Research

Single semiconductor nanocrystals (or quantum dots), molecules, and defects in semiconductors can serve as sources of single photons, but using these photons in quantum-photonic applications requires the photons to be generated with high efficiency and to be indistinguishable from one another. Indistinguishability is the greatest challenge, because it requires that the rate of photon emission is much greater than the rate of decoherence within the emitter. This has generally been addressed by cooling the emitters to cryogenic temperatures, but we are pursuing another approach that may eventually enable quantum light sources at room temperature. In this approach, the photon-emission rate is dramatically enhanced by coupling the emitters to tightly confined optical fields around plasmonic metal nanostructures. If the coupling is strong enough, hybrid light-matter states (or polaritons) are formed, potentially enabling single-photon optical nonlinearities at room temperature.

Ultrafast laser pulses can be used to excite and monitor mechanical vibrations of plasmonic metal nanoparticles with frequencies in the GHz – THz range and with amplitudes in the Ângström range. When the particles are suspended in liquid, the vibrations serve as a probe of the fluid dynamics of the surrounding liquid and the properties of the solid-liquid interface at ultrafast time scales and ultrashort length scales. In this way, we have uncovered complex phenomena in simple liquids, including viscoelasticity in small-molecule liquids and slip on the single-nanometer length scale.

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. Using time-resolved optical measurements, we are measuring the dynamical processes in systems that are engineered to produce long-lived energetic states that can drive photocatalytic reactions.

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) or a femtosecond Ti:Sapphire laser (KMLabs), 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).