Laser-induced Electron Emission Research Articles

Subadditive femtosecond laser-induced electron emission from a GaAs tip

Subadditive femtosecond laser-induced electron emission from a GaAs tip

Heating of nanoparticles and their environment by laser radiation and applications

This review considers the fundamental dynamic processes involved in the laser heating of metal nanoparticles and their subsequent cooling. Of particular interest are the absorption of laser energy by nanoparticles, the heating of a single nanoparticle or an ensemble thereof, and the dissipation of the energy of nanoparticles due to heat exchange with the environment. The goal is to consider the dependences and values of the temperatures of the nanoparticles and the environment, their time scales, and other parameters that describe these processes. Experimental results and analytical studies on the heating of single metal nanoparticles by laser pulses are discussed, including the laser thresholds for initiating subsequent photothermal processes, how temperature influences the optical properties, and the heating of gold nanoparticles by laser pulses. Experimental studies of the heating of an ensemble of nanoparticles and the results of an analytical study of the heating of an ensemble of nanoparticles and the environment by laser radiation are considered. Nanothermometry methods for nanoparticles under laser heating are considered, including changes in the refractive indices of metals and spectral thermometry of optical scattering of nanoparticles, Raman spectroscopy, the thermal distortion of the refractive index of an environment heated by a nanoparticle, and thermochemical phase transitions in lipid bilayers surrounding a heated nanoparticle. Understanding the sequence of events after radiation absorption and their time scales underlies many applications of nanoparticles. The application fields for the laser heating of nanoparticles are reviewed, including thermochemical reactions and selective nanophotothermolysis initiated in the environment by laser-heated nanoparticles, thermal radiation emission by nanoparticles and laser-induced incandescence, electron and ion emission of heated nanoparticles, and optothermal chemical catalysis. Applications of the laser heating of nanoparticles in laser nanomedicine are of particular interest. Significant emphasis is given to the proposed analytical approaches to modeling and calculating the heating processes under the action of a laser pulse on metal nanoparticles, taking into account the temperature dependences of the parameters. The proposed models can be used to estimate the parameters of lasers and nanoparticles in the various application fields for the laser heating of nanoparticles.

Laser-Induced Ion Formation and Electron Emission from a Nanostructured Gold Surface at Laser Fluence below the Threshold for Plasma Formation.

The laser formation of positive and negative ions on a nanostructured metal surface is observed at laser fluence below the plasma formation threshold. The laser radiation energy dependences of the yield of positive and negative Au ions and charged clusters as well as electrons from the laser-induced nanostructures on the surface of gold are obtained at laser fluence below the plasma formation threshold using a pulsed laser with a wavelength of 355 nm and a pulse duration of 0.37 ns. It is shown that the ratio of the signals of positive and negative ions is constant over the entire range of the laser radiation energies, while the ion signal dependence on the laser radiation energy is described by a power function with an exponent of 9. The role of gold nanoparticles with a size of less than 5 nm in the formation of Au ions and charged Au clusters is discussed.

Capacitive effect in ultrafast laser-induced emission from low conductance diamond nanotips

Single crystal diamond nanotips reveal a new behavior for ultrafast laser-induced electron emission. Under tightly focused femtosecond laser illumination, electron yield shows a saturation with the laser intensity. When the DC bias is sufficient for dark field emission, large optical intensities can switch off the emission occurring between laser pulses during a few hundred μs, because of the low conductance of the diamond tip. We propose a macroscopic model to combine a capacitive effect with the different conduction and emission mechanisms. This study shows that non-metallic photocathodes offer different perspectives from the conventional metallic ones.

Very thin N-doped nanostructured carbon films on quartz and sapphire substrate: Photoelectron emission properties

Very thin N-doped nanostructured carbon films were deposited on quartz and sapphire substrate by radio-frequency reactive magnetron sputtering using carbon target and gas mixture of Ar and N2 or N2+H2 reactive gasses. Rutherford backscattering spectroscopy and Elastic recoil detection analytical methods determined the concentration of elements in the films. Scanning electron microscopy was used to investigate the surface morphology of nanostructured very thin carbon films. Raman spectroscopy was used for the determination of chemical structural properties of the thin nanostructured carbon films. Pulsed laser induced electron emission method was used for the study of photoelectron emission properties of nanostructured carbon films. Measured bunch charge results of fabricated transmission photocathodes showed better photoelectron emission properties of very thin nanostructured carbon films prepared on sapphire substrates. Effects of substrate and technology of very thin nanostructured carbon films on the properties of photo-induced electron emitters as backside illuminated transmission photocathode are discussed.

Analysis of two-color laser-induced electron emission from a biased metal surface using an exact quantum mechanical solution

Photoelectron emission from nanotips via strong-field lasers is a powerful technique for coherent control of ultrafast electron dynamics, and so is important in many areas, including time-resolved electron microscopy and free-electron lasers. The effects of dc bias are typically ignored, though. Exact solution of the time-dependent Schr\odinger equation reveals that, under large dc bias, strong modulation persists in the emission current, the magnitude of which increases significantly. In the authors' model, the dynamics of the multiphoton excited states depend strongly on the applied dc field. This work suggests a practical approach to strong control of high-current photoemission.

Femtosecond Laser-Induced Electron Emission from Nanodiamond-Coated Tungsten Needle Tips.

We present femtosecond laser-induced electron emission from nanodiamond-coated tungsten tips. Based on the shortness of the femtosecond laser pulses, electrons can be photoexcited for wavelengths from the infrared (1932nm) to the ultraviolet (235nm) because multiphoton excitation becomes efficient over the entire spectral range. Depending on the laser wavelength, we find different dominant emission channels identified by the number of photons needed to emit electrons. Based on the band alignment between tungsten and nanodiamond, the relevant emission channels can be identified as specific transitions in diamond and its graphitic boundaries. It is the combination of the character of initial and final states (i.e., bulk or surface-near, direct or indirect excitation in the diamond band structure), the number of photons providing the excitation energy, and the peak intensity of the laser pulses that determines the dominant excitation channel for photoemission. A specific feature of the hydrogen-terminated nanodiamond coating is its negative electron affinity that significantly lowers the work function and enables efficient emission from the conduction band minimum into vacuum without an energy barrier. Emission is stable for bunch charges of up to 400electrons per laser pulse. We infer a normalized emittance of <0.20 nm rad and a normalized peak brightness of >1.2×10^{12} A m^{-2} sr^{-1}. The properties of these tips are encouraging for their use as laser-triggered electron sources in applications such as ultrafast electron microscopy as well as diffraction and novel photonics-based laser accelerators.

Ionization by an Oscillating Field: Resonances and Photons

We describe new exact results for a model of ionization of a bound state in a 1d delta function potential, induced by periodic oscillations of the potential of period $$2\pi /\omega $$ . In particular we have obtained exact expressions, in the form of Borel summed transseries for the energy distribution of the emitted particle as a function of time, $$\omega $$ and strength $$\alpha $$ of the oscillation of the potential. These show peaks in the energy distribution, separated by $$\hbar \omega $$ , which look like single or multi-photon absorption. The peaks are very sharp when the time is large and the strength of the oscillating potential is small but are still clearly visible for large fields, and even for time-periods of a few oscillations. These features are similar to those observed in laser induced electron emission from solids or atoms (Phys Rev Lett 105:257601, 2010). For large $$\alpha $$ the model exhibits peak-suppression. The ionization probability is not monotone in the strength of the oscillating potential: there are windows of much slower ionization at special pairs $$(\alpha ,\omega )$$ . This shows that ionization processes by time-periodic fields exhibit universal features whose mathematical origin are resonances which pump energy into the system represented by singularities in the complex energy plane. All these features are proven in our simple model system without the use of any approximations.

Laser-induced electron emission from Au nanowires: A probe for orthogonal polarizations

Photoelectron field emission, induced by femtosecond laser pulses focused on metallic nanotips, provides spatially coherent and temporally short electron pulses. The properties of the photoelectron yield give insight into both the material properties of the nanostructure and the exciting laser focus. Ultralong nanoribbons, grown as a single crystal attached to a metallic taper, are sources of electron field emission that have not yet been characterized. In this report, photoemission from gold nanoribbon samples is studied and compared to emission from tungsten and gold tips. We observe that the emission from sharp tips generally depends on one transverse component of the exciting laser field, while the emission of a blunted nanoribbon is found to be sensitive to both components. We propose that this property makes photoemission from nanoribbons a candidate for position-sensitive detection of the longitudinal field component in a tightly focused beam.

Optical Control of Young\u2019s Type Double-slit Interferometer for Laser-induced Electron Emission from a Nano-tip

Interference experiments with electrons in a vacuum can illuminate both the quantum and the nanoscale nature of the underlying physics. An interference experiment requires two coherent waves, which can be generated by splitting a single coherent wave using a double slit. If the slit-edge separation is larger than the coherence width at the slit, no interference appears. Here we employed variations in surface barrier at the apex of a tungsten nano-tip as slits and achieved an optically controlled double slit, where the separation and opening-and-closing of the two slits can be controlled by respectively adjusting the intensity and polarization of ultrashort laser pulses. Using this technique, we have demonstrated interference between two electron waves emitted from the tip apex, where interference has never been observed prior to this technique because of the large slit-edge separation. Our findings pave the way towards simple time-resolved electron holography on e.g. molecular adsorbates employing just a nano-tip and a screen.

Early-stage effects of residual charges in a metal target on emitted electrons induced by femtosecond laser–metal interactions

Electron emissions from a metal target surface may be induced due to the irradiation of the target by a femtosecond (fs) laser pulse. The emitted electrons will leave behind residual charges (which are positive) in the metal target near its surface. The residual charges may affect the evolution of the emitted electrons, which is called the “residual charge effect”. An intuitive belief could be that the residual charge effect is insignificant, because the huge number of free electrons in the interior region of the metal may quickly neutralize the residual charges. In this paper, the early-stage (at a time scale of less than ∼1 picosecond) residual charge effect has been investigated. The study shows that contrary to the above intuitive belief, the early-stage residual charge effect is very significant under the studied conditions, which has greatly slowed down the expansion of emitted electrons and enhanced their recombination back into the surface of the target. The study implies that to accurately study the early-stage fs laser-induced electron emission and other closely related processes, the residual charge effect should not be neglected.

Laser-initiated explosive electron emission from flat germanium crystals

Flat Sb-doped germanium (100) crystals were investigated in the triode configuration under pulsed tunable laser illumination (pulse duration tlaser = 3.5 ns and photon energy hν = 0.54–5.90 eV) and under DC voltages &amp;lt;104 V. Large bunch charges up to ∼1 μC were extracted from the cathodes for laser pulses &amp;gt;1 MW/cm2 corresponding to a high quantum efficiency up to 3.3% and cathode currents up to 417 A. This laser-induced explosive electron emission (EEE) from Ge was characterized by its voltage-, laser power- and hν-sensitivity. The analysis of the macroscopic surface damage caused by the EEE is included as well. Moreover, we have carried out first direct measurements of electron energy distributions produced during the EEE from the Ge samples. The measured electron spectra hint for electron excitations to the vacuum level of the bulk and emission from the plasma plume with an average kinetic energy of ∼0.8 eV.

Ultrafast strong-field photoelectron emission from biased metal surfaces: exact solution to time-dependent Schrödinger Equation.

Laser-driven ultrafast electron emission offers the possibility of manipulation and control of coherent electron motion in ultrashort spatiotemporal scales. Here, an analytical solution is constructed for the highly nonlinear electron emission from a dc biased metal surface illuminated by a single frequency laser, by solving the time-dependent Schrödinger equation exactly. The solution is valid for arbitrary combinations of dc electric field, laser electric field, laser frequency, metal work function and Fermi level. Various emission mechanisms, such as multiphoton absorption or emission, optical or dc field emission, are all included in this single formulation. The transition between different emission processes is analyzed in detail. The time-dependent emission current reveals that intense current modulation may be possible even with a low intensity laser, by merely increasing the applied dc bias. The results provide insights into the electron pulse generation and manipulation for many novel applications based on ultrafast laser-induced electron emission.

Laser-induced delayed electron emission of Co−4 anions

SynopsisThe cryogenic trap for fast ion beams (CTF) located at the Max-Planck-Institut für Kernphysik has been used to study laser-induced delayed electron emission of various molecular and metal-cluster anions. We will discuss recent experiments investigating the radiative cooling respectively heating of Co−4 anions produced in hot and cold ion sources. The experiments will be continued at the cryogenic storage ring (CSR), which is presently being commissioned, to disentangle laser-induced electron emission from atom or cluster dissociation.

Electron dynamics and prompt ablation of aluminum surface excited by intense femtosecond laser pulse

Thin aluminum film homogeneously heated by intense IR femtosecond laser pulses exhibits on the excitation timescale consequent fluence-dependent rise and drop of the IR-pump self-reflectivity, followed by its final saturation at higher fluences F > 0.3 J/cm2. This prompt optical dynamics correlates with the initial monotonic increase in the accompanying laser-induced electron emission, which is succeeded by its non-linear (three-photon) increase for F > 0.3 J/cm2. The underlying electronic dynamics is related to the initial saturation of IR resonant interband transitions in this material, followed by its strong instantaneous electronic heating via intraband transitions during the pump pulse resulting in thermionic emission. Above the threshold fluence of 0.3 J/cm2, the surface electronic heating is balanced during the pump pulse by simultaneous cooling via intense plasma removal (prompt ablation). The relationship between the deposited volume energy density in the film and its prompt electronic temperature derived from the self-reflection measurements using a Drude model, demonstrates a kind of electron “liquid–vapor” phase transition, driven by strong cubic optical non-linearity of the photo-excited aluminum.

Modeling ultrashort laser-induced emission from a negatively biased metal

A theoretical study of ultrashort laser-induced electron emission from a negatively biased metallic cathode has been performed. Classical as well as tunneling electron emission mechanisms are considered. It was found that electron emission is governed by an interplay of processes inside as well as above the cathode. A hybrid model is proposed, where the electron distribution within the target is retrieved from Boltzmann scattering integrals, while the charge distribution above it is studied by a Particle-In-Cell simulation. The results indicate that non-equilibrium effects determine the initial emission process, whereas the space charge above the target suppresses the effectively emitted charge.

Laser-induced electron emission from a tungsten nanotip: identifying above threshold photoemission using energy-resolved laser power dependencies

We present an experiment studying the interaction of a strongly focused 25 fs laser pulse with a tungsten nanotip, investigating the different regimes of laser-induced electron emission. We study the dependence of the electron yield with respect to the static electric field applied to the tip. Photoelectron spectra are recorded using a retarding field spectrometer and peaks separated by the photon energy are observed with a 45% contrast. They are a clear signature of above threshold photoemission (ATP), and are confirmed by extensive spectrally resolved studies of the laser power dependence. Understanding these mechanisms opens the route to control experiment in the strong-field regime on nanoscale objects.

Generalized model for ultrafast laser induced electron emission from a metal tip

In this paper, we will present a brief review of the recently developed non-equilibrium heating and time dependent tunneling model to study the dynamic processes in using an ultrafast laser to induce electron emission from a metallic tip ranging from the multiphoton to the optical tunneling regime. Due to the short time scale of the ultrafast laser pulse, the lattice is found to be in a non-equilibrium condition and a single temperature model is no longer valid. The ultrafast laser heating enhances the electron emission through both the multiphoton and optical tunneling processes rather than over-barrier emission due to thermal heating. The paper is focused on the methodology of how these two effects (non-equilibrium heating and time-dependent tunneling) are combined in a self-consistent model. The model shows a smooth transition of the emitted charge as a function of laser field, ranging from the multiphoton emission regime at low laser field to the optical tunneling regime at high laser field. The paper will conclude with some discussion of future work.

Plasmonic Near-Electric Field Enhancement Effects in Ultrafast Photoelectron Emission: Correlated Spatial and Laser Polarization Microscopy Studies of Individual Ag Nanocubes

Electron emission from single, supported Ag nanocubes excited with ultrafast laser pulses (λ = 800 nm) is studied via spatial and polarization correlated (i) dark field scattering microscopy (DFM), (ii) scanning photoionization microscopy (SPIM), and (iii) high-resolution transmission electron microscopy (HRTEM). Laser-induced electron emission is found to peak for laser polarization aligned with cube diagonals, suggesting the critical influence of plasmonic near-field enhancement of the incident electric field on the overall electron yield. For laser pulses with photon energy below the metal work function, coherent multiphoton photoelectron emission (MPPE) is identified as the most probable mechanism responsible for electron emission from Ag nanocubes and likely metal nanoparticles/surfaces in general.

Ultrafast laser-induced electron emission from multiphoton to optical tunneling

Based on a time-dependent quantum model, a relation between the onset of the optical tunneling regime and the metal work function is determined. In the multiphoton regime, the number of photons required for absorption is reduced from $n=3$ (at pulse length $\ensuremath{\tau}&gt;20$ fs) to $n=2$ (at $\ensuremath{\tau}&lt;8$ fs) due to the energy uncertainty principle. The phase of the laser is important for optical tunneling, but is only manifest in the multiphoton regime when the number of laser cycles is close to or less than 1. The effect of the field gradient at the tip can be important when the radius of the tip is 40 nm or smaller. The extension of the model to include nonequilibrium electron distribution due to ultrafast laser excitation is discussed. Comparisons with other models and experimental findings are presented.