super-Gaussian Profile Research Articles

Magnetic field enhanced terahertz emission from metallic nanostructures with response to laser spatial profile distribution

This communication proposes an analytical model that exhibits terahertz (THz) radiation generation from spatially corrugated noble-metal spherical and cylindrical nanoparticles (NPs) placed under the influence of an externally applied static magnetic field. This scheme involves the resonant excitation of THz radiation through the beat-wave of two lasers in the presence of ponderomotive nonlinearities. Effect of magnetic field as well as laser intensity profiles, which include super, cosh, flat-top and ring-shaped Gaussian profiles, have been investigated. The incorporation of the magnetic field induces substantially more dynamic laser-metal coupling and influences the surface plasmon resonance condition associated with electrons of the NPs, thereby leading to stronger THz emission. Our results also demonstrate that the spatial profile of the laser affects the THz output, as it impacts the excitation and dynamics of the NPs. Additionally, we find that both the shape, size and interparticle-separation of NPs play crucial roles in enhancing THz generation. Furthermore, we explore the effects of varying other parameters like electric field strength and beam waist of incident lasers. These findings provide valuable insights for the design and optimization of THz sources based on NP systems, offering new avenues for applications in spectroscopy, imaging, communication and biomedical sciences.

Designing the Second Target Station with a Coupled Neutronics-Mechanical Optimization Workflow

The Second Target Station (STS) at the Spallation Neutron Source of the U.S. Department of Energy’s Oak Ridge National Laboratory is being designed to become the world’s highest peak brightness source of cold neutrons. As the STS project evolves, neutronics and other engineering analyses will inform many design iterations. To facilitate this process, a fully automated optimization workflow was developed to convert a parameterized computer-aided-design model of the target into an unstructured mesh geometry model and then to run a neutronics calculation and (optionally) a mechanical analysis for each design iteration. This workflow enables efficient, high-fidelity modeling; simulation; and optimization of new designs, as has been demonstrated for the optimization of the STS neutron moderators. In this paper, we present the results of our first major effort to automate the design optimization process for a spallation target. In the first analysis, the goal is to find optimal dimensions of a monolithic tungsten target coupled with an optimal super-Gaussian proton beam profile to deliver maximum brightness of the resulting neutron beams while maintaining good mechanical properties of the target. In the second analysis, geometric and beam parameters are optimized for an alternative design with tungsten plates, which can reach superior mechanical performance without compromising the neutronics performance.

Manipulating melt pool thermofluidic transport in directed energy deposition driven by a laser intensity spatial shaping strategy

ABSTRACT A three-dimensional thermofluidic coupling transport model is proposed to investigate the influences of different spatial laser intensity profiles (SLIPs), including circular super-Gaussian profile (C-SGP), transverse elliptical Gaussian profile (TE-GP) and longitudinal elliptical Gaussian profile (LE-GP), on the thermofluidic transport characteristics within the melt pool. The results demonstrate that the SLIPs dramatically influence the melt pool geometries, temperature gradient (both in magnitude and direction) at the solidification interface, and fluid flow dynamics. Under the TE-GP strategy, the highest average temperature gradients are observed at the solidification interface. The LE-GP strategy yields the smallest magnitudes and narrowest variation range of the temperature gradient direction angles. The heat transport of the melt pool under the C-SGP and TE-GP strategies are jointly dominated by convective and conductive heat transfer, while those under the LE-GP strategy are dominated by convective heat transfer. Marangoni convection is strongest in the LE-GP strategy and weakest in the TE-GP strategy.

Combined super-Gaussian pump intensity profile model and its application to a solid-state laser

The modeling of a combined super-Gaussian profile distribution for fiber-coupled laser diode beam intensity is investigated in this paper. By fitting the measured beam intensity distribution to the combined super-Gaussian model, the coefficient of determination is better than that of the Gaussian function and super-Gaussian function with order of 2. Applying this theoretical model to the fiber-coupled laser diode end-pumped Nd:GdVO4 laser at 1342 nm, the theoretical calculations are in good agreement with the experimental results.

Sub-kHz-linewidth laser generation by self-injection locked distributed feedback fiber laser

We presented an integrated all-fiber sub-kHz-linewidth distributed feedback fiber laser (DFB-FL) assisted by self- injection locking. A π phase-shifted fiber Bragg grating (π-FBG) was employed to achieve single longitudinal mode operation, which was UV-inscribed in Er/Yb co-doped fiber with super-Gaussian apodization profile. The experiment results have shown that the apodized π-FBG based laser cavity has got the higher slope efficiency and the narrower linewidth, compared with the normal grating, which are consistent with the simulation one very well. To further compress the linewidth, we have employed the self-injection locking mechanism, which the linewidth of output laser could reach to around 350 Hz with 10-m delay fiber. After integrated encapsulation, the relative intensity noise (RIN) peak of the DFB-FL was reduced to below −125 dB/Hz. The beat frequency fluctuation was less than 15 MHz within 1000 s and power fluctuation was less than 1% in 7 h.

Modulated super-Gaussian laser pulse to populate a dark rovibrational state of acetylene.

A pulse-shaping technique in the mid-infrared spectral range based on pulses with a super-Gaussian temporal profile is considered for laser control. We show a realistic and efficient path to the population of a dark rovibrational state in acetylene (C2H2). The laser-induced dynamics in C2H2 are simulated using fully experimental structural parameters. Indeed, the rotation-vibration energy structure, including anharmonicities, is defined by the global spectroscopic Hamiltonian for the ground electronic state of C2H2 built from the extensive high-resolution spectroscopy studies on the molecule, transition dipole moments from intensities, and the effects of the (inelastic) collisions that are parameterized from line broadenings using the relaxation matrix [A. Aerts, J. Vander Auwera, and N. Vaeck, J. Chem. Phys. 154, 144308 (2021)]. The approach, based on an effective Hamiltonian, outperforms today's abinitio computations both in terms of accuracy and computational cost for this class of molecules. With such accuracy, the Hamiltonian permits studying the inner mechanism of theoretical pulse shaping [A. Aerts et al., J. Chem. Phys. 156, 084302 (2022)] for laser quantum control. Here, the generated control pulse presents a number of interferences that take advantage of the control mechanism to populate the dark state. An experimental setup is proposed for in-laboratory investigation.

Final amplifier of an ultra-intense all-OPCPA system with 13-J output signal energy and 41% pump-to-signal conversion efficiency.

Optical parametric chirped-pulse amplification (OPCPA) using high-energy Nd:glass lasers has the potential to produce ultra-intense pulses (>1023 W/cm2). We report on the performance of the final high-efficiency amplifier in an OPCPA system based on large-aperture (63 × 63-mm2) partially deuterated potassium dihydrogen phosphate (DKDP) crystals. The seed beam (180-nm bandwidth, 110 mJ) was provided by the preceding OPCPA stages. A maximum pump-to-signal conversion efficiency of 41% and signal energy up to 13 J were achieved with a 52-mm-long DKDP crystal due to the flattop super-Gaussian pump beam profile and flat-in-time pulse shape.

Exploring the Benefits of Ablation Grid Adaptation in 2D/3D Laser Ablation Inductively Coupled Plasma Mass Spectrometry Mapping through Geometrical Modeling.

This study aims to investigate the potential benefits of adapting the ablating grid in two-dimensional (2D) and three-dimensional (3D) laser ablation inductively coupled plasma mass spectrometry in a single pulse mapping mode. The goals include enhancing the accuracy of surface sampling of element distributions, improving the control of depth-related sampling, smoothing the post-ablation surface for layer-by-layer sampling, and increasing the image quality. To emulate the capabilities of currently unavailable laser ablation stages, a computational approach using geometrical modeling was employed to compound square or round experimentally obtained 3D crater profiles on variable orthogonal or hexagonal ablation grids. These grids were optimized by minimizing surface roughness as a function of average ablation depth, followed by simulating the post-ablation surface and related image quality. An online application (https://laicpms-apps.ki.si/webapps/home/) is available for users to virtually experiment with contracting/expanding orthogonal and hexagonal ablation grids for generic 3D super-Gaussian laser crater profiles, allowing for exploration of the resulting post-ablation surface layer roughness and depth.

Hybrid PIC–fluid simulations for fast electron transport in a silicon target

Ultra-intense laser-driven fast electron beam propagation in a silicon target is studied by three-dimensional hybrid particle-in-cell–fluid simulations. It is found that the transverse spatial profile of the fast electron beam has a significant influence on the propagation of the fast electrons. In the case of a steep spatial profile (e.g., a super-Gaussian profile), a tight fast electron beam is produced, and this excites more intense resistive magnetic fields, which pinch the electron beam strongly, leading to strong filamentation of the beam. By contrast, as the gradient of the spatial profile becomes more gentle (e.g., in the case of a Lorentzian profile), the resistive magnetic field and filamentation become weaker. This indicates that fast electron propagation in a solid target can be controlled by modulating the spatial gradient of the laser pulse edge.

Self-focusing and defocusing phenomena of super-Gaussian laser beams in plasmas carrying density gradient

Considering nonlinearities due to relativistic mass variation and ponderomotive force driven depression in the electron density, a model is developed for the self-focusing of a super-Gaussian laser beam in inhomogeneous plasmas. Since paraxial ray approximation is not appropriate for the beams of super-Gaussian profile, the formulation is developed based on the moment theory. To have an in-depth understanding of self-focusing and defocusing phenomena and to develop generalized treatment, three different types of plasma density profiles namely linear, tangent and exponential density profiles, are considered and the results are compared. Due to the saturating nature of nonlinear permittivity with laser intensity, the beam is found to undergo periodic focusing because of competition between the diffraction divergence and nonlinear self-focusing effects. The ponderomotive nonlinearity enhances the self-focusing. As the super-Gaussian beam moves into higher density plasma region up to several Rayleigh lengths, stronger self-focusing is achieved and the value of minimum spot size decreases.

Quasi-1D sedimentation of Brownian particles along optical line traps

We studied the Brownian motion of single 1 and 3 µm particles moving along optical line traps oriented in the gravity axis. The finite linear traps are formed using holographic optical tweezers which redistributes the incoming light beam into an elongated super Gaussian profile. Direct observation of the particle’s motion along the gravity axis allowed us to distinguish the non-equilibrium sedimentation of the particle, when the average velocity is 〈uz〉≠0, followed by equilibrium sedimentation when 〈uz〉=0 and the particle establishes the so-called gravitational length. We found that the bottom edge of the linear trap functions as a virtual “floor”, and subsequent Boltzmann distribution analysis of the equilibrium particle trajectories allows the estimation of the particle’s mass in the hundreds of femtograms range. In parallel, we also induced the physisorption of a macromolecule (hyaluronic acid) onto polystyrene microparticle in solution, detecting an increment of the particle’ mass of around 2 picograms. This suggests that the present technique can be used in surface chemistry, as it detects analyte absorption in solutions containing colloids by estimating the mass of both, the colloidal particle and the layer formed around it. On the other hand, we also found that when the longitudinal confinement established by the optical trap length is smaller than twice the particle gravitational length, particles remain suspended indefinitely even in the presence of a density mismatch with the medium, i.e., the notion of sedimentation disappears. Finally, we observed that the optical forces along the linear trap do not alter the Brownian motion of the particles significantly but can exert an extra pushing force on the smallest particles (1 µm) considered.

Influence of Curvature Feature on Laser Heating during Tape Placement Process for Carbon Fiber Reinforced Polyether Ether Ketone Composite

The curvature feature makes the irradiance and absorptivity change, resulting in an uneven power density distribution, which affects the quality of composite parts. In this study, a theoretical model-based Super-Gaussian profile beam in the laser irradiation area was established to obtain the heat flux distribution on the curved surface. The effect of curvature on the surface scattering reflection, temperature distribution, and surface morphology were investigated and verified the validity of the theoretical model. Furthermore, the influence of the laser intensity distribution, laser inclination and curvature radius on the power density distribution and distribution uniformity were studied. Research indicated that the power density increases as the distance from the origin increase resulting from the variation of the irradiance and absorptance along the circumference. The flatter the intensity distribution of the laser beam in the height direction, the less uniform the power density distribution. Accordingly, the typical Gaussian profile beam significantly ameliorates the power density distribution. This research provides a novel understanding of using heat sources during laser heating thermoplastic tape placement.

Simulated LCSLM with Inducible Diffractive Theory to Display Super-Gaussian Arrays Applying the Transport-of-Intensity Equation

We simulate a liquid crystal spatial light modulator (LCSLM), previously validated by Fraunhofer diffraction to observe super-Gaussian periodic profiles and analyze the wavefront of optical surfaces applying the transport-of-intensity equation (TIE). The LCSLM represents an alternative to the Ronchi Rulings, allowing to avoid all the related issues regarding diffractive and refractive properties, and noise. To this aim, we developed and numerically simulated a LCSLM resembling a fractal from a generating base. Such a base is constituted by an active square (values equal to one) and surrounded by eight switched-off pixels (zero-valued). We replicate the base in order to form 1 ×N-pixels and the successive rows to build the 1024×1024 LCSLM of active pixels. We visually test the LCSLM with calibration images as a diffractive object that is mathematically inducible, using mathematical induction over the N×N-shape (1×1, 2×2, 3×3, …, n×n pixels for the generalization). Finally, we experimentally generate periodic super-Gaussian profiles to be visualized in the LCSLM (transmission SLM, 1024×768-pixels LC 2012 Translucent SLM), modifying the TIE as an optical test in order to analyze the optical elements by comparing the results with ZYGO/APEX.

Modeling of whole-phase heat transport in laser-based directed energy deposition with multichannel coaxial powder feeding

The whole-phase heat transport model of laser-based directed energy deposition (L-DED) with multichannel coaxial powder feeding is the basis of investigating the underlying physical mechanisms, such as molten pool evolution, thermal history, grain growth, stress and deformation evolution. The existing heat transport models usually ignore or simplify some important physical processes, which limits the simulation accuracy of the models to some extent. In this paper, a modeling framework for whole-phase heat transport for coaxial L-DED is developed. Considering the interaction among the laser beam, coaxial multipowder stream and substrate, a comprehensive whole-phase heat transport model including a parametric model of powder-scale laser-powder coupling and deposition heat transport model is developed. A parametric model for laser-powder coupling can enable one to avoid the influence of the parameters of the coaxial multichannel powder-fed head on the number and structure of the model. The spatial relations between the laser beam, coaxial multipowder stream and substrate are modeled based on homogeneous transformation theory, which effectively addresses coordinate transformation on the complex spatial position and pose. The influence of two typical spatial laser beam profiles (SLBPs): the Gaussian profile (GP) and super-Gaussian profile (SGP), on the whole-phase heat transport is studied. The results show that under the conditions of the SGP and GP, the peak temperature of the coaxial multipowder stream on the deposition surface is below the melting point. The peak temperature distribution and powder heat flux for the coaxial multipowder stream shows a distribution with a lower center, higher sides and cruciform tower-like shape. The peak temperature and heat flux of the coaxial multipowder stream under the SGP are less than that under the GP. Dimensionless number analysis indicates that under both the SGP and GP, the Marangoni convection and thermal conduction jointly dominate the heat transport behavior within the molten pool. Compared to the GP, the Marangoni effect and the heat accumulation capacity of the molten pool under the SGP are weaker, but the heat dissipation capacity of the molten pool is stronger. The use of the SGP is favorable for reducing the thermal deformation of various parts during L-DED.

High-efficiency near-infrared optical parametric amplifier for intense, narrowband THz pulses tunable in the 4 to 19 THz region

Dynamic control of material properties using strong-field, narrowband THz sources has drawn attention because it allows selective manipulation of quantum states on demand by coherent excitation of specific low-energy modes in solids. Yet, the lack of powerful narrowband lasers with frequencies in the range of a few to a few tens of THz has restricted the exploration of hidden states in condensed matter. Here, we report the optimization of an optical parametric amplifier (OPA) and the efficient generation of a strong, narrowband THz field. The OPA has a total conversion efficiency of > 55%, which is the highest value reported to date, with an excellent energy-stability of 0.7% RMS over 3 h. We found that the injection of a high-energy signal beam to a power amplification stage in an OPA leads to high-efficiency and a super-Gaussian profile. By difference-frequency generation of two chirped OPA signal pulses in an organic nonlinear crystal, we obtained a THz pulse with an energy of 3.2 μJ, a bandwidth of 0.5 THz, and a pulse duration of 860 fs tunable between the 4 and 19 THz regions. This corresponds to an internal THz conversion efficiency of 0.4% and a THz field strength of 6.7 MV/cm. This approach demonstrates an effective way to generate narrow-bandwidth, intense THz fields.

Nonlinear Breit-Wheeler pair production in collisions of bremsstrahlung γ quanta and a tightly focused laser pulse

Experimental efforts toward the detection of the nonperturbative strong-field regime of the Breit-Wheeler pair creation process plan to combine incoherent sources of GeV $\gamma$ quanta and the coherent fields of tightly focussed optical laser pulses. This endeavour calls for a theoretical understanding of how the pair yields depend on the applied laser field profile. We provide estimates for the number of produced pairs in a setup where the high-energy radiation is generated via bremsstrahlung. Attention is paid to the role of the transversal and longitudinal focussing of the laser field, along with the incorporation of a Gaussian pulse envelope. We compare our corresponding results with predictions from plane-wave models and determine the parameters of focused laser pulses which maximize the pair yield at fixed pulse energy. Besides, the impact of various super-Gaussian profiles for the laser pulse envelope and its transverse shape is discussed.

Influence of Spatio-Temporal Couplings on Focused Optical Vortices

Ultra-intense laser pulses with helical phases are of interest in laser-driven charged particle acceleration and related experiments with extreme light. However, such optical vortices can be affected by the presence of residual spatial-temporal couplings. Their field distributions after propagating in free-space and in the focal plane of an ideal focusing mirror were assessed through numerical modeling, based on the Gaussian decomposition method for a 25 fs pulse with a Supergaussian spatial profile. The wash-out of the central hole in the doughnut-shaped profile in the focal plane corresponds to the rotation of the phase discontinuity.

Thermal Behavior and Power Scaling Potential of Membrane External-Cavity Surface-Emitting Lasers (MECSELs)

Membrane external-cavity surface-emitting lasers (MECSELs) have great potential of power scaling owing to the possibility of double-side cooling and a thinner active structure. Here, we systematically investigate the limits of heat transfer capabilities with various heat spreader and pumping parameters. The thermal simulations employ the finite-element method and are validated with experimental results. The simulations reveal that double-side cooling lowers the temperature by about a factor of two compared to single-side cooling when diamond and silicon carbide (SiC) heat spreaders are used. In comparison, the benefit for a thermally worse conductive heat spreader is larger, i.e. a fourfold decrease for sapphire. Furthermore, we investigate the limits of power scaling imposed by the intrinsic lateral heat flow of the heat spreaders that sets how much the pump beam diameter can be enlarged while having efficient cooling. To this end, the simulations for sapphire reveal a limit for the pump beam diameter within the hundred micrometer range, while for SiC and diamond the limit is more than double. Moreover, pumping with a super-Gaussian beam profile could further reduce the temperature rise near the center of the pump area compared with a Gaussian beam. Finally, we investigate the benefits of double-side pumping of thick membrane gain structures, revealing a more homogeneous axial temperature distribution than for single-side pumping. This can be crucial for gain membranes with thicknesses larger than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 1\,\mu \text{m}$ </tex-math></inline-formula> to fully exploit the power-scaling ability of MECSEL technology.

Performance Improvement of Ultra-Short Distributed Feedback Fiber Lasers by Engineering of Coupling Coefficient Profiles

The performance of distributed feedback erbium-doped fiber lasers considering modified Gaussian profiles for coupling-coefficient variations in Bragg gratings is analyzed and compared to that of such lasers with conventional coupling-coefficient profiles, based on solving the generalized coupled-mode equations. With increasing and decreasing coupling coefficients near the cavity front-ends and π-phase-shift, respectively, the novel modified Gaussian profiles, are proved to represent not only flattened intra-fiber intensity distribution but also extremely-effective enhancement and reduction on the output power and linewidth, respectively. Moreover, using engineered profiles the lasing is even possible for small strengths for which the lasing is not accessible using constant profiles. Unidirectional operation of such a laser can be achieved using unbalanced modified Gaussian profiles by increasing and decreasing the strength of the half-grating on each side of the π-phase-shift while the total strength remains the same. To facilitate fabrication, 2-step profiles for coupling-coefficient variations, relying on approximations to the modified Gaussian and super-Gaussian profiles are also proposed and evaluated. For a 1-cm-long grating with an integrated strength of 6 and pumped with 250 mW, lasing is impossible using a constant-coupling profile, however, it is possible using the 2-step profiles. The lasers based on the 2-step profiles provide 90 μW and 230 μW output power with 28% and 19% decrease compared to those based on the original modified Gaussian and super-Gaussian profiles with the same length and strength. The corresponding enhancement in linewidth for the 2-step profiles is less than 5% in both cases.

Effects of pumping profiles on the temperature distributions in doubleend pumped solid-state lasers: A comparison study

In this work, the effects of pumping profiles on the temperature distributions in solid-state lasers pumped by laser diode from two different end faces were studied. With different pump spot radii of , various exponent factors were examined to evaluate the behavior of temperature distributions due to Gaussian and super-Gaussian profiles. For a Gaussian pumping (n=2), the maximum temperature difference on each rod face was calculated to be 295.5 K and 226 K, respectively, at a total pump power of 92.8 W (46.4 W per face). While at the same pumping power, the temperature decreased as the n exponent increased. A good match was obtained between the proposed model and the previous works listed in the literature.