Off-resonance laser fields may be used to manipulate and spatially confine atoms and molecules through various mechanisms. We model electromagnetic fields using the FDTD method and use the results for full quantum mechanical calculations of molecular trapping.

Ultra-high vacuum (UHV)- tip-enhanced Raman spectroscopy (TERS) combines the imaging properties of scanning tunneling microscopy (STM) with the chemical information provided by surface-enhanced Raman spectroscopy (SERS). It has been recently demonstrated that low temperature UHV-TERS is capable of single molecule vibrational spectroscopy with sub-nm spatial resolution. We are interested in further developing this technique and in developing a fundamental theoretical framework to understand the mechanism(s) responsible for the sub-nm resolution and explore potential applications ranging from single-molecule molecular machines to light-induced transport and current-driven Raman Spectroscopy. Several of these applications are discussed elsewhere in our web-site. We highlight here two ongoing joint theoretical/experimental studies:
publication

The excitation and subsequent decoherence of nanoparticle plasmon resonances creates an electronic nonequilibrium that equilibrates rapidly into a high electronic temperature. The latter equilibrates slowly (on a picosecond time scale) with the lattice temperature, hence opportunities to observe both a nonequilibrium and a thermonic current. We combine a theory of plasmon-mediated electronic nonequilibrium with ab initio transport calculations and TERS experiments carried by Northwestern collaborators to explore light-induced current in molecular junctions.

Tip-enhanced Raman spectroscopy (TERS) has the ability to probe surface chemistry with both nanometer spectroscopic and submolecular spatial resolution. Collaborative experimental efforts are currently focused on combining TERS with picosecond and femtosecond pulses, allowing for observation of novel chemistry at unprecedented temporal and spatial resolutions. Towards this end, our theoretical calculations aid experimentalists in elucidating the short and long time scale effects of laser-induced heating in both pulsed and continuous wave (cw) TERS systems.

Dye-sensitized solar cells and Quantum dot solar cells offer more inexpensive ways to harness solar energy to generate electricity. However, efficiencies are below 15%. Plasmonic nanoparticles are able to boost injection efficiencies and recent experimental and theoretical work has shown demonstrated marginal increases in efficiency. Our work shows that there is an optimum coupling between plasmons and electronic excitations in the presence of light that can lead to the best possible efficiencies. We are trying to find out how to obtain this optimum for specific geometry and architectures of solar cells. Plasmonic excitations in metal nanoparticles relax to produce hot electrons that can tunnel out and ionize molecules and perform useful chemical reactions at much better efficiences due to the nanoscale of metal particles. We are trying to model these processes in the context of water splitting reactions and other useful photodegradation of organic molecules at electrodes.

In related research, we developed genetic algorithms to determine the optimal shape, size and mutual arrangement of metal nanoparticles for solar cell applications. The genetic algorithms use the Maxwell-Liouville method to couple the quantum mechanical dye molecules to the classical description of the nanopartilces and light.

It has been shown in recent studies that photodetectors using CdSe quantum dots are capable of reaching very high internal quantum efficiencies of close to 100%, but they have external quantum efficiencies of approximately 1%. Recent experimental results show that adding the quantum dots onto a layer of gold nanoislands increases the absorption of the films by 35%, which is attributed to plasmon-enhanced absorption of the quantum dot. We collaborate with an overseas experimental group to understand and optimize such photodetectors. Our numerical research uses a home-developed Maxwell-Liouville formulation to model the hybrid metal-semiconductor construct and understand its operation mechanism.

We propose a new method to control the translational, rotational and torsional motions of polyatomic molecules on the footing of plasmon-enhanced laser fields near the surface of metal nanoparticles. We exploit the ability of nanoparticles to enhance electric fields in order to achieve alignment and focusing of molecules that show observable torsions in their electronic ground state, controlling not only their translational and rotational motions, but also their internal structure. For one application, we explore the opportunity to directly decompose mixtures of nuclear spin isomers of symmetrical molecules or enantiomers of axially chiral molecules.

Surface-enhanced Raman spectroscopy (SERS) can yield precise chemical information and single molecule resolution due to the signal-enhancing capabilities created within the ~1 nm3 ‘hot spot’ junctions of metallic nano-structures. By implementing a two-color experiment, we are able to initiate atomic scale surface-molecule chemistry using multiple off-resonance wavelengths, while measuring these effects with an optimal wavelength that yields high signal-noise ratios. We report unexpected spectral fluctuations in the SERS signal of gold nanotag dimers functionalized with a reporter molecule, trans-1,2-bis(4-pyridyl)-ethylene (BPE). The origin of these fluctuations is largely unknown and is a topic of much scientific deliberation. Through a collaborative experiment-theory effort, we investigate mechanisms such as hot electron activated charged states, molecular surface-site ‘hopping’, and photo-induced isomerization that may contribute significantly to this observed phenomenon.