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Modifying Materials with the Power of Light

Thomas Cummings
Advisors: Dr. John Noé and Prof. Harold Metcalf,
Laser Teaching Center, Department of Physics and Astronomy, Stony Brook University
Richard Migliaccio, East Coast Optical Technologies

The two topics studied this summer have a common theme - they both involve the interaction of materials with intense focused light. The first study, now completed, involved refractive magnification of solar irradiance with various magnifying lenses and reading glasses, and testing the effect of the enhanced light on materials. Amplifying the Sun's irradiance has many useful applications such solar furnaces, energy storage systems and renewable energy sources. The second set of experiments, which is on-going, involves making measurements of diffuse reflectance of certain ceramic materials, which provides information on their optical absorbance. The goal is to find the most suitable material for producing engraving masters by laser drilling with a high-powered pulsed YAG laser, in the shortest possible time.

The solar irradiance at noon on a clear day in this area is expected to be nearly 1 kW/m2. This value was confirmed by measurements with a standard photodetector. The magnifying glasses increased this irradiance by the ratio of the area of the lens to the area of the final spot, which was typically 500-fold. The reading glasses, which have a longer focal length and somewhat smaller diameter than the magnifiers, increased the irradiance by only about 30-fold. This final irradiance was not sufficient to burn darkened paper, which was found (by observations with defocussed spots) to require ~ 40 kW/m2. Measured sizes of the focussed spots were in general somewhat larger than predictions based on the thin lens formula and measured focal lengths of the lenses.

For the diffuse reflectance measurements, a simple goniometer was constructed, which allowed the photodetector head of a EG&G Model 550 radiometer to move in an arc of radius 30 cm around the scattering sample. For the initial study the sample was a bright white fully-diffuse (Lambertian) test material; this was illuminated with a 15 mW HeNe laser beam incident normal to the surface. Scattered light readings ranged from 9.9 µW/cm2 with the detector nearly normal to the surface to 0.04 µW/cm2 at a very oblique angle. The angular distribution of the scattered light was in good agreement with Lambert's Cosine Law. Measurements and analysis of diffuse reflectance on the standard white surface are currently being extended to additional angles of incidence. The project will be continued following the end of the formal REU program with similar measurements with a 1.06 micron laser, which is the wavelength of the high-powered YAG laser to be used for the laser cutting.

This study was supported by NSF Grant No. PHY 02-43935.

Thomas Cummings Summer 2003
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