Advanced polarization control for optimizing ultrafast laser micro-processing

The ability to control and manipulate the state of polarization of a laser beam is becoming an increasingly desirable feature in a number of industrial laser micro-processing applications. Being able to control polarization would enable the improvement of the efficiency and quality of processes such as the drilling of holes for fuel-injection nozzles, the processing of silicon wafers or the machining of medical stent devices. This thesis presents novel, liquid-crystal-based optical setups for controlling the polarization of ultrafast laser beams, and demonstrates how such optical setups can be used to improve laser micro-processing efficiency and quality. Two experimental strategies were followed: the first used dynamic control of the polarization direction of a linearly polarized beam; the second generated beams with complex polarization structures. Novel optical analysis methods were used to map the polarization structures in the focal region of these laser micro-processing setups, using Laser Induced Periodic Surface Structures (LIPSS) produced on stainless steel sample surfaces at low laser fluence (around 1.5J/cm²), close to the ablation threshold of steel (i.e. 0.16J/cm²). This helped to characterize and calibrate the optical setups used in this thesis. The first experimental method used a fast-response, analogue, liquid-crystal polarization rotation device to dynamically control the direction of linear polarization of a laser beam during micro-processing. Thanks to its flexibility, the polarization rotator could be set-up in various synchronized configurations, for example keeping the polarization direction constantly perpendicular to the beam scanning motion. Drilling and cutting tests were performed on thin (~0.4mm thick) stainless steel sheets using a 775nm femtosecond laser at 24J/cm². The experimental results showed a consistent improvement in the micro-processing quality when the polarization direction was synchronized with the beam scanning motion. The sidewall surface roughness and edge quality of the machined structures were improved significantly, with the dimensions of ripples and distortions divided by a factor of two. The overall processing efficiency was also increased compared to that produced by linear or circular polarizations. The second experimental method used a digital, Liquid-Crystal On Silicon (LCOS) Spatial Light Modulator (SLM) to generate polarization structures with a cylindrical geometry, or Cylindrical Vector Beams (CVBs). A Jones matrix analysis was used to model the optical setup and predict the ability to produce CVBs in this way. The setup was implemented in a 775nm femtosecond laser micro-processing bench and the resulting polarization analyzed with a polarizing filter, demonstrating a polarization purity better than 84%. The amplitude and polarization properties in the focal region of the setup were studied using LIPSS produced on the surface of stainless steel samples at low fluence (1.5J/cm²), to check that the expected state of polarization had been achieved. An analytical model of the experimental setup was developed to explain the experimental results. The model predictions were in agreement with the experimental results and clarified how the polarization and phase structures affect the focal properties of the produced laser beams. Various types of CVBs were used with a high laser beam fluence (24J/cm²) for micro-machining 0.2-0.4mm thick stainless steel plates. A comparative analysis of micro-machining with radially, azimuthally, circularly and linearly polarized beams was carried out. It was shown that a radially polarized beam was more efficient at drilling and cutting high-aspect-ratio features when the plate thickness was above 0.2mm. The gain in processing speed was better than 5% compared with a circularly polarized beam and better than 10% compared with an azimuthally polarized beam, under the chosen processing parameters. However the processing speed was similar for all these polarization states (radial, azimuthal and circular) when machining 0.2mm thick plates. It was also shown that a radially polarized beam improved the processing quality, reducing the distortions affecting the edge quality of the machined structures

Creation of electrospray emitters from a flat polytetrafluoroethylene sheet through the use of laser micro-manufacturing

From IOP Publishing via Jisc Publications RouterHistory: ppub 2021-05, open-access 2021-05-01Publication status: PublishedAbstract: Electrospray propulsion is a form of electrostatic propulsion that shows potential for use as a thruster on small satellites. Electrospray emitters traditionally use arrays of capillaries, however, this paper investigates the suitability of manufacturing an emitter by laser drilling holes in sheets of polytetrafluoroethylene (PTFE), also known as Teflon. This is done by examining factors such as the machining time, hole shape and degree of taper in holes drilled through different methods. Experimental work was done to determine the shape of holes drilled in a flat sheet, and a computational model was used to simulate the performance of these drilled holes. These results were used to judge if this would be a valid method to manufacture electrospray thrusters, and how these thrusters would compare to more traditional emitters. Based on these results, it was concluded that a PTFE sheet with a width of 0.81 mm would be sufficient to achieve electrospray at voltages lower than those needed for traditional emitters

Barrier potential for laser written graphitic wires in diamond

Diamond substrates supporting an internal array of conductive graphitic wires inscribed by a femtosecond pulse laser, are useful for the detection of ionising radiation in a range of applications. Various parameters involved in the laser fabrication process were investigated in this paper to understand their impact on the electrical properties of the wires. The study revealed an effect, whereby the wires exhibit insulating behaviour until a barrier potential is overcome. When high enough voltages are applied, the wires display ohmic behaviour. The magnitude of the barrier potential, which in some cases exceeds 300 V, is shown to be strongly dependent on the laser fabrication parameters. Through process optimisation, the potential barrier may be minimised and effectively removed, coinciding with reduced values of the wire resistance

Antiretroviral-naive and -treated HIV-1 patients can harbour more resistant viruses in CSF than in plasma

Objectives The neurological disorders in HIV-1-infected patients remain prevalent. The HIV-1 resistance in plasma and CSF was compared in patients with neurological disorders in a multicentre study. Methods Blood and CSF samples were collected at time of neurological disorders for 244 patients. The viral loads were >50 copies/mL in both compartments and bulk genotypic tests were realized. Results On 244 patients, 89 and 155 were antiretroviral (ARV) naive and ARV treated, respectively. In ARV-naive patients, detection of mutations in CSF and not in plasma were reported for the reverse transcriptase (RT) gene in 2/89 patients (2.2%) and for the protease gene in 1/89 patients (1.1%). In ARV-treated patients, 19/152 (12.5%) patients had HIV-1 mutations only in the CSF for the RT gene and 30/151 (19.8%) for the protease gene. Two mutations appeared statistically more prevalent in the CSF than in plasma: M41L (P = 0.0455) and T215Y (P = 0.0455). Conclusions In most cases, resistance mutations were present and similar in both studied compartments. However, in 3.4% of ARV-naive and 8.8% of ARV-treated patients, the virus was more resistant in CSF than in plasma. These results support the need for genotypic resistance testing when lumbar puncture is performe