Laser-induced Field Emission Research Articles

Laser-induced field emission from a tungsten nanotip by circularly polarized femtosecond laser pulses

We have investigated emission patterns and energy spectra of electrons from a tungsten nanotip induced by circularly polarized femtosecond laser pulses. Variations of emission patterns were observed for different helicities of circular polarization while the energy spectra remained almost identical. The physics behind this difference in emission patterns is the change in propagation directions of surface electromagnetic waves on the tip apex. The energy spectra showed the same spectroscopic signatures as the linearly polarized laser in a strong-field regime, which are a low-energy peak and a plateau feature. The low-energy peak is due to a delayed electron emission with respect to a prompt emission. The experimental data and plasmonic simulations support our previous conclusion, where the observed delayed emission processes originate from an inelastic rescattering process. This work demonstrates that the use of circular polarization is an easy means to add extra knobs to control the spatial and temporal emission from a nanotip at the nanometer and femtosecond scale. It could find applications as a helicity-driven subcycle optical switch.

Optimization of a Plasmon Enhanced Field Emitter Array Using a Nano-Tip-Based Plasmonic Double-Gate Structure

A nano-tip-based plasmonic double-gate structure (NPDS) excited by the surface plasmon polariton (SPP) resonance of the gate electrode has recently been proposed to achieve the specifications of the UV laser-induced copper cathode. The function of the NPDS is to generate a high bunch charge via near-infrared laser-induced field emission without increasing the cathode size. In this study, we report detailed numerical studies of the proposed NPDS to elucidate the physical mechanism of SPP propagation and light-tip coupling via internal SPP resonance. The simulation results show the relationship between the internal SPP resonance and the tip-light coupling in the NPDS, and the feasibility to achieve significant field emission enhancements with the NPDS. Understanding the internal SPP field in the plasmonic double-gate structure and the light field-tip coupling process in the NPDS is of practical importance in designing field emitter devices and could become an important factor in future high-brightness free-electron laser cathodes and high-sensitivity optical biosensors.

First results on laser-induced field emission from a CNT-based nanotip

We present the first demonstration of ultrafast laser-induced field emission and measurement of the energy distribution of electrons from a nanotip based on a carbon nanotube (CNT). Our experimental setup extends the studies performed on conventional tungsten or gold tips by using this new innovative tip. The carbon tip consists of concentric carbon layers in the shape of a cone, and has been previously studied as a very good candidate for cold field emission. The first laser-induced field emission from a CNT-based nanotip has been observed and we measured the energy spectrum as well as the polarization dependance of the emission. We also characterize the damage threshold of the tip, when illuminated by a high repetition rate femtosecond laser. These first results are encouraging further studies of electron emission from CNT-based carbon nanotips.

Five picocoulomb electron bunch generation by ultrafast laser-induced field emission from metallic nano-tip arrays

Laser-induced field emission from metallic field emitter array cathodes excited by femtosecond near infrared laser pulses is explored. When 50 fs laser pulses irradiated a 1.2 × 105-tip emitter array under a DC field emission bias, electron bunches with bunch charge up to 5.2 pC were observed. The variation of the bunch charge at different laser intensities and polarizations indicated that electrons were produced from the field emitters by a photofield emission process. The result demonstrates the feasibility of metallic field emitter array cathodes for high-charge short-pulse electron source applications.

Energy Distribution Curves of Ultrafast Laser-Induced Field Emission and Their Implications for Electron Dynamics

Energy distribution curves of laser-induced electron pulses from a tungsten tip have been measured as a function of tip voltage and laser power. Electron emission via tunneling through and/or excitation over the surface barrier from photoexcited nonequilibrium electron distributions are clearly observed. The spectral shapes largely vary with the emission processes and are strongly affected by electron dynamics. Simulations successfully reproduce the spectra, thus allowing direct insight into the involved electron dynamics and revealing the temporal tunability of electron emission via the two experimental parameters. These results should be useful to optimize the pulse characteristics for many applications based on ultrafast laser-induced electron emission.

Laser-induced field emission from a tungsten tip: Optical control of emission sites and the emission process

Field-emission patterns from a clean tungsten tip apex induced by femtosecond laser pulses have been investigated. Strongly asymmetric field-emission intensity distributions are observed depending on three parameters: (1) the polarization of the light, (2) the azimuthal and (3) the polar orientation of the tip apex relative to the laser incidence direction. In effect, we have realized an ultrafast pulsed field-emission source with site selectivity of a few tens of nanometers. Simulations of local fields on the tip apex and of electron emission patterns based on photo-excited nonequilibrium electron distributions explain our observations quantitatively. Electron emission processes are found to depend on laser power and tip voltage. At relatively low laser power and high tip voltage, field-emission after two-photon photo-excitation is the dominant process. At relatively low laser power and low tip voltage, photoemission processes are dominant. As the laser power increases, photoemission from the tip shank becomes noticeable.

Enhancement of laser-induced field emission in tungsten due to a metastable d band

A possible theory explaining the experimentally observed large enhancement of the field emission current from a tungsten needle exposed to a laser beam is presented. Calculations based on the density functional theory indicate a large density of 5d states and less dense 6p states above the Fermi level. The incident laser beam of 2.5 eV pumps the electrons to an unoccupied 6p band, some of which then de-excite to 5d states at lower energies, forming metastable states since the radiative decay from 5d states to the ground state 6s band is forbidden. On the other hand, the tunneling time from the metastable state is shorter than that from the ground state because of a smaller surface barrier, which may lead to the observed enhancement of the emitted current.

IMPLICATIONS OF RELATIVISTIC CONFIGURATIONS AND BAND STRUCTURES IN THE PHYSICS OF BIO-MOLECULES AND SOLIDS

Beyond the second row of elements in the Mendeleev periodic table, the consideration of the relativistic effect is important in determining proper configurations of atoms and ions, in many cases. Many important quantities of interest in determining physical and chemical properties of matter, such as the effective charge, root mean square radii, and higher moments of radii used in many calculations, e.g. in the determinations of legend stabilization bond energies depend on whether the treatment is relativistic or not. In general, these quantities for a given l-orbital having two different j-values, e.g. [Formula: see text] and [Formula: see text], differ from each other, hence, making it necessary to treat them as separate orbitals. This also necessitates characterizing bands with their j-values in many instants and not l-values, particularly for “d” and f-orbitals. For example, in Au , [Formula: see text] and [Formula: see text] are to be dealt with as two distinct bands. The observed enhancement of laser induced field emission in W, which is not understood in terms of non-relativistic band-structures, can be explained in terms of the expected relativistic band structure. Spin-orbit coupling, which is the manifestation of the relativistic effect, is a prime factor in facilitating intersystem crossing in bio-molecules.

Ponderomotive electron acceleration using surface plasmon waves excited with femtosecond laser pulses

We report on the ponderomotive acceleration of electrons using surface plasmon (SP) waves launched on Ag and Au films. High-energy electrons, up to 2 keV, are generated in the high spatial gradient of the SP field. Acceleration gradients of ∼8GeV∕m are produced using 30GW∕cm2, 800 nm amplified 30 fs laser pulses. Investigation of the photoemission characteristics of these metal films reveals a distinct transition between the multiphoton regime and a laser-induced field emission regime. Results of the experiment are in good agreement with those predicted with test particle code, which is based on finite-difference time-domain simulation and incorporates the Drude dielectric function and photoemission properties of the metallic films.

Evanescent-wave acceleration of ultrashort electron pulses

High-power femtosecond laser pulses have been used to excite surface plasmons in 500 Å silver and gold films. Nonlinear excitation results in the emission of electron bunches through multiphoton excitation at low power and laser-induced field emission at high power. The energies of photoelectrons are found to extend as high as 0.4 keV. Calculations show that these high energies are due to ponderomotive acceleration in an evanescent field extending from the metal film out into the vacuum. The theoretical calculations suggest that femtosecond electron pulses with relativistic energies can be generated using longer wavelengths or by developing the surface morphology.