Pulsed laser ablation is a valuable tool that offers a much cleaner and more flexible etching process than conventional lithographic techniques. Although much research has been undertaken on commercially available polymers, many challenges still remain, including contamination by debris on the surface, a rough etched appearance and high ablation thresholds. Functionalizing polymers with a photosensitive group is a novel way and effective way to improve the efficiency of laser micromachining. In this study, several polyurethane films grafted with different concentrations of the chromophore anthracene have been synthesized which are specifically designed for 248 nm KrF excimer laser ablation. A series of lines etched with a changing number of pulses and fluences by the nanosecond laser were applied to each polyurethane film. The resultant ablation behaviours were studied through optical interference tomography and Scanning Electron Microscopy. The anthracene grafted polyurethanes showed a vast improvement in both edge quality and the presence of debris compared with the unmodified polyurethane. Under the same laser fluence and number of pulses the spots etched in the anthracene contained polyurethane show sharp depth profiles and smooth surfaces, whereas the spots etched in polyurethane without anthracene group grafted present rough cavities with debris according to the SEM images. The addition of a small amount of anthracene (1.47%) shows a reduction in ablation threshold from unmodified polyurethane showing that the desired effect can be achieved with very little modification to the polymer.
We report the combined use of optical sorting and acoustic levitation to give particle sorting. Differing sizes of microparticles are sorted optically both with and without the aid of acoustic levitation, and the results compared to show that the use of acoustic trapping can increase sorting efficiency. The use of a transparent ultrasonic transducer is also shown to streamline the integration of optics and acoustics. We also demonstrate the balance of optical radiation pressure and acoustic levitation to achieve vertical sorting.
Whilst the main strength of optical trapping techniques is arguably its precision and dexterity, the complimentary
technique of acoustic trapping offers massive scalability and potentially larger forces. Acoustic traps commonly use
ultrasonic standing waves to trap particles within the nodes of a pressure field, often over distances upwards of a few cm.
Here, an acoustic Bessel beam has been created using a piezoelectric cylinder whereby particles are trapped within the
entire 14 mm-diameter of the transducer (1.5 cm2 trapping area). In optics, Bessel beams have the ability to trap
particles over axial distances of several hundred microns. In this acoustic case, the Bessel function shape of the field is
formed within the entire length of the cylinder (10 mm). Polymer spheres ranging from 1 μm to 100 μm in diameter are
trapped simultaneously within the nodes of the standing wave field, in this case the concentric rings of a Bessel beam.
The smaller particles within this field (< 5m) have also been trapped optically using a single beam optical tweezer, as
the acoustic force scales such that it becomes comparable to that of the optical trap. This allows for a large range of
particle sizes to be simultaneously trapped in a single device, and for large arrays (hundreds of mm2) to be formed
acoustically within which particles can be individually optically trapped. This result demonstrates the complementarity
of optical and acoustic trapping which makes it possible to trap large area arrays of particles whilst retaining the
dexterity to manipulate individual particles.
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