Laser Incident Angle Research Articles

Influence of laser process parameters on the characteristic of 30CrMnSiA steel substrate and adhesively bonded joints

The effect of laser process parameters including single pulse energy (E), dimples per square millimetre (D) and laser incidence angle (θ) on the properties of 30CrMnSiA steel substrate and the tensile shear strength of adhesively bonded joints was investigated. The characterization of the substrate was established by surface morphology, the dimension (diameter and depth) of dimples and surface roughness (Ra and Rz). Single lap joints were fabricated and tensile tests were then performed. The fracture surfaces were observed and analyzed. The results indicated that the dimple dimension and surface roughness increased as the single pulse energy increased. The increase of dimples per square millimetre made surface roughness improve. The larger incidence angle reduced the actually absorbed energy changing the shape and size of dimples. The diameter of dimples increased while the depth decreased as incidence angle increased. The modified substrate morphology as well as increased dimple dimension expanded the contact area between the adhesive and adherend. The shear strength was enhanced and the maximum value was 24.67 MPa increasing by 363% compared to the adhesive joints without surface treatment. The mechanical interlocking between the adhesive and laser textured morphology was discovered in the fracture surface demonstrating the improvement of shear strength.

Manipulating birefringence in AgCl thin film loaded by silver nanoparticles under normal and oblique incident angles

We have induced anisotropy in AgCl thin film, which is loaded by silver nanoparticles, by irradiating a single low power laser beam at wavelength 532 nm. The induced anisotropy in Ag–AgCl thin film is due to the formation of gratings in the direction of light polarization for both normal and oblique angles of laser incidence. In Ag–AgCl thin film, which was irradiated by oblique laser beam, complex grating with different line-spaces is formed. It turns out that the line-spaces of the grating depend on the incident angle. Therefore, by changing the incident angle of the laser beam, the morphology and optical properties of anisotropic nanostructures formed on Ag–AgCl thin film can be manipulated. Linear birefringence and linear dichroism are measured in these nanostructures by probe beams at different wavelengths in the visible region. Our results show that the sample which is irradiated normally has the largest linear birefringence and linear dichroism compared to the oblique incident samples. In the normal incident sample, the maximum value of linear birefringence is seen at wavelength 632.8 nm (Δn = 0.35), while the largest absolute value of linear dichroism is observed at wavelength 532 nm, which is the same wavelength as the incident laser beam.

Laser Induced Nano and Micro Structures of Molybdenum Surface Applied in Multistage Depressed Collector for Secondary Electron Suppression

The laser processing molybdenum metal surface method was first proposed to enhance the efficiency of multistage depressed collectors (MDCs). In this study, the secondary electron yield (SEY), surface geometrical morphologies and chemical states of laser processed Mo metal samples were characterized. For the first time, the effects of laser parameters and incident angle of primary electrons on the SEY property of laser induced nano and micro structures of molybdenum surface were investigated. The influence rule of laser processing patterns, power, pitch spacing, scanning speed on surface morphologies and SEY were systematically explored. A maximum SEY of laser processed Mo metal less than one was achieved. The Mo 3d and O 1s core level spectra of Mo metal before and after laser processing were analyzed in this study. Furthermore, the corresponding oxidation states of Mo as well as the relative distribution were elucidated.

Double optimal density gradients for harmonic generation from relativistically oscillating plasma surfaces

The influence of plasma density gradients on the harmonic generation process from relativistically oscillating mirror (ROM) is studied experimentally. It is observed that the harmonic intensities from orders of 21st to 24th first drop and then reach the maximum value with the increase of plasma scale length L. 2D particle-in-cell simulations are performed to investigate the intensities of high-order harmonics at different L values, which show that there are two optimal scale lengths Lopt for efficient ROM harmonic generation. The two optimal Lopt values are interpreted by a quasi-1D analytical model. By matching the potential energies provided by laser pulse and charge displacement in the plasma with an exponential distributed density profile, the model can quantitatively predict the optimal Lopt at the given laser incidence angle and intensity. Our work is beneficial for better understanding the role of L in ROM harmonic generation and the parameters affecting the optimal Lopt.

Sensitivity Analysis of Laser Quenching Parameters of ASTM 1045 of Disk Laser Based on Response Surface Method

Laser hardening is an important branch of laser surface hardening technology, which is widely used in metallurgy, transportation, machinery manufacturing, aerospace and other fields. At present, relying entirely on experience or process trial-and-error method, it can not effectively reveal the transient mechanism of laser quenching of disk laser, which is not conducive to shortening the research and development cycle and saving costs. The numerical simulation provides an effective way to obtain the dynamic evolution law of multi-field coupling in laser quenching process. In this paper, a thermo-mechanical coupling model of ASTM 1045 laser quenching process by disk laser is established. In the model, the temperature-dependent physical parameters were calculated by CALPHAD method. The transient law of temperature and microstructural transformation during quenching was obtained by solving the model. The formation and transformation degree of martensite were characterized by the dynamic changes of the depth and width of quenched transformation layer. The quenching structure and transformation hardening law were observed by Axioskop 2 SEM and Zeiss-IGMA HD FE-SEM to verify the accuracy of the simulation results. On this basis, the process parameters of laser quenching were sampled by Monte-Carlo method based on response surface methodology. The sensitivity effects of different process parameters on the temperature field and phase change field of laser quenching were analyzed, which laid a theoretical foundation for the optimization of process parameters. (1) Establish a thermo-mechanical coupling model for quenching process of ASTM 1045, and the sensitivity of the system is calculated. (2) The model can accurately reproduce the transient process of temperature and phase transformation field during laser quenching. (3) The laser power and laser incident angle are positively correlated with the response value. (4) The spot diameter and scanning speed are negatively correlated with response value. (5) The model can effectively calculate the sensitivity of laser quenching, predict the width and depth of quenching layer, and optimize process parameters.

Investigation of laser active reconnaissance system performance based on its parameters

Laser active reconnaissance technology is one of the important technologies in the field of optoelectronic countermeasures. However, the Laser active reconnaissance effect is affected by various kinds of factors. According to the process of laser active reconnaissance, the laser active reconnaissance model is established. Furthermore, the echo power and the relationship between the power and various factors are simulated and analyzed. The results show that the echo power is subject to laser divergence angle, atmospheric visibility, angle of incidence and other factors. Among them, the divergence angle has the greatest influence on echo power, followed by the laser incident angle. The slight increase of laser divergence angle and laser incident angle will cause the echo power to drop sharply. However, the slight increase of receiving aperture of cat eye optical window will lead to the echo power increase rapidly. Therefore, in order to obtain the maximum laser echo power in specific application environment, it is necessary to select appropriate parameters for laser active reconnaissance system.

Eavesdropper's ability to attack a free-space quantum-key-distribution receiver in atmospheric turbulence

The ability of an eavesdropper (Eve) to perform an intercept-resend attack on a free-space quantum key distribution (QKD) receiver by precisely controlling the incidence angle of an attack laser has been previously demonstrated. However, such an attack could be ineffective in the presence of atmospheric turbulence due to beam wander and spatial mode aberrations induced by the air's varying index of refraction. We experimentally investigate the impact turbulence has on Eve's attack on a free-space polarization-encoding QKD receiver by emulating atmospheric turbulence with a spatial light modulator. Our results identify how well Eve would need to compensate for turbulence to perform a successful attack by either reducing her distance to the receiver, or using beam wavefront correction via adaptive optics. Furthermore, we use an entanglement-breaking scheme to find a theoretical limit on the turbulence strength that hinders Eve's attack.

Scan Line Intensity-Elevation Ratio (SLIER): An Airborne LiDAR Ratio Index for Automatic Water Surface Mapping

Owing to the characteristics of how a laser interacts with the water surface and water column, the measured Light Detection and Ranging (LiDAR) intensity values are different with respect to the laser wavelength, the scanning geometry and the reflection mechanism. Depending on the instantaneous water condition and the laser incidence angle, laser dropouts can appear, causing null returns or empty holes found in the collected LiDAR data. This variable intensity response offers a valuable opportunity for using airborne LiDAR sensors for automatic identification of water regions, and thus, we previously proposed an airborne LiDAR-based ratio index named the scan line intensity-elevation ratio (SLIER). Over the water surface, airborne LiDAR data are always found to have a high fluctuation of the intensity value and low variation of the elevation along each scan line, and thus, the water region has a higher SLIER value compared to the land. We examined the SLIER on a multispectral airborne LiDAR dataset collected by Optech Titan and a monochromatic airborne LiDAR dataset collected by Optech Galaxy on a natural rocky shore and a man-made shore. Our experiments showed that SLIER was able to provide a high separability between land and water regions and was able to outperform the traditional normalized difference water index (NDWI) for estimation of the water surface. With the use of SLIER as a mechanism for training data selection, our case studies demonstrated an overall accuracy of 98% in the use of either monochromatic or multispectral LiDAR data, regardless of the laser channel being used.

Plasmonic enhancement and directional emission for side-illumination tip-enhanced spectroscopy

Realization of single-molecule sensitivity is crucial for tip-enhanced spectroscopy (TES) and its applications. Plasmonic enhancement and directional emission of TES, which are related to the improvement of detection sensitivity, have been theoretically investigated. Calculated enhancement factors for Raman and fluorescence spectroscopy are of the order 1010 and 104 respectively. Directional TES emission reveals an optimal collection angle of ±25º, although tradeoffs must be made between the emission and laser incident angles to ensure high-sensitivity TES. The proposed method enhances detection sensitivity by considering the contributions of plasmonic enhancement and directional emission of TES, useful for the design of high-sensitivity TES platforms.

Experimental study of pulsed laser bending process parameters of 6061 aluminum alloy sheet

The effect of process parameters on pulsed laser bending of 6061 aluminum alloy sheet was investigated experimentally under oblique incidence. The results of single factor experiment showed that the bending angle increased with the increase of duty cycle, frequency, scan times and sheet width, decreased with the increase of scan speed, spot diameter and sheet thickness, and increased first and then decreased with the increase of the distance from the scanning line to the free end. The bending angle of water cooling was smaller than the natural cooling, but the bending angle increment of water cooling exceeded the natural cooling after 55 scans. When the laser incident angle was 15°, the bending angle reached 102.35° after 60 scans under natural cooling condition. The combination of optimal processing parameters and the order of factors affecting the laser bending angle were determined by orthogonal experiment. The order of influencing factors was duty cycle, scan speed, spot diameter, frequency and the distance from the scanning line to the free end.

Maximizing magnetic field generation in high power laser–solid interactions

In order to understand the transport of fast electrons within solid density targets driven by an optical high power laser, we have numerically investigated the dynamics and structure of strong self-generated magnetic fields in such experiments. Here we present a systematic study of the bulk magnetic field generation due to the ponderomotive current, Weibel-like instability and resistivity gradient between two solid layers. Using particle-in-cell simulations, we observe the effect of varying the laser and target parameters, including laser intensity, focal size, incident angle, preplasma scale length, target thickness and material and experimental geometry. The simulation results suggest that the strongest magnetic field is generated with laser incident angles and preplasma scale lengths that maximize laser absorption efficiency. The recent commissioning of experimental platforms equipped with both optical high power laser and X-ray free electron laser (XFEL), such as European XFEL-HED, LCLS-MEC and SACLA beamlines, provides unprecedented opportunities to probe the self-generated bulk magnetic field by X-ray polarimetry via Faraday rotation with simultaneous high spatial and temporal resolution. We expect that this systematic numerical investigation will pave the way to design and optimize near future experimental setups to probe the magnetic fields in such experimental platforms.

Influence of single-beam expanding scanning laser circumferential detection system parameters on detection capability

Aiming at the high requirement for pulse-repetition frequency of the existing single-beam synchronous scanning circumferential detection, which is difficult to use practically. The method of single-beam expanding scanning laser circumferential detection is proposed. Based on the principle of single-beam expanding scanning laser circumferential detection, the mode of scanning has an inherent defect of periodic detection blind area in the detection field. The method of one-way spreading laser line beam into fan-shaped beam is proposed. The analytical expression of the lowest scanning frequency and the pulse frequency are derived. Echo characteristics of cylindrical target and the section attenuation coefficient are analyzed. Mathematic model of cylindrical target echo power of pulsed expanding laser beam is established. The mathematical model of section attenuation coefficient of cylindrical object is established, and the variation of the section attenuation coefficient when the center line and the edge of the beam have different positions relative to the cylindrical target is analyzed. The expression of the position having the smallest section attenuation coefficient and the expression of largest angle between the adjacent pulse laser beams are obtained, then the influence of system parameters on the section attenuation coefficient is also discussed. The emphasis is placed on the influence of pulse frequency, beam angle and incidence angle on the ability to detect different diameter targets. As the laser pulse frequency increases, the detectable target diameter is smaller and the detection ability is stronger. Increasing the beam angle and lowering the laser incident angle are beneficial to reducing the minimum laser pulse frequency required to discover the target. The methods of calculating maximum beam angle and minimum pulse frequency under typical conditions of the detection system are presented. When the incident angles are <inline-formula><tex-math id="M1">\begin{document}${\text{π}}/3$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181860_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181860_M1.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M2">\begin{document}${\text{π}}/4$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181860_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181860_M2.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M3">\begin{document}${\text{π}}/6$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181860_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181860_M3.png"/></alternatives></inline-formula>, the maximum beam angle and the lowest pulse frequency are calculated for a cylindrical target with a diameter of 0.18 m at a detection distance of 6 m, the minimum pulse frequency decreases effectively after beam expansion. The results show that the pulse repetition frequency will be effectively reduced by slightly expanding the beam. This study may provide theoretical basis for designing and optimizing the single-beam pulsed laser circumferential detection.

Simulation on tip-assisted focusing of laser energy for sub-surface photon heating

The extensive confinement of electromagnetic field and the resulting localized heating effect induced by metallic nanotips are of great interest which promise novel applications including material surface processing and laser-assisted manufacturing. In this work, the sub-surface heating effect of silicon and monolayer graphene induced by gold nanotip-light interaction is studied through numerical simulations. The optimized optical enhancement around a free-standing gold tip is firstly studied with respect to the incident laser wavelength and incident angle. The maximum electric intensity enhancement is up to ∼66 times within sub-10 nm region around the tip apex which is obtained under a 532 nm p-polarized laser incidence. With the presence of silicon under the nanotip, the optical field in the gap is enhanced further, generating a heat source at the sub-surface (∼2 nm depth) of silicon. The heat source energy density is as high as 3 × 1015 W m−3 with a moderate incident intensity of 2 × 107 W m−2. As for gold nanotip-monolayer graphene configuration with a separation distance of 1 nm, the enhanced electric field can penetrate into monolayer carbon atoms and induce intensive heating by 55 times in the graphene and by 104 times across the carbon layer surface, which could be used for localized modifications of graphene surface like surface cleaning and property tuning. Simulation results in this work provide a guide for the laser-assisted manufacturing/modification not only in the bulk materials but also in the 2D nanostructures.

Terahertz wave generation from liquid water films via laser-induced breakdown

Understanding the physics of terahertz (THz) wave generation from water is essential for developing liquid THz sources. This letter reports detailed experimental measurements of THz wave emission by focusing intense laser pulses onto water films. The simulation based on a ponderomotive force-induced dipole is supported by the observation of the THz intensity dependence on the laser incidence angle. This work provides fundamental insights into the THz wave generation process in water and an alternative perspective for studying laser-induced breakdown in liquids.

Thermal effects of the narrow band filter irradiated by laser at oblique incidence

Based on the Maxwell equations and the heat conduction theory, the thermal effects of the 10.6μm narrow band filter are analysed under the irradiation of the pulsed laser at oblique incidence. The temperature field and the drift effect of the narrow band filter are compared and discussed at different incident angles and polarizations. Results show that,with the increase of incident angle,the amplitude and position coordinates of the peak temperature rise have changed greatly.The peak value of the temperature rise decreased gradually with the increasing incident angles. The position coordinates of the peak temperature rise gradually approaches to the interface of the filter and air with the increasing incident angle.The difference of thermal effects between the P and S polarized laser irradiation becomes more and more obvious. The passband drift characteristics under different laser incident angles are compared. When the incident angle is small, the central wavelength deviation of the filter reached over 200nm, and the transmittance decreased to less than 6%.With the increase of the incident angle, the melting damage threshold of the filter is getting bigger and bigger. When the incident angle is greater than 30°, the difference of the melting damage threshold between P and S polarized light becomes larger and larger.The incident angle and polarization state of the laser should be considered in order to realize the laser damage to the narrow band filter with a small power.

Tabletop laser-driven gamma-ray source with nanostructured double-layer target

Laser-driven gamma-ray source potentially offers a compact, cost-effective, ultra-short, and ultra-bright alternative to conventional gamma-ray sources based on large-scale particle accelerators. Based on the laser-driven approach, we use multidimensional particle-in-cell simulations to demonstrate that a nanostructured double-layer target, which consists of a nanostructured foam coated on top of a metal substrate, can absorb laser energy into high-energy electrons in the nanostructured foam, and then efficiently convert it into copious gamma photons via the nonlinear Compton scattering process enabled by the solid-density substrate, which acts as a plasma mirror to reflect the laser pulse. The effects of different nanostructures in the foam target and the oblique laser incidence are presented. It is shown that the conversion efficiency of gamma photons increases when the size of nanoparticles decreases or the filling factor of nanoparticles increases in nanostructured foam target, but decreases when the laser incidence angle increases. At realistic conditions with nanostructured foam and non-normal incidence, the double-layer target still exhibits an unprecedentedly high conversion efficiency in high-energy gamma-ray production due to the laser reflection by the plasma mirror, which can be two and even three orders of magnitude higher than that of the single-layer target without the substrate using currently available lasers with intensity of 1021 .

A Multifunctional Dye‐doped Liquid Crystal Polymer Actuator: Light‐Guided Transportation, Turning in Locomotion, and Autonomous Motion

AbstractA strip of a liquid crystal elastomer doped with a near‐infrared dye with one side crosslinked monodomain and the other crosslinked polydomain along the thickness behaves like a multifunctional photoactuator without the need for a support. A flat strip with two ends fixed on substrate surface forms a moving bump under laser scanning, which can be used as light‐fueled conveyor to transport an object. Cutting off and laser scanning the bump with two free ends makes a soft and flexible millimeter‐scale crawler that can not only move straight and climb an inclined surface, but also undergo light‐guided turning to right or left as a result of combined out‐of‐plane and in‐plane actuation. Based on the self‐shadowing mechanism, with one end of the strip fixed on substrate surface, it can execute a variety of autonomous arm‐like movements under constant laser illumination, such as bending–unbending and twisting, depending on the laser incident angles with respect to the strip actuator.

A Multifunctional Dye-doped Liquid Crystal Polymer Actuator: Light-Guided Transportation, Turning in Locomotion, and Autonomous Motion.

A strip of a liquid crystal elastomer doped with a near-infrared dye with one side crosslinked monodomain and the other crosslinked polydomain along the thickness behaves like a multifunctional photoactuator without the need for a support. A flat strip with two ends fixed on substrate surface forms a moving bump under laser scanning, which can be used as light-fueled conveyor to transport an object. Cutting off and laser scanning the bump with two free ends makes a soft and flexible millimeter-scale crawler that can not only move straight and climb an inclined surface, but also undergo light-guided turning to right or left as a result of combined out-of-plane and in-plane actuation. Based on the self-shadowing mechanism, with one end of the strip fixed on substrate surface, it can execute a variety of autonomous arm-like movements under constant laser illumination, such as bending-unbending and twisting, depending on the laser incident angles with respect to the strip actuator.

Rapid Evaluation of All-Optical Modulation in Quantum Cascade Laser Using Photoluminescence Spectrum

All-optical modulation in a quantum cascade laser (QCL) can be rapidly evaluated using a photoluminescence (PL) spectrum. The PL spectrum reflects the electron transition rate of the sub-bands of the active region of a QCL. The peak position, amplitude, and displacement of the PL spectrum can be used to determine the optimal wavelength, spot position, the laser incident angle, optical power, and displacement in the electric field of the near-infrared laser. This PL spectrum approach is rapid, accurate, and simple. Furthermore, it can be used to optimize high-speed all-optical modulation of QCLs for use in free-space optical communication and molecular-detection applications.

Nanosecond pulsed laser irradiation of sapphire for developing microstructures with deep V-shaped grooves

A nanosecond pulsed Nd:YAG laser was used to irradiate a sapphire substrate to produce 3-dimensional microstructures with sharp V-shaped grooves. In initial experiments, where single grooves were machined, a maximum taper angle of ∼79° was obtained. At constant laser parameters, the taper angle remained constant. As the taper angle increases from a flat surface, the irradiated area increases while incident fluence decreases; once the incident fluence approaches the ablation threshold, the taper angle becomes constant. The taper angle could be controlled by the laser fluence, scanning speed and incident angle of the laser beam. Using these results, a kind of surface microstructure, comprised of micropyramids with steep walls, was successfully machined for optical measurement applications. The surface roughness, transmittance and crystallinity of the microstructure surface could be controlled by the laser scanning speed. By applying the taper formation mechanism proposed in this study, the micromachining of sharp microstructures with steep walls on various hard brittle materials becomes possible.