Double resonant plasmonic nanoantennas for efficient second harmonic generation in zinc oxide

We demonstrate the efficient generation of second harmonic light in zinc oxide (ZnO) by utilizing double resonant plasmonic nanoantenna arrays. The antenna design is based on two gold dipole rods with plasmonic resonances at $\omega$ and $2\omega$, enabling strong localization of light at the fundamental frequency $\omega$ within the ZnO, as well as improved reemission of the second harmonic generation (SHG) at $2\omega$ into the far field. Wavelength-dependent SHG measurements show that the intensity of the far-field signal strongly depends on the properties of the ZnO substrate: While a bulk-ZnO substrate causes the SHG signal emitted from the nanoantennas to decrease, a thin-film ZnO substrate provides a strongly enhanced signal from the double resonant antennas. Comparing the wavelength-dependent results from the double resonant antennas with single dipole rods, the enhancement of the SHG intensity is more than twofold. Our experimental results confirm theoretical calculations of the SHG obtained from the double resonant antenna arrays.

Figure 1

Sample design for the double resonant antennas on ZnO. (a) Schematic of the fabricated double resonant structure consisting of two gold nanorods with a small gap. (b) Scanning electron microscopy image of a fabricated antenna array. (c),(d) Schematic cross section of both samples with different substrate configurations.

Figure 2

Measured extinction spectra of different antenna arrays in dependence of the fundamental antenna length on bulk ZnO substrate (sample A). For each array, two resonances are apparent. The resonance of the SHG antenna remains stationary, while the fundamental resonance can be tuned. The corresponding gap size between both antennas is 40 nm.

Figure 3

Measured SHG intensities (filled squares) of double resonant antenna arrays on sample A (left) with the bulk ZnO substrate and sample B (right) with the thin-layer ZnO for various wavelengths. The black line represents a Lorentz fit of the measured data.

Figure 4

(a) Comparison between the SHG intensities from a double resonant nanoantenna and single dipole rod antennas. The SHG signal from the sample can be increased by a factor of two due to the resonance of the shorter antenna at about 700 nm. The solid line shows the corresponding Lorentz fit. (b) Cross section of the calculated electric field amplitudes for resonant excitation from both antenna structures. The strong fields at the end of the antenna structures arise from the resonant excitation of the LSPPs. The field overlap between both modes (at the fundamental and SHG wavelengths) leads to an improved radiation of the SHG signal to the far field.

Figure 5

(a) Experimental data showing the ratio of the measured SHG signal intensity from the double resonant antennas compared to the single dipoles for different antenna lengths $L_f$. Shifting the fundamental antenna resonance results in a weaker overlap of the generated frequency-doubled signal with the SHG antenna resonance. (b) Simulated SHG intensities from the corresponding FEM calculations for different lengths of the fundamental antenna. The insets show the spectral overlap of the extinction spectra for the fundamental and SHG antenna for the two marked points.