Characterizing the optical near‐field in the vicinity of a sharp metallic nanoprobe by angle‐resolved electron kinetic energy spectroscopy

Abstract

The effect of the near-field distribution of sharply etched, nanometer-sized gold tapers on angle-resolved kinetic energy spectra of electrons photoemitted by ultrafast laser pulse irradiation is investigated both experimentally and theoretically. In the experiments, the enhancement of the local electric field at the tip apex is sufficiently large to enable optical field induced tunneling of electrons tunnel out of the metal tip. Strong field gradients near the tip apex, with a decay length shorter than the quiver amplitude, accelerate the electrons to high energies within less than one optical cycle. This electron emission is confined to a narrow cone angle around the taper axis, while low-energy quiver electrons cover a much broader angular range. This subcycle acceleration manifestly alters the energy distribution of the emitted electrons, resulting in pronounced plateaus in their kinetic energy spectra. The electron motion in the curved vectorial electric field is analyzed and it is shown that observed changes of both the kinetic energy spectra and their angular distribution depend sensitively on the near-field decay length and curvature, which indicates that such angle-resolved kinetic energy spectra of photoemitted electrons give information on the optical near-field distribution in the vicinity of nanometer-sized field emitters.

k-space imaging of the eigenmodes of sharp gold tapers for scanning near-field optical microscopy

“…Theoretically, it has been shown that the fields at the tip apex decay with a length slightly shorter than the apex radius [26–28]. Taking image charge effects into account [21–22], the decay in k-space closely resembles the behavior in real space.…”

Plasmonic nanofocusing – grey holes for light

“…As a consequence, the usual quiver motion of an electron in the oscillating laser field can be suppressed if the near-field decay length l F is shorter than the electron quiver amplitude l q = e⋅f ⋅E 0 m 2 . The spatial adiabaticity parameter = l F l q is thus an important characteristic for the motion of electrons in the near field around nanostructures [124,143,144]. Near-field gradient acceleration hence provides an interesting experimental concept for controlling the trajectories and kinetic energy distributions of electron pulses generated by strong-field photoemission from metallic nanotapers.…”

Multiphotonk-resolved photoemission from gold surface states with 800-nm femtosecond laser pulses

“…While above-threshold PE has been observed for different materials and sample shapes [2][3][4][5][6][7][8][9], studies of k-resolved multiphoton PE from flat surfaces are still scarce. Recently, two-and three-photon PE spectra have been measured in angle-resolved experiments on Ag(111) [10] and Cu(110) [11] using a relatively low laser intensity, corresponding to a power density of the order of 10 GW/cm 2 at a wavelength of λ = 400 nm.…”

“…50 fs, carrier envelope phase phenomena and the intrinsic loss of energy resolution can be safely neglected. In terms of peak intensity, at 100 GW/cm 2 tunneling phenomena are still negligible for a flat metal surface at infrared wavelengths (Keldysh parameter γ > 1 [12]), so that a multiphoton regime can be assumed, at variance with the case of emission from sharp objects like nanotips, where a strong field enhancement is induced by the small curvature [8,9,16,17]. For instance, ultrafast demagnetization [18] and optical manipulation of magnetic order experiments [19][20][21], of growing importance in the last decade, are well within the multiphoton regime.…”