Neuhauser Group
UCLA Department of Chemistry and Biochemistry
Nanopolaritonics
The interactions between surface plasmons, which are collective electron excitations at the surface of a metal, and molecular exictations, lead to many interesing phenomena.
On one hand, plasmonic systems can be used as sensors which utilize the strong fields around metal nanostructures, as in the case of surface enhanced Raman spectroscopy (SERS). The strong fields at the surface can be focused into nearby analyte molecules, resulting in drastically increased sensitivity, and possibly non-linear effects.
On the other hand, a nearby molecule can be used to influence plasmon transfer along devices. The study of the control of plasmons via molecules and other dipolar matter is called nanopolaritonics.
For example, surface plasmon-polariton transfer through a chain of gold nanoparticles is highly sensitive to presense of a nearby dipolar molecule. We have numerically investigated the interaction of a dipolar molecule on surface plasmon-polariton (SPP) transfer through a linear array of gold nanoparticles using classical point plasmons for the metal nanoparticles, coupled to a two-level molecule described quantum mechanically with linear response. We demonstrated that an xy-oriented molecule can scatter incident x-polarized SPPs into y-polarized ones, over a frequency range centered at the molecule's resonance, via a phenomenon similar to a Fano resonance.
This effect can be used to gate plasmon transfer through a switch geometry, as shown in the figure on the right. Depending on dipole moment and orientation, a nearby dipolar molecular (shown as a purple triangle) can direct incident x-polarized SPPs into either the upper or lower output of a fork junction of spherical gold nanoparticles.
Spintronics
The use of spin as opposed to charge to store, process and transmit information has great technological potential. Certain molecular systems, such as repetitive ring systems, are expected to exhibit spin-birefringence wherein different spin polarizations have difference transmission spectra through the wire; alternatively, incident spins can be flipped upon transmission.
In particular, we have shown that polyacetylene molecular wires/ring devices with bonded chlorogold groups exhibit significantly spin-flipped currents. That is, incident x-polarized spin current is partially flipped into y-polarized spins
This effect is a result of the spin-orbit coupling due to the gold. In essence, the symmetry between forward and reverse moving states is broken, resulting in a phase-lag for the spins.
Density functional theory
We also develop and implement improved time-dependent density functional theory (TDDFT) approaches suitable for nanoscale device modeling. These include:
- Orbital-free DFT methods
- Hydrodynamic tensor DFT
- Quantum Drude friction