UV Laser Ablation of Ferroelectrics
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UV LASER ABLATION OF FERROELECTRICS R. F. HAGLUND, Jr.,* J. H. ARPS,* K. TANG,* A. NIEHOF** and W. HEILAND** *Haglund, Arps and Tang Department of Physics and Astronomy, Vanderbilt University,
Nashville, TN 37235 **Niehof and Heiland Osnabriick, Germany
FB Physik, BarbarastraBe 7, Universitait Osnabrtick, D-4500
ABSTRACT We have investigated laser ablation of excited atoms from the ferroelectrics LiNbO3 and KNbO 3 at 308 nm . Comparisons of the yields for 0*, K* and Nb* from pure and undoped KNbO 3
show the effects of changing intensity, surface condition and irradiation time on the yield of
excited atoms. Below about 20 GW.cm" 2 , the mechanisms for production of excited atoms differ among the various species; above that intensity, the production of a dense electron-hole
plasma appears to impart a collective character to the ablation mechanism. Introduction and Motivation Lithium niobate has been for many years the workhorse of nonlinear optics technology, and is currently in wide use for frequency up-conversion, acoustooptic devices [1] and optical switching [2]. More recently, potassium niobate has attracted attention as a ring resonator for frequency doubling diode lasers into the blue region of the spectrum [3]. These applications naturally generate concern for high-intensity laser-induced damage to the surfaces of this material, and for the potential reduction in ablation yield from selective doping. In addition, because these materials are not easily processed by ordinary chemical or mechanical techniques, lasers [41 and ion beams [5] are being considered for micromachining and direct writing in optoelectronic circuits [61 made from these materials. Last but surely not least, the unusual electronic structure of these materials offers intriguing possibilities for creating the localized lattice distortion which is the precursor of laser-induced desorption and ablation. Experimental Apparatus and Measurements The experimental setup is shown in Figure 1. Samples of commercial LiNbO3 and Czochralski-grown KNbO3 were mounted in an ultrahigh vacuum chamber with a base pressure of 10-9 torr. Some of the KNbO3 samples were doped with Na (-1 %) and Mg (-500 ppm). A Lumonics Hyper-Ex 460 laser with a maximum energy of 180 mJ at 308 nm provided the ablating pulses; the pulse length was of order 12 ns FWHM. Laser light was focused onto the target at normal incidence by a 30-cm focal length lens to form a focal spot approximately. 150 gim x 250 gim. To avoid artifacts from changes in the focal spot, the laser pulse energy was adjusted over the range from 60 to 180 mJ/pulse by changing the discharge voltage on the laser head; the voltage-vs.-pulse energy curve was calibrated by means of a Scientech ultraviolet calorimeter. These pulse energies, for the measured focal spot, correspond to an intensity range of 13-36 GW.cm-2, well above the plasma formation threshold. An Apple Macintosh microcomputer controlled the timing of the laser pulse and the entire data acquisition sequence. Mat. Res. Soc. Symp. Proc. Vol. 201.
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