International Workshop on Atomic Interactions in Laser Fields - Abstracts
Francesco Fuso
INFM - Unita di Pisa Universita, Dipartimento
di Fisica
Universita di Pisa, Via F. Buonarroti 2, I-56127 Pisa, Italy
e-mail: fuso@difi.unipi.it
In the last few years, pulsed laser deposition (PLD) has become one of the most appealing techniques for rowin thin films and multilayers of innovative materials on the laboratory scale. Particularly successful is the application of PLD to refractory materials, as high temperature superconductive ceramics, but, thanks to its flexibility, the technique can be applied also to metals, including shape-memory alloys routinely produced in our laboratory. One of the key points in PLD is the high kinetic energy acquired by the ablated material, due to the highly effective interaction between the UV laser pulse (typically, excimer lasers are used) and the target. When a buffer gas is injected into the deposition chamber, the high kinetic energy promotes reactive-collisional processes involving the plume of ablated material and the buffer gas. Those processes are fundamental for the attainment of the correct stoichiometry in the deposited layers especially in PLD of ceramics, where molecular oxygen is used as the buffer gas. The modifications of dynamics and composition of the plume during its expansion have been investigated in the past through a variety of methods, including emission spectroskopy [1] and ion-mass spectrometry [2], but none of them was specifically focused on the effects of collisional processes on the buffer gas, preventing a deep understanding of the relevant collisional dynamics.
Thanks to its flexibility and to the detailed information that can provide, absorption spectroscopy can provide brilliant results in this kind of analysis. Space and time-resolved capabilities, required to monitor transient processes as PLD, can be easily achieved by sending the probe beam. tuned on the transition of interest, across the plume at different positions and recording the transmitted intensity as a function of time after the arrival of the ablating laser pulse. We have already applied absorption spectroscopy to the atomic oxygen transition 3s 5S2 -> 3p 5P3 [3], that allowed us to map the density of atomic oxygen produced through O2 dissociation during plume expansion. More recently, we have investigated molecular oxygen absorption spectroscopy during PLD of ceramics (the superconductive YBCO) and metal (the shape-memory alloy NiTi) targets in order to derive detailed information relevant for both a deep understanding of PLD basic mechanisms and for the development of in-situ diagnostics tools for the process.
Different transitions of the O2 atmospheric band b 1Σg+ (ν' = 0) ← X 3Σg- (ν'' = 0) [4], located in the wavelength range 7594-7734 Å, are employed. These transitions, accessible by using commercially available diode laser sources, were already exploited in remote sensing applications [5, 6] and, more recently, to test the symmetrization postulate [7], but, at the best of our knowledge, they have never been used in the diagnostics of transient processes. Indeed, their weakness (they involve magnetic dipole and electric quadrupole transitions) requires special care to enhance measurement sensitivity, which must be pushed towards the 10-4 - 10-3 range. As the probe beam source, the setup exploits a free running Sharp LT030MD laser diode, operated at temperatures around 40 °C in order to work in the wavelength range of interest. Tunability on the order of ±5 GHz around the center of the investigated transition is accomplished by varying the injection current. Wavelength reference is obtained by sending part of the laser beam into a standard absorption spectroscopy setup, based on an 80 cm multi-pass cell filled with pure O2 in the pressure range 10-800 mbar. The remaining part of the beam is coupled to a single-mode optical fiber and sent into the deposition chamber at different positions with respect to the target. After crossing the plume, the probe beam is detected by a low noise photodiode connected to a digital oscilloscope triggered by the ablating laser shot. The system allows a space resolution of 2 mm (corresponding to the diameter of the probe beam) and a time resolution in tens of nanoseconds range, suitable for a detailed mapping of absorption in the typical conditions of PLD. The same beam-launching system can be also used with a probe beam tuned on the atomic oxygen transition [3], so that atomic and molecular oxygen absorption measurements can be easily carried out in the same experimental conditions and a comparison can be readily performed.
Data are acquired for different choices of process parameters (ablation laser fluence and molecular oxygen pressure in the deposition chamber). Besides a detailed mapping of O2 and O relative density during PLD, the tunability of the probe beam enables investigation of the absorption lineshape in order to derive the loca1 translational temperature reached by oxygen in the effective conditions of the experiment. The results, compared with predictions based on different models for PLD in the presence of a buffer gas, including the drag and the shock-wave models, shed light on the complex phenomena occurring in PLD, and set the basis for the development of an accurate in-situ diagnostics of the process.