Posted in | Nanomaterials

New Study Explains the Mechanism of Ultrafast Electron Photoemission by Metallic Nanostructures

The mechanism of photoemission of electrons from metallic nanostructures under ultrafast laser excitation has now been explained by the results of the Russian-Japanese experiment.

When Metallic nanoparticle ensembles are irradiated by powerful laser pulses of femtosecond (1 fs = 10-15 seconds) duration, they emit short bunches of electrons.

Now, at Lobachevsky University, researchers have long been exploring the plasmon effect—the excitation through the light of collective electron oscillations in nanoparticles as well as the amplification of the light field related to these oscillations in the purlieu of the nanoparticle, which has an important role to play in this process. The effective electron photoemission from a metal is provided by the plasmon amplification of the light field.

The possibilities of viable applications of plasmon nanostructures are related to their use as ultrafast photocathodes to produce pulsed sources of high-brightness coherent X-ray radiation as well as to create microscopes that have a high temporal resolution.

The electron photoemission from metallic nanoparticles is typically accompanied by terahertz radiation emission (its range in the scale of electromagnetic waves is between microwaves and light), making it feasible to utilize this radiation as a tool for exploring photoemission.

The intensity of terahertz radiation depends non-linearly on the intensity of the laser pulse and demonstrates a high nonlinearity order (from 3 to 6 in various experiments). Although the mechanism of terahertz radiation generation by photoelectrons is not fully understood, it is believed that the high order of nonlinearity is explained by the multi-photon nature of electron emission, that is, by the need to transfer energy from several laser photons to the electron for performing the work to release the electron from the metal.

Michael Bakunov, Head, Department of General Physics, Lobachevsky University

In order to test the theory of the mechanism of multi-photon photoemission, Lobachevsky University researchers along with their Japanese colleagues from Osaka University, Shinshu University, and Tokyo Institute of Technology, carried out an experiment in which powerful ultrashort light pulses of different wavelengths—from 600 to 1500 nm—were used to irradiate the same kind of metallic nanostructure, an assortment of gold nanorods (“golden nanoforest”).

The result was unexpected. In spite of the fact that the quanta energy varied over twofold, the order of nonlinearity was more or less the same (4.5-4.8) for wavelengths ranging between 720 and 1500 nm. Moreover, the order of nonlinearity was even higher (6.6) for a wavelength of 600 nm (with the maximum quantum energy).

These results disprove the hypothesis of multi-photon emission of electrons. At the same time, the experimental dependences are in good agreement with the tunnel emission mechanism, whereby electrons are made to escape from the metal by a plasmon enhanced light field.

Michael Bakunov, Head, Department of General Physics, Lobachevsky University

The results of the study performed by Russian and Japanese researchers have been reported in one of the top scientific journals, Scientific Reports.

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