Attosecond control of electrons emitted from a nanoscale metal tip

Abstract

Attosecond science is based on steering electrons with the electric field of well controlled femtosecond laser pulses. It has led to the generation of extreme-ultraviolet pulses with a duration of less than 100 attoseconds (ref. 3; 1 as = 10(-18) s), to the measurement of intramolecular dynamics (by diffraction of an electron taken from the molecule under scrutiny) and to ultrafast electron holography. All these effects have been observed with atoms or molecules in the gas phase. Electrons liberated from solids by few-cycle laser pulses are also predicted to show a strong light-phase sensitivity, but only very small effects have been observed. Here we report that the spectra of electrons undergoing photoemission from a nanometre-scale tungsten tip show a dependence on the carrier-envelope phase of the laser, with a current modulation of up to 100 per cent. Depending on the carrier-envelope phase, electrons are emitted either from a single sub-500-attosecond interval of the 6-femtosecond laser pulse, or from two such intervals; the latter case leads to spectral interference. We also show that coherent elastic re-scattering of liberated electrons takes place at the metal surface. Owing to field enhancement at the tip, a simple laser oscillator reaches the peak electric field strengths required for attosecond experiments at 100-megahertz repetition rates, rendering complex amplified laser systems dispensable. Practically, this work represents a simple, extremely sensitive carrier-envelope phase sensor, which could be shrunk in volume to about one cubic centimetre. Our results indicate that the attosecond techniques developed with (and for) atoms and molecules can also be used with solids. In particular, we foresee subfemtosecond, subnanometre probing of collective electron dynamics (such as plasmon polaritons) in solid-state systems ranging in scale from mesoscopic solids to clusters and to single protruding atoms.

Extreme-ultraviolet light generation in plasmonic nanostructures

“…Numerous characteristic nonlinear optical phenomena, such as above-threshold and strongfield photoemission, have become accessible using local field enhancements in metal nano-tips [12,62,13,14,15], resonant optical antennas [24,28,16,81], nanoparticles [17], rough surfaces [63], and plasmonic waveguides [26,15]. These experimental studies are paralleled by increasing theoretical efforts of describing strong-field effects at surfaces and in optical near fields [82,83,64], with a particular emphasis on nanostructure-enhanced high-order harmonic generation (HHG) [46,65,66,84,67,85,86,87,88,89,90].…”

“…Recently, even higher nonlinear effects such as above-threshold and strong-field photoemission and acceleration were observed in plasmonic and other nanostructures [12,62,63,17,15,64], closing the gap to strong-field and attosecond physics. [13,14] Another key example of this development was the report of high-harmonic generation (HHG) in plasmonic bow-tie antennas [24], that is, the integration of the most prominent strong-field effect of gaseous media [20,33] into a nanostructure. Despite triggering widespread theoretical [46,65,66,67] and experimental [26,28] efforts, subsequently, this effect has remained rather elusive.…”

“…Moreover, multiphoton and strongfield photoemission from the waveguide walls [13,63,14] will provide additional electrons capable of collisional excitation. We have found that upon moderate waveguide modification by using incident powers in excess of 200 mW, the steepness of the threshold slope could be influenced, potentially by introducing roughness and thus altering the contributions from surface-emitted electrons.…”

“…Recently, also higher-order nonlinear optical phenomena have become accessible in such experiments. Specifically, the so called strong-field regime was reached in nanostructures using local intensities sufficient to trigger field-driven ionization and electron accelerationeffects that have been shown in the context of the strong-field photoelectric effect at metal nanotips, -antennas and -particles [12,13,14,15,16,17]. Furthermore, the spatial confinement and inhomogeneity of the enhanced near-fields often lead to alternative, nanostructure-specific scalings of the observed processes, which are at variance from those exhibited by their classical-non-field-enhanced-counterparts.…”

The impact of the carrier envelope phase-dependence on system and laser parameters

“…Theoretical calculations have shown that the (high energy) parts of PES coming from electron recollisions are more sensitive to CEP than the (low energy) parts from directly emitted electrons [15,12,16,17,18]. Experimentally, the dependence of high-energy PES on the CEP has been explored for atoms [13,19,20], dimer molecules [21], and nano-tips [22,23], providing challenging motivations for the theoretical investigations.…”