OBSERVATIONS OF RUBY-LASER BEAM-INTENSITY PATTERNS WITH DYE-SENSITIZED PHOTOPOLYMERS

The first use of a new dye-sensitized photopolymerization techniques 2 with a ruby laser3 occurred through the collaboration of two exhibitors during the Demonstration and Exhibits Session of the Ninety-Eighth Annual Meeting of the National Academy of Sciences at Washington, April 24, 1961. Spatial distributions 4 of the light intensity in the beam emerging from the ruby laser was observed by use of this technique. The three-dimensional button-shaped solid plastic "photographs" (see Fig. 1) provide intensity information versus position on the end of the ruby, since the depth of exposure (height of image) is monotonically related to beam intensity. Typically, only a central core of the ruby participates in the laser action. Resolution and speed limitations associated with the first experiments prevented the recording of filament structure4 5 observed from this ruby by other means. Shown in Figure 1 is a photograph of the first plastic "buttons" obtained for different levels of excitation of the ruby laser. The flat button shown at (A) was produced by fluorescent radiation from the laser when operated slightly below oscillation threshold. A central "spike" was superposed upon this button whenever laser action occurred. The base diameters for the spikes shown in (B), (C), (D), and (E) ranged from 4 to 7 mm, the heights from 6 to 14 mm. A larger fraction of the ruby crystal participates in laser action at higher pump powers. The 1.25 X 0.375 inch ruby was excited by a General Electric FT 506 helical flash lamp driven by a 336-Mf bank of capacitors. With this capacitor bank, the oscillation threshold was 200 joules. The liquid solution was positioned 1.5 inches from the partially silvered end of the ruby. It is likely that this separation impaired the resolution of the system somewhat. The laser apparatus employed for the experiments is shown in Figure 2. Experiments were performed with two monomer-activator systems. The one that appeared to be particularly promising (see Fig. 1) consisted of an approximately 40% solution of acrylamide, containing a saturation quantity of methylene bis(acrylamide) as cross-linking agent, about 0.001% methylene blue, and N,N,N',N' tetramethyl ethylenediamine as electron donor. Another monomer-activator system was composed of approximately 40% aqueous calcium acrylate solution with methylene blue and tetramethyl ethylenediamine. The sensitivity of these monomer-activator systems to radiation in the absorption region of the photoreducible dye is strongly dependent upon the concentration of residual oxygen in the aqueous solution and upon the concentration of the electron donor for a given dye concentration. Although methylene blue has its maximum absorption at 6,650 A, the extinction coefficient is still about one sixth of its maximum at the output wavelength of the laser, namely 6,943 A. The mechanism by which free radicals are generated in the monomer-activator system is explained in references 1 and 2. As would be expected, a large proportion of cross-linking agent is neces-