This is a second harmonic generation experiment in which the light
is scattered in all directions rather that as a narrow coherent beam.
The technique can be easily applied to study a very wide range of
materials because electrostatic fields and phase matching are not
required. Other advantages are that polarization analysis gives
information about the tensor properties, and spectral analysis of
the scattered light gives information about the dynamics. The main
disadvantage is that the signal tends to be much weaker than in
coherent harmonic generation experiments.
Bacteriorhodopsin is a protein which undergoes a reversible color
change when it absorbs a photon. Bacteriorhodopsin and its various
mutants have been investigated for photonics applications including
signal processing, 3-D data storage, holographic storage, and spatial
light modulators. The work in this laboratory is aimed at producing
an optically configurable spatial light modulator suitable for
optoelectronic neural network applications. Large scale neural
networks require the massive parallelism that optics can provide.
The simplest example of frequency conversion is Second Harmonic Generation,
where a laser beam propagating through a nonlinear optical medium generates
a beam of light at twice the original optical frequency (for example, an
invisible infrared laser beam is converted into a beam of green light).
In such "parametric" processes, the nonlinear optical medium responds on
a time scale as short as femtoseconds, but large effects often require
light intensities greater than 1 MW/cm2.
In the present experiments, the quadratic and cubic nonlinearities of
isolated molecules are determined from measurements of the intensity
of the frequency-doubled light beam, produced when a strong electric
field is applied to a gas sample through which a laser beam passes.
This experiment gives very accurate measurements of the nonlinear
optical properties of small molecules. These measurements are
important because they are a direct test of theoretical calculations
for the same molecules.
This unique research facility uses periodic phase matching in a
periodic electrode array to permit accurate gas phase hyperpolarizability
measurements for a wide range of molecules, using cw and pulsed lasers,
with wavelengths over the near-infrared and visible. The experiment
currently uses a Nd:YAG laser which emits infrared light pulses with
a power of 10 kW. The strength of the second harmonic signal generated
in the sample is increased up to 10,000 times by using an electrode array
in which the field periodically changes sign. The sample cell and
sample handling apparatus is constructed to operate at temperatures
up to 200 C. This allows gas phase measurements of molecules such
as para-nitroaniline, which are closely related to chromophores of
immediate practical interest.
The cubic nonlinearity of isolated molecules is determined from
measurements of the depolarization of a laser beam as it propagates
through a sample to which a transverse electric field has been applied.
When the results of this experiment are combined with the results of
the electric-field-induced second harmonic generation experiment,
they allow one to clearly distinguish the nuclear and electronic
contributions to the nonlinear optical response of a molecule
(effects of the motion of the atomic nuclei as compared to effects
of the motion of the electrons around the nuclei). The experiment
uses argon-ion, dye, and He-Ne lasers producing continuous low power
light beams in the near infrared and visible. The apparatus measures
birefringent phase shift with nanoradian sensitivity.
A wide range of coherent and nonlinear interactions occur when
laser light is resonant with strong optical transitions in atoms.
Laser sources and spectroscopic techniques for cooling and trapping
rubidium atoms in a magneto-optic trap are being developed for the
production and study of cold atoms. Much of the development work
in this project has been done by undergraduate students in the
Research Experience for Undergraduates program supported by the
National Science Foundation.