At The Queen's University of Belfast (Northern Ireland), work on pulsed-laser deposition (PLD) of superconducting films, magnetic films, and soft x-ray mirrors is proceeding in parallel with more fundamental PLD plume studies (see Laser Focus World, Oct. 1994, p. 103). Queen's researchers are using a range of diagnostic techniques based on fast-gated two-dimensional charge-coupled-device (CCD) detectors. Plume images are recorded using an Oriel Instruments (Belfast, N. Ireland) Instaspec V fast-gated intensified CCD (ICCD).
A plume is formed when 1-4 J/cm2 of energy from a 248-nm KrF excimer laser with 20-ns pulse duration hits the target. The spectrally resolved plume emissions were imaged 2 µs after ablation of a superconductive yttrium barium copper oxide (YBCO) target in 180-mTorr ambient oxygen. The compact air-cooled ICCD was operated at -25°C with a gate-on time of 20 ns. Images were recorded as single laser events: the spectrally integrated emission of the plume, the 603-nm chemiluminescent emission of YO from the reaction of atomic yttrium with ambient oxygen at the plume edges, and the 440-nm Y+ ion emission, which is greatest at the high-temperature expanding plume front. The plumes extend about 2 cm from the target surface.
Other spectroscopic techniques used by the researchers include laser-absorption spectroscopy and laser-induced fluorescence (LIF). To obtain images with these methods, the ablated material is backlit by the expanded beam of a short-pulse dye laser with a narrow bandwidth—tuned to an absorption resonance of the plasma species. Recording the two-dimensional profile of the transmitted beam provides information on the total number of absorber particles along the line of sight.
Simultaneous imaging of the resulting LIF at right angles to the probe dye-laser beam determines the exact location of these absorbers along the line of sight. Such orthogonal two-dimensional imaging makes it possible to determine the three-dimensional distribution of absorbing species within the expanding plume at the instant of the probe laser pulse. Spectroscopy data are complemented by data from other ICCD-based diagnostics such as time-resolved Mach-Zehnder interferometry to measure electron density.
Capturing spatially resolved two-dimensional images of such transient systems requires appropriate fast-gated ICCD devices. Queen's physicists Tom Morrow and Ciaran Lewis, in collaboration with Oriel Instruments research physicist Raied Al-Wazzan, are developing new techniques and diagnostic devices for convenient one-, two-, and three-dimensional mapping within the rapidly expanding plume.
Their current work, supported by the UK Engineering and Physical Sciences Research Council, aims to provide quantitative data to validate and further develop computer simulation codes of laser-induced plasma plumes. Long-term objectives are to provide practical and cost-effective monitoring of critical plasma parameters and, ultimately, provide feedback control during pulsed-laser deposition.