removes the mechanical Cu support leaving a final free-standing gold structure. Unprecedented feature sizes of smaller than 10 μm are achieved with surface roughness of 0.3 to 0.7 μm. Formation of interfacial mixing volumes between the two metals is not found confirming the viability of the approach.

Laser-Induced Periodic Surface Structures (LIPSS) on Polymers Processed with Picosecond Laser Pulses

Based on a literature review, it was concluded that Laser-induced Periodic Surface Structures (LIPSS) on polymers are produced when applying laser sources operating either in the ultraviolet wavelength and nanosecond pulse duration, or radiation of wavelengths ranging from 265nm to 1045nm and pulse durations in the femtosecond regime. LIPSS were not reported when using picosecond laser sources. The purpose of this paper is to study whether (and if so which) LIPSS form on polymers when picosecond pulsed laser source is applied. Low Spatial Frequency LIPSS (LSFL) and High Spatial Frequency LIPSS (HSFL) have been obtained on polycarbonate and on polystyrene when applying picosecond laser pulses at a wavelength of 343nm on single spots and on processed lines. When using a wavelength of 515nm, LSFL and HSFL have been produced only on polycarbonate, but also led to porousness of the structured area.

Highly flexible short-pulse generation and high sensitivity sensing with stimulated Brillouin scattering in optical fibers

We demonstrate two novel types of applications using the same generator as a source for Brillouin multi-wavelength generation. The first is as a highly flexible picosecond pulsed laser source which is tunable in time duration, repetition rate and emission wavelength for optical clock applications in telecommunication. The second application is a high sensitivity distributed Brillouin sensors (DTS) to lower costs and widens the market sector. We demonstrate tunability of the pulsed laser source from ~15 ps down to ~3.5 ps over the whole telecommunications C-band by simply controlling the number of Stokes waves being generated forming a phase-locked Brillouin frequency comb. The repetition rate is the Brillouin frequency shift of ~10 GHz which can be tuned by changing the gain fiber within the cavity. An increase in the standard temperature sensitivity of DTS of ~1.3 MHz/°C by 6 fold is also demonstrated. This increase is of great importance in DTS, since the detection of any variation can be made faster, which can enhance the functionality of such sensors.

Coherent Laser Source for High Frame-Rate Optical Time-Stretch Microscopy at 1.0 μm

We demonstrate a coherent picosecond pulsed fiber laser for the high frame-rate optical time-stretch microscopy at 1.0 μm. The spectrum of a picosecond pulsed laser is commonly broadened before the time-stretch imaging, which however will degrade its stability and coherence. As a result, it is required to enhance the degraded signal-to-noise ratio by averaging, which would compromise the frame rate on the other hand. Instead of pursuing such kind of spectrum-broadened picosecond pulsed laser sources, we propose a pulse train extracted directly from an all-normal dispersion mode-locked fiber laser with a rectangle-shaped optical spectrum. It delivers stable and coherent performance for the serial time-encoded amplified microscopy at 1.0 μm. With this robust picosecond pulsed laser, real-time stain-free flow imaging with a frame rate of 26.25 MHz and a spatial resolution of <; 2 μm is demonstrated. Featured with the compact configuration and good coherence property, it is a promising picosecond pulsed fiber laser source for the ultrafast interferometric time-stretch microscopy at 1.0 μm.

Thermal laser micro-adjustment using picosecond pulse durations

Thermal laser micro forming (LμF) is a non-contact process for the precise adjustment of micro-scale metallic components. Micro-electronic systems often comprise functional components which require highly accurate micro-scale adjustment after fabrication. Such functional components are typically difficult to access and highly sensitive to mechanical force. The application of lasers offers the potential for controlled and repeatable micro-adjustment of these components in a contact free process.The use of a picosecond pulsed laser source reduces the heat diffusion depth when compared to longer pulse durations, resulting in less thermal loading of the bulk substrate. When combined with high repetition rates, localized heat accumulation in micro-scale components can be achieved and utilized in a thermal forming process.In this work, the micro-adjustment of 1000μm×300μm stainless steel actuator arms using a mode locked fibre laser with a maximum pulse energy and duration of 3μJ and 20ps respectively was conducted. The effect of pulse overlap, laser power, irradiation strategy and number of irradiations on the net bend angle is presented.

Picosecond Laser Machining of Metallic and Polymer Substrates for Fluidic Driven Self-Alignment

Fluidic self-alignment of micro-components relies on creating a receptor site that is able to confine a liquid droplet. When a micro-component is brought in contact with the droplet, capillary forces move the component to its final position. A method to stop the advancing of a liquid from a receptor site, consists of creating geometrical features, such as edges around the site. A picosecond pulsed laser source was used to create suitable edges in a metallic and a polyimide substrate. Subsequently, the self-alignment capabilities of these sites were tested. The receptor sites in polyimide showed the highest success rate.

Refractive Third-order Nonlinearity in Vanadium-oxide Phthalocyanine Micro-crystals

We report a study on the resonant third-order nonlinear optical response of vanadyl-phthalocyanine micro-crystals. The refractive and absorptive contributions to the nonlinear response at several wavelengths across resonance were determined by employing the z-scan technique with a tunable picosecond pulsed laser source. The samples studied were cyclohexane suspensions of vanadium-oxide phthalocyanine micro-crystals 40 nm in diameter. These micro-crystals represent an interesting alternative for the production of solid state samples of good optical quality through relatively simple techniques. Experimental results show that for wavelengths above 610 nm, the sample presents saturation of absorption, while for shorter wavelengths, induced absorption is found. Around 610 nm however, the absorptive contribution to the nonlinearity vanishes while a finite refractive contribution remains. This wavelength region is therefore potentially useful for all-optical switching applications.

A new design for an ultrahigh vacuum surface analysis chamber with optical scatter measurement capability

An ultrahigh vacuum (UHV) surface analysis chamber coupled with a visible light-scattering apparatus is described. This combination of UHV surface analysis capabilities and bidirectional reflectance distribution function measurements permits investigation of effects of various surface treatments such as gas adsorption and ion bombardment on optical scatter. Stray light reflections from the UHV windows and components are temporally rejected by use of a picosecond pulsed laser source and synchronized, time-gated detector. This system allows measurement of optical scatter levels in the range of 10−7 sr−1.