Doctoral Defense
Friday, April 10, 2015
10:00 AM FH 580
All are welcome to attend.
APPLICATIONS OF INDUCED GRATINGS IN
NONLINEAR MEDIA
Ujitha Abeywickrema
University of Dayton
Abstract
Materials exhibiting effective nonlinearity through refractive index modulation at relatively low optical powers can be
exploited for various applications. Examples of such materials include liquids where the refractive index is modified
through heating, and photorefractives where the refractive index modulation is caused by the induced space charge
field due to optically generated charges and their redistribution. Optical probing techniques of these and related
effects include digital holography, holographic interferometry, and diffraction.
First, the effect of self-phase modulation of a focused laser beam in a thermal medium such as a liquid is studied
using a low power probe beam. Beyond self-phase modulation, thermal blooming occurs, due to bubbles generated
in the liquid. These bubbles are characterized using the same probe and digital holography. An application of these
bubbles to nanoparticle agglomeration and transport for drug delivery systems is proposed.
Next, the use of recording materials such as photorefractive lithium niobate for implementing real-time phase shifting
holographic interferometry is examined in detail. Holographic interferometry is a convenient tool for 3d
characterization of deformations of an object. The hologram of an object is first written in the material using a
reference beam, and then read out by the same reference beam and light from the deformed object. It is shown that
the use of both Bragg and non-Bragg orders during conventional two-beam coupling in a photorefractive material
facilitates the simultaneous generation of phase shifts necessary for this type of holographic interferometry.
In certain applications involving liquid crystals, the spatial modulation of the director axis can yield improved energy
coupling in hybrid liquid crystal – photorefractive devices. Nanoscale engineering of the director axis is possible using
the surface corrugation in photorefractives induced by the space charge field through the piezoelectric effect. This
surface corrugation is characterized by monitoring the diffraction of a probe beam from the surface of the
photorefractive material, taking care to eliminate any Fabry-Perot effects and diffraction from the volume grating in
the material.