Research

 Current projects

Strong quantum-mechanical coupling using plasmons

Excitation of plasmon resonances in metal nanostructures can lead to strong localization of optical fields in nanometer-scale volumes near the particles.  These enhanced near fields, in turn, have been widely explored for enhancement of optical processes, from absorption and emission to Raman scattering and harmonic generation. We have made unambiguous experimental measurements demonstrating that a single semiconductor nanocrystal, or quantum dot, coupled to the plasmon resonance in a metal nanostructure can produce (1) induced transparency in the plasmon extinction (the high-cooperativity regime), and (2) splitting into distinct plasmon-exciton hybrid states (the strong-coupling regime). Current collaborative research involves (1) chemical synthesis and assembly methods of nanoparticle assemblies that show intermediate and strong coupling with high yield; (2) demonstration of ultrafast, low-energy optical nonlinearities by modulating induced transparency; (3) strong coupling to isolated electronic or vibrational transitions in single molecules; (4) time-domain measurement and control of strongly-coupled light-matter states; (4) coupling of individual luminescent defects in silicon to plasmonic nanostructures.

Funding: AFOSR, NSF, NIST

 

Unconventional nanoscale fluid dynamics using metal-nanoparticle vibrations

When light from a laser pulse is absorbed by a metal nanoparticle, it rapidly expands; this, in turn, results in coherent vibration of the entire nanoparticle.  Plasmon resonances in noble-metal nanoparticles enable sensitive, all-optical measurement of these vibrations: as the particle vibrates, the plasmon frequency oscillates, resulting in changes in the transmission of a second, probe laser pulse. Using pump-probe measurements, we have measured the frequency and damping of vibrations for bipyramidal gold nanoparticles in simple liquids, observing a significant viscoelastic response, in contrast to the Newtonian fluid dynamics that are usually assumed to hold.  Extending these measurements to a range of liquid compositions and temperatures has demonstrated a breakdown of the slip boundary condition that is conventionally assumed to hold at liquid-solid interfaces, revealing a slip length on the nanometer scale. We are currently extending these measurements to investigate viscoelastic effects in the bulk modulus of simple liquids, investigate nonlinear mechanical response at the nanoscale, observe non-Newtonian and slip effects in pure water, and ultimately understand how bulk fluid-mechanical properties emerge from interactions among molecules in the liquid.

Funding: NSF

 

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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).

 

Microscope