Ultrafast electron emission from sharp nanotips

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Tungsten nanotip. left Scanning electron micrograph of an etched tungsten tip. Note there is residual salt pollution leftover from the electrochemical etching process. These are removed via current flash cleaning. right 10,000x magnification of the tip apex with 85,000x magnification in the inset.
adapted from: Mina R. Bionta, “New experiment for understanding the physical mechanisms of ultrafast laser-induced electron emission from novel metallic nanotips”. Ph.D. Thesis. Université Paul Sabatier—Toulouse III (2015).
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Photoelectron spectra. Each set of spectra were taken with the same applied peak laser intensity: left: 6x1011 W cm-2, right: 1x1012 W cm-2. Changing the tip material or laser repetition rate (mean power) changes the dominant emission mechanism. The blue spectra show narrower spectra indicating thermally enhanced field emission. The green spectra show multiphoton emission characterized by ATP peaks separated by the 1.55 eV photon energy of the laser pulse. The red spectra show much broader energies indicating a transition into the optical tunneling regime and the emergence of a spectral plateau for high intensity indicating electron recollision and rescattering. CC: carbon cone nanotip. SCW: single crystal tungsten nanotip. Ag: silver nanotip.
adapted from: Mina R. Bionta, “New experiment for understanding the physical mechanisms of ultrafast laser-induced electron emission from novel metallic nanotips”. Ph.D. Thesis. Université Paul Sabatier—Toulouse III (2015).
 

I investigated the interaction of a sharp nanotip with an ultrashort laser pulse which generates photoemission of electron wavepackets. An electron can be emitted from a sharp nanotip system by many different mechanisms. Each mechanism gives a unique signature that can be identified by the photoelectron energy spectrum.

I observed the emission of photoelectrons from various nanotips based on different materials: tungsten, silver, and a new type of carbon-based nanotip formed around a single carbon nanotube. I confirmed the observation of above threshold photoemission (ATP) peaks from a tungsten nanotip. I detected the first laser induced electron emission from a carbon cone based on a single carbon nanotube. I observed a plateau in the electron spectra from a silver nanotip, the signature of electron recollision and rescattering in the tip. Various studies were performed in function of the voltage applied, repetition rate of the laser, laser polarization, energy and wavelength of the laser in order to understand these phenomena. From spectral features we were able to extract information about the system such as the enhancement factor of the laser electric field near the nanotip and the probability of above threshold photon absorption. Comparisons of the various spectra observed allowed us to spectrally identify the mechanisms for photoemission for tip based systems.

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