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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
and
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.
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