Resonant-Plasmon-Assisted Subwavelength Ablation by a Femtosecond Oscillator

We experimentally demonstrate the use of subwavelength optical nanoantennas to assist a direct nanoscale ablation using the ultralow fluence of a Ti:sapphire oscillator through the excitation of surface plasmon waves. The mechanism is attributed to nonthermal transient unbonding and electrostatic ablation, which is triggered by the surface plasmon-enhanced field electron emission and acceleration in vacuum. We show that the electron-driven ablation appears for both nanoscale metallic as well as dielectric materials. While the observed surface plasmon-enhanced local ablation may limit the applications of nanostructured surfaces in extreme nonlinear nanophotonics, it, nevertheless, also provides a method for nanomachining, manipulation, and modification of nanoscale materials. Collateral thermal damage to the antenna structure can be suitably avoided, and nonlinear conversion processes can be stabilized by a dielectric overcoating of the antenna.

Figure 1

SEM images of an Au nanoantenna with ${\mathrm{SiO}}_2$ dielectric coating in perspective view. (a) Pristine, (b) after 50 min of laser irradiation.

Figure 2

SEM images and EDS maps. SEM images of bow-tie nanoantennas after 30 min of laser irradiation in the (a) $x\text{−}y$ plane and (b) the $x\text{−}z$ plane. (c)–(j) EDS element maps of the laser exposed nanostructure in the $x\text{−}y$ plane (left row) and in the $x\text{−}z$ plane (right row), which shows that the generated nanostructure in the gap contains indeed Si and O, as indicated in the red circles in (c)–(f).

Figure 3

SEM images of bow-tie nanoantennas with different heat capacities and with a 5-nm coating after modification by 8 fs (a),(c) and 16 fs (b),(d) laser pulses from an oscillator. The thicknesses of nanoantennas in (a),(b) has been reduced to 50 nm while the thickness in (c),(d) is 135 nm. All the samples are illuminated by the same number of laser pulses.

Figure 4

Top-view (a) and enlarged side-view (b) SEM image of a pure Au nanoantenna after laser ablation. The thickness of the antenna is 135 nm. EDS map showing the Au content of an ablated (c) and pristine (d) Au antenna.

Figure 5

(a) Experimentally measured extinction spectra of plasmon resonant Au nanoantennas before (green squares) and after (blue circles) laser irradiation. Thermal melting reshapes the bow-tie nanoantennas into nanospheres, and thus the plasmonic resonance disappears. (b) The increase of heat capacity induces the transition from thermal melting to near-field-enhanced nonthermal ablation, leading to the deposit of a low-density thin film in the feed gap. Extinction spectra of nanoantennas after near-field ablation (blue circles) still show the plasmonic resonance, yet attenuated with respect to the pristine antennas (green squares).

Figure 6

Simulations of near-field distribution of the antenna with (a) and without (b) ${\mathrm{SiO}}_2$ coating. (c) Gap-plasmon-enhanced surface THG from ${\mathrm{SiO}}_2$-coated (green squares) and metallic (blue circles) antenna as a function of the laser polarized direction. (d) Temporal evolution of surface THG with long-term irradiation (green curve, coated antenna; blue curve, metallic antenna). (e) Electron density at the surface of fused silica (blue) and the laser intensity envelope (red). (f) Representative trajectories of electrons tunneling at different phases. Some electrons are sent back to the Au surface, while others remain in vacuum. The black arrow indicates the direction of electron velocity. The electron density also decreases along the black arrow, meaning that most of the quiver electrons are close to the surface after a single laser cycle. (g) Energy (black curve) and trajectory (red curve) of the most energetic returning electrons.

Figure 7

Numerical simulations of the near-field enhancement $|E/E_0|$ in the gap region of a pristine (a) and an ablated (b) gold bow-tie nanoantenna. The geometrical parameters for simulations (dashed white curves) are in excellent agreement with the real size of the nanostructures (embedded false-color SEM images) before (a) and after (b) ablation. Note that $r_0$ and $r_1$ are the radius of curvature at the tips, $g$ is the gap size, ${ϵ}_{\mathrm{Au}}$ and ${ϵ}_{\mathrm{air}}$ denote the dielectric constant of gold bulk and air, respectively.

Figure 8

Schematic illustration of the electrostatic ablation mechanism. The plasmonic near-field enhancement at the surface layer results in strong ionization and hot electron emission. After the pulse, the electrons remaining in the vacuum create an intense electrostatic field, which may then pull individual ions out of the surface.