Single Crystal Growth with CO2 Laser-Heated Floating-Zone
By the 1970s, the telecommunications industry was increasingly interested in growing single crystal oxides of small-diameter and high-purity due to their advantageous material properties for fiber optical applications. The floating zone technique was the preferred choice, but could not be applied due to inefficient RF heating of the oxides. A solution to this problem was presented in 1972 by John Haggerty, who used CO2 laser radiation to create the floating zone in a fiber drawing process. Further improvements by Martin Fejer and Robert Feigelson in 1980 helped to establish the Laser-Heated Pedestal Growth (LHPG) method as shown in the figure above.
LHPG is a method used in industry and material research, notably for high melting point materials, because of its flexible and cost-effective fabrication of single crystal fibers. The laser beam is guided into a closed chamber, where it hits a reflaxicon that converts the laser beam to a hollow cylinder shape. The beam is then guided to a parabolic mirror that focuses the radiation over the pedestal source.
The fiber drawing takes place in three steps. First, the focused laser radiation creates a small melt zone on top of the pedestal. Second, a seed crystal is introduced into the melt zone, creating solid-liquid interfaces at the pedestal and seed. And third, the melt zone and solidification at the seed are fed by the continuous pulling of the fiber. Under these controlled conditions, it is energetically favorable to maintain the seed’s microstructure during solidification, which enables the continuous growth of the single crystal. Stable thermodynamic conditions, and thus a stable CO2 laser source, are essential for success.
In practice, however, small thermal fluctuations lead to a variation of the fiber diameter. This can be overcome by using a stabilized laser source, such as Access Laser’s AL50ST, and by monitoring the melt zone.
To read the complete paper by Sandro Eckert, a Sales & Application Engineer at Access Laser, click here.
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