Laser Focal Spot Research Articles

Compact Laser-Induced Fluorescence Detector with Adjustable Laser Focal Spot for Multiple Purposes.

In many research fields, the demand for miniaturized laser-induced fluorescence (LIF) detection systems has been increasing. This work has developed a compact LIF detector, employing a laser diode as the excitation source and a photodiode as the photodetector with an adjustable laser focal spot, to meet the diverse requirements of various observation targets, such as capillaries, PCR tubes, and microfluidic chips. It features the functionalities of background fluorescence correction, the adaptive adjustment of the dynamic range, and constant power control for the laser. The influence of the excitation power on the detection limit was studied through experiments, and the configuration results for LED/LD as light sources and 487/450 nm wavelengths were compared and optimized. A fully integrated, compact, modular epifluorescence LIF detector was subsequently constructed, measuring 40 × 22 × 38 mm3 in total size, with a cost of USD 320, and achieving a detection limit of 0.4 nM for fluorescein sodium. Finally, the detector was integrated into a nucleic acid detection system with a microfluidic chip on the Chinese Space Station (CSS) and was also tested with PCR tubes and capillaries, proving its broad practicality and adaptability to various analytical systems.

Tailoring bistability in optical tweezers with vortex beams and spherical aberration

We demonstrate a bistable optical trap by tightly focusing a vortex laser beam. The optical potential has the form of a Mexican hat with an additional minimum at the center. The bistable trapping corresponds to a nonequilibrium steady state, where the microsphere continually hops, due to thermal activation, between an axial equilibrium state and an orbital state driven by the optical torque. We develop a theoretical model for the optical force field, based entirely on experimentally accessible parameters, combining a Debye-type nonparaxial description of the focused vortex beam with Mie scattering by the microsphere. The theoretical prediction that the microsphere and the annular laser focal spot should have comparable sizes is confirmed experimentally by taking different values for the topological charge of the vortex beam. Spherical aberration introduced by refraction at the interface between the glass slide and the sample is considered and allows us to finetune between axial, bistable, and orbital states as the sample is shifted with respect to the objective focal plane. We find overall agreement between theory and experiment for a rather broad range of topological charges. Our results open the way for applications in stochastic thermodynamics, as they establish a control parameter—the height of the objective focal plane with respect to the glass slide—that allows us to shape the optical force field in real time and in a controllable way. Published by the American Physical Society 2024

The influence of laser focusing conditions on the direct laser acceleration of electrons

Direct laser acceleration of electrons during a high-energy, picosecond laser interaction with an underdense plasma has been demonstrated to be substantially enhanced by controlling the laser focusing geometry. Experiments using the OMEGA EP facility measured electrons accelerated to maximum energies exceeding 120 times the ponderomotive energy under certain laser focusing, pulse energy, and plasma density conditions. Two-dimensional particle-in-cell simulations show that the laser focusing conditions alter the laser field evolution, channel fields generation, and electron oscillation, all of which contribute to the final electron energies. The optimal laser focusing condition occurs when the transverse oscillation amplitude of the accelerated electron in the channel fields matches the laser beam width, resulting in efficient energy gain. Through this observation, a simple model was developed to calculate the optimal laser focal spot size in more general conditions and is validated by experimental data.

Forensic Advantages and Disadvantages of Raman Spectroscopy Methods in Various Banknotes Analysis and the Observed Discordant Results

This work discussed and highlighted advantages and disadvantages of Raman spectroscopy methods in the analysis of various security areas of banknote around the world, trending in the current literature. Various methods applied on the three main security areas of banknotes investigation were considered – ink, prints and substrate security areas. Specifically, the advantages and disadvantages envisage from the various methods focuses on laser focal spot size used, Raman signal detection on the selected security areas and the samples detection efficiency of the various methods reported. The research also explains some discordant results observed in the various works conducted on similar security areas of different banknotes. Overall, Ink and print security analysis are the most widely analyzed security areas employed in the various researches, while the spectral analysis of substrate security area was the least explored area. The Raman forensic advantages of banknotes investigation outweighed the envisage disadvantages, and the most common disadvantage notice in the various methods are the fluorescent effect and single note detection (per analysis) limitation in the various methods.

Dependence of spectral purity of Gd plasma emission around 6.7 nm on laser irradiation conditions

Gadolinium is a compelling candidate for the next-generation 6.x nanolithography light source. We present a systematically study on the spectral purity at 6.7 nm in the gadolinium laser produced plasma. Variations in extreme ultraviolet spectra are observed over a wide range of laser focal spot sizes and laser pulse energies due to the combined effect of plasma temperature and opacity. When varying laser focal spot size, ionic populations of Gd ions at different electron temperatures were estimated by the collisional-radiative model for the explanation of changes in the spectral profile. When adjusting the laser pulse energy, modifications in the electron density profile were measured through Nomarski interferometry, and the related optical depth was quantified assisted with a one-dimensional radiation transfer model. Results show that, appropriate laser energy and focal spot size are necessary to achieve the high spectral purity, and with a lens with focal length of f = 175 mm and a laser pulse energy of 100 mJ, a spectral purity of Gd plasma up to 3.7% over range of 5.5–10 nm is achieved.

Azimuthally extreme-ultraviolet focal splitter by modified spiral photon sieves

Extreme Ultraviolet (EUV) radiation is a short-wavelength light source that has important applications in many fields, such as optical communication, particle manipulation, and ultrahigh resolution imaging. However, the highly absorptive nature of EUV light makes it challenging to design suitable focusing optics, such as focal splitters, to properly manipulate the energetic light. Here, we propose modified spiral photon sieves to transform EUV laser light into azimuthally splitting focusing. A genetic algorithm was used to design and optimize the azimuthally focal splitters. A capillary discharge EUV laser at 46.9 nm was used to verify the effectiveness of our proposed method, and PMMA targets were used to record the focused laser spot. The profile of the recorded patterns measured by atomic force microscopy shows that the focal spots in the experiment are diffraction-limited and agreed with the theoretical analysis. The proposed technique provides a new way for manipulating EUV light and further extends the applications ranging from EUV to soft x rays.

Effect of Focal Spot Size on AlSi10Mg Alloy Parts Fabricated by Laser Powder Bed Fusion: Process Window, Mechanical Properties, and Microstructure

This study investigated the effect of the laser focal spot size on the process window, part properties, and microstructure of an AlSi10Mg alloy fabricated via the laser powder bed fusion (LPBF) process. In LPBF, process windows of different spot sizes (60 μm and 120 μm) were established to fabricate dense AlSi10Mg parts with an excellent density of 99.96% using optimal process parameters. The tensile results indicated that parts made using a large spot size had greater ductility because their coarse grain structure with an average grain size of 41 μm, as measured by the electron backscattered diffraction (EBSD) technique, reduced the resistance of dislocation movement, thus enhancing their elongation. In contrast, parts made using a small spot size had stronger yield strength because their finer grains with an average grain size of 14.6 μm enhanced their strength. The differences in their grain structures were attributed to their different cooling rates. In addition, X-ray diffraction (XRD) analysis showed a slight shift towards a lower diffraction angle at the Al-(200) peak of the parts fabricated using a smaller spot size owing to its finer grains. Moreover, the LPBF-fabricated parts using a large spot size of 120 μm exhibited a smoother surface roughness with a Sa value of 1.69 μm compared to those parts with a Sa value of 4.36 μm using a small spot size.

Precise Focal Spot Positioning on an Opaque Substrate Based on the Diffraction Phenomenon in Laser Microfabrication.

The precise positioning of the laser focal spot on the substrate is an important issue for laser microfabrication. In this work, a diffraction pattern-based focal spot positioning method (DFSPM) is proposed to achieve the precise positioning of the laser focal spot on opaque substrates. A series of diffraction patterns of laser focus under-positioning, exact positioning and over-positioning were obtained to investigate the cross-section light distribution of the laser focal spot. According to the monotonic tendency of FWHM to exhibit light intensity at the focal spot cross-section away from the focal plane, the FWHM threshold of polynomial fitted curves was used to determine the exact positioning of laser focus. The ascending scanning method was used to obtain the diffraction patterns at various vertical positions and the FWHM threshold of light distribution at the exact position. The polynomial fitted curves verify the FWHM monotonic tendency of light intensity distribution at the focal spot cross-section along the optical axis. Precise positioning can be achieved with a 100 nm adjustment resolution. This work was expected to provide references for laser microfabrication on opaque materials.

A Computational Study of Solid Si Target Dynamics under ns Pulsed Laser Irradiation from Elastic to Melting Regime

The dynamic behavior of solid Si targets irradiated by nanosecond laser pulses is computationally studied with transient, thermοmechanical three-dimensional finite element method simulations. The dynamic phase changes of the target and the generation and propagation of surface acoustic waves around the laser focal spot are provided by a finite element model of a very fine uniformly structured mesh, able to provide high-resolution results in short and long spatiotemporal scales. The dynamic changes in the Si material properties until the melting regime are considered, and the simulation results provide a detailed description of the irradiated area response, accompanied by the dynamics of the generation and propagation of ultrasonic waves. The new findings indicate that, due to the low thermal expansion coefficient and the high penetration depth of Si, the amplitude of the generated SAW is small, and the time and distance needed for the ultrasound to be generated is higher compared to dense metals. Additionally, in the melting regime, the development of high nonlinear thermal stresses leads to the generation and formation of an irregular ultrasound. Understanding the interaction between nanosecond lasers and Si is pivotal for advancing a wide range of technologies related to material processing and characterization.

Raman Scattering for Label-Free Chemical Imaging

Raman spectroscopy provides chemical information by detecting light scattered from a monochromatic source (such as a laser) at energies that correspond to molecular vibrations. Because Raman spectroscopy commonly uses visible lasers, the spatial resolution is approximately the same as what can be seen with an optical microscope. First demonstrated in the 1970s, coupling Raman spectroscopy with microscopes enabled the chemical information to be obtained from a focused laser spot. By moving the laser across the sample and recording the Raman spectrum at each location, images can be generated from changes in intensity at different Raman shifts that spatially characterize the molecules present. From the development of the Raman microprobe to today, advances in instrumentation have increased the speed, sensitivity, and spatial resolution of Raman microscopy. This article covers the fundamentals of Raman microscopy and how technological advances are enabling a variety of applications.

Probing magnetization dynamics of iron oxide nanoparticles using a point-probe magneto-optical method

Magnetic nanoparticles (MNPs) are promising as local heat generators for magnetic hyperthermia under AC magnetic fields. The heating efficacy of MNPs is determined by the AC hysteresis loop area, which in turn is affected by the dynamic magnetic properties of the nanoparticles. Whilst inductive-based AC magnetometers can measure the average magnetic behavior of samples, the use of the magneto-optical Faraday effect with a focused laser spot allows point-probe measurements to be made, and without some of the magnetic field limitations imposed by inductive methods. In this work, the AC magnetic properties of different sized iron oxide MNPs in suspension were measured by AC magnetometry and AC susceptibility techniques. AC hysteresis loops measured by magneto-optical magnetometry were validated using a commercial inductive AC magnetometer, and compared to the magnetization relaxation behavior revealed by fitting the AC susceptibility data. The spatial sensitivity of the point-probe magneto-optical method is also demonstrated by measuring the AC hysteresis loop from large (>1 μm) MNP aggregates dried onto glass slides. These aggregated particles are found to be magnetically softer than in their suspension form, suggesting interparticle coupling mechanisms could occur when the nanoparticles form dense aggregates.

Water repellence of biomimetic structures fabricated via femtosecond laser direct writing

Femtosecond laser direct writing based on two-photon polymerization (TPP), which is a sub-classification of vat photopolymerization, has the advantages of unlimited geometrical freedom and high resolution, and has attracted significant interests from scientists and engineers in recent years. In the present study, TPP fabrication was performed using a laser equipment with wavelength of 514 nm. The collimated beam diameter before the focusing lens was measured as 5 mm and the focused laser spot diameter was calculated as 280 nm. Key TPP processing parameters were investigated for the fabrication of nanowires, and micro/ nanoscale biomimetic structures, including cone and pinecone structures. The finest nanowire has a diameter of 105 nm and is achieved at the laser power of 0.8 μW (near the laser polymerization threshold). For the three-dimensional (3D) structures, laser power, layer thickness and hatch spacing would influence the fabrication accuracy. Generation of a stiff 3D cone structure would require a minimum laser power of 1.4 μW when the hatch spacing and layer thickness are 200 nm. The optimum processing parameters (laser power 2.2 μW, hatch space and layer thickness 100 nm) can generate structures with dimensions close to the design values. For pinecones with nano features, the scanning strategies play a significant role in the nano fabrication accuracy. A row of pinecones and cones were fabricated and tested for their water repellence performance. The relationship between the performance and the nano cone density was investigated. The pinecone structures exhibit superior performance to the ones with no nano-scale structures, indicating that improved hydrophobicity can be achieved by designing the micro/nano hierarchical structures. The increased number density of the nano cones in a micro/nano hierarchical structure was found to profoundly enhance the water repellence of the structures. The mechanisms were discussed with Wenzel and Cassie models.

Study on the Duration of Laser-Induced Thin Film Plasma Flash

The accuracy of judging whether the film is damaged directly affects the accuracy of the measurement of the film laser damage threshold. When judging the film damage by the traditional plasma flash method, there is a problem of misjudgment caused by the failure to distinguish the film and air plasma flash. In order to eliminate misjudgment, the two flashes are accurately distinguished by the difference in the duration of the air and film plasma flash. This paper aims to obtain the theoretical and experimental values of the duration of the film plasma flash (tf) and analyze the factors affecting it. Firstly, taking single-layer hafnium oxide and aluminum oxide thin films as examples, when the wavelength of the incident laser is 1064 nm, the diameter of the laser focusing spot is 0.08 cm, the energy of the incident laser is 100 mJ, and the pulse width of incident laser is 10 ns, the tf of hafnium oxide, and aluminum oxide thin films are 542.7 and 299.6 ns, respectively. Secondly, the experimental study of tf was carried out. Through six experiments, the following results were obtained: (1) With the increase in incident laser energy, the tf of both films increases; (2) The tf of the hafnium oxide film is longer than that of the aluminum oxide film. (3) The experimental parameters are put into the calculation model, and the theoretical results are in good agreement with the experimental tf values. Finally, it is found that tf increases with the increase in incident laser energy and incident laser pulse width, and decreases with the increase in focusing spot diameter.

Partition of Omega-like facility into two configurations of 24 and 36 laser beams to improve implosion performance

An Omega-like beam configuration is considered where the 60-beam layout can be separated into two independent sub-configurations with 24 and 36 laser beams, each minimizing direct drive illumination non-uniformity. Two different laser focal spot profiles, one associated with each configuration, are proposed to apply the zooming technique in order to increase the laser-target coupling efficiency. This approach is used by 1D hydrodynamics simulations of the implosion of a direct-drive capsule characterized by a relatively large aspect ratio A = 7 and an optimized laser pulse shape delivering a maximum of 30 TW and 30 kJ, with different temporal pulse shapes in each of the two sets of beams. It is shown that zooming allows for an optimistic 1D thermonuclear energy gain greater than one while without zooming the thermonuclear gain remains largely below one. While this is incompatible with the as-built Omega laser, it provides a promising option for a future intermediate-energy direct drive laser system.

In search of ways to improve the properties of a laser-accelerated heavy ion beam relevant for fusion fast ignition

Ion fast ignition (IFI) is one of the proposed options for inertial fusion in which the ignition of nuclear fuel is initiated by an intense ion beam. In this paper, the properties of a laser-accelerated heavy ion beam are investigated for the possible use of such a beam as a fuel igniter in the IFI scenario. Using a two-dimensional particle-in-cell code, detailed studies of laser-driven heavy ion acceleration were carried out to determine the possibility of improving the properties of the heavy ion beam relevant for IFI by the appropriate selection of certain laser and target parameters. In simulations, a 1-ps laser with an energy of 150–250 kJ irradiated targets with a variety of atomic mass numbers, areal densities, thicknesses, and densities. For each of the sets of laser and target parameters considered in the paper, the parameters of the heavy ion beam relevant for IFI were determined and discussed. It was found that for realistic laser driver parameters, the IFI requirements are best met by ion beams with moderate ion mass numbers (A ∼ 50–100), such as the beam of Cu ions. It was shown that by optimizing the laser focal spot, as well as by properly matching the energy and power of the laser to the target areal mass density, it is possible to significantly improve the properties of the heavy ion beam relevant for IFI and, in particular, bring a many-fold increase in the intensity, fluence, and energy of the beam.

Probing bulk electron temperature via x-ray emission in a solid density plasma

Bulk electron temperatures are calculated for thin Cu targets irradiated by the petawatt class Vulcan laser, from the Kα yield obtained using highly oriented pyrolytic graphite crystals. Cu-Kα emission studies have been used to probe the bulk electron temperature. A 30–80 eV core temperature extends homogeneously over distances up to ten times the laser focal spot size. Energy shifting has been observed due to different ionization states produced for different temperatures in the plasma. Polarization dependencies of plasma temperature are observed through the production of x-rays in different targets. 2D PIC simulations were performed to measure the polarization dependency of bulk electron temperature, which supports our experimental results. This paper could be of importance in understanding the different behavior of laser coupling at different polarizations and their role in x-ray production.

Frequency-selective spin-wave propagation in magnonic waveguide with a local laser-heated region

We report on the spin-wave propagation along a magnonic waveguide with a local area of decreased magnetization, which is induced by heating produced with a focused laser spot. A phase-sensitive Brillouin light scattering technique is used to image how the spin wave propagates along the waveguide with a local heat landscape. Frequency-selective signal propagation along the waveguide is demonstrated. Micromagnetic simulations reveal intermodal interference variation in the region after the heated area. The proposed way to reconfigure the magnetization landscape can be used in magnonic devices with frequency-selective spin-wave transport.

Formation and manipulation of 2D colloidal crystals driven by convective currents and electrostatic forces

Colloidal crystal plays a major role in novel optical devices such as waveguides, photonic crystals, and colloidal crystal lasers, among others. It has been known that colloidal crystals may self-assemble due to optical, electric, or chemical potentials, and light-induced thermal effects, such as thermophoresis and convective currents. In this work, the formation and manipulation of a self-limiting assembly of monolayer colloidal crystals using a 20 nm absorbing thin film of titanium and a CW-laser emitting at 1070 nm is reported. The colloidal particles, dragged by thermally induced convective currents, self-assembling over the titanium thin film around the laser focal spot. The self-assembled monolayer colloidal crystal tends to form a circular shape. Using numerical simulation, we obtained a potential well associated with the convective currents that spatially govern the radial extension of the monolayer colloidal crystal. We also show, that by changing the pH of the solution i.e., by tuning the electrostatic interaction, the monolayer colloidal crystal can be formed or destroyed. In addition, it was possible to create a larger monolayer colloidal structure covering an area of 600 μm2 using an array of six laser spots.

Femtosecond laser regulatory focus ablation patterning of a fluorescent film up to 1/10 of the scale of the diffraction limit.

Patterned quantum dots (QDs) and perovskites have attracted a great deal of attention in the fabrication of optoelectronic device arrays for transistors, image sensors and displays. However, the resolution of current patterning technologies is insufficient for nanopatterned QDs and perovskites to be integrated in advanced optoelectronic and photonic applications. Herein, we demonstrate a femtosecond laser regulatory focus ablation (FsLRFA) patterning technique of a fluorescent film involving both semiconductor core-shell QDs and perovskite up to 1/10th of the scale of the diffraction limit. Annular lines with a 200 nm-width are obtained after the irradiation of the femtosecond laser. Moreover, the combination of ablated different geometries enables the laser focal spot as brushes for FsLRFA patterning technology to fabricate delicate and programmable patterns on the fluorescent film. This technology with nanoscale resolution and patterning capability paves the road toward highly integrated applications based on QDs and perovskites.

Hybrid laser processes for thick silver coating fabrication on AlN substrate

Aluminum nitride (AlN) are particularly suitable as integrated circuits (ICs) substrates due to its high thermal conductivity and excellent electricity insulation. However, its poor weldability with metals limits its usage. Recent research on surface metallization of AlN provides possible solutions to tackle this defect. Nevertheless, these solutions show some shortages such as complicated processes or insufficient electrical conductivity. In this paper, we report a method that consists of laser induced surface metallization and laser sintering of silver (Ag) coatings. A nanosecond laser was applied to induce a 10 μm thick aluminum (Al) layer from the AlN substrate. Afterwards, laser sintering of Ag layers was implemented, which could enhance the conductivity and the bonding performance between layers. With optimized laser parameters applied, both the electrical conductivity and the bonding tests demonstrated excellent physical properties. Finally, simulation and EDS analysis illustrated the melting evolution and confirmed a metallurgical combination of Al and Ag, thus enhancing bonding strength. Thanks to the small size of focused laser spot, electrical circuits width could be greatly narrowed if these findings were applied; hence highly dense ICs on AlN substrate become potentially available.