Research

 Current projects

Plasmon-exciton coupling

Plasmonic metal nanostructures can serve as nanoscale optical cavities, confining optical fields well below the diffraction limit. Quantum dots, molecules, or other emitters placed in these localized field will couple strongly to the emitters. In the weak-coupling regime, the radiative rate of the emitter is strongly enhanced, potentially enabling colloidal nanocrystals or defects in silicon to serve as efficient, room-temperature sources of indistinguishable single photons. In the intermediate-coupling regime, the coupled system has the potential to serve as an ultrafast, ultra-low-power, nanoscale all-optical modulator. In the strong-coupling regime, the coupled system has the potential to enable the transduction and ultrafast manipulation of quantum information at room temperature. We are currently working on making all of these potential phenomena a reality.

Funding: AFOSR, NSF, NIST

 

Nanoscale fluid dynamics using metal-nanoparticle vibrations

Ultrafast laser pulses can be used to excite and monitor mechanical vibrations of plasmonic metal nanoparticles. When the particles are suspended in liquid, their vibrational frequencies and damping rates serve as a probe of the fluid dynamics of the surrounding liquid and of the solid-liquid coupling. Using this technique, we have directly probed viscoelastic effects in simple, molecular liquids, and we have measured slip at the solid-liquid interface, with viscoelasticity enhancing this nanoscale slip. We are currently quantifying viscoelasticity and slip in a series of simple liquids, towards a microscopic understanding of these high-frequency, nanoscale phenomena. We are also investigating the mechanisms responsible for mechanical energy dissipation within the metal nanoparticles themselves and potential nonlinear effects in the optically measured nanoparticle vibrations, towards applications for in-situ molecular sensing.

Funding: NSF

 

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Charge and energy transfer at the nanoscale

Chemically synthesized nanoparticle assemblies and molecular arrays can mimic the key processes involved in photosynthesis, particularly solar energy capture, energy transfer, and charge transfer, potentially enabling them to efficiently generate liquid fuels directly from sunlight. We have measured charge transfer at the single-particle level in a model system of colloidal nanocrystals with single adsorbed molecules on their surfaces, removing the inhomogeneities present in ensemble measurements, and inferring the wavefunctions involved in charge transfer. We are also investigating charge- and energy-transfer dynamics in a series of custom molecular arrays, with the goal of producing ultra-long-lived energetic intermediate states that can drive catalytic reactions.

 

 

Capabilities

Transient-absorption spectroscopy

Broadly tunable 100-fs, 2-kHz pump pulses (based on Spectra-Physics Spitfire Pro amplified Ti:Sapphire system and TOPAS optical parametric amplifier); broadband visible / NIR probe; delay times up to 3 ns; automated data acquisition using Ultrafast Systems HELIOS system.

Single-particle microscopy

Home-built system for dark-field scattering and luminescence spectroscopy of single particles: excitation with ~50-ps pulsed diode lasers at 420 nm and 510 nm (PicoQuant LDH, also operable cw), time-correlated single-photon counting with < 30 ps timing resolution and autocorrelation capability (PicoQuant PicoHarp 300 electronics and MPD PDM single-photon counting avalanche photodiodes), grating spectrometer with back-illuminated CCD detector (Princeton Instruments PIXIS), sample positioning with 50 mm travel and ~50 nm repeatability (Alio integrated XY linear stage).