• Open Access

Ultrafast Multiphoton Thermionic Photoemission from Graphite

Shijing Tan, Adam Argondizzo, Cong Wang, Xuefeng Cui, and Hrvoje Petek
Phys. Rev. X 7, 011004 – Published 17 January 2017

Abstract

Electronic heating of cold crystal lattices in nonlinear multiphoton excitation can transiently alter their physical and chemical properties. In metals where free electron densities are high and the relative fraction of photoexcited hot electrons is low, the effects are small, but in semimetals, where the free electron densities are low and the photoexcited densities can overwhelm them, the intense femtosecond laser excitation can induce profound changes. In semimetal graphite and its derivatives, strong optical absorption, weak screening of the Coulomb potential, and high cohesive energy enable extreme hot electron generation and thermalization to be realized under femtosecond laser excitation. We investigate the nonlinear interactions within a hot electron gas in graphite through multiphoton-induced thermionic emission. Unlike the conventional photoelectric effect, within about 25 fs, the memory of the excitation process, where resonant dipole transitions absorb up to eight quanta of light, is erased to produce statistical Boltzmann electron distributions with temperatures exceeding 5000 K; this ultrafast electronic heating causes thermionic emission to occur from the interlayer band of graphite. The nearly instantaneous thermalization of the photoexcited carriers through Coulomb scattering to extreme electronic temperatures characterized by separate electron and hole chemical potentials can enhance hot electron surface femtochemistry, photovoltaic energy conversion, and incandescence, and drive graphite-to-diamond electronic phase transition.

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  • Received 5 July 2016

DOI:https://doi.org/10.1103/PhysRevX.7.011004

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Shijing Tan, Adam Argondizzo, Cong Wang, Xuefeng Cui, and Hrvoje Petek*

  • Department of Physics and Astronomy, and Pittsburgh Quantum Institute, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

  • *Corresponding author. petek@pitt.edu

Popular Summary

Graphene, a two-dimensional sheet of carbon atoms, is a remarkable material. It is strong, transparent, flexible, and an excellent conductor of electricity. These properties make it an attractive material for a wide range of applications that include solar cells, touch screens, and even tennis racquets. When graphene sheets are stacked, graphite is formed. Understanding the behavior of electrons within graphite could have implications for light harvesting in photosynthetic systems and may also explain how graphite is turned into diamond when energized by a light pulse from a laser. We found that shining intense laser light on graphite immediately creates a dense gas of electrons with temperatures comparable to that of the surface of the sun. Remarkably, the electron temperature achieved depends on the color of the light, with less energetic photons creating hotter temperatures.

Our experiment exposes graphite to an ultrafast, wavelength-tunable laser. We measure the energy of electrons emitted from the graphite as the laser wavelength changes from ultraviolet (290 nm) to infrared (880 nm). As the wavelength decreases, the energy of electrons increases dramatically. This counterintuitive result comes from the excited electrons in graphite scattering within 25 fs to create a temperature as high as 5500 K. We argue that the response comes not from single electrons absorbing multiple photons but from electrons scattering off of one another within the dense electron gas in graphite.

We predict that similar behavior might show itself in other materials composed of one-atom-thick layers such as graphite. The hot electron gas might also lead to efficient chemistry on the surface of some graphitic materials and could explain how laser light can transform graphite into nanodiamonds.

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Vol. 7, Iss. 1 — January - March 2017

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