Laser Focal Spot Research Articles

Measurement of thermal conductivity in laser-heated diamond anvil cell using radial temperature distribution

ABSTRACT Thermal conductivities of planetary materials under extreme conditions are important input parameters for modeling planetary dynamics such as accretion, geodynamo and magnetic field evolution, plate tectonics, volcanism-related processes etc. However, direct experimental measurements of thermal conductivity at extreme conditions remain challenging and controversial. Here we propose a new technique of thermal conductivity measurement in laser-heated diamond anvil cell (LH-DAC) based on radial temperature distribution around laser focal spot, mapped by imaging tandem acousto-optical tunable filter (TAOTF). The new technique provides much more information about heat fluxes in the laser-heated sample than existing static heating setups, and does not require dynamic numerical modeling using heat capacities in contrast to dynamic pulsed heating setups. In the test experiment, thermal conductivity of γ-Fe at conditions relevant to cores of terrestrial planets was measured.

Laser shock adhesion test numerical optimization for composite bonding assessment

The present work presents the latest development of laser shock adhesion test (LASAT) technology, targeting the weak bond detection in bonded aeronautic structures. This problematic is still holding back a wider use of bonding, which could however be a significant breakthrough in the way of assembling parts. By mechanically loading the bondline thanks to laser-induced shock waves, LASAT acts as proof test to reveal the presence of local weaknesses. In the present paper, focus is made on the optimization of the laser shock parameters regarding the assembly to test. Objective is to avoid loading too much the composite, thus avoiding damage, to increase the test performances. Numerical modelling is used, following a specific methodology, to understand the phenomena and identify the key parameters.The basic laser shock configuration was first investigated. Due to the stress distribution, this setting allows one to test a bond whose strength is equal or below 40% of composite inter-laminar strength. The effects of the laser focal spot on the stress distribution are also quantified. A 4 mm diameter shows good performances for the assembly to test. For the first time, three different optimizations are proposed: tunable pulse duration, double pulses on the front face and symmetrical laser shocks. They are first theoretically described. Numerical results then support these configurations’ performances. The double pulse solution makes it possible to test a bond strength equal or inferior to 80% of composite inter-laminar strength, when symmetrical pulses enable to reach 100% thanks to a sharp stress distribution. These results are validated by experimental evidence that is also presented. Finally, the present work offers helpful information for the development and deployment of LASAT for aeronautic bonded structures.

Microphotoelectrochemical Surface-Enhanced Raman Spectroscopy: Toward Bridging Hot-Electron Transfer with a Molecular Reaction.

The rational design and applications of plasmon-mediated chemical reactions (PMCRs) are fundamentally determined by an understanding of photon-electron-molecule interactions. However, the current understanding of the PMCR of plasmon-decayed hot electron-mediated reactions remains implicit, since there has not been a single measurement of both hot-electron transfer and molecular transformation following photon excitation. Therefore, we invented a method called microphotoelectrochemical surface-enhanced Raman spectroscopy (μPEC-SERS) that uses an ultramicroelectrode (UME) whose dimensions match those of the focused laser spot. This system can simultaneously record the photocurrent (∼picoamps) of hot-electron transfer with a high signal-to-noise ratio and the SERS spectra of a molecular reaction in the same electrode area. The responses of the photocurrent and SERS spectra to laser illumination can correlate the surface reaction activated by hot electrons with the SERS spectral changes. A typical PMCR ofp-aminothiophenol (PATP) on a Ag UME was used to illustrate that the correlation of the photocurrent with the spectral changes is capable of revealing the reaction mechanism in terms of the formation of activated oxygenated species. The laser power-, laser wavelength-, and surface roughness-dependent photocurrents link the formation of activated oxygenated species to the hot-electron transfer. Further comparisons of the photocurrent with the conventional electrochemical current of the oxygen reduction reaction indicate that the activated oxygenated species are oxidative in transforming PATP to p,p'-dimercaptoazobenzene, which is supported by a density functional theory (DFT) calculation. Therefore, μPEC-SERS could be a powerful tool for investigating PMCRs and other systems involving photon-electron-molecule interactions.

Laser-based surface pre-treatment for metal-plastic hybrids using a new process strategy

Abstract In order to join metal-plastic hybrid parts so as to achieve lightweight constructions, innovative joining technologies are essential. For this, thermal joining is a promising approach due to the absence of any joining agent. The joint strength can be increased by surface pre-treatment of the metal with continuous wave laser radiation. Large joining areas, however, are challenging in terms of productivity, efficiency and geometrical flexibility when using conventional beam trajectories. A more promising approach is the use of continuous beam trajectories, which can be easily combined with on-the-fly processing. More precisely, a circularly oscillating laser focal spot is geometrically independent of every possible in-plane load direction, and causes lower thermal distortion of the metal sheet than using conventional beam trajectories. Therefore, the circular beam oscillation was characterized for the processing of large areas. In this context, the resulting topography of the metal was analyzed with respect to the surface roughness and a design criterion was mathematically derived and experimentally validated. Additionally, the joint strength was compared to the strength resulting from conventional beam trajectories. For verification, steel substrates were joined with a carbon-fiber-reinforced thermoset. The laser-based pre-treatment using the circularly oscillating laser focal spot led to a similar joint strength, while being more productive when combined with on-the-fly processing, compared to conventional structuring trajectories. Consequently, the processing time can be significantly reduced.

Mapping photocathode quantum efficiency with ghost imaging

Measuring the quantum efficiency (QE) map of a photocathode injector typically requires laser scanning, an invasive operation that involves modifying the injector laser focus and rastering the focused laser spot across the photocathode surface. Raster scanning interrupts normal operation and takes considerable time to setup. In this paper, we demonstrate a novel method of measuring the QE map using a ghost imaging framework that correlates the injector laser spatial variation over time with the total charge yield. Ghost imaging enables passive, real-time monitoring of the QE map without manually modifying the injector laser or interrupting injector operation. We first demonstrate the method at the UCLA Pegasus photoinjector with the help of a digital micromirror device (DMD) and a piezoelectric mirror to increase our control of the overall transverse variance of the illumination profile. The reconstruction algorithm parameters are fine-tuned using simulations and the results are validated against the ground truth map acquired using the traditional rastering method. Finally, we apply the technique to data acquired parasitically from the LCLS photoinjector, showing the feasibility of this method to retrieve a QE map without interrupting normal operation.

Active Feedback 3D Single-Molecule Tracking

Single molecule tracking has the potential to revolutionize the study of biological systems. However, most current single molecule imaging and tracking methods are limited to two dimensions. Meanwhile, real-time 3D single molecule tracking methods, which use active feedback to "lock-on" to single molecules in solution have until now been limited to observation times of 500 msec or less, greatly limiting their utility. Recently we developed a real-time 3D single particle tracking method known as 3D Dynamic Photon Localization Tracking (3D-DyPLoT). 3D-DyPLoT dynamically scans a focused laser spot in 3D with a tunable acoustic gradient lens and a 2D electro-optic deflector and has demonstrated real-time tracking at high diffusive speeds (up to 20 µm2/s) and low photon count rates 10 kHz. Here we demonstrate that the high tracking speed, sensitivity, and robustness of 3D-DyPLoT make it ideally suited active feedback 3D single molecule tracking. By implementing optimized tracking parameters based on single molecule model simulation, 3D-DyPLoT can continuously track single fluorescent dye molecules for several minutes at a time, two orders of magnitudes longer than previously reported methods. We demonstrate the application of this new single molecule tracking microscopy to track the free, 3D diffusion of DNA, RNA and proteins in solution. With two-color observation channels, 3D-DyPLoT was also applied to observing the transcription of single DNA molecules in solution by monitoring the real-time production of mRNA. This real-time 3D single molecule tracking method promises to be powerful tool for capturing the dynamics of single biomolecules at high speeds and over 3D distances. (Supported by NIH R35GM124868 and NSF 1847899).

Modeling and analysis for the position and posture errors of laser focal spot in large-scale fabrication via two photon polymerization

The position and posture errors of laser focal spot significantly affect the fabrication quality and functionality of microstructures in two photon polymerization, especially over a large area. These errors can be greatly reduced by using ultra-precision positioning device, such as piezoelectric stage and scanning galvanometer. However, the position and posture errors cannot be neglected in large-scale fabrication using large stroke linear stages which are necessary tools in most instances. In the present work, the relationship between the geometric errors of large stroke positioning platform and the position and posture of laser focal spot were revealed and modeled by homogeneous transformation matrix. Key errors affecting on the position and posture of focal spot were found out by sensitivity analysis and compensated during the construction of machine tool. Proof-of-concept experiments were performed in a millimeter-level (1 mm × 1 mm) fabrication of microfluidic whose surface was measured by an optical profiler. The measuring results showed that arithmetical mean height (Sa) and root mean square height (Sq) of the measured surface were less than 100 nm and maximum height (Sz) of the measured surface was less than 500 nm. The compensated machine tool demonstrated a high precise performance within a large continuous area. This work was expected to provide references for the rational layout of optical components in optical system or error compensation in laser process equipment over a large area.

Water-based helical laser hole-cutting in nickel super-alloy GH4049 assisted by longitudinal and transverse magnetic fields with/without ultrasonic assistance

A novel helical laser hole-cutting (HLHC) technology based on coupled assistance of water-based ultrasonic vibrations and external longitudinal/transverse magnetic fields is reported for cutting holes in nickel super-alloy GH4049 sheets. Effects of external unidirectional transverse and longitudinal magnetic fields on HLHC performance were correspondingly analyzed and compared with and without water-based ultrasonic assistance. The influence of the loading direction of an external magnetic field and the magnetic field intensity of an external longitudinal/transverse magnetic field on HLHC performance is first reported based on systematic corresponding comparisons with and without water-based ultrasonic assistance. The effects of external longitudinal/transverse magnetic fields and beam expanding ratios on actual laser focal spot sizes were analyzed and discussed. It was demonstrated that the latest reported HLHC technology assisted by water-based ultrasonic vibrations and external longitudinal magnetic fields could generate deeper holes with relatively high cutting efficiency, smaller hole taper, higher hole aspect ratio, smaller hole circularity deviation, better hole wall quality, much better performance for relieving residual stress, thinner recast layer, more uniform refinement of grains, and better performance for improving micro hardness. Generally, a stronger external longitudinal magnetic field coupled by water-based ultrasonic vibrations was more helpful for cutting deeper and higher-quality holes. It was shown that the change of loading direction of an external magnetic field with/without ultrasonic assistance did not greatly alter recast layer formation and micro hardness performance for the workpiece helically cut. Compared to the transverse magnetic assistance, a corresponding longitudinal magnetic field coupled with water-based ultrasonic vibrations was more effective for micro hardness improvement for this reported HLHC technology. When using the same water-based ultrasonic vibration assistance, a stronger longitudinal magnetic field was more effective for relieving residual stress for the workpiece helically cut (a stress reduction percentage of around 96.5% was reported in this work). The average laser focal spot size obviously decreased with laser beam expanding ratio, but the external longitudinal/transverse magnetic assistance did not greatly alter the actual laser focal spot size.

Fast 3D movement of a laser focusing spot behind scattering media by utilizing optical memory effect and optical conjugate planes

Controlling light propagation intentionally through turbid media such as ground glass or biological tissue has been demonstrated for many useful applications. Due to random scattering effect, one of the important goals is to draw a desired shape behind turbid media with a swift and precise method. Feedback wavefront shaping method which is known as a very effective approach to focus the light, is restricted by slow optimization process for obtaining multiple spots. Here we propose a technique to implement feedback wavefront shaping with optical memory effect and optical 4f system to speedy move focus spot and form shapes in 3D space behind scattering media. Starting with only one optimization process to achieve a focusing spot, the advantages of the optical configuration and full digital control allow us to move the focus spot with high quality at the speed of SLM frame rate. Multiple focusing spots can be achieved simultaneously by combining multiple phase patterns on a single SLM. By inheriting the phase patterns in the initial focusing process, we can enhance the intensity of the focusing spot at the edge of memory effect in with 50% reduction in optimization time. With a new focusing spot, we have two partially overlapped memory effect regions, expanding our 3D scanning range. With fast wavefront shaping devices, our proposed technique could potentially find appealing applications with biological tissues.

Surface topography following pulsed laser texturing: Implications for adhesion and wettability

A model for predicting surface topography following nanosecond pulsed laser texturing of metals is applied to calculating the areal surface roughness, Sa, average ablation depth, Da, and Wenzel roughness factor or adhesion area ratio, r, for a range of representative cases relating to adhesion and wettability. Optimisation of the laser scanning strategy, number of laser passes and focused spot size is performed by considering the ratio of increases in Sa and r with respect to the average ablation depth: and where Highest values of and are achieved where the pulse separation distance is equal to the focused spot size in both the scanning and lateral directions. Increases in Sa, r and can be achieved by performing multiple laser passes, while r and can be increased independently of Sa and by reducing the focused laser spot size. These results suggest that r and Sa can be optimised effectively and independently in line with a given application. Finally, laser texturing experiments are performed on AA 6082 aluminium alloy and 316L austenitic stainless steel specimens with the aim of confirming model outcomes, after which tensile tests are performed on adhesive-bonded joints prepared with the same laser treatments. Further to demonstrating the benefits of laser texturing, it is shown that optimum results are not necessarily associated with a single value of Sa or r, but are instead a compromise between maximising these parameters and limiting the ablated volume.

Technological and optical methods for increasing the spatial resolution of thermochemical laser writing on thin metal films

Various techniques, effects, and factors are examined that enable one to increase the spatial resolution of thermochemical direct laser writing on thin metal films. A few specific conditions of experimental laser processing with the focused beam of a CW green laser are investigated therewith. One of the goals of this work was to find a suitable metal and its thickness that would make it possible to implement the through-oxidation regime. When properly adjusting a value of laser exposure, this regime was found to be very useful for increasing the spatial resolution of the laser writing. This is due to bleaching of an oxidized film near the peak of a focused laser spot. Apart from the experimental results concerning the technology of laser processing of the metal films, in this work, there are also considered some ways of reducing a focused spot size used for the direct laser writing.

Spatially resolved optical excitation of mechanical modes in graphene NEMS

Emerging applications in nanoelectromechanical systems (NEMS) made from two-dimensional (2D) materials demand simultaneous imaging and selective actuation of the mechanical modes. Focused optical probes to measure and actuate motion offer a possible solution, but their lateral spatial resolution must be better than the size of the resonator. While optical interferometry is known to have excellent spatial resolution, the spatial resolution of the focused, laser-based optical driving is not currently known. Here, we combine separately scanned interferometry and optical drive probes to map the motion and forces on a suspended graphene nanomechanical resonator. By analyzing these maps with a force density model, we determine that the optical drive force has a spatial resolution on the order of the size of the focused laser spot. Using the optical force probe, we demonstrate the selective actuation and suppression of a pair of orthogonal antisymmetric mechanical modes of the graphene resonator. Our results offer a powerful approach to image and actuate any arbitrary high-order mode of a 2D NEMS.

Scanning laser terahertz near-field reflection imaging system

We developed a scanning laser terahertz (THz) near-field reflection imaging system using a fiber-coupled femtosecond laser source for high spatial resolution measurements without any sub-wavelength probes. In this system, THz wave pulses generated at the laser focusing spot in a non-linear optical crystal interact with a sample set on the crystal. Using this system, we obtained the THz reflection images of metallic structures with a resolution of ∼λTHz/34 (λTHz = 680 μm) and also observed paraffin-embedded human breast tissue of several 10 μm in size. These results promise that this system can be suitable for industrial applications which require micron-scale resolution.

Enhanced relativistic-electron beam collimation using two consecutive laser pulses

The double laser pulse approach to relativistic electron beam (REB) collimation in solid targets has been investigated at the LULI-ELFIE facility. In this scheme two collinear laser pulses are focused onto a solid target with a given intensity ratio and time delay to generate REBs. The magnetic field generated by the first laser-driven REB is used to guide the REB generated by a second delayed laser pulse. We show how electron beam collimation can be controlled by properly adjusting the ratio of focus size and the delay time between the two pulses. We found that the maximum of electron beam collimation is clearly dependent on the laser focal spot size ratio and related to the magnetic field dynamics. Cu-Kα and CTR imaging diagnostics were implemented to evaluate the collimation effects on the respectively low energy (≤100 keV) and high energy (≥MeV) components of the REB.

Control of laser light by a plasma immersed in a tunable strong magnetic field.

The interaction between laser light and an underdense plasma immersed in a spatio-temporally tunable magnetic field is studied analytically and numerically. The transversely nonuniform magnetic field can serve as a magnetic channel, which can act on laser propagation in a similar way to the density channel. The envelope equation for laser intensity evolution is derived, which contains the effects of magnetic channel and relativistic self-focusing. Due to the magnetic field applied, the critical laser power for relativistic self-focusing can be significantly reduced. Theory and particle-in-cell simulations show that a weakly relativistic laser pulse can propagate with a nearly constant peak intensity along the magnetic channel for a distance much longer than its Rayleigh length. By setting the magnetic field tunable in both space and time, the simulation further shows that the magnetized plasma can then act as a lens of varying focal length to control the movement of laser focal spot, decoupling the laser group velocity from the light speed c in vacuum.

Optimization of laser-nanowire target interaction to increase the proton acceleration efficiency

We report on improved proton acceleration from the interaction of a short pulse high intensity laser (>1020 Wcm−2) with nano-engineered targets. Planar targets (from 7 to 20 μm) with protruding gold nanowires having different total areal densities, lengths, and diameters, ranging from 3% to 60% of the size of the laser focal spot were used during an experimental campaign at the 3 J, 30 fs HERCULES laser facility. The results show the importance of the average number of nanowires per focal spot, N, on laser energy absorption. We show that the proton acceleration is significantly improved by using 1 nanowires per focal spot. Detailed analysis indicates that 1 nanowire per focal spot optimizes the interaction between laser pulse and nanowires, in which the wings of the pulse pull out electrons from the wires forming a plasma with density that allows for deep penetration of the laser pulse into the array. When moving away from this optimum in both directions, N ≪ 1 and N ≫ 1, the laser pulse-nanowire coupling is either too weak or unfavorable for obtaining maximum proton energy. Proton spectra are compared to simulations using 2D-3V particle-in-cell code which reproduces the experimental data with good agreement.

Colorizing Ti-6Al-4V surface via high-throughput laser surface nanostructuring

In this work, an innovative high-throughput laser surface nanostructuring process was developed to enhance the hydrophobicity and reduce the reflectivity of the Ti-6Al-4V surface using a nanosecond pulse laser. The laser textured Ti-6Al-4V surfaces exhibit combined anti-reflective, highly hydrophobic and mechanically enhanced effects. In the meantime, the anti-reflective Ti-6Al-4V surfaces show different colors when treated by different laser processing parameters. Compared with the existing ultra-short laser-based surface-texturing methods which often rely on the generation of micro-scale patterns using a focused laser spot and very fine spatial resolution, this innovative laser surface treatment method significantly increases the processing speed and reduces the production cost. This novel process will bring a new avenue for controlling the surface color, wettability and mechanical property of the important engineering surfaces.

Realising single-shot measurements of quantum radiation reaction in high-intensity lasers

Modern laser technology is now sufficiently advanced that collisions between high-intensity laser pulses and laser-wakefield-accelerated (LWFA) electron beams can reach the strong-field regime, so that it is possible to measure the transition between the classical and quantum regimes of light–matter interactions. However, the energy spectrum of LWFA electron beams can fluctuate significantly from shot to shot, making it difficult to clearly discern quantum effects in radiation reaction (RR), for example. Here we show how this can be accomplished in only a single laser shot. A millimetre-scale pre-collision drift allows the electron beam to expand to a size larger than the laser focal spot and develop a correlation between transverse position and angular divergence. In contrast to previous studies, this means that a measurement of the beam’s energy-divergence spectrum automatically distinguishes components of the beam that hit or miss the laser focal spot and therefore do and do not experience RR.

Bremsstrahlung emission profile from intense laser-solid interactions as a function of laser focal spot size

The bremsstrahlung x-ray emission profile from high intensity laser-solid interactions provides valuable insight to the internal fast electron transport. Using penumbral imaging, we characterise the spatial profile of this bremsstrahlung source as a function of laser intensity by incrementally increasing the laser focal spot size on target. The experimental data shows a dual-source structure; one from the central channel of electrons, the second a larger substrate source from the recirculating electron current. The results demonstrate than an order of magnitude improvement in the intensity contrast between the two x-ray sources is achieved with a large focal spot, indicating preferable conditions for applications in radiography. An analytical model is derived to describe the transport of suprathermal electron populations that contribute to substrate and central channel sources through a target. The model is in good agreement with the experimental results presented here and furthermore is applied to predict laser intensities for achieving optimum spatial contrast for a variety of target materials and thicknesses.

Plasmonic Trapping-Induced Crystallization of Acetaminophen

We demonstrate that plasmonic trapping can control crystallization of acetaminophen from its aqueous solution. Irradiation of a focused continuous-wave near-infrared laser to a plasmonic Au nanolattice supporting a thin film of a saturated solution allowed acetaminophen molecules to crystallize in annular distribution with the center of the focal spot. The annularly distributed crystals can be spatially manipulated by changing the position of the laser focal spot. The annular pattern is rationalized by competition between electrical field gradient force as an attractive force to the focal spot and thermophoretic force as a repulsive force. It is also found that, upon stopping the laser irradiation, the crystals first transformed to highly concentrated droplets rather than directly dissolving to the solution. Relaxation of the droplets by self-diffusion to the solution followed to the crystal/droplet transformation. These two-step dissociation dynamics indicate that not only plasmonic trapping of the molec...