Ultrafast Plasmon-Enhanced Hot Electron Generation at Ag Nanocluster/Graphite Heterojunctions

Hot electron processes at metallic heterojunctions are central to optical-to-chemical or electrical energy transduction. Ultrafast nonlinear photoexcitation of graphite (Gr) has been shown to create hot thermalized electrons at temperatures corresponding to the solar photosphere in less than 25 fs. Plasmonic resonances in metallic nanoparticles are also known to efficiently generate hot electrons. Here we deposit Ag nanoclusters (NC) on Gr to study the ultrafast hot electron generation and dynamics in their plasmonic heterojunctions by means of time-resolved two-photon photoemission (2PP) spectroscopy. By tuning the wavelength of p-polarized femtosecond excitation pulses, we find an enhancement of 2PP yields by 2 orders of magnitude, which we attribute to excitation of a surface-normal Mie plasmon mode of Ag/Gr heterojunctions at 3.6 eV. The 2PP spectra include contributions from (i) coherent two-photon absorption of an occupied interface state (IFS) 0.2 eV below the Fermi level, which electronic structure calculations assign to chemisorption-induced charge transfer, and (ii) hot electrons in the π*-band of Gr, which are excited through the coherent screening response of the substrate. Ultrafast pump–probe measurements show that the IFS photoemission occurs via virtual intermediate states, whereas the characteristic lifetimes attribute the hot electrons to population of the π*-band of Gr via the plasmon dephasing. Our study directly probes the mechanisms for enhanced hot electron generation and decay in a model plasmonic heterojunction.

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Ultrafast Microscopy: Imaging Light with Photoelectrons on the Nano-Femto Scale

Experimental methods for ultrafast microscopy are advancing rapidly. Promising methods combine ultrafast laser excitation with electron-based imaging or rely on super-resolution optical techniques to enable probing of matter on the nano–femto scale. Among several actively developed methods, ultrafast time-resolved photoemission electron microscopy provides several advantages, among which the foremost are that time resolution is limited only by the laser source and it is immediately capable of probing of coherent phenomena in solid-state materials and surfaces. Here we present recent progress in interference imaging of plasmonic phenomena in metal nanostructures enabled by combining a broadly tunable femtosecond laser excitation source with a low-energy electron microscope.

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Using Ultrafast Laser Pulses and Computational Modeling to Understand Nonproportionality in Spectroscopic Gamma-Ray Detectors

Richard Williams
Thursday, March 23, 2017 - 12:00pm to 1:30pm

Under gamma-ray or charged-particle excitation, scintillation light yield is a complicated function of carrier diffusion and cooling in the track along with kinetic rate terms depending on local excitation density. Up to 1021 electron-hole pairs/cm3 are produced in an initial track radius of about 3 nm. Extracting the fundamental rate constants directly from such conditions would require solving the diffusion and cooling problems in complex track structures first. Laser interband photon density response and time-resolved pump-probe studies are surrogate experiments that...

The Calisthenics of Surface Femtochemistry

  • By Aude Marjolin
  • 27 October 2016

Application of a femtosecond spectroscopy technique to a copper surface has allowed the desorption of carbon monoxide molecules to be tracked with unprecedented detail.

In this view point article, Hrvoje Petek comments on the work of Ken-ichi Inoue et al. on the multidimensional nonequilibrium dynamics of CO as it desorbs from a Cu(100) surface.  He opens by saying that "it has long been a chemist’s dream to catch a chemical reaction in the act", with the aim "to observe and interfere with this process in mid-course and thereby control its outcome".

Reflections on Nano and Femto Imaging

  • By Aude Marjolin
  • 12 February 2016

"Scanning near-field optical microscopy combined with pump–probe spectroscopy can resolve ultrafast dynamics at the nanoscale."

In this short article, Hrvoje Petek reflects on a new technique that combines the nanometre resolution of near-field microscopy with the femtosecond resolution of pump–probe spectroscopy. This technique has been developed by Markus Raschke and colleagues at the University of Colorado at Boulder and submitted in the present issue of Nature Nanotechnology.

First Detection of a Fundamental Particle of Light-Matter Interaction in Metals: the Exciton

  • By Aude Marjolin
  • 1 June 2014

PQI faculty Hrvoje Petek and Sean Garrett-Roe have become the first to detect a fundamental particle of light-matter interaction in metals, the exciton. The team has published its work in the June 2014 online issue of Nature Physics.

Mankind has used reflection of light from a metal mirror on a daily basis for millennia, but the quantum mechanical magic behind this familiar phenomenon is only now being uncovered.