Incident Angle Of Laser Beam Research Articles

Nonlinear third harmonic generation at crystalline sapphires.

Third harmonic generation (THG) is a nonlinear optical phenomenon which can be applied in diverse research areas including interfacial studies, sub-wavelength light manipulation, and high sensitivity bio-molecular detection. Most precedent studies on THG have focused on dielectric and metallic materials, including silicon, gold, and germanium, due to their high nonlinear susceptibility. Sapphire, a widely-used optical substrate, has not been studied in depth for its third harmonic characteristics, despite its excellent optical transmission in the UV-visible range, high thermal conductance, and superior physical and chemical stability. In this research, we comprehensively studied THG at thin air-dielectric interfaces of sapphire wafers by controlling the wafer cutting planes, focusing depth, incidence angle, laser intensity, and input polarization of the input laser beam. These findings can lead to broader use of third harmonics for high-precision sapphire characterization, such as surface quality inspection, crystallinity determination, interfacial studies, delamination check, and real-time monitoring of crack propagation.

Influence of laser beam incidence angle on laser lap welding quality of galvanized steels

Based on the characteristics of laser welded structural parts of auto bodies, the influence of variation in laser beam incidence angle on the lap welding performance of galvanized auto-body sheets was studied. Lap welding tests were carried out on the galvanized sheets for auto-body application at different laser beam incidence angles by using the optimal welding parameters obtained through orthogonal experiment. The effects of incidence angle variation on seam appearance, cross-sectional shape, joint mechanical properties and microstructure of weldments were analyzed. In addition, the main factors influencing the value of incidence angle were investigated. According to the results, the weld seams had a good appearance as well as a fine, and uniform microstructure when the laser beam incidence angle was smaller than the critical incidence angle, and thus they could withstand great tensile and shear loads. Moreover, all tensile–shear specimens were fractured in the base material zone. When the laser beam incidence angle was larger than the critical incidence angle, defects like shrinkage and collapse tended to emerge, thereby resulting in the deteriorated weldability of specimens. Meanwhile, factors like the type and thickness of sheet, weld width as well as inter-sheet gap all had a certain effect on the value of laser beam incidence angle. When the sheet thickness was small and the weld width was narrow, the laser beam incidence angle could be increased appropriately. At the same time, small changes in the inter-sheet gap could greatly impact the value of incidence angle. When the inter-sheet gap was small, the laser beam incidence angle should not be too large.

Development of Laser Scanner for Full Cross-Sectional Deformation Monitoring of Underground Gateroads.

The deformation of underground gateroads tends to be asymmetric and complex. Traditional instrumentation fails to accurately and conveniently monitor the full cross-sectional deformation of underground gateroads. Here, a full cross-sectional laser scanner was developed, together with a visualization software package. The developed system used a polar coordinate measuring method and the full cross-sectional measurement was shown by 360° rotation of a laser sensor driven by an electrical motor. Later on, the potential impact of gateroad wall flatness, roughness, and geometrical profile, as well as coal dust environment on the performance of the developed laser scanner will be evaluated. The study shows that high-level flatness is favorable in the application of the developed full cross-sectional deformation monitoring system. For a smooth surface of gateroad, the sensor cannot receive reflected light when the incidence angle of laser beam is large, causing data loss. Conversely, the roughness surface shows its nature as the diffuse reflection light can be received by the sensor. With regards to coal dust in the measurement environment, fine particles of floating coal dust in the air can lead to the loss of measurement data to some extent, due to scattering of the laser beam.

Study on laser welding of austenitic stainless steel by varying incident angle of pulsed laser beam

In the present work, AISI 304 stainless steel sheets are laser welded in butt joint configuration using a robotic control 600W pulsed Nd:YAG laser system. The objective of the work is of twofold. Firstly, the study aims to find out the effect of incident angle on the weld pool geometry, microstructure and tensile property of the welded joints. Secondly, a set of experiments are conducted, according to response surface design, to investigate the effects of process parameters, namely, incident angle of laser beam, laser power and welding speed, on ultimate tensile strength by developing a second order polynomial equation. Study with three different incident angle of laser beam 89.7deg, 85.5deg and 83deghas been presented in this work. It is observed that the weld pool geometry has been significantly altered with the deviation in incident angle. The weld pool shape at the top surface has been altered from semispherical or nearly spherical shape to tear drop shape with decrease in incident angle. Simultaneously, planer, fine columnar dendritic and coarse columnar dendritic structures have been observed at 89.7deg, 85.5deg and 83deg incident angle respectively. Weld metals with 85.5deg incident angle has higher fraction of carbide and δ-ferrite precipitation in the austenitic matrix compared to other weld conditions. Hence, weld metal of 85.5deg incident angle achieved higher micro-hardness of ∼280HV and tensile strength of 579.26MPa followed by 89.7deg and 83deg incident angle welds. Furthermore, the predicted maximum value of ultimate tensile strength of 580.50MPahas been achieved for 85.95deg incident angle using the developed equation where other two optimum parameter settings have been obtained as laser power of 455.52W and welding speed of 4.95mm/s. This observation has been satisfactorily validated by three confirmatory tests.

Periodic nanostructure induced on PEN surface by KrF laser irradiation

There are several kinds of periodic surface structures, ranging from dots to ripples. The most common pattern is a ripple-like structure, where the direction of the ripples is parallel with the main polarisation of the laser beam. The distance between individual ripples (their period) depends on several factors, namely: the chemical structure of the polymer, wavelength of the laser irradiation and the angle of laser beam incidence. Oriented polyethylene naphthalate (PEN) foils with a thickness of 50 μm were used. The samples were irradiated with a KrF laser under laser beam incidence 0-45° to the laser beam. Selected polymer samples were also exposed by irradiation through a contact mask. Surface roughness and the dimensions of the ripple-like structures were measured by atomic force microscopy in tapping mode. The concentration of the elements on the surface was obtained from Angle Resolved X-ray Photoelectron Spectroscopy (ARXPS) spectra measured by X-ray Photoelectron Spectroscopy (XPS) spectrometer. The surface morphology and dimensions of prepared structures were evaluated and compared with those prepared on pristine samples without mask. The transition between the irradiated surface under the slits and the shielded surface in between the slits was evaluated. The optimal input parameters for nanopattern with high periodicity were determined. The shape and the surface roughness of the ripple pattern depend strongly on the angle of incidence of the laser beam. The surface roughness and ripple width increase with the angle of laser beam incidence. Irradiation with laser leads to changes in chemical composition of the PEN surface layer. Even at initial phase of pattern formation a significant increase of carbonyl and carboxyl groups was detected.

Optical laue diffraction on photonic structures designed by laser lithography

Two-dimensional photonic crystals with square symmetry C4v were obtained using the laser lithography method. The structure of these samples was studied by scanning electron microscopy. Optical Laue diffraction for monochromatic light was studied experimentally depending on the incidence angle of laser beam and lattice constant. Interpretation of the observed diffraction patterns is given in the framework of the Laue diffraction mechanism for an one-dimensional chain of scattering elements. Red thresholds for different diffraction orders were determined experimentally and theoretically. The results of calculations are in an excellent agreement with experiment.

Femtosecond laser-controlled self-assembly of amorphous-crystalline nanogratings in silicon

Self-assembly (SA) of molecular units to form regular, periodic extended structures is a powerful bottom-up technique for nanopatterning, inspired by nature. SA can be triggered in all classes of solid materials, for instance, by femtosecond laser pulses leading to the formation of laser-induced periodic surface structures (LIPSS) with a period slightly shorter than the laser wavelength. This approach, though, typically involves considerable material ablation, which leads to an unwanted increase of the surface roughness. We present a new strategy to fabricate high-precision nanograting structures in silicon, consisting of alternating amorphous and crystalline lines, with almost no material removal. The strategy can be applied to static irradiation experiments and can be extended into one and two dimensions by scanning the laser beam over the sample surface. We demonstrate that lines and areas with parallel nanofringe patterns can be written by an adequate choice of spot size, repetition rate and scan velocity, keeping a constant effective pulse number (Neff) per area for a given laser wavelength. A deviation from this pulse number leads either to inhomogeneous or ablative structures. Furthermore, we demonstrate that this approach can be used with different laser systems having widely different wavelengths (1030 nm, 800 nm, 400 nm), pulse durations (370 fs, 100 fs) and repetition rates (500 kHz, 100 Hz, single pulse) and that the grating period can also be tuned by changing the angle of laser beam incidence. The grating structures can be erased by irradiation with a single nanosecond laser pulse, triggering recrystallization of the amorphous stripes. Given the large differences in electrical conductivity between the two phases, our structures could find new applications in nanoelectronics.

Ultralow-frequency Raman system down to 10 cm(-1) with longpass edge filters and its application to the interface coupling in t(2+2)LGs.

Ultralow-frequency (ULF) Raman spectroscopy becomes increasingly important in the area of two-dimensional (2D) layered materials; however, such measurement usually requires expensive and nonstandard equipment. Here, the measurement of ULF Raman signal down to 10 cm(-1) has been realized with high throughput by combining a kind of longpass edge filters with a single monochromator, which are verified by the Raman spectrum of L-cystine using three laser excitations. Fine adjustment of the angle of incident laser beam from normal of the longpass edge filters and selection of polarization geometry are demonstrated how to probe ULF Raman signal with high signal-to-noise. Davydov splitting of the shear mode in twisted (2+2) layer graphenes (t(2+2)LG) has been observed by such system in both exfoliated and transferred samples. We provide a direct evidence of twist-angle dependent softening of the shear coupling in t(2+2)LG, while the layer-breathing coupling at twisted interfaces is found to be almost identical to that in bulk graphite. This suggests that the exfoliation and transferring techniques are enough good to make a good 2D heterostructures to demonstrate potential device application. This Raman system will be potentially applied to the research field of ULF Raman spectroscopy.

Investigating the Emission Characteristics of Single Crystal YAG When Activated by High Power Laser Beams

Limitations associated with light emitting diodes (LEDs) operating under high current densities due to the efficiency droop has created a need to look for alternative light sources; here we report investigations on the potential of laser diodes (LDs) for high brightness lighting solutions. High power laser diodes require phosphor targets with certain performance criteria such as high thermal conductivity, efficiency and structural geometry. Here we examine the possibility of using single crystal YAG:Ce phosphor materials as potential targets for generation of light via laser diodes. We report on the emission properties of the crystals with different sizes and examine the effect of laser beam incident angle incident on crystal target emission.

High-quality and controllable hollow beam generation by a liquid-core optical fiber

We present the generation of high-quality and beam parameter controllable hollow beams with a liquid-core optical fiber (LCOF) filled with CS2 by adjusting off-axis orientation and LCOF structure. The dependences of the dark spot size (DSS), ring beam width (RBW) and transverse field intensity distribution of the hollow beam on the incidence angle of laser beam, the length and core diameter of the LCOF are investigated experimentally, and analyzed theoretically by using Zemax simulation. The results show that the quality of the hollow beam is related to the times of total internal reflection of skew rays in the fiber core; the DSS and RBW are closely controlled by the incidence angle and core diameter. High-quality thin-ring hollow beam is achieved experimentally by the LCOF of the core diameter of 100μm and the length of 1m with the incidence angle of less than 10°. The hollow beam generated has high stability, and is weakly dependent on the bending of the LCOF.

Surface plasmon polariton boosted photorefractive scattering in indium tin oxide coated Fe-doped lithium niobate slabs

To study the impact of surface plasmon polaritons on photorefractive effect, indium tin oxide (ITO) thin films were deposited onto Fe-doped lithium niobate (Fe: LiNbO3, LN) slabs. Striking scattering near surface normal (SNSN) of the slabs was observed, increasing with the incident angle of laser beam. The SNSN was so strong that it depleted the transmitted beam. Based on a tentative physical picture of photovoltaic (PV) effect induced electrostatic modification, the electron density of skin layer of ITO film was raised to such a level that surface plasmon polaritons (SPPs) were excited in LN/ITO interface via phase grating mediation. Consequently, the localized intensified electromagnetic fields exerted on the LN slabs and resulted in the SNSN. The impact of the SPPs and SNSN on the photorefractive effect can be seen from over 2.5 times SNSN enhancement in power and as high as 6.0 times boosting of the ratio of the SNSN power to the remaining transmitted one.

Experimental investigation of plume expansion dynamics of nanosecond laser ablated Al with small incident angle

Experimental study of expansion dynamics of pulsed-laser ablation plasmas from Al is presented. A systematic investigation of plasma plume expansion is done. The laser beam is focused on the target with an incident angle between 0° and 20°. The results show that the plume growth is almost normal to the target surface, irrespective of the incident angle of the laser. Besides, the time evolution of the plasma plume geometry ratio at different incident angles shows that the incident angle of laser beam influences very slightly its shape at later delay time. The results imply that when the incident angle is small (ranging from 0° to 20°), the influence of the incident angle on the plume expansion is rather trivial.

Experimental characterization of bi-directional terahertz emission from gold-coated nanogratings

The THz radiation emission of Au-coated nanogratings (fused silica substrate, 30 nm Au layer thickness, 500 nm grating constant) upon fs laser irradiation (785 nm, 150 fs, 1 kHz, ≤1 mJ/pulse) is observed in both directions along the laser beam axis (forward and backward) and for both, illumination of the Au/air or the Au/silica interface. THz radiation along the laser beam propagation is emitted in a narrow solid angle of about 15° fwhm independent on the laser pulse fluence, the angle of incidence and the nanograting profile. The bar width and groove depth of the nanograting as well as the angle of laser beam incidence strongly affect the THz radiation yield. The energy of single THz light pulses is measured absolutely (2 fJ in the 0.3–0.38 THz range) using a highly sensitive and fast superconducting transition edge sensor. The bi-directional emission of THz radiation is in agreement with the model assumption of surface plasmon polaritons propagating simultaneously on both Au layer interfaces (Au/air and Au/silica).

Research on key influence factors of laser overlap welding of automobile body galvanized steel

In views of structure characteristics of the auto-body parts, the influences of the beam incident angle and joint gap on the performance of laser overlap welded joints were investigated. The experimental results indicate that there were the critical values of beam incident angle and joint gap during laser overlap welding of galvanized steel. The thickness of sheet and the width of joint had a certain influence on the critical beam incident angle and the limit joint gap. With regard to thicker sheet, the limit joint gap can increase appropriately, but the critical beam incident angle should not be too big. With narrow weld width, the laser beam incident angle can increase appropriately, but the joint gap should not be bigger. Additionally, the critical beam incident angle and the limit joint gap were varied with the thickness of the upper sheet. The tensile-shear tests show that the maximum tensile-shear strength of the joint can be obtained with an optimized beam incident angle and joint gap.

Polarization and angular effects of femtosecond laser-induced conical microstructures on Ni

In this paper, through femtosecond laser pulse irradiation with various polarizations and incident angles, we create arrays of nanostructure-covered conical microstructures (NC-CMs) on Ni. We show that the shape of CMs depends significantly on the polarization and incident angle of the laser beam, and find that the size of nanostructures is distributed asymmetrically on the CMs at off normal incidence. We suggest that nonuniform energy deposition owing to the polarization and the incident angle of laser beam plays an important role in the asymmetric nanostructure distribution and shape of CMs on Ni.

Photon-Drag Effect in Single-Walled Carbon Nanotube Films

We observed an interaction of single-walled carbon nanotube films with obliquely incident nanosecond laser radiation in visible and infrared regions generating unipolar voltage pulses replicating the shape of the laser pulses. The photoelectric signal significantly depends on the laser polarization and has maximum value at the laser beam incidence angle of ±65° and at the film thickness of 350 nm. The results are explained in the framework of the photon-drag effect.

Effect of laser beam incidence angle on the thermoelastic generation in semi-transparent materials

When a laser beam is absorbed in a semi-transparent material, a volume acoustic source is created owing to penetration of the laser beam inside the material and to thermoelastic transduction. Many experimental and theoretical studies have been conducted to better understand this ultrasound generation process with normal laser light incidence on the material surface. The purpose of this paper is to analyze the effects of the asymmetry caused by oblique incidence of a laser line source on the generation of acoustic waves in semi-transparent isotropic materials. Experiments on a glass plate demonstrate that such an obliquely incident laser light strongly affects bulk acoustic waves generation. Compressional and shear waves are enhanced and the loss of symmetry of the acoustic source causes asymmetrical behavior of the acoustic waves. Surprisingly, compressional-wave amplitude decreases whereas shear-wave amplitude increases in the region where the electromagnetic energy is refracted. This feature is explained by semi-analytical calculations.

Investigations of electrochemical polymerization processes of thin poly(pyrrole) films and its application to anion sensor based on surface plasmon resonance

A study of the response behavior of anion to surface plasmon resonance (SPR) based sensor, in which a Poly(pyrrole) (Ppy) modified thin gold film was used as a sensor chip is described. In situ surface plasmon resonance and electrochemical methods were used to investigate the electropolymerization and doping/dedoping processes of thin Ppy film. The electropolymerization of pyrrole was carried out under cyclic voltammetric conditions, and simultaneously monitored by in situ SPR. It has revealed that the transition between the reduced state and oxidized state of the Ppy, corresponding to the doping/dedoping of anions, can lead to very distinct changes in SPR signal at a fixed angle of incident laser beam. Furthermore, it has demonstrated that the concentration, the charge and size of anions, as well as the film thickness play important roles in the ingress/egress process of anions. Based on this, this combination of experimental approaches can be used to detect Cl−. SPR signal exhibited a good linear relationship with the concentration of Cl−.

Application of ultrasonic wave propagation imaging method to automatic damage visualization of nuclear power plant pipeline

Hundreds of kilometers of pipeline, especially the elbows and welded joints in nuclear power plants (NPPs), are susceptible to aging and other types of damage. Therefore, the condition of a piping system requires regular inspection. We propose an automatic damage visualization technique using a laser ultrasonic scanning system and an ultrasonic wave propagation imaging (UWPI) method for the regular inspection process. Ultrasonic wave propagation movies (UWPMs) in thick and complex NPP pipes could be successfully visualized in a straight pipe with a crack, a welded pipe with multiple open and inner cracks at the welds, and an elbow pipe with wall thinning. A UWPM with ultrasonic time information could enable the discrimination of damage-induced wavefields from the wavefield not related to damage by providing the appearance time, source location, wavefront curvature, and propagation direction of the damage-induced wavefield. For the specimens tested, a 1 mm long crack in a 9 mm thick pipe could be localized regardless of the relative orientation of a sensor to the crack direction; 4 mm long 1.5 mm deep inner crack in a 4 mm thick welding bead could be detected despite the presence of the welding bead; wall thinning with gradual property variations could also be detected. In addition, since this method did not require laser focusing or reference data from the undamaged condition, and allows a large laser beam incident angle, it was very advantageous for the automatic scanning of the curved pipe surfaces. This study demonstrated that the proposed laser ultrasonic system equipped with the UWPI method is a useful inspection tool for NPP piping system management.

Innovative design for optical porous silicon gas sensor

A simple system for the detection of analytes in the atmosphere employing a freestanding nanostructured porous silicon optical microcavity was designed and built. The system is based on the measurement of optical transmittance changes as a function of the incidence angle of a monochromatic laser beam. Using a matrix formalism and effective medium theories we calculate the optimal conditions for the system so as to obtain the best sensitivity of the sensor. We tested the system over a detection area of the porous silicon microcavity of 100 μm 2 with isopropyl alcohol vapor diluted in a N 2 stream. With the system proposed we were able to detect changes in the concentration of isopropyl alcohol as small as 1 ppm, which is equivalent to a change in the effective refractive index of about 3 × 10 −6. The response time is very fast, lower than 0.5 s.