Changes to the Microscopy Setup

To accommodate flat samples instead of a capillary-held droplet, the imaging setup was modified in several ways. A sample mount was designed and 3D printed to hold the 15×15 mm chips without obstructing the camera view and laser path, as can be seen in fig. 3.17a. It was itself mounted onto a Thorlabs kinematic prism mount (KM100B/M) instead of the original mounting plate to allow for simple adjustment of the sample tilt.

An adapter was made from aluminum to connect to a set of three translation stages (SLC Series, SmarAct, Germany) and designed so that the assembly would meet the target weight of the stages in combination with a Thorlabs anodized aluminum post. The translation stages were computer controlled and thus facilitated the automated scanning of the sample.

This is an important feature for thin-film or micro-volume samples as only few shots can be acquired for each position. To simplify scanning of the features on the micro-well chips, a

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Figure 3.18:LabView user interface for targeting individual well structures. The module SmarActStagesAtmcontrolled the piezo stages that positioned the sample, while the module ChipScan was used to calibrate the distances in thexz-plane and the vertical shift alongy

by taking three or more reference positions.

script to calibrate the distance between adjacent wells and to automatically compensate for shifts in vertical sample position, produced by slight tilts of the chip, was implemented in LabView. A second camera was incorporated into the setup to monitor the sample from below, such that calibration in the horizontalxz-plane, wherez is directed along the illumination laser beam, could be performed using this camera view, and calibration in the xy-plane was performed according to the main camera. An image of the control software can be seen in fig. 3.18. Correct illumination of the well-chips was crucial to be able to distinguish the well features from the unmodified surface. The best results were achieved with a diffuse illumination by pointing a high power LED lamp at a white surface (a sheet of paper) positioned right next to the PIRL focusing lens. The chip was held at a small angle around thex-axis, with the edge of the chip facing the main camera being slightly further up than the edge facing away, to avoid out-of-focus contributions from the substrate and clipping of the illumination laser beam. Without this tilt, strong horizontal stripes would show up on the interference image.

Representing the second big change compared to the previous section, the new PIRL III (PIRL-III-ND, Light Matter Interaction Inc., Canada) was used for the experiments presented here, and a sketch of the adjusted setup can be found in fig. 3.17b. This laser had significantly more pulse energy (up to 1.35 mJ) and a longer pulse duration ((400±50) ps) than the

Chapter 3 Time-Resolved Imaging of Laser Ablation Plumes

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Figure 3.19:PIRL III beam profile in the focus of aF= 150 mm lens. a) Using a 200µm pinhole spatial filter to clean up the beam. b) Without spatial filtering. The profiles on the top and right side are shown along the red and green line drawn on top of the beam profile image, respectively. The top right shows an integrated radial profile. The annotated distances are in mm, one pixel of the WinCam beam profiler corresponds to 17µm.

PIRL I, which was necessary to be able to ablate full micro-volumes from the chips. According to the specifications, the emission spectrum was centered at 2917 nm with a full width half maximum (FWHM) of 28 nm. AF = 500µm lens was used right at the PIRL III output to prevent excessive divergence of the beam, and the telescope / spatial filter assembly was set to unity magnification by using two plano convex lenses withF = 100 mm, which were adjusted to yield a collimated output beam. Triggering was implemented very similar to what was described for PIRL I: the PIRL III was set to continuous operation and its trigger output was used to trigger the delay generator. A shutter with a larger aperture was used for these experiments and placed within the spatial filter / telescope assembly right after the pinhole. The delay generator would trigger the shutter control (VCM-D1, UniBlitz, USA) and microchip laser (FDSS 532-Q, CryLas GmbH, Berlin, Germany). Because the trigger output had a large delay and jittered considerably with respect to the actual output pulse, a fast photodiode (Thorlabs DET10) was inserted into the laser enclosure right behind the final mirror, and the delay between the PIRL and the microchip laser pulse determined on an oscilloscope.

It was found that merely recording the average output power of the laser was sufficient and pulse to pulse fluctuations were not taken into account for the data analysis in this section.

The shot-to-shot pulse energy was still monitored, but now, in lieu of a good position for a beam-splitting mirror, by pointing the photo-diode at the back-side of the shutter to collect stray light. This method was not as reliable as using a direct beam, as the shutter operation

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3.3 Ablation with Initial Spatial Confinement

changed the amount of stray light reaching the detector. Nonetheless it was used to monitor trends in the beam power which appeared due to warm-up of the system. Rough estimates were achieved by calibrating the average diode reading to that of a power meter placed in front of the focusing lens. Single shot mode had to be used for the diode readout during calibration to fix the influence of the shutter movement on the amount of detected stray light.

The larger pulse energy of the PIRL III as compared to the PIRL I facilitated the choice of a larger focus diameter. This also allowed positioning of the focusing lens considerably further away from the sample, which greatly simplified alignment and increased the Rayleigh range. A lens with a focal length of F = 150 mm was chosen and placed before the last mirror in the setup, which approximately matched the focus FWHM to the largest well diameter to be introduced in the experiment. Figure 3.19 shows the beam profile in the focus as measured by a WinCamD beam profiler (DataRay Inc., USA). In fig. 3.19a, a 200µm pinhole was used to spatially filter the beam, similar to the setup described in section 3.2.1.

This way, an almost Gaussian beam profile with 1/e² diameters of 243µm×215µm was achieved. However, the pinhole tended to be damaged when large fluences were used due to the high laser fluence in the focus of the telescope assembly. Figure 3.19b shows the focus beam profile when no spatial filter was used. In this case, a strongly asymmetric double peak was formed, and the plume images showed significant distortions based on the irregular intensity distribution. The experiments presented in section 3.3.2 were performed with the profile shown in fig. 3.19a. The stated fluences were calculated according to eq. (3.18) and correspond to the pulse energy transmitted to the sample position.