Efficient Nonlinear Light Emission of Single Gold Optical Antennas Driven by Few-Cycle Near-Infrared Pulses

Individual nanometer-sized plasmonic antennas are excited resonantly with few-cycle laser pulses in the near infrared. Intense third-harmonic emission of visible light prevails for fundamental photon energies below 1.1 eV. Interband luminescence and second harmonic generation occur solely at higher driving frequencies. We attribute these findings to multiphoton resonances with the $d$-band transitions of gold. The strong third-order signal allows direct measurement of a subcycle plasmon dephasing time of 2 fs, highlighting the efficient radiation coupling and broadband response of the devices.

Figure 1

Spectrally integrated nonlinear emission from 48 individual linear gold antennae of different arm length $l$ and feed gap $g$. The arm width $w$ is kept constant at 60 nm. Lateral and vertical distances between antennas are set to $4\mu \mathrm{m}$ and $3\mu \mathrm{m}$, respectively. The nanostructures are raster scanned through the focus of a 7.8-fs laser pulse with a spectrum extending from 0.85 to 1.4 eV. The visible intensity is depicted in a color scale as a function of position. The dashed line is a guide to the eye illustrating the regions with antennae in resonance with the driving field. Inset: schematic drawing of the antenna geometry studied in this data set.

Figure 2

Typical nonlinear emission spectrum from a single gold optical antenna excited with the total bandwidth of the 7.8-fs pulse. SHG: second harmonic generation, TPPL: two-photon photoluminescence, THG: third-harmonic generation. Inset: scanning electron microscope image of the antenna. A bowtie-shaped geometry is featured in this figure which is composed of two opposing nanotriangles (triangle height: 390 nm, triangle base: 360 nm, gap: 60 nm).

Figure 3

Nonlinear emission from resonant gold nanoantennas depending on excitation photon energy. (a) Third-harmonic emission (THG, blue line) of a linear device with arm length $l=320\mathrm{nm}$, width $w=60\mathrm{nm}$ and feed gap $g=40\mathrm{nm}$, excited with a pump spectrum (EXC) confined below 1.1 eV (dashed red line). (b) The two-photon photoluminescence (TPPL) and second harmonic generation (SHG) from a specimen with $l=240\mathrm{nm}$, $w=60\mathrm{nm}$ and $g=40\mathrm{nm}$, excited with a pump photon energy above 1.1 eV (magenta dashed line) is depicted as a green line. The peak spectral intensity of the TPPL/SHG signal is weaker by a factor of 19.5 as compared to the THG emission in (a). (c) and (d) Sketches of the band structure of gold illustrating the relevant pump and emission processes for the conditions in (a) and (b), respectively. Arrow lengths indicate photon energies in excitation (up) and emission (down). Colors correspond to the processes indicated in (a) and (b).

Figure 4

Dephasing of a single resonantly driven gold nanoantenna: Fourier-filtered third-harmonic emission intensity as a function of delay time and frequency for (a) the bare fused silica substrate and (b) an elliptical gold nanoantenna (arm length $l=250\mathrm{nm}$, width $w=100\mathrm{nm}$, and feed gap $g=40\mathrm{nm}$). Integrated emission spectra are depicted as white lines. Interferometric third-order autocorrelation traces from (c) the substrate and (d) the resonant antenna. Careful analysis and comparison of the experimental data (dotted lines) with simulations (red lines) yield a reference pulse duration of $t_p=9.8\mathrm{fs}$ and a plasmon dephasing time of ${\tau }_{\mathrm{pl}}=2.0\mathrm{fs}$.