University of Zagreb. Faculty of Science. Department of Physics.
Abstract
U ovome radu proučavan je efekt laserske ablacije dobivene s dva, vremenski korelirana, laserska pulsa. Kao glavni parametar javlja se vrijeme kašnjenja između laserskih pulseva τ, koje je varirano između 0 i 50 μs. Eksperiment se bazira na mjerenju vremenski ovisne apsorpcije, odnosno emisije ablacijskog oblaka titana u vakuumu u ovisnosti o τ. Mjereni su apsorpcijski i emisijski spektri atoma Ti, te emisijski spektri atoma Ti i iona Ti+, te je praćeno njihovo vremensko odvijanje. Za mjerenje apsorpcije korištena je laserska apsorpcijska spektroskopija pomoću optičkog rezonatora (laspor), a za mjerenje emisije klasična emisijska spektroskopija pomoću monokromatora. Rezultati apsorpcije i emisije pokazuju višestruko pojačanje signala prilikom dvostruke ablacije za dani τ u odnosu na jednostruku. U apsorpcijskim se mjerenjima, za određene prostorno-vremenske paremetre, javlja dopplerovo cijepane apsorpcijskih linija što je indikacija povećane paralelne brzine (u odnosu na površinu mete) ablatiranih čestica, odnosno posljedica eksplozije faze. Također je razvijen i model koji simulira apsorpcijska mjerenja, a pomoću kojega je moguće odrediti i neke dodatne parametre (npr. Raspodjela brzina, te oblik funkcije gustoće ablatiranih čestica). Mjereni su i volumeni kratera nastali na površini mete nakon ablacije, te njihova dubina i radijus, za razna vremena τ. Za najmanja vremena τ dominira efekt zasjenjenja za drugi puls pa volumen nije najveći iako je meta za drugi puls najzagrijanija (najmanji prag za ablaciju). Također se za veća vremena kašnjenja (2 μs) dobivaju napareni filmovi titana na Si substrat s najmanjom hrapavošću i najvećom debljinom.In this work the effect of laser ablation which is induced with two, time separated, laser pulses was studied. As a main parameter in dual-pulse laser ablation delay time between the laser pulses τ appeared and it was scanned within interval 0 – 50 μs. Experiment is based on measurement of time-dependent absorption and emission from plasma plume of titanium ablated in vacuum in respect to τ. Absorption and emission spectra of Ti, and emission spectra of Ti+ were also measured as well as its temporal dependence. Absorption measurements were performed by means of cavity ring-down spectroscopy while emission measuremenst were performed by means of classical monocromator. Results shows many-fold increase of measured signal in dual-pulse in comparison to single-pulse laser ablation. For given temporal and spatial parameters absorption line profiles appeared doppler-splitted. It is indication of increased velocity component parellel to the target surface, i.e. increased phase explosion. The model was developed for simulation of absorption measurements. From the model parameters such are velocity distributuin of ablated species, or anisotropy function can be assessed. Otherway those parameters cannot be obtained directly from the experiment. Volumes, radii and depths of the craters induced by laser ablation were also measured in respect to τ. It was found that largest volumes were given for shorters τ when ablation threshold is lowest, except that for the shortest τ where screening effect dominates for the second pulse hence energy delievered to the target was decreased. Ti films deposited on si depend on τ and on regime of ablation (single or dual). The thickest and smoothed layer was made with τ = 2 μs
