Observing the Enhanced Backscattering of Light

Maanit Desai ...; John Noe, Harold Metcalf, Laser Teaching Center, 
Dept of Physics and Astronomy, Stony Brook University

Why is the sky blue?  Why does one's finger glow red when
exposed to an intense light source?  Studying how light scatters is
important to understanding many fascinating properties about matter
and nature.  More specifically, the understanding of how light
interacts with complex media such as colloidal solutions or biological
tissue has many important implications and applications.  A
fascinating phenomenon observed when light undergoes multiple
scattering in complex media is a narrow but well defined intensity
cone which appears in the exact backscattering direction.  This
enhanced, or coherent, backscattering effect (CBS), is caused by
photons undergoing a random walk and self-interfering constructively
at angles very close to 180 degrees.  The width of the backscattering
cone has been theorized and shown to be dependent on the root-mean
path length that the photons take inside the scattering medium.

The goal of this study is to use a relatively low-cost setup including
a polarized 10 mW HeNe laser, an anti-reflection coated non-polarizing
50-50 cube beam-splitter, a quarter-wave plate, various lenses, and an
Electrim EDC-1000N CCD camera to observe the enhanced backscattering
effect in various media, and to relate these observations to theory.
In the past, more expensive setups involving powerful argon-ion lasers
and photon-counting photomultiplier tubes or cooled CCD cameras have
been used.  The beam splitter makes it possible to observe scattered
light moving backwards directly towards the source, and the quarter-
wave plate and linear polarizer help to suppress reflections and
single-scattered light.

So far this summer the necessary optical components have been
gathered, individually tested and set up, and some preliminary
observations made.  The angular divergence of the laser beam was
reduced to ~150 micro-radians (uR) by using two converging lenses to
expand the beam diameter from ~1 mm to ~7 mm.  The scattered light was
focussed on to the CCD camera with a lens of focal length 19 cm; this
should give an angular resolution of ~37 uR per pixel. After careful
cleaning and alignment of the optical elements, the background light
observed in the camera has been reduced to an insignificant level, and
a search is underway for evidence of the expected "cone."

This study was supported by the Simons Foundation and NSF Grant PHY 00-98044.