Petru-Vlad Toma

Additive manufacturing has recently become a key enabling technology in industrial fields, ranging from customized products for everyday usage to aerospace applications and small-batch industrial tooling. The future prospects extend up to the biofabrication of human organs. Ensuring the quality and repeatability of this process requires a systematic and comprehensive investigation of the underlying physical phenomena. In particular, melt-pool evolution is a critical feature, since irregularities in its spatial profile can influence microstructural evolution and weaken the integrity of the manufactured part. Microscale defects arising from balling and keyhole phenomena, often associated with recoil pressure, can severely degrade the quality of the resulting scanned track. This paper reviews the current state of optical approaches for melt-pool characterization and feature monitoring relevant to industrial laser additive manufacturing for process control and quality improvement, with a special focus on pyrometry and high-speed imaging. A single high-speed camera was generally used in experiments for melt-pool feature extraction, but two cameras were used to bypass emissivity values, which are otherwise difficult to obtain. Mathematical models were introduced to provide complementary information about melt-pool features, while artificial intelligence algorithms were used in other cases to process optical information. New melt-pool imaging databases and classifiers are expected in the near future to enable fast selection of appropriate process parameter windows, eliminating costly trial-and-error experiments.

We present a study of the scattering of a monochromatic helical laser beam, described by a Laguerre-Gauss solution of the Maxwell equations, on an electron sheet, initially at rest in the focal plane of the laser; the interaction is described in the framework of a local plane-wave approximation. We calculate the scattered electromagnetic field observed in an arbitrary point at a large distance from the laser focal spot, by adding coherently the contributions of each point in the electron sheet. Due to the interference effects, the radiation is emitted only into the forward direction, within a narrow cone, and it has a spatial structure that we analyze theoretically and numerically. For circularly polarized incident fields, the structure is also helical, with a helical index which depends on the helical index of the incident radiation and the harmonic order. These structures can be observed experimentally, as each harmonic order is emitted with a different frequency, and within cones of different opening angles. Our findings are in agreement with experimental results in the literature which demonstrate the generation of orbital angular momentum carrying photons by radiation scattering on electrons.

Orbital angular momentum of light and the associated Laguerre–Gauss modes have garnered increasing interest due to the many applications they promise in the fields of telecommunications and quantum information, as well as charged particle acceleration. Here, we use the numerical simulation of the dynamics of a large number of identical, charged, relativistic, but classical particles, forming a statistical ensemble and seen as test-particles in a time-dependent external electromagnetic field, to get insight into the process of a head-on collision with a pulsed Laguerre–Gauss mode. We have identified two steady-states regimes (low a0 and large a0), where the distribution of the final angular momentum gained by the particles as a function of their initial coordinates is well-behaved and does not change its structure with variations of the reduced amplitude. For small reduced amplitudes a0 of the incoming electromagnetic field, we have also found an analytic approximation of the expression for the angular momentum transferred to the particles after the pulse has left their region. Using both numerical integration and the analytic approximation, we have investigated the effects of the field’s gradient, initial phase of the field, polarization type, and azimuthal order m.

In this work we investigate the transfer of orbital angular momentum from a Laguerre-Gauss laser pulse to an ensemble of electrons, initially at rest. We explored the dependence of the transfer mechanism on the polarization, as well as on the dimen-sionless parameter a₀. A scaling behaviour with a₀ was noticed. Two distinct regimes were highlighted, associated with a change in the value of the power law exponent from 1 to 2 in the case of linearly polarized beams and from 1 to 4 in the circularly polarized case.