Pulsed Laser Beam Research Articles

Nanoscale local modification of PMMA refractive index by tip-enhanced femtosecond pulsed laser irradiation

Tip-enhanced optical effects have enabled a wide palette of nanoscale investigation techniques and recently they were successfully used to modify and manipulate matter with nanoscale spatial resolution. We contribute to this latter area by reporting a viable method to modify the refractive index of polymethyl methacrylate (PMMA). Local surface changes of the refractive index are accomplished by focusing a femtosecond pulsed near-infrared laser beam on the apex of a metalized nanosized tip, traditionally used in scanning probe microscopy (SPM). The refractive index is changed over a circular area with diameter <200 nm under 790 nm illumination, representing a four-fold increase in resolution compared to the current state-of-the-art. These results are obtained at low laser pulse energies (<0.5 nJ), which represents a two orders of magnitude advantage over previously reported methods. Besides enabling modifications of the refractive index with high lateral resolution, this method can pave the way towards other important applications such the fabrication of photonic crystal lattices and surface waveguides.

Terahertz shaping technology based on coherent beam combining

The generation of terahertz (THz) waves by focusing a femtosecond pulsed laser beam at a distance is able to overcome the strong absorption properties of air and has rapidly attracted the attention of industry. However, the poor directionality of the THz wave radiation generated by this method is not conducive to THz wave applications. By controlling the morphology of the ultrafast laser-excited plasma filament and its electron density distribution through coherent beam combining technology, we achieve direct THz beam shaping and are able to obtain THz wave radiation of Gaussian or arbitrary transverse distribution. The novel experimental approach proposed in this paper opens up the research field of direct THz wave shaping using plasma. Moreover, it innovates multi-parameter convergence algorithms and, by doing so, has the potential to find beam patterns with higher energy conversion efficiency and break the energy limit of THz waves emitted by lasers at high power.

Characterization of a Continuous Beam Cold Atom Ramsey Interferometer

The use of atom interferometers in inertial systems holds the promise of improvement of several orders of magnitude in sensitivity over sensors using current technology such as micro-electro-mechanical (MEMS) devices or ring laser gyroscopes (RLGs). This paper describes the construction and characterization of an atomic interferometry system for eventual use in a dual-atom-beam accelerometer/gyroscope sensor. In contrast with current state-of-the-art atomic sensors which use pulsed cold atom sources and pulsed laser beams, the investigated apparatus relies purely on continuous atomic and laser beams. These differences can result in a sensor with reduced complexity, a smaller physical footprint, and reduced power consumption. However, these differences also introduce challenges resulting from laser and atomic beam divergences and from velocity averaging due to both longitudinal and transverse velocity spreads. In this work, we characterize our rubidium-based atom beam system and show that Ramsey-style interference can still be observed. The implications for future research are also outlined and discussed.

All-optical, non-contact, local measurement of both Young’s modulus and Poisson’s ratio in metals using a combination of Rayleigh and leaky surface acoustic waves

In non-destructive evaluation (NDE), measuring ultrasound (US) longitudinal and shear wave speeds is the main method to determine two independent mechanical moduli (Young’s modulus and Poisson’s ratio). Most US techniques use time-of-flight measurements. However, when the local sample thickness is unknown or the sample geometry is complex, bulk-wave propagation speeds cannot be accurately defined. Here we show that a properly shaped beam of nanosecond laser pulses can be used to efficiently excite (without material ablation) two types of surface acoustic waves. In addition to a conventional surface, or Rayleigh, wave, a leaky surface wave (LSAW) can be launched in the near field of the excitation region. We present the theoretical background, numerical simulations, and experimental results clearly showing that both elastic constants can be reconstructed locally by tracking Rayleigh and LSAW waves. Spatially resolved elastic properties can be obtained using local values of wave speeds obtained by sample scanning. Non-contact optical detection of propagating waves at the sample surface, in which a fiber-optic Sagnac interferometer was used, is a key piece of the method. It does not require acoustic coupling and allows remote measurements in the near field of the laser source with micron-scale resolution.

Influence of laser parameters on corrosion resistance of laser melting layer on C45E4 steel surface

The key to further improve the corrosion resistance of the laser melting layer on steel surface is to select appropriate laser parameters. In this paper, a single factor laser melting experiment on C45E4 steel surface by using a pulsed laser beam at a wavelength of 1064 nm and electrochemical analysis were conducted to explore the influences of laser parameters including single pulse energy density, spot overlap ratio and number of laser scans on the corrosion resistance of the laser melting layers. The optimal laser parameter combination was obtained: single-pulse energy density of 3.82 J·cm−2, spot overlap rate of 80 % and 4 number of laser scans. The thickness of the optimal laser melting layer prepared with the optimal laser parameters was only 10.85 μm, and it had a maximum self-corrosion potential of 1.031 V, a minimum corrosion current of 3.451 × 10−9 A·cm−2 and a maximum charge transfer resistance of 1541.2 Ω·cm−2. During the salt spray corrosion test, the corrosion rate of the optimal laser melting layer was only one third of that of the traditional hot alkaline blackened layer with a thickness of 12.04 μm. The results showed that the corrosion resistance of the optimal laser melted layer was significantly improved compared to that of the conventional thermally alkaline blackened layer. The main reason was that the determination of the optimal single-pulse energy density, spot overlap ratio and number of laser scans in turn, made the optimal laser melting layer had a stable Fe3O4-FeO corrosion resistance structure, effectively reduced the surface roughness, microcrack density and oxidation leakage point, and prevented the C45E4 steel surface from being over-oxidized.

Transfer-Printed Environmental-Friendly Anisotropic Filter with Laser-Controlled Micropores for Efficient Oil/Water Separation

Water is indispensable for sustaining life on Earth. Oil–water mixtures/or emulsions from industrial waste and other sources are a serious environmental concern for both human beings and aquatic life. Specially treated meshes and textiles with opposing wettability for oil–water separation have been widely reported as a solution to this challenge. Nonetheless, such membranes are hindered by certain drawbacks, including high manufacturing costs, usage of harmful chemicals, and lack of diverse applicability. Here, we report a facile method to fabricate Janus oil–water separation membrane with a controllable pore structure that has a unique directional flux rate. The superhydrophobic (SHB) layer of the membrane is formed by transfer-printing (TP) carbon soot particles onto a polydimethylsiloxane (PDMS)-coated paper surface. Meanwhile, a spin-coated thin layer of chitosan on the other side of the film served as a hydrophilic (SHL) and underwater oleophobic face. A pulsed laser beam is used to produce micropores with conical structures. The separation ability of the membrane for both light oil–water and heavy oil–water mixtures is thoroughly investigated. Moreover, the significance of the pore shape and the size is also elucidated. The flexible Janus membrane showed high thermal stability and ideal (i.e., 99.8%) separation efficiency. The membrane can be produced over a 151 cm2 size range. Besides having flexibility and superior performance, the fabricated membrane is environmentally friendly and economically viable. This work establishes a scalable basis for efficient and low-cost oil/water membranes from non-porous substrates.

Machine-learning-assisted analysis of transition metal dichalcogenide thin-film growth

In situ reflective high-energy electron diffraction (RHEED) is widely used to monitor the surface crystalline state during thin-film growth by molecular beam epitaxy (MBE) and pulsed laser deposition. With the recent development of machine learning (ML), ML-assisted analysis of RHEED videos aids in interpreting the complete RHEED data of oxide thin films. The quantitative analysis of RHEED data allows us to characterize and categorize the growth modes step by step, and extract hidden knowledge of the epitaxial film growth process. In this study, we employed the ML-assisted RHEED analysis method to investigate the growth of 2D thin films of transition metal dichalcogenides (ReSe2) on graphene substrates by MBE. Principal component analysis (PCA) and K-means clustering were used to separate statistically important patterns and visualize the trend of pattern evolution without any notable loss of information. Using the modified PCA, we could monitor the diffraction intensity of solely the ReSe2 layers by filtering out the substrate contribution. These findings demonstrate that ML analysis can be successfully employed to examine and understand the film-growth dynamics of 2D materials. Further, the ML-based method can pave the way for the development of advanced real-time monitoring and autonomous material synthesis techniques.Graphical

Single-event effects calibration using two-photon absorption and a CMOS image sensor

This paper proposes a method to get an indication of the amount of electron-hole pairs that are necessary to trigger a single-event effect (SEE) in electronics by using two-photon absorption (TPA) and a CMOS image sensor. One of the most important materials in electronics is silicon, which is also the photosensitive part of CMOS image sensors. TPA can occur in it when two photons with an energy lower than the bandgap energy interact with the same electron within a short time window. This can be done by focusing a pulsed laser beam on a sensitive node in the electronics, generating a cloud of electron-hole pairs and thus simulating a SEE. We propose to first measure the necessary laser energy to generate a SEE with TPA in the electronics, to then focus the laser beam with the same laser energy in the CMOS image sensor. With the CMOS image sensor, we will be able to tell how many electrons were detected, which gives a good indication of how many electron-hole pairs were needed for the SEE. This paper first describes TPA, then characterises different CMOS image sensors to finally compare those results with heavy-ions.

Highly Flexible Infrared Emitter with Spatially Controlled Emissivity for Optical Security (Adv. Mater. Technol. 4/2023)

Infrared Emitters In article number 2200808, Myeongkyu Lee and co-workers present a Ge2Sb2Te5 (GST)- based infrared emitter that is fabricated on a flexible substrate such as plastic and paper, and its emissivity is space-selectively controlled using a spatially modulated pulsed laser beam. The multispectral properties of GST enable different visible and thermal images to be simultaneously printed in the same area of the emitter. This provides a promising feature for optical security, including anti-forgery.

Mathematical Modeling of Melting Tungsten Exposed to Pulsed Laser Beam

Mathematical Modeling of Melting Tungsten Exposed to Pulsed Laser Beam

General theory of light propagation and triplet generation for studies of spin dynamics and triplet dynamic nuclear polarisation

A general theory is presented describing the photo-excitation dynamics of transient, paramagnetic triplet states of aromatic molecules including their absorption properties in a molecular host crystal when irradiated by a pulsed laser beam. It is applied to the system of a pentacene doped naphthalene single crystal, where the photo-excitation of pentacene results in a highly non-equilibrium population of intermediate electronic triplet states. These paramagnetic states can be addressed to align the proton spins of the naphthalene host at moderate experimental conditions using dynamic nuclear polarisation (DNP). The photo-excitation profile and the resulting triplet state distribution, which is determined by the material and the laser parameters, leads to a corresponding proton polarisation profile. Measurements of the resulting polarisation inhomogeneity by spatially resolved neutron transmission confirm the theory. The theoretical model then allows the extraction of crucial parameters that enable quantitative investigations of the dynamics of spin systems. For applications of triplet DNP with pentacene-doped naphthalene crystals, polarisation maximum, homogeneity and build-up rate are key factors, and the theory allows the identification of optimal experimental parameters depending on the priorities placed on these factors.

Asymmetry of keyhole and weld pool geometry in PLBW of tailor-welded steel sheets with edge misalignment: Numerical modeling and experimental validation

Edge misalignment in butt welded joints is difficult to avoid due to factors such as poor processing accuracy, the presence of assembly tolerances, and welding distortion. It reduces the performance of the joints, which is inextricably associated with molten pool behaviors during the welding process. The purpose of this paper was to investigate the effects of the edge misalignment on weld pool dynamic behaviors by developing a multi-phase CFD model to reproduce the pulsed laser beam welding (PLBW) on a couple of low-carbon steel plates with a thickness of 1.8 mm, which had been verified with experimental weld pool profiles. Both experiments and numerical simulations took into account different scales of edge misalignment. The results demonstrated that the keyhole quickly penetrated the workpiece and remained quasi-steady with the continuous emission before shutting down when the laser intensity was dropped. The flow patterns within the weld pool were characterized by a vertical downward flow near the keyhole, Marangoni flows on both the top and root surfaces, together with a flow from the higher-assembled side to the lower. Those were driven by recoil pressure, surface tension gradient, and gravity, respectively. Moreover, both the weld pool and the keyhole showed a clear inclination toward the assembled lower side, as measured by positive slopes with values ranging from 5 to 25. When the sheet edges became more misaligned, the size of the weld pool shrunk because less laser energy reached the surface of the workpiece. At the termination of the third pulse cycle, the maximum weld width at the top surface (TW) and root surface (RW), as well as the penetration depth, were reduced by 5.281 %, 25.495 %, and 2.303 %, respectively. A larger scale of weld misalignment contributed to a more inclined weld pool, marked by lower slope values.

A new precursor route for the growth of NbO2 thin films by chemical vapor deposition

Niobium dioxide (NbO2) exhibits metal-insulator transition (Mott transition) and shows the potential for application in memristors and neuromorphic devices. Presently growth of NbO2 thin films requires high-temperature reduction of Nb2O5 films using H2 or sophisticated techniques such as molecular beam epitaxy and pulsed laser deposition. The present study demonstrates a simple chemical route of the direct growth of crystalline NbO2 films by chemical vapor deposition using a freshly prepared Nb-hexadecylamine (Nb-HDA) complex. X-ray diffraction studies confirm the NbO2 phase with a distorted rutile body-centered-tetragonal structure and the film grown with a highly preferred orientation on c-sapphire. X-ray photoelectron spectroscopy confirms the +4 oxidation state. The present method offers facile growth of NbO2 films without post-reduction steps which will be assumed to be a cost-effective process for NbO2 based devices.

Influence of pulsed laser beam welding in vacuum on the mechanical properties of non-grain oriented electrical steel sheets

Abstract Major components of electrical machines are iron cores made of stacked electrical steel sheets. In this paper, pulsed laser beam welding in vacuum is used to join the individual electrical steel sheets. Three different non-grain-oriented electrical steel alloys are investigated and joined by individual spot welds. The influence of pulsed laser beam welding in atmosphere as well as in vacuum on the welding result and the tensile strength of the welded joint is discussed. The results show that the insulation coating of the electrical sheets has significant effects on the grain structure as well as the hardness of the weld spots. These effects can be reduced with decreased working pressure, which reduces the power loss of iron cores. The reduction of the working pressure and the associated reduction of the vapor plume, which appears to reduce the energy input into the component depending on the material and coating, results in an increase of the melt volume and the joint cross sectional area. The tensile strength of the performed welds was tested and interpreted with respect to the influencing process parameters.

Design of an ultrafast pulsed ponderomotive phase plate for cryo-electron tomography

Ponderomotive phase plates have shown that temporally consistent phase contrast is possible within electron microscopes via high-fluence static laser modes resonating in Fabry-Perot cavities. Here, we explore using pulsed laser beams as an alternative method of generating high fluences. We find through forward-stepping finite element models that picosecond or shorter interactions are required for meaningful fluences and phase shifts, with higher pulse energies and smaller beam waists leading to predicted higher fluences. An additional model based on quasi-classical assumptions is used to discover the shape of the phase plate by incorporating the oscillatory nature of the electric field. From these results, we find the transient nature of the laser pulses removes the influence of Kapitza-Dirac diffraction patterns that appear in the static resonator cases. We conclude by predicting that a total laser pulse energy of 8.7μJ is enough to induce the required π/2 phase shift for Zernike-like phase microscopy.

Ultralarge suspended and perforated graphene membranes for cell culture applications.

With its high mechanical strength and its remarkable thermal and electrical properties, suspended graphene has long been expected to find revolutionary applications in optoelectronics or as a membrane in nano-devices. However, the lack of efficient transfer and patterning processes still limits its potential. In this work, we report an optimized anthracene-based transfer process to suspend few layers of graphene (1-, 2- and 4-layers) in the millimeter range (up to 3 mm) with high reproducibility. We have explored the advantages and limitations for patterning of these membranes with micrometer-resolution by focused ion beam (FIB) and picosecond pulsed laser ablation techniques. The FIB approach offers higher patterning resolution but suffers from the low throughput. We demonstrate that cold laser ablation is a fast and flexible method for micro-structuring of suspended graphene. One promising field of application of ultimately thin, microporous graphene membranes is their use as next-generation cell culture supports as alternative to track-etched polymer membranes, which often exhibit poor permeability and limited cell-to-cell communication across the membranes. To this end, we confirmed good adhesion and high viability of placental trophoblast cells cultivated on suspended porous graphene membranes without rupturing of the membranes. Overall, there is high potential for the further development of ultrathin suspended graphene membranes for many future applications, including their use for biobarrier cell culture models to enable predictive transport and toxicity assessment of drugs, environmental pollutants, and nanoparticles.

Photonic crystal cavity-mediated improved absorptive nonlinearity of C-4-hydroxy-3-methoxphenilcalix[4]resorcinarene.

The insertion of a nonlinear material at the photonic crystal cavity leads to a strong interaction between localized photonic modes and the introduced material's electronic state, leading to exceptionally improved nonlinear optical properties at low input power. We report the enhanced nonlinear absorption and optical limiting properties of C-4-hydroxy-3-methoxphenilcalix[4]resorcinarene (CHMPCR) in a one-dimensional polymeric photonic crystal cavity. Open aperture z-scan measurements with a nanosecond pulsed laser beam having a repetition rate of 10 Hz at 532 nm from a polymeric-based microcavity demonstrate a 4.5 times enhancement in the nonlinear absorption coefficient of CHMPCR relative to its reference film. The phenomenal increase in nonlinear response could be due to the enhanced local field-assisted effective two-photon absorption arising from the interaction of CHMPCR with intensified confined light emerging from the resonant excitation at the cavity. The low optical limiting threshold and enhanced nonlinear absorption of the CHMPCR-based photonic crystal cavity make it a promising candidate for realizing cost-effective optoelectronic devices.

Method to obtain 3D point clouds of tunnels using smartphone LiDAR and comparison to photogrammetry

Remote sensing methods, for example, photogrammetry and laser scanning can be employed to scan rock masses by digitizing underground spaces or slopes. To reconstruct a 3D model by photogrammetry, plenty of photos should be captured. This process is time-consuming and can be dangerous when the rock mass is unstable. Therefore, rapid capturing methods are needed to reduce the acquisition time. This can be achieved with LiDAR scanners that capture a 3D point cloud with a high-speed pulsed laser beam. However, the cost of laser scanners is high, which limits their usability. Apple’s iPhone 12 Pro Max smartphone is equipped with a LiDAR sensor and is much cheaper than conventional laser scanners. Several mobile applications for 3D models using the smartphone Lidar have been developed, making the scanning process easy. Therefore, smartphones can be used for rapid scanning of unstable surfaces of tunnels and slopes. In this study, 3D point clouds (3DPC) of a tunnel wall of the Underground Research Laboratory of Aalto University (URLA) are obtained by a high-resolution digital camera and smartphone LiDAR. The models are analyzed and compared. The results show the quality of the smartphone LiDAR sensor is adequate for generating 3D models of underground spaces.

Laser-printed emissive metasurface as an optical security platform

Optical security is a promising application of metasurfaces because light has large degrees of freedom in metasurfaces. Although many different structures/materials have been proposed for this purpose, the fabrication of dynamic metasurfaces in a straightforward and scalable manner while maintaining a high security level remains a significant challenge. Herein, a metasurface consisting of a phase-changing Ge2Sb2Te5 (GST) layer and a thin metal back reflector is presented to space-selectively and dynamically control the infrared emission of the surface by a spatially modulated pulsed laser beam. Unlike conventional laser processes using a focused beam, the employed laser printing is an expanded beam-based parallel process that enables the fabrication of wafer-sized emission patterns. Owing to the multispectral responses of GST, mutually independent visible and infrared images can be printed in one region. Grayscale emission patterns can also be obtained by gradually modulating the spatial profile of the laser beam, which makes the replication of laser-printed emission patterns extremely difficult. We also demonstrate that colors images can be obtained by depositing IR lossless layer on the GST surface. All these features indicate that the presented emissive metasurface has the potential for use as an effective platform for anti-counterfeiting.

Effect of welding heat input conditions on the dynamic behavior of pulse laser beam welding molten pool for Ti6Al4V thin plate with clearance

Effect of welding heat input conditions on the dynamic behavior of pulse laser beam welding molten pool for Ti6Al4V thin plate with clearance