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

Hirofumi Yanagisawa, Christian Hafner, Patrick Doná, Martin Klöckner, Dominik Leuenberger, Thomas Greber, Jürg Osterwalder, and Matthias Hengsberger
Phys. Rev. B 81, 115429 – Published 17 March 2010
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  • INTRODUCTION
  • METHODOLOGY
  • OPTICAL CONTROL OF FIELD-EMISSION SITES…
  • VOLTAGE AND POWER DEPENDENCE OF EMISSION…
  • CONCLUSIONS
  • ACKNOWLEDGEMENTS
    • References

    Abstract

    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: (i) the polarization of the light, (ii) the azimuthal, and (iii) 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 photoexcited 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 photoexcitation 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.

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    • Received 28 January 2010

    DOI:https://doi.org/10.1103/PhysRevB.81.115429

    ©2010 American Physical Society

    Authors & Affiliations

    Hirofumi Yanagisawa1,*, Christian Hafner2, Patrick Doná1, Martin Klöckner1, Dominik Leuenberger1, Thomas Greber1, Jürg Osterwalder1, and Matthias Hengsberger1

    • 1Physik Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
    • 2Laboratory for Electromagnetic Fields and Microwave Electronics, ETH Zürich, Gloriastrasse 35, CH-8092 Zürich, Switzerland

    • *hirofumi@physik.uzh.ch

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    Vol. 81, Iss. 11 — 15 March 2010

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    Images

    • Figure 1
      Figure 1
      (Color online) Schematic of the experimental setup (a). A tungsten tip is mounted inside a vacuum chamber. Laser pulses are generated outside the vacuum chamber. An aspherical lens is located just next to the tip to focus the laser onto the tip apex. Emitted electrons are detected by a position-sensitive detector in front of the tip. The polarization angle θP is defined in the inset, where the laser beam propagates toward the reader’s eye (see the text for further description). (b) shows a schematic defining the orthogonal angle between the laser propagation direction and the tip axis. The right angle is where x is maximum. (c) shows tip positions in (x,y) coordinates for different θ, found while the tip apex is kept in the focus of the laser. The angle which gives maximum x is defined as θ=0 for convenience.Reuse & Permissions
    • Figure 2
      Figure 2
      (Color online) Schematic illustration of the excitation and interference (a) of surface EM waves and (b) of the model tip. The radius of curvature of the tip apex is 100 nm, and the length is 700 nm. (c) represents the field distribution of the focused beam used in the simulation at a certain time. The beam waist is 1 μm and the wavelength is 800 nm. Small arrows indicate the field direction, and the field strength is represented by a linear color scale: highest field values are represented in yellow (white). The calculated time-averaged field distribution around the model tip is shown in (e) for θP=0° together with a magnified picture in the vicinity of the tip apex. (e) shows the time-averaged field distribution around a longer model tip together with a magnified picture in the vicinity of the tip apex; the tip length is twice as large as that of (d). (f) represents the time-averaged field distributions around the model tip simulated by incident laser light with wavelengths of 750, 800, and 850 nm.Reuse & Permissions
    • Figure 3
      Figure 3
      (Color online) Electron emission patterns for two orthogonal azimuthal orientations (φ) of the tip without laser [(a) φ=0° and (c) φ=90°], and with laser irradiation [(b) φ=0° and (d) φ=90°]. Vtip indicates the dc potential applied to the tip and PL indicates the laser power measured outside the vacuum chamber. The insets in (a) and (c) show the front view of the atomic structure of a tip apex with a curvature radius of 100 nm, based on a ball model, in which green areas with white edges indicate the field-emission sites and the dashed red arrow indicates the laser propagation direction. The inset in (b) shows a schematic side view of the laser-induced field-emission geometry, in which green (gray) vectors indicate intensities of electron emission and the white arrow indicates the laser propagation direction. A dashed white line denotes a mirror symmetry line of the atomic structure in each picture. In (c) and (d) specific regions of interest, marked by dashed red (light gray) rectangles, are blown up on the right-hand side.Reuse & Permissions
    • Figure 4
      Figure 4
      (Color online) θ dependence of LFEM images at [φ=0°,θP=0°], which were taken at Vtip=−1500 V and PL=20 mW. θ is varied from θ=−12° to θ=12° by 4° steps. Schematics for the experimental configuration are shown at the top, in which red (gray) arrows indicate the laser propagation direction. The corresponding FEM images are also shown below, which were taken at Vtip=−2200 V. The white dashed lines in the pictures denote a mirror symmetry line of the atomic structure. The total yield Sright from right side of each image and the total yield from left side Sleft with respect to the white dashed line were taken. The ratio of Sright to Sleft is plotted in the graph. Blue circles are for LFEM and black squares are for FEM.Reuse & Permissions
    • Figure 5
      Figure 5
      (Color online) Comparison of measured and simulated laser-induced FEM (LFEM) images for different light polarization angles θP and for different azimuthal orientations φ of the tip. The leftmost column gives the FEM images without laser irradiation for four different azimuthal angles (Vtip=−2250 V). For the same azimuthal angles, observed LFEM images are shown as a function of polarization angle θP in 30° steps (Vtip≈−1500 V and PL=20 mW). The simulated LFEM images from the photofield-emission model, in which Vtip and PL were set as in the corresponding experiments, are shown on the right-hand side of the observed LFEM images. The color scale and laser propagation direction are the same as in Fig. 3. This figure is a reproduction of Fig. 3 from Ref. 20.Reuse & Permissions
    • Figure 6
      Figure 6
      (Color online) (a) Time evolution of laser fields over a cross section of the model tip while propagating through the tip apex from left to right. The polarization vector is parallel to the tip axis (θP=0°). Small black arrows indicate the field direction, and field strength is represented by a linear color scale: highest field values are represented in yellow (white). The time-averaged field distribution for tungsten and gold tips is shown in (b) and (c) where the model tip of Fig. 2b is employed for both. The dielectric functions of tungsten and gold for the wavelengths between 700 and 900 nm are plotted by black dots in (d) and the values at 800 nm are highlighted by red circles. (e) shows the time-averaged field distributions around the tungsten tip for three different polar angles: θ=−12°, 0°, and 12°. In (f) the time-averaged field distributions are given in a front view of the model tip for different polarization directions θP (θ=0°). The laser propagation direction is indicated by dashed red arrows and is the same as in the experiment.Reuse & Permissions
    • Figure 7
      Figure 7
      (Color online) (a) A schematic of field emission from a Fermi-Dirac distribution, where an electron with a normal energy W is emitted. The surface barrier above W is shown by a cross-hatched area. (b) and (c) show schematics of photofield emission from a nonequilibrium electron distribution and optical field emission from a Fermi-Dirac distribution, respectively.Reuse & Permissions
    • Figure 8
      Figure 8
      (Color online) The calculated dc field distribution around the model tip is shown in (a). A wirelike shape was employed for the simulation of relative dc fields. The color scale is the same as in Fig. 6. The relative dc field distribution at the tip apex of (a) is shown as a function of angle θc, which is defined in (a). The obtained work-function profile along a (001)-(011)-(010) curve is shown in (c) as a function of θc.Reuse & Permissions
    • Figure 9
      Figure 9
      (Color online) (a) Experimentally obtained LFEM image and simulated LFEM image at [φ=0°,θP=0°] for both photofield-emission and optical field-emission models. (b) shows line profiles extracted from the observed FEM images (red line with squares), LFEM images [dashed blue (light gray)], and from LFEM images simulated by the photofield-emission model (green solid line) and the optical field-emission model (black dashed line) at [φ=0°,θP=0°]. The whole scanned line corresponds to the unfolded rectangle indicated by the dashed blue line in the FEM figures above, and the corresponding sides are indicated by white arrows. Each line profile has been normalized by the maximum value.Reuse & Permissions
    • Figure 10
      Figure 10
      (Color online) Laser power and tip voltage dependence of the electron emission patterns at [φ=0°,θP=0°]. On the vertical axis laser power and on the horizontal axis tip voltage are plotted. The inset shows the electron emission patterns where the laser beam is displaced from the tip apex downward by distances of 1 and 16 μm, taken at the 90 mW laser power. The definition of distance d is also shown on the right-hand side of the inset. The time-averaged field distribution around the tip when the laser beam is displaced by a distance of 0.5 μm is also shown in the inset, where the longer model tip shown in Fig. 2e was used. The green dashed circles highlight the left-side electron emission sites (see Sec. 4B).Reuse & Permissions
    • Figure 11
      Figure 11
      (Color online) (a) Simulated electron energy distribution curves. The spectrum at the top of (a) shows the simulated electron energy distribution of field-emitted electrons with the parameters representing the point where maximum laser intensity can be observed for φ=0°, θP=0°. The parameters are the following: the work function is 4.45 eV, a dc field of 1.32 V/nm corresponding to a tip voltage of −1500 V, and S1 is 1.6×10−6 for 20 mW laser power. The energy distribution for lower dc fields but with the same laser power is also shown in (a). The vacuum level Evac is defined as 0 eV, and the Fermi level EF is 4.45 eV below Evac. The energies corresponding to one-, two-, and three-photon excitations from EF (1PPE, 2PPE, and 3PPE) are also indicated by vertical dashed lines. The simulated LFEM images at the corresponding tip voltage and laser power are shown in (b).Reuse & Permissions
    • Figure 12
      Figure 12
      (Color online) FN plots for FEM and LFEM (PL=10, 20, 30, 40, and 50 mW). The vertical axis is signal i over Vtip2 on a logarithmic scale. The signal i is the total yield of electron emission from the right-hand (310)-type facet of each image in Fig. 10. The horizontal axis is 1/Vtip.Reuse & Permissions
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