Marc Hauer

Novel applications for laser ablation of photopolymers

Abstract The ablation characteristics of various polymers were studied at low and high fluences. The polymers can be divided into three groups, polymers containing triazene groups, polyesters with cinnamylidenemalonyl groups, and polyimide (PI) as reference polymer. The polymers containing the photochemically most active group (triazene) are also the polymers with the lowest threshold of ablation and the highest etch rates, followed by the designed polyesters and then PI. The triazene-polymer (TP) was studied at low fluences with additional techniques. UV–Vis spectroscopy and TOF-MS reveal that the triazene-chromophore decomposes also upon irradiation with fluences below the threshold of ablation. At the threshold fluence, a pronounced change is detected, i.e., an approximately 10 times higher decomposition rate. Nanosecond surface interferometry was applied to detect changes of the surface morphology of the TP and PI after irradiation with fluences above the threshold of ablation. In the case of the TP, no swelling of the surface is observed and etching starts and ends with the laser pulse, while a very pronounced swelling is detected for PI. The clear difference between PI and the designed polymers can be explained by a pronounced thermal part in the ablation mechanism of PI, while photochemical activities are more important for the TP. The combination of phase masks and the TP allows an efficient fabrication of three-dimensional topographies using laser ablation. The TP also reveals superior properties for applications in the near-IR. The carbon-doped polymer shows properties that are useful for the application of polymers in laser plasma thrusters for microsatellites.

Designed polymers for laser-based microthrusters: correlation of thrust with material, plasma, and shockwave properties (Plenary Paper)

The micro laser plasma thruster (μLPT) is a micropropulsion device, designed for the steering and propelling of small satellites (10 to 100 kg). A diode laser is focused on a two-layer polymer tape, where it forms a plasma. The thrust produced by this plasma is used to control the satellite motion. Three different polymers (GAP, PVN and PVC) doped with carbon and/or IR-dye were investigated for their performance as fuel polymer. The different dopants for GAP seem to have only little influence in the ablation properties. The most pronounced differences are observed in the fragment ejection detected in the shadowgraphy measurements and the crater appearance. For all carbon doped polymers, the ablation spots have a similar rough morphology. The shadowgraphy measurements of PVN reveal, that the shockwave and particle plume propagates faster as in the case of the other polymers. The particle plumes showed a very different expansion behavior for all polymers, whereas the plasma temperature and electron density measurements showed no significant difference. Only PVC displayed a slower almost linear drop of the plasma temperature over time. The thrust measurements showed the best results for GAP.

Polymers designed for laser applications: fundamentals and applications

The ablation characteristics of various polymers were studied at low and high fluences. The polymers can be divided into three groups, i.e. polymers containing triazene groups, designed ester groups, and reference polymers, such as polyimide. The polymers containing the photochemically most active group (triazene) exhibit the lowest threshold of ablation (as low as 25 mJ cm-2) and the highest etch rates (e.g. 250 nm/pulse at 100 mJ cm-2), followed by the designed polyesters and then polyimide. Neither the linear nor the effective absorption coefficients reveal a clear influence on the ablation characteristics. The different behavior of polyimide might be explained by a pronounced thermal part in the ablation mechanism. The laser-induced decomposition of the designed polymers was studied by nanosecond interferometry and shadowgraphy. The etching of the triazene polymer starts and ends with a laser pulse, clearly indicating photochemical etching. Shadowgraphy reveals mainly gaseous products and a pronounced shockwave in air. The designed polymers were tested for applications ranging from microoptical elements to polymer fuel for laser plasma thrusters.

Laser ablation of a triazene polymer studied by ns-interferometry and shadowgraphy

Nanosecond-interferometry and shadowgraphy is used to observe the dynamic behavior of the etching process during and after the irradiation pulse. Commercially available polymers exhibit quite often poor laser ablation properties for irradiation wavelengths >=248nm. At these wavelengths the absorption is due to the quite photostable aromatic groups. A photolabile triazene polymer was selected to compare the influence of a photolabile group on the laser ablation process. The photochemical active triazene reveals a strong absorption band at 332 nm and is responsible for the observed high etch rates and the low threshold for 308 nm irradiation. The absorption coefficients at 193 nm and at 308 nm are comparable, allowing to study the influence of the different absorption sites by ns-interferometry and shadowgraphy measures. The etching of the triazene polymer starts and ends with the laser beam. No surface swelling, which is assigned to photothermal ablation, is detected for fluences above the threshold of the ablation. The expansion of the laser ablation induced shockwave was measured for the photolabile triazene polymer and the photostable polyimide. The speed of the shockwave increases with fluence and is higher for irradiation with 193 nm than for 308 nm. A shockwave with equal or higher velocity is observed for the triazene polymer than for the polyimide.

Designed polymers for laser-based microthrusters: correlation of thurst with material, plasma, and stockwave properties

The micro laser plasma thruster (μLPT) is a micropropulsion device, designed for the steering and propelling of small satellites (10 to 100 kg). A diode laser is focused on a two-layer polymer tape, where it forms a plasma. The thrust produced by this plasma is used to control the satellite motion. Three different polymers (GAP, PVN and PVC) doped with carbon and/or IR-dye were investigated for their performance as fuel polymer. The different dopants for GAP seem to have only little influence in the ablation properties. The most pronounced differences are observed in the fragment ejection detected in the shadowgraphy measurements and the crater appearance. For all carbon doped polymers, the ablation spots have a similar rough morphology. The shadowgraphy measurements of PVN reveal, that the shockwave and particle plume propagates faster as in the case of the other polymers. The particle plumes showed a very different expansion behavior for all polymers, whereas the plasma temperature and electron density measurements showed no significant difference. Only PVC displayed a slower almost linear drop of the plasma temperature over time. The thrust measurements showed the best results for GAP.