M Raynaud et al talk about their recent research, published in JPCM, which looks to increase both the photoemission yield and the photoelectron energy of the nano-scale object array targets. You can read the full paper here.
Due to their unique optical properties, metallic nano-particles are very attractive. They hold the promise of opening the way to controlling light on the nanometer length scale and the femtosecond time scale. Surface plasmons (SP) are the collective oscillations of free electrons that are confined evanescently on a metal surface. In a grating, SP coupled with photons from a short laser pulse can induce a strong acceleration of electrons via modified photoemission mechanisms. In order to improve the electron sources in terms of emissivity, energy and beam spread, we try to take advantage of metallic nanoparticles organized within a network to enhance the overall plasmon resonance of the system. This point is of considerable interest for the processes and devices that exploit hot-electron emission, e.g. in photo-catalysis, photovoltaic devices and optoelectronics.
In our paper recently published in JPCM, we have studied the photoelectron emission induced by short-pulse (25 fs) laser excitation of a regular two-dimensional array of gold-coated nano-scale objects (N-O) within a range of moderate laser intensities ~ 1010 W/cm2 and varying the laser wavelength (266-800 nm). The aim is to obtain a target with an efficient plasmon structure and to understand the role played by the parameters of the excitation, in particular the laser wavelength.
We observe that by using a periodic gold N-O array target, a SP is excited and the electron emission is dramatically modified. The yield of the electron current emitted from the N-O array target is enhanced by at least a factor of 50 with respect to the single-crystal sample. In the N-O array target one also observes higher kinetic energies than can be expected based on the photoelectric balance. This behavior can be qualitatively interpreted by considering both space charge effects and electron ponderomotive acceleration within the local induced surface plasmon field in vacuum, whereby the relative contributions of these two mechanisms depend on both the laser wavelength and intensity. In addition, we have determined for the first time, the wavelength dependence of the photoemission from the N-O array target when the laser excites a surface plasmon. These results are well interpreted by means of a model that describes the SP-stimulated electron emission process as a two-step mechanism. The first step considers the electronic transitions from the metallic conduction band towards the solid continuum that are induced by the surface plasmon field. The second step of the model is based on a classical description of the motion of the freed electrons along the two spatial components of the inhomogeneous oscillating surface plasmon field in the region outside the target surface and leads to the final spectra. It has validated the experimental conclusion attributing the variation of the electron energy with the laser wavelength with the variation of the electron ponderomotive acceleration in the local induced surface plasmon field. Finally, we also demonstrate that with increasing laser intensities, we can obtain electrons with kinetic energies as high as roughly 300 eV at moderate laser intensities (~5 1010 W/cm2). Such a high energy value cannot be reached with the gold single crystal under similar irradiation conditions.
The main experimental and theoretical activities of this paper have been carried out at the University of Bordeaux, France, while the targets were prepared at the Czech Technical University in Prague.
About the authors
N. Fedorov, G. Geoffroy, G. Duchateau, H. Jouin and P. Martin are members of the Centre Laser Intense et Applications (CELIA) at the University of Bordeaux. N. Fedorov is a young Physicist who was in charge, along with G. Geoffroy and P. Martin, of the experiment. H. Jouin is Professor of Physics at the University of Bordeaux, he was involved in the modelisation with G. Duchateau and M. Raynaud which belong to the Laboratoire des Solides Irradiés at the Ecole Polytechnique.
L. Štolcová, J. Proška, and F. Novotnỷ are members of the Czech Technical University in Prague. M. Domonkos belongs to the Institute of Physic, ASCR, v.v.i. in Prague. They realized and characterized the N-O array targets.
This work is licensed under a Creative Commons Attribution 3.0 Unported License. Figures taken from N Fedorov et al 2016 J. Phys.: Condens. Matter 28 315301. © 2016 IOP Publishing Ltd. All rights reserved.
Categories: Journal of Physics: Condensed Matter