Effective Electron Temperature Measurement Using Time-Resolved Anti-Stokes Photoluminescence.

Anti-Stokes photoluminescence of metal nanoparticles, in which emitted photons have a higher energy than the incident photons, is an indicator of the temperature prevalent within a nanoparticle. Previous work has shown how to extract the temperature from a gold nanoparticle under continuous-wave monochromatic illumination. We extend the technique to pulsed illumination and introduce pump-probe anti-Stokes spectroscopy. This new technique enables us not only to measure an effective electron temperature in a gold nanoparticle (∼103 K under our conditions), but also to measure ultrafast dynamics of a pulse-excited electron population, through its effect on the photoluminescence, with subpicosecond time resolution. We measure the heating and cooling, all within picoseconds, of the electrons and find that, with our subpicosecond pulses, the highest apparent temperature is reached 0.6 ps before the maximum change in magnitude of the extinction signal.

Circular Dichroism Measurement of Single Metal Nanoparticles Using Photothermal Imaging.

Circular dichroism (CD) spectroscopy is a powerful optical technique for the study of chiral materials and molecules. It gives access to an enantioselective signal based on the differential absorption of right and left circularly polarized light, usually obtained through polarization analysis of the light transmitted through a sample of interest. CD is routinely used to determine the secondary structure of proteins and their conformational state. However, CD signals are weak, limiting the use of this powerful technique to ensembles of many molecules. Here, we experimentally realize the concept of photothermal circular dichroism, a technique that combines the enantioselective signal from circular dichroism with the high sensitivity of photothermal microscopy, achieving a superior signal-to-noise ratio to detect chiral nano-objects. As a proof of principle, we studied the chiral response of single plasmonic nanostructures with CD in the visible range, demonstrating a signal-to-noise ratio better than 40 with only 30 ms integration time for these nanostructures. The high signal-to-noise ratio allows us to quantify the CD signal for individual nanoparticles. We show that we can distinguish relative absorption differences for right circularly and left circularly polarized light as small as gmin = 4 × 10-3 for a 30 ms integration time with our current experimental settings. The enhanced sensitivity of our technique extends CD studies to individual nano-objects and opens CD spectroscopy to numbers of molecules much lower than those in conventional experiments.

Explosive, oscillatory, and Leidenfrost boiling at the nanoscale.

We investigate the different boiling regimes around a single continuously laser-heated 80 nm gold nanoparticle and draw parallels to the classical picture of boiling. Initially, nanoscale boiling takes the form of transient, inertia-driven, unsustainable boiling events characteristic of a nanoscale boiling crisis. At higher heating power, nanoscale boiling is continuous, with a vapor film being sustained during heating for at least up to 20µs. Only at high heating powers does a substantial stable vapor nanobubble form. At intermediate heating powers, unstable boiling sometimes takes the form of remarkably stable nanobubble oscillations with frequencies between 40 MHz and 60 MHz, frequencies that are consistent with the relevant size scales according to the Rayleigh-Plesset model of bubble oscillation, though how applicable that model is to plasmonic vapor nanobubbles is not clear.