- Coherent coupling between single semiconductor nanocrystals and plasmonic metal nanostructures
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. Our goal is to produce even stronger, coherent interactions so that we can observe new phenomena that emerge as a result of the interactions. In particular, we have predicted that a single semiconductor nanocrystal, or quantum dot, can lead to nearly complete cancellation of the optical scattering by a metal nanostructure. In addition, this induced transparency can be eliminated or even reversed when the system is excited by short laser pulses with appropriate energy. More exotic phenomena, such as entanglement of quantum-dot populations and coherent harvesting of solar energy, may be possible when multiple nanoparticles are joined in controlled assemblies. We are working on observing these phenomena experimentally, using a combination of bottom-up and top-down methods to produce controlled assemblies of metal nanoparticles and semiconductor nanocrystals, and using single-particle spectroscopy and electron microscopy to correlate optical properties of individual assemblies to their nanometer-scale structure.
- Nanomechanics and 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 water-glycerol solutions. The results show that there is a significant elastic response in the liquids, in contrast to the Newtonian fluid dynamics that are usually assumed to hold for simple liquids. Our demonstration of this effect constituted the first direct, mechanical measurement of viscoelastic effects in simple liquids. We are currently extending these measurements to different liquids and different solvents, with the aim of investigating viscoelastic effects in the bulk modulus of simple liquids, observing non-Newtonian effects in pure water, and ultimately understanding how bulk fluid-mechanical properties emerge from interactions among molecules in the liquid.
Funding: NSF CAREER
Semiconductor nanocrystals have been studied for several years as gain materials for solution-processable lasers with emission throughout the visible and near-infrared spectral range, but have been limited by impractically high laser thresholds. We have shown that semiconductor nanoplatelets, or colloidal quantum wells, can exhibit amplified spontaneous emission with low threshold and high gain coefficient. The superior properties of these nanoplatelets can be attributed in part to lower rates of Auger recombination in the platelets, a process in which one exciton (bound electron-hole pair) recombines non-radiatively by giving up its energy to another exciton. We showed that the Auger recombination rate can be further reduced by growing core-shell nanoplatelets, demonstrating a previously unobserved non-monotonic dependence of Auger rate on shell thickness. We produced a laser by integrating nanoplatelets with a photonic-crystal cavity, and showed that it had the lowest threshold of any room-temperature laser that has been demonstrated so far. Further development of these gain materials may lead to compact lasers at wavelengths that are currently difficult to access, and nanoscale light sources for chemical sensing and for on-chip optical communication.
- 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).