The electrostatic dipole interaction between polar molecules in liquids such as water is strong enough to drive long-range orientation ordering of the molecules. The first evidence for the existence such orientation order comes from recent hyper-Rayleigh scattering (HRS) experiments.
The HRS spectrum for dipolar liquids contains an extremely sharp peak, with the polarization signature of a longitudinal polar collective mode, which disappears when the mean dipole coupling strength is reduced below a threshold value. This peak is interpreted as HRS from long-lived domains with ferroelectric order. These domains are of nanometer size and persist for 100 ns or longer. The observations are consistent with theoretical models predicting predominantly toroidal order for the dipoles, where each domain is a planar vortex with the dipoles in the vortex core pointing along the axis.
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.