Our research is in the broad area of interaction of light with matter comprising condensed matter physics, molecular physics, nonlinear optics, laser spectroscopy, and surface sciences. In particular, we have been active in search for understanding of nonlinear optical effects, developing novel linear and nonlinear optical techniques for material studies, and applying them to material science research. We initiated the field of nonlinear optics in liquid crystals and applications of nonlinear optics to characterization of liquid crystals. We pioneered the development of optical second harmonic generation and sum-frequency generation as powerful and versatile spectroscopic tools for surface and interface studies and their applications to many neglected, but important, areas of surface science. More recently, we have been developing optical sum-frequency generation as a novel sensitive spectroscopic technique for probing molecular chirality and for chiral microscopy. We are also engaging in optical characterization of nanostructures and studies of plasmonics and metamaterials.
Sum-frequency generation (SFG) is a process in which two input beams at frequencies w1 and w2 mix in a medium and generates an output beam at the sum frequency w = w1 + w2 . [See Y. R. Shen, Nature 337, 519 (1989).] As a second-order nonlinear optical process, it is only allowed under electric-dipole approximation in media without inversion symmetry. At surfaces or interfaces where inversion symmetry is necessarily broken, SFG is naturally allowed with monolayer sensitivity, and can therefore be used as an effective surface probe for such media. If either w1 (w2) or w1 + w2 or both are scanned over resonances, SFG should be resonantly enhanced and thus yield a SF spectrum for the medium. It can be used to probe electronic as well as vibrational transitions in a medium.
Schematic diagram describing the SFG process and beam geometry
for SFG in the transmission and reflection directions.
Our current interest in surface and interfacial studies using SFG focuses on polymer surfaces and how they induce alignment of a liquid crystal film, which is of key importance in liquid crystal industry. Considering that SFG is the only technique that can yield vibrational spectra of liquid surfaces, we are also using the spectroscopic technique to probe the structures of various water interfaces at the molecular level and molecular adsorbates at such interfaces as well as ultrafast surface dynamics. These studies are highly relevant to many crucial problems in physics, chemistry, biology, and environmental science. We collaborate with Dr. Glenn Waychunas at LBNL studying water/oxide interfaces of importance to geoscience. For solid surfaces, we employ the technique to study surface melting of ice and surface phonons of oxides, problems that have hardly been explored.
We have recently demonstrated that SFG can be used as chiral vibrational and electronic spectroscopy similar to the conventional circular dichroism spectroscopy, but with much higher sensitivity. We have obtained chiral vibrational and electronic SFG spectra from a molecular monolayer, which would be impossible to obtain with circular dichroism spectroscopy. We are now working on further improvement of the sensitivity of the technique and on possibility of carrying out chiral microscopy on biological systems in collaboration with Prof. Haw Yang’s group in chemistry. Further work will include photo-induced chirality changes and chiral dynamics.
In collaboration with Prof. Xiang Zhang’s group and researchers at Hewlett-Packard, we are studying metamaterials that exhibit negativce refractive index in the near-infrared range. Light propagation in such a medium is predicted to have many interesting behaviors including imaging not restricted by diffraction limit, optical switching of power links, backward harmonic generation, etc. worth investigating. Electromagnetic waves associated with metals, i.e., plamonics, are at the heart of metamaterials. This topic is also an interest of our study.
We are also interested in optical characterization of nanostructures. Dr. Feng Wang in our group has developed different techniques, Rayleigh scattering, Raman scattering, two-photon-excited fluorescence, and modulated absorption spectroscopy to investigate electronic and phonon properties of single carbon nanotubes. The measurements will be extended to doped nanotubes, interaction between nanotubes, nonlinear responses and dynamics of nanotubes.